Turbomachinery for the world's largest nitrogen plant - MAN Diesel ...

Turbomachinery for the world’slargest nitrogen plantEnhanced Oil Recovery to increase the outputin the Cantarell oil field, Mexico

Turbomachinery for theworld’s largest nitrogen plantEnhanced Oil Recovery toincrease the output in theCantarell oil field, MexicoDr.-Ing. L. TuranskyjMAN Turbomaschinen AGGHH BORSIGOberhausen, GermanyB.A. Keenan BSc C.Eng. FIMechEBOC Process Gas Solutions,Guildford, Surrey, EnglandPaper presented at the“Exposición Latinoamericana delPetróleo” Maracaibo, Venezuela,26-28 June 2001SummaryAs part of a project to increasethe production output of PEMEX’sCantarell offshore oil field, situatedin the Gulf of Mexico, the world’slargest ever nitrogen plant has beenconstructed. It was designed, built,owned and operated by Companiade Nitrogeno de Cantarell S.A. deC.V. (CNC) an international consortiumof BOC Gases of Great Britain,Westcoast Energy of Canada,Marubeni of Japan, Linde Germanyand ICA Fluor Daniel of Mexico.For this project, MAN TurbomaschinenAG GHH BORSIG was awardedthe order to supply, supervise theerection of and commission the aircompressors for the air separationplant and the nitrogen productcompressors including their driversfor the delivery of the high puritynitrogen to the oil field. The totalinstalled driver power for compressionexceeds 400 MW.This paper contains a brief descriptionof the plant as a whole, detailsof the turbomachines supplied andthe results of their workshop trials.IntroductionMethods of introducing nitrogenunder high pressure into oil fieldswith a low well pressure or to increaseproduction can be usedfor enhanced oil recovery either ifthere is insufficient gas available forreinjection, or the associated gasis to be used for another purpose.Petroleos Mexicanos (PEMEX),the Mexican state oil company, isutilizing these methods to increaseits future production capacity in theCantarell oil field.2

PEMEX’s production output to datefrom its main oil field accounts for athird of total domestic oil production.The Cantarell oil field is located offshorein the Bay of Campeche, inthe southern Gulf of Mexico (Fig. 1).Roughly 1.4 million barrels per dayof crude oil have been producedhere on average in previous years.In addition to other measures toincrease the daily output to around2 million barrels per day, the diminishingwell pressure of the crude oilis to be increased to approx. 84 barand maintained at that level byinjecting highly pure nitrogen intothe rock formations containing theoil. To do this, around 1.3 millionNm 3 /h or roughly 40,300 tonnes perday of nitrogen are required, whichare produced from atmosphericair in an air separation plant andfollowing compression in turbocompressorsto 121 bar is transportedto the offshore injection facilities ina 36” subsea pipeline approx.92 km long.source: www.enr.comFig. 1: Location of the Cantarell Oil FieldThe scope of the project won comprises,as shown in Fig.2, a naturalgas fired gas turbine COGEN powerplant, an air separation plant toproduce the nitrogen, seawater andtreated water cooling systemsincluding the subsea water pipes,turbocompressor sets and pipelinesto the platform. It was constructedon the Atasta peninsula on the Gulfof Mexico, not far from Cuidad delCarmen, by CNC, an internationalconsortium made up of the followingcompanies:BOC Gases/Great BritainMarubeni Corporation/JapanWestcoast Energy Inc./CanadaLinde AG/GermanyICA Fluor Daniel S.de R.L. de C.V./Mexico.The last-named company is a jointventure between Constructoras ICA(Mexico) and Fluor Daniel (USA).At the end of 1997, ICA Fluor Daniel,in consultation with BOC, awardedthe contract to supply the turbocompressorsets on either side of the airseparation plant to MAN TurbomaschinenAG GHH BORSIG, whichwas also contracted to superviseerection of the compressors and tocommission them.Fig 2: Plant Schematic3

