Subsea Arches – Cable Systems Engineering for LCOE Reductions

Event:
FOWT
Date:
April 2018
Author:
G. Gardner, P. Ward, R. Tapias, T. Brown

Hywind’s installation in 2017 was a watershed moment for floating offshore wind which
marked the transition of the technology from pilot projects into early demonstration of
commercialisation. This transition was naturally accompanied by the subject of scale. The
advent of large floating wind farms introduces the opportunity to leverage economies of
scale in order to achieve the reduction in the Levelised Cost of Energy (LCOE). One
approach to achieving this is through optimisation of the cable configuration.

The touch down point is a critical area for failure of cables particularly in highly dynamic
applications. In order to mitigate fatigue and over bending issues at the array cable touch
down point, subsea arches are often considered. These structures isolate the motion of the
floating wind turbine unit from the cable touch down point, vastly reducing the loading at
the critical bending location. Subsea arches present added cost to a project through
materials, additional foundations and extended installation durations. Optimisation of the
cable design to minimise their requirement may therefore be a key area for cost reduction
as a significant number of subsea arches may be required in large arrays.

The study investigates several avenues for reducing the cost of these components. The
most effective cost reduction for any component in a system is to remove the need for the
component. Through a parametric study, global analysis is performed to assess the fatigue
and strength limits of an array cable without a subsea arch in varying water depths and
environments. Several typical turbine vessel motions are used for the study. An envelope
is developed to define the geographical and environmental limitations for no-arch designs.

Considering increasingly challenging environments further work is performed to assess the
requirements for subsea arch size and location. The cable configuration is optimised for
the varying environments and water depths and trends in optimised cost presented. The
varying requirement from the mooring system and the impact on cost are also discussed
and taken into consideration during the optimisation process. Further options for cost
reduction through standardisation, manufacturing and installation techniques are also
discussed.

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