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Severe Wind and Rolling affecting hull strength & stability of Container ship

Purpose of IMO severe wind and rolling criterion is to evaluate the vesselís ability to survive severe beam seas and wind. Such a situation may be assumed to occur should the vessel lose steering or propulsive capability in a storm, in which case it will likely end up wallowing in beam seas.

If the vessel can roll freely back to leeward under the combination of all of these forces (assumed wind, beam seas and vessels momentum gathered in a roll) without rolling past its range of positive stability (capsize point), or taking on water though open down flooding points (such as the Engine room air inlets), it passes the requirement.

Most cargo stowage and stability calculation software provide Alarms / Highlights for stability conditions not complying with IMO severe wind and rolling criterion. Compliance must be confirmed.

Forces on containers during transportation
Cargo transportation often involves different modes of transport, by road, rail and/or sea. Often, combined cargo transport units like containers will set out by road, continue by sea and perhaps conclude as rail freight. Cargo transported by combined means must be stowed and secured so as to withstand the forces involved in all the modes of transport used.

In adverse weather, outboard and partially exposed container stacks may be exposed to wind. The degree of force depends on the velocity and direction of the wind, and the profile of the stacks affected. The higher the stacks, the greater the surface area and consequently force generated.

When container ships are exposed to heavy weather, accidents often involve overturned container stacks in the aft stacks. This is despite acceleration at the fore end being greater than in the aft.

Aft stack problems

One reason for the problems with aft stacks can be that, due to the required line of sight from the bridge, stack stowage height aft of the superstructure is greater than it is to the fore.

Another reason may be that large, wide container ships can suffer stern slamming when there are following or quartering seas. This can drastically raise forces on the securing equipment in the aft container stacks. The intensity of acceleration and forces during sea transports is affected by different types of seas. Sheltered waters, shallow seas and deep oceans cause different levels of acceleration.

Early avoidance essential

To avoid excessive acceleration and forces, course and speed may have to be adjusted to ease the shipís motion in heavy seas. Early avoidance of adverse weather and sea conditions is always recommended. Taking into account the actual stability conditions, it may also be necessary to ballast or de-ballast the ship, thus improving its behaviour and avoiding excessive acceleration.

Factors for consideration:
Other relevant factors:

Related Topics

Draft, Trim and Heel affecting Hull Strength and Stability of containership

Visibility from Bridge affecting navigation of containership

Propeller immersion affecting navigation of containership

Shearing forces, Bending moments and Torsional moment affecting Hull Strength and Stability of containership

Other factors affecting Hull Strength and Stability as necessary

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IMO Intact Stability Criterion for containership

Corrected GoM for Hull Strength and Stability of containership

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