A practical long-term hydro-structure calculation framework for global responses of gravity-type cages under design waves

The present study proposes a practical calculation framework for long-term hydro-structure analysis of gravity-type fish cages under design wave actions. For global extreme responses of fish cages, a two-part distribution model is introduced to accurately extrapolate the short-term extreme responses and quantify the short-term variability, where the Generalised Pareto distribution provides a simple and effective model performance tuning for describing the response distribution tails compared with the core-only distribution model. The fatigue damages of fish cages are estimated using a deterministic method by replacing irregular wave simulations with a set of equivalent regular waves to avoid computing costs, while maintaining a reasonable estimation reliability. The rationality and feasibility of the methodological framework are demonstrated through a case study. The results of the analysed case indicate that the short-term variability of extreme responses of the fish cage is non-negligible and must be considered in determining appropriate design safety levels. By comparing the results with those obtained using an equivalent regular wave design method, the regular wave analysis tends to underestimate the extreme internal force responses of the collar and net twine, while overestimating the mooring force. Furthermore, the collar is more susceptible to fatigue failure risk compared to the net twine and mooring line, especially at the lower and upper flanges.