Forces and Stresses on Ships [Compatibility Mode]

FORCES AND STRESSES ON SHIPS A study of how forces and stresses affect ships p

FORCES AND STRESSES „

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Do you know the difference between forces and stresses? Forces are external external, e e.g g you exert a force if you push or pull on a trolley St Stress – is i it just j t what h t you feel f l if exams are on tomorrow?

Static Forces on Ship p

D Dynamic i Forces on Ship -6 degrees of freedom

The movements of the 6 6-degrees of freedom

Stress „

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Stress is “internal” – so you feel y stressed inside you For steel structures, like the material that ships are made of of, stress builds up until a breaking point, then it breaks. breaks

Stress … cont cont’d d „

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Stress = Force/Cross Sectional Area You can reduce stress by increasing the crosscross-section area area, ii.e. e for ships ships, use thicker plates or larger sections.

Two categories g of Stresses „

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Structural Stresses = stresses affecting ff the h whole h l ship h Local Stresses = stresses affecting particular parts of ship

St Structural t l Stresses St

Longitudinal g Stresses in Still Water „

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Although upthrust (buoyancy) is equal to the weight of the ship, the distribution of weight & buoyancy is not uniform throughout the length of the vessel. Thi gives This i rise i to differences diff in i load l d throughout the length in the form of a. bending b di momentt b. shear forces

NonNonuniform weight i ht distribution along ship causes bending moment and shear forces

Bending Moment „

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Bending moment causes the hull to experience tensile stresses on one side of the neutral axis and compressive stresses on the other. This type of bending stresses are at the maximum at the deck and keel. Longitudinal structural members are designed to take these stresses.

Bending Stress

Shearing Forces „

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Shearing forces result in shear stresses in the affected cross section. They are at a maximum on the neutral axis. axis

Shearing Stress

Result of Stress „

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For you, too much stress causes headache,, sleeplessness, p , and even panic. For a ship ship, all these stresses may lead to deformation, buckling, or cracks in extreme cases cases.

Longitudinal g Stresses in Seaway „

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These are wave wave--induced, causing hogging gg g & sagging gg g Hogging is when the crest of a wave is amidships causing the midship to be amidships, pushed up due to greater buoyancy there momentarily Sagging happens when a wave trough is amidships amidships, with the crests at the ends

hogging & sagging „

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When a ship is in seaway, the weight distribution remains unchanged, g , but the distribution of buoyancy is altered. This causes the size and location of bending moment and shearing force to change (compared to stress in still water) and may be more damaging on the vessel. vessel

hogging & sagging

Longitudinal g Stresses due to incorrect Loadinq „

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Incorrect loading is caused by poor loading g sequence q or loading g arrangement –e.g. commencing loading g from the forward end It causes the weight distribution to be very different from the buoyancy distribution, magnifying the bending moment & shearing forces

Racking „

When a vessel is rolling, the accelerations on the ship p structure are liable to cause distortions in the transverse section. Transverse bulkheads, beam knees and tank side brackets help p to prevent p racking. g

Racking

Water Pressure „

Water p pressure acts perpendicular p p to the surface, increasing with depth. The effect is to p push the ship's p side in,, and the bottom up.

Water Pressure

Drydocking „

Upthrust p of keel blocks in dock tends to set up the keel, resulting in stresses g of shape p of which mayy cause a change the transverse section.

Drydocking

Ship p on Keel Blocks in Dock

L Localised li d St Stresses

Pounding „

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Heavy pitching & heaving may subject p of the vessel to severe the forepart blows from the sea, especially in the g p condition. lightship Strengthening of bottom occurs up to 25% of length from forward forward.

Pounding

Pounding in seaway

Panting „

This is a stress occurring g at the ends of the vessel due to variation in water pressure on the shell plating p p g as the vessel pitches in a seaway.

Panting

Localised Loading „

Localised heavyy weight, g , e.g. g engine g room, or ore in the hold, may give rise to localised distortion of the parts p affected.

Localised Loading

Ends of Superstructure „

These mayy represent p major j discontinuities in the ship's structure, giving g g rise to localised stress resulting g in cracking.

Ends of Super-Super structure

Profile of a ship

The profile is not like this

Note the “tapering” p g of the superstructures Tapers here

Deck Openings „

Holes cut in the deck plating, eg y , masts,, etc,, create areas of hatchways, high local stress due to lack of p g continuityy created byy the opening.

Deck openings p g create stresses

Other localised stresses „ „

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vibration due to propellers stresses in vicinity off h hawse pipes, windlass, winches bilge keels

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The End