This paper studies a method for absorbing wave energy via the relative oscillation between a floating body and an on-board, actively controlled motion-compensated platform. One feature of such a system is that the floating device does not need a reaction from the sea-bottom or another floating/submerged body. Frequency-domain analysis is used to investigate practical energy absorption strategies for a heaving point absorber utilizing a compensator. In particular~ to prevent the oscillations of one of the system elements from being too large, constrained optimal control is applied. Calculations indicate that with proper weighting of the constraint terms the oscillations of the interacting masses and the required control forces could be restricted to realistic amplitudes, in exchange for some loss in the absorbed power.
Of the large number of wave energy devices that have evolved to date, on-shore and near-shore systems (e.g. Whittaker et aI, 1995; Hotta, 1995; Falcgo, et el, 1995; Randlov, et al, 1995; Sjostrom, 1995) have probably received the most widespread attention. In many of these devices, an oscillating primary converter absorbs wave energy by reacting against the sea-bottom via a fixed supporting structure or via an extensible pipe or tether. Except for certain shallow-water regions with localized focusing of wave energy (due to topography-induced wave refractions), greater energy is generally available in deeper waters. Deep-water operation commonly requires floating devices. Floating devices offer significant structural economy due to reduced impact loads in extreme waves. As may be expected, however, it is difficult to obtain a reaction from the sea-bottom in deep water. In some cases, a deeply submerged plate/sphere is provided for this purpose. In others, such as the floating oscillating water columns (OWC) in JAMSTEC's "Mighty Whale" (Washio et al, 1998), the primary converters react against a floating hull.
