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ACaseStudyinOffshoreGas

PipelineDepressurization

A ec C en,P.E.,AmirA wazzan,P .D.

McDermottSubsea

Engineering

Houston,Texas,U.S.A.

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Introduction

PapersObjectives

Challenges HydrateManagementMethodology

Conclusions

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Q&A2

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A m or conc rn in off hor oil &

productionsystems

waterandlighthydrocarbonmolecules

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oc agescanoccurveryrap y3

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WhenWillGasHydratesForm?

Hydrate can form in two fundamental ways:

Slow cooling (flowing and/or shut-down)

Rapid cooling (Joule-Thompson Effect)

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1 ft3of hydrates ~ 178 ft3of gas

5

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To :

eterm ne t e mens on o t e restr ct onorifice (RO) used to depressurize thep pe ne a ter an unp anne s ut own

Study the liquid discharge rate duringde ressurization and

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-

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- ,

Transports gas condensate from an

processing facility as cons s s o approx ma e y

methane, CGR = 40 bbl/mmscf

Depressurization after an unplannedshutdown

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Pipeline Profile

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rough terrain

Concerns on hydrate formation Difficulties in OLGA modeling during

shutdown and depressurization

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Case#

OperatingTemperature

Flowrate(mmscfd)

Shore ArrivalPressure

(C) (bar/psi)

1 13 (Winter) 500 120 /1,7402 13 (Winter) 1,000 85/1,233

3 35 Summer 500 120/1 740

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Close onshore emergency shutdown valveESDV with continued roduction

Activation of offshore instrumented overpressure

rotection s stem IOPPS shuts down thepipeline when the system pressure reaches 135bar (1,958 psi)

Shut-in for 3 days

Commence depressurization

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One-Sided Depressurization

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RO Size(inch)

TemperatureDownstream of

DischargeRate Downstream

DepressurizationTime to Reach

the RO (C) of RO (ft3/h)

1.25 -58 1,050 79.01.50 -59 2,940 55.5

1.75 -59 3,325 40.6

Design temperature downstream of the RO = -70C

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Minimum

Case#

epressur zat on

Time to Reach 30bar (hr)

TemperatureDownstream

of RO (C)

ea qu

DischargeRate (ft3/h)

ota ns ore

LiquidDischarged (bbl)

1 55 -59 2,940 1,400

2 53 -52 1,750 675

3 51 -34 1,050 600

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Simulation Results (Case 1)Steady State Temperature Profile90

70

80

50

60

e(

C)

20

30

Temperatur

Onshore Processing Facility

0

10

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Distance (mile)

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Simulation Results (Case 1)Pressure Profiles

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Simulation Results (Case 1)Pressure Trend Upstream of the RO

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Simulation Results (Case 1)Temperature Trend Downstream of the RO

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-

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One-Sided vs. Two-Sided

Depressurization

CaseDepressurizationTime to Reach 30

MinimumTemperature

Peak LiquidDischar e

bar (hr)

RO (C)Rate (ft3/h)

One-

sided55.0 -59 2,940

Two-

sided55.5 -59 2,940

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One-Sided vs. Two-Sided

Depressurization

Identical minimum temperature

Identical peak liquid discharge rate Gas compressibility plays a prominent role

in maintaining the pipeline pressure

Elevation difference

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1.5-inch RO is selected to conduct

Increased liquid volume will elongate

depressurization times

,low production rate, higher onshore arrival

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To be continued

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Conclusions continue

or e wors case qu pro uc on case

(Case 1), it takes 55 hours to depressurize thepipeline from settle out conditions to 30 bar

(435 psi)

-1.5-inch RO yielded similar results to those of

-

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McDermott Subsea Engineering

this work

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Contact:Aleck Chen, P.E.Flow Assurance En ineerin

McDermott Subsea Engineering: 757 N. Eldridge Pkwy, Houston, TX 77079, U.S.A.: (281) 870-5896 : (281) 870-5840

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.