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Filename and location14/03/2006 1

Cooling Systems

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Purpose of Cooling Systems

Prevention of over-heating of process equipment

Prevention of product damage

Ensure desired performance of process equipment

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Cooling Systems

Generally, the types of cooling available offshore are- Closed Circuit Cooling Water (CCCW)

Direct seawater

Air cooling

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Typical Cooling Loads

Process cooling - Crude oil ~ 0.6-0.7 kW/(m3/h.oC)

Condensate ~ 0.3-0.4 kW/(m3/h.oC)

Gas (no condensation) ~ 500-800 kW/(m3/h.oC)

Produced water cooling ~ 1.2 kW/(m3/h.oC)

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Utility Cooling - Instrument/plant air after cooling ~ 350 kW/(106Sm3/h.oC)

Turbine lube oil / seal oil cooling ~ 10% of driver rating(kW)

Compressor lube oil / seal oil cooling ~ 10% of driver rating

Pump motor cooling ~ 10% of motor rating

Diving facilities ~ 200kW

Air conditioning - small ~ 150kW

medium ~ 300kW

large ~ 500kW

Typical Cooling Loads

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Schematic of a Typical CCCW System

SEA WATERSUPPLY

OPEN

DRAINS

GLYCOLMAKE-UP

WATERMAKE-UP

LP FLARE

SEAWATER

RETURNTC

TCV

CCCW CIRCULATINGPUMPS

FC

TURBINE

GENERATOR

COOLERS

FCV

CRUDE OILLOADING

COOLERS

COMPRESSIONCOOLERS

PLATE HEATEXCHANGERS

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Closed Circuit Cooling Water

(CCCW) Systems

Utilises a closed circuit of glycol and water mixture which

is cooled by exchange with sea water

Type of glycol used is dependent on glycol that is used on

the platform, i.e. TEG or MEG. Typical glycol concentration is 30 - 40% w/w

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Corrosion inhibitors are injected into the cooling medium

Permits use of carbon steel piping

Use of CCCW is limited to situations where contamination

by seawater of the cooled stream is to be avoided I.e. compressor seal / lube oil cooler, crude shipping pump coolers

Closed Circuit Cooling Water

(CCCW) Systems

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CCCW System Design

To determine total cooling loads, Q, summate allcoincident loads and add an allowance for run-up of

largest spare machine.

To determine flowrate, m, use inlet and outlet

temperatures, T and specific heats, CP of glycol solution

i.e.

TmCQ P=

Pump design capacity is approximately 1.25*calculated

total load flowrate as a safety margin

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CCCW System Design

Header sizing is based on pressure drop and velocity offluid

Non-boiling liquid should be approximately 15oC below bubble

point

The sea water circulating pumps should have a flat head

characteristic in order to avoid excessive design pressureon the plate heat exchangers

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CCCW System Design

Heat Exchangers - For initial sizing of heat exchangers use -

CCCW 32oC in and 18oC out

Seawater 15oC in and 27oC out

Header Tanks -

Size based on thermal expansion and the design pressure of the

system to cater for tube rupture on the heat exchangers

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Direct Seawater Cooling System

TO SEAWATER

DUMP HEADER

FROM SEAWATER

FILTER PACKAGE

INLET GAS

COOLER

TEST

SEPARATOR

COOLER

FLASH GAS

AFTER

COOLERDEW

POINT

COOLER

CRUDE OIL

COOLER

PIPELINE COMPRESSOR

AFTER COOLER

FLASH GAS

COOLERS

BOOSTER

COMPRESSOR

COOLER

PLATFORM

TOILETS

SEWAGE

TREATMENT

UNIT

HVACSYSTEM

DESALINATION

PACKAGE

SEAWATER

CCCW

EXCHANGER

EQUIPMENT

OVERFLOWS

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Direct Seawater Cooling Systems

- Advantages

Results in a lower overall system weight and space

compared to CCCW

Due to lower operating temperature and hence larger temperature

difference and elimination of the seawater/CCCW heat exchangers

Process outlet temperatures down to 18oC are possible

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Corrosion resistant (more expensive) materials needed for

construction

E.g. cupro-nickel

Colder skin temperatures may cause hydrate or waxing

problems in exchangers

Higher maintenance / downtime

Seawater to be used must be treated with hypochlorite and

filtered to 80 - 120m

Flow rates may be high due to lower return temperaturelimitations

Direct Seawater Cooling Systems

- Disadvantages

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Air Cooling Systems - Advantages

Can still be used in the event of CCCW failure Ideal for emergency and life support equipment

Low power requirement

Only occasionally are air coolers considered for offshore

cooling duties

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Large in size and weight occupying a lot of space

Expensive to operate

Required temperatures lower than ~ 65oC are not possible

Low efficiency

Variable cooling rates difficult to achieve

Air Cooling Systems - Disadvantages