Conceptual Design of a Polymer Based Joint between Tether and Foundation in Tidal Energy Power Plant : Concept generation and development of a polymer based joint

Abstract: This master thesis treats the development of a new component in a tidal energy power plant. The technology that the component should be used in extracts energy from tidal an low velocity currents. This is done by that a turbine is placed on a kite which is pushed forward in the water due to the lifting force acting on the wing. A tether connects the kite with a bottom joint that is placed on a foundation at the seabed. The bottom joint used today is heavy and expensive, which was the main reason to that this thesis was initiated. In this work, the possibility of using a polymer based design for the connection between the tether and foundation was investigated. The optimal outcome of the project was that the polymer solution should provide a spring function to the power plant. A lot of conventional product development methods have been used in the project. The project was divided into five parts: planning, product specification, concept generation, concept choice and conceptual design. In order to understand what was required of the solution in terms of the spring function, a model that aimed to represent what effect a spring function would have on the power plant was developed. According to the model, a spring function in the tether direction could increase the velocity of the kite in its trajectory. The model is based on some simplifications which is assessed to need further investigation. The spring function was translated to that the component should be able to elongate as a response to the force acting in the tether direction. In the concept choice phase it was chosen to proceed with a design similar to that of a bend stiffener. It was decided that a spring function was not required of the component, though desirable. A material selection was performed and the most optimal material for a single part that should be able withstand the tension, allow rotation, and provide a spring function was concluded to be TPU(ester, aromatic, Shore 50D). In the attempt to understand what was needed to be considered if the spring function should be solved by a material response in a polymer component, relevant theory was collected. A numerical analysis in Abaqus was performed which indicated that such a solution was unreasonable. It was then decided to proceed with the development of a bend stiffener, where the tether should be connected directly to the foundation. The thesis finally concludes with a conceptual design of a bend stiffener. The most suitable material for a bend stiffener was concluded to be TPU(ether, aliphatic, Shore 60D). The initial dimensions were determined by the maximum angle and tension combination that the tether would be exposed to. A static analysis was performed in Abaqus to illustrate the function of the product. The analysis indicated that a bend stiffener could provide the required function. However, the stress in the component became high, which indicated that the bend stiffener material might fail due to the applied load. In order to fully evaluate this, it was concluded that a more accurate material model was required.