The SEA-Clam wave energy device has undergone the metamorphic changes presented in the historical development. Each variation leads to more complex hydrodynamic and motion responses. The most recent novel Oscillating Water Column (OWC) form of the device has multiple internal free surfaces, the motions of which are required to predict power take-off once a device specification with structural integrity has been identified. Motion equations for structure and the large number of degrees-of-freedom of the free surfaces modelled as massless plates are derived and presented in a very compact form. This paper also proffers two simpler models consistent with standard offshore engineering calculations. These are investigated within since structural integrity of device requires further refinement; as demonstrated in the companion paper. Quality checks of the hydrodynamic analyses are explained and applied to justify the numerical investigations undertaken. Sea spectra for the possible operational site of South Uist are used to generate motion transfer functions for associated wave frequency range. The peak pitch response of this large annular shaped structure is a main concern regarding survivability. The analyses undertaken reflect conceptual rather than detailed design status of the device. (C) 2015 Published by Elsevier Ltd.