A modelling methodology for multi-body systems with application to wave-energy devices

Time-domain modelling of wave-energy devices is an important step in the design and evaluation of potential devices and to provide a basis for model-based control design. This is due to the need for information on the device's transient response characteristics; even when linear potential theory is assumed when modelling hydrodynamic loads, significant non-linearities may be present in the system due to the power-take-off (PTO), mooring, and control subsystems. In this paper, an approach for modelling multi-body marine systems is presented. Newton–Euler equations with eliminated constraints (NE-EC) are utilised to capture the rigid-body dynamics of the constrained multi-body system. This results in the convenient integration of active loads (as opposed to inter-body constraint forces) acting on the multi-body system and provides a form suitable for control design. This paper considers active loads originating from hydrodynamic, PTO, and mooring components. The model developed is validated on a two-body system and is shown to have good agreement across the range of important operational frequencies.