Availability assessment of a raw gas re-injection plant for the production of oil and gas. Carlo Michelassi, Giacomo Monaci

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1 16 th IMEKO TC4 Symposum Explorng New Fronters of Instrumentaton and Methods for Electrcal and Electronc Measurements Avalablty assessment of a raw gas re-njecton plant for the producton of ol and gas Carlo Mchelass, Gacomo Monac GE Ol & Gas, Va F. Matteucc, 2 - I Florence, Italy, carlo.mchelass@ge.com GE Ol & Gas, Va F. Matteucc, 2 - I Florence, Italy, gacomo.monac@ge.com Abstract- Drven by the needs of hghly compettve markets, the performance requrements of today s producton systems are becomng ncreasngly demandng. Hgh avalablty s one of the key requrements for new machnes and ndustral plants. The objectve of the present study s the assessment of the avalablty of a Gas Re-njecton Plant used for the producton of ol and gas n the Kashagan feld (Kazakhstan). In order to assess the avalablty performance of ths complex system, a broad set of studes has been carred out to gather accurate nformaton at the level of detal consdered sutable to meet the avalablty analyss target. Followng ths prelmnary work, the avalablty analyss was performed usng a smulaton approach. The RBD (Relablty Block Dagram) technque, n conjuncton wth Monte Carlo smulatons, allowed many dfferent elements to be evaluated smultaneously to determne the plant avalablty. I. Introducton The avalablty of a system s a non-lnear functon that depends on many parameters, ncludng relablty, mantanablty and logstcs. Relablty s determned from an analyss of the expected lfe of components and the functonal confguraton of the system; mantanablty can be estmated from the tme to repar or restore faled equpment, and the frequency and duraton of scheduled mantenance tasks; fnally, logstcs can be evaluated from the spare parts warehouse polcy establshed to support the mantenance actvtes. In order to assess the avalablty performance of a complex system such as a raw gas re-njecton plant, a broad set of studes s needed to obtan the correct nputs to populate a model. These prelmnary studes nclude, among others: Relablty Data Collecton to gather relablty nformaton at the falure mode level Mantanablty Analyss, carred out at the falure mode level to dentfy the tasks and the assocated tme requred to carry out correctve mantenance Falure Modes and Effects Analyss (FMEA) coupled wth a Crtcalty Analyss (CA) followng nternatonal standards applcable to the Ol & Gas ndustry. The FMEA and CA are supported by a varety of desgn and specfcaton documents. The avalablty analyss that followed the prelmnary studes can be dvded nto three man phases. In the frst step, the Plant and Instrument Dagrams (P&IDs) and other relevant documentaton were analyzed to dentfy those components whose falure could cause system unavalablty. As a general approach, ths analyss ncluded every ndvdual component ncluded n the GE Ol & Gas scope of supply. For example, n the overall system avalablty evaluaton, even the mpact of small ppng, flanges and manual valves was consdered. The second step was the constructon of the plant Relablty Block Dagram (RBD), whch represents a relablty model of the system. It was constructed by connectng the dfferent blocks representng the components evaluated n the frs step. Snce some redundant components have dfferent relablty confguratons, dependng on the falure mode consdered, the RBD structure could nclude smultaneous seres-parallel connectons. Fnally, n the last step, smulatons based on the Monte Carlo method were run. Ths methodology s one of the most frequently used technques for the smulaton of complex systems because t allows takng nto account the non-lnear effects of relablty, mantanablty and logstcs n the assessment of the system relablty and avalablty performance. Smulatons were performed by smultaneously consderng two dfferent operatng stages of the plant: normal operaton and start up.

