Floating offshore solar platforms (FOSPs) are envisaged to become Ultra Large Floating Structures (ULFS) with horizontal dimensions of several kilometers. The payload of these structures comprises mainly the photovoltaic panels as well as cabling and limited electrical equipment. Thus the payload per unit area of these FOSPs is very low. Therefore it is likely that these platforms will become lightweight and flexible ULFS. Such a lightweight flexible ULFS is expected to act as a damping lid on the water surface influencing the ocean waves. The problem is to find analytical or semi-analytical solution for highly simplified modelled structure with varied stiffness interacting with surface waves. One possible way to solve the problem is to treat the structure as the boundary condition of the surface waves and apply Green’s function method to determine velocity potential and structure response.