Guidelines on Selection and Design of Cooling Options for Offshore Applications Gas compression and cooling is most common in offshore facilities. Gas compression is required to meet export gas pressure, or to deliver high pressure lift gas or injection gas to reservoir, to recover more oil. Because of compression, gas temperatures go up necessitating cooling. The cooling duty can be in the range of 5 MW to 40+ MW, depending on operating conditions. In offshore applications, space and weight are critical; which makes selection and design of cooling system very important. This article discusses various cooling options for gas compression, which include air cooling, direct sea water cooling and the indirect sea water cooling. Various types of heat exchangers, e.g. shell and tube heat exchangers, plate type heat exchangers and printed circuit heat exchangers are considered for water cooling options. This article 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 each option is also considered, to understand overall impact.
T E C H N O L O G Y Offshore Equipment
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as from gas liquid separator is sent to the compression train for compression. A typical two stage compression train is shown in Figure 1. Before compression, entrained liquid is removed in fine separator. Hot compressed gas is cooled at every intermediate and final stage. Cooled compressed gas is either exported to an onshore plant or the gas can be used as lift gas or injection gas Figure 1 to the reservoir, to recover more oil. Hot compressed gas can be cooled via air cooling, direct sea water cooling or indirect sea water cooling. For each option application, design details, merits and demerits are discussed in detail in this article.
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Air Cooling Option In this option, hot compressed gas is cooled using fin fan air coolers. This option is preferred for new platforms or on onshore plants, where space is not a constraint. Fin fan air coolers can be kept on top deck of offshore platform.
Selection Criteria Air cooling is a popular and simple option for many gas platforms, which are also known for trouble free operation. This does not involve big sea water lift pumps, filtration units, break tank, sea
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water fouling / plugging and exchanger cleaning etc. Temperature control and maintenance are simple and down time is minimal. At the same time it requires highest foot print of offshore platform and higher height of the fin fan air cooler; which in turn adds substantial structure weight and cost to the project. There are other small issues like hot air circulation and fan noise. Still, air cooler is an attractive option when cooling / injection water is not easily available for gas cooling.
Design Cases During production life cycle as time passes, reservoir pressure depletes and gas production also drops. During later years, air cooler inlet temperatures are high because of high compression ratios but cooling heat duty drops because of reduction in mass flow rate. It is not always necessary that highest heat duty is considered in design case. Sometimes low inlet temperature to cooler is also considered for design case because of low LMTD available. In special designs, same air coolers are designed for parallel and series operation, for single and two stage compression respectively during production life cycle. In critical service like twister, air coolers are also designed for one fail case. Generally overall finned heat transfer coefficient for compressor air cooler is between 25 to 30 W/m2K.
The height of air coolers and nearby equipment to be located must be considered carefully to ensure sufficient air supply to the tube bundles. Either grating or full opening below air coolers are preferred to supply more air to fin fan air coolers. Air inlet design temperature needs to be selected carefully to avoid big overdesign. Generally average temperature of the hottest month can be considered as air inlet design temperature. For hot air circulation 1 or 2 degree higher can be considered for forced draft fan, and can later be verified by CFD. In cold regions, special consideration needs to be given to avoid hydrate formation or any condensation.
Direct Sea Water Cooling Option In this option hot compressed gas is cooled using direct sea water in shell and tube heat exchangers. This option is preferred when space is a major constraint.
Figure 2
Selection Criteria As compared to air coolers, direct sea water cooling requires a significantly smaller offshore footprint.
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Either sea water is returned back to sea or it can be used as injection water to recover more oil from the reservoir. The hotter the injection water the better it is for the injection purpose. Thus after gas cooling sea water can be used as injection water. Otherwise dedicated sea water lift pumps, filtration units, chemical / corrosion inhibitor injection units, break tank etc are required for the service. PCHE (Printed circuit heat exchanger) is not suitable for direct sea water cooling, as small pores can be blocked by dirty sea water and scaling. Plate exchangers are limited by design pressures and it can also be blocked by solid particles. Thus only shell and tube exchangers are suitable for direct sea water gas cooling.
