Selecting HEX in Offshore

17 YEARS OF PROCESS DESIGN EXPERIENCE IN

UPSTREAM AND DOWNSTREAM HC

IChemE UK

Guideline on Selection and

Design of Cooling Options for

Offshore Application

BY MANISH V SHAH

CHARTERED ENGINEER &

FELLOWSHIP FROM

[email protected]

http://hc-mvs.devhub.com/

Mobile +91 94299 26447

GUIDELINE ON SELECTION AND DESIGN OF

COOLING OPTIONS FOR OFFSHORE APPLICATION

Gas compression and cooling, is most common in offshore facilities.

Gas compression is required to meet,

Export gas pressure,

To deliver, high pressure lift gas, or Injection gas, to reservoir, to

recover more oil.

Because of compression, gas temperature goes 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.

2

This article discusses, various cooling options, for gas compression; which includes..

air cooling,

direct sea water cooling,

Indirect sea water cooling.

Following types, of heat exchangers, are considered for water cooling options..

shell and tube heat exchangers (S & T HX),

plate type heat exchangers (PHE),

printed circuit heat exchangers (PCHE) 3



4

G

T

G

T

Feed gas

from surge

drum

Air cooling option

Export / Injection

gas Air

cooler

Air

cooler



5

G

T G

T

Feed gas

from surge

drum

Direct sea water cooling option

Export / Injection

gas

Direct sea water cooler ( Shell & Tube heat

exchanger )

Sea water

supply

Injection water or return back to

sea

Sea water system

1) Sea water lift pumps

2) Sea water Filtration unit

3) Chemical Injection

systems



6

G

T

G

T

Feed gas

from surge

drum

Indirect sea water cooling option

Export / Injection gas

Indirect sea water cooler

Shell & Tube heat exchanger OR

PCHE

Sea water

supply Injection water

or return back to

sea

Sea water system




systems

Potable water system

1) Potable water package

2) Potable water pumps

3) Potable water tank

Plate & Frame

HX

Potable water

tank and pumps



This article provides, broad guidelines, on selection & design, of various

offshore cooling options.

Comparisons of these options are made with respect TO

space,

relative cost,

fouling,

material selection,

availability,

reliability,

and maintenance considerations.

Number of equipment, required in each option, is also discussed, to

understand overall impact.

7



8

G

T

G

T

Feed gas

from surge

drum

Air cooling option

Export / Injection

gas Air

cooler

Air

cooler



AIR COOLING OPTION

Selection Criteria for air cooling option :

Air cooler is attractive option, when sea water is NOT available.

New offshore platform, where space and weight are NOT a

constraint.

Air cooler can be kept on top deck.

9

AIR COOLING OPTION

Advantages of air cooling option :

No sea water system or potable water system. (Thus less no. of

equipments.)

Air cooling control is simple and easy.

No fouling, corrosion or blocking issue.

Less maintenance or cleaning.

Highest availability and reliability.

No pump handling or their maintenance.

Less costly compared to direct sea water cooling option.

10

AIR COOLING OPTION

Disadvantages of air cooling option :

Biggest foot print and

High height of air coolers, adds, big structure and their cost.

Minor Issues like .

Fan Noise

Hot Air Recirculation

11

AIR COOLING OPTION

Design considerations for compressor gas cooler :

In special design, same air coolers are design for parallel and series

operation, for single and two stage compression respectively.

In critical service like twister, air coolers are also design for one fail

case.

12

AIR COOLING OPTION

Construction features of air cooler :

Forced draft For easy removal of fan or motor, forced draft fan is best

choice for maintenance.

CFD can be used, to calculate hot air recirculation, for

forced draft arrangement.

Induced draft For critical applications, if temperature approach, is less

than 15C (?), then Induced draft is selected. {Temperature

approach is, difference between, PROCESS outlet

temperature, and air inlet temperature}

Tube thickness Min. 2 mm

Bundle width

and length

Max. bundle width 3m { To be selected, to suite transport

requirement, from fabrication house to offshore platform. }

13

AIR COOLING OPTION

Construction features of air cooler :

Tube fine Extruded aluminium fins, are recommended, for sea weather

condition.

MOC fans

and lovers

Aluminium fans and lovers are recommended, for sea weather

condition.

Max. Motor

power

To avoid gear drive fans, motor power is restricted to 37 KW.

No. of fans More than one fan per bay {In the event of one fail case,

partial cooling duty is possible.}

Surrounding Height of air cooler, and near by equipment, to be located

carefully, to ensure sufficient air supply, to the tube bundles.

Either grating, or full opening, below air coolers are

recommended, to ensure, sufficient air supply to air cooler. 14

AIR COOLING OPTION

Air cooling Application :

New offshore platform, where space and weight are NOT a

constraint.

