TECHNICAL QUESTIONS & ANSWERS PART-13

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1998

Q. Why is dry-docking referred to as a major event in the maintenance of a ship? As a chief engineer describe different steps that need consideration while planning a dry docking project of a ship due for its first special survey.

Answer:-

1. Dry docking is a process whereby a ship is brought into a dock which is then emptied of sea water so that work can be performed on the exterior part of the ship below the waterline

2. The purpose of dry docking is to examine underwater hull and fittings, assess their condition and carry out required repair and maintenance

3. As per the M.S. cargo ship construction & survey rules (INDIA) 1991, each & every ship has to dry dock twice in 5 years in which the intermediate dry dock should be at two & half years but can be extended up to 3 years. Intermediate dry dock can be replaced by the in water survey but ship has to be dry docked at 5 years’ interval.

4. The M.S. cargo ship construction & survey rules 1991 also states that propeller shaft & the tube shaft driving screw propellers shall be taken out & surveyed at the intervals not exceeding two & half years, but the Principal officer can extend it up to 3 years. But, depending on the tail shaft system like oil bath or water bath, keyless propeller or keyed propeller, the survey requirement may be extended. One of dry dockings has to coincide with the special survey, which is done once in 5 years

5. Dry-docking is referred to as a major event in the maintenance of a ship because of reason that without it the underwater portion of ship cannot be assessed and due to the extent of survey/items to be examined during dry docking.

6. Following checks and repairs are carried out in dry dock:

  • Condition of underwater hull up to load water line i.e., bottom and side shell for damages, shell-opening edges for wasted and corners for possible cracks
  • Rudder for damage and leakage, drain plugs to be opened to find evidence of leakage, locate the leak by air/hydro-test, repair and retest, rudder bearing condition including pintle, lock nut tightness and pintle clearance and smoothness of rudder movements to ascertain if rudder is required to be removed for necessary repairs. Rudder trunk and stern frame for general condition and possible damages/cracks.
  • Condition of oil seals for stern glands, rope guards, extent of shaft drop and condition of propeller blades, condition of storm valves, sea tubes, sea inlet and outlet tubes and valves (if sea connections due for survey now or before the next docking survey).
  • Condition of sea chest gratings, compressed air/steam pipe condition should be checked and condition of sacrificial anodes in sea chests should be checked
  • Condition of forward part of the vessel for chaffing with chains/damages with anchors including those on bulbous should be verified
  • Condition of bilge keel for damages, possible cracks in way of bilge keel butts and anodes should be verified
  • Chain cables if ranged and anchors if lowered should be examined
  • If docking coincides with the special survey (i.e. within 15 months of due date of Special Survey) the following SS items normally examined in dry dock should be surveyed and credited towards special survey:
  • Shell plating and TM (thickness measurement) of bottom shell
  • Anchors & chains including calibration of cables
  • Chain lockers
  • Scuppers, sanitary discharges and valves
  • No hot work is permitted in tankers and gas carriers without gas freeing the tanks and vessels are clear from ports. As such repair list for docking for these kind of ships should not only reflect defective items but also items which are likely to be defective before next DS (Docking Survey), i.e. within the next two and half years to avoid the vessel operating with a large numbers of condition of class.
  • It is essential that not only dates for docking are planned, but also details of repairs should be completed well before docking is due, discussed with repair workshops. Materials ordered and made available before repairs commenced.
  • A properly planned docking would not only reduce the docking and repair time but would also ensure proper repair at a lower cost.

7. Each classed vessel is subject to a specified programme of periodic surveys after delivery. These are based on a 5-year cycle & consist of the annual surveys an intermediate survey & a class renewal / special survey.

8. Special survey which must take place every 5 years & is a far more thorough inspection of the vessel, its machinery & fabric. Special surveys are costly &, requiring time in a dry-dock with the vessel out of service for some time, depending on what the classification surveyor finds in his inspection

 The special survey requires machinery dismantled to check it for wear & tear, the thickness of the plate checked for corrosion to assure that it remains within the acceptable limits. The opportunity will be taken to check parts of the ship that cannot be checked in normal circumstances. All the vessel’s systems like refrigeration, electrical, control engineering & main & auxiliary machinery will be examined. If the vessel is a tanker or the gas carrier, the special items of the equipment appertaining to the vessel’s type, such as cargo pumps & inert gas systems will be examined. In short, the Special Survey may be likened to the major health check for the vessel, as it gets older

9. As dry dock is very important event in the ships life, it is very costly affair too. So dry dock planning to be carried out in advance

10. Chief engineer should consider following things while planning for dry dock:

1. Prepare an official repair list; include proper photocopies of plans or diagrams of parts to repair. Send the repair list to office

2. Ensure all plans are onboard

i. dry dock plan

ii. propeller push graph and plan

iii. Rudder fitting diagram and plan

iv. Tail shaft arrangement and plan

v. Steel plans (shell expansion plans, mid ship section plan, scantlings and frame plan)

3. Check painting requirement for hull and advise master accordingly for raising requisition for paints.

4. Check the last anchor chain report and accordingly prepare for anchor and anchor chain inspection

5. Make a complete list of items to be surveyed and send it to superintendent for arranging surveyor for this.

6. Carry out a gauging inspection / thickness measurement and establish the scope of steel repairs.

7. Make a complete list of all certificates expiring; list of conditions of class to be dealt with is clear, list of new applicable regulations to be attended to is available.

8. Modifications /fabrications if any as per new regulations should be prepared

9. Make up to date list of spare parts available on board. Raise requisition accordingly

10. Job allotment

11. Job timing schedule

12. Safety

13. Check last tail shaft survey report. Check recent water content in stern tube lub oil. If stern tube shaft liners (spare) or seals are required, then inform superintendent and raise requisition accordingly.

14. CE has to check precision tools inventory. He should have poker gauge in his custody. Any short coming should be reported to superintendent