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IChemE SYMPOSIUM SERIES NO. 153 # 2007 IChemE

THE BP CLAIR PLATFORM: A CASE STUDY OF APPLICATION OF LAYOUTIN THE CONTROL OF EXPLOSION HAZARDS

Vincent Tam1 and Simon Coleman2

1e-mail: [email protected]: [email protected], bp

One of the prime tools in the control of major hazards, such as gas explosion is layout of equipment.

An offshore production facility, unlike its onshore counterpart, very often has a limited footprint.

Equipments are often arranged on many levels separated by decks. This creates potentially situ-

ations where both congestion and confinement could lead to high explosion overpressures. In

this paper, we describe the general principle of layout, gave examples based on the recently

built bp Clair platform, the application of these principles and some novel approach to layout

that led to a facility that is able to tolerate virtually all worst-case explosion events as understood

using the current state of the art tools.

KEYWORDS: explosion, control, layout, case-study, bp clair

INTRODUCTIONThe layout of equipment, pipework, walls and decks on anoffshore platform is known to have an impact on the severityof a potential explosion on the platform (IGN 1993). Thesame is true for an onshore processing plant. This paperillustrates the process and methodology used in optimizinglayout against gas explosion hazards in a recently completedoffshore production platform, Clair. Our experience is thatdecisions made in the early phase of the project, i.e.Concept Selection and Early FEED is critical for successfulapplication in the control of potential explosion hazards.

Clair is the first bp facility to operate in the UKCSdesigned in Houston, USA, by Mustang Engineering. Thisprecedent has created much discussion in the industry inthe UK. Some of the discussions centered round costsaving, some on maximizing learning across differentcultures. It is not the purpose of this paper to delve into thedetails of differences in working practices and approaches indesign engineering in general. Rather we focus on theprocess of the Clair design against gas explosion hazardsgiven the engineering organization. Our experience is thatno one deliberately set out ignoring major hazards in thedesign of a new platform and Mustang Engineering is ofno exception. The path to produce a fit-for-purpose designfor the UKCS might be different to that taken by, say, aUK engineering contractor; but it is the end results thatcount. In this paper, we will touch upon some of thefactors that drive the Clair design and highlight some ofthe differences when compared with a conventional UKCSplatform specifically on gas explosion hazard.

Past experience showed that gas explosion hazardshave the potential to form a significant part of the totalrisk both to personnel and to the Temporary Refuge on anoffshore platform. The cost in reducing this risk to a toler-able level was recognized to be potentially high, particularlyif changes in design were to be made at the latter stages, e.g.in retrospective upgrading of blast walls. The BP approachon the Project was to eliminate, prevent and minimize the

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potential for gas explosion hazards at an early stage, bydesigning these out as far as possible. This approachbegan in the design of Bruce at the end of the 80’s andembedded into our design process since the design of bpETAP (Paterson 1998) where the process of PECT wasapplied. This will be discussed in more detail later.

CLAIR PHASE 1 DEVELOPMENTThe Clair Oil Field is one of the largest, yet to be exploited oilfield in the UKCS. Figure 1 gives the location of the Clair OilField, the location of the Phase 1 Development platform andthe export pipeline routes to shore. The Clair facility describedin this paper form Phase 1 of the development of the field andis located 45 miles west of the Shetland Islands in 140 m ofwater. Phase 1 is expected to have a recoverable reserve ofsome 250 million barrels of oil and a plateau production rateof 60,000 b/d of oil and 20 MMSCFD of gas. Two pipelinesbring the Clair oil and gas production to reception facilities atthe Sullom Voe terminal in Shetland.

The Clair platform consists of a single, 4-leg steeljacket, with integrated topsides comprising wells, drillingand oil and gas production facilities, associated utilities,power generation, safety systems and accommodation for120 persons. It has a service life of 25 years.

There will be some 15 production wells, which will begas lifted and 8 water injection wells with the wellheadslocated in a central wellbay. The well fluids will be processedin 2 oil separation trains, each consisting of 3 stages ofseparation and a single 3-stage gas compression train. Oilis exported via a dedicated 22 inch pipeline to the SullomVoe oil terminal and gas in exported to the BP West of Shet-lands Pipeline system (WoSPS) via a 10 km 6 inch pipeline.

