Revised land use planning arrangements around large scale petroleum depots  RR511

Health and Safety Executive
Revised land use planning
arrangements around large scale
petroleum depots Prepared by Environmental Resources Management Ltd
for the Health and Safety Executive 2007
RR511
Research Report
Health and Safety Executive
Revised land use planning
arrangements around large scale
petroleum depots Dr Andrew Franks CEng MIChemE
Environmental Resources Management Ltd
Manchester Office
Suite 8.01
8 Exchange Quay
Manchester M5 3EJ
Following the incident at the Buncefield Oil Storage Depot in December 2005, the Health and Safety Executive (HSE)
commissioned Environmental Resources Management (ERM) to assist in reviewing HSE’s approach to providing land­
use planning (LUP) advice in the vicinity of similar installations.
Proposals for revised arrangements for provision of LUP advice have been developed. Two options are presented. The
proposals are based on a review of information on the effect of blast on building occupants, observations of the blast
damage at Buncefield, and a review of some of the justification underpinning certain aspects of the current
arrangements. Both of the options proposed would result in greater restriction on development of land in the vicinity of
those sites affected.
For both options, the proposed system would operate within a defined geographical area around affected sites. The
sensitivity of the size of this area to certain factors has been investigated.
This report and the work it describes were funded by the Health and Safety Executive (HSE). Its contents, including any
opinions and/or conclusions expressed, are those of the author alone and do not necessarily reflect HSE policy.
HSE Books
© Crown copyright 2007
First published 2007
All rights reserved. No part of this publication may be
reproduced, stored in a retrieval system, or transmitted in
any form or by any means (electronic, mechanical,
photocopying, recording or otherwise) without the prior
written permission of the copyright owner.
Applications for reproduction should be made in writing to:
Licensing Division, Her Majesty’s Stationery Office,
St Clements House, 2­16 Colegate, Norwich NR3 1BQ
or by e­mail to hmsolicensing@cabinet­office.x.gsi.gov.uk
Acknowledgements
The assistance of personnel in HSE’s Hazardous Installations
Directorate, and in particular Dr David Painter and
Mr John Murray, is gratefully acknowledged. ii
CONTENTS
CONTENTS
III
EXECUTIVE SUMMARY
V
1
INTRODUCTION
1
2
CURRENT ARRANGEMENTS
2
2.1
2.2
LAND USE PLANNING ZONES
DEVELOPMENT TYPES AND HSE’S ADVICE
2
4
3
REQUIREMENT FOR REVIEW OF CURRENT ARRANGEMENTS
8
4
OBJECTIVE OF REVISED ARRANGEMENTS
10
5
OPTIONS FOR REVISED LUP ARRANGEMENTS
11
5.1
5.2
CHANGES TO LUP ZONES
CHANGES TO SENSITIVITY LEVEL DEFINITIONS
15
15
6
DEVELOPMENT OF PROPOSALS FOR REVISED ARRANGEMENTS 16
6.1
6.2
6.2.1
6.2.2
6.2.3
6.3
6.4
REVISION OF LUP ZONE BOUNDARIES
REVISION OF SENSITIVITY LEVEL DEFINITIONS
Small housing, hotel / hostel and retail / leisure developments
Workplaces
Summary of Recommendations
OPTIONS
APPLICATION
16
17
17
19
20
20
20
7
IMPACT OF THE PROPOSED ARRANGEMENTS
21
7.1
7.2
SUMMARY OF IMPACTS
ADVANTAGES AND DISADVANTAGES
44
44
8
SENSITIVITY
46
iii
iv
8.1
8.1.1
8.1.2
8.2
8.2.1
8.2.2
8.2.3
8.2.4
8.2.5
8.3
8.3.1
8.3.2
8.4
SOURCE TERM
Sensitivity to Source Geometry
Sensitivity to Fuel Composition
VAPOUR DISPERSION
Release Duration
Topography
Weather
Presence of Ignition Sources
Source Term
EXPLOSION PROPERTIES
Explosion Size
Explosion Strength
SUMMARY
46
47
50
50
51
51
51
51
51
52
52
54
56
9
SUMMARY AND CONCLUSIONS
59
10
REFERENCES
61
APPENDIX 1 REVIEW OF RESEARCH ON EXPLOSION EFFECTS
62
APPENDIX 2 BUILDING DAMAGE OBSERVATIONS
69
APPENDIX 3 ESTABLISHING REVISED ZONE BOUNDARIES
75
APPENDIX 4 CASE SOCIETAL CONCERNS - WORKPLACES
78
EXECUTIVE SUMMARY Following the incident at the Buncefield Oil Storage Depot in December 2005, the Health and
Safety Executive (HSE) commissioned Environmental Resources Management (ERM) to assist
in reviewing HSE’s approach to providing land-use planning (LUP) advice in the vicinity of
similar installations.
At the time of writing the detailed causes of the incident are still under investigation, although a
series of reports have been published by the Major Incident Investigation Board (MIIB) (MIIB,
2006a; MIIB, 2006b; MIIB, 2006c; MIIB, 2006d).
The work performed by ERM has comprised the following:
• performing a review of the current LUP arrangements;
• consideration of the various options for revised LUP arrangements in the light of the
experiences at Buncefield;
• development of the selected options for revised arrangements;
• investigation of the impact of the proposed revised arrangements upon the LUP advice
provided by HSE to local authorities; and,
• examination of the sensitivity of the proposed options to a range of factors associated
with the features of the major hazard sites of interest.
The Health and Safety Executive (HSE) provides land use planning (LUP) advice to local
authorities regarding applications for development in the vicinity of major hazard installations
and major hazard pipelines. This report relates only to HSE’s approach to providing LUP
advice around major hazard installations.
At present HSE employs a system for determining the advice to be given that comprises the
following elements:
• establishing three concentric (but not necessarily circular) zones around the site in
question, termed the inner, middle and outer zones;
• assigning development proposals to one of four ‘Sensitivity Levels’; and,
• determining the advice to be given by essentially using a matrix that shows which
development Sensitivity Levels are considered by HSE to be inadvisable in which
zones.
For major hazard sites handling or storing flammable substances a ‘protection based’ approach
is used to determine the zone boundaries. In most cases the protection based approach involves
considering the consequences of a selected event. Zone boundaries are set at the range to
different levels of harm that would be expected to result from the event:
• the boundary between the inner and middle zones is usually set at the range at which members of a typical population could be subjected to a level of harm corresponding to a significant likelihood of death; • the boundary between the middle and outer zones is usually set at the range at which a typical, exposed population could experience a dangerous dose; v
• the outermost edge of the outer zone is usually set at the range at which a sensitive or
vulnerable population could experience a dangerous dose.
The requirement for review of the current arrangements arises from the experience of the
Buncefield incident. As a result of the MIIB investigations, it is known that:
•
•
•
•
•
a storage tank was filled to overflowing with petrol;
as the petrol overflowed from the tank a substantial vapour cloud was generated;
the vapour cloud spread over parts of the site and to areas beyond the site boundary;
the vapour cloud was ignited, resulting in a vapour cloud explosion (VCE); and,
the VCE caused extensive damage to buildings, particularly those close to the site of the
explosion.
At present it is not clear why the vapour cloud exploded with such force. Although accidents
involving VCEs have occurred before and the VCE phenomenon has been the subject of much
scientific investigation, an explanation for the strength of the VCE experienced at Buncefield is
currently lacking.
At present HSE performs protection based assessments of sites like that at Buncefield (storage
facilities for petroleum products). This assessment involves considering the potential for a
VCE. However, on the basis of current scientific understanding of the way in which VCEs
occur, the potential for a VCE at a site like Buncefield would be limited to those parts of the
facility that provided sufficient confinement or congestion to generate a VCE, such as the tanker
loading rack, giving rise to relatively small hazard ranges. The protection based assessment
would therefore be likely to be based on other hazards, typically large pool fires.
Proposals have been developed for revised arrangements for HSE’s provision of LUP advice.
The proposals have been developed by:
• reviewing published HSE research on the effects of explosions (Jeffries et al., 1997;
Galbraith 1998);
• considering the levels of damage to buildings observed around the Buncefield site;
• considering the principles upon which HSE’s advice is based; and,
• revisiting the justifications underlying the Sensitivity Level definitions for proposed
developments in the light of the other activities.
The specific objectives of the proposed revised arrangements are to:
• establish LUP zone boundaries that take into account observations arising from the
Buncefield incident; and,
• provide a decision making framework for use with the revised LUP zones that takes into
account the particular nature of the VCE hazard.
The proposed arrangements are protection based. The objective of this and other protection
based approaches is to:
vi
“achieve a separation between developments and the site which provides a very high degree of
protection against the more likely smaller events, whilst also giving very worthwhile (sometimes
almost total) protection against unlikely but foreseeable larger-scale events.” (HSE, 1989).
It is not the objective of the proposed arrangements to provide complete protection for future
developments from damage, or for their occupants from harm. Neither is it the objective of the
proposed arrangements to suggest what should be done about existing land use around affected
sites.
It was judged that any revised approach should have regard for the current state of knowledge
regarding events at Buncefield, and should be somewhat precautionary in nature.
Two options are presented:
• Option A: On the basis of blast damage observations at Buncefield, increase the size of
the LUP zones only; or,
• Option B: Increase the size of the LUP zones and revise the development Sensitivity
Levels.
Under the current system the Buncefield site has the inner, middle and outer zone boundaries set
at distances of 120 m, 135 m and 185 m respectively. The proposed system would increase
these distances to 250 m, 300 m and 400 m respectively.
The proposed changes to the Sensitivity Levels under Option B would place greater restrictions
on the types of development that HSE would not advise against in the inner zone.
It is recommended that the proposed arrangements are applied to those sites which are similar to
Buncefield in a number of important respects. These similarities would indicate that on the
basis of what is known at present, the site has the potential to give rise to a comparable VCE.
If implemented, it is recommended that the proposed arrangements are applied to sites with the
following characteristics:
• the site is classed as either ‘upper tier’ or ‘lower tier’ under the Control of major Accident Hazards Regulations 1999 (the COMAH Regulations); • the site stores petroleum in vertical, cylindrical, non-refrigerated above-ground storage
tanks with side walls greater than 5 m in height; and,
• the filling rate of the storage tanks is greater than 100 m3 / hour.
These criteria align with those used by the Buncefield Standards Task Group, a joint COMAH
Competent Authority and Industry committee (HSE, 2006).
In comparison with the current system, the proposed changes will, if implemented, result in:
• significantly larger zones for the sites affected (for both Options A and B);
vii
• as a result of the increased area within the zones, a larger number of consultations
concerning proposed developments in the vicinity of affected sites (for both Options A
and B); and,
• more restrictive advice concerning developments in the inner zone (for Option B).
Furthermore, it is recognised that there may be significant implications in the proposed system
for other types of site. These sites include bulk LPG (liquefied petroleum gas) storage,
gasholders, other flammable liquid storage and other facilities presenting a hazard with an
associated significant likelihood of death in the inner zone (including VCEs and other hazards
such as fireballs). A review of the implications of the study findings for LUP arrangements
applicable to other types of site is recommended.
An investigation of the sensitivity of the size of the proposed LUP zone boundaries to various
factors was undertaken. The factors considered included:
• the rate at which vapour is formed and the conditions under which this occurs (the
source term);
• the way in which the cloud moves (the dispersion of the vapour);
• the quantity of fuel involved in the explosion (the explosion size); and,
• the violence of the explosion (the explosion strength).
The factors considered to have the greatest influence on the size of the zones were:
• the geometry of the storage tanks at the site (this affects the way the liquid contents
behave in the event of a tank being overfilled);
• the duration of a release (this affects the size of the flammable vapour cloud formed);
• the topography of the tank surroundings (features in the surroundings such as slopes and
obstacles have a marked effect on the behaviour of the vapour cloud); and,
• the weather conditions at the time that a release occurs (calm, low wind speed
conditions favour formation of a large cloud).
It is important to note that:
• several of these factors (tank geometry, time to ignition and topography) are strongly
site-specific;
• it is conceivable that a combination of differences in these factors could lead to an
explosion with more serious consequences than Buncefield;
• in the time available it has not been possible to quantify the impact of all of these
factors on land-use planning zone boundaries; and,
• the reasons for the strength of the explosion at Buncefield are not yet understood,
further investigation may indicate that there are additional factors that are important.
viii
1
INTRODUCTION
Following the incident at the Buncefield Oil Storage Depot in December 2005, the Health and
Safety Executive (HSE) commissioned Environmental Resources Management (ERM) to assist
in reviewing HSE’s approach to providing land-use planning (LUP) advice in the vicinity of
similar installations.
At the time of writing the detailed causes of the incident are still under investigation, although a
series of reports have been published by the Major Incident Investigation Board (MIIB) (MIIB,
2006a; MIIB, 2006b; MIIB, 2006c; MIIB, 2006d).
The work performed by ERM has comprised the following:
• Performing a review of the current LUP arrangements;
• Consideration of the various options for revised LUP arrangements in the light of the
experiences at Buncefield;
• Development of the selected options for revised arrangements;
• Investigation of the impact of the proposed revised arrangements upon the LUP advice
provided by HSE to local authorities; and,
• Examination of the sensitivity of the proposed options to a range of factors associated
with the features of the major hazard sites of interest.
1
2
CURRENT ARRANGEMENTS
The Health and Safety Executive (HSE) provides land use planning (LUP) advice to local
authorities regarding applications for development in the vicinity of major hazard installations
and major hazard pipelines. This report relates only to HSE’s approach to providing LUP
advice around major hazard installations.
Major hazard installations are defined as those sites that are required to have Hazardous
Substances Consent (DETR, 2000), obtained from the appropriate Hazardous Substances
Authority (usually part of the Local Authority). The Hazardous Substances Consent (or
Consent for short) defines the quantities of dangerous substances that the site can hold, and may
also define the size of the largest storage vessels, the locations of vessels, the locations of areas
used to store transportable containers and the maximum temperatures and pressures within
storage and process vessels.