Offshore platform, where sea water is NOT available. (or expensive

option)

15

16

G

T G

T

Feed gas

from surge

drum

Direct sea water cooling option

Export / Injection

gas

Direct sea water cooler ( Shell & Tube heat

exchanger )

Sea water

supply

Injection water or return back to

sea

Sea water system




systems



DIRECT SEA WATER COOLING OPTION Selection Criteria for direct sea water system :

As compared to air cooler, direct sea water cooling, requires smaller

offshore footprint.

Either sea water, is retuned back to see, or it can be used as injection

water, to recover more oil from reservoir.

Hotter the injection water, better for the injection purpose.

After gas cooling, sea water can be re-used for injection purpose.

For sea water SYSTEM, following equipment are necessary

sea water lift pumps,

filtration units,

chemical and corrosion inhibitor injection units.

17

DIRECT SEA WATER COOLING OPTION

Selection Criteria for compressor gas cooler:

PCHE (Printed circuit heat exchanger), is not suitable, for direct sea

water cooling, as small pore can be blocked by, dirty sea water and scaling.

Plate exchanger is limited by design pressure.

Thus only shell and tube exchanger, is suitable for, direct sea water

cooling.

18


Construction features of S & T gas cooler :

Fluid allocation :

Shell Vs Tube :

To avoid shell side difficult cleaning and shell

removal, cooling water is kept at tube side.

This facilitates tube side mechanical cleaning;

and it also helps, in reducing fouling rate, by

maintaining good velocity inside the tubes.

TEMA type : AEN is best choice for tube side mechanical

cleaning, without removal of tube side piping.

No shell side mechanical cleaning is required.

19



Tube MOC : For sea water, Titanium tubes are best

choice.

Shell MOC : Generally it can be Duplex or Inconel based,

on gas composition. (OR Shell MOC is same

as other process equipments.)

20


Design Parameters for S & T gas cooler :

Pressure drop @

shell side

(Process gas )

Heat transfer is controlled by shell side,

thus try to maximise pressure drop on shell side,

but it is limited by tube vibration.

Pressure drop

@ tube side

(Sea water)

1 bar or more.

To reduce fouling, sea water velocity to be kept,

as high as possible (Minimum 1 m/s to 3 m/s or

more)

21


Sea water return temperature, to be selected based on following criteria.

a) Sea water return temperature, to be set, as per local regulation, and

to save marine life.

b) If sea water, is returned back to sea, temperature can be between 40

to 45C.

c) Lower sea water return temperature, bigger the pump and filtration

units.

d) To avoid excessive fouling, maximum tube skin temperature, shall be

restricted, to 60C in clean condition.

e) To allow multiple tube pass, sea water return temperature, should be

lower than, process outlet temperature. This also helps, in

increasing tube side velocity; and thus reducing fouling. 22


Advantages of Direct Sea water cooling :

Smaller foot print, as compared to air cooling & indirect cooling options.

Less no. of equipment, as compared to indirect cooling option (as

potable water package is NOT required).

Disadvantages of Direct Sea water cooling :

Highest costly, because of exotic material and high pressure thick shell.

Fabrication and supply is also limited. (Few vendors)

Because of fouling, frequent cleaning is required; thus less

availability and reliability. 23


Direct sea water cooling Application :

Offshore platform, where sea water is available, for gas cooling.

Offshore platform, where space and weight are constraint.

24

25

G

T

G

T

Feed gas

from surge

drum

Indirect sea water cooling option

Export / Injection gas

Indirect sea water cooler

Shell & Tube heat exchanger OR

PCHE

Sea water

supply Injection water

or return back to

sea

Sea water system




systems

Potable water system

1) Potable water package

2) Potable water pumps

3) Potable water tank

Plate & Frame

HX

Potable water

tank and pumps



INDIRECT SEA WATER COOLING OPTION Selection Criteria for indirect sea water system :

The indirect sea water cooling system, eliminates high tube skin

temp, as sea water, is cooling, low temp potable water.

Maximum potable outlet water temperature is 60C.

Thus the indirect system, is less likely, to have scaling and corrosion

problems, on sea water side. Thus availability and reliability, of

indirect system, is much higher, than direct system.

As compared to direct system, sea water return temperature, can be

higher, for indirect system. Thus requirement, of sea water flow is

less, for indirect system.

For sea water cooler, plate type is best choice.

For the compressor gas cooler, shell and tube exchanger (S & T HX)

and Printed Circuit Heat Exchangers (PCHE) are two choices. 26

INDIRECT SEA WATER COOLING OPTION 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 higher velocity in

narrow gaps and titanium plates. (titanium is less prone to fouling.)

Plate type exchanger, occupies less space and weight.

Plate type HX is cheaper, than the conventional S & T HXs.

