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Ship stresses
- 1. SHIP STRESSES TeamIrian: Raktim Maitra Piyush Pratik Sahoo Md. Faraz Ansary Md. Shajeed Khan
- 2. The ship at sea or lying in still water is constantly being subjected to a wide variety of stresses and strains. These forces may initially be classified into: STATIC FORCES DYNAMIC FORCES
- 3. STATIC FORCES • These are the forces due to difference in weight and buoyancy which occur at various points along the length of the ship
- 4. Vessel at rest Static force
- 5. Dynamic 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.
- 8. Forces produce stresses in the ship’s structure which may be divided into two categories: •Global stress – affects the whole ship •Local stress- affects a particular part of a ship
- 9. HOGGING • If the buoyancy in midship is greater the ship will hog.
- 10. SAGGING • If the buoyancy in the mid-ship region is lesser than weight the ship will sag
- 11. RACKING • 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.
- 12. STRESSES DUE TO WATER PRESSURE • Water pressure acts perpendicular to the surface and increases with depth
- 13. STRESSES DUE TO DRY-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.
- 15. SLAMMING OR POUNDING • While heaving or pitching, the forward end leaves and re-enters the water with a slamming effect. • the slamming down of the forward region on to the water is known a pounding. • Flat areas of plating are brought into violent contact with water at a very acute angle. • There is a loud bang and the ship shudders. • Momentum of the ship receives a check and energy is imparted to the ship girder to make it vibrate. • This is also known as pounding and is a result of Dynamic forces.
- 16. PANTING • 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
- 17. Arrangements to resist Panting • In the forepeak side stringers are fited. • The side singers meet at the fore end. • beams are fited at alternate frames
- 18. Stresses 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.
- 19. Deck opening creates areas of high local stress due to lack of continuity of Structure
- 20. Thank you and have a great day everyone!
Editor's Notes
- forces within the ship result from structural weight ,cargo machinery and running of machinery. External forces include:- =Hydrostatic preassure of water on hull =Action of wind and waves to withstand the above forces the ship is built in such a manner that at all times ship is able to resist all stresses/strains throughout the structure.
- 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.
- Dynamic forces result from ship motion and action of wind and waves.
- 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. the static and dynamic forces create transverse and loca; stresses in the ship structure. longitudinal stresses are greatest in magnitude and result in bending of the ship along its legth.
- 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.
- 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.
- when the ship is rolling, it is accelerated and decelerated, resulting in forces in the structure tending to distort it. this condition is known as Racking and its greatest effect is felt wen the ship is in light or ballast condition. to resist this condition Brackets, beam knees joining horizontal ad vertical items of structre are used.
- Water pressure increases with depth and tends to set in the ship's plating below the water line. Transverse section of a ship is subjected to static pressure from the surrounding water. Considerable distortion of structure can occur in absence of adequate stiffening. This is a result of Static forces.
- This is a result of Static forces.
- additional stiffening must be fited in pounding region to reduce the possibility of damage to the structure. to reduce pounding effect additional structural strength must be provided forward perpendicular aft for 25-30%of the ships length. the shell plating in either side of the keel is increased in thickness, depending upon ships minimum draught. frame spacing is reduced, full and half hieght intercoastal side girders are fitted and solid floors are installed in every frame space. with longitudinal framing, longitudinal spacing is reduced. intercoastal side girders are fited and tranverse floors are installed at alternate floors.
- the movement of waves along a ship causes fluctuation in water preassure on the plating this tends to create in and out movement of shell plating kwn as panting. this requirs special strenghtining in forepeak and aftpeak regions to counteract the stress.
- The structure of a ship is strengthened to resist the effects of panting for 15% of ships length in the forepeak side stringers are fited to the shell plating at intervals of 2m below the lowest deck the side stringers meet at fore end which in many ships a horizontal stringer is fited to the collison bulkhead in line with each shell stringer, this forms a ring around the tank channel. beams are fited in alternate frames in linw with the stringers and connected to frames by bracket.