Plant conceptsource: Fluor DanielPemex awarded the CNC consortium,led by BOC, a gas supply contractfor a period of 15 years. This decisionwas based on the lowest gas pricecoupled with design, constructionand operating experience. Key tothis award was experience of thecomponents including the machinery,minimum fuel consumption measuredin energy units per unit capacityof nitrogen,and a predicted highlevel of availability of the plant.The facility was built on a green fieldsite provided by Pemex. There wasno infrastructure or utilities, with theexception of natural gas which isused to fuel the gas turbines. Owingto this lack of infrastructure at theerection site, the plant in its entirety,including the gas turbine cogenerationplant, was designed as anisland solution.Fig. 3: Nitrogen Production PlantIn designing the plant, it was recognisedthat minimum energy usagecombined with the best economicsolution could only be accomplishedby a high level of integration whichwould balance the optimum airseparation cycle with the optimumenergy supply cycle. As BOC hadexperience successfully designing,building and operating such facilitiese.g. their integrated gas turbinecogeneration and ASU facility atGresik Indonesia, another facilityusing MAN GHH BORSIG’s compressorsand steam turbines, BOCled this activity. Using proprietarycomputer programs, in excess of40 combined energy and ASUcycles were evaluated includingvariation of process cycles, variationsof compressor types and coolingarrangements, simple cycle, combinedcycle, and cogeneration gasturbine arrangements before theoptimum solution was decided on anNPV and experience basis. The solutioneventually implemented as themost economical is based on a fourASU-train version as follows:– Four (three operating/one spare)gas turbine generators with heatrecovery steam generators (HRSGs)connected downstream to the gasturbine exhausts to generate steam– four motor driven intercooled aircompressors,– four air separation plants toproduce the nitrogen and– four steam turbine driven intercoolednitrogen compressors.Splitting the total production betweenfour units that can be operated independentlyalso provides the requiredflexibility of the nitrogen plant in theevent that the oil field requirementsvary.Fig. 3 shows a photograph of thenitrogen production plant. The fourgas turbine units – gas turbines,generators and HRSGs – are on theleft hand side of the picture. Themachine house can be seen oppositethese, in the middle of thepicture. The four turbocompressorsets – four air compressors withmotor drive and the four nitrogencompressor sets with condensingsteam turbines, gear units and lowandhigh-pressure nitrogen compressors– are housed here. At theright-hand side of the picture are thefour air separation facilities, and thesea water recooling system withcooling towers in all is located inthe left-hand section. The area asa whole is roughly 6 km from thecoast. Water is piped from 5 kmoffshore of a sump at the beachwhere it is pumped to the coolingtowers for use in the intercoolersand condensers.4

TurbomachineryMAN Turbomaschinen AG GHHBORSIG supplied the following 16turbomachines for the nitrogen plant:Four intercooled axial flow air compressorsconsisting of two back toback axial stage groups and oneoverhung radial stage driven by ABBtwo pole synchronous motors, andfour intercooled centrifugal nitrogencompressor trains. Each nitrogencompressor train consists of a lowpressure casing driven by a condensingsteam turbine through a stepup gearbox with each low pressurecasing directly connected to a highpressure casing. The following is amore detailed description of theseturbomachines:Axial flow main aircompressorsMAN GHH BORSIG is the marketleader in the construction of axialcompressors for industrial applications.Axial compressors were developedat the start of the 1950s andFig. 5: Axial Flow Compressor Arrangementhave since been introduced verysuccessfully into various market segmentsof the process industry. Whiletheir primary use initially was in blastfurnace applications, the emphasishas shifted over time increasingly tothe chemical and petrochemicalsector, and also to the “air separationplant” segment of the market.Thus since 1977 MAN GHH BORSIGhas supplied a total of 14 identicalaxial flow compressors, each with36 MW (48275 HP) drive power, tothe world’s largest oxygen plant inSasolburg, South Africa. Thesecompressors have now successfullyclocked up around 2 million operatinghours. The compressor design isdescribed in detail in [1].The axial flow compressors used forthe air separation plant on Atastaare virtually identical to those for theSasolburg installation. Each of thefour machines operates at over50 MW (67050 HP) with suctionvolumes in excess of 600,000 m 3 /h(353100 acfm) depending on plantdemand and ambient conditions.The power required is supplied by asynchronous motor directly driving thecompressor at 3,600 rpm and startedby a soft starter. Its rated drive outputis 52 MW (69733 HP).The air is taken in via a radial inletcasing (Fig. 4) and following compressionin seven axial stages and afinal radial end stage in the low-pressuresection, is conveyed to the intercoolerlocated beneath the compressor(Fig. 5). Following further compressionin the intermediate-pressuresection (six axial stages and radialend stage) and further stage of intercooling,final compression is carriedout in the overhung-mounted radialstage with axial intake.Fig. 4: Intercooled Axial Flow Compressor5

