Fatigue Analyses in Fixed Offshore Jackets using Spectral Method

Fatigue, is defined as structural failure under alternative load cycles in a long period of time. This experience is important when cyclic loads exert to the structures. In this study a model was developed to estimate fatigue life under irregular waves for fixed offshore jackets using S-N curves and Palmergen-Miner method. In this study valid codes and most authentic international regulations, such as ABS and API codes of practices were employed. The inputs to the model are wind velocity, wave spectral density and structural details including S-N curves, type of connections and welding situation in different joints. The model can estimate the fatigue life of the structure based on the worst environmental condition. A case study carried out for a typical jacket in Persian Gulf. A dynamic analysis using SAP2000 was undertaken with an exciting force extracted from a proposed spectrum for sea state of the study area. The results showed that the spectral analysis of fatigue life is more accurate than time domain analysis using zero- up crossing technique. The result of the model also shows promise for optimum design of steel offshore jacket type platforms choosing proper type of connections and high quality welding in joints.