Installation of a dynamic power cable for a floating offshore wind turbine

Abstract

The latest Conference of Parties at Paris resulted in an important agreement that 27 percent of the total European energy consumption has to come from renewable energy in 2030. Offshore wind energy is an important resource for renewable energy. The shortage of shallow water areas pushed offshore wind farms into deeper waters. Floating Offshore Wind Turbines (FOWTs) become more economically viable in deeper waters than bottom founded wind turbines. The goal of this research is to explore multiple installation methods for the pull-in of a dynamic power cable into a FOWT and to compare them with respect to workability determined by different limitations. The FOWTs transfer its energy using dynamic power cables. The dynamic cable configuration used in this research is a pliant wave supported by distributed buoyancy modules (DBMs). The properties of the dynamic cable and the configuration have been chosen in cooperation with cable supplier JDR. For the FOWT a design of a vertical axis wind turbine combined with a tri-pod substructure is used. VBMS\'\ Ndurance is chosen as reference cable lay vessel (CLV) for the installation. The first method is pull-in of the dynamic cable in a pre-installed FOWT. A CLV performs the pull-in directly after laying of the dynamic cable towards the FOWT, with the tether supporting the dynamic cable configuration being installed afterwards. The second method is a post-lay pull-in of the dynamic cable into the FOWT with aid of the CLV. The third method is a post-lay pull-in of the dynamic cable from the FOWT only. In the last two methods, the dynamic cable is picked after wet storage on the seabed. The difference between these methods lies in the shape in which the dynamic cable is wet-stored. The DBMs and tether are already installed during the wet storage period of method two and three. In addition, free lay of the dynamic cable and overboarding of it prior to the pull-in operation is modelled as a reference case. In the first two pull-in method the dynamic cable transfers from the CLV towards the FOWT using a winch control system. The system controls the pay-out and pull-in rates of the winches on the CLV and FOWT, respectively. The system ensures a minimum seabed clearance and avoids that the cable configuration becomes too tight. Ansys AQWA and OrcaFlex are used to model every aspect of the different methods and operations. In addition, an external Python script is used to control the rates of the winches. All dynamic cable analysis are performed using a regular wave approach with three different wave heights and periods for every direction, distributed uniformly in 30 degree bins. The results show that all pull-in methods are suitable to install the dynamic cable into the FOWT. Based on these results the third pull-in has the highest workability. This is explained by the fact that the pull-in is performed by using the FOWT only. Therefore only the FOWT influences the cable motions, whereas in the other methods also the CLV adds to the dynamic cable behaviour. The FOWT has a more stable motion behaviour in environmental conditions compared to the CLV. For this reason the interaction of the cable with the seabed is reduced and a higher workability is obtained. Cable compression is a limitation which is a common issue in cable installation analyses. In this research it is the most common failure mechanism for all methods in environmental conditions. A correlation between compression and large vertical cable accelerations is found in the free lay model and the pull-in method 1. These vertical accelerations are mainly caused by the buoyant cable section which lies in the splash zone. Compression in the two post-lay methods is caused by interaction with the seabed during recovery of the cable. The buoyant part of the cable is modelled with the DBMs uniformly distributed. At the transition point, where the cable diameter changes due to the DBMs, problems with exceeding of the maximum curvature are found, since the cable has no contact with the chute. Avoiding of exceeding of this maximum curvature is possible by introducing a structure which supports the DBMs at the stern of the vessel.