Material aspects of wooden towers for offshore wind turbines

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.


Introduction
Through the initiative of WorldWideWind [1], the concept of the Counter Rotating Axis Floating Tilted Turbine (CRAFTT) has been proposed.The design of generators [2] and the dynamics [3] have just recently been addressed.For a new concept, many additional engineering questions arise, for example: What would be a suitable material for the wind turbine tower?Previous long-term experiences in wood towers for vertical axis wind turbines are positive [4,5], and shows that wood towers is a viable alternative to conventional steel towers on land.These two types are shown in Figure 1.For larger turbines offshore, the demands on long-term properties are more severe.Recent developments of largescale wooden buildings show promise regarding upscaling to some degree.One example is the 17-floor building Mjøstårnet, where the load carrying frame is made of timber [6].Another is the 100 m Hannover-Marienwerder wind turbine tower made of wood [7].There are even prospects of building wooden skyscrapers in the future [8].What makes offshore installations special compared with towers on land is that maintenance and repair is more difficult, and the inevitable presence of water and hence moisture.Therefore, a longer expected lifetime is desired to increase inspection intervals.The main concerns are then fatigue and creep over time.Secondly, moisture is a well-known challenge for all load-carrying wooden structures.Degradation in the presence of water can be addressed, as illustrated by the fact that wood is historically the go-to material to build ships.If properly cared for, microbial degradation at sea can be avoided, but the softening of the wood is an issue that should be carefully considered in design, especially for wooden offshore structures.The sorption of moisture inevitably leads to loss of stiffness and strength, increased creep deformations and decreased fatigue resistance [9,10].Since this effect may be significant, two alternatives present themselves.Either the moisture is kept out by an impermeable barrier, or the effects of moisture on the mechanical properties is accounted for the design process.Treating the bulk of the wood to make it hydrophobic is generally not an option, since the hydrophilic wood polymer (most notably hemicellulose) is omnipresent and abundant in the material, and any chemical impregnation would neither be practical nor environmentally justifiable.In the following, we will address these concerns qualitatively, and suggest some approaches to approach them.First, fatigue is discussed, then the effect of moisture on deformation.Fatigue and failure of potentially weak links, such as joints of different geometries must be addressed specifically.Here, we focus on the material aspects of load carrying members and not on joints and geometrical features.

Fatigue in wood
The first design step is to prevent unwanted oscillations by dynamic FE analyses [4], preceded by validating analytic calculations if possible [3].If eigenfrequencies coincide with the cyclic excitation, high loads and stresses will develop, which can lead to failure.Secondly, design against static failure is performed, where the maximum stresses should be well below strength values.In Eurocode for design of wood structures, the ultimate limit state concerns strength, including parameters for load variation, moisture and material variations with corresponding partial coefficients [11].In contrast, the serviceability limit state concerns also stiffness and the resulting deformations.However, limit state design is prioritized, and design against deformations are generally based on numerical calculations and not by following validated design codes for wooden structures.Fatigue is not dealt with to the same extent in buildings, as the static loading dominates, and the wood is known to be relatively fatigue resistance, in contrast to many metallic materials, which can show individual cracks propagate until they attain a critical size leading to catastrophic failure.Wood is a fibrous aligned material, bearing resemblance to fibre composites, another fatigue resistant material, which is preferred in wind turbine rotor blades specifically because their resistance to fatigue, cf.e.g.[12].The microstructures of aligned fibre reinforced plastics and wood alike tend to prevent or slow down damage accumulation and crack growth during cyclic loading.The wood grains (longitudinal direction) should then be aligned in the main loading direction, corresponding locally to the direction of the maximum principal stress.
Since wood is a porous anisotropic material, all material directions (longitudinal, transverse and radial) need to be considered, in addition compression or tensile loading [13].Multiaxial fatigue analysis requires a large number of material parameters that are very costly to quantify.One can therefore start to consider uniaxial fatigue along the grain (longitudinal direction), provided the wood is adequately oriented in the structural members.An example of an S-N curve for birch in tension is shown in Figure 2(a).Fatigue damage is seldom observed in longitudinal loading.However, in off-axis loading, cracks develop at an earlier stage and can grow progressively until failure.Such a transverse fatigue crack growth is shown in Figure 2(b).S-N curves are readily implemented in most commercial FE softwares.It is advisable to identify a practical fatigue limit from S-N data, and use the corresponding stress value in a limit stress criterion.As is the case for strength, stiffness and creep, also the fatigue behavior is susceptible to the moisture level, and furthermore to moisture variations (mechanosorptive effects).Thus, testing under service conditions is generally needed, since predictive schemes have not been validated.Just as for various types of composite materials, each new type of wood needs to be characterized separately.Although there are ways to predict wood stiffness and static strength as a function of density based on their porous microstructure [15], the generalization to fatigue strength has, to our knowledge, not been investigated.
Just like static strength, the presence of knots will most likely affect the fatigue strength, since the knots act as defects from which fatigue damage can initiate.The size effect in fatigue of wood is largely unexplored, and similar techniques as for size effects from strength grading could be adopted [16].

