1. Managing the potential gas hydrate risk in subsea hydraulic control lines Alan Demange, Richard Rowntree – Castrol Offshore

2. 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)

3. 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

4. If ALL elements are present…
What can we do next ?
Work on ensuring no gas ingress and assume no hydrates will form
or…
Assume gas ingress and evaluate the actual hydrate risk and manage it !

5. Hydrate formation & dissociation curve
Source: “Introduction to Hydrates and Hydrate Modelling”, by Beryl Edmonds, InfoChem Computer Services Ltd.
Focus on hydrate dissociation curve

6. Evaluating & Managing the Hydrate Risk
1. Benchmarking with pure methane
5000
6000
7000
8000
9000
10000
11000
12000
13000
-10 -8 -6 -4 -2 2 4 6 8
T/C
P/psia
Results from laboratory testing at Heriot-Watt University
Hydrate risk
Hydrate free

7. 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 2 4 6 8
T/C
P/psia
Hydrate free
Hydrate risk

8. 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

9. 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

10. 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

11. Thank you for your attention…
Any Questions ?