A series of model tests for the BBDB (Backward Bent Duct Buoy) type OWC (Oscillating Water Column) wave energy device has been conducted in order to study wave drift forces in regular waves at the ocean engineering basin, MOERI/KORDI. The BBDB models are designed for 2-D and 3-D lower hull configurations with an orifice type air-duct located at the middle of the top lid. Numerical simulation using HOBEM (Higher Order Boundary Element Method) for wave drift force responses has been carried out and the results are compared with those of the model tests. Finally, the mechanisms of reverse drift phenomena of a BBDB type wave energy device are discussed.
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 probable 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 motion resonance ranges in surge and pitch motions as well as heave, due to the bent duct, it has the advantage in view of wave absorption power comparing with the conventional cylinder type OWC wave energy device. In addition, McCormick et al. (1992) 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.