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TECHNICAL PAPER
BOP Tethering and Motion
Measurements – Enabling Safe
Subsea Well Decommissioning
J. Lodhia
AOG March 2019
BOP Tethering and Motion Measurements – Enabling Safe Subsea Well Decommissioning
14th March 2019
J Lodhia
Learn more at www.2hoffshore.com
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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
Learn more at www.2hoffshore.com
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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 WellheadLearn more at www.2hoffshore.com
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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 LifeLearn more at www.2hoffshore.com
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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
Learn more at www.2hoffshore.com
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BOP Tether System
Clump Weights
BOP StackTensioning
System
Tether Lines
Primary aim is to reduce BOP stack motionsLearn more at www.2hoffshore.com
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Effect of BOP Tether on Bending along Conductor
Approx. 3 times reduction in bending loadLearn more at www.2hoffshore.com
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Unfactored Fatigue Results with BOP Tether System
Fatigue lives improved by a factor of 185xLearn more at www.2hoffshore.com
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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?
Learn more at www.2hoffshore.com
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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
Learn more at www.2hoffshore.com
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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
Learn more at www.2hoffshore.com
16 of 21
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
Learn more at www.2hoffshore.com
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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
An
gu
lar
Ra
te (
de
g/
s)
Acc
ele
rati
on
(m
/s2
)
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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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
Learn more at www.2hoffshore.com
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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
Learn more at www.2hoffshore.com