This paper proposes the practical application of a reliability based procedure for fatigue inspection planning of fixed offshore structures. This procedure is based on a simplified probabilistic crack growth model for welded tubular joints available in the literature, which encompasses fracture mechanics and S-N curves and uses the results of the original fatigue design. Current techniques for reliability updating are implemented to re-evaluate the fatigue failure probability after inspection campaigns. Target reliability values are defined to be compatible with the simplicity of the procedure. Actual applications illustrate the inspection costs reduction that can be obtained.
Fatigue is an important limit state in the design and operation of steel offshore structures. The fatigue design is usually based on Miner's rule and S~N curves but alternatively a fracture mechanics approach can be employed. There are several uncertainties concerning its evaluation. In-service inspections, using nondestructive tests, are planned and performed in order to assure an adequate safety level and to gather more information about the fatigue process. Fatigue inspection results can be basically summarized in detection or no detection of cracks. Until recently, fatigue inspection planning was based mainly on engineering judgment and usually the results of previous inspections were not accounted for the next ones. However, since the ~ reliability analysis has become a practical and widely spread tool this situation has changed. Some important works on probabilistic inspection planning of offshore structures have been published recently (e.g., Madsen et al., 1987; Kirkemo, 1988, Madsen et al., 1989; Jiao, 1992). These works take into account inspections results and show that it is possible to establish a rational fatigue inspection planning. Relevant practical results of probabilistic inspection planning can be seen also in Pedersen et al. (1992).
