BEYOND THE HORIZON: The decommissioning dilemma

We are beginning to see over the North Sea decommissioning horizon. The technical challenges are enormous, and the situation is neither pleasant nor satisfactory. For naval architects and marine technologists, some of the challenges to remove giant redundant steel and concrete structures are awesome.

Whereas the Gulf of Mexico has already witnessed the removal of many hundreds of small steel platforms, the situation in the North Sea is in its infancy. Several platforms have been taken out from southern waters and returned to shore, but the option of dumping at sea is simply not available under the terms of the European Sintra agreement, also known as OSPAR 98/3.

A policy of rigs-to-reefs disposal may have positive environmental benefits in the Gulf of Mexico, whereas in Europe, the Brent Spar fiasco upset oil companies and contractors, and probably has set back programs for the inevitable removal and disposal of North Sea platforms.

In the North Sea, water depths up to nearly 200 meters in the northern sectors have required steel platform jackets weighing up to 30,000 tons, and concrete jackets with a mass of around 1 million tons. The removal of these giant structures will absorb millions of dollars, and perhaps endanger lives despite a rigorous Safety Case standard, and will require technical ingenuity - but to what environmental advantage?

OSPAR 98/3 is a European cuckoo that is unsustainable on a scientific evaluation of environmental problems and a decision more politically driven, than environmentally, and will cost British taxpayers $30 billion to implement.

A labor of Hercules

Removal is not simply a reverse procedure to installation, especially for deepwater platforms. Furthermore, will vessels used 20 or more years ago still be in service or will new ones have to be built or techniques developed to ensure a safe delivery to some reception site ashore?

Steel jackets were installed by three methods: crane lift off a barge, barge launch, or self-flotation and down-flooding. Topsides were installed in units weighing between a few hundred tons and 10,000 tons. The large units were structurally configured to be lifted by twin cranes. De-construction might require subdivision into smaller units and hence a longer time span.

Minimal first cost is now the driver, rather than any gains from earlier onstream oil and gas. Concrete structures pose the severest problems: it may not be so simple to refloat what was delicately ballasted into place like a lunar lander.

Years of soil adhesion must be overcome through a combination of enforced excess buoyancy and under-skirt jetting by water or compressed air. The danger of the "champagne cork" effect is well known, with a possibility of losing control of stability as the structure rises to the surface. Contractors have expressed confidence, but no actual concrete platform has yet been recovered. At the moment, OSPAR 98/3 provides for the possibility of derogation for concrete platforms, but attitudes could harden. It is noteworthy that Norway has not signed the accord.

Reverse procedures

North Sea jackets were installed in a single piece, so can procedures be reversed? We are working against gravity. After 20 years, there will be uncertainties regarding mass and associated center-of-gravity (in three planes) which could have a major impact upon acceptable crane-lift capacity. In turn, that means uncertainty for the stability of associated craft.

OSPAR 98/3 permits derogation for the "bottles," or lowest part of the structure, but at the same time 40 or more steelwork members having substantial scantlings must be cut beneath deep hostile waters. It may be possible to renew self-flotation, if sufficient integrity of a structure can be confidently established. However, a barge launch is not reversible!

The crane vessels used to install and hookup platforms during the 1970s are getting old and some have been converted to new purposes. Ships have a performance different than that of semisubmersibles in terms of operability, lifting capacity, and outreach and upreach of the crane hook. The removal of smaller jackets and their topsides from shallower waters present a different task to those in northerly waters. The future for redundant platforms, whether recycling of materials or re-use at another site, also has a profound impact on the method and procedure chosen for removal.

The new horizon

The North Sea is poised for decommissioning on a grand scale, with early action for fields such as Maureen and Frigg. The deconstruction process will need to continue for at least another 15 years. Potentially, there are over 200 structures to remove, unless many attain derogation status. Crane vessels will be needed in one form or another, but what of temporary buoyancy aids, catamaran lift ships, and submersible barges?

Various formats have been proposed and some schemes are nearing fruition. The prime technical challenge for vessel designers is the ensuring of safe stability at all times as a procedure unfolds. Buoyancy aids might be tried on a smaller platform instead of using a conventional crane barge (some of the naval architecture problems will be reviewed in a later article in this publication).

With regard to these challenges, I am not a computer Luddite! Computers play a valuable role in checking the myriad of parametric values and combinations in a proposed removal scheme. That is the second stage. In novel circumstances, a first stage is to check the feasibility through physical model testing when "what-if" scenarios can be raised and debated between management, sea-going staff, and technocrats. That same testing can be used later for training purposes.

The scale of deconstructing North Sea platforms is a magnitude greater than for the Gulf of Mexico. Is it really necessary to spend as much as $30 billion? Are there cheaper alternatives to crane vessels? Are contractors ready for sustained action in the northerly waters?

William Penney
Associate Professor in Offshore Operations
Southampton Institute, UK