Floating offshore wind turbines (FOWTs) are an opportunity for large energy consumers in the oil and gas industry to reduce emissions. As the oil and gas structures are often installed in deep waters, the connecting power cables conventionally laying on the seabed have very long transmission distances leading to power losses and large cable sizes. In the present study, the novel concept of a suspended power cable between a FOWT and a Floating Production Storage and Offloading Unit (FPSO) in a large water depth of 1000 m is investigated. In this study, the power cable is kept floating between the sea surface and the seabed without touching neither of them. The power cable configuration is varied. A catenary configuration is investigated, as well as two configurations with subsea buoys attached at different locations along the cable. The OC3-Hywind 5 MW reference FOWT is set up with a deepwater mooring system and a spread moored FPSO is modeled having characteristics similar to existing FPSOs. Simulations are carried out in the analysis program OrcaFlex. Environmental conditions for the Campos Basin, Brazil, are assumed. The different configurations are evaluated in a steady-state analysis. The largest tensions occur for the catenary configuration, whereas it shows the lowest cable excursions and hang-off declination. A suspended configuration with buoys attached results in lower tensions that are below common limits but has larger excursions. This setup is studied further with a dynamic analysis. The tension at floater hang-off increases compared to steady-state results. The floater motions and the current seem to be the main factors influencing the suspended cable. The design of a suspended cable configuration is a balance between cable tensions and excursions, versus the amount and distribution of buoyancy attached.
Floating offshore wind turbines (FOWTs) are an opportunity for Floating Production Storage and Offloading Units (FPSOs) to reduce emissions. To avoid long connecting power cables with long transmission distances between a FOWT and an FPSO, the novel concept of a suspended power cable in a large water depth of 1000 m is investigated in this study. The power cable is kept floating between the sea surface and the seabed without touching either of them. A catenary configuration, as well as two configurations with subsea buoys attached at different locations along the cable, are investigated. The OC3-Hywind 5 MW reference FOWT is set up with a deepwater mooring system and a spread moored FPSO is modeled having characteristics similar to existing FPSOs. Steady-state and dynamic simulations are carried out in the numerical software OrcaFlex. The different configurations are firstly evaluated in steady-state analyses. The largest tensions are observed for the catenary configuration, whereas it shows the lowest horizontal cable excursions. Buoys attached along the center section of the cable lift it into regions with strong currents. This results in a large horizontal excursion of the cable and large tensions. The suspended configuration with buoys attached evenly over the cable results in significantly lower tensions than the other two configurations. It is studied further with dynamic analyses. The tensions at the floater hang-offs increase by a maximum of 24% compared to steady-state results indicating that dynamic analysis is crucial for the design of suspended cable configurations.
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