A Retrospective of Platform Development in Cook Inlet, Alaska

Abstract

ABSTRACT Some twenty years ago a number of unique offshore platforms were installed in Cook Inlet, Alaska. These structures are, to date, still the only oil and gas platforms operating in an ice environment. Although originally designed for a field life of twenty years, higher oil prices, and also the discovery of field extensions, now make it likely that several of these platforms will remain in place for as long as fifty years. The paper provides a retrospective of the design and installation methods that were developed for these platforms and describes the inspection procedures and corrosion preventive measures that have been taken to prolong the life of these structures. INTRODUCTION During the early 1960's four oil fields and one gas field were developed in Cook Inlet, Alaska. These fields, see Figures 1 and 2, were placed on production with fourteen self contained drilling and production platforms, see Table 1, which were installed in a four year period from 1964 to 1968. A fifteenth platform was added in 1986. The rapid development of these five fields was an extraordinary accomplishment because environmental conditions in Cook Inlet make this area one of the more difficult offshore areas to operate in. The combination of ice, tides, current, bitter cold and earthquakes made the design, fabrication and installation of these platforms a formidable technical challenge. One reason why this development was able to proceed as rapidly as it did was that twenty years ago there still existed an aura of cooperation between the oil industry, the responsible governmental agencies, and the public. It is questionable whether the same type of development could be accomplished today in two or three times the length of time, if at all. The discovery of the immense Prudhoe Bay field on the North Slope of Alaska shortly after the initiation of production from Cook Inlet has tended to overshadow these accomplishments and has made this area, in a remote corner of the world to begin with, a bit of a forgotten offshore area. Yet, even today, some 20 years later, these Cook Inlet platforms are still the only fixed offshore drilling and production platforms anywhere in the world that operate in an ice environment. The initial development plans for the Cook Inlet fields anticipated an economical field life of about 20 years and the platform designs were based on this assumption. These twenty years, however, have now passed and, because of new field extensions and secondary recovery projects, it now appears that several of the fields have a further remaining economic life of 25 years or more. This implies that the platform structures will also need to remain in place for this period of time because replacement, at least with current oil prices, would not be economical. That this platform life extension is possible is attributable to, amongst others, the conservative design criteria assumptions that were used, and also to an early recognition that special measures were needed to arrest a high rate of corrosion of the underwater portions of the platform structures.