Explain fully the procedure taken before dry-docking a vessel and the precautions taken
before undocking.
In many companies it is the responsibility of the marine engineers to inspect the hull of the ship
on entering the graving dock. It is essential on such occasion to make a thorough examination to
ensure that all necessary work is carried out. The shell plating should be hosed with fresh water
and brushed down immediately to remove the salt before the sea water dries. The plating must
be carefully checked for distortion, birching, roughness, corrosion and slack rivets.
In the case of welded ships the buts and seams should be inspected for cracks. The side shell
maybe slightly damaged due to rubbing against stays. After inspection and repairs the plating
should be wired brush and painted. Any sacrificial anodes must be checked and replaced if
necessary, taking care not to paint over the surface.
The ship side valve and cocks are examined, glands repacked and greased. All external grids are
examined for corrosion and freed from any blockage. If service wastage has occurred the grid
maybe built up with welding. The shell boxes are wire brushed and painted with an anti-fouling
composition. If the double bottom tanks are to be cleaned, the tanks are drained by unscrewing
the plugs fitted at the after end of the tank. This allows for complete drainage since the ship lies
at a slight trim by the stern. It is essential that these plugs should be replaced before undocking
new gunmetal always is fitted.
The after end must be examined with particular care. The propeller shaft is measured by
inserting a wedge between the shaft and the packing. If this wear down exceeds about 8mm the
bearing material should be renewed, 10mm being regarded as an absolute maximum. There
should be little or no wear down on an oil lubrication stern tube. The wear down in this type is
usually measured by means of a special gauge as the sealing ring is not allowing the insertion of
a wear down wedge. The efficiency and safety of the ship depends to a great extent on the case
taken in carrying out such an inspection. The anchor chain should be flashed out on the dock
floor and inspected. The chain should then be sand blasted and the ends changed over before
being pick up.
DRYDOCKING OF A VESSEL
REASONS:
1) periodical docking for CSI and classification survey to assess and ascertain the extent of wear
and type of the underwater parts of the hull-shell, plating, welded seams, rudder, rudder pintle
clearances, propeller, tail shaft wear down, sacrificial anodes, sea chest, sea suction and
overboard discharge valves, sanitary discharges, storm valves and anchor chains.
2) occasional docking when it is not possible to inspect or repair a suspected damage to any
underwater part with the vessel afloat.
PREPARATIONS:
1) A detailed repair specification covering docking survey or inspections and accepted repair
pertaining to these items.
2) A repair and survey specification covering overhaul of deck, engine, electrical, navigation,
communications and accommodation equipment, repairs to hull plating hatch covers, cargo
gear, cleaning and painting approved alternations or additions to vessels equipment etc. This
specification is for items that would-be dealt with concurrently with docking surveys and
repairs.
3) all repair items to be marked out physically and spares stores required to be arranged for.
4) Ballast condition to comply with the dockyard.
5)cleaning and gas freeing of tanks for possible manning
PRECAUTIONS:
1) Fire lines tested before docking and line pressure ensured during the entire docking period
2) Potable fire extinguisher checked and ready at the area of any hot work. To confirm with the
yard that they would be providing fire watch and extinguishers for any hot work undertaken by
then.
3) Tanks and enclosed spaces to be checked for gas free certificate obtained for man entry and
hot work from approved government chemist.
DOCKING:
1) The yard prepares the block for the vessel to rest on, from the docking plans of the ship. The
docking plan shows strengthened areas on hull for supporting the vessel in dock with minimum
stress, location of sea openings and double bottom tank plugs. Arrangement is made to ensure
none of the blocks would cover sea openings, bottom plugs, etc.
2) Vessel enters dock with draft and trims conditions acceptable to the yard under the pilot age
of a dock master.
3) The dock gates are closed and water is pumped out. The ship hull aft first touches the blocks,
the keel forward following. Trim is therefore an important factor as the pumping of the water
must be rapid to allow minimum time between the stern resting and the keel.......
4) When water level in the dock lowers to sea suction draft, the ships power is switched off and
ensuring air bottles are pressed up, and shore power and fire lines are connected promptly.
5) All sea water lines are allowed to drain into the dock to prevent ingress of water while
removing any connection.
6) Cooling water connection for domestic fridge and air conditioning plants cooling are
connected
7) Sewage plant to be in operation where applicable or facilities made available ashore and
toilets locked out.
DOCKING INSPECTION WITH CSI AND CLASSIFICATION SURVEYORS:
1. Ship’s hull to be inspected for any damage like indentations in way of strakes and frames
2. Inspect ship hull for corrosion and wastage. An ultrasonic gauging of the hull may be
necessary depending on the extent of corrosion and/or age of vessel. Specification for gauging
would be as CSI classification unless generally in way of wind and weather strakes-two below
about a quarter from either end of vessel.
3. Inspect welded seams for corrosion
4. Inspect sacrificial anodes for wastage
5 Rudder should be drained and air pressure tested for leaks
6. Rudder pintle bush clearances taken, recorded and compared with last readings for extent
and rate of wear
7. Inspect propeller blades for physical damage and cavitation corrosion
8. Propeller wear down should be recorded and compared with previous readings
9. inspect sea chest grating and chambers for any damage and wastage of anodes of fitted
10. Check sea suction/discharge and storm/sanitary valves for wear and tear.
SAFETY PRECAUTIONS:
1. Fire main pressure and fire watches maintained at all times
2. Supervision to ensure personal and fire safety practices adhered to
3. Warning to be posted on electrical starter or breakers for equipment under repair, tank valve
locked up and notices displayed as necessary. proper lock out procedures followed, checklist
4. make sure lifting gear like wire slings, chain blocks have proper certification before being
taken into service.
5. A check list should be made and verified before flooding the dock for undocking the vessel.
list to include fit of bottom plugs, sea gratings, propeller ropes guard, rudder, anodes, sea
suction and discharges
6. Tank conditions checked and stability worked out for undocking draft and trim, and to verify
conditioned with dock master, the same condition as the vessel went up on dock
7. After flooding dock to sea chest level, open and check sea suction valves for any abnormality
like leaky joints or packing
8. Main engine crankshaft deflections are taken before and after docking to check out any
deviations from standard readings.
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