Abstract:

The research on utilizing offshore jacket platforms in the East China Sea as offshore CO₂ sequestration and injection platforms is a critical component for the implementation of the East China Sea CCS project. Therefore, it is particularly important to perform accurate fatigue analysis and calculation for the existing jacket platforms. The spectral fatigue analysis method, based on the assumption of linearity of both structural system and wave loading mechanism, can effectively capture the randomness of environmental wave conditions when calculating wave loads and the subsequent structural responses on offshore jacket structures. This method is commonly utilized for the design of offshore jacket platforms and is also suitable for the reassessment of existing jacket structures. Although this method is highly suitable to be applied in offshore platform design and fatigue analysis for deepwater scenarios where nonlinearities between wave height and wave force are not very severe, it has still been widely utilized for the design and assessment of shallow water jacket platforms without carefully considering the possible calculation errors due to strong nonlinear factors between wave heights and wave forces. These errors primarily result from difficulties in choosing appropriate wave heights for a series of wave periods required for producing transfer functions between random wave spectra and structural stress response spectra, due to the significant nonlinearity between wave heights and wave forces mentioned above. This study focused on the potential calculation errors in the spectral fatigue analysis method for shallow water platforms. It presented optimized calculation results from research aimed at reducing the errors in the spectral fatigue analysis method and proposed a new technical approach. This approach could produce more accurate transfer functions between sea state spectra and structural stress response spectra by using the joint probability density function of wave height and period, thereby reducing errors in the spectral fatigue analysis of shallow water platforms and enabling rational utilization of individual local sea state data. Therefore, fatigue damage and service life at the tubular joints of offshore jacket structures could be calculated more accurately. The improved spectral fatigue analysis was applied to the design assessment of jacket platforms in the East China Sea. The results of the fatigue analysis were compared and discussed with those obtained by conventional spectral fatigue analysis methods. In addition, the wave probability density function used for computing fatigue damage in the existing design software was only effective for the narrow-band spectra, and it caused additional errors for the broad-band spectra. This study also discussed improvements in the calculation of fatigue damage for the broad-band spectra. This improved spectral fatigue analysis method can be applied to the structural fatigue life analysis and calculation of the offshore CO₂ injection retrofitted jacket platforms.

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