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Measures to Prevent Container Ship Hull Stress and Torsional Moment

Due to a variety of weather conditions, a combination of forces exerted upon a container ship and its cargo during long sea passages. Such effects may arise from pitching, rolling, heaving, surging, yawing, swaying, or combining any two or more. A large container ship while encountering these forces its Hull stress and Torsional moment will come into border if due consideration is not being given while planning container stowage.

A container vessel cargo loading thus involves many complexities, and careful consideration, if not performed during cargo planning and handling stages it may be fatal at sea. If the Master of a containership observes the stresses beyond required limits, even though applying the best Ballast and Fuel layout, the Terminal planner / Central planner must be informed and cargo stow plan appropriately modified. He should note Staggered stowage on different cargo holds, and heavy stowage around end fore and aft Bay would cause critical stress against Hull strength.

containerships operational matters
Oil Tanker Safety Guide
Most modern ships now employ a Hull-stress monitoring system that assists in measuring stresses in selected locations of a hull girder. The system usually includes a database in which measurements are recorded for subsequent reference.

Motions on a seaway
Motions on a seaway

How to Avoid Hogging and sagging?

On a large containership, those responsible for loading the ship have to take care to avoid straining the vessel’s hull. If too much weight is placed amidships the vessel will sag. As the vessel cannot submerge her load line mark amidships she will not be able to load her full cargo. If excess weight is placed at the ends of the ship and not enough in the middle the vessel may hog. If a vessel in such a condition were loaded with a full deadweight cargo, her load line marks amidships would indicate she could carry more cargo.

Hogging stress

Sagging stress

With large ships this deformity can be feet rather than inches, and apart from the apparent tension on the hull and the puzzles already mentioned, it may also increase the draft which is often critical for a deep-draft ship getting in and out of port. Ships are somewhat flexible structures, and the bending may not do much permanent harm; but if bent severely the vessel may become permanently distorted, which is undesirable from many points of view.

To help ships’ officers and those accountable for making the essential computations avoid this bending they must, of course, be supplied with the essential data, gadgets, and calculators. Any such longitudinal stresses will be exacerbated by the vessel pitching when end on to the waves.

With smaller general-cargo ships the problem of hogging and sagging is not so likely to be dangerous, but that of stability can well be. Ship stability can be defined as the ability of the ship to return to the upright when slightly inclined, and instability can result from too much top weight or, conversely, too little bottom weight.

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