ABSTRACT

As the world is recognising the threat of the climate emergency and recent geopolitical tensions highlight a need for more energy independence, offshore renewable energy development is becoming strategic for many countries. The rapid deployment of offshore wind in the last decades and the ever-increasing industry interest in floating offshore wind turbines suggest that this technology is likely to play a major role in the future energy landscape. Floating wind is expected to unlock access to the most significant resources of offshore wind across the globe, especially to many countries lacking the shallow water necessary for the current fixed offshore wind foundation.

Among the various concept families of floating wind platforms, partly inherited from O&G, the Tension-Leg-Platform (TLP) has the particularity of being restrained by pre-tensioned lines into a position of positive buoyancy. This configuration confers to the mooring system significant importance in resisting aero-hydrodynamic loads which enables a reduction of the size of the platform hull. However, a thorough understanding of the extreme loading cycle in the mooring lines becomes essential in the design process.

Since such a system is very stiff in the vertical degrees of freedom, it is sensitive to high-frequency loads which can generate a resonant response. The literature has indeed highlighted the contribution of high-frequency wave loads, such as third-order ringing loads, to the risk of slack-line events which can be critical for the structural integrity of the foundation system. Predicting these loads through high-fidelity time-domain approaches such as fully non-linear Boundary Element Method (BEM) or Navier-Stokes CFD is expensive and hardly suitable for a stochastic design process. Therefore, access to lower-fidelity engineering numerical models that can predict high-frequency wave loads with a sufficient degree of accuracy is important for the deployment of TLP technologies. While frequency domain BEM (Boundary Element Method) solvers can be used to calculate second-order diffraction forces (via Quadratic Transfer Functions, QTFs), the study of third-order high-frequency wave loads is still a research field since the '90s with the publications of the FNV and Rainey's strip theory approaches and Malenica & Molin semi-analytical diffraction solution.

This article, therefore, aims to analyse the high-frequency motion response of a 10MW floating wind TLP as predicted by some of these numerical engineering approaches and compare them to measurements from a tank testing campaign carried out at EDF R&D, a study currently lacking in the literature. The motion and mooring tension transfer functions obtained from monochromatic runs are compared and show that including a third-order wave loading model is crucial for accurately capturing the third-order springing response. Furthermore, as mentioned in the literature, it is observed that there is a strong coupling between the tower's first bending mode and the pitch response, which is captured both experimentally and numerically.

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