The vision of a sustainable future with reduced CO2 emissions and the increased energy demand have stimulated the development and research of renewable energy sources the last years. The wind energy industry is rising on a very large scale in Europe, with UK being the global leader in offshore wind. The higher wind speeds over the sea and the lower visual and acoustic impact turns the interest towards offshore sites. At the same time the challenges due to more severe weather conditions, because of the simultaneous wind and wave loading are increasing. To enhance the reliability of such offshore structures and reduce electricity production cost and maintenance cost, it is essential to update the aero-elastic codes used for the design, the analysis and the optimization of wind turbines.
The need for a sustainable and affordable energy, renders important the technological innovation and demands for development of more advanced tools in this field. Next generation wind turbines that will require faster installation and less maintenance as well as optimisation of wind farm layouts will drive the cost of electricity down. Furthermore, the accurate representation of the structure during the design process can lead to significant material savings.
The design of the support structure of a wind turbine (tower-transition piece-monopile) plays an important role in the dynamic response of offshore wind turbines. Although the offshore oil and gas industry gives an insight into the dynamic behaviour of offshore structures, the assessment of offshore wind turbines differs significantly in some aspects. Apart from the wave excitation, the loading on the rotating blades due to the wind introduces at least two more excitation frequencies, the rotational frequency (1P) and the frequency caused by the blades passing in front of the tower (3P). The first natural frequency of oil and gas platforms is higher than the wave excitation. In the case of an offshore wind turbine, the first natural frequency is between the wave and the third harmonic of the rotational frequency. The narrow band of the support structure natural frequency renders the dynamic behaviour of the support structure sensitive to changes in the geometry. Consequently the challenge for offshore wind turbines is increased due to the requirement that their natural frequency should avoid a) wave excitation and b) blade passing excitation.
For shallow water depths the most commonly used sub-structures and foundations are:
- Suction bucket
- Gravity base
Moving into deeper water, where steadier and stronger wind is provided, different solutions are required. For medium water depths (around 50m) Jacket foundations are installed, while for deep waters the floating concept is the only feasible solution.
Damping is the dissipation of energy from a structure that is vibrating. The accurate estimation of the damping on a wind turbine has a pronounced effect on the load and lifetime prediction as well as on the dynamic response of the system. The total damping of the first bending mode is a combination of aerodynamic, structural, hydrodynamic, soil damping and damping due to damping devices.
Exact values of the net support structure damping are site dependent due to the influence of the soil damping on the structure’s response. In reference  measurements of all sources of damping in an offshore wind turbine at Horns Rev 1 and the Burbo offshore wind farms resulted in a logarithmic decrement of about 10% (excluding aerodynamic damping). It was concluded that the available damping is more than what it is used in the simulations. In the same study the case of the side-side aero-elastic damping is examined, and its importance to decrease cross-wind loads due to wind-wave misalignment is pointed out. In reference  the estimated logarithmic decrement considering only the additional offshore damping (no aerodynamics included) is 14-15%.
Studies have shown that especially in the case of wind-wave misalignment, the cross-wind fatigue loads at the tower bottom are extremely sensitive to the damping applied in the simulation models. A reduction in the cross-wind accumulated fatigue, compared to simulations with a logarithmic decrement of 6% (a typical value used for design of the support structure), can be achieved by increasing the values of the net support structure damping in the models. Additionally, at a typical design damping of 6%, the simulated cross-wind fatigue loads are very sensitive to the wind-wave misalignment angle; however, this sensitivity decreases when higher damping values are applied. Therefore, an accurate choice of this damping value is necessary in order to accurately predict the cross-wind fatigue loads on the support structure.
DESIGN OF SUPPORT STRUCTURE
The design of the support structure of an offshore wind turbine requires the load calculation from various operational conditions (normal operation and extreme events), as prescribed in the design load cases of the IEC 61400-3 standards. The purpose is to examine all possible wind-wave combinations that the offshore wind turbine will be subjected to during its lifetime.
Because of the low aerodynamic damping experienced in the cross-wind direction, the side-side fatigue at the support structure due to wave loading misaligned with the wind can become a significant design factor. Several studies have demonstrated the importance of wave directionality during the design process and therefore the misalignment between the wind and wave directions should be included in the design, if misalignment conditions are present in the site of installation. The effect of misalignment on the fatigue, including the probability density function of misalignment angles has been investigated in reference . An increase in the fatigue damage accumulation due to waves perpendicular to the wind direction is also reported in reference . However, measurements from full scale wind turbines, justifying these findings are not very often reported.
The over-dimensionalization of the support structure due to high estimated fatigue loading might result in a non-economically feasible design. For example for a 3.6MW offshore wind turbine, reduction in the wall thickness of the monopile up to 20% and implementation of higher damping, based on site measurements can result in 1.5 higher lifetime than the baseline design (37 years). The thickness reduction results in saving around 120 tons of steel from the substructure and approximately 81600-90000 euros per wind turbine, based on the current offshore steel plate prices, as provided by the specialized steel mills and steel trading companies.
The increased number of offshore wind farms that are currently under development requires reliable load prediction for enhanced component reliability. The importance of accurate simulation models and representation of damping is pointed out during the design phase of the support structure of an offshore wind turbine, for material savings and thus reduction of the cost of electricity.
 Tarp-Johansen, N. J., Andersen, L., Christensen, E. D., Morch, C., Kallesoe, B., & Frandsen, S. (2009). Comparing Sources of Damping of Cross-Wind Motion. European Wind Energy Conference EOW. Stockholm.
 Damgaard, M., Andersen, J. K., Ibsen, L. B., & Andersen, L. V. (2012). Natural frequency and damping estimation of an offshore wind turbine structure. International Offshore and Polar Engineering Conference ISOPE. Rhodes.
 Norton, E. J., & Quarton, D. C. (2003). Recommendations for design of offshore wind turbines (RECOFF).
 van der Meulen, M. B., Ashuri, T., van Bussel, G. J., & Molenaar, D. P. (2012). Influence of nonlinear irregular waves on the fatigue loads of an offshore wind turbine. The Science of Making Torque from Wind. Oldenburg.