Answer Construction Question 18

Question 18: With reference to Underwater Inspection in Lieu of Dry docking

A. Explain in detail, how an underwater survey is carried out;

B. State the requirements to be fulfilled before an underwater survey is acceptable to the surveying authority;

C. Construct a list of the items in order of importance that the underwater survey authority should include.

Answer:  A. Preparation: To facilitate underwater survey, plans must be submitted showing the external features of the hull below the sheer strake with a key plan indicating the location of frames, bulkheads, weld lines, openings etc. In order that the survey can plan the action beforehand.

Cleaning: The hull plating surface is to be cleaned prior to the in-water survey by a particular system called 'Brush kart'. It is hydraulically operated vehicle with three brushing heads. It is drawn by a driver to clear the plating surface of all forms of marine fouling.

Survey Vehicle: It is a self propelled, steerable survey vehicle fitted with the following

(a) A long range low light TV camera to aid steering and check for hull distortion;

(b) A close view high resolution color TV Camera, to give a true picture of the state of the vessel coating and for inspections of weld seams;

(c) A 35mm still camera;

(d) An ultrasonic probe to measure plate thickness;

(e) A depth meter;

(f) Speed indicator;

(g) An umbilical cable from survey vehicle to survey Boat for electrical power supply to hydraulic motor and information transfer;

(h) Buoyancy spares and compressed air bottle for floating of the vehicle.

Survey Boat: It Houses the surveyor and consists of-

(a) A control containing TV monitors;

(b) Plate thickness print-out;

(c) Audio cassette recorder;

(d) Video cassette recorder;

(e) Play back unit;

(f) Drive communication system;

(g) Vehicle control system and all associated instruments.

Operation:

(a) The survey vehicle is taken to the datum line by a drive;

(b) With the aid of TV monitors and using the shell expansion plan as a map, the vehicle may be guided, from control console, over the bottom and sides of the hull by following weld-runs and other features such as inlets and tank plugs. Pictures and navigational information are relayed back and video recorded along with plate thicknesses giving the surveyor and integrated visual record of all relevant information. The vehicle will also provide pictures of stern frame, rudder, propeller, bilge, keels and hull openings.

(c) Drivers are also used for measurement of stern bearing wear down, pintle clearance and to inspect stern seals, anodes and rudder stock palm coupling bolts.

(d) The video recording, audio recording of the conversation between the surveyor and the driver and thickness printout and analyzed to obtain a clear knowledge of the ship's underwater condition.

Conditions and Requirements:

(a) This may be carried out for vessel having self-polishing paint in under water hull.

(b) The hull plating surface is to be cleared prior to the in-water survey by a particular system called "Brush kart". It is hydraulically operated vehicle with three brushing heads. It is drawn by a diver to clear the plating surface of all forms of marine fouling.

(c) To be carried out under water surveillance of a surveyor with.

(d) Ship is to be brought to suitable draught.

(e) Ship to be anchored possibly in sheltered water

(f) The water should be so choosen where visibility is good.

(g) The survey may be carried in lieu of any one of the two dockings required in a 5-year period on ships less than 15 years old.

(h) Beam of vessel to be greater than 30m (or less as agreed in special areas) and

List of items:

(a) Underwater hull up to load water line;

(b) Bottom and side shell for damages;

(c) Shell opening edges inspected for damage/wastage and corners for possible cracks;

(d) Condition of oil seal for stern tube;

(e) Rope guard, condition of propeller blades;

(f) Sea tubes;

(g) Sea water inlet and outlet valves;

(h) Sea chest grating;

(i) Sacrificed anodes;

(j) Compressed air, steam pipe lines to sea chest;

(k) Forward part of vessel for chaffing with chain, damage, with anchor including those on bulbous bow;
(l) Condition of bilge keel, possible cracks;

(m) Boot topping region for rubbing against quays and floating debris (region b/w ballast and loaded WL);

(n) Tank drain plugs for dents.