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58 JANUARY/FEBRUARY 2013 Visit our website at www.safan.comOffshore

Equipment

TECHNOLOGY

Guidelines on Selection andDesign of Cooling Options forOffshore Applications

G as from gas liquid separator is sent to thecompression train for compression. Atypical two stage compression train isshown in Figure 1. Before compression,

entrained liquid is removed in fine separator. Hotcompressed gas is cooled at every intermediateand final stage.Cooled com-pressed gas iseither exportedto an onshoreplant or thegas can be usedas lift gas orinjection gasto the reservoir,to recover more oil. Hot compressed gas can becooled via air cooling, direct sea water cooling orindirect sea water cooling. For each option appli-cation, design details, merits and demerits arediscussed in detail in this article.

Air Cooling OptionIn this option, hot compressed gas is cooled using

fin fan air coolers. This option is preferred for newplatforms or on onshore plants, where space is not aconstraint. Fin fan air coolers can be kept on top deckof offshore platform.

Selection CriteriaAir cooling is a popular and simple option for

many gas platforms, which are also known for trou-ble free operation. This does not involve big seawater lift pumps, filtration units, break tank, sea

OffshoreEquipment

Gas compression and cooling is most common in offshore facilities. Gas compression isrequired to meet export gas pressure, or to deliver high pressure lift gas or injection gas toreservoir, to recover more oil. Because of compression, gas temperatures go up necessitatingcooling. The cooling duty can be in the range of 5 MW to 40+ MW, depending on operatingconditions. In offshore applications, space and weight are critical; which makes selection anddesign of cooling system very important. This article discusses various cooling options for gascompression, which include air cooling, direct sea water cooling and the indirect sea watercooling. Various types of heat exchangers, e.g. shell and tube heat exchangers, plate type heatexchangers and printed circuit heat exchangers are considered for water cooling options. Thisarticle provides broad guidelines on selection and design of various offshore cooling options.Comparisons of these options are made with respect to space, fouling, material selection,availability, reliability and maintenance considerations. The equipment required in eachoption is also considered, to understand overall impact.

Figure 1

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water fouling / plugging and exchanger cleaningetc. Temperature control and maintenance are sim-ple and down time is minimal. At the same time itrequires highest foot print of offshore platform andhigher height of the fin fan air cooler; which in turnadds substantial structure weight and cost to theproject. There are other small issues like hot aircirculation and fan noise. Still, air cooler is an attrac-tive option when cooling / injection water is noteasily available for gas cooling.

Design CasesDuring production life cycle as time passes, res-

ervoir pressure depletes and gas production alsodrops. During later years, air cooler inlet tempera-tures are high because of high compression ratiosbut cooling heat duty drops because of reductionin mass flow rate. It is not always necessary thathighest heat duty is considered in design case.Sometimes low inlet tem-perature to cooler is also con-sidered for design casebecause of low LMTD avail-able. In special designs, sameair coolers are designed forparallel and series operation,for single and two stage com-pression respectively duringproduction life cycle. In criti-cal service like twister, aircoolers are also designed forone fail case. Generally over-all finned heat transfer coeffi-cient for compressor air cooleris between 25 to 30 W/m2K.

The height of air coolers and nearby equipmentto be located must be considered carefully toensure sufficient air supply to the tube bundles.Either grating or full opening below air coolersare preferred to supply more air to fin fan aircoolers. Air inlet design temperature needs to beselected carefully to avoid big overdesign.Generally average temperature of the hottestmonth can be considered as air inlet design tem-perature. For hot air circulation 1 or 2 degree

higher can be consid-ered for forced draftfan, and can later beverified by CFD. Incold regions, special

consideration needs to be given to avoid hydrateformation or any condensation.

Direct Sea Water Cooling OptionIn this option hot compressed gas is cooled

using direct sea water in shell and tube heatexchangers. This option is preferred when spaceis a major constraint.

Selection CriteriaAs compared to

air coolers, directsea water coolingrequires a signifi-cantly smaller off-s h o r e f o o t p r i n t .

Figure 2


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Either sea water is returned back to sea or it can beused as injection water to recover more oil from thereservoir. The hotter the injection water the better itis for the injection purpose. Thus after gas coolingsea water can be used as injection water. Otherwisededicated sea water lift pumps, filtration units,chemical / corrosion inhibitor injection units, breaktank etc are required for the service. PCHE (Printedcircuit heat exchanger) is not suitable for direct seawater cooling, as small pores can be blocked by dirtysea water and scaling. Plate exchangers are limitedby design pressures and it can also be blocked bysolid particles. Thus only shell and tube exchangersare suitable for direct sea water gas cooling.

Process ParametersHeat transfer is controlled by shell (gas) side, thus

the need to maximise pressure drop on gas side, butit is limited by vibration. To reduce fouling and tubeskin temperature, sea water velocity needs to be keptas high as possible (minimum 1 m/s and maximum3+ m/s). Thus high pressure drop (1+ bar ) is pre-ferred on tube (sea water) side. Lower tube skintemperature lowers fouling tendency. Generally over-all dirty heat transfer coefficient can be between 300to 500+ W/m2K for direct sea water cooler using shelland tube heat exchanger.

