Procedimiento para el análisis del conjunto industrial

In this chapter, the characteristics of offshore wind turbines that affect their dynamic and fatigue performance were reviewed. Offshore wind turbines mounted on monopiles were selected as the focus of the research, as they have the largest percentage of installed capacity, and failure of offshore wind turbines was discussed. An overview of the research on the influence of damping and the uncertainties associated with it was given. Soil-structure interaction and its importance in monopile performance was discussed. This was followed by a review of the literature on scour effects on the monopiles. The weaknesses of grouted connections were outlined. Finally, fatigue was considered. The following conclusions could be drawn from the literature review:

47 Knowledge on offshore wind energy structures is based on the onshore wind and more mature experience and knowledge gained in the oil and gas industry. However, the two industries are considerably different concerning their loading and other details of the structural design. Monopiles are the most common type of support structures for offshore wind turbines to date. The harsh environment that surrounds monopile offshore wind turbines creates vibration concerns. Three main areas of influence exist in the vibration of offshore wind turbine: inertia, damping and stiffness. Inertia is fairly well characterised. However, damping and stiffness effects attracted higher attention in the literature due to the complexities in their prediction.

Damping is one of the main drivers in the fatigue of offshore wind turbines. Energy dissipation through aerodynamic damping has proven to be the main contributor to the total damping present in the structure in operation. However, research has shown that estimation of aerodynamic damping is a complex task and measurements show variations in the results. It was also argued that soil damping can be higher than normally anticipated. Thus, the uncertainties in the actual amount of damping present in offshore wind turbines can affect their fatigue life significantly and require further research.

Soil-structure interaction, scour and transition piece performance affect the stiffness and hence, the dynamics and fatigue of offshore wind turbines. Soil-structure interaction between the monopile and soil is critical. The theories are all adopted from oil and gas platforms, which have significantly different characteristics. The current techniques for the application of p-y curves in the design of monopiles have shown to lack integration of the long-term soil-structure interaction elements, especially under high amplitude loads, which can put the predicted life of these structures in question. Various recent approaches for the analysis of soil-structure interaction and consideration of these effects were suggested. Yet, the extent of these effects on the long-term performance of offshore wind turbines requires further research.

Scour primarily increases the bending moment in monopiles. Tower top displacement was also shown to experience significant increase as a result of scour. Additionally, monopiles are designed to have their first natural frequency in a narrow band between the blade passing frequency (2P or 3P) and the rotor frequency (1P). Scour depth of 1.3Dpile is the recommended value for the design of offshore wind turbines, which could alter the natural frequencies of monopiles. This has significant effects on the dynamic response and long-term performance of these structures. However, research has shown that scour depth can be variable and the backfilling processes were shown to compensate some of the adverse effects of scour. The uncertainties regarding the scour depth and backfilling process requires further attention to the long-term implications of scour. A large number of monopiles with grouted connections have been reported for connection damage or failure. The design of these connections is adopted from oil and gas industry. In offshore wind

48 turbines, grouted connections undergo a combination of large axial and bending loads, which is different from oil and gas platforms. Moreover, the bending action in these connections is very complex and therefore better modelling and analysis of these connections is crucial. Shear keys or conical connections are now recommended to be used in offshore wind turbines to increase the strength of these connections. However, it was also shown that over the life of a monopile, the wearing of grout could occur which increases the chances of fatigue life reduction.

Monopiles experience various dynamic loads throughout their lifetime, which highlights the importance of fatigue design for these structures. The real loads are too complex for simulations and have to be simplified. Wind and wave loads are combined and analysed on the basis of time- domain analysis. Time-domain fatigue analysis of offshore wind turbine is used as a benchmark for fatigue predictions. The fatigue life of offshore wind turbine can vary due to different sources of uncertainty (damping, soil stiffness and scour). Fatigue loads were shown to be sensitive in offshore wind turbines with soft soils. The influence of damping on the fatigue life of offshore wind turbines’ monopiles in parked and operational conditions showed that fatigue damage is significantly affected by the level of damping. A complete study of fatigue life changes due to different damping levels has been recommended on several occasions. In addition, scour was shown to have an influence on the fatigue of monopiles in parked condition. However, under operational conditions the effect of scour has not been studied thoroughly. Combination of scour and backfilling over the life of offshore monopiles was shown to be advantageous for their fatigue. As both scour and backfilling processes are unknown and uncertain, design of offshore wind turbines can be on the over-conservative side. Their influence on the fatigue of monopile - supported offshore wind turbines in operation still remains unclear. It is evident from the literature that knowledge of the fatigue life under operational conditions through a complete fatigue life analysis, with all the uncertainties surrounding various influential parameters in the subject such as damping, scour and backfilling, is rather limited.