Sea water return temperature to be selection Criteria
Heat transfer is controlled by shell (gas) side, thus the need to maximise pressure drop on gas side, but it is limited by vibration. To reduce fouling and tube skin temperature, sea water velocity needs to be kept as high as possible (minimum 1 m/s and maximum 3+ m/s). Thus high pressure drop (1+ bar ) is preferred on tube (sea water) side. Lower tube skin temperature lowers fouling tendency. Generally overall dirty heat transfer coefficient can be between 300 to 500+ W/m2K for direct sea water cooler using shell and tube heat exchanger.
a) Generally it can be 40 to 45ºC or as per local regulation to save marine life (if sea water is returned back to sea). Lower sea water return temperature means bigger the sea water pumps and filtration units. b) To avoid excessive fouling, maximum tube skin temperature should not be higher than 5O/60°C in clean condition. c) Sea water return temperature should be lower than process outlet temperature, to allow multiple tube pass, this helps in increasing tube side velocity. With increase in sea water temperature or decrease in water velocity, both will increase fouling tendency in sea water. The advantage of the direct water system includes less deck area and less number of equipment, due to elimination of the potable water system. However, direct sea water cooling option is not preferred choice because of the following issues: 1. Shell and tube exchanger with titanium tubes at sea water side and high pressure duplex /
For exchanger cleaning, additional equivalent space as exchanger size is NOT required, as mechanical cleaning is not required for shell type. Vibration free design is a challenge for the exchanger designer. Double segmental baffle, no tubes in window, vapour belt feed device (annular distributor),
Inconel shell at process side are highly expensive. There can be also limitation on supply and fabrication of this type of exchangers. 2. Gas compression generally occurs at gas temperatures between 90 to 160ºC or higher. It means, in some part of cooler; always tube skin
Process Parameters
Offshore Equipment
J shell or rod baffles should be evaluated carefully to ensure vibration free design.
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temperature at sea water side will exceed 5O/60°C temperature. High tube skin temperature will accelerate fouling and corrosion severely at sea water side; even sea water return temperature is maintained at 4O°C. If sea water is used for direct process gas cooling and other purpose, then non hazardous consumers should be separated using break tank from hazardous consumers (process gas cooler). In case of tube rupture in gas cooler, high pressure process gas should not travel to non hazardous consumers. Additional break tank, pumps and water inside break tank will take additional space and weight on offshore platform. Due to fouling / blocking, regular cleaning of sea water cooler is always a concern to plant operator and maintenance engineer. This means less availability and reliability. If sea water is not used for injection purpose, then sea water is returned back to see. Thus dedicated sea water lift pumps, filtration units, chemical / corrosion inhibitor injection units are required for this service. To reduce high skin temperature / fouling as well to limit sea water return temperature; sea water flow increased substantially, which requires bigger pumps, power, filtration units etc. During later life, cooling heat load will reduce, still same sea water flow rate need to be kept to maintain good velocity. This will do over cooling on gas side and power wastage for sea water pumps.
or 3 x 50% exchangers. This however increases cost, weight and the deck area requirement. 3. Maintain very high tube velocity (3+m/s) in titanium tubes and low sea water return temperature (35-38ºC), both will reduce fouling considerably. But these will result into high pressure drop and bigger sea water flow rates. Thus bigger pumps and filtration units and power requirement. To achieve higher velocity one can go for F shell with multiple tube pass, if possible.
Sea Water Fouling and Scaling There are two basic types of fouling that can occur in seawater systems: marine growth and scale formation. Marine fouling can be controlled. Most operators have a continuous running system backed up with shock dosing of biocide (say once every couple of weeks). Two types of continuous running systems are used: hypochlorite generation or copper ion systems. Scale formation is caused due to the seawater getting too hot. To prevent this temperature rise on the seawater needs to be limited and the exchanger design needs to be reviewed for hot spots (tube skin temperature at sea water side). Typically, the wall temperature in contact with the sea water should be maintained below 50/60ºC.
Indirect Sea Water Cooling Option In this option, Hot Compressed gas is cooled using closed loop potable water, and potable water is cooled by sea water in plate exchanger. This option is preferred, when space is a constraint.
