FORCES AND STRESSES FORCES AND STRESSES ON SHIPSON SHIPSON SHIPSON SHIPS

A study of how forces and A study of how forces and stresses affect shipsstresses affect shipspp

FORCES AND STRESSESFORCES AND STRESSESFORCES AND STRESSES FORCES AND STRESSES

Do you know the difference between Do you know the difference between forces and stresses?forces and stresses?

Forces are external e g you exert aForces are external e g you exert aForces are external, e.g you exert a Forces are external, e.g you exert a force if you push or pull on a trolleyforce if you push or pull on a trolleyStSt i it j t h t f l ifi it j t h t f l ifStress Stress –– is it just what you feel if is it just what you feel if exams are on tomorrow?exams are on tomorrow?

Static Static Forces Forces ononon on ShipShippp

D iD iDynamic Dynamic ForcesForcesForces Forces on Shipon Ship-- 6 6 degreesdegreesdegrees degrees of of freedomfreedom

The movements of the 6The movements of the 6--degrees of freedomdegrees of freedom

StressStressStressStress

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

Stress cont’dStress cont’dStress … cont dStress … cont d

Stress = Force/Cross Sectional AreaStress = Force/Cross Sectional Area

You can reduce stress by increasing You can reduce stress by increasing the crossthe cross section area i e for shipssection area i e for shipsthe crossthe cross--section area, i.e. for ships, section area, i.e. for ships, use thicker plates or larger sections.use thicker plates or larger sections.

Two categories of Two categories of ggStressesStresses

Structural Stresses Structural Stresses ff h h l hff h h l h= stresses affecting the whole ship= stresses affecting the whole ship

Local Stresses Local Stresses = stresses affecting particular parts of = stresses affecting particular parts of shipshipshipship

St t l StSt t l StStructural StressesStructural Stresses

Longitudinal Stresses in Longitudinal Stresses in ggStill WaterStill Water

Although upthrust (buoyancy) is equal to Although upthrust (buoyancy) is equal to the weight of the ship, the distribution of the weight of the ship, the distribution of weight & buoyancy is not uniform weight & buoyancy is not uniform throughout the length of the vessel. throughout the length of the vessel. Thi i i diff i l dThi i i diff i l dThis gives rise to differences in load This gives rise to differences in load throughout the length in the form ofthroughout the length in the form of

b di tb di ta. bending momenta. bending momentb. shear forcesb. shear forces

NonNon--uniform uniform

i hti htweight weight distributiondistributiondistribution distribution along ship along ship causes causes bendingbendingbending bending moment and moment and shear forcesshear forces

Bending MomentBending MomentBending MomentBending Moment

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

Bending StressBending StressBending StressBending Stress

Shearing ForcesShearing ForcesShearing ForcesShearing Forces

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

Shearing StressShearing StressShearing StressShearing Stress

Result of StressResult of StressResult of StressResult of Stress

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

Longitudinal Stresses in Longitudinal Stresses in ggSeawaySeaway

These are waveThese are wave--induced, causing induced, causing hogging & sagginghogging & sagginggg g gg ggg g gg gHogging is when the crest of a wave is Hogging is when the crest of a wave is amidships causing the midship to beamidships causing the midship to beamidships, causing the midship to be amidships, causing the midship to be pushed up due to greater buoyancy pushed up due to greater buoyancy there momentarilythere momentarilythere momentarilythere momentarilySagging happens when a wave trough Sagging happens when a wave trough is amidships with the crests at theis amidships with the crests at theis amidships, with the crests at the is amidships, with the crests at the endsends

hogging & sagginghogging & sagginghogging & sagginghogging & sagging

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

hogging & sagginghogging & sagginghogging & sagginghogging & sagging

Longitudinal Stresses due Longitudinal Stresses due ggto incorrect Loadinqto incorrect Loadinq

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

RackingRackingRackingRacking

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

RackingRackingRackingRacking

Water PressureWater PressureWater PressureWater Pressure

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

Water Water PressurePressurePressurePressure

DrydockingDrydockingDrydockingDrydocking

Upthrust of keel blocks in dock tends Upthrust of keel blocks in dock tends ppto set up the keel, resulting in stresses to set up the keel, resulting in stresses which may cause a change of shape of which may cause a change of shape of y g py g pthe transverse section.the transverse section.

DrydockingDrydockingDrydockingDrydocking

Ship on Keel Blocks in Ship on Keel Blocks in ppDockDock

L li d StL li d StLocalised StressesLocalised Stresses

PoundingPoundingPoundingPounding

Heavy pitching & heaving may subject Heavy pitching & heaving may subject the forepart of the vessel to severe the forepart of the vessel to severe ppblows from the sea, especially in the blows from the sea, especially in the lightship condition.lightship condition.g pg pStrengthening of bottom occurs up to Strengthening of bottom occurs up to 25% of length from forward25% of length from forward25% of length from forward.25% of length from forward.

PoundingPoundingPoundingPounding

Pounding in seawayPounding in seawayPounding in seawayPounding in seaway

PantingPantingPantingPanting

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

PantingPantingPantingPanting

Localised LoadingLocalised LoadingLocalised LoadingLocalised Loading

Localised heavy weight, e.g. engine Localised heavy weight, e.g. engine y g , g gy g , g groom, or ore in the hold, may give rise room, or ore in the hold, may give rise to localised distortion of the parts to localised distortion of the parts ppaffected.affected.

Localised LoadingLocalised LoadingLocalised LoadingLocalised Loading

Ends of SuperstructureEnds of SuperstructureEnds of SuperstructureEnds of Superstructure

These may represent major These may represent major y p jy p jdiscontinuities in the ship's structure, discontinuities in the ship's structure, giving rise to localised stress resulting giving rise to localised stress resulting g g gg g gin cracking.in cracking.

Ends of Ends of SuperSuper--structurestructurestructurestructure

Profile of a shipProfile of a shipProfile of a shipProfile of a ship

The profile is not like thisThe profile is not like thisThe profile is not like thisThe profile is not like this

Note the “tapering” of the Note the “tapering” of the p gp gsuperstructuressuperstructures

Tapers hereTapers here

Deck OpeningsDeck OpeningsDeck OpeningsDeck Openings

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

Deck openings create Deck openings create p gp gstressesstresses

Other localised stressesOther localised stressesOther localised stressesOther localised stresses

vibration due to propellersvibration due to propellersf hf hstresses in vicinity of hawse pipes, stresses in vicinity of hawse pipes,

windlass, wincheswindlass, winchesbilge keelsbilge keels

The EndThe End