Major hazard pipelines (HSE, 1996) are defined in accordance with the kind of material they
convey (whether or not it is a ‘dangerous fluid’ under the terms of the legislation). What
constitutes a dangerous fluid depends on the pipeline operating conditions as well as the nature
of the material conveyed.
The decision-making algorithm used to determine the appropriate advice in a given case is
embodied in decision support software called PADHI (Planning Advice for Developments near
Hazardous Installations). The PADHI tool is described in a document that may be downloaded
from the HSE website (HSE, undated).
2.1
LAND USE PLANNING ZONES
The methodology within PADHI requires that three concentric zones are established around the
installation (termed the inner, middle and outer zones, as shown in Figure 1). The outermost
edge of the outer zone is the ‘consultation distance’ (CD). The size of these zones can vary
significantly from site to site, depending on the type of installation under consideration.
2
Inner Zone
Site
Middle Zone
Outer Zone
Consultation Distance
Figure 1 Land Use Planning Zones
Similarly, the method by which the zone sizes are established depends on the type of site. For
some sites a ‘risk based’ approach is used, whereby the zone boundaries correspond to different
values of the risk of an individual receiving a ‘dangerous dose’ or worse.
A dangerous dose is considered to cause all of the following effects to an exposed population:
•
•
•
•
severe distress to almost everyone;
a substantial proportion requires medical attention;
some people are seriously injured, requiring prolonged treatment;
any highly susceptible people might be killed.
Exposure to a dangerous dose equates to a probability of fatality of around 1-5% in a typical
population.
Hence:
• the boundary between the inner and middle zones corresponds to an individual risk of
dangerous dose or worse of 10 chances per million per year (1) (cpm);
• the boundary between the middle and outer zones corresponds to an individual risk of
dangerous dose or worse of 1 cpm (2);
• the outer edge of the outer zone (the CD) is set at an individual risk of dangerous dose
or worse of 0.3 cpm (3) .
(1)
(2)
(3)
This may also be written as either 1 in 100,000 chances per year, or 1 x 10-5 chances per year.
This may also be written as either 1 in 1,000,000 chances per year, or 1 x 10-6 chances per year.
This may also be written as either 3 in 10,000,000 chances per year, or 3 x 10-7 chances per year.
3
In general, a ‘risk based’ approach is used when the site in question stores or processes toxic
substances such as chlorine.
For other sites (typically those handling flammable substances) a ‘protection based’ approach is
used. In most cases the protection based approach involves considering the consequences of a
selected event (whereas a risk based approach considers both the consequences and likelihood
of a range of events). Zone boundaries are set at the range to different levels of harm that would
be expected to result from the event:
• the boundary between the inner and middle zones is usually set at the range at which
members of a typical population could be subjected to a level of harm corresponding to
a significant likelihood of death;
• the boundary between the middle and outer zones is usually set at the range at which a
typical, exposed population could experience a dangerous dose;
• the outermost edge of the outer zone is usually set at the range at which a sensitive or
vulnerable population could experience a dangerous dose.
DEVELOPMENT TYPES AND HSE’S ADVICE
2.2
Proposed developments are assigned to one of four sensitivity levels, as shown in Table 1.
More detailed definitions are provided in the PADHI document.
Table 1 Development Sensitivity Levels
Level
1
2
3
4
Description
Based on normal working population
Based on the general public – at home and involved in normal activities
Based on vulnerable members of the public (children, those with mobility
difficulties or those unable to recognise physical danger)
Large examples of Level 3 and large outdoor examples of Level 2.
In each case there are some ‘exclusions’. For example, small housing developments of 1-2
dwellings are ‘excluded’ from Level 2 and placed into Level 1.
For a given development proposal, advice is then generated by essentially using the decision
matrix shown in Table 2.
Table 2 Decision Matrix
Sensitivity Level
Inner Zone
1
DAA
2
AA
3
AA
4
AA
AA: Advise against
DAA: Don’t advise against
Middle Zone
DAA
DAA
AA
AA
Outer Zone
DAA
DAA
DAA
AA
It should be noted that HSE’s role in the planning process is advisory. The decision as to
whether a proposed development receives planning permission rests with the local authority,
4
taking into account the advice supplied by HSE. The local authority has to weigh in the balance
all of the factors relating to a proposed development (economic, social, environmental, safety)
in reaching its decision.
However, HSE has the option to have an application ‘called-in’ for determination by the
Secretary of State where it believes that the risks are substantial and the local authority has not
given due weight to its advice in coming to their decision.
The following factors would favour calling in the application:
• Any proposals for significant residential development or development for vulnerable
populations in the inner zone;
• Proposals where the risk of death exceeds the tolerability limit for a member of the
public of 1 x 10-4 per year (HSE, 2001);
• Proposals where there are substantial numbers of people exposed to the risk, giving rise
to a high degree of societal concern;
• Proposals where the endangered population is particularly sensitive, (e.g., the
development is a hospital, school or old people’s home);
• Whether there have been previous call-ins in similar circumstances;
• Whether there are issues of national concern as opposed to merely of local importance;
or,
• Whether there is clear evidence that the case concerned is being used to challenge
HSE’s risk criteria for land-use planning and failure to meet that challenge would
damage HSE’s credibility; or where a decision against HSE’s advice could, by
precedent, set aside parts of the relevant legislation.
For the purposes of this paper, Sensitivity Level 1 developments are of particular interest, since
under the current system they would not be advised against even if they were at a location
adjacent to a major hazard site boundary (see Table 2 above). Further details of the definition of
Sensitivity Level 1 developments have been extracted from the PADHI document and are
displayed in Table 3. Those development types which are assigned to Level 1 as a result of
being excluded from other Levels are shown in Table 4.
5
Table 3 Sensitivity Level 1 Developments
Reference
DT1.1 Workplaces
Examples
Offices, factories, warehouses, haulage
depots, farm buildings, non-retail
markets, builder’s yards.
DT1.1 x1
DT1.1 x2
Sheltered workshops, Remploy
DT1.2 Parking
Areas
Car parks, truck parks, lock-up
garages.
DT1.2 x1
Car parks with picnic areas, or at a
retail or leisure development, or
serving a park and ride interchange.
Development Detail and Size
Workplaces (predominantly non-retail), providing
for less than 100 occupants in each building and
less than 3 occupied storeys – Level 1
EXCLUSIONS
DT1.1 x1 Workplaces (predominantly non-retail)
providing for 100 or more occupants in any
building or 3 or more occupied storeys in height Level 2 (except where the development is at the
major hazard site itself, where it remains Level 1).
DT1.1 x2 Workplaces (predominantly non-retail)
specifically for people with disabilities - Level 3.
Parking areas with no other associated facilities
(other than toilets) – Level 1
EXCLUSIONS
DT1.2 x1 Where parking areas are associated with
other facilities and developments the sensitivity
level and the decision will be based on the facility
or development.
6
Justification
Places where the occupants will be
fit and healthy, and could be
organised easily for emergency
action. Members of the public will
not be present or will be present in
very small numbers and for a short
time.
Substantial increase in numbers at
risk with no direct benefit from
exposure to the risk.
Those at risk may be especially
vulnerable to injury from hazardous
events and / or they may not be able
to be organised easily for
emergency action.
Table 4 Sensitivity Level 1 Developments - Exclusions from Other Levels
Reference
Examples
DT2.1 x1 (Exclusion from DT2.1, Infill, backland development.
Housing)
DT2.2 x1 (Exclusion from DT2.2, Smaller - guest houses, hostels, youth
Hotel
/
Hostel
/
Holiday hostels, holiday homes, halls of
residence,
dormitories,
holiday
Accommodation)
caravan sites, camping sites.
DT2.3 x1 (Exclusion from DT2.3, Estate roads, access roads.
Transport Links)
Any railway or tram track.
DT2.4 x1 (Exclusion from DT2.4,
Indoor Use by Public)
Development Detail and Size
Justification
Developments of 1 or 2 dwelling Minimal increase in numbers at risk. units - Level 1. Accommodation of less than 10 beds Minimal increase in numbers at risk. or 3 caravan / tent pitches - Level 1
Single carriageway roads – Level 1
Minimal numbers present and mostly
a small period of time exposed to
risk.
Associated
with
other
development.
Railways – Level 1
Transient population, small period of
time exposed to risk. Periods of time
with no population present.
Development with less than 250 m2 Minimal increase in numbers at risk.
total floor space – Level 1
7
3
REQUIREMENT FOR REVIEW OF CURRENT
ARRANGEMENTS
The requirement for review of the current arrangements arises from the experience of the
Buncefield incident. As a result of the MIIB investigations, it is known that:
•
•
•
•
•
a storage tank was filled to overflowing with petrol;
as the petrol overflowed from the tank a substantial vapour cloud was generated;
the vapour cloud spread over parts of the site and to areas beyond the site boundary;
the vapour cloud was ignited, resulting in a vapour cloud explosion (VCE); and,
the VCE caused extensive damage to buildings, particularly those close to the site of the
explosion.
Flammable vapour will burn if it is mixed with sufficient air and ignited. Current thinking is
that:
• a fuel-air cloud that burns in the open (i.e. in the absence of any confining structures
such as walls or roofs) will burn relatively slowly and generate little if any blast
pressure; and,
• in order to generate damaging levels of blast pressure, the fuel-air cloud needs to be at
least partially confined (i.e. located within walls and/or a roof) or be located within a
congested region (i.e. engulfing obstacles such as pipes, steel frames, vessels etc.).
It is also known that ignition of a fuel-air cloud within a small confined volume can produce an
explosion which propagates into that (larger) part of the cloud which is outside the confined
volume. This is termed ‘bang box’ ignition.
It is also important to note that the characteristics of the blast produced by a VCE differ
somewhat from those of the blast produced by a ‘conventional’ explosive such as TNT.
At present it is not clear why the vapour cloud exploded with such force. Although accidents
involving VCEs have occurred before and the VCE phenomenon has been the subject of much
scientific investigation, an explanation for the strength of the VCE experienced at Buncefield is
currently lacking.
At present HSE performs protection based assessments of sites like that at Buncefield (storage
facilities for petroleum products). This assessment involves considering the potential for a
VCE. However, on the basis of current scientific understanding of the way in which VCEs
occur, the potential for a VCE at a site like Buncefield would be limited to those parts of the
facility that provided sufficient confinement or congestion to generate a VCE, such as the tanker
loading rack, giving rise to relatively small hazard ranges. The protection based assessment
would therefore be likely to be based on other hazards, typically large pool fires.
It seems possible that the incident at Buncefield may alter current scientific understanding of
VCE mechanisms. This in turn may cause HSE (and risk assessment practitioners generally) to
re-think their approach to the assessment of sites storing large quantities of flammable
8
substances. However, a detailed understanding of events at Buncefield is at least some months
(and possibly, if further experimental investigations are required, some years) away.
9
4
OBJECTIVE OF REVISED ARRANGEMENTS
The specific objectives of the proposed revised arrangements are to:
• establish LUP zone boundaries that take into account observations arising from the
Buncefield incident; and,
• provide a decision making framework for use with the revised LUP zones that takes into
account the particular nature of the VCE hazard.
The proposed arrangements are protection based. The objective of this and other protection
based approaches is to:
“achieve a separation between developments and the site which provides a very high degree of
protection against the more likely smaller events, whilst also giving very worthwhile (sometimes
almost total) protection against unlikely but foreseeable larger-scale events.” (HSE, 1989).
It is not the objective of the proposed arrangements to provide complete protection for future
developments from damage, or for their occupants from harm. Neither is it the objective of the
proposed arrangements to suggest what should be done about existing land use around affected
sites.
10 5
OPTIONS FOR REVISED LUP ARRANGEMENTS
The principal features of the system used by HSE for providing LUP advice are:
•
•
•
The zones around the site;
The development categories; and,
The decision matrix.
Therefore revised arrangements could involve alterations to one or more of these features.
An initial list of options was developed by HSE and presented to ERM for consideration. These
options are listed in Table 5, together with the author’s comments in each case.
11
Table 5 Comments on Options for Revision of LUP Arrangements
Option
1. No change to existing planning zones and CDs [consultation distances].
Use existing LUP matrix for LUP advice.
2. Change LUP zones (for Buncefield or similar sites) plus existing LUP
matrix.
2.1(a), (b) Change LUP zones (for Buncefield or similar sites) using VCE
calculated on foreseeable loss of containment (LOC) amount, plus existing
LUP matrix. Either (a) base upon existing methodology or (b) base upon
varying stepped values of assumed congestion up to a maximum value.
2.1 (c) Change LUP zones (for Buncefield or similar sites), plus existing
LUP matrix. Base upon (c) observed overpressure at Buncefield.
ERM Comment
Given the potential number of casualties (had the accident occurred when
nearby buildings were occupied) and the level of public concern, this is not
considered to be a viable option.
A revision to the way the LUP zones are set is probably necessary in view
of the nature of the incident (i.e., a VCE not a pool fire). However, if this
were used in conjunction with the existing matrix, Sensitivity Level 1
developments would still ‘not be advised against’ within the Inner Zone,
right up to the site boundary. Hence in some ways this option does not take
into account the experience at Buncefield (i.e., the extensive damage to
commercial / office buildings and potential numbers of casualties).
Therefore the author considers that a revision to the matrix (or at least
Sensitivity Level definitions) is required, as well as revision of the zone
boundaries.
A fuller understanding of the event sequence and mechanism of the
Buncefield explosion may alter views about what constitutes a ‘foreseeable’
event. It is understood that, based on current views about what is
foreseeable, HSE already conducts assessments of potential VCE events at
such establishments. See also comments on Option 2 above.
This option has some advantages in that it is based on experience as
opposed to conjecture about the event sequence and mechanism of the VCE.
However, estimates of overpressure obtained from observed damage are
subject to uncertainty. This could be avoided by using distances to
observed damage levels to set zone boundaries rather than distances to
estimated overpressure levels. See also comments on Option 2 above.
