the design of bp etap platform against gas explosions

Journal of Loss Prevention in the Process Industries 13 (2000) 73–79www.elsevier.com/locate/jlp

Short Communication

The design of BP ETAP platform against gas explosions

Ken Paterson1,a, Vincent H.Y. Tama,*, Thanos Morosa, David Ward-Gittosb

a BP Amoco, Sunbury-on-Thames, TW16 7LN, UKb Brown and Root Engineering, Leatherhead, KT22 7AB, UK

Abstract

Risks of personal injury from gas explosion, together with fire and smoke ingress, were among the key hazards that the EasternTrough Area Project (ETAP) team intended to design out as far as possible. This paper describes the process ETAP followed toachieve this. The process involved the early application of the appropriate advance technology and personnel at the concept selectionstage and right through different stages during design, and an integrated team including explosion specialists.

All major design decisions on explosion optimisation were made at the early stage of front-end engineering design (FEED),resulting in a relatively straightforward detailed design phase. These early design decisions had the effect of not only reducing gasexplosion consequences, but simplifying layout, e.g. reducing pipe run and structures. The end result is a design which givesinherently low risk to personnel and Temporary Refuge impairment without the uncertainties of high cost of late remedial work totake account of high explosion loads, and consequent project delay. 1999 Elsevier Science Ltd. All rights reserved.

Keywords:Gas explosions; Offshore platform; Design process; Design against gas explosions; BT ETAP Project; Management of uncertainties;Safety case

1. Introduction

This paper describes the explosion analysis workundertaken on the BP Eastern Trough Area Project(ETAP). Past experience showed that gas explosion haz-ards formed a significant part of the total risk both topersonnel and to a temporary refuge on an offshore plat-form. The cost in reducing this risk to a tolerable levelwas recognised to be potentially high, particularly ifchanges in design were to be made at the latter stages,e.g. in retrospective upgrading of blast walls. The BPAmoco approach on the project was to minimise gasexplosion risk at an early stage, designing it out as faras possible. Building on the experience gained in theAndrew Project (Tam & Langford, 1994), ETAPdeveloped a design process, which was found to be suc-cessful in meeting the project objective. This paperdescribes the design process undertaken and includessome general guidelines for future facilities. While spe-cific examples given in the paper may not be generally

* Corresponding author. Tel:+44-(0)1932-76-2724; fax:+44-(0)1932-76-4127.

1 Currently at: BP Amoco Exploration, Dyce, Aberdeen, UK.

0950-4230/00/$ - see front matter 1999 Elsevier Science Ltd. All rights reserved.PII: S0950-4230 (99)00019-4

applicable, we believe the philosophy and the processdescribed is of wider applicability.

1.1. Project description of ETAP project

The BP ETAP was managed by an alliance, which forthe topside facilities included among others the oper-ator—BP Amoco; design and project management con-tractor—Brown and Root Ltd.; fabrication contractors—AMEC Process and Energy Ltd., Kvaerner Oil and Gasand Consafe Engineering UK; and installation—Heere-mac.

The Eastern Trough Area consists of seven reservoirswith a total estimated reserve of 400 million bbl of oiland 1.1 trillion cubic feet of sales gas. The central pro-cessing facilities (CPF) for the ETAP development islocated over the largest reservoir at the Marnock field.The CPF provides central processing facilities for thefive subsea production manifolds (serving Skua, Egret,Heron, Machar and Monan), and a normally unattendedproduction platform (serving the Mungo reservoir).

Processing at the CPF consists of separation, com-pression, gas re-injection, gas dehydration and waterinjection. Oil is exported through the Forties pipelineand gas via the central area transmission system.

The CPF consists of two platforms connected by

74 K. Paterson et al. / Journal of Loss Prevention in the Process Industries 13 (2000) 73–79

bridges: a 7000 t deck for the quarters platform (QU)and a processing drilling riser (PdR) platform, whichconsists of two modules and weight about 14 000 t, seeFig. 1. This paper is concerned with the explosion risksassociated with the PdR platform. Explosion risks on thenormally unattended platform on the Mungo field is thesubject of a separate paper (Moros, Tam, Webb & Pater-son, 1995).

