The motions of floating BBDB(backward-bent duct buoy) wave energy absorbers with an orifice type air-duct located at the middle of the top lid in regular waves are calculated taking account of the oscillating surface-pressure due to the pressure drop in the air chamber above the oscillating water column within the scope of the linear wave theory. A series of model tests has been conducted in order to verify the motion responses in regular waves at the ocean engineering basin, MOERI/KORDI. The pneumatic damping through an orifice-type duct for the BBDB wave energy device are deduced from experimental research. Numerical simulation for motion responses of the BBDB wave energy device considering pneumatic damping coefficients has been carried out and the results are compared with those of model tests.
During last two decades various ideas have been proposed to utilize wave energy, and the system based on OWC concept has been turned out to be the most promising candidate for commercialization (McCormick, 1981; Masuda et al., 1986) because of its simplicity of operational principle and easiness of construction. The BBDB (Backward Bent Duct Buoy) type OWC wave energy device has been proposed by Masuda (1987) where the basic principle of the oscillating water column has been maintained, but the oscillating mass of the water has been increased by using a L-shape bent duct, a short vertical and long horizontal inner duct (Fig. 1). Since the BBDB has the broad resonance ranges due to surge, pitch and heave motions, it has the advantage in view of wave absorption power comparing with the conventional cylinder type OWC wave energy device. McCormick et al. (1992) and Kim et al. (2006) have experimentally shown the presence of reverse wave drift of the BBDB with an air-duct that makes the buoy drift into the waves over specific frequency ranges. The reverse drift force of the BBDB has also been found numerically by Hong et al.(2004).