The Arctic Cone Exploration Structure: A Mobile Offshore Drilling Unit for Heavy Ice Cover

Abstract

INTRODUCTION The offshore oil and gas industry has been required by circumstances in recent years to place increasing emphasis on the very hostile, relatively remote, oil provinces. The Alaskan Beaufort Sea, ranks near the top of the list of new offshore development areas in terms of both remoteness and the harshness of its environment. Nevertheless, this area and the nearby Chukchi Sea offer among the best prospects for major oil finds of the U.S. Outer Continental Shelf waters remaining to be explored. Two lease sales have been held to date in the Beaufort Sea offshore Alaska's North Slope, the most recent being OCS Sale No. 71, which was held in October 1982. The first major offshore sale in this area took place in December 1979. The majority of tracts leased in the 1979 sale are in water depths of less than 50 ft and can be explored using gravel islands. However, over 70% of the tracts made available for Lease Sale 71 are in water depths beyond 50 ft. A similarly high percentage of tracts scheduled for future lease sales in the Beaufort and Chukchi Seas are in depths beyond the range of economical development from gravel islands. Even within the 50 ft depth contour, multi well drilling programs do not look attractive from gravel islands. Industry has responded to this situation in the last year and a half by offering designs for a number of different caisson based mobile drilling units. These have generally been designed for water depths between the range of 25 and 70 ft and for operations in relatively sheltered waters where they will not be exposed to the most severe ice conditions. This paper reports on the results of a rig development program which began in 1981 to design a mobile drilling unit which could operate beyond the 50 ft depth contour in the most exposed ice conditions. The Arctic Cone Exploration Structure (ACES) project has produced the design of what is likely to be the prototype for heavy-duty, bottom-founded mobile rigs for arctic offshore drilling. The current ACES rig design has focused on developing the drilling capability for the water depth range of 50 to 110 ft. Approximately 68% of tracts made available during the recent OCS Sale 71 fell within this depth range. The latest phase of the ACES rig design was sponsored by Exxon Company, U.S.A.; Shell Oil Company; and Standard oil Company of California/Chevron. An earlier conceptual design phase of the program included three additional U. S. oil and gas companies. Brian Watt Associates, Inc. (BWA) of Houston, Texas, was the prime contractor for the design. Zapata Off-Shore Company of Houston was responsible for the drilling related systems. The objective of this paper is to show the level of effort which has gone into the development of the ACES mobile rig design. The results of that effort are presented, including the design criteria and performance objectives used.