Time-dependent reliability analysis of mooring lines for fish cage under corrosion effect

Moored fish cage structures are forced to move into the offshore area due to the shrinking availability of sheltered near-shore sites and increasing environmental impacts of aquaculture, and this provokes a significant challenge for the reliability design of mooring system. The uncertainties involved in the predictions of applied loads and structural strength are analyzed for calculating the reliability level of mooring system. In this study, a probability corrosion model is used to describe the uncertainty of corrosion depth and the joint probability density of significant wave height and spectral peak period is selected to depict the randomness of sea states. The non-linear finite element model is developed to investigate the minimum breaking strength of mooring chains and is validated through comparing with the formulas providing by the International Association of Classification Societies (IACS) Unified Requirements concerning for Materials and Welding. A validated numerical model is applied to calculate the tension force of mooring system for both the single-cage system and the multi-cage system at several sea states, and the response surface method is utilized to display the limit state function to calculate the reliability level in terms of the uncertain metocean variables. The results indicate that the reliability level decreases significantly with the increasing corrosion depth and the correlation of corrosion model for mooring chains has a remarkable impact on the failure probability of mooring system. In addition, the failure probability of mooring system for the four-cage system is much higher than that for the single-cage system.