12 Option
2.2(a), (b) Change LUP zones (for Buncefield or similar sites) using VCE
calculated on extremely large loss of containment (LOC) amount, plus
existing LUP matrix. Either (a) base upon existing methodology or (b) base
upon varying stepped values of assumed congestion up to a maximum
value.
2.2 (c) Change LUP zones (for Buncefield or similar sites), plus existing
LUP matrix. Base upon (c) observed overpressure at Buncefield.
3. Change LUP zones [in one of the ways described in Option 2] plus new
LUP matrix. New LUP matrix could take into account propensity to
damage. Develop advice DAA/AA [Don’t Advise Against / Advise
Against] based upon overpressure and building design.
3.1 and 3.2 as 2.1 and 2.2 [but with revised LUP matrix].
4. No change to existing policy, but review all planning applications on a
case by case basis [either] (a) around Buncefield or (b) elsewhere
5. Moratorium on all planning applications within (a) existing CDs; (b)
proposed new CD based on Option 2 above; or (c) inner zone.
6. Create a ‘cordon sanitaire’ around Buncefield and all other sites.
7 (a) For the above calculations [Option 2 or 3] change zones for Buncefield
only
ERM Comment
Current understanding of VCE mechanisms indicates that, in the case of a
very large release, the magnitude of the explosion becomes limited by the
size of the confined / congested volume available rather than by the size of
the cloud. At a site similar to Buncefield the available confined / congested
volumes are typically small (e.g. loading racks) and therefore this approach
is likely to give the same results as 2.1. See also comments on Option 2
above.
It is not clear how this option differs from 2.1 (c). See comments on Option
2.1 (c) above.
A combination of revised zones and a revised decision framework is, in the
author’s view the most promising way forward. It may not be necessary to
revise the decision matrix, one possible solution may be to alter the
sensitivity level definitions to reflect building vulnerability to blast.
See comments on 2.1, 2.2 and 3 above.
A basis for decision making would still need to be established, even if it
was only available within HSE. This would require development of one of
the other options.
Until further information becomes available, it is not clear whether the
existing zones are large enough for options 5 (a) or 5 (c) (i.e., it is possible
that zones should be larger than they are at present).
Although a precautionary approach is considered to be appropriate under
the circumstances, a complete moratorium seems indiscriminate and may
result in further distress among people living or working in the affected
zones. It is the author’s view that a moratorium should not be dismissed as
a possibility, but should only be considered if a workable, more refined
approach cannot be developed in the required time scales.
This does not seem to be a likely way forward in the short term.
At present there does not seem to be a good reason for considering
Buncefield alone. It is understood that there have been accidents in France
and the USA which bear some similarities to Buncefield, so there is
evidence to indicate that the circumstances are not unique.
13 Option
7 (b) For the above calculations [Option 2 or 3] change zones for all 97 oil
storage sites
7 (c) For the above calculations [Option 2 or 3] change zones for some of
the 97 based upon gasoline for sites where tanks are floating roof and filled
by pipeline
7 (d) For the above calculations [Option 2 or 3] change zones for all sites where there is a foreseeable risk of VCE.
ERM Comment
This seems a more logical approach than 7 (a).
This is viewed as the favoured approach, although a firm basis for
identifying which sites should be included needs to be developed.
If a VCE is foreseeable on the basis of current understanding, then HSE will
already have taken this into account. However, it is possible that as the
investigation proceeds the view of what is ‘foreseeable’ will change,
requiring a broader re-evaluation of the approach taken with VCE hazards.
It is recommended that this is considered further as more information
becomes available.
14 Overall, it was judged that any revised approach should have regard for the current state of
knowledge regarding events at Buncefield, and should be somewhat precautionary in nature. In
summary, it was recommended that consideration be given to amendment of both the zones and
the development sensitivity level definitions.
5.1
CHANGES TO LUP ZONES
It was decided that observations of the damage incurred at Buncefield could be used to estimate
maximum ranges to damage levels of interest. These distances would then be used to establish
LUP zone boundaries. For example, the inner zone distance would correspond to the maximum
range to serious structural damage. Establishing zone boundaries on the basis of observed
damage rather than an estimate of the overpressure required to cause the observed level of
damage avoids the uncertainty inherent in the overpressure estimate.
An analytical approach to establishing LUP zones (e.g. by using an explosion model to calculate
distances to overpressures of interest) is not considered feasible at present, owing to the lack of
understanding of the explosion mechanism involved.
5.2
CHANGES TO SENSITIVITY LEVEL DEFINITIONS
In addition, the definition of Sensitivity Level 1 (S1) developments was revisited to ensure that
the nature of the vapour cloud explosion hazard and the effect of such events on building
occupants were addressed adequately.
15 6
DEVELOPMENT OF PROPOSALS FOR REVISED
ARRANGEMENTS
The proposed revised arrangements have been developed by:
• Reviewing published HSE research on the effects of explosions (Jeffries et al., 1997;
Galbraith, 1998), (see Appendix 1);
• Considering the levels of damage to buildings observed around the Buncefield site (see
Appendix 2);
• Revisiting the justifications underlying the Sensitivity Level definitions for proposed
developments in the light of the first two activities.
The review of published HSE research found that a model for predicting the effects of blast
from VCEs on buildings and their occupants (Jeffries et al., 1997) contained flaws which raised
questions over the validity of the model results. Further development of the model would be
necessary to rectify these problems.
6.1
REVISION OF LUP ZONE BOUNDARIES
Some limited but useful data are available which correlate levels of building damage to the
harm suffered by their occupants (Galbraith, 1998). These data result from observations of
bomb damage sustained during the Second World War.
In conjunction with the observations of damage to buildings, the data have been used to form a
judgement concerning the likelihood of harm (in broad terms) to the occupants of different
buildings. Damaged buildings were assigned to one of four harm categories: high, medium, low
and minimal. The first three of these categories correspond to the three harm levels used when
performing a protection based assessment (see Section 2.1). The ‘minimal’ category indicates
that the potential for harm to occupants arising from damage to the building was such that it
would not be of interest for the purposes of setting LUP zone boundaries.
The distances to buildings in the different harm categories were measured and used to establish
zone boundaries. This resulted in the proposed zone boundary distances displayed in Table 6.
The process is described in detail in Appendix 3.
Table 6 Proposed Zone Boundaries
Zone
Inner
Middle
Outer
Distance (m)
250
300
400
16 6.2
REVISION OF SENSITIVITY LEVEL DEFINITIONS
The Buncefield explosion and subsequent fires resulted in significant damage to property;
particularly to those buildings situated relatively close to the installation (see Appendix 2). For
the Northgate and Fuji buildings, from the information available it seems likely that, had people
been present in those parts of these buildings closest to the site, they would have been exposed
to a level of harm presenting a significant likelihood of death (see Appendix 3).
These observations have prompted the author to review the HSE definitions of Sensitivity Level
1 developments, since such developments would not be advised against anywhere within the
consultation distance, even within the inner zone (see Table 2). Hence such developments could,
under existing arrangements, be placed at locations where levels of individual risk were
relatively high.
Essentially Sensitivity Level 1 developments have been defined as:
• small examples of housing, holiday / hostel and retail / leisure developments (see Table
4); and,
• office, factories and workplaces for less than 100 occupants and less than 3 stories in
height (see Table 3).
The justifications for placing these types of development into Sensitivity Level 1 are discussed
below.
Throughout these discussions, it is important to note that the HSE calculates risk in terms of the
individual risk of experiencing a ‘dangerous dose or worse’, as opposed to the individual risk of
fatality (see Section 2.1). Exposure to a dangerous dose corresponds to approximately a 1-5%
probability of fatality to a typical cross-section of the population (with only the most vulnerable
or sensitive individuals becoming fatalities). However, the proportion of ‘or worse’ within the
risk of dangerous dose or worse varies as a function of distance from an installation and with the
hazards presented by the installation.
For most types of hazard, as the site of interest is approached the proportion of ‘or worse’ (e.g.,
fatality) within the risk of dangerous dose or worse is increased. Also, the proportion of ‘or
worse’ tends to be higher with fire and explosion hazards than with most toxic hazards. Indeed,
for some types of fire or explosion event (including a VCE like that experienced at Buncefield),
in the near field the risk of dangerous dose or worse is almost entirely a risk of fatality.
6.2.1 Small housing, hotel / hostel and retail / leisure developments
Several smaller developments of various types are assigned to Sensitivity Level 1 as a result of
an ‘exclusion’ from another Level (see Table 4). These include:
• housing developments for 1 or 2 dwellings;
• holiday / hostel accommodation of less than 10 beds or 3 caravan / tent pitches; and,
• retail / leisure development with less than 250 m2 total floor space.
17 For each of these development types, the justification given within the PADHI document (HSE,
-) is that there is ‘minimal increase in numbers at risk’. A more detailed discussion of the
justification is presented in HSE’s risk criteria document for land use planning (HSE, 1989).
This document provides a second reason for not advising against such developments:
“the ‘dangerous dose’ would only be fatal for a small fraction of the population, and a fraction
of the population of two houses is probably zero.” (HSE, 1989; para. 85).
Although this statement relates to small housing developments, the document makes it clear that
holiday / hostel accommodation is placed in the same category as housing (HSE, 1989; para.
81); and retail / leisure development is assumed to be “similar in significance” to housing (HSE,
1989; para. 82).
These justifications relate to the increment of societal risk associated with a proposed
development (the case societal risk). The first relates to the number of people exposed to the
risk, the second to the numbers of people who might become fatalities if exposed to a dangerous
dose.
For the first of the justifications given (‘minimal increase in numbers at risk’) it appears that the
low case societal risk associated with such developments has been given greater weight than the
relatively high individual risk to which people could be exposed. This arises from a tension
between two of the stated principles of HSE’s involvement in land-use planning, that:
“The risk from a hazardous installation to an individual employee or member of the public
should not be significant when compared with other risks to which he is exposed in everyday
life.” (Advisory Committee on Major Hazards, 1984) para. 21;
and:
“The separation of a hazard from a built-up area need not mean… that the intervening land is
completely sterilised or available only for agricultural purposes. Depending on the degree of
hazard, it may be possible for it to be allocated for other purposes involving low population
densities, including some industrial developments.” (Advisory Committee on Major Hazards,
1984), para. 84.
Under the current arrangements, the second of these principles overrides the first for some of
those developments falling into Sensitivity Level 1. However, for a site with the potential for a
VCE like that experienced at Buncefield, the risk of ‘dangerous dose or worse’ close to the site
is likely to be almost entirely a risk of fatality. In view of this, it is proposed that for certain
developments currently placed within Sensitivity Level 1, the balance between the two
principles stated above is shifted in favour of the first, and that these developments should be
assigned to Sensitivity Level 2.
The second justification is essentially that, in a small number of exposed people, it is probable
that none of them would be particularly sensitive or vulnerable to the hazard in question.
Therefore, if this group of people was exposed to a dangerous dose, it is unlikely that any
fatalities would result. However, this argument does not consider the proportion of ‘or worse’
within the ‘dangerous dose or worse’. As discussed above, for a VCE like that experienced at
18 Buncefield, the ‘or worse’ proportion is likely to be high close to the event. Hence, even
amongst a small group of people, some fatalities could result. This supports the proposal that
the types of developments in question should be re-assigned to Sensitivity Level 2.
This issues discussed in this Section have broader implications and are not specifically related to
the Buncefield accident or the VCE hazard. However, it is considered prudent to address them
in the context of this study, in view of the significant likelihood of fatality to which people close
to sites of interest could be exposed.
6.2.2 Workplaces
The first part of the justification given for assigning workplaces to Sensitivity Level 1 is that
these are ‘places where the occupants will be fit and healthy, and could be organised easily for
emergency action’ (see Table 3).
The first part of the justification (‘occupants fit and healthy’) appears to suggest that amongst a
group of workers, it is likely that the proportion that could be considered particularly sensitive
or vulnerable would be lower than in the population at large. Therefore, if this group of workers
was exposed to a dangerous dose, the resulting number of fatalities would probably be lower
than for a group of the same size but representing a ‘typical’ cross-section of the general
population.
However, as with one of the justifications discussed in the previous Section, this argument does
not consider the proportion of ‘or worse’ within the ‘dangerous dose or worse’. As discussed
above, for a VCE like that experienced at Buncefield, the ‘or worse’ proportion is likely to be
high close to the event. Hence, even amongst a ‘fit and healthy’ group of people, some fatalities
could result.
The second part of the justification appears to be that the risk to people in a workplace would be
significantly reduced as a result of taking emergency action. This assumes that:
•
•
effective emergency action (such as evacuation or sheltering) is possible; and,
there is sufficient time for this action to be taken.
With regard to the first of these points it is not clear what would constitute effective emergency
action for occupants of workplaces close to a site presenting a VCE hazard. Evacuation of
building occupants could place them in the flammable cloud. However, remaining within a
building could place the occupants at greater risk of injury from blast effects.
Secondly, even if some kind of emergency action was possible, a VCE could occur rapidly, with
little or no opportunity for raising the alarm.
In view of this, it is recommended that consideration is given to alteration of the definition of
Sensitivity Level 1 developments within the revised arrangements. A more detailed discussion
of the case societal risk aspects of workplace developments is presented in Appendix 4.
19 6.2.3 Summary of Recommendations
The recommended alterations to the Sensitivity Level definitions are as follows:
• all workplaces that are routinely occupied for a significant proportion of the time (i.e. –
offices and factories), regardless of size, are re-assigned to Level 2;
• workplaces or areas that are visited intermittently (warehouses, timber yards, outdoor
storage areas, haulage depots, farm buildings) with no associated office facilities and
car parks with no associated amenities remain in Level 1;
• housing developments of 1 or 2 dwelling units are re-assigned to Level 2;
• smaller hotel / holiday / hostel accommodation is assigned to Level 2; and,
• retail development with less than 250 m2 floor space is assigned to Level 2.
