Suspended Power Cable Configurations for Floating Offshore Wind Turbines in Deep Water Powering an FPSO

Abstract

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.

Optimization of bend stiffeners of a suspended inter-array power cable between two floating offshore wind turbines

“…When subject to current in the transverse direction, the cable turned almost into a horizontal catenary. Schnepf et al [5] reported that this led to large tension and a large departure angle at the hang-off points. An extensive range of departure angles needs to be considered in the design process of bend stiffeners.…”

“…Their parametric study revealed that the bend stiffener is unable to limit the bending when the end diameter is close to the cable's outer diameter, leading to excessive fatigue damage values. Schnepf et al [5] investigated a suspended power cable with different configurations between a FOWT and a Floating Production Storage and Offloading Unit (FPSO). In their study, the cable was observed to move upwards into regions with strong currents when the buoys' distribution was concentrated on the midspan of the cable.…”

“…An extensive range of departure angles needs to be considered in the design process of bend stiffeners. The feasibility study on suspended interarray power cables between two FOWTs was conducted by Schnepf et al [6]. The lowest values of the minimum bending radius were reported at connection points between the cable and bend stiffeners attached to the buoys.…”