Fatigue Assessment Of Deep Water Catenary Moorings

Since there is a need for energy sources, the deeper water is more and more attended by people. The deepwater platform is usually in position by mooring system which combined by mooring lines. With incessantly variable motion of the platform caused by various kinds of horrible ocean conditions especially by wave loads, alternated stress is produced in the mooring line. With the number of alternated stress cycles increasing, fatigue damage will be caused. The industry has experienced several mooring line failures over the years. Therefore, the investigation of fatigue damage is of great significance.Spar platform has been fully studied for the gas and oil exploration in deepwater in recent years. Mooring lines in a typical mooring system which can be designed to hold the vessel on location are the important component commonly used by the offshore industry. Now, there are few studies on the fatigue damage analysis of deepwater mooring line. The purpose of this paper is to present a rational method for analyzing, designing or evaluating the fatigue damage of deepwater mooring line. These study could serve as reference in the deepwater mooring line design and have a significant impact on the theory and technical developments of the deepwater engineering.Take the representative single mooring line of the Deep Draft Multi-Spar platform as the fatigue research object, and the mooring line are made up of chain-wire-chain. The environmental condition in South China Sea is defined as that combination of wind, waves, and current for which the fatigue damages of mooring line are evaluated. Two environmental directions at 0 degree and 90 degree are supplied in this case, and every long-term sea state can be divided into 56 short-term sea states. The analysis of Spar platform response is carried out in which the wave force is calculated under diffraction theory. The response of the mooring line is calculated under nonlinear finite element method. The stress time histories of the three connecting links between the top of chain and topside, chain and wire on the mooring line in each short-term sea state are calculated, and then the rain flow counting method is employed to obtain the fatigue load spectrum in each short-term sea state. The Miner linear cumulative rule is used to compare the fatigue damage of the three connecting links in long-term sea state, and then the fatigue damages of mooring line is evaluated. The results show that the connecting links between the top of chain and topside, bottom chain and wire ropes on the mooring line can be subjected to more additional fatigue damages than others, and precautionary measures should be taken to avoid mooring failure due to fatigue damages in the fatigue design.