Cathodic Protection Experience In Cook Inlet, Alaska

Abstract

Abstract: Amoco Production Company operates four of fourteen platforms in the Cook Inlet of Alaska. Amoco's ten years of underwater corrosion and cathodic protection experience in the Cook Inlet environment are reviewed. Tests show that 90-100 ma/sq.ft. are necessary to polarize the Cook Inlet structures where high tidal velocities are prevalent. Further, impressed current densities of 60 ma/sq.ft. maintain the cathodic protection as evidenced by the presence of a cathodic scale in the lower portions of the splash zone of the structure legs. An underwater corrosion coupon program was run to verify underwater protection and effects of structure shielding are described. Field data on protection are also confirmed by laboratory data. Introduction: The Cook Inlet is a marine estuary near Anchorage, Alaska. Amoco Production Company operates four of the fourteen offshore drilling and production platforms located in Cook Inlet. These four structures are jointly owned by Amoco, Atlantic Richfield, Phillips, Skelly and Standard of California. Platform Baker was installed in 1965, while Platforms Anna, Bruce and Dillon were installed in 1966. At the Amoco-operated platforms, water depths range from 65' to 110' (mean low water) with tidal ranges as high as 30' (-5' to +25' MLLW). Peak tidal currents are 10 FPS (7 MPH). The Cook Inlet is characterized in winter months by large floating pans of ice generally l' to 2' thick and as much as ½ mile across. Seasonal variation in fresh water run-off dramatically affects the estuary turbidity and salinity. Glacial silt concentrations as high as 440 ppm usually make the water opaque. Salinity varies as much as 22% between summer and winter. Recognizing further that the water temperature varies from 28° F in the winter to 55° F in late summer, the observed resistivity of the Cook Inlet waters is from 0.43 ohm-meters at 28° Fin the winter to 0.29 ohm-meters at 55° F during periods of peak run-off in the summer. The waters of Cook Inlet are also oxygen-saturated to the ocean floor year-round. Platform Construction: Jackets of the four Amoco-operated platforms are of similar construction. As depicted in Figure 1, the structures are four-legged with cross-bracing located well below the water surface to minimize ice loading. 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 the atmosphere. Finally, the legs were provided with a ½" thick sacrificial corrosion wrap of A-36 steel in the tidal splash zone. This material is external to the structural portion of the leg. Average submerged unpitted structure surface areas vary-from 23,000 sq.ft. on Platform Bruce to over 50,000 sq.ft. on Platform Baker. All calculated current densities have been based upon unpitted surface areas although the actual surface areas can be expected to be significantly greater due to pitting.