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TRANSCRIPT
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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
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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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- ,
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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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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