Environmental contour method is an efficient method for predicting the long-term extreme response of offshore structures. The traditional environmental contour is obtained using the joint distribution of mean wind speed, significant wave height, and spectral peak period. To improve the accuracy of traditional environmental contour method, a modified method was proposed considering the non-monotonic aerodynamic behavior of offshore wind turbines. Still, the modified method assumes constant wind turbulence intensity. In this paper, we extend the existing environmental contour methods by considering the wind turbulence intensity as a stochastic variable. The 50-year extreme responses of a monopile-based offshore wind turbine are compared using the extended environmental contour methods and the full long-term method. It is found that both the environmental contour method and the modified environmental contour method, with the wind turbulence intensity included as an individual variable, give more accurate predictions compared with those without. Using the full long-term method as a benchmark, this extended approach could reduce the nonconservatism of the environmental contour method and conservatism of the modified environmental contour method. This approach is effective under wind-dominated or combined wind-wave loading conditions, but may not be as important for wave-dominated conditions.

Water wave impact on a wet deck is an important issue in ocean engineering, and the plate-shaped structure in the splash zone tends to suffer tremendous impact loads. This work presents a method for predicting the wave slamming uplift force on a fixed plate with different inclined angles. Both numerical simulation and the scale model test of the wave impact loads on an inclined plate were performed, and a good agreement was obtained. In addition, the influence of three important wave parameters on the slamming uplift force was systematically investigated: relative deck width B/L_S, relative wave height Δh/H_1/3, and the plate’s inclined angle α. The results indicate that the three parameters can significantly influence the wave slamming uplift force. Finally, a developed empirical equation is proposed for estimating the wave slamming uplift force on the inclined plate.

A new type of offshore wind tension leg platform (TLP) connected with a series of buoys (Serbuoys-TLP) is proposed. With the consideration of coupling effect, derivations on the stiffness of the Serbuoys-TLP mooring lines are given. The complicated coupling motion characteristics of the TLP with buoys system are investigated by means of experiments and numerical analysis. The suppressive efficiency on the surge under some condition is nearly 68%, when the wave period is the common wave period of the East China Sea (6 s). Namely, the suppressive effect of series buoys on surge motion response of TLP is analyzed. Through several aspects of suppressive effect on the surge including wave properties, submerge volume and position of buoys are investigated. The modal analysis method is also adopted to interpret the coupled motion response. In the end, the responses of TLP and Serbuoys-TLP are simulated under actual sea conditions with the consideration of wind, wave and current. Based on the parametric study using the modal analysis combined with hydrodynamic analysis, the conclusion can be drawn that the surge of TLP can be effectively suppressed by the addition of a series of buoys in the Serbuoys-TLP.

In the present work, a crashworthy device for a monopile offshore wind turbine has been proposed, which consists of the inner two-layer rubber torus and the outer thin steel shell. The performance of the crashworthy device against ship impact has been investigated experimentally. Based on the prototype of a 4 MW monopile wind turbine in the East China Sea, the scale ratio of the test model has been designed to be 1/50. The test ship model has been simplified as a “rigid car” equipped with a high-frequency force sensor in the front, which is available for changing the ship mass with different weights. The ship-impact velocity can be accurately controlled by a motion platform driven by a direct current machine. The effect of the key design parameters of the crashworthy device on its anti-impact performance has been tested and compared under typical ship impact cases. The results indicate that the crashworthy device can effectively reduce both the ship impact force and the top nacelle acceleration, and the physical mechanism that has been clarified. The outer thin steel shell can significantly use its structural deformation to absorb the ship impact energy, which is beneficial for reducing the structural damage of the offshore wind turbine (OWT)’s tower. The inner rubber torus can effectively prolong the ship impact duration, which is available for smoothing the impact force. Finally, the porous design for the outer steel shell of the crashworthy device has been proposed and tested.