A heuristic approach for inter-facility comparison of results from round robin testing of a floating wind turbine in irregular waves

This paper introduces metrics developed for analysing irregular wave test results from the round robin testing campaign carried out on a floating wind turbine as part of the EU H2020 MaRINET2 project. A 1/60th scale model of a 10 MW floating platform was tested in wave basins in four different locations around Europe. The tests carried out in each facility included decay tests, tests in regular and irregular waves with and without wind thrust, and tests to characterise the mooring system as well as the model itself. While response amplitude operations (RAOs) are a useful tool for assessing device performance in irregular waves, they are not easy to interpret when performing an inter-facility comparison where there are many variables. Metrics that use a single value per test condition rather than an RAO curve are a means of efficiently comparing tests from different basins in a more heuristic manner. In this research, the focus is on using metrics to assess how the platform responds with varying wave height and thrust across different facilities. It is found that the metrics implemented are very useful for extracting global trends across different basins and test conditions

Benefits of using a spectral hindcast database for wave power extraction assessment

When considering deployment of wave energy converters at a given site, it is of prime importance from both a technical and an economical point of view to accurately assess the total yearly energy that can be extracted by the given device. Especially, to be considered is the assessment of the efficiency of the device over the widest span of the sea-states spectral bandwidth. Hence, the aim of this study is to assess the biases and errors introduced on extracted power classically computed using spectral data derived from analytical functions such as a JONSWAP spectrum, compared to the power derived using actual wave spectra obtained from a spectral hindcast database

Analyse expérimentale et numérique de jet critique dans une vague déferlante.

In the continuity of the work presented during the last Journ´ees in Cherbourg, we are interested in the kinematics of the fluid in a breaking wave. In particular, we try to reproduce critical jets whose mechanism of appearance is still poorly known. The kinematics of the fluid is generated by sloshing in a tank ; this is an efficient way to inject a lot of energy into the fluid. If one is able to anticipate theoretically/numerically the appearance of critical jet at the free surface –in particular within the framework of the potential theory of flows– it is advisable to verify experimentally the reality of the results of the calculation. This is the objective of an experimental campaign conducted at Ifremer. A rectangular tank, fixed on a hexapod is put in forced motion. The movement is a horizontal oscillation cycle. A wide range of critical jet configurations is accessible.Dans la continuité des travaux pr´esent´es lors des dernières Journ´ees de l’Hydrodynamique à Cherbourg, on s’intéresse à la cinématique du fluide dans une vague déferlante. En particulier on cherche à reproduire des jets critiques dont le mécanisme d’apparition est encore mal connu. La cinématique du fluide est générée par le ballottement dans un réservoir (sloshing) ; c’est un moyen efficace pour injecter beaucoup d’énergie dans le fluide. Si l’on est en mesure d’anticiper théoriquement/numériquement l’apparition de jet critique à la surface libre –notamment dans le cadre de la théorie potentielle des écoulements– il convient de vérifier expérimentalement la réalité des résultats du calcul. C’est l’objectif d’une campagne expérimentale menée `a l’Ifremer. Une cuve rectangulaire, fixée sur un hexapode est mise en mouvement forcé. Le mouvement est un cycle d’oscillation horizontale. Une large gamme de configurations de jet critique est accessible

Etude expérimentale pour l'optimisation des performances d'une plage d'amortissement parabolique

An optimisation study was run before rebuilding the damping beach of the Ifremer Ocean Engineering Basin. Trials were conducted at a ¼ geometrical scale at Ecole Centrale de Marseille in a wave tank reproducing the original performance of the beach. A series of modifications was studied : incidence, device between the beach and the adjacent wall, porosity. Porosity was shown as the most efficient tool to enhance the beach absorption and full scale measurements revealed that the resulting damping coefficient was less than 10 % for all usual periods and amplitudes.La refonte de la plage d’amortissement du bassin de génie océanique de l’Ifremer Centre de Brest a suscité une étude d’optimisation afin d’en améliorer les performances. Des essais à l’échelle ¼ ont été conduits dans le canal de l’Ecole Centrale de Marseille afin de reproduire dans un premier temps la forme de l’ancienne plage et confirmer son comportement. Une série de modifications a ensuite été étudiée : changement d’assiette, ajout d’appendices entre la plage et le mur, ajout d’une porosité. C’est cette dernière option qui a été retenue et transposée à l’échelle 1 sur la nouvelle plage dont le taux de réflexion a ainsi été abaissé à un niveau inférieur à 10 % pour la gamme de périodes et amplitudes usuelles

