Platform Design and Construction in Cook Inlet, Alaska

Severe icing from every direction for 8 months of the year, -40F temperatures, 30-ft tides and 60-mile winds, 8-knot currents, zero visibility under water and an earthquake from time to time - and still Platform C stands fast in the Middle Ground Shoal field of Cook Inlet, Platform C stands fast in the Middle Ground Shoal field of Cook Inlet, Alaska.

Introduction

The conditions that make offshore oil operations such a hazardous and costly venture are nowhere more evident than in Cook Inlet, Alaska. In this offshore area (Fig. 1 ), ice, tides, current, bitter cold and earthquakes combine to produce one of the greatest challenges in offshore oil engineering encountered to date.

The challenge of this new offshore frontier began in the fall of 1963 with the discovery by Shell Oil Co. (as operator for SAS - itself, Atlantic Richfield Co., and Standard Oil Co. of California) of the Middle Ground Shoal field. The discovery well was drilled from floating drilling equipment, as were several other exploratory wells in the Inlet. With the discovery of commercial production, however, it became necessary to provide facilities for year-round drilling and production. It became readily evident that the production. It became readily evident that the conventional Gulf-Coast templet type of platform could not be turned in this environment and that a radically new kind of platform was required to withstand the winter ice. The tower-type structure with only four large-diameter legs protruding through the ice zone seemed to fit these conditions best.

The first one of this new generation of platforms was designed and installed by Shell as operator for the SAS group during 1964 in the Middle Ground Shoal field. It was followed by several other platforms in the same field and in newly discovered other fields (see Fig. 2), so that at the end of 1968, only 5 years after the discovery of the first oil field in Cook Inlet, there were 14 self-contained platforms in operation.

This rapid development was all the more remarkable because little or no information was initially available with which to establish design criteria to counteract the extremely great lateral forces from the massive ice floes that form in the winter. Considerable initiative, ingenuity and determination on the part of numerous individuals from both the oil companies and the contractors were required to overcome these engineering unknowns and make the Cook Inlet venture a success.

Environmental Conditions

Cook Inlet, in southern Alaska, penetrates some 170 miles inland from the Gulf of Alaska (see Fig. 1). Although weather conditions are somewhat tempered by the proximity of the ocean, the average annual temperature is only about 35F. Minimum ambient winter temperature is minus 40F.

Due to the particular shape of the Inlet and its nonhem latitude, tides are among the highest in the world and range up to 30 ft. The resulting high current, 10 to 12 ft/sec, keeps the water turbulent most of the time, and so much sand and glacial flour is in suspension that underwater visibility is nil. Water temperatures range from a high of 55F in the summer to a low of 29F in the winter.

Upper Cook Inlet, the area of oil activity, is covered with ice during the winter months.

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