Effects of moisture
Although moisture is present to a very high degree in native wood, the mechanical performance of a living tree is excellent.If cracks develop due to severe loads in e.g.storms, living wood tissue can heal and grow reaction wood which makes the tree recover and continue to carry load.For dried timber, however, the presence of moisture normally reduces strength and stiffness and leads to viscoelastic deformations compared with those of fully dry wood.The effect is significant in the transverse directions (radial, tangential and in shear).This can be attributed to the breakage of hydrogen bonds in the presence of water molecules in hydrophilic biopolymers in the cell wall, most notably hemicellulose.The stress transfer between the load carrying cellulose microfibrils is then impaired.
For large offshore wood structures, the self-weight and the influence of moisture will be considerable.For long-term service, this means that, in addition to fatigue from load fluctuations, there will be significant static loads which most likely well lead to creep deformations.If these deformations become too large, the dynamic properties of the structure will change and stress redistrubtions can lead to failure.Creep is therefore an issue to consider in design, especially since creep is significantly accelerated in the presence of moisture in wood [17].Furthermore, a particularity of wood is that a changing moisture content leads to accelerated creep, even beyond the rate for the highest moisture contents.This effect, known as mechanosorptive creep, is currently under significant investigation, since it has important engineering implications in a variable climate.To fully account for this effect in predictive models, the characterization of many material parameters is needed [18], and is yet to be practical in engineering design.For a constant climate, the approach by Bengtsson et al. [19] may be adopted.Creep data in the main material directions are then fitted to an appropriate number of parameters (see Figure 3).Although the load is predominantly unidirectional for slender structures, small misalignments and proximity to joints can lead to non-negligible off-axis stresses, which is why an orthotropic creep model is recommended, just as is the case for fibre composites.There is a striking lack of shear creep data for wood, despite its importance in overall dimensional stability of structures.A simplified method to obtain the shear creep has been proposed [20].This would then make the orthotropic creep tensor complete, ready to be implemented in FE codes.It should also be noted that most published creep data is limited in time for practical reasons, and extrapolation for practical usage times is inevitable.It is the same situation for fatigue, although a time-temperature-moisture superposition can be envisaged to accelerate testing by increasing the surrounding temperature [21].
To mitigate creep in wood in moist environments, the obvious action is to dry out the wood and seal it with a barrier to prevent any further moisture absorption.This approach is for instance taken for wood fibre based insulators in power transformers, before sealing the system with a dielectric hydrophobic oil in a water-tight container.Another application area where large-scale sealing with a barrier coating has proven successful is aluminium coating in the extrusion process in manufacturing of high-voltage underwater cables.Such long cables which span the bottom of seas have an immense surface, and the coating rather effectively prevent the growth of water trees over time.It is therefore not far-fetched to believe that it would be possible to effectively coat dried wood with aluminium sheaths, preventing moisture ingress over prolonged times.A wooden core fully enclosed by aluminium sheaths forms a sandwich structure, not unlike those proven useful in aircraft wings with a balsa wood core.Aluminium has also high specific mechanical properties, which has made it a preferred material in components in structures subjected to fatigue, most notably in aeronautical and automotive applications.These technologies are yet to be explored to minimize moisture absorption over extended periods of time in wood members in offshore towers for wind turbines.

Concluding remarks
This presentation highlights some of main material aspects that needs to be addressed if wood should be used in towers for large offshore wind turbines, such as in the CRAFTT concept.A qualitative assessment of the key features of wood under long-term static and cyclic loading in a moist climate is thus presented.Since inspection and repair intervals will be longer offshore, fatigue is more of concern than for wooden wind turbine towers on land.The higher levels of moisture necessitate more effective ways to prevent moisture uptake in the wood, and models to predict creep deformations due to high selfweight of large towers.Since wood is a highly variable material, a significant amount of testing is needed to determine material parameters that can be used in design.Also, models for fatigue and creep need to be balanced, i.e. not too detailed to require an unpractical number of material parameters to be characterized, and not too simplified so that they fail to make predictions at a reasonable accuracy.It is hoped that this short review will provide some ideas of which research topics that could be prioritized in order to facilitate the use of wood in towers for offshore wind turbines.Since wood is renewable in contrast to steel, commonly used in towers, a more sustainable use of raw materials can be achieved for large future wind farms.Experiences from the ongoing rapid development of wood frames for high-rise buildings can also help pave way for the use of wood towers for wood turbines at sea.