1.2. Design process in general

There are a number of stages involved in a topsidefacility project. It begins with ‘concept selection’ duringwhich a number of design options were studied. Whena preferred option has been arrived at FEED takes place.The purpose of FEED was to develop the design to asufficient level to provide a cost estimate of certain toler-ance (typically within a range of 15%). The detaileddesign that followed developed the details to allow fabri-cators to construct the platform.

Our experience showed that the most effective contri-bution from safety specialists and the application ofadvance explosion modelling tools took place in conceptselection and early FEED. This is in contrast with theconventional approach where rough and ready methodsare used during early design stages; safety specialists andadvance modelling tools tend to be applied near the latterstage of FEED or more likely at detailed design stages.

Fig. 1. An elevation of the ETAP PdR platform, viewed from south, showing the four process decks.

2. Explosion design process

In this section, the process followed by ETAP isdescribed.

2.1. Process starts at concept stage

2.1.1. Concept defined the general shape of theplatform

Initially, a number of concept options were evaluatedfor their explosion risk potentials and the ease withwhich the design could be modified to bring the riskpotential to within the project target.

The concept option chosen was based on a four deckprocess module with a separate accommodation moduleconnected by two bridges. For large process modules,the explosion over-pressure loading is a main driver forthe layout and consequently the overall structural con-figuration. Each process deck was further divided intothree compartments by blast walls (Fig. 1).

The size and dimensions of these compartments wereestablished based on a minimum width concept to facili-tate flame travel. An aspect ratio parameter was used todetermine the height of the decks. The Andrew Project(Tam & Langford, 1994) found that the aspect ratio fora three open-sided volume area should be less than 3 inorder that gas explosion pressure remained manageable.On ETAP, a maximum aspect ratio of 2.5 was used.

75K. Paterson et al. / Journal of Loss Prevention in the Process Industries 13 (2000) 73–79

The process of defining the compartment size early inthe conceptual design of the project effectively estab-lished an optimum arrangement for the topside facility.The structural longitudinal trusses framed the perimeterof these compartments. Each compartment was dividedby stainless steel blast walls and transverse trusses wereplaced at regular intervals, see Fig. 2.The minimum width concept negated the need for anyadditional central spine trusses. Uniform spacing and aconstruction depth of 1.5 m for primary girders spanning20 m ensured structural support with both adequatestrength and ductility properties for the blast conditions.Secondary in-deck trimmer beams spanning 4.8 m withonly one lateral restraint proved to be the optimum sol-ution for deck construction.

2.1.2. Assess a realistic over-pressure target—not toexceed target

Based on results and experience gained in previousprojects and a number of simulations, a project target of‘not to exceed 1.5 bar’ was set. This allowed each disci-pline to focus their effort to optimise their design.

It is worth noting that using the technology availableat the time, our experience suggested that a maximumexplosion over-pressure could be controlled to less than1 bar for the theoretical worst case (the volume filled,with a stoichiometric cloud and ignition at the top centreof blast wall) in all areas on the CPF.

The large-scale experiments, Fire and Blast for TopSide Structure Phase 2 (Selby & Burgan, 1998), wasunder preparation at the time and results would be avail-able during the detailed design phase of the project. Thepotential impact of ‘unexpected’ results needed to bemanaged. The project concluded that given an uplift ofa maximum over-pressure target of 1.5 bar, a more strin-gent limit on aspect ratio (2.5 instead of 3), and withthe combination of inventory isolation and control, the

Fig. 2. Main structural trusses of Level 3 of the ETAP PdR platform showing locations of two main blast walls.

adverse impact of the Phase 2 experiment would be man-ageable.

2.1.3. Use of appropriate expertise and tools at thisstage

At the stage of comparing options, no calculationswere carried out. This assessment was based on thejudgement of the explosion experts and experience ofprevious projects. However, calculations were carriedout to define the above ‘not to exceed’ design targets.

2.2. Front-end engineering design (FEED)

A major effort to optimise equipment layout and struc-tural definition took place at the very early stage ofFEED. Contrary to the conventional approach, we useda computational fluid dynamics (CFD) explosion code(FLACS) as part of the design. The challenge to ETAPwas to make this work without increasing additionalelapse time in the design process. In practice, there wasno delay.

