The petroleum industry has gained much experience in the past decade in exploring and drilling for oil in offshore open water environments all over the globe. Drill ships, fixed platforms and semi-submersible moored platforms have been specially designed and constructed for exploring for offshore oil and producing oil from sub-sea fields.
The Arctic has recently become the focal point of interest because of the large onshore finds on the north slope. The U.S. Government has also predicted large but unproven oil fields on the outer Continental Shelf of Alaska and plans lease sales in the Bering, Chukchi and Beaufort Seas. Should the offshore fields in the Arctic be proven to be abundant, the oil industry will be required to construct offshore platforms which must withstand very large forces due to movements in the ice fields surrounding the structures. As an example, the Beaufort Sea Continental Shelf varies from 50 to 80 kilometers offshore. It consists of a flat shelf and a steep outer shelf deepening to about 36.5 meters at 18.5 kilometers offshore. Ice covers the Beaufort Sea for about nine months of the year. The inner zone of fast ice reaches a thickness of about 2 meters. However, seaward of this zone of fast ice lies Arctic pack ice containing 2 to 4 meter multi-year ice floes.
Conventional platforms designed and used in ice-free waters are incapable of withstanding the large lateral forces generated by large ice fields and ice floes. Moreover, it is beyond the present state-of-the-art to design deep water platforms for Arctic environments because of the yet undefinable character of an Arctic ice field. Arctic ice fields contain not only uniform ice sheets of certain physical properties but also contain single and multi-year pressure ridges which could have drafts as deep as 30 meters.
Currently, there are two proven methods for drilling offshore in the Arctic. These methods involve constructing either a gravel or ice island for use as a drilling platform. Both of these techniques are limited to shallow water depth up to approximately 10 meters. Beyond a 10 meter depth man-made ice or gravel islands become infeasible. Conventional deep water drilling platforms designed to withstand ice forces and overturning moments due to high ice forces would be, indeed, massive and expensive.
Attempts are being made to develop new methods of dealing with the problem of structures in ice-covered waters. One of the most simplistic and promising approaches is to design a structure with sloping sides in a conical configuration. This type of structure takes advantage of the fact that ice is much weaker in bending than it is in crushing by sloping the surface interface, thereby changing the force vector and inducing a bending-crushing mode of failure rather than a uniaxial crushing mode of failure. The resultant forces against the structure vary with the angle of the cone and the friction of the ice against the cone.
USPTO Backgrounds
USPTO Backgrounds