Fatigue Life of Offshore Steel Structures Under Stochastic Loading

The fatigue life of offshore steel structures under stochastic loading is studied. Of special interest is the problem of fatigue damage accumulation, and in this connection, a comparison between experimental results and results obtained using current codes and specifications. Fatigue test series are performed, with various types of stochastic loadirtg that are realistic in relation to offshore structures. Four different load spectra, with irregularity factors ranging from $\sim 0.70-1.00\text{,}$ have been applied. The experimental investigation comprises test series on both full‐scale tubular joints and smaller weided test specimens. The materials that have been used until now have been ordinary offshore structural steels, with a yield stress of J. Strain Analysis $f_y\simeq 360-410\mathrm{MPa}\text{.}$ The test series show a significant difference between constant amplitude and variable amplitude fatigue test results. For the welded plate test specimens, approximately 100 fatigue tests have been carried out, and the values of the Miner sum that, were obtained in the variable amplitude test series, generally vary in the range 1/3–2/3. On the basis of these results, a modified fatigue damage accumulation formula is proposed. For the full‐scale tubular joints, the number of test results is currently too limited to draw final conclusions. However, the results obtained until now indicate a value of the Miner sum of $M\sim 0.7-0.8$ for the variable amplitude tests.