Development of a Novel Ice-Resistant Semisubmersible Drilling Unit

ABSTRACT A multiyear program was initiated by ARCO Alaska, Inc. to assess the operational feasibility of drilling operations year-round in the ice-covered waters of the Bering, Chukchi, and Beaufort Seas. ARCO Alaska, Inc. is considering several alternative concepts for year round drilling in the Bering Sea. One such concept, the Ice-Resistant Semisubmersible Drilling Unit, is a design concept of Mitsui Engineering and Shipbuilding Company. The design is intended to operate in broken, continuous, and ridged sea ice, and withstand severe open water sea conditions. The requirement to operate in two dissimilar environments results in a unit that is somewhat unusual when compared to typical semisubmersible drilling units. INTRODUCTION Mitsui has many years of ice technology experience. In order to meet the engineering challenge in the ice-covered seas, Mitsui is carrying out various research and development programs of its own with the cooperation of Japan Oil Corporation and Japan Foundation for Shipbuilding Advancement. Mitsui's R&D activities are not restricted to theoretical studies and research at its office, but are carried out in ice-covered Okhotsk Sea coast and Saroma saline lagoon in Hokkaido, a northern part of Japan, where in winter, the sea is covered by fast ice and pack ice. ARCO Alaska and Mitsui have embarked upon a joint development agreement to develop a unit based in both background technologies. The ARCO Alaska-Mitsui program comprises five phases:Preparation of a preliminary proposal by Mitsui.Preparation of environmental criteria and unit design criteria by ARCO Alaska.Preliminary design of the Ice-Resistant Semisubmersible by Mitsui.Physical model detailed design, construction, and testing in open water basin and ice basin to confirm unit performance predicted for the preliminary design of Phase 3.Design confirmation for the preliminary design of Phase 3 based on the evaluation of Phase 4 results by the two companies. DESIGN CHARACTERISTICS The Ice-Resistant Semi submersible resembles typical semisubmersible designs but with important differences:Although it is somewhat larger than the average semi submersible, about the same as the largest new designs, it has no struts or bracing members which pass through the waterplane. This minimizes interaction with the ice.Each of its four columns has a compound inverted cone-shaped section at the top. The riser protector also has a similar conical shape. In ice, the unit is ballasted so that these cones are at ice level.The unit is designed to operate as a bottom supported unit in water depths up to 65 feet. This capability increases the unit's range of water depth operations. The design is based on four columns with minimum interconnecting bracing to facilitate passage of ice through and around the unit. Downward-compounded cones are included at the top of the columns to minimize ice loads at the design ice operating draft, and a riser protector is included. Two operating drafts are provided for the two dissimilar environments. In open water the operating draft is 21.5m (70.5 ft), and the column shape is cylindrical to minimize wave induced motions.