Accurate modeling of low-frequency (LF) wave loads and responses is important but challenging in the design of spar-type floating offshore wind turbines (FOWTs), especially for shallow and intermediate water depths. A 6 MW spar-type FOWT system designed for a water depth of 100 m is investigated both experimentally and numerically to study the nonlinear hydrodynamics responses in waves. The second-order LF wave loads are modeled in the time-domain tool FAST using two different methods based on full Quadratic Transfer Functions (QTF) and Newman's approximation, respectively. To account for viscous effects in calculating the LF responses, linear and quadratic damping models calibrated from model tests are comparatively studied and the latter is found to be more suitable and easier to implement in the time-domain simulations. It is demonstrated that it is crucial to include the 2nd order difference-frequency wave excitation in the analysis of motions in both horizontal and vertical planes. With the considered water depth, a full QTF method is seen to greatly improve the prediction of LF responses compared with Newman's approximation. In general, it is observed that the wave-frequency (WF) responses are higher in amplitude than the LF responses for surge and heave, while the LF pitch motions are higher than the corresponding WF contribution.

低频（LF）波浪载荷和响应的精确建模非常重要，但在设计翼梁式浮式海上风力涡轮机（FOWT）时尤其具有挑战性，特别是对于浅水和中等水深的情况。通过实验和数值研究了设计用于水深100 m的6 MW翼梁式FOWT系统，以研究波浪中的非线性流体动力响应。在时域工具FAST中使用分别基于完全二次传递函数（QTF）和Newman近似的两种不同方法对二阶LF波载荷进行建模。为了在计算低频响应时考虑粘性效应，对通过模型测试校准的线性和二次阻尼模型进行了比较研究，发现后者在时域仿真中更合适且更易于实现。结果表明，在分析水平面和垂直平面中的运动时，必须包括二阶差分频率波激励。在考虑了水深的情况下，与纽曼近似法相比，使用完整的QTF方法可以大大改善LF响应的预测。通常，可以观察到，对于喘振和起伏，波频（WF）响应的幅度高于LF响应，而LF俯仰运动高于相应的WF贡献。