Design of Mooring Systems for Large Floating Wind Turbines in Shallow Water
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- Institutt for marin teknikk 
Hywind Scotland is the first operational floating wind turbine park in existence. It was developedby Equinor and is located off the coast of Scotland at a water depth of approximately 105m.In this thesis, a time domain model of a semi-submersible floating wind turbine is used to checkthe feasibility of floating wind turbines placed at 70m water depth. If this is proven feasible, newareas for floating wind turbine parks will become available for development.This thesis describes relevant literature for mooring systems, and preexisting offshore wind turbineconcepts. Rules and regulations for floating wind turbines are described with respect toULS, ALS and line slack.The environmental conditions for this thesis are empirical data from the cite of Hywind Scotland,given by a metocan design basis from Equinor. The design requirements for floating windturbines states that it needs to be able to withstand a storm with a 50-year return period, andcurrent with a 10-year return period.The time domain model is created in SIMO-RIFLEX using SIMA, and is a simplification of a preexistingmodel crated by Qiang Wang. The aim of the simplification is to create a model with reducedsimulation time which is able to efficiently and accurately perform mooring analyses fordifferent systems. This was achieved by removing structures representing the wind turbine, andmodeling the semi-submersible floater to behave as a full turbine. By comparing the simplifiedmodel with the original model, it is shown that the excited motions are similar, the simplifiedmodel uses 64% of the original simulation time.To provide feasible mooring systems for a wind turbine in shallow water, chain mooring systemsfollowing the catenary equations and taut mooring systems compiled of polyester ropesare analyzed. It is found that for a water depth of 70m a full chain system is not feasible, and asignificant reduction in axial stiffness is needed to meet the requirements for ULS. The simulationsshows that a taut mooring system with 800m lines is feasible. Shorter lines can withstandULS, but the dimensioning factor is slack, and longer lines are required.