Infragravity waves and moored floating structures

Up to this day, design recommendations or practices for floating structures, such as DNVs RP-C205 and F205 for example, do not account for the existence of low frequency free Infragravity (IG) waves. These are low-frequency water waves which represent an additional excitation for (moored) floating structures. In this study, the influence of free IG waves on moored floating structures response is investigated and compared to second-order low-frequency wave forces, as the standard low frequency excitation, for two selected important free IG events in 2013 and 2014 at the SEM-REV (Site d'Experimentation En Mer - Récupération de l'Energie des Vagues) test site near Le Croisic, Pays-de-la-Loire, France. The wave forces and motion response for two example floater-mooringsystems are calculated and compared for the different frequency ranges separately and combined. Using the latest developments in the wind-wave framework WAVEWATCHIII®, e.g. the parameterized implementation of free IG sources at the reflecting boundaries, directional wave spectra including the low-frequency IG wave band are used. This study has been conducted between November 2014 and October 2015 in the scope of the axis 7 «Sea motions and interaction with marine structures» of the cluster of Excellence LabexMER "A changing ocean"

Interaction houle-courant en bathymétrie variable via une approche de type boussinesq

Modeling wave-current interaction needs a method combining accurate kinematics and reasonable computing efforts. Refined waves kinematics are described by a Boussinesq-type approach allowing a variable bathymetry. This method is adapted to take current effects into account. Results on Doppler shift, waves blocking and velocity profile are reported in a twodimensional case.La modélisation de la cinématique de sites à forts courants exposés à la houle requiert une approche équilibrée entre complexité du modèle et finesse de la description recherchée. Afin de représenter la cinématique de l’écoulement à l’échelle de chaque vague, une formulation de type Boussinesq originale permettant la présence de bathymétrie variable est employée. Celle-ci doit être adaptée afin de prendre en compte les effets du courant. Les résultats sur le décalage Doppler, le blocage des vagues ainsi que sur les profils de vitesses sont présentés dans un cas bidimensionnel

Interaction houle-courant par profondeur finie : impact sur la cinématqiue

The aim of this large scale basin experiment on wave current interactions was a better understanding of vertical velocity profiles changes due to progressive surface waves. The instrumentation deployed including wave gauges, pressure sensors and current meters allowed precise measurements along the basin. In particular, the effect of waves on the homogenization of vertical profile was measured, and the effect of current on the underestimate of wave height from bottom mounted pressure sensors was demonstrated. However, the current flow without wave was bi-dimensional and weakly sheared, and no study on the current instability as observed in previous experiment could be achieved.Ce travail expérimental sur les interactions houle-courant avait pour but une meilleure connaissance des profils de courant sur la colonne d’eau en fonction des conditions de houles et de la profondeur d’eau relative. L’instrumentation déployée (sondes limnimétriques, capteurs de pression, courantomètres) ont permis de mettre en évidence d’une part l’impact de la superposition d’une houle et d’un courant sur le profil vertical de l’écoulement moyen, et d’autre part l’effet du courant sur le calcul de la hauteur de la houle à partir de capteurs de pression sur le fond. Cependant, pour les cas étudiés lors de cette campagne expérimentale, l’écoulement en l’absence de vagues restait bien bidimensionnel et peu cisaillé, ce qui ne nous a pas permis de mener une étude sur les instabilités telles qu’on avait pu les observer lors d’une précédente campagne, cependant trop peu instrumentée pour une étude détaillée

Numerical and experimental modeling of offshore wind energy capture: Application to reduced scale model testing

International audienceHorizontal axis wind turbines installed on board floating supports stand among the principles of offshore wind energy capture. This document describes the experimental device and methodology for analyzing the performance of a reduced scale model of a 12 MW horizontal axis floating wind turbine. The particularity of this turbine is that it is mounted on a floater conceived according to Eolink's concept, where several masts connect the nacelle with the floater. The tests are performed at Ifremer's ocean basin. For that purpose, a wind generator and a model at scale 1:50 are developed. The experimental results are compared with numerical estimations obtained with the blade element momentum method

Wave and turbulence measurements at a tidal energy site

This work presents the analysis of wave and turbulence measurements collected at a tidal energy site. A new method is introduced to produce more consistent and rigorous estimations of the velocity fluctuations power spectral densities. An analytical function is further proposed to fit the observed spectra and could be input to the numerical models predicting power production and structural loading on tidal turbines. Another new approach is developed to correct for the effect of the Doppler noise on the high frequencies power spectral densities. The analysis of velocity time series combining wave and turbulent contributions demonstrates that the turbulent motions are coherent throughout the water column, rendering the wave coherence-based methods not applicable to our dataset. To avoid this problem, an alternative approach relying on the pressure data collected by the ADCP is introduced and shows appreciable improvement in the wave-turbulence separation