Lazzerini, Guido (2023) Design and optimization of innovative offshore wind turbine floating platforms and mooring lines. [Tesi di dottorato]

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Abstract

The research activities carried out in this work were focused on the design and optimisation of floating offshore wind turbines. The floating offshore wind resource has a very large expected impact on the overall energy production scenario. According to an estimate by DNV, the global installed floating offshore wind capacity by 2050 is estimated to be 260 GW. Anyway, the exploitation of such valuable resource remains a technological challenge due to the harsh environment in which they operate and higher costs of installation and maintenance. To reach the ambitious de-carbonisation targets set by European regulations, and envisioned by DNV, the use of the wind resource at offshore sites in the Mediterranean (and also on the Italian coast) seems inevitable. Nevertheless, the installation in these sites presents peculiar conditions, which can be summarised as: lower average wind speed (implying lower energy production) compared to the North Sea, high seabed depths and the absence of an established supply-chain for offshore constructions. It is essential to develop routines and tools to ease the design and optimisation of such systems, in order to reduce the initial investment and thus the cost of produced energy, and to increase the competitiveness of the sector compared to other forms of renewable energy production. In this work, routines were developed for the preliminary design and optimisation of floating platforms and mooring systems, following the most common industry standards and guidelines (DNV-ST-0119, IEC 61400-3-2). A Python™ framework has been developed in which the processes of modifying the geometry and configuration of the platform and mooring lines and calculating the performance of the systems by means of time-domain simulation, carried out using open-source software, are implemented. The optimization of such systems is performed through non-gradient-based algorithms. The combination of such heuristic optimisation algorithms (e.g. genetic algorithms) and performance calculated through numerical simulations in the time domain offers a powerful tool for the design and optimisation of floating platforms and mooring lines, as it allows a large number of design variables to be explored and optimal solutions to be found in the presence of strongly non-linear objective functions. In a first case study, the optimisation environment was used to search for a floating platform and mooring line configuration to meet the requirements of an innovative control type for floating offshore wind farms. In this type of control, by increasing the yaw motion of the entire system, an increase in the wake mixing of the upwind turbines can be achieved to increase the flow velocity and thus the energy production of the turbines in the wake. In a second case study, design routines were used to develop a floating platform with an innovative shape and characteristics. The preliminary design phase was verified with numerical simulations with increasing fidelity and experiments in the towing tank of the University of Naples.

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