Fig. 6: Nitrogen LP CompressorFig. 7: Nitrogen HP CompressorFig. 8: Condensing Steam TurbineThe wide range of compressorperformance data required can bedelivered with just four synchronouslyadjustable rows of stator bladesat the inlet to the low-pressuresection. The reasons for the superbpartial load characteristics lie in themoderate aerodynamic loading ofthe blading of the low- and intermediate-pressuresection togetherwith the stage characteristics of theHP stage.The dimensions of the axial flowcompressor can best be characterizedby the distance betweenbearings of around 6 m, the outerdiameter of the largest centrifugalimpeller of 1.64 m and the transportationweight of roughly 180 tonnes.Centrifugal type nitrogenproduct compressorsEach of the four centrifugal compressortrains designed to API Standardoperates at up to 48 MW dependingon the well demand profile and compressesin excess of 335,000 Nm 3 /h(300 million standard cubic feet perday) at pressures up to 121 bara(1755 psia).The low-pressure compressor, twotimes intercooled, contains four impellerswith a nominal outer diameterof 750 mm (Fig.6). The two externalvolutes with a circular cross-sectionare typical of the cast casing with ahorizontal split joint and are connectedto the vaneless diffusers of therespective stages.The high-pressure compressor isprovided with two stages of intercooling(Fig.7) and contains threestage groups, each comprising twoimpellers with an outer diameter of660 mm. The casing of cast steel isdesigned as a barrel with a singlecover. On this compressor also,volutes with a circular cross-sectionand a connected conical diffuseroptimize efficiency in transferring thecompressed nitrogen to the coolerslocated beneath the machine. Theinitial stages of the three HP stagegroups have vaned diffusers. The HPcompressors are constructed withoutan internal casing; in place ofthis, the internals are connected viaspecial tie-bolts to form the axiallyremovable barrel.Both compressors have single drygas seals at the shaft ends to sealthe gas chambers relative to atmosphere.To achieve optimum efficiency of theselected compressor configuration,the flow-conducting componentswere produced with a particularlyhigh-grade surface finish, especiallyon the HP compressor.Because of the power required bythe compressors, their rotor lengthsand, for the nitrogen compressor, thepressure at which it worked, specialattention was paid to the rotordynamics.The design was carried out inaccordance to API 617 includingboth unbalance response analysisand log decrement analysis to confirmstability.Steam turbinesEach of the steam turbine driversfor the nitrogen compressors obtaintheir steam from waste heat boilersconnected to each of the gas turbines.Each steam turbine can expandmore than 160 t/h of high pressure/high temperature steam to produce upto 55 MW (73755 HP) at 3,200 rpm.6

