A Retrospective of Platform Development in Cook Inlet, Alaska

Abstract

Summary About 20 years ago a number of unique offshore platforms were installed in Cook Inlet, AK. To date, these structures are still the only oil and gasplatforms operating in an ice environment. Although originally designed for a20-year field life, higher oil prices and the discovery of field extensionsmake it likely that several of these platforms will remain in place for as longas 50 years. This paper reviews the design and installation methods developedfor these platforms and describes the inspection procedures and thecorrosionprevention measures that have prolonged the lives of these structures.prolonged the lives of these structures. Introduction During the early 1960's four oil fields and one gas field were developed in Cook Inlet, AK. These fields (Figs. 1 and 2) were placed on production with 14self-contained placed on production with 14 self-contained drilling andproduction platforms (Table 1), which were installed during 1964-68. Anotherplatform was installed in 1986. The rapid development of these five fields wasan extraordinary accomplishment because environmental conditions in Cook Inletmake it one of the most difficult offshore areas in which to operate. Thecombination of ice, tides, current, bitter cold, and earthquakes made thedesign, fabrication, and installation of these platforms a formidable technicalchallenge. This rapid development was possible 20 years ago because an aura ofcooperation existed among the oil industry, the responsible governmentalagencies, and the public. It is questionable whether the same type ofdevelopment could be accomplished today in two or three times the length oftime, if at all. The discovery of the immense Prudhoe Bay field on Alaska's North Slope shortly after production from Cook Inlet began has overshadowedthese accomplishments and made this remote corner of the world an almostforgotten offshore area. Yet, even today, the Cook Inlet platforms are the onlyfixed offshore drilling and production platforms anywhere in the world thatoperate platforms anywhere in the world that operate in an ice environment. Theinitial development plans for Cook Inlet fields anticipated an economic fieldlife of about 20 years, and the platform designs were based on this assumption. These 20 years, however, have passed and, because of new field extensions andsecondary recovery projects, it now appears that several of the fields have afurther remaining economic life of 25 years or more. This implies that theplatform structures must also remain in place for this period of time becausereplacing place for this period of time because replacing them, at least atcurrent oil prices, is not economical. That this platform life extension ispossible is attributable to the conservative design-criteria assumptions thatwere used and to an early recognition that special measures were needed toarrest the high corrosion rate of the underwater portions of the platformstructures. This paper provides a retrospective of the design and installationmethods developed for these unique platforms and describes the inspectionprocedures and corrosion prevention measures that have been taken to preventionmeasures that have been taken to prolong the lives of these structures. prolongthe lives of these structures. Environmental Conditions Cook Inlet is located in southern Alaska (Fig. 1) and penetrates 170 miles[270 km] inland from the Gulf of Alaska. Table 2 summarizes the area'senvironmental conditions. Because of the shape of Cook Inlet and its northernlatitude, tides are among the highest in the world, ranging up to 30 ft [9 m]. The resulting high currents keep the water turbulent most of the time, and somuch sand and glacial flour is in suspension that underwater visibility is nil. Upper Cook Inlet, the area of oil activity, is covered with ice during thewinter months. Ice can be expected as early as November and as late as May. Theinlet is ice-free the remainder of the year. During periods of ice cover, thetides move the ice periods of ice cover, the tides move the ice up and down theinlet at essentially the same speed as the water current. Bottom and subsurfacesoil conditions vary greatly, ranging from soft unconsolidated clays on thewest side of the inlet to boulder-covered, extremely stiff clays on the middleand the east side. Cook Inlet is in the Pacific earthquake belt and, asevidenced by the widespread damage from the 1964 Good Friday earthquake, structures in the inlet are subject to potentially large earthquake loadings.potentially large earthquake loadings. Environmental Forces Winter sea ice is the principal environmental force imposed on Cook Inletstructures. In 1963, when the first oil field was discovered, very littleinformation was available on the characteristics and strength of the sea icegenerated during the winter months in Cook Inlet. P. 146