6.3
OPTIONS
Since the proposed revised zone boundaries are considerably larger than those normally
encountered for those sites of interest (see Table 6), the view could be taken that it would be
sufficiently precautionary to change the zone boundaries without changing the Sensitivity
Levels. This results in two options for revised arrangements:
• Option A: Increase the size of the LUP zones only; or,
• Option B: Increase the size of the LUP zones and revise the development sensitivity
levels.
6.4
APPLICATION
It is recommended that the proposed arrangements are applied to those sites which are similar to
Buncefield in a number of important respects. These similarities would indicate that on the
basis of what is known at present, the site has the potential to give rise to a VCE.
If implemented, it is recommended that the proposed arrangements are applied to sites with the
following characteristics:
• the site is classed as either ‘upper tier’ or ‘lower tier’ under the Control of major Accident Hazards Regulations 1999 (the COMAH Regulations); • the site stores petroleum in vertical, cylindrical, non-refrigerated above-ground storage
tanks with side walls greater than 5 m in height; and,
• the filling rate of the storage tanks is greater than 100 m3 / hour.
These criteria align with those used by the Buncefield Standards Task Group, a joint COMAH
Competent Authority and Industry committee (HSE, 2006).
20 7
IMPACT OF THE PROPOSED ARRANGEMENTS
The impact of the proposed options has been illustrated by considering a series of hypothetical
developments at different locations around a hypothetical site. Under the current system the
hypothetical site has the inner, middle and outer zone boundaries set at distances of 120 m, 135
m and 185 m respectively (these distances correspond to those currently set for the Buncefield
site). The proposed revisions to the system would increase these distances to those presented in
Table 6. The selected locations are displayed in Figure 2. With regard to these locations, it
should be noted that:
•
•
•
•
Location 1 is within the current inner zone;
Location 2 is within the current middle zone;
Location 3 is within the current outer zone; and,
Locations 4, 5 and 6 are all outside the current consultation distance.
Although the current middle zone is quite ‘narrow’, it has been assumed for the purposes of the
illustrations that Location 2 is able to accommodate the hypothetical developments
considered (1).
Different types of development have been represented by symbols. A key to the symbols used
in the illustrations is presented in Figure 3, Figure 4 and Figure 5.
The illustrations are provided in the following figures:
•
•
•
•
•
•
•
•
•
•
Figure 6 Comparison of Existing and Revised LUP Zones;
Figure 7 Illustration 1: Current Situation, Inner Zone;
Figure 8 Illustration 2: Current Situation, Middle Zone;
Figure 9 Illustration 3: Current Situation, Outer Zone;
Figure 10 Illustration 4: Current Situation, Beyond CD;
Figure 11 Illustration 5: Option A (Revised Zones Only), Inner Zone;
Figure 12 Illustration 6: Option A (Revised Zones Only), Middle Zone;
Figure 13 Illustration 7: Option A (Revised Zones Only), Outer Zone;
Figure 14 Illustration 8: Option B (Revised Zones and Sensitivity Levels), Inner Zone;
Figure 15 Illustration 9: Option B (Revised Zones and Sensitivity Levels), Middle
Zone; and,
• Figure 16 Illustration 10: Option B (Revised Zones and Sensitivity Levels), Outer Zone.
The illustrations are also summarised in Table 7, where differences with the current system are
shown by highlighted cells.
(1)
There are additional rules within the current policy to address developments which straddle zone boundaries.
21 Site
1
2
3
4
5
6
Key
Inner Zone
Middle Zone
Outer Zone
Figure 2 Location of Hypothetical Developments
22 Symbo
Symboll
Symbol
Description
Description
Factory, providing for less than 100
occupants in each build
buildiing and less than
3 occupied storeys.
Factory, providing for more than 100
occupants in each building or more than 3
occupi
occupied storeys.
Offices, provi
providing for less than 100
occupants in each build
buildiing and less than
3 occupied storeys.
Offices, provi
providing for more than 100
occupants in each building or more than 3
occupi
occupied storeys.
Housing: Deve
Devellopments of 1 or 2
dwelling units
Housing: developments of more than
1 or 2 un
uniits, up to and incl
ncludi
uding 30
dwe
dwellling uni
units and at a density of no
more than 40 per hectare.
Developments for use by the general
public: Development wi
with less than
250 m2 total floor space
Developments for use by the general
public where total floor space is from
250 m2 up to 5000 m2.
Figure 3 Key to Symbols (Part 1)
23
Symbol
Symbol
Description
Description
Institutional: Hospital, where the site
being developed is less than 0.25
hectares.
Institutional: Hospital, where the site
being developed is larger than 0.25
hectares.
Institutional: School (not 24 hour
care), where the site being developed
is less than 1.4 hectares.
Developments for use by the general
public where total floor space is
greater than 5000 m2.
Housing: developments of more than
30 dwelling units or at a density of
more than 40 per hectare.
Developments for use by the
general public Predominantly openair developments likely to attract the
general public in numbers greater than
1000 people at any one time
Workplace: Warehousing with no
associated offices / factory.
Workplace: Outdoor storage with no
associated offices / factory.
Figure 4 Key to Symbols (Part 2)
24
Symbol
Symbol
Symbol
Symbol
Descri
Descript
ptiion
Description
Parking areas wi
with no other
associated facilities (other than
toi
toilets).
Workplace: Farm buildi
buildings with no
associated offices / factory.
Factory, regardless of number of
occupants in each buildi
building and number
of occupied storeys.
Offices, regardl
regardless of number of
occupants in each buildi
building and
number of occupied storeys.
Housing: developments of up to and
including 30 dwelling uni
units and at a
density of no more than 40 per
hectare. Includes small developments
of 1-2 uni
units
Developments for use by the general
public all developments where total
floor space is up to 5000 m2. Includes
developments wi
with less than 250 m2
floor space.
Figure 5 Key to Symbols (Part 3)
25 Existing Zones
Revised Zones
Site
Site
Figure 6 Comparison of Existing and Revised LUP Zones
26 Sensitivity Level 1
9
Sensitivity Level 2
8
Site
Sensitivity Level 3
8
Sensitivity Level 4
8
Current Inner Zone
Key
Inner Zone
Middle Zone
Outer Zone
Figure 7 Illustration 1: Current Situation, Inner Zone
27
Sensitivity Level 1
9
Sensitivity Level 2
9
Site
Sensitivity Level 3
8
Sensitivity Level 4
8
Current Middle Zone
Key
Inner Zone
Middle Zone
Outer Zone
Figure 8 Illustration 2: Current Situation, Middle Zone
28
Sensitivity Level 1
9
Sensitivity Level 2
9
Site
Sensitivity Level 3
9
Sensitivity Level 4
8
Current Outer Zone
Key
Inner Zone
Middle Zone
Outer Zone
Figure 9 Illustration 3: Current Situation, Outer Zone
29
Beyond the Current CD
No consultation with HSE required
Site
Key
Inner Zone
Middle Zone
Outer Zone
Figure 10 Illustration 4: Current Situation, Beyond CD
30 Sensitivity Level 1
9
Sensitivity Level 2
8
Site
Sensitivity Level 3
8
Sensitivity Level 4
8
Proposed Inner Zone
No change to Sensitivity Levels
Key
Inner Zone
Middle Zone
Outer Zone
Figure 11 Illustration 5: Option A (Revised Zones Only), Inner Zone
31
Sensitivity Level 1
9
Sensitivity Level 2
9
Site
Sensitivity Level 3
8
Sensitivity Level 4
8
Proposed Middle Zone
No change to Sensitivity Levels
Key
Inner Zone
Middle Zone
Outer Zone
Figure 12 Illustration 6: Option A (Revised Zones Only), Middle Zone
32
Sensitivity Level 1
9
Sensitivity Level 2
9
Site
Sensitivity Level 3
9
Sensitivity Level 4
8
Proposed Outer Zone
No change to Sensitivity Levels
Key
Inner Zone
Middle Zone
Outer Zone
Figure 13 Illustration 7: Option A (Revised Zones Only), Outer Zone
33
Sensitivity Level 1
9
Sensitivity Level 2
8
Site
Sensitivity Level 3
8
Sensitivity Level 4
8
Proposed Inner Zone
Revised Sensitivity Levels
Key
Inner Zone
Middle Zone
Outer Zone
Figure 14 Illustration 8: Option B (Revised Zones and Sensitivity Levels), Inner Zone
34
Sensitivity Level 1
9
Sensitivity Level 2
9
Site
Sensitivity Level 3
8
Sensitivity Level 4
8
Proposed Middle Zone
Revised Sensitivity Levels
Key
Inner Zone
Middle Zone
Outer Zone
Figure 15 Illustration 9: Option B (Revised Zones and Sensitivity Levels), Middle Zone
35
Sensitivity Level 1
9
Sensitivity Level 2
9
Site
Sensitivity Level 3
9
Sensitivity Level 4
8
Proposed Outer Zone
Revised Sensitivity Levels
Key
Inner Zone
Middle Zone
Outer Zone
Figure 16 Illustration 10: Option B (Revised Zones and Sensitivity Levels), Outer Zone
36
Table 7 Summary of Illustrations
Location
Current
Zone
1
Inner
2
Middle
Development Description
Current
Sensitivity
Level
Factory, providing for less than 100 occupants in each
1
building and less than 3 occupied storeys.
Offices, providing for less than 100 occupants in each
1
building and less than 3 occupied storeys.
1
Housing: Developments of 1 or 2 dwelling units
Developments for use by the general public:
1
Development with less than 250 m2 total floor space
Workplace: Warehousing with no associated offices /
1
factory.
Workplace: Outdoor storage with no associated offices /
1
factory.
Workplace: Farm buildings with no associated offices /
1
factory.
Parking areas with no other associated facilities (other
1
than toilets).
2, 3, 4
All other developments in Sensitivity Levels 2, 3 and 4
Factory, providing for less than 100 occupants in each
1
building and less than 3 occupied storeys.
Offices, providing for less than 100 occupants in each
1
building and less than 3 occupied storeys.
1
Housing: Developments of 1 or 2 dwelling units
Developments for use by the general public:
1
Development with less than 250 m2 total floor space
Workplace: Warehousing with no associated offices /
1
factory.
37 Current
Advice
Proposed
Zone
Option A
Advice
Option B Option B
Sensitivity Advice
Level
8
2
9
Inner
9
9
Inner
9
2
8
9
9
Inner
Inner
9
9
2
2
8
8
9
Inner
9
1
9
9
Inner
9
1
9
9
Inner
9
1
9
9
Inner
9
1
9
8
9
Inner
Inner
8
9
2, 3, 4
2
8
8
9
Inner
9
2
8
9
9
Inner
Inner
9
9
2
2
8
8
9
Inner
9
1
9
Location
3
Current
Zone
Outer
Development Description
Current
Sensitivity
Level
Workplace: Outdoor storage with no associated offices /
1
factory.
Workplace: Farm buildings with no associated offices /
1
factory.
Parking areas with no other associated facilities (other
1
than toilets).
Factory, providing for more than 100 occupants in each
2
building or more than 3 occupied storeys.
Offices, providing for more than 100 occupants in each
2
building or more than 3 occupied storeys.
2
Housing: developments of more than 1 or 2 units, up to
and including 30 dwelling units and at a density of no
more than 40 per hectare.
Developments for use by the general public where total
2
floor space is from 250 m2 up to 5000 m2.
3, 4
All other developments in Sensitivity Levels 3 and 4
Factory, providing for less than 100 occupants in each
1
building and less than 3 occupied storeys.
Offices, providing for less than 100 occupants in each
1
building and less than 3 occupied storeys.
1
Housing: Developments of 1 or 2 dwelling units
Developments for use by the general public:
1
Development with less than 250 m2 total floor space
Workplace: Warehousing with no associated offices /
1
factory.
Workplace: Outdoor storage with no associated offices /
1
factory.
Workplace: Farm buildings with no associated offices /
1
factory.
38 Current
Advice
Proposed
Zone
Option A
Advice
Option B Option B
Sensitivity Advice
Level
9
1
9
Inner
9
9
Inner
9
1
9
9
Inner
9
1
9
9
Inner
8
2
8
9
Inner
8
2
8
9
Inner
8
2
8
9
Inner
8
2
8
8
9
Inner
Inner
8
9
3, 4
2
8
8
9
Inner
9
2
8
9
9
Inner
Inner
9
9
2
2
8
8
9
Inner
9
1
9
9
Inner
9
1
9
9
Inner
9
1
9
Location
4
Current
Zone
Beyond
CD
Development Description
Current
Current Proposed
Sensitivity
Advice
Zone
Level
9
Parking areas with no other associated facilities (other
1
Inner
than toilets).
9
Factory, providing for more than 100 occupants in each
2
Inner
building or more than 3 occupied storeys.
9
Offices, providing for more than 100 occupants in each
2
Inner
building or more than 3 occupied storeys.
9
Housing: developments of more than 1 or 2 units, up to
Inner
2
and including 30 dwelling units and at a density of no
more than 40 per hectare.
9
Developments for use by the general public where total
2
Inner
2
2
floor space is from 250 m up to 5000 m .
9
Institutional: Hospital, where the site being developed is
3
Inner
less than 0.25 hectares.
9
Institutional: School (not 24 hour care), where the site
3
Inner
being developed is less than 1.4 hectares.
9
3
Housing: developments of more than 30 dwelling units or
Inner
at a density of more than 40 per hectare.
9
Developments for use by the general public where total
3
Inner
floor space is greater than 5000 m2.
8
Institutional: Hospital, where the site being developed is
4
Inner
larger than 0.25 hectares.
8
4
Inner
Developments for use by the general public
Predominantly open-air developments likely to attract the
general public in numbers greater than 1000 people at any
one time.
Factory, providing for less than 100 occupants in each
1
Not
Inner
building and less than 3 occupied storeys.
Applicable
Inner
Offices, providing for less than 100 occupants in each
1
Not
building and less than 3 occupied storeys.
