Question

Describe the In-water survey to classification society requirements of the external underwater structure of a very large crude carrier.

Answer. Figure.

APPLICATION: May be carried out in lieu of any one of the two dockings required in a 5 year period on ships less than 15 years old. Beam of the vessel to be greater than 30m.(or less as agreed in special cases) and a suitable high resistance paint must have been applied. To be carried out under the surveillance of a surveyor with the ship at suitable draught in sheltered water, the in-water visibility is to be good and the underwater hull clean.

The hull plating surface is cleaned prior to in-water survey by a particular system called brush kart. This is a hydraulically powered vehicle with three brushing heads. It is drawn by a driver over the hull surface to clean the plating of all forms of marine fouling.

SURVEY VEHICLE (scan): A self propelled steerable survey vehicle, fitted with a long range low light TV camera to aid steering and check for hull distortion, a close view high resolution color TV camera to give a true picture of the state of
hull coating and for the inspection of weld seams. A 35 mm still camera is also fitted. An ultrasonic probe is provided to measure plate thickness and other equipment includes a water depth meter and a speed indicator. Power is supplied and information relayed by means of an umbilical cable from the vehicle to survey boat. Vehicle is being floated by buoyancy space with compressed air supply from a bottle.

SURVEY BOAT;

It houses a control console containing TV monitors, plate thickness printout, audio cassette recorder, video recorder and play back unit, driver communication system, vehicle control system and associated instrumentation.

OPERATION :

The survey vehicle (scan) is taken to the starting datum by the diver. With the aid of TV monitors and using the shell expansion plan as a map, the vehicle may be guided from the control console over the bottom and sides of the hull by following weld runs and by reference to other features such as inlets and tank plugs. Pictures and navigational information are relayed back and video recorded along with plate thickness giving the surveyor an integrated visual record of all relevant information. The vehicle will also provide pictures of stern frame, rudder, propeller bilge keel, hull openings. Divers are also used to measure stern bearing wear down, pintle clearance and to inspect stern seals anodes and rudder stock palm coupling. The video recording and audio recording of the conversation between the surveyor and the diver and the thickness print outs are obtained and can be analyzed in detail.

PREPARATIONS:

To facilitate underwater survey plans must be submitted showing the external features of hull below the sheer strake together with a key plan indicating the location of the frames, bulkheads, weldliness, openings etc, in order that the surveyor can plan his action beforehand.