Abstract: The research on converting the existing offshore jacket platform in East China Sea to a platform for offshore CO₂ storage and injection is an important research project for the implementation of the project of East China Sea CCS. Therefore, it is particularly important to perform accurate fatigue analysis and calculation for the existing jacket platforms. Spectral fatigue analysis approach is commonly utilized for offshore jacket platform design and also is effective for reassessment of jacket structures. This method is a computationally efficient to describe the random nature of environmental ocean wave conditions during calculating wave loads on the offshore Jacket structures and subsequently structural responses. However, its fundamental theory is based on the assumption of linearity of both structural system and wave loading mechanism. Although this method is critically appropriate to be applied in offshore platform design and fatigue analysis for deep water scenarios where wave and force nonlinearities are not very onerous, it has still been widely utilized for 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 for shallow water platforms. This paper presents the results of the research of investigating the errors from 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 for 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. This new technical approach of fatigue analysis can be applied in appraisal of the existing jacket platforms for the CO₂ injections in the East China Sea.

Key words: jacket, spectral analysis, fatigue damage, wave loads, probability density