Applicable
39 Option A
Advice
9
Option B Option B
Sensitivity Advice
Level
9
1
8
2
8
8
2
8
8
2
8
8
2
8
8
3
8
8
3
8
8
3
8
8
3
8
8
4
8
8
4
8
9
2
8
9
2
8
Location
Current
Zone
Development Description
4
Beyond
CD
(contd.)
Housing: Developments of 1 or 2 dwelling units
Current
Sensitivity
Level
1
Developments for use by the general public:
Development with less than 250 m2 total floor space
Workplace: Warehousing with no associated offices /
factory.
Workplace: Outdoor storage with no associated offices /
factory.
Workplace: Farm buildings with no associated offices /
factory.
Parking areas with no other associated facilities (other
than toilets).
Factory, providing for more than 100 occupants in each
building or more than 3 occupied storeys.
Offices, providing for more than 100 occupants in each
building or more than 3 occupied storeys.
Housing: developments of more than 1 or 2 units, up to
and including 30 dwelling units and at a density of no
more than 40 per hectare.
Developments for use by the general public where total
floor space is from 250 m2 up to 5000 m2.
Institutional: Hospital, where the site being developed is
less than 0.25 hectares.
Institutional: School (not 24 hour care), where the site
being developed is less than 1.4 hectares.
Housing: developments of more than 30 dwelling units or
at a density of more than 40 per hectare.
Developments for use by the general public where total
floor space is greater than 5000 m2.
40 1
1
1
1
1
2
2
2
2
3
3
3
3
Current
Advice
Proposed
Zone
Option A
Advice
Not
Applicable
Not
Applicable
Not
Applicable
Not
Applicable
Not
Applicable
Not
Applicable
Not
Applicable
Not
Applicable
Not
Applicable
Inner
9
Inner
9
2
8
Inner
9
1
9
Inner
9
1
9
Inner
9
1
9
Inner
9
1
9
Inner
8
2
8
Inner
8
2
8
Inner
8
2
8
Inner
8
2
8
Inner
8
3
8
Inner
8
3
8
Inner
8
3
8
Inner
8
3
8
Not
Applicable
Not
Applicable
Not
Applicable
Not
Applicable
Not
Applicable
Option B Option B
Sensitivity Advice
Level
8
2
Location
Current
Zone
4
Beyond
CD
(contd.)
5
Beyond
CD
Development Description
Current
Sensitivity
Level
Institutional: Hospital, where the site being developed is
4
larger than 0.25 hectares.
4
Developments for use by the general public
Predominantly open-air developments likely to attract the
general public in numbers greater than 1000 people at any
one time.
Factory, providing for less than 100 occupants in each
1
building and less than 3 occupied storeys.
Offices, providing for less than 100 occupants in each
1
building and less than 3 occupied storeys.
Housing: Developments of 1 or 2 dwelling units
1
Developments for use by the general public:
Development with less than 250 m2 total floor space
Workplace: Warehousing with no associated offices /
factory.
Workplace: Outdoor storage with no associated offices /
factory.
Workplace: Farm buildings with no associated offices /
factory.
Parking areas with no other associated facilities (other
than toilets).
Factory, providing for more than 100 occupants in each
building or more than 3 occupied storeys.
Offices, providing for more than 100 occupants in each
building or more than 3 occupied storeys.
Housing: developments of more than 1 or 2 units, up to
and including 30 dwelling units and at a density of no
more than 40 per hectare.
41 1
1
1
1
1
2
2
2
Current
Advice
Proposed
Zone
Option A
Advice
Option B Option B
Sensitivity Advice
Level
8
4
Not
Applicable
Not
Applicable
Inner
8
Inner
8
4
8
Not
Applicable
Not
Applicable
Not
Applicable
Not
Applicable
Not
Applicable
Not
Applicable
Not
Applicable
Not
Applicable
Not
Applicable
Not
Applicable
Not
Applicable
Middle
9
2
9
Middle
9
2
9
Middle
9
2
9
Middle
9
2
9
Middle
9
1
9
Middle
9
1
9
Middle
9
1
9
Middle
9
1
9
Middle
9
2
9
Middle
9
2
9
Middle
9
2
9
Location
Current
Zone
5
Beyond
CD
(contd.)
6
Beyond
CD
Development Description
Current
Sensitivity
Level
Developments for use by the general public where total
2
floor space is from 250 m2 up to 5000 m2.
Institutional: Hospital, where the site being developed is
3
less than 0.25 hectares.
Institutional: School (not 24 hour care), where the site
3
being developed is less than 1.4 hectares.
3
Housing: developments of more than 30 dwelling units or
at a density of more than 40 per hectare.
Developments for use by the general public where total
3
2
floor space is greater than 5000 m .
Institutional: Hospital, where the site being developed is
4
larger than 0.25 hectares.
4
Developments for use by the general public
Predominantly open-air developments likely to attract the
general public in numbers greater than 1000 people at any
one time.
Factory, providing for less than 100 occupants in each
1
building and less than 3 occupied storeys.
Offices, providing for less than 100 occupants in each
1
building and less than 3 occupied storeys.
Housing: Developments of 1 or 2 dwelling units
1
Developments for use by the general public:
Development with less than 250 m2 total floor space
Workplace: Warehousing with no associated offices /
factory.
Workplace: Outdoor storage with no associated offices /
factory.
42 1
1
1
Current
Advice
Proposed
Zone
Option A
Advice
Option B Option B
Sensitivity Advice
Level
9
2
Not
Applicable
Not
Applicable
Not
Applicable
Not
Applicable
Not
Applicable
Not
Applicable
Not
Applicable
Middle
9
Middle
8
3
8
Middle
8
3
8
Middle
8
3
8
Middle
8
3
8
Middle
8
4
8
Middle
8
4
8
Not
Applicable
Not
Applicable
Not
Applicable
Not
Applicable
Not
Applicable
Not
Applicable
Outer
9
2
9
Outer
9
2
9
Outer
9
2
9
Outer
9
2
9
Outer
9
1
9
Outer
9
1
9
Location
Current
Zone
6
Beyond
CD
(contd.)
Development Description
Current
Current Proposed Option A
Sensitivity
Advice
Zone
Advice
Level
9
Workplace: Farm buildings with no associated offices /
1
Not
Outer
factory.
Applicable
9
Parking areas with no other associated facilities (other
1
Not
Outer
than toilets).
Applicable
9
Factory, providing for more than 100 occupants in each
2
Not
Outer
building or more than 3 occupied storeys.
Applicable
9
Offices, providing for more than 100 occupants in each
2
Not
Outer
building or more than 3 occupied storeys.
Applicable
9
Housing: developments of more than 1 or 2 units, up to
Not
2
Outer
and including 30 dwelling units and at a density of no
Applicable
more than 40 per hectare.
9
Developments for use by the general public where total
2
Outer
Not
floor space is from 250 m2 up to 5000 m2.
Applicable
9
Institutional: Hospital, where the site being developed is
3
Not
Outer
less than 0.25 hectares.
Applicable
9
Institutional: School (not 24 hour care), where the site
3
Not
Outer
being developed is less than 1.4 hectares.
Applicable
9
Housing: developments of more than 30 dwelling units or
3
Not
Outer
at a density of more than 40 per hectare.
Applicable
9
Developments for use by the general public where total
3
Not
Outer
2
floor space is greater than 5000 m .
Applicable
8
Institutional: Hospital, where the site being developed is
4
Not
Outer
larger than 0.25 hectares.
Applicable
8
Outer
4
Not
Developments for use by the general public
Predominantly open-air developments likely to attract the
Applicable
general public in numbers greater than 1000 people at any
one time.
KEY: 9 Don’t Advise Against; 8 Advise Against; Not Applicable – no requirement to consult HSE
43
Option B Option B
Sensitivity Advice
Level
9
1
1
9
2
9
2
9
2
9
2
9
3
9
3
9
3
9
3
9
4
8
4
8
7.1
SUMMARY OF IMPACTS
In comparison with the current system, the proposed changes will, if implemented, result in:
• significantly larger CDs and zone boundaries for the sites affected (for both Options A
and B);
• as a result of the increased area within the CD, a larger number of consultations
concerning proposed developments in the vicinity of affected sites (for both Options A
and B); and,
• more restrictive advice concerning developments in the inner zone (for Option B).
Furthermore, it is recognised that there may be significant implications in the proposed
arrangements for other types of site. These sites include bulk LPG storage, gasholders, other
flammable liquid storage and other facilities presenting a hazard with an associated significant
likelihood of death in the inner zone (including VCEs and other hazards such as fireballs). This
is because the points raised in Section 6.2 concerning possible means of and time available for
emergency action are equally applicable to such sites. A review of the implications of the study
findings for LUP arrangements applicable to other types of site is recommended.
7.2
ADVANTAGES AND DISADVANTAGES
The advantages and disadvantages of the proposed system are displayed in Table 8.
44
Table 8 Advantages and Disadvantages
Advantages
Precautionary in the light of current understanding.
Based on accident experience, less
contentious than using predictive models,
particularly in view of current uncertainties.
Provides increased separation for sensitive or
normally occupied developments (due to the
larger CD), particularly to those in office /
factory developments excluded from the
inner zone (due to changes in Sensitivity
Level definitions).
Relatively quick and straightforward to
implement.
D
isadvantages
The system relies on interpretation of
damage and historical information, which is
open to debate. In particular, the historical
data relate to experience with ‘conventional
explosives’ which produce blast with
different characteristics to that from a VCE.
The system may not be precautionary enough
(i.e. it is possible that a VCE at another
comparable site could be worse than that at
Buncefield).
The change gives rise to inconsistency –
those sites affected by the proposed system
will use the revised set of Sensitivity Level
definitions, other sites will use the current
definitions.
The increase in CDs and more restrictive
advice may generate an adverse reaction
from local authorities, developers, etc.
For those sites affected, it will not be
possible to use the existing PADHI software
to determine the appropriate advice. This is
likely to have resource implications for HSE.
On balance it is considered that the proposed system reflects the best that can be achieved with
the time and information available.
45 8
SENSITIVITY
In Table 8, it was observed that the proposed system may not be sufficiently precautionary (i.e.
that a VCE at another comparable site could be worse than that at Buncefield). In view of this,
an investigation of the sensitivity of the size of the boundaries to various factors was
undertaken. The factors considered included:
• the rate at which vapour is formed and the conditions under which this occurs (the
source term);
• the way in which the cloud moves (the dispersion of the vapour);
• the quantity of fuel involved in the explosion (the explosion size); and,
• the violence of the explosion (the explosion strength).
8.1
SOURCE TERM
The manner in which vapour was generated during the Buncefield incident is described in the
third progress report published by the MIIB (2006c).
In summary, liquid overflowed from the tank through eight vents around the tank roof. This
liquid flowed down the surface of the conical roof until it reached the roof edge. At this point
the liquid struck a deflector plate (designed to direct firewater onto the tank wall). The deflector
plate directed a proportion of the liquid on to the tank wall, but the remainder cascaded over the
top of the plate, forming airborne droplets. The proportion of the liquid directed on to the wall
flowed downwards until it struck a wind girder running circumferentially around the tank. As
the liquid struck the girder a proportion of it was directed away from the tank wall, again
forming a shower of droplets. This is illustrated in Figure 17, copied from the third progress
report (MIIB, 2006c).
The formation of showers of droplets enhanced the evaporation of the liquid fuel, forming a
dense cloud of vapour. Calculations performed by the Health and Safety Laboratory (HSL)
suggest that, after primary droplet break-up was complete, falling liquid and vapour rapidly
reached equilibrium at the centre of the showers. Equilibrium between vapour and liquid
represents an upper bound to the amount of evaporation from the liquid that may occur under a
given set of conditions.
46 Liquid Flow
Deflector
Deflector Plate
Liquid breaks
into droplets
Wind
Wi
nd Girder
Tank
Figure 17 Mechanism of Vapour Formation
8.1.1 Sensitivity to Source Geometry
The geometries of the tank and of the showers of liquid are important parameters in determining
the rate of vapour formation. Of particular importance are the number and width of the
cascades of liquid droplets. In the Buncefield case there were eight such cascades, one for each
vent in the tank roof (see Figure 18). The width of the cascade is determined principally by the
spreading of the flowing liquid over the tank roof prior to reaching the roof edge.
On the basis of discussion with HSL experts involved in the investigation, it is thought that
moderate changes in liquid flow rate from the vent only produce small changes in the width of
the flow. Hence, given that the total filling rate is of the order of hundreds of cubic metres per
hour, the rate of vapour generation is not considered to be sensitive to moderate changes in this
rate.
In addition, since the liquid droplets and vapour reached equilibrium after falling a relatively
short distance, it seems likely on the basis of simulation work performed to date by HSL that the
rate of generation of vapour would not be particularly sensitive to tank height.
47 Spreading liquid
Falling droplets
Vent
Tank
Figure 18 Liquid Overflowing from the Buncefield Tank (Plan View)
If a tank of this design (1) were to possess a larger number of roof vents (as might be
encountered with a tank of larger diameter), more droplet cascades would be formed in the
event of overfilling, resulting in higher rates of vapour formation. This is illustrated in Figure
19.
(1)
The tank is described as having had a cone roof with an internal floating deck.
48
Figure 19 Liquid Overflowing from a Larger Diameter Tank (Plan View)
The presence of features on the outer surface of the tank (distributor plates, wind girders) seems
to be an important factor in promoting the formation of free—falling liquid droplets. Such
features cause liquid flowing down the side of the tank to detach from the surface of the wall.
It should be noted that petroleum storage tanks vary in design. Some tanks with a cone roof do
not have a distributor plate running around the edge of the roof. The number and arrangement
of vents within a cone roof may vary. Not all tanks with a floating deck have a roof.
The behaviour of liquid overflowing from a tank with a floating deck but no roof is harder to
predict. Liquid would be expected to begin overflowing at a relatively low point in the tank
wall. The liquid would flow down the tank wall until meeting an obstruction (such as a wind
girder or an access platform) at which point it would be likely to detach from the wall and form
a shower of droplets. This is displayed in Figure 20.
