Ultrasonic cleansing

A method of cleaning a surface of an underwater structure, in which focussed beams of acoustic energy are directed at a surface to be cleaned with an energy level sufficient to cause cavitation of the water adjacent the surface and/or direct mechanical disruption at the surface, thereby dislodging matter to be removed. In one embodiment a part-spherical array (1) of acoustic transducers (2) is arranged to direct the beams to a common focus (4). In an alternative embodiment, separate focussing means may be used with a flat array. An array may be hand held or mounted to an underwater vehicle. Part-cylindrical or parabolic arrays may also be used. The method is useful for cleaning subsea structures, e.g. oil rigs, at below 15 metres depth.