In order to design a floating OWC-type wave energy converter such as Backward Bent Duct Buoy (BBDB) optimally, it is necessary to develop the numerical method in time domain considering motions of BBDB, air flow in air chamber and rotation of air turbine and mooring system in waves. In this paper, as the first step to develop this numerical method, firstly, equations of motion of a floating OWC-type wave energy converter in waves using the impulse response matrix by body velocity and air pressure in air chamber are shown. Secondly, calculation results in frequency domain by 3D boundary element method for a diffraction problem and radiation problems by forced motion with unit body velocity and unit air pressure in air chamber for BBDB are shown. Experiments on exciting forces and radiation forces for heave and pitch mode are also carried out and calculation results are compared with experimental results.
Many types of wave energy converters(WEC) that are based on various concepts have been proposed in recent years (Cruz(2008)). Various methods evaluating energy removed from the waves by such converters are also shown (Falnes(2002)). Recently, authors are trying the research with the aim of the practical application of Backward Bent Duct Buoy(BBDB) which is one of floating oscillating water column type WEC. This device was invented by Masuda(1986) and consists an air chamber, horizontal duct, buoyancy chamber and turbine as shown in Fig.1 This device has some advantages, that is, i)the primary conversion efficiency is higher than other floating OWCs, ii)as the wavelength for which primary conversion efficiency is maximum is about four times the length of the BBDB, a longer floating structure is not required. iii)as BBDB slowly advances against wave propagation direction in particular wave frequency band, the mooring force and mooring cost are deduced in irregular sea.