中等波陡长峰不规则波作用下单桩周围波场研究

摘要

单桩基础是海上风电行业应用最多的基础型式，且为适应更深水域中更大容量的风电机组，其直径预计在近期还将继续增大。大直径单桩使单桩附近海上作业的规划面临新的挑战，因为此时已不能再把单桩视为“透明”结构。因此，为保证作业安全，需要计入单桩存在效应的、准确的桩周波场估计方法。本文针对长峰不规则入射波作用下的单桩周围波场（WFAM）开展研究，通过物理试验改变波陡与波浪绕射数，考察不同不规则波要素下的波场特性，从而拓展了以往主要关注单桩爬高的试验研究。首先，把试验所得单桩周围波场的线性传递函数（LTF）与线性理论相比较，发现二者在高能频段吻合良好；而在低频段与高频段线性相干较小，说明这些频段的非线性效应显著。其次，通过超越概率分析与有效值计算考察水面高程时间序列，发现单桩周围波场的波峰超越概率与线性理论并不相符，而波高超越概率则可用线性理论较好地预测。此外，绕单桩顺时针／逆时针传播、而非沿径向传播的第 II 类波，对波峰特性有明显影响。

Abstract

Monopile foundations are the most built foundations in the offshore wind industry. The diameter is also expected to increase in the near future to accommodate larger wind turbines in deeper water depths. The large monopile diameter imposes additional challenges in planning marine operations near the monopile, as the monopile can no longer be treated as a transparent monopile. Therefore, proper wave field estimation around the monopile that also accounts for the monopile existence is needed to assure the safety of the operations. An investigation of the wave field around a monopile due to long crested irregular incident waves is provided in the present study. An experimental study was performed with different irregular wave properties, varying the wave steepness and the wave diffraction number. The present study extends the available experimental studies that focus primarily on the runup of the monopile. Initially, Linear Transfer Functions (LTFs) of the wave field around a monopile from the experimental study were compared to the linear theory and found to match well in the high energy frequency range. Small linear coherences are also seen in the low and high frequency ranges, indicating pronounced nonlinear effects in those frequency ranges. Further, the elevation time series is investigated via exceedance probability analysis and significant value computation. It is found that the crest exceedance probability of the wave field around a monopile does not correspond to the linear theory, while the wave height exceedance probability can be predicted well using the linear theory. Moreover, Wave Type II, which travels in the clockwise/anti-clockwise direction around a monopile and not in the radial direction, influences the crest properties.