Material aspects of wooden towers for offshore wind turbines

Abstract

Possible new innovative materials for Counter Rotating Axis Floating Tilted Turbines are studied and discussed. The 40 MW version of the Counter Rotating Axis Floating Tilted Turbine (CRAFTT) will reach as far as 80 m below sea surface and up to 400 m above. The CRAFTT is an integrated design for floating offshore wind with two turbines on the same tilted shaft where the lower turbine is mounted directly on a rotating mast integrated with floater. The upper turbine will reach altitudes of 400 m. The system is designed to be a direct drive system, eliminating need for gearbox, taking advantage of the double air-gap speed of generator. With the generator placed at lower end as ballast the incentive to reduce weight for wings, tower and blades increase. Furthermore, wood is an attractive option as it enables both low CO2 impact production and higher degree of reusability. However, fatigue properties from both mechanical and thermal cycling needs to be addressed in order to evaluate new structural materials in the context of floating wind turbines. Starting from scratch without any preconceived notions, one could consider timber as a potential option for the tower. In such a preliminary and qualitative deliberation, one can consider that the use of wood as the main load-carrying material in large structures has been proven during the last decade by the development of new high-rise wooden buildings, with even higher buildings with timber as the main structural component expected in the future, The tall wooden buildings have been made possible since wood has the advantage of having high specific mechanical properties, i.e. high strength and stiffness with respect to density in the grain direction, in addition to being renewable. Another advantage is that wood is less sensitive to fatigue than many metallic materials, since its hierarchical microstructure prevents the propagation dominant cracks when loaded in the longitudinal direction. Design against fatigue is crucial in wind turbine structures given the inevitable cyclic loading. As all materials, wood certainly has its drawbacks, the foremost being its sensitivity to moisture, which is of obvious concern in off-shore applications. Moisture has a softening effect, resulting in creep, and moisture may trigger chemical or microbial degradation. The development of barrier coatings of aluminium has shown to be very efficient in e.g. high-voltage cables and food packaging, making them impermeable to moisture and air. Such techniques should be applicable also in wood constructions. This presentation highlights the main points specific for wood as a construction material in the design of wood towers for wind turbines in offshore locations, which need to be addressed in design.