Fig.8 shows a section through atypical, reaction-type condensingsteam turbine for mechanical drives.With an isentropic efficiency rating ofalmost 85%, this turbomachine alsomeets the high standards expectedwith regard to optimum energy utilizationby the overall plant.The 23-stage turbine blading is dividedinto four sections. The rotor bladesof the two-stage control wheel, theHP and IP part and the first threestages of the LP part have integratedshrouds, into which damping wiresare caulked in a circumferential direction.Only the last four rotor bladesof the LP part are free-standing. Allblades up to the end stage are heldin the rotor by T roots or double-Troots. The end stage has a fingerroot fixing which is attached to therotor by two pins. The guide vanesare connected in segments on therotor side to riveted shrouds, againstwhich sealing strips caulked into theshaft run.Four control valves are used forpartial load control. These are connectedto one another by a balancebeam control system, which is actuatedby hydraulic oil. The controlFig. 9: Nitrogen CompressorTrain Arrangementvalves are coordinated so that avirtually linear connection is createdbetween the steam throughput andvalve stroke.Special features of the steam turbinedesign include the welded jointbetween the cast turbine casingand welded exhaust casing, and thebearing housings set up separatelyto the turbine casing. The first ofthese features contributes substantiallyto the tightness of the horizontalsplit joint, even when the turbineis started up and stopped frequently;the second facilitates simple alignmentof the casing and rotor, whichis retained even during transientoperating conditions.The nitrogen compressor trainarrangement is shown in Fig.9.Plant dynamicsand controlThe definition of plant dynamicshere means the analysis of transientoperating states of turbomachineryinstallations such as occur duringstart-up and shutdown, in controlprocesses and in the event ofsystem malfunction. MAN GHHBORSIG began at an early stageto analyse the behaviour of turbomachinerysystems under aforementionedoperating conditions [2], [3].One aim of these investigations is toensure a mode of operation for compressorinstallations that excludesunstable events such as “surging”even for transient operating states.In dynamic simulation, the thermodynamicattributes of the compressorsin the form of performancedata, the characteristics of the driveunits, the mass moments of inertiaof all rotating components, the storagevolumes of the gas coolers andthe gas-carrying pipelines, plus theattributes of the controllers used,and the corresponding valves andfittings as related to one another bymeans of a system of differentialequations, which are solved digitally.As a result of this simulation, all theimportant time-dependent parameters,such as speeds, pressures,volume flows etc. in the transientbehaviour of the installation areavailable for further evaluation. Byvarying elements of the plant, e.g.the arrangement and attributes ofvalves and fittings, this behaviour canbe influenced in a targeted mannereven at the plant design stage.7

of mechanical and thermodynamictrials are precisely stipulated invarious internationally approved acceptancecodes for the respectivemachine types. Customers andthose they commission to carry outacceptance therefore devote particularattention to these test runs.Fig. 10: Nitrogen Compressor LoopOne example of using this simulationsystem is shown in Fig. 10, showingthe system design of the nitrogencompressors. To avoid “surging” ofindividual stages or stage groups intransient processes, recycle valvesare provided for certain stagegroups, which open automaticallywhen required. Fig.11 shows as theresult of such simulation how thetransient effects of a shutdown process– in this case by means of turbinetripping – can be traced in theperformance map of the 6th stagegroup of the nitrogen compressor set.The turbomachinery and relatedunits are controlled and monitoredby the proven turbolog DSP machinecontrol and protection system, whichwas developed in-house. Linked tothe digital control and monitoringsystem of the overall plant,turbolog DSP protects the air andnitrogen compressors against surging,controls the position of thevariable stator vanes of the axialflow compressors, the speed of thesteam turbines, monitors and controlsall the other components, andlast but not least handles the visualizationof the operating data requiredfor smooth, uninterrupted production.Workshop trialsWorkshop trials of the compressorsand their drivers serve to demonstratethe guaranteed mechanicalintegrity of the finished product aswell as the guaranteed thermodynamicproperties prior to dispatch tothe site as it has long been recognisedthat it is easier and more costeffective to fix a problem in the factoryrather than in the field. For suchlarge units the cost of one weeksdelay can result in over $1 million inlost revenue. These test runs areto be regarded as a comprehensivequality assurance measure at theend of the design and manufacturingprocess. The procedure and contentFig. 11: Result of Dynamic SimulationMAN GHH BORSIG has met thegrowing market demand for test runsto be carried out by continuouslyexpanding the number of test standsavailable for such tests [4]. Driveoutputs of 14 MW (electric motors),16 MW (steam turbines) and 25 MW(gas turbines) are now available inthe plant for works test purposes,along with an open-air area for compressortrials involving combustiblegases. The following programme oftrials was conducted for the turbomachinesdescribed in Chapter 3:Mechanical testing of all– compressors as per API 617,– steam turbines as per API 612 and– gear boxes as per API 613,and thermodynamic test runs eachinvolving one axial flow compressor,one LP centrifugal compressor andone HP centrifugal compressor asper ASME PTC-10.8

