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Cargo holds and hatch cover strength requirement for a seagoing bulk carrier
To carry the maximum amount of cargo, bulk carriers are designed with a high block coefficient (Cb), ie their hulls are given a full form. Cargo holds have large unobstructed hatches to facilitate the process of cargo loading and discharging. The ship's holds also have hold bilges for drainage in the form of two wells, one on each side aft the hold. The bilges are used to collect water from the tank top and must not be more than half the height of the DB tank. They may also have high level bilge alarms to track the level of water in them.
On ships that discharge bulk cargoes using grabs, bulldozers or hydraulic hammers, the tanktop (the deck of the cargo hold) has to be additionally strengthened during construction.
Bulk carriers built after 1998 are fitted with water level detectors and audible and visual alarms as follows:
- In the aft of the cargo hold one alarm to warn when the water level is above the inner bottom reaches 0.5 m a second alarm to warn when the water level reaches a height of either 15% of the cargo hold or 2 m
- an alarm in the ballast tank(s) forward of the collision bulkhead to warn when the water level in the tank reaches 10% of the tank capacity. This alarm, along with the one fitted in the ballast hold, can be deactivated when the compartment is used for carriage of a liquid
- an alarm in any dry or empty space (except the chain locker) that extends forward of the first cargo hold to warn when the water level reaches height of 0.1 m above the deck of the respective compartment.
Fig: Hydraulically folded steel hatch cover
Fig:Steel hatch cover arrangement on a bulk carrier
Hatch openings are covered by weathertight steel hatch covers extending to between 45 - 60% of the ship's breadth and 57 - 67% of the length of each cargo hold.
The arrangement of cargo handling equipment on a bulk carriers weather deck and the cargo space layout should be designed with the highest possible level of ship productivity in mind, making fast and efficient loading possible even in ports with limited infrastructures.
Ship safety is of paramount importance and allows for no compromise. Many elements combine to produce a safe system, starting with features such as the ships layout. Other important factors to consider when enhancing safety at sea are hatch cover strength and weathertightness.
Hatch cover tightness is not achieved by sealing alone. Attention must be paid to hull movements and coaming deflections so that restraints, locators, support pads and sealings can be arranged in an optimal way to work together for weathertight integrity.
The marine environment is corrosive and, for maximum longevity, cargo systems have to be built to withstand these demanding conditions. Emphasis is placed on manufacturing either corrosion-free or easily replaceable components. Where these measures are not enough, the design allows for sufficient corrosion margins.
From the bow (forward perpendicular) back one quarter of the ship's length, hatch covers need to withstand a load of 1.75 tonnes per metre. Aft of this, hatch covers must withstand a load of 1.30 tonnes per metre. The forward hatches have coamings at least 600 millimetres high and at least 450 millimetres high aft of this.
Modern bulk carriers use hydraulic hatch covers that, generally, open in a fore and aft direction (for folding hatch covers) or athwartships (sliding hatch covers), for example:
Mechanical Hatch Covers
The most common are steel hatch covers, which may be of folding, sliding or rolling types, fitted with securing devices to make them weathertight . They are commonly opened or closed by either a hydraulic or an electric rolling system using a single control.
Steel Pontoon Covers
Using gantries to lift and stow hatch covers, portable steel hatch covers are used and made weathertight by securing devices such as cleats, cross joint wedges, etc. A `piggy-back' type hatch cover is sometimes used on ships.
Piggy backing pontoons means that during loading some hatches will be closed with another hatch pontoon on top. When switching to load the next hatch the loading hatch must first be closed, and then the next hatch to be loaded opened with its pontoon placed on the previous hatch. To minimise the time lost to moving between hatches, extra care must be taken in the initial planning of the loading sequences.
To keep hatch covers weathertight, effective sealing is required between the coaming and the hatch cover. To achieve the seal the compression bar exerts pressure on the rubber gasket. Once properly sealed, the hatch covers are secured in position against the coaming during sea passages by a `quick acting cleat' mechanism between the hatch covers and the coaming. Cross joint wedges are used to seal panels or pontoons. These exert pressure on the adjacent pontoon top which in turn exerts pressure on the compressor bar between the pontoons to achieve weathertightness.
Fig: Hatch cover vents
After each load/discharge and before hatch closure, all coamings must be cleared of any cargo residue to avoid damage to the hatch seals. Damage at this point could easily compromise the weathertight integrity of the hatch lid, resulting in damage to cargo. Drainage pipes and non return values at the coaming corners will need to be cleared.
After each use these jacks must be inspected for possible leaks especially at the seals, which will soon show signs of leakage. The pipe connections that run under the coaming of these various joints, are also prone to leakage (in the same manner as the jacks). This, in turn, can lead to slippery decks and possible accidents, including overboard discharge causing pollution.
Chains and Rollers
On older ships, the chains will have stretched through long usage and this will, in turn, cause problems when closing hatches as the jack-up points may not centre over the jacks. Rollers will also show wear on the bearings and split pins, and replacements may be required.
Quick acting cleats and hold downs
As with all other parts of the hatch, cross joint cleats/ wedges , quick acting cleats or hold downs all need to be checked as they will suffer from wear that can compromise the watertight integrity of the lid. In the case of quick acting cleats, the rubber bush will need to be replaced at regular intervals. Owing to the natural working of the ship at sea, weather permitting a daily check of the hatch cleats (including hydraulic) should be made to ensure that they are tight. These cleats should not to be bar tight as this could cause severe compression damage of the rubber seats.
Steel hatch cover testing guideline
Corrosion prevention methods for bulk carrier
Maintenance procedure for mechanical steel hatch covers
Steel hatch cover maintaining watertightness - Classification society guideline
Structural standards & strengthening of bulk carriers
Indication of unusual motion or attitude of bulk carriers and risk management / evacuation
Deterioration of ships structure and consequences of forward flooding
Handling water ingress problems in bulk carrier, investigation and countermeasures
Our detail pages illustrated many safety aspects of Bulk carrier
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Operation of sea going bulk carriers involved numerous hazards . Careful planning and exercising due caution for all critical shipboard matters are important . This site is a quick reference to international shipping community with guidance and information on the loading and discharging of modern bulk carriers so as to remain within the limitations as specified by the classification society.
It is vital to reduce the likelihood of over-stressing the ship's structure and also complying with all essential safety measures for a safe passage at sea. Our detail pages contain various bulk carrier related topics that might be useful for people working on board and those who working ashore in the terminal. For any remarks please Contact us
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