Question

With reference to the dry-docking of a vessel:

a) Describe how the vessel is prepared for dry-docking;

b) State the pro-docking information that should be given to the drydock authority:

(c) describe the hull inspection and the normal maintenance that should be carried out in dry-dock.

Answer

Before entering drydock the ship should be upright and trimmed slightly by the stern. empty of cargo with minimum free surface in the tanks.

Arrangements to be made maintain for the safe operation and maintenance of the machinery and systems at all times. This includes cooling diesel generators, shore electrical supply, auxiliary condenser, refrigerator and fire protection.

b) Gas free certificates must be obtained for any empty fuel tanks, etc. that are to be worked in drydock. Before entering the dock, all tanks and bilges are sounded and recorded. Any transferring considered necessary should be carried out.

A dry docking condition is known and a dry docking plan available for ship and dock staff.

Since tank soundings just prior to dry docking are logged, any fuel and water consumed in dock and its effect on trim and heel may be determined from data supplied by the shipbuilder.

Tables of hydrostatic data and loading tables for compartments are available.

(c) All underwater parts of the hull are inspected for damage and corrosion. Distortion of the underwater hull can be observed. If a dent is discovered, an inspection of the inside of the ship to determine internal damage should be carried out. Serious dents must be repaired, minor dents are left. All repairs and any dents left should be logged. Cathodic protection systems should be checked for security of attachment and worn sacrificial anodes should be renewed if necessary. Particular attention should be paid at the forefoot, at the after end in way of the stern frame and on twin screw ships around propeller 'A' brackets for corrosion and cracks. Attention should be given to edges of welds, which may suffer corrosion attack. The rudder drain plug should be removed to check the rudder is watertight. The rudder wear down should be checked (maximum about 19 mm) at the tiller (steering flat). In cases-of excessive wear down renew the carrier bearing (and disc if fitted). Check rudder pintle clearances, security of palm couplings and condition of the stock (cracks). The propeller should be examined for erosion (cavitation) cracks and bent blades. CP blade bolts and cone connections should be checked. All underwater valves (injections and discharges) should be examined. Valve grids and ship's boxes should be examined from the dock bottom to ensure the grids are clear and secure, and then from the engine room, etc. to see the attachments to the hull are sound; the valve seats tight (hammer test) and the valve lids and seats free of scores and pits. Check the valve spindles are free to turn in their bridges. Check each valve overhauled by shore labour before it is boxed up. The anchors and cable are ranged on the dock floor for special surveys. Defective lengths {corrosion and wear greater than 11% cracked links) are removed and replaced. Joining shackles are parted and also checked. When re-assembling the pins require re-sealed with a lead pellet.