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

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-10 -8 -6 -4 -2 0 2 4 6 8

T/C

P/p

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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)

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-10 -8 -6 -4 -2 0 2 4 6 8

T/C

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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 ?

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