For exchanger cleaning, additional equivalentspace as exchanger size is NOT required, as me-chanical cleaning is not required for shell type. Vi-bration free design is a challenge for the exchangerdesigner. Double segmental baffle, no tubes in win-dow, vapour belt feed device (annular distributor),

J shell or rod baffles should be evaluated carefullyto ensure vibration free design.

Sea water return temperature to beselection Criteria

a) Generally it can be 40 to 45ºC or as per localregulation to save marine life (if sea water isreturned back to sea). Lower sea water returntemperature means bigger the sea waterpumps and filtration units.

b) To avoid excessive fouling, maximum tubeskin temperature should not be higher than5O/60°C in clean condition.

c) Sea water return temperature should be lowerthan process outlet temperature, to allowmultiple tube pass, this helps in increasingtube side velocity.

With increase in sea water temperature or decreasein water velocity, both will increase fouling tendencyin sea water.

The advantage of the direct water system includesless deck area and less number of equipment, due toelimination of the potable water system. However,direct sea water cooling option is not preferred choicebecause of the following issues:

1. Shell and tube exchanger with titanium tubes atsea water side and high pressure duplex /

Inconel shell at process side are highly expen-sive. There can be also limitation on supply andfabrication of this type of exchangers.

2. Gas compression generally occurs at gastemperatures between 90 to 160ºC or higher. Itmeans, in some part of cooler; always tube skin


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temperature at sea water side will exceed5O/60°C temperature. High tube skin tem-perature will accelerate fouling and corrosionseverely at sea water side; even sea waterreturn temperature is maintained at 4O°C.

3. If sea water is used for direct process gascooling and other purpose, then non hazard-ous consumers should be separated usingbreak tank from hazardous consumers (proc-ess gas cooler). In case of tube rupture in gascooler, high pressure process gas should nottravel to non hazardous consumers. Addi-tional break tank, pumps and water insidebreak tank will take additional space andweight on offshore platform.

4. Due to fouling / blocking, regular cleaning ofsea water cooler is always a concern to plantoperator and maintenance engineer. Thismeans less availability and reliability.

5. If sea water is not used for injection purpose,then sea water is returned back to see. Thusdedicated sea water lift pumps, filtration units,chemical / corrosion inhibitor injection unitsare required for this service.

6. To reduce high skin temperature / fouling aswell to limit sea water return temperature; seawater flow increased substantially, which re-quires bigger pumps, power, filtration units etc.

7. During later life, cooling heat load will re-duce, still same sea water flow rate need tobe kept to maintain good velocity. This willdo over cooling on gas side and power wast-age for sea water pumps.

Following are different ways to im-prove availability and reliability of di-rect cooling option.

1. This includes on line chemicalcleaning, use of titanium tubeswith oxide coating, provision ofextra tubes which are normallyplugged and can be used in casesome of the tubes have developedsignificant scale etc.

2. Another possibility that can beexplored is to provide more re-dundancy in terms of say 2 x 100%

or 3 x 50% exchangers. This however increasescost, weight and the deck area requirement.

3. Maintain very high tube velocity (3+m/s) intitanium tubes and low sea water return tem-perature (35-38ºC), both will reduce foulingconsiderably. But these will result into highpressure drop and bigger sea water flow rates.Thus bigger pumps and filtration units andpower requirement. To achieve higher ve-locity one can go for F shell with multipletube pass, if possible.

Sea Water Fouling and ScalingThere are two basic types of fouling that can

occur in seawater systems: marine growth andscale formation. Marine fouling can be controlled.Most operators have a continuous running systembacked up with shock dosing of biocide (say onceevery couple of weeks). Two types of continuousrunning systems are used: hypochlorite genera-tion or copper ion systems.

Scale formation is caused due to the seawater get-ting too hot. To prevent this temperature rise on theseawater needs to be limited and the exchangerdesign needs to be reviewed for hot spots (tube skintemperature at sea water side). Typically, the walltemperature in contact with the sea water shouldbe maintained below 50/60ºC.

Indirect Sea Water Cooling OptionIn this option, Hot Compressed gas is cooled using

closed loop potable water, and potable water is cooledby sea water in plate exchanger. This option is pre-ferred, when space is a constraint.

Figure 3


Selection CriteriaThe indirect sea water cooling system elimi-

nates high tube skin temperature, as sea watercools potable water with maximum (outlet) tem-perature of 60ºC. Thus the indirect system is lesslikely to have scaling and corrosion problems insea water coolers. Thus availability and reliabil-ity of indirect cooling is much higher than directcooling system. Indirect cooling option requirespotable water package, potable water tank andpumps. If sea water is not required for injectionpurpose, then dedicated sea water lift pumps,filtration units, chemical / corrosion inhibitorinjection units, break tank etc are also requiredfor this service. Compared to direct cooling, seawater return temperature can be higher for indi-rect cooling, thus requirement of sea water is lessfor indirect cooling. For sea water cooler, platetype is best choice. For the compressor gas cooler,shell and tube exchanger and Printed Circuit HeatExchangers (PCHE) are two choices.

