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

Thank you