A bottom-fixed lattice tower for offshore wind turbines
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Wind turbines have been sited away from coasts in the last decade to exploit the tremendous potential of wind energy on the vast oceans, especially in Northern Europe where there is relatively limited space on land and large sea areas. To date most of the existing offshore wind farms are located in shallower water but the current focus in the offshore wind industry is to explore the more stable wind energy in intermediate water depths, say the depths between 30-70 m. Increased water depths require novel technologies to make wind energy more economically competitive in such hostile site conditions. Among other things, the search for cost-efficient support structures is a critical challenge, because these can amount to 20-30% of the total project cost, and even more in deeper waters. This thesis aims to examine the practicability of full height lattice towers to accommodate wind turbines in intermediate water depths. Lattice towers are first geometrically optimized to determine the possibilities for cost efficient designs when respecting the ultimate limit state. Then, since it is the main design driver of offshore wind turbine support structures, the fatigue life of promising designs is assessed, using the frequency domain method. If necessary, the structures are modified. All the studies for this thesis draw upon the load values derived from the NREL 5MW Offshore Baseline Wind Turbine model, situated in the North at a water depth of 35 m. This thesis concentrates on three and four-legged lattice towers, and studies their behavior under variation of geometrical parameters; such as top distance between the legs, brace inclination, and number of segments along the tower height. To seek the most economic design each of the considered lattice tower configurations was assessed by varying the distance between the bottom legs, while maintaining the height constant. The complete series of configurations was evaluated under extreme turbine and wave loads. The towers were further quantitatively judged against the relevant criteria, including their structural performance under the ultimate limit state, the mass of the structures, and the fabrication costs, all in orderto determine the optimal solution. The best side topology for the chosen reference site was identified as the one consisting of ten X-shaped braces, with inclined diagonals of constant angle throughout. The threelegged lattice tower was found to be approximately equal in tower mass to the fourlegged type, but with simpler geometry and consequently a less complex fabrication procedure. It was also indicated that the tower mass varied as the bottom distance between the legs changed, and reached its lowest values at distances of 17-25 m for all the configurations under consideration. One of the most significant challenges encountered in the design of offshore wind turbine support structures is the fatigue assessment, since wind and waves are random and occur with significant intensities in many different frequency regions. Being timeefficient, the full frequency domain method was adopted in this work to assess the fatigue life of those promising designs first determined in the static analysis. If needed, the thickness of the tubes was modified to enable the towers to meet the fatigue criteria. After applying the joint detailing technology, the mass of lattice towers was found to be less than 50% of their counterpart; i.e., corresponding tubular-monopile hybrid support structures. The three-legged and four-legged lattice towers come out equal in material usage. However, the three-legged type wins in fabrication cost, thanks to the smaller number of members and joints. To render the geometrical parameter studies efficient, an in-house finite element (FE) method space-frame code was programmed. Using this code, FE models could be built to assess the different lattice tower configurations quickly and flexibly. The code also provides the ability to modify structures gradually, until they are able to meet specified criteria. Complex fabrication procedures are often regarded as the main drawback of lattice type wind turbine towers. A rough estimate of the fabrication time was made for the two lightest lattice towers, one three-legged and one four-legged tower. A sequence of values representing the variable efficiencies was set for each contributing parameter which plays a significant role in the fabrication cost of lattice towers. The results indicate that the three-legged lattice towers need less fabrication time than the fourlegged ones, in most cases. The comparison to a tubular-monopile structure allows for concluding that a lattice tower, especially the four-legged type, is costlier than a tubular tower with the currently standard welding rate. However, the difference can be lessened with flux core arc welding (FCAW) technology being improved. To introduce mass production to the fabrication of lattice towers will offer greater potential to reduce the manufacturing costs of these structures.