2 16 th IMEKO TC4 Symposum Explorng New Fronters of Instrumentaton and Methods for Electrcal and Electronc Measurements II. Problem Statement Today, the Caspan Sea s one of the areas of greatest nterest for ol and gas companes n spte of the hgh concentraton of contamnants found n these ol felds. In order to explot n full the ol reservors located n the northeast area of the Caspan Sea wthn the Kazakh Republc, t s necessary to safely handle the assocated gas that contans hgh percentages of H2S. Ths gas cannot be released nto the atmosphere because t s lethal. The only way to recover ths crude ol s to process the natural gas and re-nject the sour gas nto the reservors at hgh pressure. Sour gas (H2S), often found n ol felds, mposes envronmental and safety problems proportonal to the type of contamnants and ther concentraton. The only compressor manufacturer that has accrued experence n the re-njecton of sour gas at the pressures requred by Caspan Sea felds s GE Ol & Gas. The Raw Gas Re-njecton Plant sted n Kashagan, (Kazakhstan) s the most advanced project n the feld because of both the hgh re-njecton pressure nvolved, 11,022 ps (760 bar), and the amount of H2S contaned n the gas, 18%. The hgh pressure level requred the development of a new compressor model to assure meetng the re-njecton pressure at the thrd compresson stage. The GE Ol & Gas scope of supply s the RGI Offshore Plant, whch conssts of two dentcal barges. Each barge ncludes: Turbo-group contanng an MS5200D Gas Turbne equpped wth a Dry Low NOx emsson system and three barrel type Centrfugal Compressors Two Motor-compressors (each consstng of an electrc motor drvng a Recprocatng Compressor) to provde the proper pressure for the Centrfugal Compressor Seal Gas Balance of Plant (BoP) ncludng all the utltes external to the machnes, such as Process Gas, Fuel Gas, Seal Gas, Electrcal and Control System, etc. GE Ol & Gas was also asked to carry out an Operaton and Mantenance (O&M) project for the above scope of supply. Consderng the attenton to safety ssues mrrored by the dedcated safety studes carred out outsde the project tself, the O&M project ncluded a varety of analyses, ncludng an Avalablty Analyss (AA) addressng producton capablty. The fnal goal of the study was the evaluaton of the plant avalablty and the determnaton of the crtcal weak ponts to dentfy potental mprovements n the mantenance polces. The Avalablty estmate and the predcton of the performance n terms of producton are based on the system desgn (components, falure modes and frequences, repar/restoraton tmes, and system archtecture) but also on the Operaton and Mantenance polces foreseen (mantenance strateges, msson profle, logstcs, etc.). As a consequence of the unque and crtcal nature of ths techncal applcaton, the level of detal of the analyss was set at the tem level and the crtcalty of the assocated FMs (Falure Modes) was assessed. The soluton, adopted to provde mantenance feedback at the component level and suted to the desgn documents used to support the analyss, requred a major effort ncludng about 2500 blocks to set up the RBD model based on about 4700 plant tems that were analyzed n the project. A smulaton approach was selected to perform the avalablty analyss. In ths case t s based on the Monte Carlo smulaton method that generates random falure tmes from each component s falure dstrbuton. The results, whch are dffcult to solve analytcally, can be obtaned through the smulaton. On the other hand, smulaton methods also have certan drawbacks, not the least of whch s that they can be tme consumng. Furthermore, the results depend on the number of smulatons, whch mples a lack of expermental repeatablty, even though Monte Carlo smulaton s one of the most relable predcton tools. III. Prevous Analyses The modellng of the system requred that several analyses be carred out to provde the necessary data and/or nformaton n support of the dfferent phases. The most mportant nput steps n support of the avalablty model are the followng: the settng up of the Relablty Block Dagram wth the relablty lnks between the numerous tems composng the plant modelled, the relablty nput data, the correctve mantenance nput data, and the preventve mantenance nput data. A bref descrpton of all the prelmnary actvtes carred out n support of ths objectve follows. A. Equpment breakdown structure All the tems belongng to the boundary lmts were structured herarchcally. Each tem to be

