Cathodic Protection Experience In Cook Inlet, Alaska

Abstract

Underwater corrosion and cathodic protection experience in Cook Inlet, Alaska, is presented. Laboratory work, corrosion coupon studies, and field observation demonstrate that 90 to 100 milliamp/sq ft is needed to polarize the Cook Inlet structures where high tidal velocities prevail. Impressed currents of 60 milliamp/sq ft are required to maintain adequate cathodic protection against the cathodic scale found in the lower splash zone of the protection against the cathodic scale found in the lower splash zone of the structure legs. Introduction Amoco Production Co. operates four of the 14 offshore drilling and production platforms located in Cook Inlet, Alaska. These four structures are owned jointly by Amoco, Atlantic Richfield Co., Phillips Petroleum Co., Getty Oil Co., and Chevron U.S.A., Inc. Platform Baker was installed in 1965, while Platforms Anna, Bruce, and Dillon were installed in 1966. Water depths at the structures range from 65 to 110 ft (mean-low low water [MLLW]) with tidal ranges as high as 30 ft (-5 to +25 ft MLLW). Peak tidal currents are 10 ft/sec (7 miles/hr). Cook Inlet is characterized in winter by large floating pans of ice about 1 to 2 ft thick and as wide as 1/2 mile. These ice floes significantly affect corrosion and cathodic protection because corrosion products are eroded away continuously during ice conditions. The ice also precludes the use of protective measures that require physical installations, such as anodes and wiring, in the ice-impacted area. Seasonal variation in fresh-water runoff dramatically affects the water's turbidity and salinity. Glacial silt concentrations as high as 440 ppm usually make the water opaque, causing cumbersome underwater inspection operations. Salinity varies as much as 22 percent between summer and winter. Recognizing that the water temperature varies from 28 degrees F in the winter to 55 degrees F in late summer, the observed resistivity of Cook Inlet water varies from 0.43 ohm-m at 28 degrees F in the winter to 0.29 ohm-m at 55 degrees F during periods of peak runoff in the summer. The water of Cook Inlet also is saturated to the ocean floor with oxygen all year. These conditions create an operational environment that is unique in offshore operations. Amoco's 12 year's experience in this environment have been valuable. This paper summarizes Amoco's experience, describes conditions to other investigators active in Cook Inlet, and provides a general description of current trends of design criteria. Platform Construction Platform Construction Jackets of the four Amoco-operated platforms have similar construction. These structures are four-legged with cross-bracing located far below the water surface to minimize ice loading (Fig. 1). The jackets are constructed of three types of material, including Kaisaloy 50-MV high-strength steel in the submerged high-stress areas and Sheffield Lo-Temp steel in the high-stress areas exposed to atmospheric temperatures. The legs are provided with a 1/2-in.-thick, sacrificial corrosion wrap of provided with a 1/2-in.-thick, sacrificial corrosion wrap of A-36 steel in the tidal splash zone. This material is outside the structural portion of the leg. Average submerged, unpitted-structure surface areas vary from 23,000 sq ft on Platform Bruce to more than 50,000 sq ft on Platform Baker. All calculated current densities, however, were based on unpitted surface areas, although the actual surface areas may be greater because of pitting. Surface areas also vary significantly, as much as 4,400 sq ft during the 30-ft tides. JPT P. 531