SHIP STRESSESSHIP STRESSES

RishiPiravinAnmolAgilanArief

Immanuel

What is stress and What is stress and strain?strain?

• STRESS – Is defined as the load put on a piece of material or a structure

• STRAIN – Is defined as the permanent deformity or weakness caused by excessive stress

There are 3 main types of stress:

•Tensile / Tensioning•Compressive / Compression•Shear

Tensioning

Compression

tendency to pull the material apart

tendency to crush the material or to buckle

Shear

Shear is the effect of two forces acting Shear is the effect of two forces acting

in opposite directions and along in opposite directions and along

parallel lines.parallel lines.

Mechanical Properties of Metals

Elasticity - The case with which a metal may be bent or molded into a given shape.

Brittleness - The opposite of plasticity, lack or elasticity

Malleability - The property possessed by a metal of becoming permanently flattened or stretched.

Hardness - The property of a metal to resist wear and abrasion.

Fatigue - A metal subjected to continually varying loads may eventually suffer from fatigue.

Ductility - Ability to be drawn out lengthwise, the amount of the

extension measures the ductility.

Brittle Fracture• Low temperatures at or near freezing points,

•Load on material relatively light,

•Defects or faults in a weld

•Internal stresses within material ie. welding may initiate a fracture.

The ship at sea or lying in still water is The ship at sea or lying in still water is constantly being subjected to a wide constantly being subjected to a wide variety of stresses and strains, which variety of stresses and strains, which result from the action of forces from result from the action of forces from

outside and within the ship.outside and within the ship.

These forces may initially be classified into:These forces may initially be classified into:

STATIC FORCESSTATIC FORCES

DYNAMIC FORCES DYNAMIC FORCES

STATIC FORCESSTATIC FORCES

These are due to

• Internal forces resulting from structural weight, cargo and machinery weight.

• External static forces including the hydrostatic pressure of the water on the hull.

Vessel at rest

Static forceStatic force

DYNAMICDYNAMIC FORCESFORCES

They result from

• The ship’s motion at sea.• The action of wind and waves.• The effects of operating machinery.

Dynamic ForcesDynamic Forces• A ship is free to move

about six degrees of freedom.

• Three linear and three rotational.

• Various forces acting on ship are constantly varying in degree and frequency.

• This movement of the ship introduces dynamic forces which result in stresses on the ship’s structure.

• Heave is the motion of the ship when the ship have being up by a wave or sea.

• Sway is the swing of a mast or bow of a ship from side to side as the vessel progresses in a heavy sea.

• Surge is the movement forward as the bow of a ship rises and dips when it encounter waves which are strong enough to life it. The ship surge up the side of a wave, often shipping water as it passes through the crest, then dips down the other side of the wave.

• Pitch is the motion of a ship in rising the crest of a wave then descending into the following trough.

• Roll is the motion of a ship from side to side as she moves through the water.

• Yaw is where the bow of a ship falls away or sways erratically from side to side as the vessel moves through the water.

Forces produce stresses in the ship’s Forces produce stresses in the ship’s

structure which may be divided into two structure which may be divided into two

categories:categories:

•Global stress – affects the whole ship

•Local stress- affects a particular part of a ship

HOGGINGHOGGING• Hogging is when the

ship bends upwards longitudinally. This occurs when there is more weight concentrated at the ends due to uneven cargo distribution or when the vessel rides a wave crest in its middle, causing excessive buoyancy.

SAGGINGSAGGING• Sagging is the reverse of

hogging when the ship bends longitudinally in the downwards direction. This occurs when there is more weight concentrated in the mid length of the vessel due to uneven cargo distribution or when the vessel rides a wave trough in its middle causing excessive buoyancy at the ends.

RACKINGRACKING• When a ship is rolling,

the accelerations on the ship’s structure are liable to cause distortion in the transverse section.

• Greatest effect is under light ship condition.

TORSIONTORSION• A ship traversing a

wave train at angle will be subject to righting moments of opposite directions at its ends.

• The hull is subject to a twisting moment and the structure is in torsion.

STRESSES DUE TO STRESSES DUE TO WATER PRESSUREWATER PRESSURE

• Water pressure acts perpendicular to the surface and increases with depth

STRESSES DUE TO STRESSES DUE TO DRY-DOCKINGDRY-DOCKING

• Tends to set the keel upwards.

• Due to the up-thrust of the keel blocks.

• Tendency for the ship’s sides to bulge outwards.

• Bilges tend to sag.

PANTINGPANTING• This is a stress,

which occurs at the ends of a vessel due to variations in water pressure on the shell plating as the vessel pitches in a seaway. The effect is accentuated at the bow when making headway

DMS-DO

Stresses caused by localized loadingStresses caused by localized loading

Localized heavy loads may give rise

to localized distortion of the

transverse section.Such local loads

may be the machinery (Main engine) in the

engine room or the loading of

concentrated ore in the holds.

Pounding

Effect: 0.25L of the bow/stern

Pounding due to heavy pitching as the whole ship is lifted in a seaway may be subject the forepart to severe impact from the sea. It gives greatest effect when in light condition. About 30% from forward strengthened.

Deck opening creates areas of high local stress due to lack of continuity of Structure

Thank you and have Thank you and have a great day a great day everyone! everyone!