3 16 th IMEKO TC4 Symposum Explorng New Fronters of Instrumentaton and Methods for Electrcal and Electronc Measurements potentally nserted n the RBD was regstered and dentfed wth a unque TAG, and for each TAG, all the necessary nformaton for the subsequent phases such as reference drawngs, P&IDs, specfcatons, datasheets, O&M manuals, reference manufacturers and supplers, and complete dentfcaton of the TAG relablty characterstcs were collected. B. Relablty data collecton The TAG relablty characterstcs determne the approprate relablty data for each tem. The data sources are: Dedcated analyss of smlar equpment from nternal GE Ol & Gas fleet montorng tools Data from manufacturers Commercal databases such as OREDA or NPRD95 [1], [2], [3]. These data are collected at a falure mode level when avalable. Otherwse a global value s assgned to the component falure rate. C. Correctve mantenance data collecton Mantanablty was a key consderaton n the desgn of the RGI plant. Specal analyses, such as 3- D modellng, were conducted to optmze the layout of the barges to provde safe and rapd access for both correctve and planned mantenance actvtes. For the avalablty analyss, a dedcated study was performed for dentfyng the Correctve Mantenance Tme, that s the tme necessary to restore an tem back to servce after a falure. It ncludes several contrbutng factors: Tcm = t FLDT, + t TOST, + t ART, + t BIST, t AOT, j, (1) Tcm = correctve mantenance tme assocated wth the j-th falure mode of the component t FLDT = tme necessary for fault locaton and dagnoss t TOST = tme necessary for gettng the tem out of servce, t ART = actve repar tme t BIST = tme for gettng the tem n servce agan after the repar t AOT = overlap tme for actvtes conducted n parallel. All these data are collected recognzng that each falure mode of each tem requres a unque mantenance tme, dependng on the accessblty and on the barge/plant safety procedure to be followed. C. Falure Modes and Effects Analyss The purpose of the Falure Modes and Effects Analyss s to analyze the falure modes of each tem to be potentally nserted n the RBD n order to determne ther effects on the process n terms of safety, producton and the envronment. The output of the FMEA can be used as a good bass for decdng f an tem should be nserted nto the RBD. The FMEA results were ntegrated wth other general project documentaton such as Cause and Effect Dagrams, Functonal Descrptons, Component Specfcatons and Component Datasheets. D. Mantenance Plan The Mantenance Plan defnes the preventve and on-condton actvtes to be carred out on each TAG wthn the scope of supply and collects all the relevant nformaton on actvtes, sklls, duratons, tools and spare parts. By the proper flterng and analyss of these data t s possble to defne the mantenance plan to be modelled n the RBD. IV. Modellng Approach In order to estmate the producton avalablty, the entre RGI plant was modelled usng a complex Relablty Block Dagram. More than 18,000 falure modes for the elementary components along wth ther potental effects were analyzed n order to evaluate whether they should have been ncluded n the relablty model.

4 16 th IMEKO TC4 Symposum Explorng New Fronters of Instrumentaton and Methods for Electrcal and Electronc Measurements The goal of the study was to predct the producton avalablty durng two operatonal modes: Startup and Producton, wthout takng nto account the potental degradaton of producton n the event of non-crtcal or ncpent falures. Other specalzed analyses, such as Quanttatve Rsk Assessment, were performed for evaluatng the probablty of havng crtcal safety problems. Ths mples that only those tems, whch could mpact ether the ablty to start the plant or producton, were ncluded n the RBD. Detals of the decson rules for selectng the falure modes, the assocated falure rates, and ther correctve mantenance tmes are presented, whle the planned mantenance ntervals and duratons are dentfed n the Mantenance Plan study. A. Identfcaton of the Crtcal Falure Modes Workng wth all the avalable engneerng documents and customer gudelnes, the potental falure modes of each component were analyzed to determne whch of them actually could have an mpact on the global avalablty of the