Study On Remaining Fatigue Life Of Deepwater Semi-submersible Platform

Deepwater semi-submersible platform with complicated large structure and high cost iswidely used in deep sea energy exploration and often works in extremely bad environment.Security incidents of offshore platforms will not only bring great economic loss, casualtiesbut also bring serious pollution of environment. So fatigue life of deepwatersemi-submersible platform is necessary and significant to be studied.In this paper, the fatigue life of a deepwater semi-submersible is studied based on ABSOffshore Structures Fatigue Assessment Rules. Firstly, according to the global strengthanalysis, the high sress areas of the node positions are selected as key joints.The finiteelement models of the key joints are established. The strength analysis of the key joints iscalculated and fatigue hot spots are found. Then the fatigue life of the key joints is studiedbased on S-N curves and fracture mechanics, which both considering corrosion damage andcrack damage every5years. The main research conclusions of this paper are listed below:(1) The remaining fatigue life of the platform is calaulated based on S-N curve methodand fracture mechanics method every5years. The corrosion damage and crackdamage is considered. The conclusion is that the remaining fatigue life is reducedwith the service time increasing. So high-frequency repair and maintenance is neededfor the aging platform to prevent sudden accidents.(2) The remaing fatigue life becomes shorter when considering corrosion damage andcrack damage, which agrees with the actural working environment.(3) Semi-submersible platform is welded structure and cracks are inevitable.The remaingfatigue life is reduced because of crack propagation with the service time increasing.The fracture mechanics method is more reasonable to forecaste the fatigue life of theplatform.(4) The connection of the column and the brace of the semi-submersible platform is theimportant joint. It suffers from higher cyclic load and it has lower fatigue life thanthe connection of the column and the upper deck. It determines platform service life.So the routine repair and maintenance is espically needed for the aging platform.