• Energy Research

Abstract. Floating wind energy has attracted substantial interest since it enables the deployment of renewable wind energy in deeper waters. Compared to the bottom-fixed turbines, floating wind turbines are subjected to more disturbances, predominantly from waves acting on the platform. Wave disturbances cause undesired oscillations in rotor speed and increase structural loading. This paper focuses on investigating the potential of using wave preview measurement in the controller system labeled as wave feedforward control. Two wave feedforward controllers were designed: one to reduce generator power oscillations, and the other one to minimize the platform pitch motion. In this study, a software-in-the-loop wave tank experiment is presented for the purpose of investigating the potential of wave feedforward control for floating wind turbines. In the experiment, a 1:40 scaled model of the DTU 10 MW reference wind turbine is used on top of a spar platform, with the reference closed-loop functionalities. Different environmental conditions, including wind speed, significant wave height, turbulence intensity and wave spreading, were applied during the experiments to test the control performance, and their effect on the turbine dynamics in general. It was found that the feedforward controller for rotor speed reduces the power fluctuations properly with a fair control effort, while the one for platform pitch motion requires huge actuation duty. It was concluded that wind turbulence has more dominance on the global dynamic response than waves.

This document presents the metocean design basis of offshore test site �� SEM-REV �� operated by Centrale Nantes and dedicated to the test of floating wind turbines and wave energy converters. It is released in order to provide data for offshore operations and engineering in the French offshore area of South Brittany and Pays la Loire regions. It presents a metocean analysis, based on a Measure-Correlate-Predict approach in a consolidated report which provides simultaneously design data for wind, waves, current, tides and marine growth on a French site. In particular, it uses data issued from more than 10 years of continuous campaigns performed since the creation of SEM-REV test site since 2009: 2 years of wind measurements, 10 years of wave measurements and 19 ADCP campaigns for current measurement. The provided design data are operational statistics and long-term statistics such as values for return periods of 1, 5, 10, 25, 50 and 100 years and for various directional sectors. Additional information are also provided such as turbulence intensity, extreme direction change for wind. Scatter diagram, monthly distribution, beaking wave criteria��� are provided for waves. Harmonic analysis of tidal measurements enable to hindcast and forecast water elevation on site. Deeper analysis of current on the water column is also provided. Marine growth thickness is evaluated on various components such as chains, nylon and dynamic cable. Additionally, other environmental conditions such as air density, normal and extreme air and sea temperature ranges are presented. This report was appraised by Bureau Veritas Marine and Offshore which �� certifi[es] that the metocean condition of the SEM-REV Test Site has been appraised according to the following international standards : IEC 61400-1:2019 Wind turbines ��� Part1 : Design requirements IEC 61400-3-1:2019 Design requirements for fixed offshore wind turbines IEC 61400-3-2:2019 Design requirements for floating offshore wind turbines [...] The metocean design basis of SEM-REV test site presented, and the associated Measure-Correlate-Predict based method has been found in compliance with the above standards [���] ��

Previous research on real-time deterministic sea wave prediction has generally focused on evaluating the accuracy and efficiency of short-term wave fields within a specific prediction zone. However, for a real-time wave prediction system, it is necessary to provide a continuous description of the ocean wave surface based on short-term prediction segments. In this regard, we have developed algorithms for continuous wave prediction in directional wave fields based on the "practical" prediction zone. The practical prediction zone refers to the time interval available for generating a continuous wave forecast by excluding the reconstructed waves from the prediction zone proposed by Kim et al. (2023). We also introduce and discuss several important time factors, such as the update interval of the spatio-temporal wave dataset, the total computation time, and the length of the practical prediction zone. By gaining a deeper understanding of numerical modeling setups, we have established strategies to reduce computational costs, which are directly related to the accuracy of continuous wave prediction. In particular, the development of these strategies suggests guidance in specifying direction and frequency bandwidths for continuous wave prediction.

Floating marine structures implement real-time wave excitation force prediction to address optimal control issues. The accuracy of force prediction relies on adequate wave forecasting. This paper presents a comprehensive analysis of deterministic wave forecasting by considering various wave steepnesses and directional spreads. In addition, we introduce new methods for predicting wave excitation forces acting on the floating body of interest. The methods are based on a set of frequency coefficients of wave excitation forces, which are generated in conjunction with wave amplitude parameters optimized in the data assimilation and frequency response functions obtained from boundary element method tools. These approaches offer the advantage of streamlining the calculation process, eliminating the need for simulating wave surfaces through wave propagation. Moreover, for the first time, we study a prediction zone for wave excitation forces by comparing predicted forces with theoretical forces. Lastly, the force prediction is validated against experiments conducted on a captive platform model in both unidirectional and multidirectional sea states.

This paper details a comprehensive study of deterministic real-time wave forecasting in directional seas. By using wave models on the basis of a Lagrangian description, a good balance was achieved between computational efficiency and model accuracy. Nevertheless, due to the highly non-uniform spatial distribution of data and the relatively small size of data in time inherent to remote optical measurements, the initial conditions are determined through an optimization process, which is computationally demanding, especially in multidirectional sea states. Accordingly, in order to offer a real-time system of wave prediction in the case of multidirectional waves, we propose a simplified and succinct assimilation method for the process of wave reconstruction. We also develop a three-dimensional spatio-temporal prediction zone where the future evolution of wave fields can be estimated based on wave measurements. Lastly, we outline a tank-scale experimental campaign conducted to mimic the measurements of a LIDAR in a real configuration. A comparison of model performances with the experimental observations shows that in a multidirectional approach, it is necessary to consider wave components in direction as well as in frequency to achieve nearly the same accuracy as for unidirectional seas.

Due to the ever-increasing electricity demand, along with the need to reduce the dependence on fossil or nuclear resources, a growing amount of renewable energy is integrated in the energy mix of many countries. However, the cost-effective integration of wave energy remains difficult as its cost is still not competitive compared with other energy sources. This paper deals with the energy production management of a simulated wave energy converter farm based on point absorbers that could be installed in the vicinity of the SEM-REV site. The approach considers the electrothermal behaviour of the export cable in combination with techno-economic aspects. The method can be used to extend the power export capability of a test site at no cost, which could be very interesting to install more marine renewable energy (MRE) converters and/or of greater rated power. Our study shows that exploiting the thermal inertia of a wave farm submarine export cable, while also considering techno-economic aspects, may lead to an increase of 18% in the annual energy production without modifying this cable.

This study deals with the initialization of three-dimensional wave field computations. We carry out such simulations with an HOS model developed at LMF-ECN since 2002 and based on the work of West et al. (1987) and Dommermuth & Yue (1987). In such models, initial conditions for three-dimensional realistic sea state computation are obtained by linearly distributing energy density spectrum. This however implies a relaxation of the non-linear effects as proposed by Dommermuth (2000) for bi-dimensional monochromatic wave train and Tanaka (2001), over several tenths of wave periods. The present work tests those former initialization methods and exposes an alternative initialization based on a non-linear three-dimensional approach. Non-linear interaction processes are both accounted in the spectra of elevation and potential of velocity, in accordance with the formulation of Dalzell (1999) at second order in wave steepness. Non-linear energy calculation is then addressed and the efficiency of the methods as well as their possible impact on properties and statistics of the wave field are investigated.

This dataset is a deliverable of the FLOATECH project, funded under the European Union’s Horizon 2020 research and innovation programme under grant agreement No 101007142. The aim of this dataset is a result of the field experiments carried out at the Floatgen FOWT located at the SEM-REV test site.