system. Each block of the RBD represented a sngle system component whose falure rate was computed as follows: A global falure rate that s the sum of the falure rates of the assocated crtcal falure modes s consdered: λ = n j = 1 λ = falure rate assocated wth the component and j=1,..,n = component falure modes that nfluence the system avalablty The correctve mantenance tme used for each block of the RBD (representng one plant tem) s calculated usng the results of the Mantenance Plan by weghtng the correctve mantenance tme of each crtcal falure mode based on ts probablty of occurrence as follows: λ T cm, T cm, = (3) n λ λ Tcm, = global correctve mantenance tme assocated wth the -th component The preventve mantenance actvtes were modelled as blocks perodcally falng at every planned mantenance cycle, whose restoraton tme s equal to the mantenance duraton. B. General assumptons In the RBD for the avalablty assessment, the preventve and correctve mantenance ntervals as well as the assocated duratons are carred out n accordance wth the approprate O&M manuals, procedures and applcable documents. In partcular, due to the toxcty of the process gas, safety rules do not permt personnel to work on a barge whle t s runnng. Therefore, correctve mantenance actvtes can be performed only when the barge s shut down, depressurzed and purged. These operatng procedures produce mportant consequences n the RBD and ultmately, n the avalablty estmate results. In fact, snce even n the case of redundant confguratons (lke k out-of n and standby) t s not possble to repar faled unts n the presence of hazardous fluds, both the falure rate and the correctve mantenance tmes of all redundant confguratons have been calculated separately usng ndvdual RBDs, because mantenance wll be performed only when all the redundant tems have faled (except for some safety crtcal falure modes). Moreover, the assumpton that all the mantenance operatons restore the component to an as-goodas-new condton was made. The human factor was not assessed,.e., human relablty was consdered equal to 100%. External factors such as natural events, droppng of objects and mpact wth external bodes were not consdered n the avalablty analyss, whch had ndeed the am of estmatng the nherent avalablty of the GE Ol & Gas scope of supply. Some other factors, whch could cause unavalablty, are neglected, ncludng ncorrect desgn relatng to szng and materals, errors n nstallaton and/or manufacturng, and ncorrect documentaton such as procedures, specfcatons, drawngs, and reports. Nor have the spare parts strategy and logstc aspects been consdered. j= 1 (2)

5 16 th IMEKO TC4 Symposum Explorng New Fronters of Instrumentaton and Methods for Electrcal and Electronc Measurements Snce the analyss was carred out durng the desgn phase, we assumed that the plant s operated n complance wth the specfcatons and desgn condtons. C. Specfc assumptons There are some mportant consderatons regardng leaks. All components and tems that are assocated wth the process gas, fuel gas and leak gas ppng are consdered as potental sources of leaks. Ths apprasal s not prompted by the possblty of producton loss but rather, prmarly by the danger represented by fluds that could be lethal to operators. Therefore f there s any leakage, the producton system s turned off to stop the release of fluds. Hence, leakage from actuated valves and other equpment has been consdered and modelled wthn the boundary lmts of the component that the leakage derves from. Ths approach has been appled both to leakage from the component body and leakage from flanges of ppng carryng hazardous fluds. For flange couplngs n partcular, the parts count methodology was appled: the total leak rate s estmated by countng the number of flanges n the hazardous fluds ppng and then multplyng by the flange leakage frequency. Both the leakage frequences for the flange couplngs and for the nstruments were evaluated n the QRA study (Quanttatve Rsk Assessment) [4] and the same data were used n the RBD. We pad partcular attenton to the leakage falure mode of tems wthn the parallel branches. The contrbuton of leakage to the overall plant unavalablty s not neglgble consderng that all the crtcal leaks are n a seres confguraton n the global RBD, even f the leakage s from tems redundant from a functonal standpont. For those