Following are different ways to improve availability and reliability of direct cooling option. 1. This includes on line chemical cleaning, use of titanium tubes with oxide coating, provision of extra tubes which are normally plugged and can be used in case some of the tubes have developed significant scale etc. 2. Another possibility that can be explored is to provide more redundancy in terms of say 2 x 100% Figure 3
Offshore Equipment
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Selection Criteria The indirect sea water cooling system eliminates high tube skin temperature, as sea water cools potable water with maximum (outlet) temperature of 60ºC. Thus the indirect system is less likely to have scaling and corrosion problems in sea water coolers. Thus availability and reliability of indirect cooling is much higher than direct cooling system. Indirect cooling option requires potable water package, potable water tank and pumps. If sea water is not required for injection purpose, then dedicated sea water lift pumps, filtration units, chemical / corrosion inhibitor injection units, break tank etc are also required for this service. Compared to direct cooling, sea water return temperature can be higher for indirect cooling, thus requirement of sea water is less for indirect cooling. For sea water cooler, plate type is best choice. For the compressor gas cooler, shell and tube exchanger and Printed Circuit Heat Exchangers (PCHE) are two choices. Plate Type Sea Water Cooler For 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 be selected. • Sea water side fouling is reduced because of high velocity in narrow plates. • Plate type exchanger occupies less space, weight and much cheaper than the conventional shell and 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 plate type exchanger. Design Parameters for Plate Type Sea Water Cooler
Higher return temperature of the potable water reduces the cost of the sea water plate cooler as well as size of the cooling water pump and the piping. Overall Heat transfer coefficient of plate type exchanger is 4-5KW /m2K. Indirect cooling - Printed Circuit Heat Exchangers (PCHE) PCHE has the lowest weight and lowest footprint as compared to all the options, but it requires additional inlet fine filters (1 operating and 1 stand by), valves, instruments (differential pressure gauge / alarm across filters) and piping for both inlet streams of PCHE, which will take additional space and weight. Typically the PCHEs are 4 to 6 times smaller than the shell and tube exchangers for same duty. Regular cleaning of inlet filter will add maintenance cost. One has to take special design and operation considerations to prevent PCHE failure because of any flow fluctuations or thermal fatigue. Also PCHEs are very prone to blockage as they have very small pore sizes within the unit (in the order of 2 mm). PCHE blockage due to solid particle in sea water or hydrate formation on process side as gas is not dry. The PCHEs are complete welded construction and contain no gasket joints, other than for external connection to piping, hence maintenance activities are kept to a minimum. Other things to be concerned about PCHEs is boiling of the cooling medium (e.g. from loss of cooling medium flow or hot spots in the unit). Considering all these issues of PCHE offers less availability and reliability as compared to shell and tube exchanger. Vendors can guide design consultant as well the Operator Company at the design stage and also develop operating procedure for proper control to avoid these failures. Many offshore platforms are equipped with PCHE for the benefit of lowest weight and footprint. By adding 100% standby PCHE, one can increase availability, but again more piping and instruments and thus more space and weight.
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Offshore Equipment
Overall Heat transfer coefficient of PCHE type exchanger is 2-3+KW /m2K.
Final Recommendation • Select air cooler, when sea water is NOT
Generally overall heat transfer coefficient for shell and tube gas cooler is 300 – 500+ W/m2K.
available (viable), and space is NOT a constraint. Air cooler is simple & easy to use,
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and it has, highest availability and reliability, among all options. • Select indirect water cooling, when sea water is available and space is a constraint. • Direct sea water cooling, is NOT a preferred choice, because of exotic, expensive construction; 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, for onshore gas cooling application. (If sea water is only choice, then above three points are valid, PET for onshore cooling application.) ENQUIRY NUMBER:
This publication thanks Mr. Manish Shah for providing this paper Mr. Manish Shah, a Process Manger at Linde Engineering India, has a Degree in Petrochemical Engineering from MIT India. Mr. Shah is one of the Finalists for the Chemical Engineering 2012 Personal Achievement Award, has received a nomination for the John Grill Award from Worley Parsons and received appreciation letter from Linde. Mr. Shah is a Fellow Charter Engineer (UK CEng FIChemE 99929097) with 17 years of process design experience in leading various feasibility study, concept, feed, basic and detail engineering of Oil & Gas, Refinery and Petrochemical units. In the field of Heat Transfer and Process safety, Mr. Shah has presented seven technical papers in ten international conferences and two technical papers in industrial magazines. His main specialization is in exchanger
(shell and tube and air cooler using HTRI and HTFS) and Tray/packed column designing (using Hysys, Aspen Plus, Sulzer and Koch program). He is proficient in Steady state simulation using HYSYS, UniSim, Aspen Plus & PRO II. He has done extensive work in flare and blowdown system designing for high pressure oil and gas facility. He has done Process design of offshore & onshore oil and gas facilities with gas compression, gas liquid water separation, oil stabilisation, produced water system, acid gas removal and gas Dehydration. Process design of downstream industry includes SRU (Sulphur recovery unit), TGTU, ARU, SWS, LPG recovery. Process design of refinery includes CDU (Crude Distillation Unit), CFU (Condensate Fractionation Unit), SGCU (Saturated Gas Concentration Unit), LSG (Low Sulphur Gasoline), DHDS (Diesel Hydro De-Sulphurisation) and Mild Hydro cracking unit.
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