Hatch covers of ships are designed to be efficient and cost effective, as an initial investment and during service, and at the same time should suit the demands of the various types of cargo vessels. The major objective of hatch covers and coamings on ships is to prevent the ingress of water into the cargo hold and protect the goods from being damped and damaged.
Hatch covers also act as a barrier to the ship’s internal structure by enduring the green water loads in extreme weather, which can damage the internal structure of ship due to corrosion.
The various types or hatch covers that are mainly used on board are as follows –
- Lifting type
- Rolling type
- Folding type
- Sliding type
- Roll stowing type
Lifting type or lift away type hatch covers are generally used on container ships, whereas rolling type are used on bulk carriers (to be specific side-rolling). Rolling type can be divided into end rolling, side rolling, and piggy back & telescopic. Folding type hatch covers are basically used on general cargo ships. However, these may vary depending on the stowage space and the type of cargo that is carried.
Folding type: Folding type hatch covers consists of two flat type topped panels which are operated via hydraulic arms. These may be fitted on both weather deck as well as tween deck. A major advantage in the design of folding type hatch covers is its large size which means less number of panels.
Rolling type: Rolling type hatch covers consists of two covers at the end of the hatchway. Wheels are fitted which help in the sliding of the panel either athwartship in case of side rolling or longitudinally in case of end rolling. These type of hatch covers are usually fitted on large ships. Hydraulic rams are required to raise them to rolling position as they are extremely heavy and have very large dimensions.
Lifting type: Lift away type hatch covers can be classified into two types –
a) Single panel covers,
b) Multi panel covers.
Single panel cover consists of a single cover for each opening, these can be seen mainly on bulk carriers. In case of multi panel covers, a single hold is covered via multiple number of covers, these are most commonly observed in cellular container ships in the case of longitudinal joints, and for multipurpose cargo ship in case of transverse joints.
Stacking cover type: These type of facility are used on ships having relatively smaller hatch cover. It consists of a hydraulically powered lifting crane facility whose purpose is to lift the covers longitudinally and stack it together at one end or over any empty stowage tank. It is relatively cheap and is used mostly in barges.
Designing a hatch cover does not mean just designing the cover and its coaming, there are various key components which ensure that the hatch cover stays intact and effective for a very long period. The key parts are as follows –
- Bearing pads
- Securing devices: cleats & wedges
- Pontoons and panels
- Operating mechanism
- Drains & non return valves
- Compression bars
It should be ensured that the maintenance of the above mentioned components are done on a regular basis in order to ensure the safety of the cargo and the effectiveness of the hatch cover & coamings.
General considerations while designing the various components of hatch cover are as follows:
- Deck Opening: The strength of the hull is dependent on the size of the deck opening. High tensile steel must be used to provide adequate longitudinal strength even in ships where the hatch width is more than 70% of the width. Detailed structural design and strength analysis should be carried out on ships with large hatchways. Stress concentration points are usually located at the corners of the openings which can be eliminated by providing thick elliptical or parabolic plates.
- Coaming Height: Height is measured above the upper surface of the deck. They are closed by steel covers fitted with direct securing arrangements. However, the coaming can also be omitted as long as the safety of the ship is not at risk. Majority of ships have coaming height of about 1-1.8 m in order to accommodate more cargo, especially, in bulk carriers. Coamings can also be designed by sloping them inwards, creating a larger opening at the deck level.
- Hatch Corner Construction: The designer should avoid sudden discontinuity of longitudinal strength at the ends of side coaming. The girders are extended to a suitable end beyond the hatch end for structural continuity. Usually taper brackets are used to extend the hatch coaming in order to reduce stress concentration. As mentioned earlier elliptical or parabolic plates at the corners also help in reducing stress concentration which would otherwise lead to fatigue followed by fracture.
- Drainage: Proper drainage facility should be provided in order to prevent the damage of sensitive cargo. This can be done by providing a channel around the peripheral seal of the hatch cover which ensures that the water is discharged onto the weather deck through a hole in the coaming.
Hatchless Container Ships
Concept of hatchless container ships can be considered as a revolutionary change in the field of cargo ship design. This can be understood well if we look into the disadvantages of having hatch covered container ship.
- Deadweight can be increased by eliminating the weight of the heavy hatch covers.
- Since, hatch covers are located high their removal improves stability.
- Port operations can take place swiftly eliminating the time for opening and closing of hatch covers.
- Individual vertical stacks can be removed without disturbing the others. Whereas previously the containers on the hatch had to be removed first in order to get to the containers below the deck.
- Securing of containers has become easier in contrast with a conventional container ship where the containers above the hatch covers had to be secured via lashings separately.
- The maintenance of hatch covers, coamings, gaskets, cleats etc., becomes unnecessary.
However, this type of concept has got various disadvantages as well. First of all, it cannot prevent the flow of green water into the holds which leads to internal structural damage. Second, when empty it is subjected to torsion and hence these ship have relatively large torsion boxes. Third, water entering the holds need to be removed via proper drainage system or else it may cause free surface effect.
As the dimensions of the ship increases the size of the hatch cover also increases as a result it becomes difficult to provide longitudinal strength to the ship. Hence, proper longitudinal strength should be provided in the form of hatch coamings.
Hogging and sagging of the vessel can also be reduced by providing gaskets at the corners of the hatch whose rubber can absorb relative movement of the compression bars. Transverse deformation can also be reduced using gaskets. However, in order to reduce torsional deformation, resilient cleats should be fitted at the ends of the hatch covers, thus reducing the relative movement between the panels.
It is to note that a variety of factors are taken into consideration while choosing the type and design of hatch cover for a particular ship.
The Original Posted by : http://www.marineinsight.com