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# Ship Motion and Stresses

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Various ship motion and stresses

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### Ship Motion and Stresses

1. 1. Ship Motion & Stresses 01/04/19 1
2. 2. STATIC 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.
3. 3. Vessel at rest Static force
4. 4. DYNAMIC FORCES They result from  The ship’s motion at sea.  The action of wind and waves.  The effects of operating machinery.
5. 5.  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. Dynamic Forces
6. 6. 01/04/19 SHIP MOTION 6 Ship Motion SIX Degrees of Freedom of the Ship:  Rolling  Surging  Pitching  Swaying  Heaving  Yawing
7. 7. Rigid Body Motion of a Ship • Translational motion : surge, sway, heave • Rotational motion : roll, pitch, yaw • Simple harmonic motion : Heave, Pitch and Roll surge roll pitch heave sway yaw 6 degrees of freedom Ship Motion
8. 8. 01/04/19 8 Ship Motion
9. 9. • 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. • •
10. 10. • 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.
11. 11.  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
12. 12. HOGGING  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.
13. 13. SAGGING  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.
14. 14. 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.
15. 15.  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. TORSION
16. 16. STRESSES DUE TO WATER PRESSURE  Water pressure acts perpendicular to the surface and increases with depth
17. 17. 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.
18. 18.  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 PANTING
19. 19. 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.
20. 20. 01/04/19 22 Slamming or Pounding  Heavy pitching assisted by heaving as the whole ship is lifted in a seaway.  May subject the forepart to severe impact from the sea.  The greatest effect is experienced in the lightship condition  To compensate for this the bottom over 30% forward is additionally strengthened for ship > 65m length, forward draft <0.045L
21. 21. 01/04/19 23 Slamming or Pounding
22. 22. 01/04/19 24 Panting  This is a stress which occurs at the ends of a ship due to variation in water pressure on the shell plating as the ship pitches in a seaway.  The effect is accentuated at the bow when making headway.
23. 23. 01/04/19 25 Panting
24. 24. 01/04/19 SHIP MOTION 26 Ship Motion Methods used to Reduce Rolling Passive Bilge keel Passive tank; Controlled Passive tank Active Active tank Fin stabilisers: a. Fixed Fin b. Retractable: • Athwartships • Folding
25. 25. 01/04/19 27 Ship Motion Bilge Keel • 300-600 mm deep • 1/3 length of ship (midship) • Mounted at right angle to the bilge radiused plating either side (at turn of bilge) • Damp any tendency of the ship’s roll by 25 % • Constructed of 2 sections
26. 26. 01/04/19 SHIP MOTION 28 Ship Motion Passive Tank (Flume Tank) • Provides a righting or anti-rolling force as a result of the delayed flow of fluid in tank • Operation is independent of ship speed and will work when the ship is at rest • As the ship rolls, the mass of water in the tank will be moved but a moment or two after the ship rolls. • Thus, when the ship is finishing its roll and about to return, the still moving water will oppose the return roll • The water mass thus acts against the roll at each ship movement is activated by gravity
27. 27. 01/04/19 29 PASSIVE TANK
28. 28. 01/04/19 SHIP MOTION 30 Ship Motion Controlled Passive Tank (Winged tank System)  The greater height of tank at sides permits a large water build-up  Thus a greater moment to resist the roll  The rising fluid level must not however, fill the wing tank  The air duct between the two wing tanks contains valves which are operated by a roll sensing device  Differential air pressure between tanks is regulated to allow the fluid flow to be controlled and ‘phased’ for maximum roll stabilization
29. 29. 01/04/19 31 CONTROLLED PASSIVE TANK
30. 30. 01/04/19 SHIP MOTION 32 Ship Motion Active Tank It is made up of motor pump unit and valves When the ship rolls to starboard, the pump activates and pump the water to port tank to provide the anti-rolling moment When the ship rolls to port, the water is pumped to starboard to provide the stabilization moment The valves are activated according to the water flow direction
31. 31. 01/04/19 SHIP MOTION 33 Active Tank
32. 32. 01/04/19 34 Ship Motion Fin Stabiliser - General  One or more pair fitted each side of ship  Size or area is governed by ship’s breadth, draught, displacement etc.  May be retractable or fixed  They act to apply a righting moment to the ship as it is inclined by a wave or force on one side  The angle of tilt of the fin and the resulting moment on the ship is determined by sensing control system  The forward speed of the ship enables the fins to generate thrust which result in righting moment
33. 33. 01/04/19 35 PRINCIPLE OF STABILISER R: Force on fin L : Lift D : Drag