This paper introduced the concept of rather than using damping devices to dissipate wave-induced
energy from a FOWT, a WEC is implemented to capture this additional energy to increase the system
energy yield and make the system more cost effective. An initial numerical study was performed to gain a
first insight into the characteristics required of such a WEC, with the following qualitative conclusions:
Maximum energy extraction from the WEC is achieved by matching the WEC natural frequency
to the FOWT natural frequency and using low damping ratios.
Maximum motion reduction of the FOWT is achieved by shifting the WEC to a lower frequency
than the FOWT natural frequency, although this shift must not be too large.
The high variability of the absorbed power at higher values of mean absorbed power is also an issue
that may possibly be dealt with through novel control systems and PTO systems. A trade-off between the
above two outcomes would be required to ensure the final system is cost-effective.
As noted in Section 3, in this analysis the frequency-dependent added mass and damping of the WEC
were not considered. Once the geometrical shape of the WEC is specified, these characteristics will be
included in future analyses. It is acknowledged that this inclusion would change the results somewhat, but
the methodologies employed and insights gained from this study will still be valid.
This paper has presented the potential of combining wind and wave energy devices to reduce system
costs through shared infrastructure, fatigue load reduction and increase the system energy yield. The latter
is becoming increasingly important, as the myriad of restrictions of possible ocean energy offshore sites
start to limit the available space for arrays of such devices.