Thus, the process of minimising explosion over-press-ure went hand-in-hand with the process of design. Atevery stage of the design of the layout, modelling wascarried out to aid the process of minimising explosionover-pressure. This stepwise approach allowed theoptimisation to be applied at all levels starting fromcompartment shape, major equipment alignment andlocation, major pipe-work to minor pipe-work layout,and so on. One of the key factors that ensured the suc-cess of this stage was the integrated team. The explosionexpert, being part of the team, attended and obtainedrelevant information from project meetings, and resultsof modelling were presented at the next progress meet-ing, allowing the design to go at a high speed.

Explosion analysis was based on a module-filled gascloud with ignition located at the top centre of the blast


wall. The use of the same scenario throughout the designprocess allowed assessment to be made on the effective-ness of different design options through direct compari-son. Absolute calculated over-pressure values providedsome indications as to the levels of optimisationefforts required.

At the early stage of FEED, there were many changesto the layout. Some of the changes were minor, e.g. relo-cation of a piece of equipment, some were major andthe impact on explosion load was correspondingly large.Some examples are given below.

2.2.1. Remove area with high effective aspect ratioIn the earlier layouts, there were localised regions

with high aspect ratios (length to height ratio), e.g. thespace above a local equipment room which did notextend the full height. This space could allow potentiallyvery high over-pressure to develop, and consequently,this and other similar spaces were removed.

2.2.2. Pig launcher and receiver areaThe layout of this area was completely reworked. In

the initial design, the pig launchers and receivers wereorientated in a platform north–south direction. This con-figuration was found to give rise to high over-pressure(many bar) as the predominant venting direction was inthe east–west direction. The launchers and receiverswere re-aligned along the east–west direction in the finaldesign, and connecting pipe work was also simplified.The end result is that the maximum calculated over-pressure was reduced by a factor of 10. Fig. 3 showsover-pressure calculated for the two layouts.

2.2.3. Hook-up pipe-workConventionally, hook-up pipe-work is routed via

internal and external pipe-racks between vessels in dif-ferent areas of the platform. We found that this gave riseto a high density of pipe-work, particularly on the exter-nal pipe-rack which led to significant blockage of theventing area. Solutions were varied. On the cellar deck,pipe-work was routed upward and onto deck 2. Thisresulted in a reduction of vent blockage and the totalamount of pipe-work required, furthermore this pipe-work was aligned to minimise the generation of turbu-lence as seen by a developing gas explosion, thus min-imising any potential consequences of a gas explosionin this area. Another example is in the oil export area(on level 4). Here the pipe-work connected from thisarea to the compressors on the weather deck above.Instead of routing the pipe-work in the conventionalway, the pipe-work was routed through the weather deckand bottom fed into compressors. Again, the amount ofpipe-work was reduced and gas explosion conse-quences reduced.

2.2.4. Reduced leaked inventoriesAt the latter stage of FEED, effort was put into the

layout of pipe-work and detailed inventory isolation, andcontrol analysis was put in place to limit the potentialsize of the flammable gas cloud that could be developedwithin each compartment. For example, by fitting anextra ESDV, the peak inventory in the production mani-fold was reduced by 70% and the total release durationwas halved. This resulted in much smaller gas cloudsfor the same wind conditions. Fig. 4 indicates the gascloud build-up as a function of time for the originaldesign and the one with the extra emergency shut downvalve (ESDV).

2.2.5. Development of top-side structure—efficiencyuse of structure leading to lower weight

During the course of the design, the predictions ofstructural response to explosion loads were improved,allowing the cost and weight of structural steel to bereduced.

A series of structural analyses was completed includ-ing a non-linear finite element analysis of the separationarea on the process module. This led to a saving of some5% of structural steel weight for this compartment byreducing the flange widths of the primary girders andadopting lighter secondary beams.

The performance standards for different elements ofthe structure were varied to suit their functionality andthis enabled efficient usage of material, particularly inthe design of secondary beams.

