Session: New & Innovative FOW Design

Paper Number: 172808

172808 - An Improved Mass-Constructible Concrete Tension Leg Foundation Design 

Abstract: 

Tension leg platform provide a stable platform for floating wind turbines. The Provence Grand Large pilot wind farm, which is France’s first commissioned pilot farm, features tension leg floating foundations that combine a truss structure and plated buoyancy modules. The scalability of such designs will rely on steel structures yards capability, which is very much dependent on shipyard’s availability, and the anchor loads may be problematic in some deployment areas.

Eole Stab combined the dynamic performance of tension leg platforms to the scalability of concrete prefabrication in an improved tension-leg platform design that will also reduce significantly anchor loads.

The result of the research is a prefabricated, concrete, ring-shaped floater that supports a truss transition piece with improved tendon designs that reduce significantly loads and improve dramatically motion performance.

The development process uses wave tank model tests frequently to verify the adequacy of the tendon systems and the response of the floater. As swift installation methods will be critical to the large scale deployment of floating wind turbines, a 1/10 scale demonstrator will be deployed to test deployment techniques. Commercial simulation packages are used to complement tests once the behaviour is qualitatively qualified. These simulations allow to define and size systems.

Owing to the small motions of the floating wind turbine, a conventional cascade of simulations can be used :

·       diffraction-radiation for hull loads,

·       large deflection finite-element models to determine tendons and wind turbine loads,
and

·       regular finite element package to determine structural scantling.

Coupled aero-hydro-elastic models showed that unlike most floater designs, the first bending mode of the tower is not shifted. This, combined to the small motions of the floater (of 10cm of order of magnitude in power production cases) makes it possible to support turbines with no modification to the tower and control system, hence opening up again the market to marinized onshore wind turbines.

The most demanding construction work is the construction of the concrete hull. This will be made on pontoons to minimize quay infrastructure upgrade requirements and allow the use of regular docks. The total hull construction turnaround time is one month thanks to the use of prefabricated precast concrete wall and deck sections. The transition piece can be prefabricated, then assembled on the hull construction yard.

The hull is stable when tendons are not connected, making it possible to tow the complete floating wind turbine from port to installation site. As the motions of the floating wind turbines are small, O&M will be exactly equivalent to that of fixed offshore wind turbines. The use of nacelle-mounted self-erecting cranes will allow major component replacement to be made.

Results of the 15MW wind turbine are summarised below :

Wind turbine                                          IEA 15MW             

Tower base bending moment            515 MN.m

Maximum tilt                                           <1 deg

Maximum horizontal excursion        <1 m

Concrete volume in hull                      3900 m3

Hull diameter                                          60 m

Displacement                                        29 000 t

Presenting Author: Thomas Choisnet Eole Stab

Presenting Author Biography: Thomas has more than 25years experience in the offshore energy sector. He's a member of Lloyd's Register Offshore Technical Committee, of the ISSC Offshore renewables committee and a lecturer in Universities.

He participated to the technical maturation of floating offshore wind as the CTO of BW Ideol and is now the Managing Director of Eole Stab. He also led decarbonized ships designs within the CMA CGM group, and participated to the pioneering deepwater developments in West Africa in the early 2000's.
Thomas holds a MSc of ENSTA-IP Paris with major in Naval Architecture and Mechanical Engineering.

Authors:

Thomas Choisnet Eole Stab
Stéphanie Durand Eole Stab
Julien Durand Eole Stab
Philippe Durand Eole Stab