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code_aster au service de l’éolien offshore

É. Mallet, M. Capaldo, J.-L. Fléjou (EDF / R&D / ERMES) ; C. Peyrard (EDF / R&D / LNHE) ; A. Nieto-Ferro (EDF / DPNT / DCN)

Gravity Based Structure wind turbine studies

EDF will install three fixed offshore wind farms in France by 2020 (figure 1). Each farm will feature around 80 of the 6MW wind turbine from General Electric. For two of them, turbines will be installed on large monopiles driven in the soil. For the third farm, offshore Fécamp, it was decided to install the wind turbines on Gravity Based Structures (GBS). These foundations are less commonly used than monopoles, therefore investors have more interrogations regarding their design.

Figure 1

Soil-Structure interaction

First, the design of wind turbines is strongly restricted by resonance considerations, particularly when the excitation frequency (blade passing, sea waves, wind …) may coincide with the first or the second natural frequencies of the system resulting in larger amplitude of stresses and displacements. Therefore the foundation stiffness impacted by soil stiffness, plays an important part in the design. The engineering approach is mainly dictated by the DNV-GL 2013 norms and yet, this approach has several limitations. The proposed technique, based on the Boundary Element Method (BEM), enables the computation of the stiffness and damping matrices of a given soil profile. In particular, this approach has been applied to the Fécamp farm, by coupling code_aster and MISS3D (figure 2).

Figure 2

Risk of detachment

Then, the footprint of a lifted GBS is governed by stability considerations. For a GBS under extreme loadings, detachment may occur and yet there are very few recommendations on how to model it. Therefore various questions arose around this phenomenon and its potential impact on soil degradation. A precise assessment of the possibility for a GBS to detach is not easy to conduct with simplified models such as beam models. Detachment can create a hydraulic flux below the foundation. This flux is chased away as the foundation gets down. This phenomenon could lead to soil deterioration as sand may get away with the flux. Therefore there was a concern that a regular detachment, as small as it could be, may lead to a significant soil deterioration. First, code_aster was used to evaluate the basement detachment by taking into account the stiffness of the soil and the contact conditions with the foundation. Then code_aster was used to evaluate the flux under the GBS with hydromechanics modelling. These results were used in sedimentology approach to evaluate the transportation and soil degradation risk.

Under-pressures

Finally, the effect of the passing free surface waves above the foundation was also studied. The main objective was to determine the pressure under the GBS to assess the loading created on the GBS, depending on soil and waves definitions. code_aster was used to compute the pressure repartition under the GBS with hydromechanics modelling accounting for soil permeability. This study required to take into account precise information about the description of a wave, in particular concerning its period and its length as well as the attenuation of pressure variation as a function of water depth.

Floating Offshore wind turbine studies

Furthermore, the next step for development of offshore wind turbines will consist in installing turbines on floating foundations instead of using foundation fixed on the sea bed (figure 3). Floating foundations are moored by cables or steel chains, and submitted to motions induced by waves, currents and wind loads. The main interest of floating is to address the wind resource located in large water depth areas. The wind industry expects that floating foundations will be a more economical choice for depths above 50 to 60m, which corresponds to more than 50% of the offshore wind energy resource in Europe. EDF EN, as a leader of the offshore wind market in France, plans to develop a pre-commercial floating farm, featuring three 8MW turbines based on tensioned-leg platforms by the end of 2020.
EDF R&D is involved in the technical de-risking of the project and the concepts evaluation, requesting a need for simulation tools able to address the mechanical behaviour of floating wind turbines. Critical issues are identified on motions and accelerations of rotor and nacelle, as well as on dynamic response of this complex system submitted to cyclic loadings. code_aster was identified as a possible solution for floater and mooring lines modelling, taking advantage of the beam and cable finite-elements already present in the code, and of the Python interface capabilities to describe wave loads. A model of an academic floater and its catenary mooring system has been built and submitted to wave induced loadings in order to estimate floater motions and dynamic tension in the mooring lines. The results have been successfully compared to experimental data published by the DeepCwind consortium. code_aster was even more accurate than commercial and academic software dedicated to floating offshore turbine modelling. This suggests that code_aster could be used in the future to support EDF EN on floating wind. Today, some developments would be necessary to address the global system, in particular regarding rotating blades modelling under large deformation hypothesis.

Figure 3