Final report for WP4.3: Enhancement of design methods ... - Upwind

UPWIND WP4: Offshore Support Structures and Foundations
PART II: Design Methods for Floating Support Structures
A large part of the global offshore wind resource is in locations where the water is much deeper than that currently
experienced in offshore wind farm design, for instance off the coasts of the United States, China, Japan,
Spain and Norway. In these locations fixed-bottom support structures are not feasible. Therefore the possibility
of mounting wind turbines on floating support structures opens up the potential to utilise this deepwater resource.
The economic potential of floating offshore wind turbines (FOWT) is demonstrated in [70]. In order to
realise this potential, cost-effective floating wind turbine designs are needed which can compete with other energy
sources. This requires appropriate and targeted development of the design tools, methods and standards
used in the industry. In view of this, Part II of this report has its focus on design methods for floating support
structures.
In order for an offshore wind turbine to be certified, the IEC 61400-3 standard [69] requires that an integrated
loads and response analysis be performed. This type of analysis is fundamental to the design process as it enables
the structure to be optimised taking into account the fully coupled response of the whole system. Reliable
and validated design tools and methods are therefore needed which can model the dynamics and response of
floating wind turbine platforms in a comprehensive and fully integrated manner.
Chapter 6 presents a review of the current state-of-the-art in floating wind turbine design tools. An overview is
given of modelling techniques for FOWTs and the advantages and disadvantages of the various approaches
are discussed, together with recommendations for future development needs.
Chapter 7 presents a summary of the benchmarking activities performed for some of the available floating design
tools. The comprehensive testing and validation of these design tools is important for designers to have
confidence in their predictions.
Chapter 8 presents the development of advanced modelling approaches for selected aerodynamic, hydrodynamic
and mooring line simulation techniques and their applicability for integrated floating wind turbine modelling.
The combined aerodynamic, hydrodynamic and mooring line effects on floating wind turbines create
unique operating and failure design conditions which have not yet been studied in detail.
Chapter 9 presents recommendations for possible extensions to the IEC 61400-3 standard to enable applicability
to deep-water floating wind turbine designs, including the implementation of additional/different design load
cases.
6. Integrated design tools
6.1 Modelling methods for floating offshore wind turbines
In this section an overview is presented of the methods used for the numerical modelling of floating offshore
wind turbines. Different methods for the modelling of structural dynamics, aerodynamics, hydrodynamics and
mooring lines are compared and comparative strengths and weaknesses presented. The detailed equations
describing the various theories are not presented here for the sake of clarity and brevity.
6.1.1 Previous work
Frequency-domain methods are commonly used in the offshore oil and gas industries to analyse and design
floating structures. These methods have also been employed in a number of instances for the preliminary design
of floating wind turbines. Bulder et al. [71] used linear frequency-domain hydrodynamic techniques to find
response amplitude operators (RAOs) to investigate a tri-floater concept. Lee [72] used a similar process to
analyse a tension-leg platform (TLP) design. Vijfhuizen [73] used frequency domain analysis to design a barge
for a 5MW turbine including a wave energy device. Wayman [74] also performed calculations in the frequency
domain to model various TLP and barge designs.
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