WATCHING THE WORLD ABANDONING FLOATERS

With Roger Vielvoye from London Advocates of floating production systems have always made a great play of the ease with which an oil field developed with a floater can be abandoned. And just to emphasize the point, Britain's first North Sea oil field to run out of economic steam was abandoned earlier this year in just 32 days-10 days ahead of schedule.

Sept. 2, 1991

3 min read

Advocates of floating production systems have always made a great play of the ease with which an oil field developed with a floater can be abandoned.

And just to emphasize the point, Britain's first North Sea oil field to run out of economic steam was abandoned earlier this year in just 32 days-10 days ahead of schedule.

Crawford field in Block 9/28a was not one of Hamilton Bros.' best offshore investments. The North Sea Pioneer semisubmersible production vessel was commissioned in 1987, but production from the small, complex reservoir never lived up to expectations. Production finally stopped at the end of last year.

If abandonment can be handled quickly and efficiently, an operator still has an asset that can be used in another field or sold to another company. In the case of Crawford, Hamilton sold the recovered system to Ghana National Oil Corp.

Abandonment was undertaken in two stages by Stena Offshore of Aberdeen and sister company Stena Drilling with two dynamically positioned vessels working in tandem for the first time.

Tom Ehret, managing director of Stena Offshore, said field abandonment will come to the fore and prove to be a significant issue during the next 5 years as fields that started production in the 1970s become uneconomic.

Ehret says there are two issues to be considered:

When does a field become uneconomic?
How can abandonment be carried out quickly and adeptly enough to recover subsea equipment that can be sold to another user?

The debate over marginal utility of fields has just begun, he said. A new generation of contractors believes the combination of lower overheads and technologically advanced vessels and rigs will enable them to continue work in a field long after it has been deemed uneconomic by clients and contractors using dated technology.

Stena said teaming up two dynamically positioned vessels provided operational benefits that can be applied to abandonment of similar fields in which floating production vessels have been employed.

The absence of anchor wires made it possible for one of the dynamically positioned vessels, the Stena Constructor diving support vessel, to maneuver easily and quickly on all sides of the dynamically positioned Yatzy drilling rig and take full advantage of the improved operating characteristics on the leeside of the rig.

In the first phase, the Stena companies recovered control risers, laid down production and export risers, and removed the North Sea Pioneer, the single buoy mooring's floating hose, and mooring line.

The second phase involved removal of the single buoy mooring and all seabed equipment including wellheads, flow lines, risers, and control umbilicals, as well as plugging and abandoning five wells in 295-393 ft of water.

Yatzy was responsible for abandoning the subsea wells, while the diving vessel recovered 4.3 miles of pipelines and umbilicals. Stena said the ability of both vessels to maneuver and set up quickly over the various work stations provided maximum flexibility for day to day planning.