Abstract No.C181018-278

Author name(s): Qin FENG

Company: SINOPEC Offshore Oilfield Services Company, China

Spectral fatigue analysis approach is commonly utilized for offshore jacket platform design, which is also effective for the reassessment of jacket structures. This method is computationally efficient to describe the random nature of environmental ocean wave conditions while calculating wave loads on the offshore jacket structures and subsequently structural responses. Its fundamental theory is based on the assumption of linearity of both structural system and wave loading mechanism. Although it is critically appropriate to be applied to offshore platform design and fatigue analysis of deep water scenarios where wave and force nonlinearities are not very onerous, the method has still been widely utilized in the design and assessment of shallow water jacket platforms without carefully taking possible errors into account due to strong nonlinear factors between waves and forces. The source giving rise to the errors is because of the difficulties in choosing suitably correct wave heights for a series wave periods required for producing transfer functions between sea state spectra and structural response spectra. Therefore, the studies focus on the possible errors of the spectral fatigue analysis method of shallow water platforms. This paper presents the results of the investigation of the errors caused by the spectral fatigue analysis method. A new technical approach that can reduce the errors in the spectral fatigue analysis of shallow water platforms is proposed to produce more realistic transfer functions between sea state spectra and structural response spectra, which can reasonably reflect the individually local sea state data by using wave height-period joint probability density function. Hence the fatigue damage and life at the tubular joints of offshore jacket structures can be more precisely calculated. The improved spectral fatigue analysis has been applied to the design assessment of the jacket platforms in East China Sea. The results of the fatigue analysis are compared with the results of normal spectral fatigue analysis method and are discussed. In addition, the wave probability density function that is employed for computing the fatigue damage in the existing design software is only effective for the narrow band spectra, and it causes additional errors for the broad band spectra. The discussions on improving the calculation of the fatigue damage for the broad band spectra are carried out in the paper.

KEY WORDS: fatigue; spectrum; offshore; platform; jacket; wave

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