49 Overflowing
liquid
Tank
Obstruction
Shower of
droplets
Figure 20 Overflow from a Floating Deck Tank with no Roof
Depending on the tank dimensions and rate of filling, overflow could occur from several points
or, in the worst case, around the entire circumference of the tank. Such an event has the
potential to generate fuel vapour at a greater rate than that observed at Buncefield.
8.1.2 Sensitivity to Fuel Composition
Petrol is a complex mixture of hydrocarbons. The composition of this mixture is varied at
different times of the year (summer and winter), to ensure that petrol engines continue to work
properly in spite of differences in ambient temperature. In winter, petrol contains higher
proportions of the ‘lighter’, more volatile hydrocarbons butane, pentane and hexane. This was
the case with the petrol involved in the initial release at Buncefield.
The presence of a higher proportion of ‘light’ hydrocarbons tends to promote vapour generation.
Higher ambient temperatures would also tend to increase vapour formation. However, petrol
with a relatively high proportion of ‘light’ hydrocarbons is generally used when the ambient
temperatures are relatively low, and vice-versa. Hence variation in fuel composition needs to be
considered in conjunction with the likely corresponding ambient temperature.
Therefore, it seems that the potential for vapour formation from a Buncefield-type release would
not differ greatly from winter to summer. In summer the lower volatility of the fuel would in all
likelihood be balanced by a higher ambient temperature.
8.2
VAPOUR DISPERSION
Observations from CCTV camera footage and initial computer modelling runs by HSL show
that the behaviour of the cloud post-release was strongly influenced by the topography of the
surroundings. The initial density of the vapour caused it to behave very much like a liquid,
staying close to the ground, flowing down slopes and ‘piling up’ behind obstructions. It also
appears that the cloud was still spreading when it was ignited (i.e. – if ignition had not occurred
when it did the cloud would probably have spread still further).
50 Sophisticated computerised fluid dynamics (CFD) software is required to model this sort of
behaviour. Simpler gas dispersion models assume that the cloud moves over flat, unobstructed
terrain. In addition, most simple models will not work for very low wind speed conditions. The
work currently underway at HSL uses CFD models.
The relative significance of different factors is discussed qualitatively below. It has not been
possible to quantify the influence of these factors in the time available. This would require
significant effort, performing multiple CFD model runs to test the effect of different inputs.
8.2.1 Release Duration
According to the MIIB initial report (MIIB, 2006d), the release of fuel continued unchecked for
around 40 minutes before the explosion occurred. The extent and quantity of flammable vapour
within the cloud increased throughout this period. It is likely that a release of shorter duration
would have resulted in a smaller cloud in terms of the distances reached and the amount of fuel
contained.
8.2.2 Topography
One of the most important factors influencing dispersion for a release of this type appears to be
the topography of the surroundings (the presence of slopes and obstacles). A dense cloud
moving over terrain that was more level and less obstructed than at Buncefield would be
expected to spread further and more rapidly, but would not be as deep.
8.2.3 Weather
Weather (in terms of atmospheric stability and wind speed) is another significant factor. The
calm, low wind speed conditions prevailing at the time of the Buncefield incident allowed a
large vapour cloud to develop. Higher wind speeds or less stable weather would enhance
dilution and dispersion of vapour, thereby reducing the extent of the flammable portion of the
cloud.
8.2.4 Presence of Ignition Sources
As noted above, it appears from the information currently available that the cloud was still
spreading when ignition occurred. Had ignition occurred at a later time, the cloud would have
been correspondingly larger, potentially resulting in a larger explosion.
8.2.5 Source Term
A higher rate of vapour generation (for any of the reasons discussed in Section 8.1) would result
in the cloud reaching a given extent more rapidly, and could ultimately lead to a larger cloud.
51 EXPLOSION PROPERTIES
8.3
It is important to emphasise that the explosion mechanism that gave rise to such a violent
vapour cloud explosion (VCE) at Buncefield is not yet understood. Therefore it is not yet
possible to state with any confidence which factors would be important in determining the
magnitude of the explosion. The discussion below is based on the simple modelling approaches
currently available, which are in turn based on current understanding of VCE mechanisms. The
results should therefore be viewed with caution.
8.3.1 Explosion Size
Simple mathematical models of vapour cloud explosions (VCEs) such as that published by TNO
in the Netherlands (Committee for the Prevention of Disasters, 1997) provide relationships
between the energy released by the burning of the cloud and the distances to different levels of
blast overpressure.
The TNO ‘Multi-Energy’ method (Committee for the Prevention of Disasters, 1997) uses the
concept of ‘energy scaled distance’. This is obtained from:
R’
=
Where:
R’
=
R
=
=
Pa
E
=
R.(Pa / E)1/3
energy scaled distance
actual distance from centre of cloud to point of interest (m)
ambient pressure (Pa)
combustion energy within flammable cloud (J)
For a given explosion strength (intensity), a particular scaled distance relates to a specific blast
overpressure. In addition, for a given R’ doubling of the combustion energy E increases R by a
factor of (2)1/3.
In crude terms, doubling the size (volume) of the cloud doubles the energy released in the
explosion. However, the distances to overpressures of interest are not doubled but are only
increased by a factor of the cube root of two (i.e. 1.26). In other words, doubling the size of the
cloud only increases the distances of interest by 26%.
With regard to the cloud itself, the available evidence suggests that the portion involved in the
main explosion (within the Northgate / Fuji car parks) was of more or less uniform depth, but
assumed an irregular shape. Assuming the depth remains the same, doubling the volume of a
shape of uniform depth only increases the horizontal linear dimension (i.e. the ‘radius’) by a
factor of the square root of 2 (i.e. 1.41). Hence, at constant depth, doubling the volume of the
cloud increases the ‘radius’ by 41%.
It should be noted that all of these distances of interest are measured from the cloud centre
rather than the source of the release. This is shown in an idealised form in Figure 21.
52 Explosion radii
Cloud
‘radiu
s’
Source
Cloud
centroid
Figure 21 Measurement of Distances of Interest
The effect of doubling or trebling the size of the cloud on the proposed zone boundaries is
illustrated in Table 9. It should be noted that an increase in cloud size of this magnitude would
require some combination of the following factors:
•
a significant increase in the rate of generation of vapour (arising, for example, from
overfill of a larger tank, generating more or larger cascades of liquid droplets);
• a prolonged release duration (similar to or greater than that experienced at Buncefield);
• delayed ignition of the cloud, at a point further from the release (allowing a bigger
cloud to develop prior to the explosion); and,
• the same calm, low wind speed weather conditions.
53
Table 9 Effect of Increasing Cloud Size
Effect on zone boundaries
Change
in
Cloud Volume
None
Doubled
Trebled
Inner
(m)
250
350
430
Middle
(m)
300
380
430
Outer
(m)
400
500
580
8.3.2 Explosion Strength
Within the TNO Multi-Energy Method, the initial blast strength (i.e. the overpressure at the
cloud centre) is indicated by a number from 1 to 10, with 1 being the weakest and 10 the
strongest. These numbers correspond to different curves that relate overpressure to distance as
shown in Figure 22 (Committee for the Prevention of Disasters, 1997).
54 Figure 22 Multi Energy Method Blast Chart
The third progress report indicates that the overpressure in the centre of the explosion was of the
order of 700 – 1000 mbar, which corresponds approximately to an initial strength of 7.
From Figure 22, it can be seen that the curves for strengths of 6 or above converge as the
distance from the source is increased (i.e. at lower overpressures). For these explosion
strengths, this means that increasing the strength of an explosion increases the distance
associated with higher overpressures but has little or no effect on the distances to lower
overpressures.
55 For example, increasing the initial strength of an explosion from 7 to 8 increases the distance to
700 mbar by around 12%, and the distance to 600 mbar by approximately 7%. However, the
distance to 400 mbar would be virtually unchanged.
In the present case, an increase in explosion strength would be expected to produce a small
increase (around 10% or less) in the inner zone distance, but virtually no change in the middle
or outer zone distances.
8.4
SUMMARY
The influence of those factors judged to be of most significance is summarised in Table 10 and
illustrated in Figure 23. Within Table 10, the various factors have been assigned a ‘high’,
‘medium’ or ‘low’ rating according to their influence on the LUP zone sizes.
Table 10 Importance of Influencing Factors
Factor
Rating
SOURCE TERM
Tank geometry
High
Fuel composition Low
DISPERSION
Release duration
Topography
Weather
Ignition sources
EXPLOSION
Cloud size
Strength
Comment
Features of the tank design dictate the number and size of any
liquid ‘showers’ formed.
Winter fuel is more volatile than summer fuel, but ambient
temperatures are lower. In summer the fuel is less volatile but
ambient temperatures are higher.
High
High
A longer release duration allows a bigger cloud to form.
Features of the surroundings (slopes, barriers) have a marked
effect on behaviour of the dense vapour cloud.
High
Calm, low wind speed conditions favour formation of a large
cloud.
Medium The stage at which ignition of the cloud occurs (if at all) affects
the size of the resulting explosion. Ignition at a later time allows
a bigger cloud to develop before the explosion.
Medium Doubling the volume of the cloud involved in the explosion
increases zone sizes, but by less than twice.
Low Given that the explosion is already relatively strong, a further
increase in strength produces a small increase in inner zone size
and very little change in other zone sizes.
It is important to note that:
• several of these factors (tank geometry, time to ignition and topography) are strongly
site-specific;
• it is conceivable that a combination of differences in these factors could lead to an
explosion with more serious consequences than Buncefield;
• in the time available it has not been possible to quantify the impact of all of these
factors on land-use planning zone boundaries; and,
56 •
the reasons for the strength of the explosion at Buncefield are not yet understood,
further investigation may indicate that there are additional factors that are important.
57 MORE VAPOUR
Tank geometry:
more or larger
showers of droplets.
WEATHER
Calm, low wind
speed conditions.
RELEASE DURATION
Release continues for an
extended period
LARGER CLOUD
More energy available
to explosion.
LARGER EXPLOSION
Increased burn area.
Increased overpressure
distances.
TIME TO IGNITE
Longer delay prior
to ignition.
LARGER DISTANCES
Bigger zones.
MORE VIOLENT EXPLOSION
Increased overpressure distances.
TOPOGRAPHY
Configuration of
slopes and obstacles.
Figure 23 Summary of Important Factors
58 9
SUMMARY AND CONCLUSIONS
Proposals have been developed for revised arrangements for HSE’s provision of LUP advice.
The proposals have been developed by:
• reviewing published HSE research on the effects of explosions (Jeffries et al., 1997;
Galbraith, 1998)
• considering the levels of damage to buildings observed around the Buncefield site;
• considering the principles upon which HSE’s advice is based; and,
• revisiting the justifications underlying the Sensitivity Level definitions for proposed
developments in the light of the first two activities.
Two options are presented:
• Option A: On the basis of blast damage observations at Buncefield, increase the size of
the LUP zones only; or,
• Option B: Increase the size of the LUP zones and revise the development Sensitivity
Levels.
Under the current system the Buncefield site has the inner, middle and outer zone boundaries set
at distances of 120 m, 135 m and 185 m respectively. The proposed system would increase
these distances to 250 m, 300 m and 400 m respectively.
The proposed changes to the Sensitivity Levels under Option B would place greater restrictions
on the types of development that HSE would not advise against in the inner zone.
It is recommended that the proposed arrangements are applied to those sites which are similar to
Buncefield in a number of important respects. These similarities would indicate that on the
basis of what is known at present, the site has the potential to give rise to a VCE.
If implemented, it is recommended that the proposed arrangements are applied to sites with the
following characteristics:
• the site is classed as either ‘upper tier’ or ‘lower tier’ under the Control of major Accident Hazards Regulations 1999 (the COMAH Regulations); • the site stores petroleum in vertical, cylindrical, non-refrigerated above-ground storage
tanks with side walls greater than 5 m in height; and,
• the filling rate of the storage tanks is greater than 100 m3 / hour.
These criteria align with those used by the Buncefield Standards Task Group, a joint COMAH
Competent Authority and Industry committee (HSE, 2006).
In comparison with the current system, the proposed changes will, if implemented, result in:
•
significantly larger zone boundaries for the sites affected (for both Options A and B);
59 • as a result of the increased area within the zones, a larger number of consultations
concerning proposed developments in the vicinity of affected sites (for both Options A
and B); and,
• more restrictive advice concerning developments in the inner zone (for Option B).
Furthermore, it is recognised that there may be significant implications in the proposed system
for other types of site where a protection based approach is applied. These sites include bulk
LPG (liquefied petroleum gas) storage, gasholders, other flammable liquid storage and other
facilities presenting a hazard with an associated significant likelihood of death in the inner zone
(including VCEs and other hazards such as fireballs). A review of the implications of the study
findings for LUP arrangements applicable to other types of site is recommended.
An investigation of the sensitivity of the size of the proposed LUP zone boundaries to various
factors was undertaken. The factors considered included:
• the rate at which vapour is formed and the conditions under which this occurs (the
source term);
• the way in which the cloud moves (the dispersion of the vapour);
• the quantity of fuel involved in the explosion (the explosion size); and,
• the violence of the explosion (the explosion strength).
The factors considered to have the greatest influence on the size of the zones were:
• the geometry of the storage tanks at the site (this affects the way the liquid contents
behave in the event of a tank being overfilled);
• the duration of a release (this affects the size of the flammable vapour cloud formed);
• the topography of the tank surroundings (features in the surroundings such as slopes and
obstacles have a marked effect on the behaviour of the vapour cloud); and,
• the weather conditions at the time that a release occurs (calm, low wind speed
conditions favour formation of a large cloud).
60 10
REFERENCES
Advisory Committee on Major Hazards, 1984. Third Report – The control of major hazards.
HMSO.
Carter, D A, 1995. The Scaled Risk Integral – A Simple Numerical Representation of Case
Societal Risk for Land Use Planning in the Vicinity of Major Accident Hazards. In Loss
Prevention and Safety Promotion in the Process Industries, Volume II. Eds. Mewis J J, Pasman
H J and De Rademaeker, E E. Elsevier Science.
