In this study, a rigid-base single-degree-of-freedom structural model was proposed for the wind turbine with a linearly variable cross-section tower. Additionally, the concentrated spring model was introduced to simulate the flexibility of grout connection at the transition piece as well as the monopile foundation for structural modelling of an offshore wind turbine. Thus, the close-form solution of its natural frequency was accordingly obtained and was verified satisfactory. This simplified model was then used to exhibit the influence of the grout connection failure and the foundation scour on the natural frequency of wind turbine structures, showing its applicability to damage detection.
Energy has always been the driving force of national development and economic growth. For environmental protection and sustainability, countries all over the world are devoted to the exploitation of renewable energy. The wind power, which accounted for 24% of the total capacity of renewable energy in 2017 (IRENA, 2018), continues to play a more important role among all the renewable energy resources. Because of the enormous area and abundant wind energy in the ocean, the focus has been turned to the offshore wind power. At the end of 2017, the global offshore wind power capacity reached 19 GW, with an increase of 8 GW in 2016–2017. In Taiwan, the fossil fuel resources are rare and the supply is dependent on import, leading to the urgent need for renewable energy. Fortunately, Taiwan Strait has been recognized to have excellent wind resources, and thus offshore wind power has become one of the focuses of renewable energy in Taiwan. The goal is to reach an offshore wind power capacity of 520 MW in 2020 and 5.5 GW in 2025.
An offshore wind turbine could be subjected to the actions of wind, waves, currents and earthquakes, and the corresponding loads have to be transferred safely to the seabed through the support structure, including the tower, transition piece and foundation, as shown in Fig. 1. Therefore, the support structure must have adequate bearing capacity and displacement control ability. Stehly, Heimille and Scott (2017) indicated that 14% of the capital expenditures of a fixed-bottom offshore wind project is related to the support structure, not yet including the installation cost. Therefore, the design of support structure is one of the important issues in the offshore wind turbine design. If the design is insufficient, the safety and serviceability of the wind turbine cannot be assured; while if the design is too conservative, the installation cost will be increased and the profit may be thus reduced.