Abstract: In order to improve the effect of damage diagnosis for offshore platforms, and accurately determine both location and degree of damages, an improved diagnosis method based on modal strain energy was worked out, with which the mistakes of multi-judgement and mis-judgement happening in the traditional analysis and processing of modal strain energy can be avoided. Firstly, the modal strain energy based indexes under actual condition were calculated for the elements. Then the indexes were decomposed into several single damage condition indexes, and the weight coefficients of each single damage condition index to actual condition index were extracted and calculated. Finally, the diagnosis indexes were corrected according to the distribution difference of weight coefficient in different modes, and used to diagnose the damage. Finite element simulation tests of an indoor jacket offshore platform model were performed to verify the improved method. The results show that the damage diagnosis reached by using the method is consistent with the multi-damage conditions of the platform structure, which proves the method is effective and feasible for offshore platform damage diagnosis.

Key words: offshore platform, damage identification, modal strain energy(MSE), improved diagnostic index, damage degree