The most exposed area for fatigue loading in an offshore platform is the area where the deck connects with the shafts of the base structure. The fatigue loading of the deck is becoming more and more often the governing factor of the design. A feasability design study has been carried out to determine if concrete is a material for use in offshore deck structures. The conclusion is clear: Concrete trusses are a good alternative to steel for deck structures, especially when fatigue loading is the governing factor.
The design of platforms for the North Sea has been moving steadily towards use in larger depths. At the same time, the decline of oil prices since 1986 has caused a demand for less expensive structures. This has lead to platforms which are more slender and less stiff than were the first Condeep structures, and therefore more sensitive to cyclic loading. For a number of years, steel has been the obvious material for use in the deck structures, primarily because of its relatively low weight. A secondary reason is because truss structures in steel give high flexibility with respect to the placement of equipment between the trusses. The fatigue loading of the deck is becoming more and more often the governing factor of the design. This, in turn, has lead to the search for new solutions, for example, methods to uncouple the fixed connection between the deck and the shaft without sacrificing safety or increasing maintenance costs. The company Dr.techn. Olav Olsen a.s has been largely involved in the design and development of concrete offshore structures in Norway, and the experience so far is that concrete has high resistance against fatigue loading response, for instance at shaft top where the concrete meets the steel in deck structure.