Session: 06-08-01 Model Tests - I

Submission Number: 176995

Nonlinear Forces on a Flexible Spar Platform: Experimental Evidence From Phase-Manipulated Wave Tank Tests 

With the increasing size of Floating Wind Turbines (FWT), hydro-elastic effects may play a significant role in the response of the structure, and it may be necessary to include them in model testing. Our 1:40 Froude-scaled spar platform carrying the DTU 10 MW turbine consists of a flexible tower and floater connected by a load cell. It is representative of a realistic large FWT in terms of rigid body motions, hydrodynamic loads, and first bending mode frequency. The floating system has been designed with its natural frequencies of rigid body motions and structure outside of the linear wave-frequency range to mitigate wave-induced responses. However, nonlinear dynamic responses should not be overlooked, especially in harsh conditions. To quantify the importance of the so-called sub- and super-harmonic responses, we opted for an experimental approach based on phase manipulation techniques.

We carried out irregular and focused wave tests without aerodynamic loads in an ocean engineering tank at Ecole Centrale de Nantes. Both Ultimate and Fatigue Limit States conditions have been studied. We have carefully investigated the sensitivity of nonlinear dynamic responses to wave steepness. Slow-drift responses at rigid-body motions induced by second-order difference-frequency loads have been observed. We also noted a steady-state hull vibration in irregular waves. Better known as springing, this phenomenon is triggered by second-order sum-frequency loads. To apply the phase manipulation, each test was performed with at least four different phase shifts of 90°, 180°, and 270°. Assuming that the hydrodynamic forces have a Stokes-like structure, their harmonic components can be extracted by linear combinations of phase-shifted time histories. Then, low- and high-frequency responses can theoretically be isolated.

This study highlights some of the uncertainties within the experimental application of the technique. To achieve this, we estimated phase misalignment and change in energy content between the initial and phase-shifted signals using a cross-spectral analysis. For measured wave elevation, body motions, and internal loads, the misalignment is not accurately controlled in irregular waves. Given that the phases of the input signals (i.e., the wavemaker motions) are well aligned with the target and that the timing of measurements was consistent, the observed misalignment likely originates from the inherent physics of the system. For example, our measurements were slightly affected by sloshing in the wave basin, even if the surge natural period was carefully tuned to be larger than the first wave tank sloshing mode.

The phase-based reconstruction technique can recreate the weakly nonlinear part of the initial signal. We used it as a metric to quantify the validity of the Stokes expansion. It provides insight into the effectiveness of the phase-based separation method. Reconstructed focused waves agreed well with the initial signal, while reconstructed irregular waves were not satisfactory. Shorter wave events are more easily controlled compared to longer, irregular wave tests, where factors such as sloshing, wave reflection from the beach, and nonlinear wave interactions become significant and cannot be neglected. It is important to check the phase misalignment before interpreting the results. If needed, adding more phases could lower the error induced by phase misalignment. Even in a well-controlled ocean basin, different physical behaviours are at stake. In particular, transient vibratory responses in the first bending mode were observed under harsh conditions, following a large wave packet and sometimes involving a breaking event. This phenomenon, known as ringing, clearly deviates from the predictions of the Stokes expansion.

Presenting Author: Sylvain Jamet Ecole Centrale de Nantes

Presenting Author Biography: Sylvain Jamet is a PhD student in Ocean Engineering at Ecole Centrale Nantes. His PhD is about the Study of the Dynamic and Structural Response of a Floating Wind Turbine Platform. He is involved in the Horizon Europe project, FLOATFARM, where his contribution focuses on the hydroelastic response of floating structures with experimental and numerical approaches.

Authors:

Sylvain Jamet Ecole Centrale de Nantes
Seung-Yoon Han Ecole Centrale de Nantes
Vincent Leroy Ecole Centrale de Nantes
Félicien Bonnefoy Ecole Centrale de Nantes
Athanasios Dermatis Ecole Centrale de Nantes
Benjamin Bouscasse Ecole Centrale de Nantes
Guillaume Ducrozet Ecole Centrale de Nantes