ABSTRACT

Wind turbine technology has developed tremendously in the last decade resulting in the installation of large turbines with rated power generation of more than 5 MW. Such units with hub heights of more than 90 m and the rotor diameter of more 120 m or more require careful study of support structures. Bottom-supported structures can be designed cost effectively in relatively shallow water but the cost economics can be challenging at water depths of about 50 m in relatively harsh environments. This paper describes conceptual design of a positively buoyant Tension Leg Wind Turbine (TLWT) facility suitable for both moderate and deep water. It is competitive with bottom-supported structures in moderate water depths of 40 to 50 m based on both fabrication and installation costs. The added advantage of the TLWT design is that floating structure design is largely unaffected by the water depth. The design incorporates adequate positive buoyancy so that only the magnitude of ballast is modified to account for the weight of tether lengths at different water depths. In addition to the description of the design, wind turbine characteristics incorporated into design, description of design details, fabrication and installation options as well as applied ocean environment loads affecting the design of tethering system and the overall fabricated and installed costs are presented for both 50 and 100 m water depth sites.

INTRODUCTION
Wind Turbine Dynamics

The dynamic behavior of an offshore wind turbine on a bottom-supported structure is a complex subject requiring investigation of rotor blade and tower properties together with wind, wave and current excitation loads to minimize structural resonance. Froye and Dahlhaug (2011) provide a comprehensive discussion on rotor design and Jafri et al (2011) document the results of Monopod-based offshore wind turbine dynamics based on analytical and finite element modeling.

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