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Experience that DeliversExperience that Delivers
Jeff Zhang, Michael Ramanathan
AOG Perth20th February, 2014
Case Studies on Pipeline Depressurisation for Offshore LNG
Development
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1Agenda
Offshore Pipeline Depressurisation Importance of Pipeline
Depressurisation Pipeline Depressurisation Modelling Approaches
Case Studies
Liquids Dominated System Gas Dominated System
Conclusions and Recommendations
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2Offshore Pipeline Depressurisation
Reduction of pressure in offshore pipeline systems Operation
Shutdown Hydrate Management Maintenance
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3Why is Pipeline Depressurisation Important? System integrity
and operability
Subsea pipelines Topsides facilities limited space / weight
Potential risks Low temperature occurrence Liquid surge
management
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4 Standard Predefined constant composition Look-up table with
fluid physical properties Widely used for design purpose Inadequate
for depressurisation scenarios
Compositional Tracking Track fluid composition variation
Calculate in-situ fluid properties Accurate but time consuming
Robust Design Schedule & Budget
Pipeline Depressurisation Modelling Approaches
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5Case Studies
Liquids Dominated System Depressurisation Depressurisation
(Final state: Single phase - above bubble point) Depressurisation
(Final state: Multiphase - below bubble point)
Gas Dominated System Depressurisation Inlet Side
Depressurisation Outlet Side Depressurisation
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6Case Study Liquids Dominated System
Single phase
liquid system
Inlet topsides
Depressurisation
valve
Subsea pipeline system
Inlet
Riser
Inlet Riser base
Outlet topsides
Depressurisation
valve
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7Liquid Dominated System Temperature Profile (Single Phase)
30
31
32
33
34
35
36
37
38
39
40
50 55 60 65 70 75 80 85 90 95 100
T
e
m
p
e
r
a
t
u
r
e
(
C
)
Relative Pipeline Length (%)
Minimum Temperature Profile during Depressurisation (Single
Phase)
Compositional Standard
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8Liquid Dominated System Temperature Profile (Multi Phase)
30
31
32
33
34
35
36
37
38
39
40
50 55 60 65 70 75 80 85 90 95 100
T
e
m
p
e
r
a
t
u
r
e
(
C
)
Relative Pipeline Length (%)
Minimum Temperature Profile during Depressurisation (Multi
phase)
Compositional Standard
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9Liquids Dominated System Accumulated Volumes
Variable Unit
Single Phase Depressurisation
Multi Phase Depressurisation
Standard High Fidelity StandardHigh
Fidelity
GasAccumulated Volume m
3 N/A N/A 2531 26725
Condensate Accumulated Volume m
3 206 232 3928 10808
WaterAccumulated Volume m
3 12 16 170 661
Total Liquid Accumulated Volume
m3 218 248 4098 11469
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10
Liquid Dominated System Summary
Standard vs. Compositional Tracking Minimum Temperature:
Insignificantly different Liquid Surge Volumes: Critically
different
The standard approach can be used in scenarios where the system
remains in single phase after depressurisation.
The compositional tracking approach should be used in scenarios
where the system reverts to multiphase during/ after
depressurisation.
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Case Study Gas Dominated System
Multiphase
WGC system
Inlet topsides
Depressurisation
valve
Subsea pipeline system
Inlet
Riser
Inlet Riser base
Outlet topsides
Depressurisation
valve
Outlet
Riser
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12
Gas Dominated System -Initial Liquid Holdup Profile
0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%
Relative Pipeline Length (%)
Gas Dominated System Bathymetry Schematics
Bathymetry
Riser-Pipeline
Interface
Riser-Pipeline
Interface
Riser-Pipeline
Interface
Riser-Pipeline
Interface
Deep
Water
Shallow
Water
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%
L
i
q
u
i
d
H
o
l
d
u
p
(
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)
Relative Pipeline Length (%)
Initial Liquid Holdup Profile (Compositional vs. Standard) with
Bathymetry
Compositional
Standard
Bathymetry
Riser-Pipeline
Interface
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Gas Dominated System Liquid Accumulated Volume
Variable Unit
Inlet Side Depressurisation
Outlet Side Depressurisation
Standard High Fidelity StandardHigh
Fidelity
Liquid HydrocarbonAccumulated Volume m
3 533 481 46 108
Liquid Water Accumulated Volume m
3 38 42 0 0
Ratio of Total Liquid Accumulated Volume to Pipeline
Inventory
% 65 58 5 12
1. Jeff Zhang, Ian Kopperman. Modelling of Topsides
Repressurisation for Wet Gas Condensate Systems for Development of
Dry Tree Well Start-up Strategies, 16th International Conference on
Multiphase Production System, Cannes, France 12-14 June 2013.
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14
Minimum Fluid Temperature
LocationUnit
Inlet Side Depressurisation
Outlet Side Depressurisation
Standard High Fidelity StandardHigh
Fidelity
Subsea Pipeline C -5 4 6 2
Inlet Riser C -13 -3 -4 -4
Topsides Immediately
Downstream of ValveC -78 -74 -66 -59
Gas Dominated System -Minimum Fluid Temperature
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15
Minimum Temperature Profile (Inlet Side Depressurisation)
-20
-15
-10
-5
0
5
10
15
20
25
0% 1% 2% 3% 4% 5% 6% 7% 8% 9% 10%
F
l
u
i
d
T
e
m
p
e
r
a
t
u
r
e
(
C
)
Relative Pipeline Length (%)
Minimum Fluid Temperature Profile during Inlet Side
Depressurisation
Compositional
Standard
Inlet Topsides -
Riser Interface
Riser-Pipeline
Interface
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16
Minimum Temperature Profile (Outlet Side Depressurisation)
-20
-15
-10
-5
0
5
10
15
20
25
0% 1% 2% 3% 4% 5% 6% 7% 8% 9% 10%
F
l
u
i
d
T
e
m
p
e
r
a
t
u
r
e
(
C
)
Relative Pipeline Length (%)
Minimum Fluid Temperature Profile during Outlet Side
Depressurisation
Compositional
Standard
Riser-Pipeline
Interface
Inlet Topsides -
Riser Interface
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Gas Dominated System Depressurisation - Summary
Standard vs. Compositional Tracking Liquid Surge Volumes:
Insignificantly different Low Temperatures: Critically
different
The standard approach can be used as the first-pass assessment
to reduce analysis timescales.
The compositional tracking approach presents cost-saving
opportunities for project engineering design.
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Comparison - Benefits of Compositional Tracking Approach
Item Liquid Dominated SystemGas Dominated
System
Critical Scenarios
Multiphase depressurisationacross fluid bubble point
deepwater depressurisation mobilising bulk liquids
Outcomes Significantly higher liquid surge volumeSignificantly
warmer
minimum temperature
Impacts on Design
Increased engineering safeguarding to reduce
system integrity and downtime risks
Cost saving opportunities on material selections and
engineering safeguarding
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Pipeline depressurisation can impact the design and/or operation
requirements for offshore LNG development .
Compositional tracking approach gives more reasonable and
accurate predictions.
It is onerous to be applied as a standard design approach within
project schedule and budget constraints.
It is recommended as verification exercises to ensure a robust
and optimal engineering design.
Conclusions and Recommendations
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Dr. Jeff Zhang & Michael RamanathanWood Group Kenny Pty
Ltd432 Murray StreetPerth, Western Australia
[email protected]@woodgroupkenny.com
Thanks for Listening
Any Questions?
