Dynamic responses analysis for accidental collision scenarios of floating offshore wind turbine

Abstract

Offshore wind energy has been becoming one of the most attractive alternatives for fossil fuel energy in recent decades. Like all other offshore structures, offshore wind turbines are subjected to ship collision hazards as well. However, little attention has been paid to the ship-collision problem, especially for the floating offshore wind turbines (FOWTs). The FOWT is a typical rigid-flexible coupled multi-body system, and the investigation of the dynamic responses of FOWTs under ship collision scenarios is crucial and challenging. This thesis research conducts a comprehensive analysis on both external and internal dynamics of a Spar-type FOWT under ship collision scenarios, and two innovative methods are newly proposed. Firstly, by combining an analytical model of ship collision and a FOWT simulation tool, DARwind, a novel integrated method for predicting global dynamic responses of FOWT is proposed. In this integrated method, the analytical model is an extended implementation of a ship collision model, based on the conservation of momentum principle. DARwind program is an aero-hydro-servo-elastic coupled nonlinear analysis tool for FOWTs. The integrated method can analyze and predict the global dynamic responses of FOWTs during the ship collision scenario, which include the 6DOF global motions, the tower vibration, the mooring tension, the blades tip deflection, and aerodynamic performance, etc. Based on the development of the integrated method, cases of studies are conducted. Head-on collision cases in various environmental conditions are studied. The responses of global motions, mooring tensions, nacelle accelerations, blade tip deformation, and aerodynamic loads are investigated. It is found that the impact velocity can significantly influence motions and the mooring systems, especially in still water conditions. In wave conditions, the tower top deflection shows obvious change after ship collisions. Additionally, in wind-wave conditions, the edgewise blade tip deformation (inside the rotor plane) is found to be more sensitive than that in flapwise (outside the rotor plane). To further assess the safety of FOWT, the acceleration of the nacelle is examined, because the electric equipment might be sensitive to the axis acceleration. Secondly, for purpose of investigating the internal dynamics of FOWTs during ship collision scenarios, a novel fully coupled method based on the Nonlinear Finite Element Method (NLFEM) is also proposed. This method employs the user-defined subroutine in LS-DYNA code to model the hydrodynamic, aerodynamic, and mooring loads. These loads are varying over time during the simulation and are updated every 100 timesteps in LS-DYNA structural analysis to maintain accuracy and efficiency. The hydrodynamic loads are calculated in a specific way, with linear potential-flow theory, and the mooring loads are evaluated with a simplified linearized model while the aerodynamic loads are calculated based on a look-up table for thrust coefficients. This newly proposed method is able to calculate the external and the internal dynamics of FOWTs simultaneously under ship collision scenarios. With the proposed fully coupled method, cases of studies have been conducted. The influences on the impact force and structural deformation from factors such as impact velocity, tower flexibility, deformability, and environmental loads are investigated and discussed. The external dynamic responses predicted by this coupled method are also compared with those from the integrated method described above. Good agreement has been achieved in low-energy impact scenarios. But for the high-speed energy impact scenarios with critical structural damage, the accuracy of the integrated method is not very satisfactory because some basic assumptions in the method are violated. This brings some further work. In conclusion, two novel methods are newly proposed during the research of this Ph.D. project. The proposed methods are reliable to assess the dynamic performance of FOWTs under ship FWOT collision scenarios considering the wave-wind effects. The results comprehensively reveal the responses of FOWTs under ship collision scenarios from the views of both external and internal dynamics, which could help to understand the ship-FOWT collision mechanisms better. Additionally, due to the lack of ship-FOWT collision studies, this work could supplement or benefit the engineering practice in the industrial fields, such as ship-FOWTs collision modelling, crashworthiness design, especially for the large-size FOWTs which enhance the maintenance challenges.