Design Optimisation of Power Cable, Shared Electrical Line and Mooring configurations for Floating Offshore Wind Turbines

Over the preceding decades the global demand for electricity has been rising due to social and economic progress. Together with the climate change challenge, a desire for alternative solutions is opening opportunities for renewable energies. One branch of renewable energies is offshore wind technology. It is a rising force on the energy market and its deployment has strongly increased over the last years. Projects can be large, and the wind turbines can work closer to their optimum efficiency due to more consistent wind conditions compared to on land. Bottom-fixed offshore wind turbines have a high level of standardization nowadays but are limited to a certain water depth. Thus, the trend is to go further offshore with floating concepts for even better wind resources and greater social acceptance. Currently, floating offshore wind technology does not reach a low levelized cost of energy (LCOE), meaning cost competitiveness, compared to other energy sources yet, but this is expected to change by further development. Areas for cost reduction are technology and design improvements. The cost of floating offshore wind turbines is dominated by capital expenditure. Power cables are exposed to large loads due to fluid-cable-soil interactions under combined waves and currents conditions and represents a large cost. It is also worth noting that 80% of all offshore wind insurance claims are related to electrical cables. Hence, to reduce costs, the equipment should be designed to be long-lasting, durable, optimally laid out and with the least amount of impact for the environment. Therefore, the primary objective of the proposed PhD study has been to optimize the power cable configurations of floating offshore wind turbines. Numerical analysis in this PhD study has required a fully coupled simulation and optimization tool that can account for the global response of the floating wind turbine and the hydrodynamic loads on power cables for a floating offshore wind array. The research has resulted in 11 publications, of which 2 are under final review, and 2 pure conference papers. The candidate has had 4 presentations on national and international conferences. The candidate is the main author for 6 of the publications and second author for 5. Publications: • “A Comparative Study on Damage Detection in the Delta Mooring System of Spar Floating Offshore Wind Turbines”. 2023, Springer Nature. Second author • “Dynamic Power Cable Configuration Design for Floating Offshore Wind Turbines Using Gradient-Based Optimization”. 2023, The American Society of Mechanical Engineers (ASME). Main author • “Numerical Investigations on Suspended Power Cable Configurations for Floating Offshore Wind Turbines in Deep Water Powering an FPSO”. 2023, Journal of Offshore Mechanics and Arctic Engineering. Main author • “Flow over a step cylinder using partially averaged Navier-Stokes equations with application towards dynamic subsea power cables”. 2023, IOP Conference Series: Materials Science and Engineering. Second author • “Design basis considerations for the design of floating offshore wind turbines”. 2023, Sustainable Marine Structures. Second author • “Fatigue Analysis of Inter-Array Power Cables between Two Floating Offshore Wind Turbines Including a Simplified Method to Estimate Stress Factors”, Journal of Marine Science and Engineering (JMSE), Second author • “An optimisation methodology for suspended inter-array power cable configurations between two floating offshore wind turbines”. 2023, Ocean Engineering. Second author • “Feasibility study on suspended inter-array power cables between two spar-type offshore wind turbines”. 2023, Ocean Engineering. Main author • “Suspended Power Cable Configurations for Floating Offshore Wind Turbines in Deep Water Powering an FPSO”. 2022, The American Society of Mechanical Engineers (ASME). Main author Publications under review: • “Key constraints for design analysis and optimisation of inter-array power cable configurations in floating offshore wind farms”. (2024), Marine Structures. Main author • “Efficient Global Optimization of dynamic power cable configurations for floating offshore wind turbines”. (2024), Journal of Offshore Mechanics and Arctic Engineering. Main author The candidate has further been supervisor for 3 student projects. Of direct relevance to the industry, the PhD work has resulted amongst others in two power cable configuration optimization algorithms, which have been used in CoreMarine offshore wind projects. The candidate has worked closely with CoreMarine engineers in Norway and in particular in Spain who has been involved in the offshore wind industry for close to 2 decades. The candidate has had a longer stay in the CoreMarine Australian office, getting introduced to the rising offshore wind market there and supporting their projects with her research. This has contributed to bringing real life experience, context and content to the work.

The investigation has focused mainly on dynamic subsea power cables associated with floating offshore wind installations. Initially various case studies were performed and documented to understand cable design and the driving parameters. The ultimate aim with the work though has been to define a standard for- and developing efficient algorithms for optimizing dynamic power cable configurations. To support this, a framework involving design basis considerations and key constraints has been defined to bring clarity to the border conditions for the optimization and maintain the need safety and integrity. The framework defined will as outcome support our company in our work processes for dynamic cable design, in particular for floating wind projects. The framework defined can also impact on workflow and standards for the greater industry and play into the standards of the regulating bodies. The efficient optimization algorithms developed will bring advantage to the business of our company in providing a much more qualified and objective way of defining optimal and cost-effective dynamic cable configurations. The algorithms will be known in principle through publications and can impact the way the greater industry too should they choose to benefit from the principles and findings. For the evolvement of the offshore floating wind industry to reach a low levelized cost of energy (LCOE) and become competitive to other forms of energy, this work is a contribution. The sum of works like this on other aspects of the offshore wind turbines, is what realistically will enable the offshore industry through technical readiness and financial sustainability. This is indeed an important impact.

Over the preceding decades the global demand for electricity has been rising due to social and economic progress. Together with the climate change challenge, a desire for alternative solutions is opening opportunities for renewable energies. One branch of renewable energies is offshore wind technology. It is a rising force on the energy market and its deployment has strongly increased over the last years. Projects can be large, and the wind turbines can work closer to their optimum efficiency due to more consistent wind conditions compared to on land. Bottom-fixed offshore wind turbines have a high level of standardization nowadays but are limited to a certain water depth. Thus, the trend is to go further offshore with floating concepts for even better wind resources and greater social acceptance. Currently, floating offshore wind technology does not reach a low levelized cost of energy (LCOE), meaning cost competitiveness, compared to other energy sources yet, but this is expected to change by further development. Areas for cost reduction are technology and design improvements. The cost of floating offshore wind turbines is dominated by the capital expenditure of which about 10% are mooring and anchoring costs. Apart from the mooring, power cables are also exposed to large loads due to fluid-cable-soil interactions under combined waves and currents conditions. Hence, to reduce costs, the equipment should be designed long lasting, durable, optimally laid out and with the least amount of impact for the environment. Therefore, the primary objective of the proposed PhD study is to optimize the power cable, shared electrical lines and mooring configurations of floating offshore wind turbines. Numerical analysis in this proposed PhD study requires a fully coupled simulation tool that can account for the global response of the floating wind turbine, the hydrodynamic loads on power cables, shared electrical lines and mooring lines for a floating offshore wind park.