La industria lítica del nivel C4d1a de Isturitz

Offshore wind energy generation has become one of the fastest growing means of providing renewable energy in the world. The support structure is a crucial part of an offshore wind turbine. Monopiles are currently the most commonly used structure type to support offshore wind turbines. Various aspects such as soil-structure interaction, grouted connections, damping and scour can influence the dynamic response of offshore wind turbines, and can potentially change their fatigue life. This thesis presents a comprehensive study of the factors that can influence the fatigue life of monopile-supported offshore wind turbines.

In this research, the NREL 5MW wind turbine was adapted and used as a reference model to carry out comprehensive fatigue analyses. The time-domain fatigue analyses were carried out in different stages, using a combination of different software packages. The wave load was calculated in MATLAB using a relevant wave spectrum. The incoming turbulent wind field was calculated in TurbSim and used as an input in FAST to calculate the wind load. The loads obtained from MATLAB and FAST were used as input for a beam element finite element model of the wind turbine with p-y curves to model soil-structure interaction in ABAQUS. Fatigue life was calculated in MATLAB by rainflow counting the stress signals and using the recommended S-N curve from DNV (2014). In the following, a summary of the findings is presented.

8.1 Soil-structure interaction

The influence of the soil on the short-term response of the wind turbine and its implications for the fatigue life of the structure was studied using the beam and solid (3D) finite element models of the NREL 5MW wind turbine. It was found that the overall stiffness of the offshore wind turbine is significantly influenced by the stiffness of the surrounding soil. The natural frequency of the wind turbine showed a sensitivity of up to 8% to the type of soil (from loose to dense sand soil types), close to the findings by Zaaijer (2006). The stiffness of the wind turbine in the models with p-y curves showed a good match with the elastic soil material definition in the 3D models. However, p-y curves overestimated the stiffness of soil at deeper layers compared to the 3D soil models. Application of p-y curves in the finite element models results in slightly higher stresses in the monopile, which lead to conservative fatigue life estimates. Given the computational costs associated with the 3D finite element models, p-y curves are still the most appropriate method for the fatigue analysis of offshore wind turbines. In practice, reduction of the initial stiffness of p-y springs in cohesionless soil can be used in the beam element models to model the influence of soil material nonlinearities seen in the 3D models.

145

8.2 Transition piece

The influence of the transition piece on the stiffness of the wind turbine was examined using a solid element model of the Opti-OWECS 3MW wind turbine. The presence of friction at the interface between the grout and steel cylinders in the finite element model proved to be crucial in predicting the mechanics of the transition piece. However, the value of the friction coefficient did not result in a significant change in the overall stiffness of the transition piece. The stiffness of the grouted connection was found to be effectively the same for elastic or nonlinear grout material models. Consideration of the transition piece in the wind turbine assembly revealed that reduction of grout strength or changes in the contact properties between grout and steel, which can occur in the long-term, have a small influence on the first natural frequency and the maximum stress level in the monopile. Therefore, omitting the transition piece damage in studies of fatigue life in the offshore wind turbine monopile yields accurate results.

8.3 Damping

The wind turbine operational regime and damping effects on the fatigue life were examined in the time-domain for a set of environmental states, which comprised more than 90% of the probabilities of occurrence. The influence of damping in limiting fatigue damage in offshore wind turbines was shown to be substantial. Variable damping application, as opposed to constant damping, in the fatigue analysis of offshore wind turbines was shown to reduce the fatigue damage of the environmental states above the rated wind speed, which also had higher damage contributions, and to increase the predicted fatigue life slightly. Nevertheless, as there are uncertainties attached to the damping estimation, the observed small change in fatigue life makes the constant damping assumption acceptable.

