Currently, pilot farms of floating wind turbines (FWT) are to be installed in depths between 50m and 100m. At these depths a large part of mooring lines and power cable length is susceptible to be covered with marine growth thus affecting the mass and hydrodynamic properties of these slender structures. Offshore standards have some recommendations on the data required for taking into account the bio-colonization of the lines. However marine growth characteristics are largely site dependent, time dependent (seasonal variability) and also depend on the support surface, thus inducing a great variability in the quantities of biomass present on the lines. Furthermore, marine growth distribution along the lines length is generally assumed to be uniform, while in some case large amounts of marine growth might be concentrated over small portions of the lines. The purpose of this work is to assess the impact of the quantity of marine growth on the dynamic behavior of mooring lines of floating wind turbines in a shallow water site, as well as the effect of its distribution.

Measurements on the SEMREV test site were made in order to study the development of local marine species on mooring lines. Identification of the species on site allows assumptions to be made on their repartition and growth rate. These assumptions are checked against on-site observations which quantify the thickness variations with depth. Marine growth parameters are then transposed in terms of line mass and hydrodynamic properties to be used in numerical simulations.

A test case configuration is defined, based on the FLOATGEN European project's FWT to be installed on the SEMREV test site. The dynamic responses of the mooring lines are computed for this configuration using time domain numerical simulations. A modal analysis is conducted in order to observe the evolution of the natural frequencies of the mooring lines when subjected to different scenarios of bio-colonization. Influence of the bio-colonization is then assessed for real sea states.