Structural Performance Studies On Offshore Wind Turbine Subjected To Ship Impacts And Earthquakes

Offshore wind power has been in the ascendant during the past decade, becoming the main form of exploration and utilization of green energy resource in the ocean. With widespread planning and construction of offshore wind farms, the safe operation of offshore wind turbines (OWTs) will be effected by the ship collision and earthquake during the service period, and even more serious OWTs collapse could be caused. Therefore, it is of great theoretical research significance and practical value for OWTs safety designs and normal operations to carry out the damage and response study of OWTs with different types of foundation subjected to ship impact and to conduct the model experiment of OWTs on the underwater shaking table.This study is devoted to investigate the performance of OWT due to ship impact and seismic action. Firstly, the relationship between maximum collision-force and ship mass, velocity, collision angle are analyzed based on LS-DYNA, a commercial FEM tool. Secondly, a comprehensive evaluation of anti-collision ability of different OWT foundations is conducted considering various factors, and a crashworthy device is proposed to protect the monopile OWT from being struck by ships with its parameters optimized. Finally, the model experiment of OWT with tripod foundation on the underwater shaking table is carried out. The critical issues and technical problems in the case of OWTs subjected to ship impact and earthquake are systematically and deeply studied in this paper, and some conclusions of theoretical and practical values are obtained as well. The major content of this paper are concluded as follows:(1) Based on the research of collision between ships and monopile of OWTs, the results show that the maximum collision-force increased linearly with the increase of one-third power of mass, velocity and sine of collision angle of the ship in the case of the initial kinetic energy of the ships no more than about 35 MJ,35 MJ and 25 MJ, otherwise the linear relationship is not obvious. The damage rate of area is propsed to quantitatively describe the extent of the damage of the monopile foundation for OWTs, and it can reasonably reflect the damaged area and its size through the calculating analysis.(2) Through investigating and analyzing the maximum collision-force, the damage area, the maximum bending moment of piles at the seabed, the steel consumption and the maximum nacelle acceleration of the collision system between the ship and three types of foundations (monopile, tripod, and jacket) of OWTs, it is found that the jacket generates the minimum collision-force, damage area and nacelle acceleration as well as the medium bending moment and steel consumption. Therefore, the jacket foundation has the optimum comprehensive anti-impact performance.(3) In order to reduce the damage of the monopile OWT caused by ship impact, a crashworthy device, which contains a rubber mat and outer steel shell, is proposed. With the constraint conditions of device weight, the rubber hardness, the rubber thickness and steel shell thickness of the crashworthy device are optimized with consideration of the collision-force and nacelle acceleration. It is found that the most superior crashworthy device features the rubber hardness of shore A 10°, rubber thickness of 1.5 m and steel shell thickness of 10 mm. Based on first-order optimization algorithm, a method with both major structure and accessory structure into consideration is given to achieve integrated optimization design of the crashworthy device, and its flow chart is listed. The result of the integrated optimization design of the crashworthy device is consistent with that of the former.(4) The study of tripod OWT model experiment on the underwater shaking table shows that the maximum acceleration response comes at the top of the middle tower, and higher strain occurs both at the bottom of tower and the ends of brace 1, when the OWT is put into the earthquake excitation. Though wave alone has limited influence to the response of OWT, earthquake and wave together has amplified effect to the response of OWT which cannot be neglected. Hence the coupling effect of wave and earthquake action cannot be ignored.