Committee for the Prevention of Disasters, 1997. Methods for the Calculation of Physical
Effects, Part 2. CPR 14E (the Yellow Book).
DETR, 2000. Hazardous substances consent - A guide for industry. HMSO. Available at:
http://www.communities.gov.uk/index.asp?id=1143296 .
Galbraith K Lt Col, 1998. Review of blast injury data and models. HSE Books, CRR
192/1998. Available at: http://www.hse.gov.uk/research/crr_pdf/1998/CRR98192.pdf .
HSE, (-). PADHI - HSE's Land Use Planning
http://www.hse.gov.uk/landuseplanning/padhi.pdf .
Methodology.
Available
at:
HSE, 1989. Risk criteria for land-use planning in the vicinity of major industrial hazards.
HMSO.
HSE, 1996. The Pipeline Safety Regulations 1996. HSE Books L82.
HSE, 2001. Reducing risks, protecting people. HSE Books C/100.
HSE, 2006. Buncefield Standards Task Group Initial report – recommendations requiring
immediate action. Available at: http://www.hse.gov.uk/comah/buncefield/bstg1.htm .
Jeffries R M et al., 1997. Derivation of fatality probability functions for occupants of buildings
subject to blast loads - Phase 4.
HSE Books, CRR 151/1997. Available at:
http://www.hse.gov.uk/research/crr_pdf/1997/CRR97151.pdf .
MIIB, 2006a. The Buncefield Investigation - Progress
http://www.buncefieldinvestigation.gov.uk/reports/index.htm .
Report.
Available
at:
MIIB, 2006b. The Buncefield Investigation – Second Progress Report. Available at:
http://www.buncefieldinvestigation.gov.uk/reports/index.htm .
MIIB, 2006c. The Buncefield Investigation - Third Progress Report. Available at:
http://www.buncefieldinvestigation.gov.uk/reports/index.htm .
MIIB, 2006d. Buncefield Major Incident Investigation - Initial report to the Health and Safety
Commission and the Environment Agency of the investigation into the explosions and fire at the
Buncefield oil storage and transfer depot, Hemel Hempstead, on 11 December 2005. Available
at: http://www.buncefieldinvestigation.gov.uk/reports/index.htm .
61 APPENDIX 1 REVIEW OF RESEARCH ON EXPLOSION EFFECTS
A review of HSE research reports dealing with the effects of blast on building occupants was
performed (Jeffries et al., 1997; Galbraith, 1998), to assist in understanding the impact of a VCE
on people at different types of development.
Research on Fatality Probabilities for Building Occupants Subject to Blast
(Jeffries et al., 1997)
The research culminating in the HSE research report CRR 151/1997 (Jeffries et al., 1997)
sought to develop a model for prediction of the likelihood of fatality for building occupants
exposed to a vapour cloud explosion. The model was developed by W S Atkins from first
principles. It considered the effect of a blast wave on different components of a building
structure (glazing, cladding and supporting frame), together with the effects of these failures on
the occupants (impact by glass fragments, impact by cladding debris and the effect of structural
collapse).
The model was applied to a number of different structure types and the results presented as a
series of fatality probability curves.
The authors highlighted a feature of the model that makes comparison of the relative safety of
different buildings difficult. This feature is the sensitivity of the predicted fatality probabilities
to the assumed internal layout of the building.
The model assumes that, for example, glazing fragments will affect those directly behind the
breaking window, with the fatality probability decreasing as the fragments travel into the
building (see Figure 24).
62 BUILDING INTERIOR
Fatality Probability Zones
Window
90% Probability
10% Probability
50% Probability
1% Probability
Wall
PLAN VIEW
Figure 24 Pattern of Fatality Probability from Glazing Fragments
The total fatality probability (Pt) is then derived from a weighted sum of the plan areas
associated with ‘zones’ of different fatality probability, divided by the overall plan area of the
building:
Pt
=
Σ Zone area x Probability associated with zone
Total area
The sensitivity of the model arises from the assumption that internal walls are unaffected in the
event of an explosion and effectively shield areas behind them from glass fragments and debris.
Hence areas within a building with no external walls (such as hallways, corridors and internal
rooms) ‘dilute’ the overall fatality probability.
This effect can be particularly significant with some steel or concrete framed buildings that have
a central ‘core’ housing lifts, stair wells or amenities. Since building occupants tend to spend a
small proportion of the time in such internal spaces, the overall impact of the assumption is to
reduce the predicted fatality probability for certain buildings and cause some anomalies when
different building types are compared.
In addition, the authors make the following statement in relation to the prediction of fatality
probabilities:
63 “There is considerable uncertainty in the prediction of human response to different types of
trauma, and hence the calculation of the effects of an explosion is difficult. The area of human
response is the subject of a further literature search which is currently being performed in order
to try and gain a more specific understanding of the probability of a fatality occurring as the
result of a particular injury.”
The ‘further literature search’ referred to led to the publication of CRR 192/1998 (Galbraith,
1998).
Review of Blast Injury Data and Models (Galbraith, 1998)
The CRR 192/1998 (Galbraith, 1998) report critically reviews the model described in CRR
151/1997, with particular emphasis on the fatality probability predictions. The main
conclusions of the review were as follows:
• the injury criteria used by the model for predicting death from building collapse are
over-pessimistic;
• the impact criteria used for prediction of fatality from flying debris, although based on
‘the most widely accepted injury criteria available in the world literature’, are deeply
flawed because the published criteria are themselves flawed;
• the importance of glazing fragments (a significant contributor to the overall fatality
probability in the Atkins model) as a cause of fatality is grossly overestimated –
although such fragments are a major cause of injury amongst explosion casualties, they
cause virtually no fatalities;
• the model does not consider four other potential causes of fatalities in buildings:
translational (tertiary) injury; debris originating from outside (or inside) the building;
burns; and primary blast injury. Of these, translational injury is considered to be ‘a
significant potential cause of injury at the upper end of the range of overpressures of
interest’. The author indicates that the probability of fatality from the other three
mechanisms should be low for most VCEs, but is not zero.
The overall conclusion is that the predictions of the Atkins model are grossly conservative.
Usefully, the report also presents historical data on blast injuries and fatalities. During World
War II, houses damaged by blast were assigned to different damage categories, as given in
Table 11.
64 Table 11 WWII Housing Damage Categories
Category
A
B Cb
Ca
D
Description
Houses completely demolished – i.e., with over 75% of external brickwork
demolished.
Houses so badly damaged that they are beyond repair and must be
demolished. Property included in this category if 50-75% external
brickwork is destroyed, or in the case of less severe destruction, the
remaining walls have gaping cracks rendering them unsafe.
Houses rendered uninhabitable by serious damage, needing repairs so
extensive that they must be postponed until after the war, e.g. – partial or
total collapse of roof structure; partial demolition of 1 or 2 external walls
up to 25% of the whole; severe damage to load bearing partitions
necessitating demolition and replacement.
Houses rendered uninhabitable, but reasonably quickly repairable under
war-time conditions; damage sustained not to exceed minor structural
damage, and partitions and joinery wrenched from fixings.
Houses requiring repairs to remedy serious inconveniences, but remaining
habitable, e.g. – damage to ceilings and tiling; battens and roof covering;
minor fragmentation effects on walls; broken window glass. NB – cases
with <10% windows broken were not included in this category.
The report also presents data on the causes of injury in air raid victims. The data for people in
dwellings are presented in Table 12, with the contribution from bomb fragments (pieces of
weapon casing) removed.
Table 12 Causes of Injuries in Air Raid Victims in Dwellings (excluding bomb
fragments)
Fatal
Hospital
Blast
2.8
0.2
Glass
0.2
11.1
% Contribution from Cause
Other
Flying
Falling
Debris
Debris
Fall
Burns
19.0
53.7
6.8
3.1
20.5
47.7
12.3
1.3
Other
2.1
6.1
Unknown
12.2
0.8
Note that:
• the contributions from blast (primary effects) and burns are small;
• the contribution from glass to fatalities is very small (1 person in 353, or 1 person in
303 excluding bomb fragment casualties), although this mechanism contributes
significantly to injuries;
• the most significant contributions are from other flying debris and falling debris (the
latter presumably associated with structural collapse, whether partial or total);
• the small but significant contribution from falls may be associated with tertiary effects;
and,
65 • these data relate to air raids, where some warning may have been given – data for V2
explosions (not presented in the report), where there was no warning, are said to show a
lower contribution from falling debris, with an increased proportion from flying debris.
Data are also presented which relate the building damage category (see Table 11) to the number
of fatalities for V1 bomb explosions. This information is reproduced in Table 13.
66 Table 13 Casualties from V1 Bombs Related to Housing Damage
Category
A
B
Cb
Ca
D
No. of Houses
206
172
299
173
44
Killed
76
7
0
0
0
Total No. of Casualties
Serious
63
29
30
4
0
Light
20
22
19
4
0
67 No. of
Occupants
323
257
326
182
45
Killed
23.5
2.7
0
0
0
% of Casualties
Serious
19.1
11.3
9.2
2.2
0
Light
6.2
8.6
5.8
2.2
0
With regard to the data in Table 13 the following observations are made:
• fatalities are found principally in houses with A damage, with a few in category B but
none at all in the C and D categories; and,
• in category A damaged houses, over half of the occupants were not even lightly injured.
Estimates of the overpressure required to cause category A damage range from 20 psi (1.4 bar)
for a short duration blast from 1 ton of TNT, to 12 psi (0.83 bar, 830 mbar) for a slow rising
pressure wave, taking about 100 ms to reach its peak. This latter case is more akin to a VCE.
Note also that, according to current understanding, for a VCE in a region of ‘typical’
congestion, an overpressure of this magnitude would occur within the burning cloud or close to
the cloud edge. Hence the contribution from burn injuries in a Category A damaged building
will be more significant for a VCE event than for a condensed phase explosion. Burn injury
may arise either from direct contact with the burning cloud (if burning vapour enters when, for
example, windows are broken by the pressure wave) or from secondary fires started in the
building by the VCE. Escape from a secondary fire in a building that is significantly blastdamaged may be hindered by debris and / or injury.
68 APPENDIX 2 BUILDING DAMAGE OBSERVATIONS
A summary of the damage to buildings around the Buncefield site has been prepared by
studying photographic and video evidence collected as part of the HSE investigation. A map
showing building locations is displayed in Figure 25. The map also indicates the approximate
extent of the flammable cloud as indicated by burned material (presented in the first progress
report (MIIB, 2006a)) and the possible cloud extent in those regions where the position is less
certain. The damage information is presented in Table 14.
69 24
Catherine
House
22
13
23
Keystone
10
Waverley
Waverley
9
Fuji
14
8
11
15
6
North
Gate
Andromeda
5
4
7
12
3
2
RO
Env
1 Test
Limit of flammable
cloud indicated by
burned material
16
Avica
Avi
ca
Limit of flammable
cloud uncertain
Ramseys
21
Davis Wilson
W ilson
Homes
20
17
Banner
Empty
Warehouse
18a 19 ISA
BOC
18
This map is reproduced from Ordnance Survey material with the permission of Ordnance Survey on behalf of the
controller of Her Majesty's stationery office © Crown copyright. Unauthorised reproduction infringes Crown
copyright and may lead to prosecution or civil proceedings. Health and Safety Licence no 100020125 (2006).
Note: Locations 25 and 26 are not displayed on this map
Figure 25 Building Locations
70
Table 14 Building Damage
Ref.
1
Building
RO Developments – E side
2
RO Developments – N
side
RO Developments – brickclad ‘annex’ with steelclad sections either side
3
3a
3b
4
5
RO Developments – steelclad section to E side of
brick-clad ‘annex’
RO Developments – steelclad section to W side of
brick-clad ‘annex’
Northgate Building - S
side
Northgate Building – E
side
Damage Description
Broken windows (>90% of larger panes, >50% of smaller panes), frames distorted, some pushed in
completely. Interior – ceiling panels blown off, fallen light fittings.
Virtually all windows broken and frames displaced on 1st, 2nd and 3rd floor. Frames remaining but distorted
on ground floor. Some cracking of brick cladding around windows. Interior not clearly visible in photos.
E face of corner – only a few windows broken, some frames distorted, slight cracking of brick cladding in
places. Some distortion of steel cladding on vertical tubes attached to wall (presumed part of building
heating or ventilation system). Roller-shutter door pushed in.
N face of corner – almost all windows broken and frames displaced. Cracking of brick cladding, some
bricks visible on ground outside building.
Some distortion of cladding and window breakage.
Significant distortion of cladding and window breakage. Cladding visible on ground outside building.
All windows broken and frames completely displaced. Some brick cladding also removed, particularly on
ground floor and around stair well. Significant cracking of remaining brick cladding. Some displacement
of internals – light boxes and ceiling battens hanging down, furniture moved. Internal damage appears more
significant on ground floor – fallen pipes & cables visible.
(Note – on 1st and 2nd floor cladding is brick mounted in front of concrete panels. Where the brick has been
removed, it appears to have fallen to the ground outside the building, leaving the concrete panels exposed)
All windows broken and frames displaced. Approximately 2/3 of brick cladding removed. Walls blackened
at the N end (the building caught fire following the incident). Roof collapsed into upper floor at NE corner.
Significant internal damage – tangled wreckage visible. There appears to have been partial collapse of the
floor slab at one location between ground and 1st floor. Where brick has been removed, some cracking of
the concrete panels is visible.
71 Ref.
6
Building
Northgate Building – N
side, near NE corner
7
Environmental Test House
8
Fuji Building (S wall)
9
Fuji Building (E wall)
10
Catherine House
11
12
Keystone Distribution
Loading gantry
13
Fircones (single
brick-built house)
14
Waverley (warehouse to E
of site)
Andromeda (warehouse to
E of site)
15
16
Control Building
storey
Damage Description
Damage similar to E side, NE corner. At one location, in addition to the brick cladding being removed, a
concrete panel has also fallen down outside the building. An adjoining structure (loading bay) has been
destroyed apart for some horizontal steel beams projecting from the main building.
