Strategies for Fatigue Design of Deepwater Platforms

Abstract

After reviewing strategies used for designing the 1000-ft Cognac platform, this paper will describe recent developments, which have been applied to preliminary designs of fixed platforms in up to 1600 ft of water, and to compliant towers in 2000 ft. The first is a strategy in which the waterline geometry and tower dimensions are coordinated so as to minimize wave forces at the natural frequency. This was first suggested by John Nath in 1967, and has come to be known as the “Nath hole” strategy [1]. The second is rationalization of allowable hot spot stress in terms of the Weibull shape parameter, “squiggly.” For structures which have been optimized as described in the foregoing, the trend of fatigue versus natural period can be much less severe than described by the author at the BOSS-82 conference [2].