Settling of Riser Cable due to Biofouling and Change of Response Characteristics

Abstract

Lazy Wave shape and similar shapes is often adopted for riser cables of floating offshore wind turbine (FOWT). Buoyancy is used to form and maintain the arch shape of Lazy Wave part while keeping tension in the cable low to avoid increase of tension fluctuation caused by motion of FOWT. Drawback of this shape is that it is sensitive to weight changes due to biofouling. In this study, the influence of biofouling on the static equilibrium shape, shape in current and dynamic response caused by FOWT motion was investigated. In addition, attaching of vertical spring consisting of a buoy and a chain to riser cable is proposed to reduce the sink of the cable due to biofouling. The effects of the spring on the static equilibrium shape, shape in current and dynamic response were also investigated using analysis models. It became clear that the spring has enough ability to suppress deformation of the shape of riser cable due to biofouling, and also has ability to suppress the deformation in currents and the increase of maximum tension due to the motion of FOWT.