BOP Tethering and Motion Measurements – Enabling Safe Subsea Well Decommissioning

14th March 2019

J Lodhia

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Contents

▪ Subsea well P&A challenges

▪ Benefits of BOP tether system

▪ Specification of BOP tether system

▪ In-field measurements to validate response

▪ Conclusions

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P&A Challenges for Older Wells

▪ Typically old (pre-2000) 30” conductor designs

▪ Usually not designed for fatigue loading

▪ Lack of data

▪ 6th generation semi-submersibles in shallow water

▪ Prior fatigue damage?

Older Wellhead Newer Wellhead

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1st Case Study – Conventional P&A Approach

▪ Well Location: Offshore Australia

▪ Water Depth: 68m

▪ Drilling Rig: 6th generation moored semi-submersible

▪ Wellhead System: Rigid lockdown wellhead

▪ Originally installed approx. 2005

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Wellhead & Conductor Stack-up

▪ 30” x 1.0” Conductor

▪ 20”x13-3/8” Surface Casing

▪ Conductor and surface casing cemented to mudline

▪ Combined wet weight of LMRP + BOP + Subsea Tree = 232.7Te

▪ 25 day duration = 250 days target life (0.68 years) FOS=10

LP Housing:0.97m

3.35m

Mud Line Mud

HP Housing: 1.72m

11.51m

2.48m 2.38m

0m

Cement

-8.09m

30x1.5” LP Hsg Extension Joint

6.21m

4.20m

BOP: 5.30m

Subsea Tree: 2.01m

17.49m

Riser Adapter, Lower Flex Joint and LMRP: 5.98m

30x1” XOver Joint 30x1” Intermediate Joint

1 x 30” x 1.50” Conductor Joint

-19.57m

-31.05m

-42.63m

20x 13-3/8” Surface Casing

10-3/4” x 9-5/8” Casing

13-3/8” Casing Shoe -565.1m

Surface Casing TOC 0m

-4.00m Base of HD90 Quick Stab Box

-5.90m Top 13-3/8” BTC Connector

9-5/8” Casing Shoe -1630.1m

-1596.3m 7” Tubing Packer

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Unfactored Fatigue Results

Min Fatigue Life is 11 days and does not meet Target Life

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Mitigation Options

▪ Reduce conservatisms – only applicable for marginal designs

▪ More accurate data

▪ Conventional remedial actions include:

▪ BOP modifications – lighter BOP however can be costly

▪ Different vessels – availability and cost implications

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BOP Tether System

Clump Weights

BOP StackTensioning

System

Tether Lines

Primary aim is to reduce BOP stack motions

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Effect of BOP Tether on Bending along Conductor

Approx. 3 times reduction in bending load

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Unfactored Fatigue Results with BOP Tether System

Fatigue lives improved by a factor of 185x

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Specifying BOP Tether SystemKey Considerations

▪ Consider how any change in the design will affect the overall tether wire stiffness:

▪ Clump weight position

▪ Tether wire length

▪ Tether wire OD / Maximum Breaking Force

▪ How does tether wire pre-tension impact the efficiency of the system?

▪ Monitoring system can provide further assurance

▪ Bottom clump weight stability on seabed

▪ Axial loading resistance on conductor – Any additional axial load?

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Effect of Clump Weight Position

▪ 3 distances considered:• 13.5m

• 41.0m

• 45.0m

▪ Longer tether wire reduces stiffness

▪ System response is sensitive to wire stiffness

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Effect of Tether Pre-tension & Wire OD

▪ Tether wire OD reduced – Wire stiffness reduces by 87%

▪ 3 pre-tensions considered:• 5 kips

• 10 kips

• 15 kips

▪ Clump weight positions remains constant

▪ Wire pre-tension has little effect on system response

▪ System response is sensitive to wire stiffness

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Final Fatigue Results with BOP Tether System

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2nd Case Study – Monitoring BOP Stack Motions

▪ Well Location: Offshore Australia

▪ Subsea well utilised a BOP tether system

▪ Motion monitoring equipment installed subsea onto the BOP frame and subsea tree

▪ Monitoring equipment recorded the BOP stack and subsea tree movement and accelerations

▪ Data available pre- and post-BOP tether system installation for multiple deployments

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Subsea Tree Accelerations – 1st Deployment

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11/07 00:00 11/07 12:00 12/07 00:00 12/07 12:00 13/07 00:00 13/07 12:00 14/07 00:00

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Date and Time

Cooper Energy Sole-4 Tree Monitoring

STANDARD DEVIATION OF ACCELERATION & ANGULAR RATES11-13 July 2018

X Acceleration Y Acceleration X-Z Angular Rate Y-Z Angular Rate Noise Level

Logger installed

Loggerremoved

BOP landed

BOP tether system

activated

BOP tether system

presssuretopup

Noise level

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Subsea Tree Accelerations – 2nd Deployment

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Observations From In-field Measurements

▪ Use of the BOP tether system provided 2.5x reduction in motions

▪ Consistent reduction in BOP stack motions observed over multiple deployments

▪ The observed reduction in motions directly leads to improved fatigue performance

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Summary

▪ Use of 5/6th generation rig for subsea well P&A decommissioning may lead to fatigue complications

▪ Standard remedial actions may be insufficient or too costly

▪ BOP tether system offers a direct improvement on wellhead fatigue by reducing BOP stack motions

▪ Must consider wire stiffness when designing the BOP tether system

▪ In-field motion measurements confirm the effectiveness of a BOP tether system in reducing stack motions

Questions?

http://www.2hoffshore.com/