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Richard Rickett, General Manager Engineering and Procurement - March 2015
Case Study
Case Study :
Diverless Fast Track
Repair of Mooring System
Introduction
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~ 300 floating production units in use around the world
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Design = maximise uptime = minimise disconnection/reconnection
=> need to withstand fairly extreme weather conditions
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Analysis based on historical environmental data
However - weather is inherently unpredictable and expected limits can be exceeded
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Potential early deterioration of mooring system components or at worst, failure of single or multiple
mooring lines.
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Very few have facilities to detect early warning signs of deterioration or impending failure
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Regular inspection campaigns carried out to confirm integrity of mooring system
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Not uncommon for the early signs of failure to be detected during routine inspection
DOF Subsea
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Mooring System Early Failures
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Many single line failures
Several examples of multiple line failures (wire, chain and/or connections)
Major component failures (yoke, wire, chain, connections)
Vessel drift-off
Subsea hardware and risers damaged or broken, catastrophic failure
Planned $$
Emergency $$$$$
Regular
Inspection
Inspect
damage
Repair prior
to failure
Failure
Repair
3-6 months
on station / off-station
DOF Subsea
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Case Study - Maari Oil Field
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Located 80km off the coast of South Taranaki,
New Zealand, at the entry to the Cook Strait
Largest crude oil field in New Zealand
Contributes significantly to the economy and
development of the Taranaki region.
DOF Subsea
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Maari field consists of :
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Subsea wells, flowlines & control umbilicals
Maari wellhead platform
FPSO Raroa, 8 leg mooring system
Water depth approx. 100m
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Each mooring leg comprises :
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Driven anchor pile
Lower 118mm chain section 50m
Lower 110mm wire section 200m / 400m / 500m
Upper 118mm chain section 400m
Upper 110mm wire section 200m
Chain : 118mm studless links
Wire : 110mm spiral strand uncoated wire rope
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Mooring wire anomalies
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In February 2013, a 5-year ABS inspection programme was carried out by DOF Subsea Diving Support
Vessel Skandi Singapore
Survey included an ROV survey of the 8 mooring legs
Bird-caging of spiral strand wire mooring rope was observed on mooring legs 1, 2, 3 & 4
Considered likely to effect the capacity of the mooring legs
Subsequent to the inspection, the Upper Wire Section on mooring leg No. 2 parted
Temporary mooring leg installed in Aug 2013 by others
Timeline
Feb
Mar
Apr
May
June
July
Aug
Sep
Oct
Nov
Dec
Anomalies identified in this area
Cut & add
chain
DOF Subsea
Cut &
reterminate
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Engineering of Solution
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Letter of Intent for engineering signed end July 2013
Procedure and tooling review held early August
Method statement developed, design loads calculated – 65-150 tonnes
Design based on 3m significant wave height limit
Concept developed based on an existing (lower capacity) tool previously accepted by the
classification society :
Existing tool (10t pull capacity)
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New concept (150t pull capacity)
Orders for long lead hydraulic parts placed immediately
Timeline
Feb
Mar
Apr
May
June
July
Aug
Sep
Oct
Nov
10
Dec
Engineering of Solution
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Fabrication
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Fabrication of tool awarded to a New Zealand
company (New Plymouth)
Close to port of mobilisation
Fabrication commenced end Aug 2013
Tool completed in 4 weeks
Load and function testing completed 7 weeks after
contract award)
Timeline
Feb
Mar
Apr
May
June
July
Aug
Sep
Oct
Nov
Dec
Execution - Skandi Hercules
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Built
: 2010
Length
: 108 m
Beam
: 24 m
DP
: Class 2
Bollard Pull
: 350 t
Chain locker capacity
: 1150 m3
Triple drum main winch
: 500 t
Heave compensated crane
: 140 t
2 off safe Deck rail cranes
: 5t
A-frame
: 250 t
Clear Deck Area
: 1,070 m2
Accommodation
: 90 personnel
ROVs: inbuilt 3,000 m work class ROV + overside
work class ROV (Schilling UHD)
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Execution – Schilling UHD ROVs
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Depth rating
Power
Payload capacity
Through frame lift
Manipulators :
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7-function Titan T4
