Abstract— this paper describes the development of a structural dynamics add-on to wec-sim, an open-source code dedicated to the dynamic analysis of wave energy converters (wecs). when calculating the dynamic response of a body, wec-sim by default uses a rigid body dynamics approach. such an approach ignores the potential effects of structural deformation on the wec, which can in turn affect e.g. the distributed loads across the wec and / or the individual (point) load sources that depend on the dynamic response of the wec. following a similar approach to tools used in the offshore wind industry, a structural dynamic add-on was developed using code_aster as the finite element (fe) solver to enable coupled hydro-elastic, time- domain analysis. the add-on was developed and tested using an example oscillating wave surge converter (owsc) wec model, currently being developed as part of the h2020 megaroller project. in the examples studied, the inclusion of structural dynamics is shown to affect the estimated peak power take-off (pto) loads, with variations in pto force of over 10% being observed when structural dynamics are considered in the analysis