This report outlines the mechanical load profiles of a high voltage direct current dynamic cable developed within the framework of the DCDYNAMIC project. The document begins by presenting the design basis, including the selection of the cross section for the high-voltage direct current (HVDC) cable, the identification of a technically and operationally feasible cable configuration, and a description of the platform and the met-ocean data. For the purpose of this study, an existing concept of floating substation developed by Aibel AS and Hitachi Energy, with an export cable, is selected. The platform is located in the North Sea, and the corresponding environmental conditions are applied to the analysis. The report then describes the global analysis methodology, including the selection of load cases and acceptance criteria. The global analysis studies the cable system under various environmental scenarios including the influence of the platform motions. It accounts for all relevant static and dynamic loads present in the offshore environment and evaluates the cable´s response in the time domain.
The results section covers the cable configurations, the outputs of the extreme and fatigue analyses, and concludes with the evaluation of strain in the cable insulation. The extreme analysis confirms that the lazy and tether wave configurations are viable candidates for the project for the offshore environmental conditions under consideration, as they meet the acceptance criteria. In contrast, the results demonstrate that a free-hanging configuration will require special considerations in order to be feasible with respect to touchdown zone dynamics. Additionally, the fatigue analysis demonstrates that the cable’s lifetime requirements are fulfilled.
The outputs of the global analysis enable a detailed assessment of the strain within the conductor insulation, which represents a critical aspect of the project. Strain is evaluated in the insulation of the cable and for multiple configurations. A full fatigue analysis is performed to study the fatigue loading of the insulation during 30 years of operation and the result from the simulation study is presented in the form of histograms showing the strain range vs. expected number of cycles. It has been found that the largest dynamic strain ranges experienced by the insulation system during fatigue loading are less than 0.4%. The maximum dynamic strain range seen during an extreme event corresponding to 100 YRP condition is 1.4%. These results will serve as input for the experimental testing that is planned within DCDYNAMIC for which it is important to establish realistic and relevant strains representative for fatigue loads experienced by a dynamic HVDC export cable.