It was found that engineering guidance on grateddecks was inadequate. So ETAP undertook research atChristian Michelsen Research and developed a method-ology for the design of a grated deck against gasexplosion load. The result of this was published in aseparate paper (Corr & Tam, 1998). The application ofthis gave rise to results that were some 50% less thanthe design values that had previously been assumed.

The 10 m deck heights needed to aid venting, despitethe initial reservations, did improve the overall ser-viceability of the structure and lead to an efficientdesign, particularly for lift conditions.

2.3. Detailed design

At the detailed design phase, the effort spent by theexplosion team wasreducedand much of this was spenton minimising the impact of late design items, such asheating ventilation and air conditioning (HVAC) duct-ing, cable trays and electrical panels. Although thesewere included in an approximate way during FEED, itwas important that the implementation of design at thisstage followed the same vigour as FEED. With hind-sight, there would have been benefit bringing this stageof design forward as far as the explosion assessment isconcerned, to provide a better integrated design.


Fig. 3. (a) Layout A with pig launchers aligned in north-south directions giving high over-pressures of over 4 bar. (b) Layout B with pig launchersaligned in east-west directions which is along the main direction of venting giving much lower over-pressures.

Fig. 4. Gas Cloud Size (% of module filled) vs. Time from start ofthe release, with (I) and without (G) an additional esdv.

Structurally, the symmetry of the compartmentsbrought about by the aspect ratio gave rise to simplerepetitive joint details, which in turn reduced overall fab-rication costs.

By this stage of design, the project computer-aideddesign (CAD) model (PDMS) contained nearly all thedesign details. This was used directly for gas explosionmodelling work using a specially written piece ofsoftware commissioned by ETAP to translate the datainto a format that can be read by the gas explosionmodel.

2.3.1. Control and explosion over-pressure and risksFinal calculations showed that maximum gas

explosion over-pressure on key structural surfaces in allareas were well within the project target of less than 1.5bar. In the majority of areas, they were substantiallylower.

This process resulted in low individual risks as segre-gation by compartment reduced the percentage of peopleaffected by a major gas explosion on PdR. A combi-nation of controlled gas explosion size and large separ-ation distance between potential explosion sites and the


Temporary Refuge (TR) ensure that direct TR impair-ment is virtually zero.

2.4. Verification of as-built—site verification

For final verification, a site survey was carried out onthe modules just before they were floated out. We foundthat the geometric models constructed from the projectPDMS database were a very good representation of theas-built platform. This provided assurance that calcu-lations carried out during the detailed design phasewere valid.

3. Discussions

3.1. Process: PECT

To summarise, there are four main components to theETAP process that can be summarised as people, early,continuous and tool (PECT).

3.1.1. PeopleA design team was put together with all the required

skill and experience as one would expect, but with anadditional member: a gas explosion expert. It isimportant that the person commands the confidence ofthe team, as his judgement will be called upon in settingearly design targets. He/she is also required to articulatereasons for design changes and provide expertise on gasexplosions to project team members.

3.1.2. EarlyThe process of optimising the design against gas

explosion should start as early as possible. If the processstarts late, e.g. during detailed design, then scope forchange would be restricted to minor changes whichwould have limited impact. Conversely, if the processstarts at concept, then the latitude for change is wide,and could include deck configuration. In the case ofETAP, the process started at concept selection. Further-more, a large proportion of explosion design work wasdone during early FEED.

3.1.3. ContinuouslyThe process, once started, must continue unabated to

drive down risks throughout the design process. Onlythen can one be sure that all aspects of layout have beenincluded in this process. For example, the layout of thepower turbine generator on the quarters platform wasoptimised to reduce the risk to a temporary accommo-dation cabin, which was found to be necessary duringthe early stage of ETAP’s operation.

3.1.4. ToolAn appropriate tool is used throughout the process.

Since the task is to reduce risk of gas explosion throughthe design, the chosen tool must be able to estimate theeffect of layout changes and provide sufficient infor-mation to guide design.

There is a widely held view that only a simple modelis needed during early design (to provide engineers withquick answers), and a more complex tool at detaileddesign when more information is available. Thisapproach would not have met the requirement of ETAP.Though more resources were required by FLACS thana phenomenological model, FLACS provided a consist-ent tool to assess design changes and avoid ‘surprised’over-pressure predictions as one switched from a simpleto a more complex model.