The drilling arrangements comprise the DrillingEquipment Set (DES), with drill floor and derrick and theDrilling Support Module (DSM), with mud systems.Power generation is centralized and comprises 3 Solar gasturbine powered generation sets.

Figure 1. Location of Clair field, platform location and pipeline routes


The platform has been laid-out on the principle ofseparating the hazardous and non-hazardous sections bythe greatest distance vertically and horizontally, over 3main deck levels: Cellar, Production and Main decks. Thewellheads, manifolds, oil separation and pumping and pipe-line risers being located on the lower two levels in a single,un-segregated hazardous area. Gas compression is located ina separate module above Main deck level to the E of theplatform. The DES is located above the Main deck andthe DSM is located adjacent the DES to the E.

Power generation, water injection, fire and servicewater and other utilities are located in a separate segregatedarea to the W and the accommodation module, which pro-vides the Temporary Refuge, to the far W. Four 50%POBfree-fall lifeboats are located directly adjacent to the accom-modation to the W.

Figure 2 shows a representational overview of thefacilities, Figure 3 a more detailed elevation of the platformand Figure 4 shows a plan view of the platform (atproduction deck level)

PROJECT OBJECTIVEAt the outset of the design process, the project set an objec-tive that the Clair platform has a higher performance against

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gas explosion than those of previous platforms built by bp inthe North Sea. And the Project drove the design down thepath of preventing and reducing the severity of potentialgas explosion events very early in the design.

SOME HISTORICAL PERSPECTIVEThere is an established infra-structure to service the oil andproduction industry in the shallow water of Gulf of Mexico.Platforms are designed and built in large numbers. There isalso a large market for reuse of topside facilities and support-ing jackets. This drives towards building facilities with alimited number of building blocks using off-the-shelf com-ponents and standard vendor packages. With time and experi-ence of operations, the designs of these facilities wereoptimized. The Gulf of Mexico now enjoys a reputation ofproducing simple and low cost offshore facilities to servicethe large range of fields in the shallow water region of the Gulf.

It is interesting to note that the first generation ofNorth Sea platforms in the seventies were derived fromthe Gulf of Mexico design. Through many differentfactors, such as severe weather conditions, higher pro-duction rates and more recently a more severe legislativeregime, North Sea designs diverge from the Gulf ofMexico practice. The industry has been able to produce a

Figure 2. Conceptual overview of the Phase 1 platform


diverse range of facilities designed specifically to exploitspecific oil and gas fields. These facilities are often largeand complex in relation to the typical Gulf of Mexicofacilities.

Figure 3. Platfrom Elevation (

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In concept, Clair is a conventional North Seaplatform with an integrated deck. The design of Clair inthe Gulf has meant bringing together the two differentdesign cultures.

north) of the Clair platform

Figure 4. Platform plan view (production deck level)


APPLICATION AND EXTENSION OF

PAST KNOWLEDGEAs far as the approach to gas explosion hazard goes, Claircould build on the experience of a number of recent pastbp projects. The Clair process is largely adopted from thatof ETAP (Paterson 1998). There are two reasons for this:we have intimate knowledge of its application because it isa bp facility, and many inspectors in the UK HSE viewedthis as an example of industry best practice (HSE 2004).

There are four elements to the process used on ETAP.We called this process PECT. PECT stands for using theright People with leadership and relevant expertise Earlyin the design to drive down explosion risks and maintainthis Continuously throughout the design process utilizingappropriate Tools.

We also leaned through the design of bp Schiehallionand bp Andrew facilities that it is important to controlrunaway length and aspect ratio. We will discuss the appli-cation of these later.

The explosion management process on Clair can besummarised as addressing the issue at all stages of design:

. starting at Concept

. early FEED

. continuously through the design

. mop-up late changes at end of detailed design and con-struction

. Verification/confirmation of key assumptions prior tostart-up

DESIGN PROCESSIn this section we describe the process specifically on howwe addressed gas explosion during the various stages ofdesign. A more general discussion on the project manage-ment of major hazards is given in later sections.

