Mooring rope tension and jacket infrastructure safety in integrated system of offshore wind turbine and fish cage

The combination of offshore wind power and aquaculture industry has developed rapidly, which can comprehensively utilize marine space, maximize the benefits of aquaculture and offshore wind power industry, and realize the simultaneous and efficient output of clean energy and high-quality aquatic products. As one of integrated development modes of offshore wind power and cage culture, the integrated system of fish cage and wind turbine using the pile foundation of wind turbines to form the fish cage is exposed to the extreme sea states with severe waves and currents. The hydrodynamic of the fish cage and wind turbine integrated system, as well as the evaluation of the structural safety of the pile foundation, are the basis for the construction of the integrated system. It is of great necessity to conduct a more systematic study on the hydrodynamic characteristics of fish cage and offshore wind turbine pile foundations. In this paper, an integrated system of offshore wind turbine and fish cage was constructed based on the wind turbine jacket of offshore wind power in Liuao, Zhangpu, Fujian Province. The finite element toolbox, Aqua-FE™, was used to investigate the impacts of different deployment depth, wave-current incident angle and percentages of biofouling on mooring rope tension of fish cage in extreme waves and currents. The relationship between the mooring rope tension at the upper end of the upstream side and the deployment depth of fish cage was fitted by nonlinear regression. The effects of waves and currents parameters, as well as mooring line tension of the fish cage, on the stress variations of the wind turbine jacket structure were analyzed by the specialized design software SACS for ocean engineering, and the safety of the wind turbine jacket foundation structure was evaluated. The results show that the increase in the deployment depth of the fish cage could reduce the maximum tension of the mooring rope, significantly improving the stress state of the integrated system. The variation of wave-current incident angles had little effect on the safety of integrated system of offshore wind turbine and fish cage; the rise of the percentage of biofouling significantly increased the maximum tension of mooring ropes, resulting in structural failures of many foundation rods of the wind turbine jacket. In summary, the fish cage can be arranged in an appropriate water depth to reduce stress. At the same time, the fouling organisms shall be cleaned in time, and the thickness of the jacket rod near the mooring point of the fish cage and the pile-soil point rod at the bottom of the jacket shall be appropriately increased to ensure the structural safety of the integrated system. The results can provide data support for further study of the forces in integrated system of offshore wind turbine and fish cage, safety evaluation of the system, and design optimization of the system.