november 2017, st. andrews decomissioning conference...
TRANSCRIPT
IOGP Report Decommissioning Of CGBS
November 2017, St. Andrews Decomissioning Conference
John Gillies
Joint Interest Project Manager
ExxonMobil International Limited
• International Association of Oil & Gas
Producers
• Members produce 40% of global oil and
gas in 80 countries
• Share knowledge and good practices
• Official “Observer” status at OSPAR
• Decommissioning Committee papers on
Subsea Bundles & Concrete Gravity Base
Structures (CGBS)
2
About IOGP
• 27 structures (12 each UK/Norway)
• Northern North Sea, >90m water
• 17 have been used for oil storage
• Typical 300,000 tonnes (>ten times
more than large jackets)
3
CGBS in OSPAR Area
Ownership (equity tonnage)
4
• Most clustered close to UK/Norway
median; International cooperation essential
• Non OSPAR ownership similar include
Canada, Russia, Australia, othersStatoil
Petoro
Total
ExxonMobil
Shell
Centrica
Taqa
CoP
CNR
Fairfield
Gassled ENI
Apache
Chrysaor
Mitsubshi
Hess
others
Construction Scale & Context
5
• 10 million tonnes installed 1970-1990
• ~20 times weight of all OSPAR derogation
jackets
• 0.5 million truck loads if recovered to shore
• Total aggregate ~1yr. production from UK’s
largest quarry
0
2
4
6
8
10
12
Installed weight by period
Regulatory Framework
• OSPAR 98/3 framework
• 4 derogation requests approved to date
(one oil storage), four pending
Full Removal
• Weight and geometry far exceeds available
lifting vessel, draft exceeds deepest ports
• Re-float feasibility not demonstrated:
1. Re establishing integrity
2. Seabed resistance & withdrawal suction
3. Stability during de-ballasting
• 1m thick, 20m diameter, 100 wire tendons 250 tonnes tension each
• Lifting: legs 8k-tonnes each; dual crane (Thialf, S7000)
Partial Removal
Leave In Place ?
• Leg Collapse ~ 250 years?
• Caisson substantially intact 600-1000 years?
• Collision risk <1:10,000 year event
• Toppling legs feasible with 500 tonne bollard pull
• Quantity and fate of contents
Progress Since 2013 Update
Significant knowledge added:
• Survey, sampling and attic oil removal
developed and proven
• Fate models developed
• Feasibility of leg cutting tested
• Leg lifting concepts developed