As with the ETAP project design, the design teamapplied latest CFD technology right at the beginning in

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order to provide the necessary understanding to optimizevarious part of the design. This approach was used through-out the design cycle. The majority of the explosion hazardassessment work was done right up front during earlyFEED. The final stages were mainly for verification. Someaspects of design were sufficiently well understood thatgood engineering knowledge was deemed to be sufficient,such as the initial layout of equipment, aspect ratio, etc.Even so, on some occasions, we found CFD was useful asa coaching aid to ensure all parties have common under-standing of the issues involved.

Here, we describe some of the areas we addressed.Some of these would be familiar to readers and have beencovered in our previous papers (Paterson 1998, Tam 1994and Tam 1997). We have not included them in this paper.Here we concentrate on issues relating to Gulf of Mexicopractices or further development to our previousapplications.

CONCEPT AND EARLY FEED

Adopt an open, un-segregated designOne common feature of Gulf of Mexico offshore facilities istheir openness. It is not uncommon to find facilities whichare open on all sides with grated decks throughout. Thedesign philosophy has been to rely on the use of natural ven-tilation to control major hazards such as gas explosions.This, by and large, has been successful for platforms inthe shallow water in the Gulf and embedded into engineer-ing practices in the Gulf. We believe that this is one of thestrengths of the Gulf design approach in explosion hazardcontrol.

The bp Clair design team also preferred an opendesign at the outset, so there was a meeting of minds. Theissue was the degree of openness.

One concern of this design was the impact on workingenvironment, as the West of Shetlands environment is harsh.


There were many discussions relating to the concern forworking environment involving the operations team in theproject.

The project team agreed on an open design withoutwind walls for personnel protection.

Shape of facilitiesThe aspect ratio of an area is the ratio of its length to itsheight. A large aspect ratio allows an extensive flame toaccelerate through equipment. Our experience indicatesthat this ratio should be kept to below 3, and preferable2.5 or less (Tam 1994). The Clair facility resembles a rec-tangular cuboid, 20 m wide between supporting truss,50 m long and 28 m high. The aspect ratio is thereforemuch less than 2,5 in all dimensions. The large heightbetween decks was dictated in part by access requirementsfor well intervention equipment. Nevertheless, this providedbenefit in the control of explosion hazards.

Horizontal and vertical barriersClair considered two types of segregation barriers: verticaland horizontal.

Vertical barriers are blast and/or fire-rated wallswhich segregate the hazardous external spaces (e.g.process areas) into smaller areas. The initial design didnot contain any vertical barriers in order to maximizenatural ventilation in the process areas in common withthe Gulf of Mexico norm. We considered the relativebenefits and de-merits of vertical barriers between thewellbay and the utility area and between the wellbay andthe process areas. The latter was discounted, but we foundthat, in practice, the wellbay – utility area barrier providedmeans of controlling the worst-case gas cloud size andseparating this from the face of the Temporary Refuge.

Horizontal barriers are decks which segregate theprocess space further into smaller areas. However, introdu-cing these will have the impact of increasing the aspect ratio.One of our design objectives was to control the ‘critical runlength’, which is the distance above which explosion over-pressure could increase rapidly; a dividing point betweena less rapid and more rapid development of gas explosion(Tam 1994). We found that horizontal barriers couldreduce the critical run length considerably.

The design that moved forward to detailed design hadonly one vertical barrier (between wellbay and utility area),and with all horizontal decks below the Main (weather deck)grated.

Control congestionOne example is the wellbay-manifold area. In a convention-al design, this area has a high level of congestion and oftendifficult to control by alternative arrangement. This arrange-ment usually lead to this arrangement: moving from one endof the facility to the other, there is the TR, utility, wellbay/manifold, high pressure processing equipment and lowpressure processing equipment. The consequence is thathigh pressure inventories are located close to the centre ofthe facilities where natural ventilation is not the highest.