At N end, brick walls demolished and collapsed inwards, roof partly collapsed and steel roofing material
torn off. Cracking of internal block-work walls visible. Louvres in W wall damaged (pushed in). Cladding
at N end of W wall cracked and pushed in. At S end of W wall, door / louvres completely removed.
Along the S edge, almost all cladding has been removed and pushed into the building in places. The
outermost edge of the roof has collapsed. Internal damage appears significant where visible – tangled
wreckage. Some of the outermost reinforced concrete vertical frame members have been bent / cracked.
Along the E wall, all external cladding / windows have been removed (although it is not possible to tell
from the photographs how many windows there were) and internal damage appears significant. The roof is
partially collapsed.
Building comprises multi-storey offices (long axis of building pointing N-S) with what appears to be a 2­
storey annex at the SE corner.
Main part of building, E side – windows broken (almost all) and some distortion of cladding. At the SE
corner of the main building, the structure appears to have undergone partial collapse.
Annex – significant glass breakage and distortion of cladding, with more extensive damage at SE corner.
SE corner - some cracking of exterior concrete wall and distortion of small louvred panels.
Distortion / displacement of some corrugated steel roof panels and side cladding. No damage to steel frame
or process pipework.
Roof tiles displaced, battens exposed in places, close to ridge of roof. Windows boarded in photo –
presumably broken. Chimney stack cracked but still upright. Some cracking of walls. External structure
(car port) partly demolished.
Much of cladding along W wall removed. Along S wall, some distortion of cladding, a few panels
removed.
Significant distortion of cladding, pushed in in places (especially at NW corner) around 25% removed.
Some distortion of frame at NW corner also. Some (slight) displacement of stacked goods inside
warehouse. Apparently no window breakage on glazed annex to S of building, but cladding removed on
main warehouse structure.
Most windows broken on N side. Video footage shows some internal damage (mostly collapsed ceilings).
72 Ref.
17
Building
Empty warehouse to S of
site
18
18a
19
20
BP House
BOC
ISA
Avica / Ramseys / Banner
21
Davis Wilson Homes
22
High Grange (2 storey
brick house)
The Cottage (single storey
brick house)
Eaton Lodge (single storey
house)
Leverstock Green School
(approx 1.5 km to SSW)
Houses & Flats (approx
1.3 km to SSW)
23
24
25
26
Damage Description
N side – steel clad walls pushed inwards with some removal of cladding. Bowing of roof in middle of each
‘bay’. Horizontal bars to which cladding attached torn away from vertical members of frame. Some
distortion of steel vertical frame members.
A few windows broken and some damage to facia under eaves (blown in / off).
N Side – Steel cladding dislodged and collapsed outwards. Some displacement of goods on racking inside.
W Side – roller shutter doors pushed in. N side - some cladding partly removed and blow in.
Ramseys, E side – approx 20% window breakage and damage to facia on 1st floor.
Banner, E side – some displacement of roof tiles, damage to facia under eaves, little window breakage.
Brick cladding appears undamaged.
Avica, E side – steel cladding torn off and collapsed outwards. Cracking of brick cladding on lower part of
wall. Few windows in this side, but about 50% broken.
Avica, N side – more windows in this side, 50-75% broken. Some brick cladding removed at NE corner,
cracked in other places.
Displacement of some roof tiles (around 20%). A few (<10%) windows broken. Damage to facia under
eaves.
Approx. 50% of roof tiles displaced. Most windows broken, chimney stack cracked.
Roof almost completely collapsed, windows broken. One gable end wall and chimney collapsed inwards,
cracking of remaining walls.
Most windows broken, some roof tiles displaced. Lean-to garage roof fallen in.
Some glass breakage (<20% overall)
Some breakage of larger glass panes, approx 10-20%
73 Approximate distances between key buildings of interest and the cloud edge are shown in Table
15. The maximum distance to the flammable cloud edge, as measured from the central tank in
HOSL West Bund A, was approximately 250 m. For larger buildings and where there is some
uncertainty about the cloud boundary position, the nearest and furthest distances are given.
Table 15 Distance from Cloud Edge to Locations of Interest
Furthest Distance
Nearest Distance
from Cloud Edge (m)
from Cloud Edge
(m)
60-65
110-115
55-60
In Cloud
Partly In Cloud
In Cloud
Partly In Cloud
15-20
6 5-70
90-95
120-125
160-165
130-135
140-145
270-275
290-295
530-535
500-505
1 80-185
245-250
125-130
140-145
55-60
100-110
305-310
335-340
Location
RO Developments (main building) RO Developments brick-clad corner ‘annex’
Northgate Building
Environmental Test House Fuji
Catherine House Keystone Distribution Fircones
Waverley
Andromeda
Empty Warehouse BOC
ISA
Avica
High Grange
The Cottage Eaton Lodge 74
APPENDIX 3 ESTABLISHING REVISED ZONE BOUNDARIES
When using a ‘protection’ (i.e. consequence) based approach, the locations of the LUP zone
boundaries are normally set as follows:
• inner zone: significant likelihood of death from the selected event;
• middle zone: dangerous dose to a typical population from the selected event; and,
• outer zone: dangerous dose to a sensitive / vulnerable population from the selected
event.
Inspection of Table 11and Table 13 indicates that:
• building damage similar to Category A corresponds to a likelihood of fatality that is
significantly above a dangerous dose, particularly when taking into account the
observation in Appendix 1, that the contribution from burns may be increased for a
VCE relative to a condensed phase explosion; and,
• building damage similar to Category B corresponds to a likelihood of fatality (<3%)
that is close to that for a dangerous dose.
It is more difficult to identify any correspondence between building damage and a dangerous
dose to a sensitive / vulnerable population. None of the information reviewed to date has
contained data on the vulnerability of particular groups (such as children or the elderly) to
explosion effects. It is postulated that the combination of the shock of being exposed to an
explosion, together with injury from glass or other debris, could result in fatalities among
particularly susceptible individuals. In the absence of other information, this is taken to
correspond to Category Cb damage.
There are also difficulties associated with assigning the observed damage in Appendix 2 to one
of the categories in Table 11. The damage categories relate to housing, whereas the observed
damage relates to a variety of building types. Building construction can have a significant effect
on response to blast. A level of blast overpressure causing collapse of a house may result in
severe damage to the glazing and cladding of a steel- or concrete-framed structure, but not
complete collapse of the supporting frame.
Therefore, rather than attempt to assign observed damage to one of the damage categories, the
information in Table 13 has been used to form a judgement as to the potential harm to occupants
that could have arisen from the damage observed. The levels of harm assigned reflect those
anticipated in the different LUP zones, as listed above. Hence the ‘high’, ‘medium’ and ‘low’
categories relate to the inner, middle and outer zones respectively. The ‘minimal’ category
relates to the region beyond the outer zone. In doing this, it has been borne in mind that typical
masonry-built houses tend to be more susceptible to collapse than steel- or concrete-framed
structures. The results are summarised in Table 16.
75 Table 16 Potential Harm Categories Assigned
Building
RO
Developments
Northgate
Environment
Test House
Fuji
Catherine
House
Fircones
High Grange
The Cottage
Eaton Grange
Other
buildings
Harm
Category
Low
High
Medium
High
Medium
Minimal
Minimal
Low
Minimal
Minimal
Description
Low likelihood of fatality for members of
vulnerable population.
Significant likelihood of fatality.
Low likelihood of fatality for members
population.
Significant likelihood of fatality.
Low likelihood of fatality for members
population.
Some injuries possible, but fatalities unlikely.
Some injuries possible, but fatalities unlikely.
Low likelihood of fatality for members of
vulnerable population.
Some injuries possible, but fatalities unlikely.
Some injuries possible, but fatalities unlikely.
a sensitive /
of a typical
of a typical
a sensitive /
The location of different levels of harm / damage may be used to establish LUP zone
boundaries. However, one difficulty in doing this is that there are (fortunately) relatively few
examples of the more serious levels of damage / harm and that these are quite scattered
geographically. Therefore the following method has been used:
• the inner zone boundary was set at the greater of the distance to the furthest example of
‘high’ harm potential or the closest example of ‘medium’ harm potential;
• the middle zone boundary was set at the greater of the distance to the furthest example
of ‘medium’ harm potential or the closest example of ‘low’ harm potential; and,
• the outer zone boundary was set at the greater of the distance to the furthest example of
‘low’ harm potential or the closest example of ‘minimal’ harm potential.
The distances have been calculated by adding the maximum observed flammable cloud extent
of 250 m to the distance of the building from the cloud edge (see Table 15). The results are
summarised in Table 17.
76 Table 17 Zone Boundaries
Zone
Inner
Middle
Outer
Distance
from Discussion
Suspected Release
Point (m)
250
The furthest instance of ‘high’ is within the cloud, the
closest instance of ‘medium’ is partly inside the cloud.
Hence inner zone is set at the maximum observed
flammable cloud extent.
300
The nearest instance of ‘low’ is at the NE corner of RO
Developments, approximately 60 m from the cloud edge.
400
The furthest instance of ‘low’ is at the SE corner of RO
developments, approximately 115 m from the cloud edge.
The Cottage may be at a similar distance, but there is
some uncertainty about the cloud extent in this direction.
The nearest instance of ‘minimal’ is at Fircones, which is
closer than The Cottage. It is suspected that this
anomalous situation arises from shielding by the
vegetation around Fircones. The next nearest instance of
‘minimal’ is at High Grange at around 145 m from the
cloud edge, although again there is some uncertainty in
this. The 145 m distance has been used to set the outer
zone, rounding up.
77 APPENDIX 4 CASE SOCIETAL CONCERNS - WORKPLACES
The discussion in Section 6.2.2 also has implications for the societal concerns associated with workplace type developments. That is, the case societal concerns associated with these developments may be greater for such populations when exposed to a VCE hazard than when
exposed to some other hazards (such as chlorine). The inability (or lack of opportunity) to carry
out appropriate emergency action in the event of a VCE could leave larger numbers of people
exposed to the hazard. This increase in case societal concerns can be illustrated using the Scaled Risk Integral (SRI).
Historically (i.e. prior to the introduction of PADHI), HSE has used the SRI to give an
indication of the case societal concerns associated with a given development (Carter, 1995).
The SRI is calculated using: SRI
=
Where: P
=
R
=
T
A
=
=
(P.R.T)/A
population factor (see below) individual risk of dangerous dose at development (chances per million per year,
cpm)
proportion of time for which the development is occupied
area associated with development (ha)
The population factor P is obtained using:
P
=
Where:
m
=
n
=
½ [mn + (mn)2]
modifier reflecting population type (1 for house residents, 2 for a sensitive /
vulnerable population and 0.25 for people at a workplace)
number of people occupying development
The case societal concerns were judged to be ‘significant’ if the SRI exceeded 2500, with a
presumption in favour of advising against in such cases (although SRI was not the only factor
considered when formulating advice).
The SRI result is quite sensitive to the value of the quantity (mn). The origin of the m value of
0.25 for a working population is not clear, but it is thought that a value of less than 1 represents
a less sensitive or vulnerable population and reflects the justification stated in Table 3
(occupants fit, healthy and easily organised for emergency action).
It is proposed that if, as argued above, the opportunity for emergency action is limited for a
VCE hazard, then a higher value of m would be more appropriate. This would reflect the
increased vulnerability of the population to the effects of the hazard. The effect of varying m
for a working population is illustrated in Table 18.
78 Table 18 Example SRI Values
Development
Housing
Offices
Offices
Offices
Offices
Offices
Offices
Offices
Offices
Offices
Offices
Offices
Offices
No. persons
n
75
100
100
100
100
100
100
75
75
75
75
75
75
Modifier
m
1
0.25
0.5
0.75
0.25
0.5
0.75
0.25
0.5
0.75
0.25
0.5
0.75
R (cpm)
1
20
20
20
10
10
10
20
20
20
10
10
10
T
1
0.3
0.3
0.3
0.3
0.3
0.3
0.3
0.3
0.3
0.3
0.3
0.3
A (ha)
1.2
0.8
0.8
0.8
0.8
0.8
0.8
0.6
0.6
0.6
0.6
0.6
0.6
SRI
2375
2438
9563
21375
1219
4781
10688
1852
7219
16102
926
3609
8051
The ‘Housing’ entry in Table 18 illustrates the origin of the criterion value, with 30 houses on a
1.2 ha site located on the middle-outer zone boundary giving an SRI of just under 2500. An
‘Office’ development for 100 people on a 0.8 ha site well within the inner zone (at a risk of 20
cpm) gives a similar result when using a modifier of 0.25. A smaller office development at a
similar location gives a lower SRI of under 2000.
However, increasing the modifier to 0.5 or 0.75 for the working population takes the SRI above
the 2500 criterion value for either an office development for 100 people well within the inner
zone, or a smaller office development for 75 people on the inner zone boundary.
79 Published by the Health and Safety Executive 02/07
Health and Safety Executive
Revised land use planning
arrangements around large scale
petroleum depots Following the incident at the Buncefield Oil Storage Depot
in December 2005, the Health and Safety Executive (HSE)
commissioned Environmental Resources Management
(ERM) to assist in reviewing HSE’s approach to providing
land­use planning (LUP) advice in the vicinity of similar
installations.
Proposals for revised arrangements for provision of LUP
advice have been developed. Two options are presented.
The proposals are based on a review of information on
the effect of blast on building occupants, observations of
the blast damage at Buncefield, and a review of some of
the justification underpinning certain aspects of the
current arrangements. Both of the options proposed
would result in greater restriction on development of land
in the vicinity of those sites affected.
For both options, the proposed system would operate
within a defined geographical area around affected sites.
The sensitivity of the size of this area to certain factors
has been investigated.
This report and the work it describes were funded by
the Health and Safety Executive (HSE). Its contents,
including any opinions and/or conclusions expressed, are
those of the author alone and do not necessarily reflect
HSE policy.
RR511
www.hse.gov.uk