Fatigue Analysis Of New-type Deepwater Mooring Systems

With the exploitation of offshore oil and gas moving gradually to the deep water, the new type taut wire mooring system is being widely used as the alternative of the conventional catenary mooring system. Deep water mooring system is subjected to fatigue loading, Its effect on the mooring system needs to be considered. Due to the speciality of the materials, synthetic fiber ropes show special fatigue properties, including internal abrasion, hysteresis heat, creep rupture, axial compression fatigue. In this paper, domestic and oversea researches on these features and fatigue analysis of mooring systems are summarized and reviewed, theory of fatigue analysis of the mooring systems are introduced, including T-N curve, damage accumulation theory of fatigue, fatigue life prediction procedure, which can supply significant reference for the long term safety assessment study of mooring systerms in our country.Taking the advantage of mooring analysis software Ariane, a taut wire mooring system has been designed and conducted a fatigue analysis for a FPSO in the South China Sea oil fields, the long term sea states are discreted into several short term sea states.Through qusi-static analysis, the two most loaded lines are picked out and a full time domain dynamic analysis is carried out to obtain the tension response of each rope. In this paper tension ranges and cycles are extracted from tension time history based on Rain Flow Counting Method, then the fatigue damage is estimated, the above work is implemented by a program basing on matlab, then the program is used to estimate the fatigue damage and fatigue life of the mooring system. The impacts of different pretensions, T-N curves and dynamic stiffness on the fatigue life of mooring systems are studied. In addition, the fatigue life is computed in allusion to one line damaged state. Through comparing and analyzing the results, some significant conclusions are concluded, which can provide reference for the domestic engineering application of the taut wire mooring system.