Mechanical design guidelines and criteria for mooring components in wave energy devices : Finding the optimum chain and shackle parameters

Abstract: Obtaining the perfect renewable energy source is one of the most important questions of our lifetime. One renewable energy source that could be of interest in this question because of the characteristics of the power that could be extracted from it is wave energy. There has however not been enough research done to reach a technology viable enough for large scale adoption. This study was made to investigate how to formulate the optimum design guidelines and criteria for a chain and shackle in the connection line between buoy and wave energy converter (WEC). Firstly, by conducting a literature study the material and design of the system was chosen. A main goal of the report was to make it have value in the industry, because of this the choice of design and material was based on industry standards. The material choice became the austenitic stainless steel with the grade R4, and the design of choice became the stud less chain link and the forelock shackle. A value of the expected force in the buoy WEC connection line (buoy line) was extracted from sixteen different data sets given from a wave tank test done in COAST Laboratory of Plymouth University, UK. These tests were done with two different buoys, one with a cylindrical shape and one with a torus shape. They were also done with and without dampening (the dampening was equal to 59 kN). Each of these four configurations had four different tests conducted on them resulting in a total of sixteen different data sets. The force value that occurs in the buoy line from the sixteen different wave tank tests was then scaled up and used in calculating the final diameter of the chain link. A safety factor of 1.35 was used to account for the statistical uncertainties in the characteristic properties of the specific part. These calculations were based on the fact that all chains have to be proof loaded at 70 % of their minimum theoretical breaking load and that a chain should at maximum undergo a force that is equal to 25 % of its minimum breaking load. Extra material was also added to accommodate for the corrosion that will occur in submerged environments. Finally, a finite element analysis was done on one of the chains links. The results showed that the biggest amount of von Mises stress and equivalent plastic strain occur in the inner corner of the chain link. All the contact area and the “crown” were however also shown to have plastically deformed. The plastic deformation in the contact area does not discredit the design because it is a local plasticity in a small region which leads to work hardening which in turn means that the new yield strength is higher at the deformed points, this in turn means that the wave climate will only elastically deform the system under its cyclic load and that the system will not plastically deform more than the results from the proof loading. This is very positive and will give the system a prolonged lifetime. However plastic deformation in the “crown” contributes to crack initiation which with time may lead to fatigue failure and should be considered in future studies.