Jeff Zhang, Michael Ramanathan
Case Studies on Pipeline Depressurisation
for Offshore LNG Development
AOG Perth
20th February, 2014
Experience that Delivers
Experience that Delivers
Agenda
Offshore Pipeline Depressurisation
Importance of Pipeline Depressurisation
Pipeline Depressurisation Modelling Approaches
Case Studies
Liquids Dominated System
Gas Dominated System
Conclusions and Recommendations
Offshore Pipeline Depressurisation
Reduction of pressure in offshore pipeline systems
Operation Shutdown
Hydrate Management
Maintenance
Why is Pipeline Depressurisation
Important?
System integrity and operability
Subsea pipelines
Topsides facilities limited space / weight
Potential risks
Low temperature occurrence
Liquid surge management
Pipeline Depressurisation
Modelling Approaches
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
4
Schedule &
Budget
Case 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
Case Study
Liquids Dominated System
Outlet topsides
Inlet topsides
Depressurisation
valve
Depressurisation
valve
Single phase
liquid system
Inlet
Riser
Inlet Riser base
Subsea pipeline system
Liquid Dominated System
Temperature Profile (Single Phase)
Minimum Temperature Profile during Depressurisation (Single Phase)
40
39
38
37
Temperature (C)
Compositional
Standard
36
35
34
33
32
31
30
50
55
60
65
70
75
80
Relative Pipeline Length (%)
7
85
90
95
100
Liquid Dominated System
Temperature Profile (Multi Phase)
Minimum Temperature Profile during Depressurisation (Multi phase)
40
39
38
Temperature (C)
37
Compositional
Standard
36
35
34
33
32
31
30
50
55
60
65
70
75
80
Relative Pipeline Length (%)
8
85
90
95
100
Liquids Dominated System
Accumulated Volumes
Variable
Unit
Single Phase
Depressurisation
Multi Phase
Depressurisation
Standard
High
Fidelity
Standard
High
Fidelity
Gas
Accumulated Volume
m3
N/A
N/A
2531
26725
Condensate
Accumulated Volume
m3
206
232
3928
10808
Water
Accumulated Volume
m3
12
16
170
661
Total Liquid
Accumulated Volume
m3
218
248
4098
11469
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
Outlet topsides
Inlet topsides
Depressurisation
valve
Depressurisation
valve
Multiphase
WGC system
Inlet
Riser
Outlet
Riser
Inlet Riser base
Subsea pipeline system
11
Gas Dominated System Initial Liquid Holdup Profile
Initial Liquid Gas
Holdup
Dominated
Profile (Compositional
System Bathymetry
vs. Standard)
Schematics
with Bathymetry
1
Bathymetry
0.9
Riser-Pipeline
Interface
0.8
Shallow
Water
Compositional
Liquid Holdup (-)
0.7
Standard
0.6
Bathymetry
0.5
0.4
0.3
Deep
0.2
Water
0.1
0
0%
10%
20%
30%
40%
50%
60%
Relative Pipeline Length (%)
12
70%
80%
90%
100%
Gas Dominated System
Liquid Accumulated Volume
Variable
Unit
Inlet Side
Depressurisation
Outlet Side
Depressurisation
Standard
High
Fidelity
Standard
High
Fidelity
Liquid Hydrocarbon
Accumulated Volume
m3
533
481
46
108
Liquid Water
Accumulated Volume
m3
38
42
Ratio of Total Liquid
Accumulated Volume
to Pipeline Inventory
65
58
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
13 on Multiphase Production System, Cannes, France 12-14 June 2013.
Gas Dominated System Minimum Fluid Temperature
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Minimum
Fluid Temperature
Location
Unit
Subsea Pipeline
Inlet Side
Depressurisation
Outlet Side
Depressurisation
Standard
High
Fidelity
Standard
High
Fidelity
-5
Inlet Riser
-13
-3
-4
-4
Topsides
Immediately
Downstream of Valve
-78
-74
-66
-59
Minimum Temperature Profile
(Inlet Side Depressurisation)
Minimum Fluid Temperature Profile during Inlet Side Depressurisation
25
20
Inlet Topsides Riser Interface
Standard
Riser-Pipeline
Interface
15
Fluid Temperature (C)
Compositional
10
5
0
-5
-10
-15
-20
0%
1%
2%
3%
4%
5%
6%
Relative Pipeline Length (%)
15
7%
8%
9%
10%
Minimum Temperature Profile
(Outlet Side Depressurisation)
Minimum Fluid Temperature Profile during Outlet Side Depressurisation
25
Inlet Topsides Riser Interface
20
Standard
Riser-Pipeline
Interface
15
Fluid Temperature (C)
Compositional
10
5
0
-5
-10
-15
-20
0%
1%
2%
3%
4%
5%
6%
Relative Pipeline Length (%)
16
7%
8%
9%
10%
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.
17
Comparison - Benefits of
Compositional Tracking Approach
Item
Critical
Scenarios
18
Liquid Dominated
System
Gas Dominated
System
Multiphase depressurisation deepwater depressurisation
mobilising bulk liquids
across fluid bubble point
Outcomes
Significantly higher
liquid surge volume
Significantly 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
Conclusions and Recommendations
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.
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Thanks for Listening
Any Questions?
Dr. Jeff Zhang & Michael Ramanathan
Wood Group Kenny Pty Ltd
432 Murray Street
Perth, Western Australia
jeff.zhang@woodgroupkenny.com
Vaithianathan.Ramanathan@woodgroupkenny.com
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