Platforms in Cook Inlet are subjected to a severe corrosive environment from the level of high tide to the floor of the Inlet. The Inlet water has tides of 20–30 feet with velocities of 4–8 knots. The turbulent water, laden with silt and dissolved oxygen, maintains a highly depolarizing environment on all steel surfaces submer ged in the water. In winter months, ice with thicknesses up to five feet moves with the tides and erodes the steel surfaces in the tidal area of the platforms.
The platforms have been built with an erosion-corrosion allowance in the tidal zone, but with little or no corrosion allowance below low tide. Protection of the submer ged steel was expected by cathodic protection.
Cathodic protection of the submerged steel has been applied to most structures in the Inlet. The cathodic protection systems designed with criteria established from experience with platforms at other ocean locations have not been effective in control of corrosion. This paper discusses the extent and nature of damage to steel in the tidal area and experience with cathodic protection of the Mobil Granite Point platform. Tests and performance data which demonstrate the requirements for corrosion protection in the Cook Inlet enviromnent are presented. The various factors and principles which support the unusual criteria of current density, geometry and design of an effective cathodic protection system for submerged steel in Cook Inlet are discussed.
Mobil's Granite Point platform is a four-legged tower structure. The legs of the tower are 17 feet O. D. and are placed on 80-foot centers. The legs are connected at the top with 17 feet O. D. circular girders and are braced in the submer ged portion of the tower with fourteen 6 feet O. D. horizontal and diagonal braces as shown in Figure 1. Because of the sub-zero weather environment in the Cook Inlet, low temperature steels (ASTM - A537 Grade A and Grade B) were selected for the major structural components of the platform.
The erosion-corrosion damage in the tidal zone was expected to be significant and a corrosion allowance of 3/8" additional low temperature steel was provided. The additional steel covers the area around the four legs from a distance of 5 feet below to 30 feet above mean low-low water.
The area of the platform legs exposed to the tides (approximately 30 feet) are subjected to two distinct forms of metal attack. During the winter the sheet of ice which flows with the tidal current (4 to 8 knots) is crushed as it moves against the legs. The coating of paint and corrosion products is eroded by the ice and the metal surface appears clean and bright and corrosion is mild. During the summer and warmer months, corrosion attack of the tidal area increases significantly.
The tidal area of the legs has been inspected after each winter by visual inspection and measurements of metal thickness with a sonic thickness gauge. The measurements were taken on a vertical profile of the tidal area facing the movement of ice against each leg and on a horizontal profile around each leg at a level about 5 feet above mean low-low tide.