A
dry cargo ship is a type of container or bulk carrier having several
cargo holds or spaces where it can accommodate the cargo. All cargo
holds in such ships are provided with a strong built lid which is known
as hatch cover.
What is the Purpose of Hatch Cover?
Cargo
holds are fitted with hatch covers to prevent the contact of cargo with
outer atmosphere i.e. air, moisture, weather and water and to avoid
cargo from getting wet.
Another
important function of hatch cover is to maintain the water tight
integrity of the ship at all sea going condition by not allowing any
ingress of water inside the cargo hold and disrupting the stability of the ship.
Maintenance for Hatch Covers:
Hatch
covers of cargo hold are generally made from light weight steel or high
tensile steel. They are fitted over a steel bar of the hold with a
rubber packing inserted in between them to avoid water ingress.
A proper routine maintenance to be performed by qualified officer on ship which must includes-
- Examination of hatch cover, hatch beams for corrosion, cracks and material failure
- Keep Cleats, hauling wire, rollers, chains and wedges in operational condition at all time
- Keep clean hatch cover tops and all drainage holes to be kept clear
- Look for any broken or missing gasket and replace it immediately. The length of renewed gasket must be minimum 1 m
- Before renewing rubber gasket, check and rectify steel to steel fault
- Gasket rubber to be of approved type by class
- Grease all the moving parts
- Check for any hydraulic system leakage if cover is oil operated
- Oil test to be performed for hydraulic system
- Call surveyor after any major repair in the cover and its concerned parts
Testing of Hatch Covers:
After
maintenance procedure it is advised to test the water tight integrity
of the hatch cover by different methods. The three methods to check
water tightness of hold covers are:
1. Hose water Test:
In
this test a water spray from a nozzle of 12mm diameter is sprayed over
the joint of hold and cover from a distance of 1m to 1.5 m with a
pressure of 0.5 m/ second water jet.
The limitation or drawbacks of this test is that it requires two persons and hatch cover to be tested must be empty.
The leakage if very minimal cannot be identified by naked eye and cannot be performed in sub zero or cold weather.
2. Ultrasonic Test:
The
Ultrasonic testing is a more accurate method of testing water tightness
of hold and its cover. In this system an ultrasonic generator is kept
inside a closed and intact cargo hold.
A
sensor of that unit is passed all over the compression joint and any
low pressure area or point detected by the instrument can be a leakage
point.
Few drawbacks of this instrument is it is not normally kept onboard and qualified person is required to perform this test.
3. Chalk Test:
This
is the oldest or most traditional method for testing hold cover
compression, but it cannot test the water tight integrity of the hold.
A
layer of chalk powder is applied all over the steel back of the hatch
and then the hatch cover is closed and tightened to its normal values.
The
impression of chalk on the rubber packing is then studied to check lack
of compression point shown by gap in the chalk marks.
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