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Project Info

Project CodeName/Code: POSEIDON / 2041

Title: Multi-purpose floating structures for offshore wind and wave energy sources exploitation

Principal Investigator:        Professor Spiros A. Mavrakos 

NATIONAL TECHNICAL UNIVERSITY OF ATHENS (NTUA)

 

Concept And Objectives

1.1 Background                                 

EU endorsed “a binding target for 2020 to reduce greenhouse gas emissions by 20% and ensure 20% of renewable energy sources in the EU energy mix that means a 20% share of renewable energies in overall EU energy consumption by 2020; a plan to reduce EU global primary energy use by 20% by2020”. To achieve these targets, all practical forms of renewable energy conversion have to be considered. In this respect new generation of marine renewable energy conversion technologies, including offshore wind and wave energy, have to play a very important role.

For reasons linked to energy security and climate change, Europe has set ambitious targets for the production of renewable energy. In its 2^nd Strategic Energy Review (2008), EU took 150 GW off-shore wind power capacity installed as a “high” scenario for 2030 which will primarily be delivered from marine-based turbines. In order to achieve that goal there is a need to move wind farms to deeper waters located tens of nautical miles offshore, and to increase the installed capacity with individual power plant system sizes of around 1000 MW and higher. As the viability for the development of offshore wind farms in deeper waters is highly cost-dependent, the use of the available platform space for different ocean energy sources exploitation make such offshore platforms more competitive.

In the present idea we are proposing a concept of a moored floating energy conversion system that supports offshore wind and wave resources exploitation. The concentration of several devices on one floating hub has both economic and operational advantages. Wind and wave energy converters can share the electrical cable and power transfer equipment to transport the electricity to shore. Capital costs can be also reduced overall, provided that the floater will be properly designed.

1.2 Concept principle outline description

The proposal aims at developing a preliminary design concept together with experimentally validated analysis tools for the coupled hydro-aero-elastic analysis of multi-purpose floating platforms suitable for the offshore wind and wave energy resources exploitation. Hereby, Wind Turbines (W/T) will be used for capturing the wind field, whereas Oscillating Water Column (OWC) devices are foreseen for the wave energy exploitation. The numerical tools will be implemented to the preliminary design of a floating structure configuration suitable to operate at water depths of approximately 200m in the Aegean Sea. A representative but not exclusive configuration of the type of floating energy conversion systems we are looking for consists of a triangular platform supported by cylindrical floaters with the W/T mounted at deck’s center and the cylindrical OWC devices at its corners (see Fig.1). Wind power is foreseen to be captured be W/T of either horizontal or vertical axes. The OWC consists of two concentric cylinders (see Fig.2), with the water entering through the subsurface opening into the annular chamber between the two cylinders that contains air. The wave action causes the captured water column to rise and fall like a piston, compressing and decompressing the air. As a result, there is an air flow moving back and forth through a turbine coupled to an electric generator. The floater is moored with vertical tensioned tendons (TLP type mooring).  

 

1.3 Objective(s)

The primarily objective of the POSEIDON project is the development and technical assessment of a preliminary design concept for a multi-purpose floating platform suited for offshore wind and wave energy sources exploitation together with experimentally validated analysis tools that accounts for the fully coupled hydro-aero-elastic analysis of moored multi – purpose floating systems. Hereby, the dynamics of the multi-purpose floating system will be evaluated by accounting for the hydro-aero-elastic coupling of the floating structure with the W/T and the OWC devices, the dynamics of the mooring system and its interaction with the marine soil.

The improved computational tools represent a prerequisite to enhance the scientific and engineering knowledge basis for the technical evaluation of the proposed floating structure design, which:

 

• Can form a feasible engineering solution for the utilization of offshore wind and wave energy sources at the envisaged water depths.
  • Can represent a cost-effective engineering solution by increasing the anticipated energy extraction to production cost ratio when it is compared to the corresponding one applicable to separate exploitation of offshore wind and wave energy sources. Hereby, as the wind and wave energy converters can share common infrastructure (floater, electrical cable and power transfer equipment), both economic and operational advantages can be obtained by concentrating of several devices on one floating hub.
• By being moored away from land, decreases the visual pollution caused by large wind turbines and the prohibitive cost of seafloor mounted monopoles in larger water depths

 

From the engineering point of view challenging objectives included in the proposed design concept are:

 

• To arrive at a preliminary floating structure design with optimized seakeeping behavior.
  • To model and analyze the coupled dynamics of the proposed floating structure in various wind and wave conditions and sea bed soil characteristics.
  • To validate the numerical method and computer software against experimental data 
• To design a suitable mooring system for the station keeping of the structure and assess its interaction with the sea floor.