In this paper the FOWT investigated is a vertical axis
wind turbine (VAWT) mounted on the TriFloater semisubmersible floating platform combined with a
hypothetical WEC. This hypothetical WEC was
represented by an additional degree of freedom (DOF) in
heave. Although one may represent a simple point
absorber with damping and stiffness coefficients rather
than an extra DOF, this does not seem valid for the
problem investigated here. A single-body point absorber
needs a reference point which is usually the seabed. In
the case of FOWTs, water depth is substantial which
would result in unfeasible connections between the PTO
and seabed. Another issue is that for a single-body point
absorber, efficient energy extraction is obtained when the
motion amplitude of the device is large [6], counter to
what is trying to be achieved in this study. The FOWT
cannot be assumed to be a point absorber and therefore
the WEC is being modeled as extra DOFs to be
independent of water depth.
Rather than modeling a specific WEC design, trying
to optimize it within its dynamic characteristic
constraints, here the optimum damping and stiffness
coefficients were found that would represent the ‘ideal’ damping device for a given target solution. These
optimal coefficients were identified for two cases: maximum motion reduction of the FOWT; and
maximum energy extraction by the system. A range of WEC damping values were applied in the
numerical model (see Section 4) to understand the effects of the WEC on the motion of the FOWT. In this