ABSTRACT
Minimal structures of jacket platforms are commonly applied in development of marginal offshore oil and gas fields. It is subjected to continuous cyclic wave loads during services, causing fatigue damage to the platform structures. In design of this kind of structures, assessments of fatigue damage become particularly important. Spectral fatigue analysis approach is a computationally efficient method 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,but it has still been widely utilized for the design and assessment of shallow water jacket platforms with strong nonlinear mechanism. Therefore, the paper focuses on the studies on the improvement of the accuracies of the fatigue calculations by a new technical approach that can reduce the errors in the spectral fatigue analysis of shallow water platforms. This new approach can reasonably reflect the individually local sea state data by using wave height-period joint probability density function, the discussions of comparisons between the improved approach and the design code oriented method carry out by means of the analysis results. In addition, the wave probability density function which 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 broadband spectra during the computation of the fatigue damage. The appraisal on improving the calculation of the fatigue damage for the broadband spectra also carries out in the paper.
Fatigue analysis plays an important role and has become compulsory in the design of offshore jacket platforms since fatigue failure is one of the major reasons causing the welding joint defects of offshore jacket platforms(Bishop, Sherratt, 1989). According to the international authoritative institutions in offshore platform, joint damage of the structures, about 70% above is caused by fatigue. The spectral fatigue analysis method has many advantages compared with other fatigue analysis methods, such as deterministic fatigue analysis and time history fatigue analysis etc(Pierson, Mosskowitz, 1964). The major reason that the spectral fatigue analysis method is recommended is computationally efficient, assessing structures where the response can be assumed to be linear, Gaussian, stationary and random.
