Marc Le Boulluec

Tank Testing of a New Concept of Floating Offshore Wind Turbine

The WINFLO project (Wind turbine with INnovative design for Floating Lightweight Offshore) aims at the development of competitive floating offshore wind turbines, by a consortium of 3 industrial partners (Nass&Wind Industrie, DCNS and Vergnet SA) and 2 scientific partners (IFREMER and ENSTA Bretagne). The design of the floater is an innovative semi-submersible free floating platform with particular aspects. Classical steps toward the assessment of the hydrodynamic and energy production performance include numerical modeling, model scale tank testing and intermediate or full scale tests at sea. The present study describes the wave tank tests including wind generation compared to some numerical modeling results of the coupled system composed of the support floater and the wind turbine.Copyright

Numerical Study on Hydrodynamic Behavior of an Underwater Glider

Underwater gliders are AUVs used in ocean exploration and observation. They use small changes in their buoyancy to dive and to return to the ocean surface. During the change of altitude, they use the hydrodynamic forces developed by their wings to move forward. Their flights are controlled by changing the position of their centers of gravity and their buoyancy to adjust their trim and their heel angles. For better flight control, the understanding of the hydrodynamic behavior and the flight mechanics of the underwater glider is necessary. A 6-DOF motion simulator is coupled with a BEM code for this purpose. In some specific cases, the numerical study demonstrates that an inappropriate stabilizer dimension can cause a counter-steering behavior. The simulator can be used to improve the automatic flight control. It can also be used for the hydrodynamic design optimization of the devices.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 ¨

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.

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.

Hydro-Mechanical Issues of Offloading Operations Between a Floating LNG Terminal and a LNG Carrier in Side-by-Side Configuration

This paper presents a state-of-the-art methodology to build a numerical hydro-mechanical model of a floating LNG Terminal with a LNG carrier moored alongside, complying with the physical mechanical behavior and hydrodynamic phenomenology involved in such a complex system.The required numerical functions are available for intensive engineering applications in the PRINCIPIA’s software DIODORE™ dedicated to sea-keeping and mooring analysis. The methodology has been validated by comparison with non-captive model tests in IFREMER basin in the frame of the SALT 2 JIP led by PRINCIPIA.The paper describes first the academic side-by-side case that has been tested in basin, associated with the comparison between the experimental results and the numerical application using DIODORE™. In a second step, the main physical parameters of the simulation are analyzed and some gained insights are exposed.Copyright

Well Head Production Floater: Experimental and Numerical Investigation

A model of an oil production floater is experimentally tested in regular and irregular waves for both fixed and free-moving structure. Measurements mainly concern the rigid body motions and the free surface elevation at 24 locations: 6 inside a moon-pool and 18 along the walls and above an immersed pontoon. Two natural periods of oscillation are experimentally identified inside the moon-pool and compared to linear diffraction and radiation computation and analytical formulas. Outside the moon-pool the wave run-up can be significant and strongly nonlinear, some modal periods are identified and compared to simplified formulation.© 2005 ASME

Discussion on Hybrid Utilization for Offshore Wind and Wave Energy

In this paper, we will discuss the concept of combined utilization of offshore wind and wave energy in terms of both tank test and numerical simulation. There are some possibility that combined utilization of ocean renewable energies can avoid some disadvantages that single utilization of them may have, such as cost problem associated with the stability of power and power supply. In this study, we focus on the motion of an offshore wind turbine in waves associated to wave energy converters, are installed in the same area in order to improve its motion by reducing the effect of incident waves. We carried out wave tank test and numerical simulation, and investigated the motion of wind turbine in waves. In addition, numerical simulation allows to visualize the wave fields around those devices. Appropriate arrangement of them is discussed for the most efficient utilization of offshore wind and wave energy.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.

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

ABSTRACT Modeling wave-current interaction needs a method combining accurate kinematics and reasonable computing efforts. Refined wave 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, wave blocking and velocity profile are reported in a two-dimensional case.