An Offshore Wind Tower (OWT) supported by a bottom-supported jacket can become uneconomical due to several reasons including larger turbines, an increase in water depth, and very poor foundation material. An OWT-supported by a floating structure tethered to the seafloor is an obvious alternative to a bottom-supported structure. An important consideration for an OWT is offshore installation costs. A system that allows minimization of offshore installation work will reduce the overall cost. This paper describes the conceptual design of a floating structure supporting an OWT that can be transported to an installation site and then tethered to the seafloor. This hybrid Semisubmersible and Tension Leg Wind Tower (SSTLWT) is competitive in moderate water depths of 40 to 50 meters based on both fabrication and installation costs. There are several added advantages of the SSTLWT over other designs. First, the OWT support structure design is unaffected by the water depth and, second, the SSTLWT can be either tethered to the seafloor with pre-tensioned tethers or can be conventionally moored as a free-floating system. The SSTLWT is stable in free-floating mode and the design incorporates adequate buoyancy so that only the ballast is modified from one station-keeping system to another. As the catenary mooring may become challenging in shallow water, further site-specific studies will be necessary.
In addition to the description of the design, the wind turbine characteristics incorporated into the design, the description of design details, the fabrication and installation options as well as the applied ocean environment loads affecting the design of the tethering system and the overall fabricated and installed costs are presented for both 50 m and 100 m water depth sites.
