Increased emphasis on offshore operations and the trend to deeper waters have resulted in platform costs ranging from $2 to $4 million becoming commonplace. More exotic structures for use in 600 to 1,200 feet of water and costing $10 million plus are being designed. This magnitude of investment placed in hostile offshore environments makes control of corrosion mandatory. Corrosion is controlled with cathodic protection in the submerged portions, with protective wrappings in the splash zones and with coatings in the atmospheric portions. Principal developments inrecent years have been improved coatings, built-in long-life galvanic systems and permanently mounted impressed current systems. Platform designs which minimize the number of members decreasing the area to be maintained and streamlining the surfaces to better accommodate coatings and cathodic protection have aided in controlling corrosion. Present technology should be adequate for deeper waters or for floating or underwater facilities projected for the future; however, improvements in methods and materials are needed for added reliability. Further application of improved coatings and expanded use of impressed current systems should lower costs in the future.
The expansion of the offshore petroleum industry into deeper waters throughout the world is a major challenge to corrosion engineering. Offshore Louisiana operations have moved from 30 to 50 foot depths into 300 feet of water. In California's Santa Barbara channel exploratory wells are being drilled in over 1,200 feet of water. Investments for platforms which require protection are large. In the Gulf of Mexico, platforms designed for hurricane conditions cost some $2.5 million for the 200-foot depth and costs of some $10 million are projected for 600-foot depths. In contrast, "ice flow" resistant designs in about 100 feet of water for Cook's Inlet represent investments of up to $20 million. Platforms must provide service to support wellheads and production equipment for extended periods of time and can tolerate very little progressive deterioration. Corrosion protection systems can represent up to 10 percent of total platform investment. Marine environments are hostile. As shown in Figure 1, three separate zones exist: the immersed, the splash, and the atmospheric. Corrosion rates for the Gulf of Mexico may be 25 mils per year (.001 inch penetration = 1 mil) for the immersed zone, up to 55 mils per year in the splash zone, and 5 to 10 mils per year in the atmospheric zone. While surveys show these rates to be similar throughout the world, other problems related to corrosion systems are vastly different. Tides, water temperature and velocity, and wave forces all affect the design of the corrosion control system. The immersed zone is protected cathodically. Splash zone conditions are so severe that wraps of noncorrosive materials or sacrificial steel are required and the atmospheric zone is protected using coatings. Because of the severity of offshore environments, superior methods should be used, and planning, surveillance and maintenance of control systems must be complete.
Good corrosion control begins with structural design. Throughout the structure, simple, continuous shapes should be used, eliminating sharp edges, holes, crevices or internal corners that may be inaccessible to sandblasting, coatings, and even electrical current.