1. Introduction
These days, wind became more popular as one of the renewable energy sources. It is sustainable, widely-distributed, and eco-friendly. In addition, the wind power system does not produce any greenhouse gas emission during the operation. Because of these reasons, the energy resource based on conventional fuels primarily used in the past has recently been replaced by wind energy by a significant portion. According to Wind Europe’s Central Scenario [
1], 106 GW of wind power capacity was installed in Europe between 2006 and 2016. In the same period, the US and China have installed 71 GW and 156 GW, respectively. The EU targets for 323 GW of cumulative wind energy capacity by 2030, including 253 GW and 70 GW onshore and offshore, respectively. This is equivalent to 29.6% of the EU’s total electrical power demand.
The wind energy market has so far been dominated by onshore wind turbines installed on the ground. However, onshore wind has a limited potential due to its relatively poor quality and the lack of turbine install location. Accordingly, offshore wind power systems attract more attention these days, since offshore wind is stronger and is more consistent than onshore wind. Moreover, construction of large-scale wind farms is more feasible in offshore. Consequently, a variety of offshore wind turbine concepts are currently under development around the world.
Offshore wind turbines are classified into either bottom-fixed or floating depending on the install location [
2]. Bottom-fixed wind turbines have a fixed foundation on the seabed, and are generally installed in shallow water areas of up to 60-m in depth. Meanwhile, floating wind turbines are mounted on a platform floating on the water, and are usually installed in the deep-water area where bottom-fixed wind turbines are not feasible. Floating offshore wind turbines have an extensive potential compared to bottom-fixed types, since water depth or seabed condition is not a consideration for a site selection. However, they are continuously under the influence of wave and wind, which can cause movement of the platform. The consecutive oscillation resulting from the platform motion not only alters the aerodynamic performance of the turbine, but also leads to a structural vibration of the turbine components such as the blades, shaft, and tower.
Several studies have been previously undertaken to accurately predict the aerodynamic performance of floating offshore wind turbines considering the platform motion. Vaal, Hansen, and Moan [
3] investigated the effects of a periodic surge motion on the integrated loads and the induced velocity of the wind turbine rotor by using a blade element momentum theory coupled with a quasi-steady wake model and a dynamic inflow model. Tran and Kim [
4,
5] conducted numerical simulations of a floating offshore wind turbine under a periodic platform surge and pitch motions at various frequencies and amplitudes, and examined the effect of vortex-wake-blade interaction. Tran and Kim [
6] also investigated the dynamic response of a floating offshore wind turbine by using a coupled aero-hydrodynamic approach. In their study, the aerodynamic loads of the rotor, the platform dynamic response, and the mooring line tension were calculated.
Even though some meaningful results were obtained from the previous studies [
3,
4,
5,
6] for the aerodynamic load prediction of floating offshore wind turbines, the rotor blades were assumed to be rigid, and the effects of elastic blade deformation were not properly considered. Rodriguez and Jaworski [
7] developed an aeroelastic framework for evaluating the impact of the blade elasticity on the near-wake formation and its linear stability for onshore and offshore wind turbine configurations. However, in their study, only flapwise bending deformation was examined based on a linearized elastic structural model, and the blade deformations in the other directions were not considered. Since elastic rotor blades deform in every axial, lead-lag, flapping, and torsional directions, and they are all non-linearly coupled together, it is important to consider those deformations simultaneously [
8]. Torsional deformation changes the blade effective angle-of-attack, and, thus, has a significant influence on the blade aerodynamic loads and the overall rotor performance.
In the present study, the rotor aerodynamic performance, the blade elastic behavior, and the mutual interaction of the two were investigated for a floating offshore wind turbine under periodic platform motions. To calculate the aerodynamic loads produced by the rotor blades, a blade element momentum theory was applied. For the prediction of the elastic blade deformations, a structural model was developed based on a nonlinear Euler-Bernoulli beam undergoing axial, lead-lag, flapping, and torsional deformations. The aerodynamic and structural analyses were conducted simultaneously in a tightly coupled manner by exchanging the information about the aerodynamic loads and the elastic blade deformations at every time step. The motion of the floating offshore turbine was prescribed by the six degrees-of-freedom (6DoF) platform motions. At first, the calculations were conducted for a fixed-platform wind turbine, and the aerodynamic loads and the elastic blade deformations were examined under various operating conditions. Next, the aerodynamic load variations of the rigid rotor blades were calculated when the platform is in periodic motions. Lastly, the effects of periodic platform motions on the aerodynamic performance and the aeroelastic behavior of the flexible rotor blades were investigated, along with the blade root bending moments. Since the tower and the platform are relatively more rigid compared to the blades, the effects of the flexibility of the two turbine components were not considered in the present study.
4. Conclusions
In the present study, a numerical methodology for predicting the aerodynamic performance and the aeroelastic behavior of floating offshore wind turbine rotor blades involving platform motions has been developed. The aerodynamic and structural analyses were carried out in a tightly coupled manner considering the transient flows caused by the platform motions. The structural model for predicting the elastic blade deformation is based on a nonlinear Euler-Bernoulli beam undergoing axial, lead-lag, flapwise, and torsional deformations. The blade element momentum theory was applied to calculate the aerodynamic loads on the rotor blades. Applications of the present method were made for the NREL 5 MW reference wind turbine under various operating conditions.
At first, the elastic deformations and the aerodynamic loads of bottom-fixed wind turbine rotor blades were calculated for various wind speeds. The results showed that the rotor blades experience fairly large elastic deformations, and the clearance between the tower and the blades can be reduced significantly due to the flapwise deformation. It was confirmed that consideration of blade elasticity is essential in the aerodynamic analyses of large-scale wind turbines. Next, the aerodynamic load variations of a floating offshore wind turbine under various platform sinusoidal surge motions were investigated. The rotor blades were assumed to be rigid bodies. The results were validated by comparing with other predictions by FAST and a CFD method. It was found that the rotor aerodynamic loads are significantly affected by the platform motion.
Lastly, the effects of platform motions on the aerodynamic performance and the aeroelastic behavior of a floating offshore wind turbine were investigated. The study was made for the cases when the platform is in sinusoidal oscillatory motions in each of three translational and three rotational directions. It was observed that flexible rotor blades exhibit complex vibratory behaviors under the various exciting loads, including the aerodynamic, inertia, and gravitational forces. In particular, the flapwise blade deformation is strongly affected by the aerodynamic force variation associated with the platform motion. The nose-down torsional blade deformation tends to reduce the aerodynamic loads. The flapwise root bending moment of the blade is also affected by the platform motions, especially in surge and pitch. In the edgewise direction, the gravitational force is the dominant factor to the root bending moment, while the effect of platform motions is minimal.
It was concluded that the present method is well-established, and is suitable for predicting aerodynamic loads and aeroelastic deformations of floating offshore wind turbine rotor blades involving platform motions.