Grégory Germain

Numerical and experimental study of the interaction between two marine current turbines

Abstract The understanding of interaction effects between marine energy converters represents the next step in the research process that should eventually lead to the deployment of such devices. Although some a priori considerations have been suggested recently, very few real condition studies have been carried out concerning this issue. Trials were run on 1/30th scale models of three-bladed marine current turbine prototypes in a flume tank. The present work focuses on the case where a turbine is placed at different locations in the wake of a first one. The interaction effects in terms of performance and wake of the second turbine are examined and compared to the results obtained on the case of one single turbine. Besides, a three-dimensional software, based on a vortex method is currently being developed, and will be used in the near future to model more complex layouts. The experimental study shows that the second turbine is deeply affected by the presence of an upstream device and that a compromise between individual device performance and inter-device spacing is necessary. Numerical results show good agreement with the experiment and are promising for the future modelling of turbine farms.

Evaluation of the durability of composite tidal turbine blades

The long-term reliability of tidal turbines is critical if these structures are to be cost effective. Optimized design requires a combination of material durability models and structural analyses. Composites are a natural choice for turbine blades, but there are few data available to predict material behaviour under coupled environmental and cycling loading. The present study addresses this problem, by introducing a multi-level framework for turbine blade qualification. At the material scale, static and cyclic tests have been performed, both in air and in sea water. The influence of ageing in sea water on fatigue performance is then quantified, and much lower fatigue lives are measured after ageing. At a higher level, flume tank tests have been performed on three-blade tidal turbines. Strain gauging of blades has provided data to compare with numerical models.

An experimental comparison of three towed underwater video systems using species metrics, benthic impact and performance

Managing ecological systems, which operate over large spatial scales, is inherently difficult and often requires sourcing data from different countries and organizations. The assumption might be made that data collected using similar methodologies are comparable, but this is rarely tested. Here, benthic video data recorded using different towed underwater video systems (TUVSs) were experimentally compared. Three technically different TUVSs were compared on different seabed types (rocky, mixed ground and sandy) in Kingmere Marine Conservation Zone, off the south coast of England. For each TUVS, species metrics (forward facing camera), seabed impact (backward facing camera) and operational performance (strengths and limitations of equipment and video footage) were compared with the aim of providing recommendations on their future use and comparability of data between different systems. Statistically significant differences between species richness, density, cover and assemblage composition were detected amongst devices and were believed to be mostly due to their optical specifications. As a result of their high image definition and large field of vision both the benthic contacting heavy and benthic tending TUVS provided good quality footage and ecological measurements. However, the heaviest TUVS proved difficult to operate on irregular ground and was found to cause the most impact to the seabed. The lightest TUVS (benthic contacting light) struggled to maintain contact with the seabed. The benthic tending TUVS was able to fly over variable seabed relief and was comparably the least destructive. Results from this study highlight that particular care should be given to sled and optic specifications when developing a medium- or long-term marine protected area monitoring programme. Furthermore, when using data gathered from multiple sources to test ecological questions, different equipment specifications may confound observed ecological differences...

Vortex and Wake Effects on Closely Spaced Marine Risers

The present paper describes some ongoing research performed for a better understanding of the hydrodynamic loads acting on a riser placed in the wake of an upstream one. Experiments on model tests scaled with real configurations for dual riser interaction in uniform and steady current are presented. A particular attention is paid on how fluid interaction between two cylinders of equal diameter in tandem configuration can significantly modify their structural response in term of amplitude and frequency, compared to that of a single cylinder. The circulation water channel allows to reach Reynolds numbers from 5.5 103 to 5 104 . Both in-line and cross-flow responses have been studied and are presented as functions of the reduced velocity. Observations demonstrate that wake effects can be relatively strong. If the dynamic of the upstream cylinder becomes to be well understood, the dynamic of the downstream one is hence much more complex and difficult to predict. When risers become in close proximity due to wake induced oscillations, collisions between cylinders can be observed. Comparisons between experimental and numerical results of the dynamics of a single riser in a flow are also presented.Copyright

Experimental and numerical results on VIV and WIO

Vortex-Induced Vibrations (VIV) are well-known and related to the majority of cylindrical structures subjected to strong winds or currents. The VIV limit the lifetime of the structure because they increase the forces and so the fatigue. When several structures of this kind are put together in close interaction, the wake effects (Wake Induced Oscillations - WIO) sometimes involve strong instabilities. If these structures are flexible or mobile, oscillations of several diameters can be observed and collisions can occur ([4] & [6]). Such structures are widespread in the oil industry where the extraction of oil in deep water can be done by means of risers. In some cases, risers are connected to a floating support called FPSO (Floating Production Storage Offloading) and held in tension by buoys (figure 2). These buoys are located at depth where waves do not have any significant influence. However, in these areas, the magnitude of currents can be sometimes important. Consequently, engineering companies have to find solutions to prevent hydrodynamic interactions between risers and buoys. For a better understanding and characterization of wake effects, an experimental study is carried out at the Ifremer I ¨

Account of ambient turbulence for turbine wakes using a Synthetic-Eddy-Method

The present paper aims at describing the use of a Synthetic-Eddy-Method (SEM), initially proposed by Jarrin et al. [12], in the 3D Lagrangian Vortex method framework. The SEM method is used here in order to generate a far-field incoming flow with a prescribed ambient turbulence intensity. However, for the account of the diffusive term in the Navier-Stokes equations, a classical Particle Strength Exchange model with a LES eddy viscosity is used. Firstly, the general characteristics of the Synthetic-Eddy-Method will be presented together with its integration in the framework of the developed 3D unsteady Lagrangian Vortex software [27]. The capability of the ambient turbulence model to reproduce a perturbed flow that verifies any turbulence intensity I∞ and any anisotropic ratio (σu :σv :σw ) will be discussed and validated. Then, the capability of the presented ambient turbulence model to compute turbine wakes will also be presented together with first results. Finally, comparisons will be made between the obtained numerical results against experimental data [22, 23] for two levels of ambient turbulence, namely I∞ = 3% and I∞ = 15%. Although the present study was initially performed in the framework of tidal energy, its application to wind energy is straightforward.

Experimental Study of Bubbles Sweep-Down on Oceanographic Research Vessels

In this work, a specific experimental protocol is developed to study the bubble sweep-down phenomenon on oceanographic research vessel. The combination of the utilization of a wave and current flume tank and an hexapod allows the simulation of conditions under which the phenomenon can be observed at full scale. Visualizations and PIV measurement results both obtained in the bow vicinity of the Ifremer oceanographic vessel Pourquoi pas? are presented in this paper.Copyright

Caractérisation d'effets de sillage rencontrés en milieu offshore

ABSTRACT The hydrodynamic interactions between several cylinders placed in close proximity in a steady flow are the subject of many studies, because of the strong loads that they create on these structures. For flexible or mobile structures, the wake effects can produce large amplitudes motions that can lead to clashes between the cylinders. To better understand these phenomena, an experimental study was carried out, bearing on the interaction between two models of tensioning buoy supporting hybrid risers. After checking that the models used were able to correctly reproduce the VIV motion on a single cylinder, the response of two cylinders in tandem arrangement was studied. A detailed study of the observed clashing phases for several initial spacing is presented here.