A threaded connection body is welded to the wall of a pipe, seachest or other flooded cavity within the hull of a ship or floating offshore installation. A sealed cutting apparatus is mounted via a valve unit n the connection body and a cutter extended through the open valve to form an opening in the wall. After retracting the cutter and closing the valve, the cutting apparatus is replaced by an inspection unit having a camera which is extended through the valve and the opening to inspect the cavity. After retracting the camera and closing the valve, the inspection unit is replaced by a plug deployment unit which advances a plug through the valve and screws it into the connection body. The valve unit can then be removed and replaced with a cap so that the plug and the cap provide a double seal to the connection body.