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How to maintain safe stability onboard container ship during laden voyage

As the nature of container ship operation, it’s tread to be lack of stability, due to Top Heavy Load, the Master shall always take special attention for her stability. Also the Master should remind factors to cause reducing stability more such as Alternating course with Big angle of Rudder, Towing by tugs at the scene of Berthing / Un-berthing, etc.

Parametric rolling occurs because of the fine hull form of large post-Panamax container ships. The large bow flare and wide transom increase the effect. The phenomenon occurs because of changes in the waterplane area, which can cause large changes in GM as waves pass. At times, GM can become negative. A large initial GM will provide large righting levers that can lead to violent rolling. In principle, IMO designated criteria of minimum GoM for Container ship should always be considered.

Course and speed factors during rough sea conditions:

During bad weather if a ship keep maintain her course into head seas and swell, she may encounter pitching and rolling heavily. Rolling in head seas may be associated with the phenomenon known as ‘parametric rolling’ which occurs because the ship’s waterplane area changes as waves pass along the ship’s length.

Maximum rolling can occur when a wave’s length is comparable to the ship’s length. At the instant when the ship’s midship is supported by a wave crest, with the bow and stern in a wave trough, there is an instantaneous loss of waterplane, sudden and massive loss of righting force and the ship may roll to very large angles. As the wave passes along the ship’s length the situation is reversed, strong righting forces are exerted and the ship rights herself but only to roll again as the next wave passes. Effectively, the ship performs simple harmonic motion but with violent rolling. During the conditions of very severe rolling, containers stowed on deck can be subjected to massive separation forces, forces that are likely to exceed the combined strength of the securing system. Parametric rolling may be prevented by alteration of course or change of speed.

Make a good stowage plan and maintain Safe loading

While making stowage plan When Master observes deviating from the safe criteria as specified in ships class approved loading manual, even though applying the best Ballast and Fuel layout, the Terminal planner / Central planner must be informed and cargo stow plan appropriately modified. Stability may similarly affect securing equipment. If the vessel is excessively stiff she may be subject to a short rolling period, creating greater loads on all equipment and fittings. Also it has a great influence on “Lashing strength for containers on deck.

Container ship prior departure
Fig:Container ship prior departure

A ship can capsize because of the variation of the metacentric height GM, or of the righting arms GZ, in head or following seas (parametric resonance) or due to the loss of control in severe following or quartering seas (broaching-to phenomenon). Nonetheless, a ship can capsize even in port if the metacentric height is negative.

Check items upon completion of repair works

Upon completion of the repair works the vessel is to sail from the yard in a safe condition as soon as possible. It is essential however, that adequate function tests and inspections are carried out under the supervision of superintendent of all safety systems and systems that had been disturbed during the repair period.

A suitable checklist is to be compiled in order to ensure that all critical equipment, such as key navigational, propulsion, steering, cargo, ballast, fire and gas detection systems are tested. Also to be included is the replacement of bottom plugs, anodes, sea chest valves correctly set and ensuring echo sounder and Doppler transducers are cleared and written confirmation that all personnel are adequately rested before departure.

loss of containers due lashing failure
Loss of containers due lashing failure

Suitable checklist to be completed by the Master under the supervision of the Fleet Superintendent after completing repairs, before and during flooding and prior to departure from the repair facility. A further checklist on testing of safety automation and protection devices is also to be completed). The completed checklists are to be sent to the management office in advance of sailing.

Prior to departure from a repair / dry-docking facility, it is imperative that the vessel emails / faxes to the management office the completed and signed forms. Entries must be made in the deck and engine room log books upon completion.

Damage Control Information (DCI) - The Damage Control Information is a part of onboard documentation. The documentation should be clear and easy to understand. It should not include more information than this directly relevant to damage control, and should be provided in the working language of the ship.

The DCI is intended to provide the ship’s officers with clear information on the ship’s watertight subdivision and equipment related to maintaining the integrity of the watertight boundaries, so that in the event of ship damage causing flooding proper precautions can be taken to prevent progressive flooding. The DCI should consist of: Damage Control Plan, Damage Control Manual, External Watertight Integrity Plan and Internal Watertight Integrity Plan.

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