parallel confguratons n whch the number of tems runnng at a gven tme depends on external envronmental condtons, for nstance the process gas coolng system, a specfc representaton was developed to determne an equvalent relablty redundancy, consderng the average annual ambent condtons. D. RBD Archtecture The breakdown structure of the whole RBD model s set up n four dfferent levels and s brefly descrbed as follows: 1st level: overall RBD for the whole system 2nd level: clearly dentfed RBDs for each rotatng machne and BoP secton (gas turbne, centrfugal compressor, process gas, fuel gas, seal gas, leak gas systems etc.) to be assembled nto an overall RBD of the whole scope of supply 3rd level: clearly dentfed RBDs at subsystem level. Each subsystem s a machne or BoP auxlary system and appears on a sngle sheet of the relevant P&ID whenever possble. These herarchcal level RBDs are set up by the elementary blocks (TAG level) and are put together n modellng the 2nd level RBDs 4th level: each elementary Block n the RBDs represents one sngle component and contans all the nformaton relatng to the selected crtcal FMs. V. Results In order to allow the hgh-level evaluaton of the whole barge from the avalablty standpont (whch was estmated for start-up and normal operaton), the entre re-njecton barge was dvded nto four man subsystems as detaled n Table 1. System Table 1. Hgh-level system defnton Descrpton RGI Lne-up Seal Gas Electrcal and Control Balance of Plant - Gas Turbne - Centrfugal Compressors (machnes themselves and auxlary systems) - Process seal gas - Recprocatng compressors (machnes themselves and auxlary systems) - Electrcal sngle lne dagram - Control and protecton barge archtecture All other plant systems

6 16 th IMEKO TC4 Symposum Explorng New Fronters of Instrumentaton and Methods for Electrcal and Electronc Measurements Moreover, snce the offshore Kashagan project wll be expanded, thereby ncreasng the re-njecton requrements, two dfferent avalablty assessments were provded n accordance wth the followng scenaro: CASE 1 one barge s runnng and provdng 100% of the producton requred, whle the other barge s ready to start (n stand-by state) CASE 2 (two out of two barge confguraton) each barge provdes 50% of the requred producton. In other words, both of the barges are requred for nomnal producton. The falure of a sngle barge s consdered to be an event that causes an mmedate and complete loss of ts capablty to provde the requred output. So the sngle barge s consdered avalable when t s n a state n whch t s capable of provdng ts servce, whether or not t s actually n servce, regardless of the capacty level that can be provded, n accordance wth ISO3977-9, Gas Turbnes, Procurement, Part 9: Relablty, avalablty, mantanablty and safety [5]. The sngle barge s consdered unavalable when t s n Emergency Shut Down, Pressurzed Shut Down or Compressor Unloaded condton. The Avalablty for CASE 1 and CASE 2 refers drectly to the producton that the two barges can provde. Ths means that n CASE 2 the global avalablty wll also nclude events causng the shut down of one barge wth the other runnng normally so that the group provdes about the 50% of the requred producton. From ths standpont the Global Avalablty (GA) s defned as the percentage of producton n the reference perod, n accordance wth the defntons of falure and sngle barge avalablty provded above. The formula used for the Global Avalablty calculaton s: : PH PH OH GA = PH Perod Hours cumulatve montorng calendar hours. The PH s usually one or more whole system mantenance cycles, n ths case the perod n whch both the gas turbnes on the two barges have had all the preventve mantenance actvtes carred out per the O&M manual. Outage Hours number of producton loss equvalent hours. The OH s calculated as follow. OH CASE 1: OH = LPH, : LPH Loss Producton Hours the number of hours n whch the two barges are smultaneously down for correctve or preventve mantenance. CASE 2: OH = 0.5 * LPH1 + LPH2, : LPH1 Loss Producton Hours 1 the number of hours n whch only one barge s runnng and the other s down for correctve or preventve mantenance LPH2 Loss Producton Hours 2 the number of hours n whch the two barges are smultaneously shut down for correctve or preventve mantenance In order to allow the full analyss of the RAM performance of the plant, the