managing the potential gas hydrate risk in subsea hydraulic control lines alan demange, richard...
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Managing the potential gas hydrate risk in subsea hydraulic control lines
Alan Demange, Richard Rowntree – Castrol Offshore
Introduction to Hydrates
What are they ?Crystalline solid compounds (water & light gases)
How do they form ?– High pressure– Low temperature– Presence of water– Gas (e.g. C-1, methane)
Introduction to Hydrates
Can they be found in another part of the Subsea Production System ?
– High pressure: conditions in hydraulic HP control lines
– Low temperature: seabed temperatures– Presence of water: from the control fluid !– Gas: potential migration through SSSV
If ALL elements are present…
What can we do next ?
1. Work on ensuring no gas ingress and assume no hydrates will form
or…
2. Assume gas ingress and evaluate the actual hydrate risk and manage it !
Hydrate formation & dissociation curve
Source: “Introduction to Hydrates and Hydrate Modelling”, by Beryl Edmonds, InfoChem Computer Services Ltd.
Focus on hydrate
dissociation curve
Evaluating & Managing the Hydrate Risk
1. Benchmarking with pure methane
5000
6000
7000
8000
9000
10000
11000
12000
13000
-10 -8 -6 -4 -2 0 2 4 6 8
T/C
P/p
sia
Hydrate freeHydrate risk
Results from laboratory testing at Heriot-Watt University
Evaluating & Managing the Hydrate Risk
2. What is the impact of gas impurities?
Effect of 4% impurities
(using modelling techniques)
5000
6000
7000
8000
9000
10000
11000
12000
13000
-10 -8 -6 -4 -2 0 2 4 6 8
T/C
P/p
sia
Hydrate freeHydrate risk
Evaluating & Managing the Hydrate Risk
3. The Effect of Seawater Ingress
• The source– The subsea hydraulic coupling during installation and/or
disconnection
• The impact– Dilutes the hydrate inhibition properties of the control fluid =>
increases the hydrates risk
• Solutions– Take seawater ingress into consideration in risk assessment– Hydraulic coupling selection– Develop an installation procedure to reduce overall
seawater content
Evaluating & Managing the Hydrate Risk
4. The role of the control fluid
• Measure the hydrate inhibition properties of the control fluid
• Ensure consistent product quality = consistent properties
• For more extreme developments – Deeper water, Gas projects, X HP/HT developments
– Products with enhanced hydrate inhibition properties are available for pressure requirements up to 25k psi
Evaluating & Managing the Hydrate Risk
5. Summary
• Compare control fluid hydrate dissociation curve versus expected system pressures and temperatures– Clarify pressure data especially w.r.t to hydrostatic head and system
design v actual pressures.
– Requires good interfacing between SPS team and Completions
• Remember to consider the impact of gas impurities & sea water ingress!
• If system is in the “risk zone” consider control fluid with enhanced hydrate inhibition properties
Thank you for your attention…
Any Questions ?
www.castrolsubsea.com
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