With a large government offshore leasing program established for Alaska, hydrocarbon discoveries are likely in the near future in arctic waters. This paper describes a unique fixed offshore platform concept for shallow, coastal, ice infested waters. The concept is intended for the anticipated early development areas having these conditions.
The substructure consists of four conical legs extending from interconnected cylindrical bases resting on the seabed. It can be either of a gravity or gravity plus piled type depending on the local soil conditions. A conical leg section is used below the depth reached by an ice sheet having the maximum natural thickness for the area. This configuration is designed to fail thick ice features in flexure. In the depth region of the natural ice sheet formation where adfreeze may occur, a cylindrical leg section is used to limit the area exposed to ice pressure.
The deck is fabricated, fully equipped, outfitted and tested in a graving dock. It is mated with the substructure as an integrated unit and provides enclosed space for extensive facilities. Since all of the facilities are installed during onshore construction, offshore work is minimized.
The deck is transported on a single large barge to a protected inshore mating location and mounted on the substructure. Mating is accomplished by deballasting the substructure after the barge is maneuvered between the legs of the substructure. The barge is removed and the structure is then towed to the installation site as a single unit. At the installation site it is ballasted down and the bases of the substructure are grouted to the seabed.
The concept has been developed analytically to demonstrate its viability. Important criteria used in the design effort are presented in the paper. The design process has considered all major phases of platform construction and operation. These include the facilities design, structural design, foundation design, ice force analysis, shell design and transportation and installation analyses.
The occurrence of sea ice is probably the most significant environmental factor associated with northern waters. In locations where winter sheet ice forms, the ice forces will generally govern the design of an offshore structure. Sea ice limits normal surface transportation to a few months annually. It is expected to severely hamper the logistics involved in both the construction and operation of remote offshore platforms. Sea ice is inherently hazardous. Routine design methods for handling ice forces on offshore structures have not been established. These and other factors increase the risk associated with offshore arctic operations.
The arctic structure concept consists of an integrated deck mounted on a substructure having four conical legs. Figure 1 shows a general view the concept in its final configuration. The integrated deck provides a complete drilling and production facility developed to minimize costly offshore hook-up. The legs of the substructure are designed to resist sea ice forces efficiently.
