The Backward Bent Duct Buoy (BBDB) oscillating water column (OWC) wave energy converter (WEC) has been invented following the so-far most successful OWC navigation buoys in wave energy utilisation, with aims to build large and efficient OWC wave energy converters for massive wave energy production. The BBDB device could use its multiple motion modes to enhance wave energy conversion, however, the mechanism of the motion coupling and their contributions to wave energy conversion have not been well understood in a systematic manner. In particular, the numerical modelling has been very limited in exploring how these motions are coupled and how the wave energy conversion capacity can be improved. As in this part of the research of a systematic study using numerical modelling, focus is on the understanding of the hydrodynamic performance for the BBDB OWC wave energy converter. In the study, the boundary element method based on potential flow theory has been applied to calculate the basic hydrodynamic parameters for the floating BBDB OWC structure and the water body in the water column in the BBDB OWC device. With the calculated hydrodynamic parameters and the decoupled and coupled models for the BBDB OWC dynamics, it is possible to examine these hydrodynamic parameters in details and to understand how they interact each other and how they contribute to the relative internal water surface motion, a most important response in terms of wave energy conversion of the OWC devices. All these will provide a solid base for further studying the power performance of the BBDB devices for converting energy from waves as shown in the second part of the research. (C) 2019 Published by Elsevier Ltd.