Abstract
Abstract The spread mooring system for FPSO is developed to explore deep sea area, in which swell is dominant. It is known that thetension response of mooring lines in this sea area shows bimodal spectrum. Assuming normal distribution of tension profile andRayleigh distribution of tension amplitude, the power spectral density function (PSD) of the mooring tension under the bimodalstationary random process is applied for the calculation of spectrum fatigue. Three popular methods, which are simple summation method, combined spectrum method and Jioa-Moan method, are used to combine fatigue damages from bimodal spectrum characteristics. Each damage value is compared with damage using Rainflow Cycle Counting (RCC) method which is believed to be close to exact solution. Vanmarcke’s parameter and RMS(Root Mean Square) ratio are employed to assessrelative damage variations between from RCC method and from three combination methods. Finally the most reliable fatigue damage combining method for spread mooring system is suggested.Keywords : Spread mooring system(다점계류시스템), Spectrum fatigue assessment(스펙트럼 피로 평가), Power spectral density(파워 스펙트럴 밀도), Bimodal stationary random process (바이모달 정상확률과정), Rainflow Cycle Counting(레인플로우 집계)
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