4 th International Conference on Ocean Energy, 17 October, Dublin 1 Testing of a small-scale floating OWC model in a wave flume R. P. F. Gomes, J. C. C. Henriques, L. M. C. Gato and A. F. O. Falcão IDMEC, Instituto Superior Técnico, Technical University of Lisbon, Avenida Rovisco Pais, 1049-001 Lisbon, Portugal Emails: ruigomes@ist.utl.pt; joaochenriques@ist.utl.pt; luis.gato@ist.utl.pt; antonio.falcao@ist.utl.pt Abstract The oscillating water column (OWC) wave energy converter is probably the most attractive way of converting the energy from the waves into electricity due to its simplicity and reliability. Shoreline full-scale prototypes were built in the 1990s and have proved the concept. However, large- scale exploitation of wave energy can only be performed off the coast, where large arrays of floating OWCs are to be deployed. This paper describes the testing, in a wave flume, of a 1:120 th -scale model of an axisymmetric floating OWC with non-uniform tail tube cross section. The device consists in a floater pierced by a hollow cylinder opened at the bottom to the sea water and at the top to the pneumatic chamber. A turbine simulator was placed at the top of the pneumatic chamber. Several values of the turbine damping coefficient were tested to assess damping influence on the system dynamics when subject to regular waves. Two mooring configurations are tested: one constraining the floater motion to heave; the other was a slack-mooring. The motion of the floater and of the inner water column, the chamber air pressure and the reflection of waves are analysed. A comparison with numerical results based on linear wave theory is presented. Keywords: Floating oscillating water column; model testing; spar-buoy; wave energy. 1. Introduction The oscillating water column (OWC) is a type of wave energy converter (WEC) that has been object of study since the 1940s, when Yoshio Masuda started developing a navigation buoy powered by this device. It consists of a pneumatic chamber open at the bottom to the sea water and at the top to the atmosphere through an air turbine. The oscillating motion of the water column acted upon by waves produces a bidirectional flow through the turbine that drives an electrical generator. The oscillatory characteristic of the flow requires the installation of a turbine and rectifying valves or, alternatively, a self-rectifying turbine. The latter option was found to be more robust and cost effective and is widely used. The OWC concept has been extensively studied over the last thirty years. Shoreline full-scale prototypes were built in the 1980s and 1990s. These were the cases of the Kværner multiresonant OWC, Norway [1], the Pico plant in Azores, Portugal [2] and the LIMPET plant in Islay, Scotland [3]. The Pico and LIMPET plants are still operational. Those particular devices have proved the principle of operation and the extraction of energy under real sea conditions. However, the shoreline location means a lower level of energy resource and introduces limitations into the deployment of large numbers of devices, as compared with offshore locations. More recently, floating OWC devices have been studied and developed, namely the OE Buoy and the Oceanlinx Mk3 (see [4]). A 1/4 th - scale model of the OE Buoy device has been tested in the Galway Bay (Ireland) since 2006. The Oceanlinx Mk3 prototype was tested at an offshore location in 2010. The self-rectifying Wells turbine has equipped most devices (Pico, LIMPET and OE Buoy). More recently, a self-rectifying impulse turbine was installed in the OE Buoy. The Oceanlinx Mk3 device was equipped with a Denniss-Auld turbine and a HydroAir variable radius turbine. A new self-rectifying air turbine, named biradial turbine that substantially differs from previous conceptions, was presented in [5]. Laboratory experiments on an axisymmetric floating OWC in a wave flume were first reported by Whittaker and McPeake [6]. Their work was initially based on the geometry of a navigation buoy, which was optimized through the variation of several parameters. Sarmento [7] carried out experiments on a two- dimensional bottom-standing OWC to validate a hydrodynamic model [8]. More recent studies on two- dimensional fixed OWCs were focused on the effect of the front wall configuration upon the hydrodynamic efficiency [9] and on the study of the air flow in the chamber, through the use of the particle image velocimetry technique [10]. The fixed cylindrical OWC is another type of device that has been object of some experimental studies due to its simplicity and to the availability of analytical solutions in regular waves. This geometry was used to validate hydrodynamic models [11] and to study discrete control strategies [12]. The floating version of the cylindrical OWC, moored to the tank floor, was the object of studies with