Plate Type Sea Water CoolerFor sea water cooler plate type is preferred choice

because of following reasons:• Both closed loop potable water and sea water

are low pressure system, thus plate can beselected.

• Sea water side fouling is reduced because ofhigh velocity in narrow plates.

• Plate type exchanger occupies less space, weightand much cheaper than the conventional shelland tube exchangers.

• Plate type exchanger is easy to clean.• During turn down remove few plates.• Low consumption of sea water because of

lowest possible temperature approach in platetype exchanger.

Design Parameters for Plate Type Sea Water Cooler

Higher return temperature of the potable waterreduces the cost of the sea water plate cooler as wellas size of the cooling water pump and the piping.Overall Heat transfer coefficient of plate type ex-changer is 4-5KW /m2K.

Indirect cooling - Printed Circuit Heat Exchang-ers (PCHE)

PCHE has the lowest weight and lowest foot-print as compared to all the options, but it re-quires additional inlet fine filters (1 operatingand 1 stand by), valves, instruments (differentialpressure gauge / alarm across filters) and pipingfor both inlet streams of PCHE, which will takeadditional space and weight. Typically the PCHEsare 4 to 6 times smaller than the shell and tubeexchangers for same duty. Regular cleaning ofinlet filter will add maintenance cost. One has totake special design and operation considerations toprevent PCHE failure because of any flow fluctua-tions or thermal fatigue. Also PCHEs are veryprone to blockage as they have very small poresizes within the unit (in the order of 2 mm).PCHE blockage due to solid particle in sea wateror hydrate formation on process side as gas is notdry. The PCHEs are complete welded construc-tion and contain no gasket joints, other than forexternal connection to piping, hence maintenanceactivities are kept to a minimum. Other things tobe concerned about PCHEs is boiling of the cool-ing medium (e.g. from loss of cooling mediumflow or hot spots in the unit). Considering allthese issues of PCHE offers less availability andreliability as compared to shell and tube exchanger.Vendors can guide design consultant as well theOperator Company at the design stage and alsodevelop operating procedure for proper control toavoid these failures. Many offshore platforms areequipped with PCHE for the benefit of lowestweight and footprint. By adding 100% standby

PCHE, one canincrease avail-ability, but againmore piping andinstruments andthus more spaceand weight.


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Overall Heat transfer coefficient of PCHE typeexchanger is 2-3+KW /m2K.

Generally overall heat transfer coefficient for shelland tube gas cooler is 300 – 500+ W/m2K.

Final Recommendation• Select air cooler, when sea water is NOT

available (viable), and space is NOT a con-straint. Air cooler is simple & easy to use,


and it has, highest availability and reliability,among all options.

• Select indirect water cooling, when sea wateris available and space is a constraint.

• Direct sea water cooling, is NOT a preferredchoice, because of exotic, expensive con-struction; and less availability, due to sever

scaling and blockage issues.• When clean river water is available, then direct

cooling, is best option from all options, foronshore gas cooling application. (If sea water isonly choice, then above three points are valid,for onshore cooling application.)

This publication thanks Mr.Manish Shah for providing thispaper Mr. Manish Shah, a Proc-ess Manger at Linde Engineer-ing India, has a Degree inPetrochemical Engineering from

MIT India. Mr. Shah is one of the Finalistsfor the Chemical Engineering 2012 PersonalAchievement Award, has received a nomina-tion for the John Grill Award from WorleyParsons and received appreciation letter fromLinde. Mr. Shah is a Fellow Charter Engi-neer (UK CEng FIChemE 99929097) with17 years of process design experience in lead-ing various feasibility study, concept, feed,basic and detail engineering of Oil & Gas,Refinery and Petrochemical units. In thefield of Heat Transfer and Process safety,Mr. Shah has presented seven technical pa-pers in ten international conferences andtwo technical papers in industrial maga-zines. His main specialization is in exchanger

(shell and tube and air cooler using HTRIand HTFS) and Tray/packed column design-ing (using Hysys, Aspen Plus, Sulzer andKoch program). He is proficient in Steadystate simulation using HYSYS, UniSim,Aspen Plus & PRO II. He has done extensivework in flare and blowdown system design-ing for high pressure oil and gas facility. Hehas done Process design of offshore & on-shore oil and gas facilities with gas compres-sion, gas liquid water separation, oilstabilisation, produced water system, acidgas removal and gas Dehydration. Processdesign of downstream industry includes SRU(Sulphur recovery unit), TGTU, ARU, SWS,LPG recovery. Process design of refineryincludes CDU (Crude Distillation Unit),CFU (Condensate Fractionation Unit),SGCU (Saturated Gas Concentration Unit),LSG (Low Sulphur Gasoline), DHDS (Die-sel Hydro De-Sulphurisation) and MildHydro cracking unit.

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