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Clair chose to separate the manifolds from thewellbay. This considerably reduced the extent and amountof congestion in this area and has further significantbenefit in removing potential sources of high pressure gasleaks to elsewhere where natural ventilation rates arehigher. This concept was to be defined in more detailduring FEED.

Control location of HP or high inventory sourcesThe Clair philosophy has been to locate high pressure gasinventories as far away from the TR as possible, in areaswhere ventilation rates are high and open to the outside sothat potential releases could disperse out of harm’s way.Examples include:

Location of compression moduleThis module has a very low aspect ratio. It is located high upat the east end (the TR is on the west end) of the platformabove the Main deck.

Manifolds – gas liftThe production, test and gas lift manifold arrangements arelocated at the east end of the facilities on the productiondeck. This has two benefits: one is to reduce the level andextent of congestion in the wellbay area in the middle ofthe platform, and ventilation rates at the east end is higherthan the wellbay area. Figure 4 shows the layout of theplant, with the manifolds to the far E. of the platform andthe wellheads located in the central wellbay area. The flow-lines to/from the wellheads to the manifolds run longitudin-ally between the two locations within the line of the mainE-W structural beams.

FEEDDuring FEED, layout was further refined and there was afocus on leak management: elimination prevention, andreduction were addressed. Some of the examples are givenbelow:

Connecting pipework between wellhead and manifoldsIn a conventional layout, connecting pipework is often loopedto reduce stresses. This can create an array of pipes that runs inall three directions, which leads to conditions where gasexplosion, should one occur, are severe. On Clair, the connect-ing pipework runs predominantly straight from the wellbayalong the length of the platform to the manifold at the eastend. This reduces the congestion in the wellbay. As pipesrun along the along the length of the platform by the E-Wmain steel structures, they do not introduce significantincrease of turbulence generating congestion either.

Gas injection linesNot all the gas lift flowlines are needed during initialcommissioning and production. One option was to installthem as and when required during operation. However, theproject felt that this could introduce large number of potentialleak sources during operations due to on-site construction


limitations and decided that all gas lift flowlines were to bepre-installed during construction. The pre-installed linesare welded throughout and run from the manifold area tothe edge of the wellbay area. This measure removes a largenumber of flanged high pressure gas connections.

Wellhead designAll the 15 production wells will require gas lift and thereforethe A annulus on each of these will contain high pressuregas, the inventory of which could potentially fill a substan-tial part of the wellbay area if loss were to occur. BP carriedout significant work in conjunction with it’s Xmas tree/wellhead vendor on the design of the tree to minimize thenumber of potential leak sources, to increase the overallintegrity of the tree/wellhead assemblies and to includeboth annular safety valves and integral wellhead gas liftcheck valves. This means that the large volume of gas in

Figure 5. Typical Clair product

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the annulus is isolated to the maximum possible extentshould a failure of the gas lift line line and/or the connectionat the wellhead occur. Figure 5 shows a typical Clair pro-duction well arrangement.

DETAILED DESIGN

Built-in strength – eliminate potential weak pointsThe integrity of the structure is only as strong as the weakestlink. The project spent much effort to maximize the effi-ciency of the structures, as common in any other project.When weaknesses were identified, the project has a biastowards removing structural weakness rather than havingstudies to justify their adequacy. For example, there wasan occasion when the main author attended a meetingduring which engineers, who had identified a weak struc-tural member, discussed the risk of this on the integrityagainst extreme blast load. The project manager intervened.

ion wellhead/tree arrangement


He authorized the increase of size for that structural memberand the weakness was eliminated. The meeting could havelasted longer, could have resulted in a more detailed struc-tural analysis to anticipated extreme loads, and might notlead to the elimination of the weakness identified.

CONSTRUCTION

Address late changesOne identified late design change was associated with thegas turbine-driven power generators The vendor packagecontains a fuel gas knock-out pot which increases the hydro-carbon inventory and leak source count in the power gener-ation area. The associated parts of the topsides design alsocontained additional leak sources not already accountedfor in the explosion modeling.