Fig. 12: Test Arrangement of Axial Flow CompressorFor this project BOC also requiredthat one of the nitrogen HP compressorsbe tested under full load andfull pressure (121 bara/1775 psia). Acomplete, dedicated FT8 gas turbinepackage on one of the test standswith a drive output in excess of25 MW was made available as thedrive for this test [5].The five following pictures showsome of the turbomachinery to betested on the test stands:The erection of the axial flow compressorbehind the 16 MW teststand steam turbine can be seenin Fig. 12. The two job coolers wereused for the tests. The second intercooleris positioned next to the steelfoundation, whilst the first intercooler,not visible in the picture, is locatedunderneath the axial flow compressor.The two following pictures 13 and 14show the LP centrifugal compressorand the HP centrifugal compressorin the test stand. Fig. 15 shows aview of the split joint on the condensingsteam turbine.The full load test of the HP centrifugalcompressor under design basisconditions was carried out using nitrogenin a closed loop, the job intercoolersagain being used. Fig. 16shows the test set-up of the HPcompressor. The casing, with downwardsfacing intake and dischargeconnections in the original build, wasrotated 180º upwards, to simplifythe pipework for the test set-up.The gas turbine already mentionedabove was used as the drive for thefull load test, with a drive speed of5,500 rpm a 25 MW test stand gearbox was used to match the compressorspeed.Fig. 13: Nitrogen LP CompressorFig. 14: Nitrogen HP Compressor on Test FieldFig. 15: Condensing Steam Turbine on Test Field9

the case of the axial flow compressorsabove the pressure limiting linespecified by a safety relief valve aredisregarded. The overload range iswell covered for both compressors.The rotors demonstrated a high levelof stability especially during the fullload, full pressure test of the highpressure nitrogen compressor.Figs. 19 and 20 are examples ofthe Bode plots from the tests.The plant as a whole meets the highexpectations of the operating companyand its end customer PEMEX.Oil production in the Cantarell fieldhas been increased markedly by thisinvestment of one billion US $ [6].Fig. 16: Arrangement of HP Compressor for Full Load TestResultsOf the large number of results obtainedfrom the workshop trials, onlythose which apply directly to theoperating ranges originally quotedfor the air and nitrogen compressorswill be mentioned here.All guaranteed thermodynamicvalues were met without the need todraw on manufacturing or measuringtolerances. Analysis of the officialacceptance trials and the large numberof internal measurements alsocarried out yielded the overall performancedata shown in Figs. 17 and18 for the compressors. A comparisonof the tender performance dataand so called “as built” performancedata is provided.The predicted surge limit was safelyachieved both on the air compressorsand the nitrogen compressors.Even the shape of curves of themeasured characteristics is a goodmatch for the precalculated characteristiccurves, if the deviations betweenmeasured and calculated inMAN Turbomaschinen AG GHHBORSIG has contributed to thissuccess by supplying a total of16 turbomachines with an installedmaximum drive power in total of428 MW (573957 HP). The efficiencyand availability of the turbomachinessupplied, together with MAN GHHBORSIG’s competence in handlingthe order up to commissioning,qualify the company as a mayorturbomachinery supplier for futurechallenges of a similar nature.AcknowledgementsThe authors would like to congratulatethe many people who contributedto make this project such asuccess.10