Energy that Floats – Integrating Wind Turbines onto Floating Platforms a report by Denise Fisher1 and Jason Jonkman2 1. Communications Specialist; 2. Modelling and Analysis, National Renewable Energy Laboratory
The goal of powering much of the world with clean renewable energy has led to the merging of two fields – land-based wind turbines and ocean platform technology – to create a variety of concepts for offshore wind farms. Currently, most of the world’s energy-producing offshore wind turbines are anchored to bottom-mounted substructures and nearly all have been installed in less than 50m of water. However, much of the vast offshore wind resource potential in the US, China, Japan, Norway and other countries is located at depths greater than 50m. For these nations, integrating wind turbine technology with floating support platforms will enable them to harness wind resources that occur at greater ocean depths. Seeking the most economical and reliable wind turbine support structure, researchers at the National Renewable Energy Laboratory (NREL) developed specific modelling capabilities that integrate offshore floating platforms with wind turbine technology.
Coupling the two technologies, wind turbines and floating platforms, requires complex models. Land-based wind turbines are designed and analysed using simulation tools (i.e. design codes) that are capable of predicting a design’s dynamic response to wind conditions as well as calculating the extreme and fatigue loads the system can endure. Land-based wind turbine analysis relies on the use of aero–servo–elastic design codes that incorporate wind-inflow, aerodynamic, control system (servo) and structural–dynamic (elastic) models in a simulated environment. Although numerous floating platform configurations are possible for offshore wind turbines, dynamic wind turbine models that account for the wind inflow, aerodynamics, elasticity and controls while simultaneously considering waves, sea current, hydrodynamics, and platform and mooring dynamics did not exist. NREL therefore developed a complex modelling and analysis tool that includes all the code capability to analyse land- based wind turbines with the added capacity to examine the interaction of turbines mounted on floating platforms. This sophisticated simulation tool models the dynamic response of offshore floating wind turbines, incorporating all the characteristics and physical phenomena of importance with the capability of performing appropriate analyses for the variety of proposed offshore floating wind turbine systems and their configurations.
Floating Wind Turbine Concepts
Harnessing much of the vast offshore wind resource potential requires the installation of wind turbines in deeper water. Numerous floating support-platform configurations are possible for use with offshore wind turbines. Current proposals based on the variety of mooring systems, tanks and ballast options used on platforms by the offshore oil and gas industry may be classified in terms of how they achieve basic static stability in the platform’s pitch and roll. The three primary concepts are the tension leg platform (TLP), spar buoy and barge, as shown in Figure 1. These platforms provide stability primarily through the mooring system combined with excess buoyancy in the platform:
© TOUCH BRIEFINGS 2010
either a deep draft combined with ballast or a shallow draft combined with water plane area. Hybrid concepts that use features from more than one class, such as semi-submersibles, are also a possibility.
In the case of floating structures, the offshore wind industry faces many new design challenges. Because the offshore oil and gas industries have demonstrated the long-term survivability of offshore floating structures, the technical feasibility of developing offshore floating wind turbines is not in question. However, developing cost-effective offshore floating wind turbine designs that are capable of penetrating the competitive energy marketplace will require considerable thought and analysis. Simply adapting offshore oil and gas technology directly to the offshore wind industry is not economically feasible. These economic challenges led to NREL’s innovations in wind turbine conceptual design and analysis. What was needed was the development of design codes to integrate the wind turbine and floating platform designs and then to quantify and analyse the coupled architecture to resolve the fundamental design trade-offs.
Currently, most offshore wind turbines are installed in shallow water on bottom-mounted substructures. These substructures include gravity bases and monopiles used in water to a depth of about 30m, and space-frames – such as tripods and lattice frames (e.g. ‘jackets’) – used in water to a depth of about 50m. In recent years, a number of the land-based wind turbine design codes have been expanded to include the additional dynamics pertinent to these bottom-mounted offshore support structures, including models for incident waves, sea current and the foundation dynamics of the support structure. However, none of these codes accounted for mooring system dynamics and the hydrodynamic effects that occur under conditions such as wave radiation (propagation of waves travelling outward from the platform resulting from platform motion) and wave diffraction (augmentation of incident waves when they encounter a platform).
In light of previous code limitations, NREL researchers at the National Wind Technology Center (NWTC) developed a design code, which is an enhanced simulation tool that models the dynamic response of offshore floating wind turbines. The NREL design code has been verified through model-to-model comparisons and is being applied to the design and analysis of promising floating wind turbine concepts.
Development of the Simulation Code
Throughout the past two decades, NWTC researchers worked on the development, verification and validation of comprehensive aero–servo– elastic simulators. In developing the new offshore floating wind turbine modelling capability, they combined the computational methodologies of the land-based wind turbine models from the NWTC with the comprehensive hydrodynamic computer programs developed for offshore oil and gas industries by the Center for Ocean Engineering at
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