Relablty of the systems descrbed n Table 1 was also calculated for each barge, the Relablty s defned as follows: ( PH FOH) R = (5) PH PH Perod Hours, that s, the smulaton tme FOH Forced Outage Hours of the sngle system. Upper and lower lmts consderng a confdence level of 90% have been evaluated for each Relablty fgure. Actually, these confdence lmts refer only to the results of the smulatons usng the Monte Carlo methodology and are not related to the varablty of the RBD nput data. Ten thousand Monte Carlo smulatons were run coverng a complete mantenance cycle of the GE Ol & Gas scope of supply. As for the Avalablty results for the whole plant, for Case 1 t must be hghlghted that planned mantenance does not have any mpact on the global avalablty of the overall plant, snce optmzaton of the ntervals and the actvtes permts avodance of perods wth no producton at all except for the falure of a barge durng the planned shutdown of the other, thanks to the fact that the Monte Carlo approach allowed the choce of the planned mantenance ntervals to be verfed through smulaton. Consderng the confdental nature of the results of ths study, the detals of the values cannot be presented n ths artcle. (4)

7 16 th IMEKO TC4 Symposum Explorng New Fronters of Instrumentaton and Methods for Electrcal and Electronc Measurements VI. Conclusons and Development Ths study focused manly on the estmaton of the avalablty of the specfc plant descrbed n ths paper under the stated assumptons. It was based completely on the RBD technque combned wth Monte Carlo smulaton, whch s consdered at the moment to be the best process avalable for handlng the dfferent elements smultaneously actng n a non-lnear way to determne plant avalablty. When all the nformaton related to the real feedstock s fnalzed, the result can be further detaled by modellng the actual feedstock so as to also nclude the mpact of logstc delays on the avalablty. The present analyss was carred out on a new nstallaton pror to enterng nto producton; when actual feld data become avalable, the model can be enhanced based on the actual ste condtons. Updatng the model would provde the customer/operator wth a powerful and more precse nstrument for carryng out all the relevant senstvty analyses, such as evaluatng the mpact of any future changes that the user mght consder for the plant, ncludng changng the confguraton (for possble plant expansons or changes n operaton and mantenance procedures), equpment type, mantanablty or feedstock. References [1] OREDA 97, Offshore Relablty Data, 3rd Edton, DNV Det Norske Vertas. [2] OREDA 2002, Offshore Relablty Data, 4th Edton, DNV Det Norske Vertas. [3] NPRD95, Non Electronc Parts Relablty Data, RAC Relablty Analyss Center. [4] QRA 2006 Quanttatve Rsk assessment for Kashagan RGI unts, GE&NP Internal documentaton. [5] ISO3977-9: 1997, Gas turbnes, Procurement: Relablty, avalablty, mantanablty and safety. [6] Zo E., Barald P., Patell E., Assessment of the avalablty of an offshore nstallaton by Monte Carlo smulaton, Internatonal Journal of Pressure Vessels and Ppng, Vol. 83, Issue 4, Pages , [7] Borgonovo E., Marseguerra M., Zo E., A Monte Carlo methodologcal approach to plant avalablty modellng wth mantenance, agng and obsolescence, Relablty Engneerng & System Safety, Vol. 67, Issue 1, Pages 61-73, [8] Dub A., Modellng of realstc systems wth the Monte Carlo Method - A unfed system engneerng approach, The 48th Annua1 Relablty & Mantanablty Symposum - Tutoral Notes, RAMS, [9] Ceschn G.F., Carlevaro F., Racopp G., Mas A., Turbogroup Spare Part Optmzaton by Avalablty Centered Mantenance methodology: an applcaton to LTSA contract, ASME Paper #GT Turboexpo 2003, Atlanta USA, [10] Hggns Lndley R., Mantenance Engneerng Handbook ffth edton. Mc Graw-Hll, New York, [11] A. Dub, Monte Carlo Applcatons n Systems Engneerng. John Wley & Sons, New York, [12] Ml-STD Handbook for Relablty Centered Mantenance Requrements for Naval Arcraft, Weapon Systems, and Support Equpment. [13] SAE JA 1011 Evaluaton crtera for Relablty Centered mantenance (RCM) Processes. [14] SAE JA 1012 A gude to Relablty Centered Mantenance (RCM) standard. [15] CEI IEC Applcaton gude Relablty Centered Mantenance. [16] ISO 14224:2006 Petroleum, petrochemcal and natural gas ndustres: collecton and exchange of relablty and mantenance data for equpment.