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Conference programme 

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Poster session

Lead Session Chair:
Stephan Barth, Managing Director, ForWind - Center for Wind Energy Research, Germany
Carlos Wong CBJ Ocean Platform Engineering Corp, China
Co-authors:
Carlos Wong (1) F P
(1) CBJ Concrete Ocean Platform Engineering Corp, Zhuhai, China (2) , , (3) , , (4) , , (5) , ,

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Poster
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Presenter's biography

Biographies are supplied directly by presenters at OFFSHORE 2015 and are published here unedited

Dr Wong obtained his PhD from the University of Dundee, UK 1983 and returned to the industry working mainly in bridge design and construction in various countries and in international firms. He retired from Arup 2012 and set up an engineering firm in China specializing in concrete offshore platform for wind turbine, pier, depot or even city replacing reclamation. He apply his design skills to platform leading to a number of patents pending. His major bridge works include Hong Kong - Zhuhai - Macau Bridge, Stonecutter Bridge, Hong Kong Shenzhen Western Corridor, Sutong Bridge etc.

Abstract

Wind tracing rotational semi-submerged raft for multi-turbine wind power generation

Introduction

An innovative semi-submerged raft supporting multiple turbines and self orientates to facing the wind is presented herein. Spacing between wind facing turbines is taken as 2.2D so that two rows of turbines can be placed with the wind wakes of front row of turbines project to the empty space between the aft row turbines, provided that the raft can turn along with the wind direction changes, i.e., wind tracing. A triangular raft supports three turbines and a trapezoidal raft supports five turbines. Using an eccentric rotation center, the raft will always orientate to wind facing pose by itself.

Approach

To create a floating multiple-turbine raft without wind wake suffering. All turbines face into incoming wind thereby requiring the raft turns along with the wind turns. Rotation center is eccentric from the CG towards the windward side such that the raft settles only after wind force resultant passes through the rotation center and the CG. In this orientation all turbines are wind facing, hence Wind Tracing. Support raft is submerged to depth that waves have insignificant effects on the raft beams. Beam weight is balanced by buoyancy, and the raft is made with concrete/prestressed concrete using bridge skills.


Main body of abstract

Use a semi-submerged triangular raft with floaters located at the vertices supporting 3 turbines as an example. Submerged beams connect the floater bottom at depth greater than 14m wherein waves have little effects on the beams. The beam self weight is balanced by its buoyancy so that the floatation is from the floaters. A cable is attached to the bottom end of each floater, and merges to a node away from the CG into the windward side along the bisector of the triangle. The node is tied down by a mooing line to a seabed anchor to form a single tension leg structure. The rotation is by twisting of the mooring line. Power output cable is attached to one cable line and runs down the mooring line onto seabed with sufficient slackness in loosened coils to allow limited twisting. If needed, the raft performs active turn by pointing the rotor into wind for generating torque to turn back the raft to release the twisted cable and mooing line.
Two turbines are in the front and the 3rd in the aft between the two front turbines. Wakes cast in the empty space behind. Wind force resultant is fluctuating around the CG since wind is not perfectly uniform. If rotation center is the CG, the raft will oscillate due to changing sign of the torque. With the rotation center eccentric from the CG, the fluctuating force produces torque to turn the raft until the force resultant passes through both centers.



Conclusion

The patent pending invention is very stable due to the large base, and very economical due to use of prestressed concrete which can last over 100 years without major maintenance. The same raft can support 4 generations turbines within its design life, the life-long costing is even less. Using bridge construction skills the raft can be precast in segments and assembled in a harbor. The turning without the expensive turret further reduces the cost. At the present state, the installation cost is estimated to be at the upper band of that for inland turbine, making it very competitive.


Learning objectives
Prestressed concrete can be considered as building materials in line with steel for floating wind turbine support. The multi-turbine raft is more stable than single turbine floating support. It can be viewed as a giant turbine of aggregated sweeping areas of the rotors. The eccentric rotation center is more effective in wind tracing and the cabling system that generating tension leg as well as rotation axis are the highlights of the invention (patent pending).