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When an aged mooring system seeks a life extension, it is necessary to assess the remaining fatigue life of the corroded mooring chain. This paper summarizes the results of fatigue tests performed on mooring chain samples retrieved from six different fields in West Africa and North Sea. The impacts of corrosion on fatigue life on the samples were researched. The tests were managed under a Joint Development Project, “Fatigue of Corroded Chains (FoCCs JDP)”. The objectives of the JDP are (1) to derive a methodology for assessing the remaining fatigue life of corroded chain, (2) to develop guidance for performing reliable FEA of chain links to assess remaining fatigue life, and (3) to provide more rational basis to improve industry guidance on mooring line replacement criteria for life extension. Fatigue test procedure was defined by the fifteen (15) participating members. The procedure specified the testing parameters, including mean tension, tension range, and test frequency. Six sets of fatigue tests have been completed in seawater with the number of cycles to failure recorded. These chain samples were retrieved from floating production and storage units, e.g. FPSOs and FSUs, that were still in service. Fatigue data obtained from the tests were plotted against the design SN curves and results from fatigue testing of new chain. It was found that most of these samples have limited amount of fatigue capacity remained. Most interesting finding is that the sharpness of the surface feature on the corroded chain link has a significant impact on the remaining fatigue life. Another interesting finding is that the surface feature created by corrosion can be quite distinct and unique depending on the geophysical locations where the sample came from. These findings and test results may serve as references for life extension assessment of an aged mooring system.

For the design of offshore structures it is important to accurately predict wave runup and thus avoid topside inundation and minimize the wave impact on the underside of the deck structure. In this paper a three-parameter probability distribution function for nonlinear wave run-up amplitudes is presented. It builds upon previous studies and utilizes the quadratic transformation of incident waves. The parameters of this probability distribution are estimated from the data using method of L-moments and the explicit relation between the parameters and L-moments is presented. The L-moments themselves are linear combinations of ordered data and consequently they are less influenced by outliers and unexpectedly large values. Earlier theoretical models, based on simplified diffraction theory, are presented and compared with the L-moments model. A three-parameter Weibull distribution model that utilizes the method of L-moments is derived and discussed. Run-up measurements from a mini-TLP model test program are used as the basis for comparison of the three methods. This study demonstrates that the new empirical model and Weibull distribution are more robust in representing the probability distribution of nonlinear runup amplitudes especially for the weakly nonlinear cases with moderate steepness. Although the new empirical model and Weibull distribution have different probability structure their estimates are found to be fairly close.