3.2. Design rules developed during project

One of the results of this process is that a number ofrules were developed which guided design:

1. Venting area was maximised—in practice, equipmentwas arranged to present the smallest area to the vent-ing flow, e.g. arrangement of stair-towers, vessels,etc., and grating used wherever possible.

2. No cantilevers—cantilevers have tended to be anafterthought in design to accommodate late changesin equipment layout. However, the equipment has theundesirable effect of blocking explosion venting andincreased congestion. ETAP developed a disciplinethat all changes were accommodated within thedesign confines agreed at FEED.

3. Do not fill up space—it was tempting to fill up ‘free’space within compartments. As layout was guided byexplosion over-pressures, the reason for not fillingapparently empty spaces could easily be justified.

4. Move congestion to areas with high ventilation andhigh venting.

5. Align pipe run with direction of venting—6. Segregation of equipment into the compartments to

limit potential gas cloud.

3.3. Things that could be done better

The detailed design of vendors’ skids were outsidethe scope of the project. There are benefits in includingvendors’ skids in this process.

There are a number of processes which conventionallytake place at the latter half of detailed design, e.g.location of instrument panels, routing of electrical cableracking, etc. With hindsight, the optimisation of this areacould be enhanced if some of the design could be perfor-med at early FEED.


3.4. Work up front has enormous impact

As can be seen in previous sections, a significant pro-portion of gas explosion design effort was spent at a veryearly stage. This concurs with our belief that resourcesare most efficiently and effectively deployed up-front.By the detailed design phase, very little effort was need-ed.

3.5. Simple vs. complex model at early design stages

ETAP decided at the outset that design against gasexplosion would take place at an early stage of design,this necessitates the use of a CFD tool that could beused to guide design. This is contrary to the conventionalapproach where simpler models were used at an earlystage of design and a more complex tool at the detaileddesign phase. We learnt from a previous project, e.g.Andrew (Tam & Langford, 1994) that this conventionalapproach would not meet the standard set out by ETAP.

4. Conclusion

The PECT process used by ETAP was found to besuccessful in addressing gas explosion risks during earlydesign. The four elements of PECT are: integrateddesign team including explosion specialists, starting theexplosion optimisation process as early as possible andcontinuously throughout design, and lastly using theappropriate tool (not necessarily the simplest and cheap-est tool). Its application led to the more robust manage-ment of explosion risks during design. All major designdecisions on explosion optimisation were made at the

early stage of FEED resulting in a relatively straightfor-ward detailed design phase. These early design decisionshad the effect of not only reducing gas explosion conse-quences, but simplifying layout (e.g. reducing pipe run)and structural optimisation. The end result is a designwhich gives inherently low risk to personnel and TRimpairment without the uncertainties of high cost of lateremedial work to take account of high explosion loads,and consequent project delay.

Acknowledgements

The authors acknowledge the permission of BP ETAPto publish this paper. The work presented here was theeffort of many. The authors acknowledge valuable con-tributions from members of the design team, in parti-cular, George Pyett who worked on the layout of thecellar deck and Alan Charman on the upper decks.

References

Tam, V. H. Y., & Langford, D. (1994). The design of the BP Andrewplatform against gas explosions, offshore structural design. Haz-ards, Safety and Engineering Conference. ERA, London, Novemb-er.

Selby, C. A., & Burgan, B. A. (1998). The joint industry project on:Blast and fire engineering for topside structures, phase 2: Finalsummary report. Steel Construction Institute, February.

Corr, R. B., & Tam, V. H. Y. (1998). Gas explosion generated dragloads in offshore installations.J. Loss Prevent., 11, 43–48.

Moros, A., Tam, V. H. Y., Webb, S. P., & Paterson, K. (1995). Design-ing for gas accumulation and explosions in offshore modules. Casestudy: the ETAP Mungo platform, offshore structural design. Haz-ards, Safety and Engineering Conference. ERA, London, Decemb-er.

the design of bp etap platform against gas explosions

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