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5-function Rigmaster
DOF Subsea
: 3,000 m
: 150 HP
: 400 kg
: 3,500 kg
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Mobilisation - Skandi Hercules
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Vessel contract signed mid August 2013
Hercules mobilised in Singapore 3 weeks later
Constructability Review held early September
Vessel arrived in New Plymouth NZ end September
Final risk assessment held early October
Timeline
Feb
Mar
Apr
May
June
July
Aug
Sep
Oct
Nov
Dec
Repair Methodology
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Identify location for Upper Chain Section cut subsea
Lift chain section into cutting stand
Cut chain with ROV carbide blade cutter
DOF Subsea
cut location
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Repair Methodology
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Recover FPSO end upper chain section and upper wire
section
Store chain section in vessel chain lockers
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Repair Methodology
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Fit recovery clamp to wire
Recover wire to deck & secure
Cut & remove damaged wire
Clean wire end
Re-terminate with new wire socket
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Repair Methodology
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Reconnect chain to repaired Upper Wire Section
Deploy chain until end is reached
Fit end of chain through ACCD stroking tool skid and secure in place
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Repair Methodology
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Deploy chain and ACCD tool to seabed
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Repair Methodology
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Recover other end of mooring line to deck
Attach new length of chain section
Lay end of chain into ANC ACCD tool
Fit H-link to end of chain
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Repair Methodology
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Deploy chain and ANC ACCD tool to seabed
Connect 20m tether lines between HYD and ANC
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Repair Methodology
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Connect hydraulic downline
Connect crane to chain end
Stroke tool (varied 5- 30m) & recover chain
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Repair Methodology
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Once sufficient slack is pulled through, lower chain end link into H-link
Insert H-link pin
Attach locknut.
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Repair Methodology
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Release load from ANC tool, remove tethers
Slide ACCD tool off carriages
Recover ACCD tool to deck, recover carriages to deck
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Repair Methodology
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Remove pins to release ACCD ANC tool
Lift chain off tool
Recover ACCD ANC tool to deck
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Repair Methodology
All tooling is recovered, leaving only H-links and anchor shackles in the chain leg
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Challenges - Range of Weather
Leg 2 : 7.5d ops + 6.5d WOW, FPSO in place.
Leg 3 : 4d ops + 1d WOW
Leg 1 : ~4d ops + 1d WOW
Leg 4 : 2.5d ops, no weather downtime.
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Challenges – Visibility at the Seabed
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Challenges - first leg repaired with FPSO in place
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Performance - Timeline
Engineering awarded
Tooling review
Vessel contract signed
Skandi Hercules mobilised in Singapore
Constructability Review
2013
Final risk assessment
Jul
Aug
Award
31/07/2013
Sep
Oct
Nov
Engineering
Tool fabrication
Tool testing
Mobilisation in Taranaki, New Zealand
Offshore works 1 month
Complete
14/11/2013
Learning curve‐‐‐‐‐‐‐‐
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Performance - Safety
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1 non-work related injury (rolled ankle climbing out of bunk while vessel in harbour)
1 first aid case – small cut to finger while cleaning under shelving
Significant amount of deck work
All work performed without divers
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Key Success Factors
DOF Subsea
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Proper detailed Engineering assessment
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Competent and experienced team, onshore and offshore
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Preparation – testing, trial fits, site integration tests
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Right assets
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Conclusion
Fast track repair of FPSO mooring legs can be carried out safely and
effectively, without the use of divers, and with the FPSO in place.
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Acknowledgements
DOF Subsea wish to recognise and thank OMV New Zealand Limited
for permitting the use of the material contained within this presentation.
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Questions ?
Thank you
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