Plate type HX 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. 27

INDIRECT SEA WATER COOLING OPTION

Design parameters for Plate type, sea water cooler :

MOC Titanium plates

pressure drop 1 bar or higher, to maintain high velocity, and thus

low fouling.

Fouling : Zero / low fouling, with 10-20% margin on heat

transfer area.

For plate type exchanger, do NOT use high fouling

factor, of S & T HX.

temperature

approach

Min. 3C. Or more.

28


There are, two types of compressor gas cooler, for indirect cooling

option.

printed circuit heat exchangers (PCHE)

shell and tube heat exchangers (S & T HX),

29

INDIRECT SEA WATER COOLING OPTION PCHE compressor gas cooler :

Advantages of PCHE :

PCHE has lowest weight and lowest foot print, as compared to all

the options.

Typically the PCHEs are 4 to 6 times smaller, than S & T HXs for

same HEAT duty.

30

INDIRECT SEA WATER COOLING OPTION PCHE compressor gas cooler :

Disadvantages of PCHE :

PCHE requires, additional inlet fine filters, and their cleaning.

PCHE can fail, because of flow fluctuations, or thermal fatigue.

PCHEs can block, as they have very small pore size.

Cleaning is difficult, because of welded construction.

Hot spot can occur, because of loss of cooling fluid.

Considering all above issues, PCHE offers less availability and

reliability, as compared to shell and tube exchanger.

Above problems, can be eliminated with proper design, and

operating procedures. Thus more popular for offshore platform.

31


Design parameters for PCHE gas cooler :

Fouling factor Nil

Pressure drop both sides 1 bar or more

MOC : Duplex or higher. {The PCHEs are

available in a wide range of corrosion

resistant materials, such as SS316L,

Duplex and Titanium. }

32



Fluid allocation :

Shell Vs Tube :

To avoid high pressure thicker shell, and tube

vibration; process gas is put on tube side.

This will result into, lower HX weight.

TEMA Type BEM is preferred geometry.

For temperature cross service, one can select

BFM type construction, in special case.


Design parameter for S & T gas cooler :

Pressure drop @ tube

side (Process gas )

1 bar or more

{as tube side heat transfer is controlling }

Pressure drop @ shell

side (Potable water)

0.5 bar { there is no extra benefit from higher

pressure drop on shell side, as tube side HT is

controlling, and also potable water is clean. }

Mechanical cleaning No mechanical cleaning is required, either on

shell or tube side.

34


Comparison between PCHE and Shell and tube exchanger

Printed Circuit Heat Exchangers

(PCHE)

Shell and tube exchanger

Inlet Fine filters, valves and

instruments for both streams.

Not required

Cleaning and maintenance of inlet

filters

N/A (Not required)

PCHE can block or fail S & T can not block or fail

Less availability and reliability Higher availability and reliability

Smallest foot print and weight Little higher than PCHE

Cheapest option Little higher than PCHE 35

INDIRECT SEA WATER COOLING OPTION Advantages of indirect sea water cooling:

Higher availability and reliability, as compared, to direct system,

because of less fouling.

Lower sea water requirement, because of higher possible, sea water

return temperature.

Disadvantages of indirect sea water cooling:

Additional potable water system and exchangers are required.

36

INDIRECT SEA WATER COOLING OPTION Indirect sea water cooling Application :

Offshore platform, where sea water is available, for gas cooling.

Offshore platform, where space and weight are constraint.

37

Final Recommendation :

Select air cooler, when sea water is NOT available (viable), and

space is NOT a constraint. Air cooler is simple & easy to use, 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, for onshore gas cooling application. (If sea water is only

choice, then above three points, are also valid, for onshore

cooling application.) 38



TECHNICAL PAPERS AND SPECIALISATION

I have presented seven (7) technical papers at ten (10) International conferences (in Houston, Kuala Lumpur and India) as well two (2) technical papers in Industrial magazine.

I have identified a number of strategic areas, where the application of Specialist Process Design Expertise can bring substantial benefits to our clients, as well to process and safety community. Strategic area includes one stop solution for design of various types of Heat Exchangers & Tray / Packed Columns, various Simulation Tools and Fire and Cold Blowdown study for Oil and Gas Industry.

EXPERIENCE SUMMARY

Seventeen years of Process Design experience in

a. Upstream (Oil & Gas),

b. Downstream (Refinery & Petrochemicals)

c. Utility System, in leading

Concept,

Feasibility study,

FEED,

Basic and

Detail engineering phases.

THANK YOU FOR YOUR ATTENTION

By Manish V Shah

[email protected]

http://hc-mvs.devhub.com/

Mobile +91 94299 26447

Selecting HEX in Offshore

Documents

water cooling options

design of cooling options

sea sea water system

cooling system

cooling duty

various cooling options

sea water lift pumps

air cooling option advant