We assessed the impact of gas leaks in this area andthe size of flammable gas that could accumulate there. Wefound that there was not sufficient inventory and initialgas pressure to generate a cloud of sufficient size to causeimpairment of the adjacent TR boundaries

Despite this knowledge, the design was enhanced toinclude the following measures:

. gas detection arrays at the gas inlet to the turbineenclosures.

. additional shut down valves on the fuel gas supplied tothe turbine to minimize gas inventory leaked into thepower generation area.

. additional shut down initiation of the production facili-ties on confirmed gas detection in the power generationarea.

VerificationThe final stage of the explosion modelling work was verifi-cation. This mainly involved the verification of the accuracyof the geometric model used for assessing gas explosionloadings (generated during the design process). This wasdone before float out of the main deck from the constructionyard on Tyne-side during the final stages of construction/commissioning phase.

DISCUSSIONS INCLUDING SOME

LESSONS LEARNT

HAZARD MANAGEMENT PROCESSOne of the primary mechanisms for actual delivery of theexplosion management measures for Clair were the BPhazard management and the Mustang Engineering designprocesses used for the project. Essentially these elementsworked hand in hand to deliver a fit for purpose designwhich effectively manages and controls all the major acci-dent hazards, including gas explosions.

BP’s hazard management process, in simply terms, isaccomplished by:

. Identification of hazards.

. Development of an understanding of the hazards (causes,consequences, impacts, escalation likelihood, etc.).

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. Identification of the all of the available hazard manage-ment measures to eliminate, prevent, control, mitigateand emergency respond to the hazard event.

. Selection of the appropriate management measure per-formance standards such that all hazards are managedto ALARP.

This is nothing new to most operating companiesworking in the UKCS who will recognize this type ofprocess as part of the management of major hazards andin the safety case. It shows that the management of majorhazards process developed in the UKCS worked well inthe GOM setting.

In the case of BP Clair, this process was integratedinto the project design team. BP, Mustang, and WoodGroup discipline engineers were intimately involved in thehazard management process and took ownership of theprocess to ensure that the principals and concepts were inte-grated into the design of the installation.

THE PEOPLEThere was a focus on management of explosion conse-quences through the minimization of congestion and con-finement on a on-going, rather than ‘once-off’ basis. Theexplosion analysis results were used to identify areas forimprovement at discrete steps in the design. At each ofthese steps, workshops between the discipline engineersand the explosion analysis experts were held in theHouston design offices. This enabled a better understandingof explosion consequences by the design team, and a betterunderstanding of the design constraints by the explosion ana-lysts. As a result hazard management measures were ident-ified early, and were practically implemented, and werenot compromised during subsequent design development.

The Mustang design process was very receptive tothis approach. While working together, both sides realizedthat there was more in common with respect to hazard man-agement in design that previously understood prior to start-ing the process. Mustang’s engineering management teamhad excellent processes for keeping the design simple.This was augmented by BP’s understanding and analysisof major accident hazards into one effective, fit-for–purpose package.

Small focused teamThe Mustang team was relatively small and focused in com-parison to typical North Sea project teams. The team duringdetailed design was primarily located on one floor of theMustang building in Houston. This group truly functionedas a team, most of which have worked together for anumber of projects, with a common management system.This small integrated team concept helps to keep theprocess simple, reduce rework, provides strong leadership,enables communication, and generally enhances the endresult. While it may be difficult to describe the differencesbetween this approach and typical North Sea practice,those people who have seen both sides of the equationwill clearly recognize them.


Effective communication between analysis and designDue to long distance between explosion analysts in Londonand the design office in Houston, effective communicationbetween these two key groups was recognized to be key ifClair was to achieve its objective. Clair has been able todevelop an effective ‘bridging the gap’ process in periodsbetween workshops – using key personnel to interpret andeffectively communicate design requirements to the designteam.

Continuity of staffWe found that the most vulnerable period is during the earlystage of execution or construction phase. The design team inHouston was being demobilised, and a new team set up inthe UK. Our experience is that this process needs to be care-fully planned to ensure key knowledge from both UK andUS based personnel was maintained.

