Offshore Wind Turbine Foundation Design

Abstract

Offshore wind energy has greatly matured during the last decade with an annually installed energy capacity exceeding 1 GW. A key factor for further large-scale development of offshore wind energy is a cost of energy reduction. Given for example the drop in oil price since summer 2014, which has continued into 2015 it is even more important to drive down the costs of energy for renewable energy sources such as offshore wind energy in order to arrive at a sustainable future on a global level.Cost of energy reductions for offshore wind turbines (OWTs) can be achieved by optimizations on different disciplines such as the structural design, fabrication and installation. In all cases it is very important to carefully assess the mutual influences of the different disciplines and the overall costs of energy. Different subsystems of the OWT such as the foundation or control system require on one hand the involvement of specialists with different technical backgrounds and on the other hand considerations of the whole OWT system and the mutual influences of the subsystems. For example, accurate design loads are essential for cost-efficient and safe foundation designs. However, such accurate loads can only be established under proper consideration of the dynamics of the whole system requiring adequate models of the individual subsystems and environment. This is due to the fact that OWTs introduce complex interactions between individual subsystems and the environment. Hence, a thorough understanding of the overall OWT system is essential for the establishment of accurate design loads and the subsequent optimization of individual subsystems as part of an overall optimization.In the present thesis, the design of OWT foundations is approached from the perspective of a foundation designer starting with a general introduction of the design process. The complexity of this particular field is emphasized by consideration of a variety of topics covering different foundations types and aspects throughout the design process. Focus is on structural modelling, environmental modelling and load calculations as already established in literature and design practice. Methods and approaches of the selected topics are assessed with respect to their influences on the dynamics of the system and design loads in order to evaluate their applicability in the design process. The investigations comprise new as well as existing methods and approaches.In design practice, the modelling of the structure as well as of the environment is often based on simplifications. For the environmental conditions, this is e.g. due to the fact that the combined, directional wind and wave climate consists of an impractically large amount of combinations of met-ocean parameters for load calculations purposes, which is consequently handled by application of condensed wind-wave correlations. A new damage equivalent wind-wave correlation method is introduced in the present thesis and assessed against alternative methods. It is shown that only the new method allows for a damage equivalent preservation of long-term, full wave climates throughout the entire support structure, while the alternative methods may introduce severe errors due to an insufficient consideration of the dynamics throughout the whole system.In the detailed design process, condensed wind-wave correlations are typically subjected to sequential load calculation approaches in an iterative and collaborative process between foundation designer and wind turbine manufacturer. Involvement of these different design parties may be motivated by various aspects such as introduction of state-of-the-art design expertise and tools from individual fields of technology. However, the collaboration requires special load calculation methods and simplifications of individual subsystem models in the design process due to different tools, expertise and design responsibilities of both parties. It is shown in the present thesis, how various aspects, such as the load calculation approach or the foundation model in the aeroelastic analysis, influence the dynamics and may thereby potentially introduce design load errors on the conservative or non-conservative side if not considered adequately. Different types of OWT foundations have individual characteristics and show differences in the interactions with other subsystems leading to varying requirements regarding structural modelling, environmental modelling and load calculations in the design process. Hence, it is important to carefully assess particular aspects in the specific context of OWTs and individual foundation type characteristics. For example, modelling and load calculation approaches for jacket type foundations of OWTs are often inherited from existing experiences of monopile type foundations or from their counterparts in the offshore oil & gas industry. However, severe errors may be introduced due to different dynamic characteristics and loading conditions in case the inherited approaches are not adjusted adequately for the individual requirements of jacket type foundations for OWTs. For example, quasi-static foundation load calculation approaches as often applied for jacket foundations of substations or jackets from the offshore oil & gas industry may introduce severe errors when applied to jacket foundations for OWTs e.g. due to differences in the loading conditions. However, in case of monopiles for OWTs quasi-static foundation load calculation approaches are applicable despite the fact that loading conditions are similar to their jacket counterparts. This is due to differences in the structural dynamic characteristics, e.g. the pronounced coupling of local foundation modes with higher global modes for jacket foundations of OWTs do not occur for monopiles. In the present thesis, the investigations cover monopile and jacket type foundations as representatives of individual characteristics and individual requirements of different bottom-mounted foundation types for OWTs.The present thesis is complemented by various aspects from the industrial work of the author emphasizing the industrial character of the PhD project.