Members forming the frame of an offshore jacket are prone to localized damage caused by supply boat collisions, objects dropped from the deck, corrosion and the like. The analysis of the locally damaged jacket has to be carried out for the design wave loading towards checking the intact strength of the jacket and for deciding the repair measures. The nonlinear analysis of the jacket for the in-service wave could be economically carried out using super elements since this method would confine the changes needed m each iteration to those superelements containing the damaged members. The analysis of a ten-bar truss has been used as a vehicle to illustrate the mechanics of the analysis strategy.
The behaviour of offshore jackets, even under in-service storm loadings for which they are designed, is nonlinear. The non-linearities arise due to the physical nonlinearities of the foundation soil, though the steel jacket is essentially linear. However, when a few of the jacket members get dented due to collision with moving objects, the behaviour of these damaged members becomes nonlinear. The physical nonlinearitles introduced In the jacket are thus due to the few damaged members only when the jacket is analysed under In-service design wave loadings. The analysis of the as-constructed jacket under design wave loadings becomes mandatory whenever divers discover denting In a few Jacket members during Inspection. The maintenance of the jacket calls for the determination of the Intact strength and the extent of repairs to the extant Jacket for restoring It to the designed strength. The nonlinear analysis of the jacket has also been described by Ueda, Rashed and Nakacho (1985) by an idealised structural unit method. In this paper, the superfinite element analysis has been suggested for the nonlinear analysis of the jacket for in-service wave loads.
