Fatigue vulnerability of sea cage to storm wave loads

Severe damage and losses during extreme marine events are the primary hindrance for the development of offshore cage culture. Fatigue rupture subject to cyclic wave loads is the major failure mechanism of HDPE sea cages. The internal stress generated by hydrodynamic loads could be much lower than the material yield strength but results in anisotropic cumulative damage. Present work explores structural vulnerability of sea cage to storm waves by theoretical analysis, hydro-elastic modeling, storm wave simulation, and post-disaster damage validation. The collar stress, tension of net pen, and mooring line are correlated to the ambient wave height and have direct implications for the required fatigue loading cycles with respect to given material strength. The flotation collar pipe was identified as the most fragile component with the lowest fatigue thresholds. The stress reaching 73% and 62% of ultimate rupture strength corresponds to the probable structural failure within 8–16 min and 1–3 h during a typhoon event. Degradation of HDPE due to oscillations, oxidation, and bio-erosion causes acute decrease in its status quo strength with increase in service time. Most of the typical circular fish cages deployed along China’s coastline may not withstand a storm with 4–5 m significant wave height, which is quite lower than the manufacture standard. This study provides a practical methodology to predict storm damage risk for mariculture or other coastal ocean facilities subject to cyclic hydrodynamic loads before preventive measures should be taken.