It was found that the fatigue damage is driven by a combination of hydrodynamic and aerodynamic loads for an operational wind turbine. For a non-operational turbine, the hydrodynamic loads had a more pronounced effect on the fatigue damage. In the stationary (non- operational) wind turbine, the blade pitch angles had a significant effect on the fatigue damage. Fatigue damage was driven by the wave load when the blades were feathered, while the contribution of aerodynamic loads was visible in the increased damage in the case of pitched-out- blades. Although the wind turbine experiences higher aerodynamic loads in operation, the presence of aerodynamic damping has a more significant influence on limiting the fatigue damage in the wind turbine structure. It is necessary to note that such variations in the fatigue damage in the operational and non-operational scenarios are based on the considered water depth and a more pronounced contribution of the hydrodynamic loads to the fatigue damage is expected in deeper waters. In addition, caution must be taken when applying the conclusions for the non-operational

146 scenarios considered in this research, as they are limited to the hypothetical prolongation of the fault scenarios.

For an increase of 7% in the overall damping of the system, a reduction of fatigue damage by up to 75% was observed for different environmental states. The predicted fatigue life of offshore wind turbines showed an almost linear increase with the level of damping, from 16 years at 4% overall damping to 53 years at 11% damping. From a practical point of view, supplemental damping (e.g. in the form of tuned mass damper) can extend the fatigue life of the offshore wind turbine substantially. In the case of significantly prolonged breakdown, a higher damage must be anticipated in the wind turbine structure. The reduction in the fatigue life was more substantial when higher damping was present in operational conditions.

A simplified method was proposed for monopile-supported offshore wind turbines in shallow to intermediate water depths, which uses a reference set of time-domain simulations for one damping level and the dynamic amplification of an equivalent single degree of freedom system, to predict the fatigue life for a range of damping levels. The limitations of this method for deeper waters were discussed and should be cautiously considered in practice. This method can be used to significantly reduce the computational costs involved in performing an accurate and well- grounded economic optimisation study of added damping in offshore wind turbines located in shallow to intermediate water depths.

8.4 Scour and backfilling

The influence of scour depths of up to 1.5Dpile (i.e. 9m) was studied using static, modal and fatigue analyses of the wind turbine. The first and second natural frequencies of the wind turbine showed only small changes. The static results showed that the occurrence of scour around the pile increases the maximum longitudinal stress in the pile by 9% and shifted its location down by up to 6m, as a result of the higher exposed length of the monopile and reduced soil resistance. It should be noted that such deviations in the stress are subject to the soil profile, reference wind turbine and location used and higher variations in the maximum stress can be expected at higher water depths and stiffer soil profiles.

In the time-domain fatigue analysis, the stress in the pile was considered at different depths to investigate the influence of the shift in the location of the maximum stress. Scour depth of 1.5Dpile caused a reduction of 29% in the fatigue life of the wind turbine at the originally considered point in the pile, while at the critical (lower) location, the fatigue life dropped by up to 45%. Accordingly, it was suggested that the fatigue life variability due to the shift in the location of maximum stress should be considered when designing offshore wind turbines, as it can influence the fatigue damage in the structural details significantly.

147 Backfilling of the scour hole was studied for different material densities and backfilled depths. Backfilling resulted in the recovery of the natural frequency and bending stresses, which appeared to be to a large degree independent of the backfilled material density. Backfilling with a relative material density of 40% (compared to the original soil) showed an almost complete recovery of the dynamic properties and fatigue life. Linear interpolation of the fatigue life for different natural or artificial backfilling scenarios showed that backfilled depth and period are crucial factors in the extension of the fatigue life in an offshore wind turbine. The effect of backfilling is not typically considered in the design of offshore wind turbines, but it has been proven as an important factor to benefit the operational life of the wind turbine.

The changes caused in the response of the offshore wind turbine were shown to be mainly of static or quasi-static nature, as only small changes in the first natural frequency occurred. The static variations in the longitudinal stress were compared with the variations in the time-domain simulations, using the standard deviation of the stress as proxy for the dynamic variations, and a close match was found. Consequently, it was suggested that if the changes in the natural frequency of the wind turbine are small, the fatigue life could be closely predicted by the static changes of the response. An approximate fatigue analysis method was proposed to predict the fatigue life using the statically acquired stress ratios, which requires a reference time-domain fatigue analysis. The calculated fatigue life ratios from the static analysis showed a good match with the full time - domain simulation results. In practice, this method reduces the computational costs associated with the scour/backfilling design optimisation studies substantially.

148