HAZARD CONTROL VERSUS THE APPLICATION

OF QUANTIFIED RISK ASSESSMENT (QRA)As stated above, the focus of the project was on controllinghazards, rather than minimized quantifying risks. The fourmaxims were followed: elimination, prevention, controland mitigate, in that order. QRA is not widely used at theGulf of Mexico and not part of the normal design toolkitin Mustang Engineering. The project design team’s beliefwas that if the hazards are controlled well, then riskswould be automatically low- this is borne out in practiceon Clair.

QRA was therefore only used as verification tooltowards the latter half of design and to support the necessaryinputs to the Safety Case.

CONCERNS OF THE OPEN NATURE

OF THE DESIGNDuring the early phases of design, there were many discus-sions as to the pros and cons of an open design and in par-ticular with respect to the potentially harsh workingenvironment in the open wellbay and process area. Nowthat Clair has been on station for several monthskey offshorepersonnel have expressed that they are in no doubt that theright decision as to maintaining maximum openness hadbeen made.

VENDOR PACKAGESTreating vendor packages as ‘black boxes’ could have animpact on carefully thought out control strategies (Paterson1997). The example we gave earlier of the power generationpackage shows the importance of understanding the impactof hazards associated vendor packages. There is also a needfor vendors to appreciate their role in project’s hazard man-agement and to work closely with project design team andwilling to make modifications to the design of the packages.It is far more effective, in both cost and function, to removethe hazard at source than incorporating control and mitiga-tion measures round it.

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This was highlighted some years ago and only limitedprogress in certain areas has been made. This seems to be anarea where a concerted joint industry approach is needed toforce the pace.

LIMIT STATE PERFORMANCELimit States was advocated for design against gas explosion(Corr 1999). Two limit states were recommended: theDesign (or serviceability) Case and the Extreme (or duct-ility) Case. The former relates to more frequent but lowseverity events and the latter for rare but high severityevents. A well-designed facility is robust against bothtypes of events.

Clair’s approach to preventative design has led toboth these two limits states being met comfortably. Theanalysis and assessment work carried out indicates that thedesign exceeds the stated performance such that it is ableto withstand all worst-case explosion events except one(this being a catastrophic failure of the gas export riserand its shutdown valves. Though this event has verysevere consequences also has an extremely low likelihood).

SUMMARYThe bp Clair facilities design has been completed. It is on-station and operating. The design and explosion analysisprocess that has evolved over the past few projects havebeen effectively implemented in the design office in theGulf of Mexico. Explosion hazards have been successfullycontrolled in the Clair design. Although the Gulf ofMexico design team had little pre-conception of a NorthSea design norm, we were able to implement some effectiveenhancements in layout and, such as the wellhead/ manifoldarea. The design’s performance required for the two limitstates is comfortably exceeded and barring a completerupture of the main gas export riser, the facility is robustagainst all credible explosion events. One issue that hasdogged us, however, over the last few projects is the appli-cation of explosion hazard control process to vendor equip-ment packages. We believe that this could be moreeffectively addressed by the industry as a whole.

REFERENCESCorr, R B, Frieze P A, Tam, V H Y and Snell, R O, 1999, Devel-

opment of Limit State Approach for Design of Offshore Plat-

forms, Proceedings Conference on Fire and Explosion

Engineering, ERA Technology, Leatherhead, Dec 1999.

HSE Report, 2004, A critical review of post Piper-Alpha devel-

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Report no: rr089 2004.

Paterson, K, Tam, V H Y, Moros, A, and Ward-Gittos, D, 1998,

The Design of BP ETAP Platform Against Gas Explosions,

7th Annual Conference on Offshore Installations: Fire and

Explosion Engineering, London, Nov. 1998, ERA Technol-

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Steel Construction Institute, 1993, Interim Guidance Notes for

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Explosions and Fire, A report published by The Steel Con-

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Tam, V H Y and Corr, R B, 1999, Development of a limit state

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Design - Hazards, Safety and Engineering, London,

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