Static stresses and Dynamic stresses of a ship

 

Static stresses

Hogging:

• Longitudinal bending stresses could be caused while loading the ship in still water.
• Hogging is a stress that causes the ship's hull or keel to bend upwards at amidships.
• Look at the effect when the load concentration is more on the ends of the vessel.
• The ship structure bend downwards at the end.

• Sagging: 

  

• When load concentration is more at the centre, the structure tends to bend downwards in the middle.
• This action is called sagging.
• The vessel is designed with the certain allowable maximum bending moment and shear forces which are tabulated in the approved ship's stability booklet.
• The loading and distribution of weights will be monitored by chief officer of the vessel according to the prescribed limits with the help of a load indicator and stability booklet.
• He is responsible for minimizing this 'SAG' and 'HOG' of the vessel.

Water pressure:






Act on the sides and bottom of the ship.
If the ship's sides are not strengthened, it will tend to bend inwards, as shown.

Drydock: 






• When the ship is in a dry dock, a ship's sides are no longer supported by water pressure.
• The only things holding the ship are keel blocks, bilge blocks and side shores.
• The result is that the sides of the ship tend to bulge outwards and the bottom tends to sag.
• To counteract drydocking stress, the entire bottom of a ship is strengthened.

Localised stress: 

When a heavyweight is loaded over a small area or if there is a concentration of weight in a particular area, it causes localized stresses.

Localized stress is counteracted by extra strengthening.




Dynamic stresses

Panting stress:



• Fluctuation in water pressure causes in and out movement of a ship's side plating at bow and stern.
• This movement results in panting stress.
• It is more pronounced at the bow, which pushes the bow ahead into the water or swell.
• This stress is compensated by fitting panting beams with a distance of two metres between every alternate frames.









• The panting beams are connected to the frames by beam knees and supported by a wash plate at the Centre.
• Side stringers, in line with panting stringers, are fitted throughout this deep framing region.
• Breast hooks fitted to support the radiused stem plate and to prevent in and out movement of the side shell.




Pounding stress:









• Pounding stress occurs at the bottom plating of a ship near the bow during excessive pitching.
• To compensate slamming down of a ship's bottom, outer bottom plating is thickened and connections to inner shell and inner bottom girder are strengthened.









• The thickness of plates near the pounding is increased by thirty percent and transverse frame spacing is reduced from nine hundred millimeters; to seven hundred millimetres.
• Solid floors fitted at every frame space are welded to the bottom shell and transverse floors are fitted at alternate frames.




Racking stress:

• When a ship rolls in a seaway, there is a tendency for the ship's side to get deformed.
• This deformation is caused when the deck moves laterally relative to the bottom structure.
• This in turn causes the side shells to move vertically relative to each other.
• Such deformation is called racking stress.

• This stress is more pronounced on the corners of the ship due to wave action on the hull.
• Racking stresses are reduced by fitting beam knees and tank side brackets.
• But the most effective way to resist racking is to fit transverse bulkheads.

The principle of buoyancy or the Law of floatation

 Once a ship starts floating, the amount of water displaced is equal to the weight of the ship.

This principle of floatation is known as the law of buoyancy or the law of floatation.

According to this law of floatation:

• A body is acted upon by an upward force when it is completely or partially submerged in a fluid; this upward force is equal in magnitude to the volume of fluid displaced by the floating body.
• Imagine a ship placed on the water surface. When the ship is loaded with weights in different locations, concentration of weight is at one point called G.
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• • The force of gravity acts vertically downwards through this point and is called the center of gravity of a ship. This force is equal to the weight of the ship.
  • When the ship floats in water, an upward force acts on the ship, opposing the center of gravity. This force is called the buoyancy force. It is produced by the water around the ship.
  • The buoyancy force is equal to the magnitude of the weight of the water, displaced by the ship. This force enables the ship to float.
  • Buoyancy B is always at the geometric center of the underwater volume.

  

  
  

  • Reserve buoyancy is the intact volume of the ship above the waterline and up to the uppermost continuous deck. It is the space available for displacement when any additional weight is added to the ship.
  • Reserve buoyancy is expressed as a volume in cubic meters (or cubic feet).
  • The freeboard deck is the uppermost continuous deck. Freeboard is the distance from the freeboard deck to the waterline.
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  • The freeboard determines the reserved buoyancy.
  • If a ship has a large freeboard, it is said to have large reserve buoyancy.
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  • A ship with a large freeboard can withstand a significant amount of flooding of compartments in case of damage and still remain afloat.
  • A ship with a small freeboard can withstand a comparatively small amount of flooding of compartments in case of damage before it sinks.
  • The freeboard determines the angle at which the deck edge immerses if the ship is heeled.
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  • The deck edge of the vessel with a large freeboard will immerse at a relatively large angle of heel.

  
  

  • A vessel with a small freeboard will ship more seas on deck in heavy weather than a ship with a large freeboard.
  • This endangers personnel, deck cargo and deck fittings such as hatchways.
  • When the ship tilts to one side, the center of gravity remains the same.
  • However, the center of buoyancy shifts from B to B1, as shown, because the underwater volume is more on the heeling side of the ship.

  

  
  

  • This buoyancy force acts upwards through the point B1. Metacenter M is the point where the line passing through B1 meets the line passing through 'G'.
  • Metacentric height is the distance from the center of gravity to the metacenter. The height of metacenter is the distance from the keel to the metacenter.

what is ships Gross tonnage and Net tonnage

 

What is gross tonnage and tonnage and why they are important ?

The Gross tonnage is calculated using a formula that takes into account the ship's volume in cubic meter below the main deck in the enclosed spaces above the main deck this volume is then multiplied by a coefficient which results in a dimensionless number all measurements used in the calculation of molded dimensions in the drawing that tonnage is given a different color within gross tonnage which is more or less the whole ship to indicate the difference between Net Tonnage amd Gross tonnage.

In order to minimize the daily expenses of a ship. The ship owner will keep the GT as low as possible one way of doing this is by keeping the depth small some more cargo can be placed on Deck as a consequence dangerous situations can occur as the loss of Reserve buoyancy can result in a loss of stability and in more water on Deck.

The Net tonnage is also a non dimensional number that describes the volume of the cargo space. 

The NT is derived from the GT by subtracting the volume is occupied by crew, the navigational equipment, the propulsion equipment, partly workshops and ballast.

The NT may not be less than 30% of the GT.

Framing System

 

   It is the various methods used to stiffen the bottom shel and side, plating of a ship against the compressive forces of the sea.

   Types:

1. Transverse Framing System.
2. Longitudinal Framing System.
3. Combined Framing System.

1. Transverse Framing System:

• A framing system which is often adopted for the ships experiencing severe racking stress and having dominance over sagging and hogging stress.
• Deck beams are used to support deck and beams are supported by deck girders, knees are used to connect beam to ship side framing.
• Transverse frames at 3 m interval used to support the ship side plating through the length of the ship.
• Floor plates at 3 m intervals with center and side girders used to stiffen the double bottom space. 

2. Longitudinal Framing System : 

• A framing system which is often adopted for tankers and must be used for vessels greater than 198 metres in length.
• Longitudinal stiffeners are used along the ships sides and throughout the tanks length.
• Side and bottom transverses are used to support the longitudinals against compressive loading.
• Longitudinal bulkheads are provided to divide the cargo tank into center tank and wing tanks.
• Griders are used to support deck and bottom plating.
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• 3. Combined Framing System :
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• A framing system with best compromise to suite the INSITU structural demand to counter act the composite stress and strain.
•  Decks are supported by deck transverse with deck longitudinals interconnected the deck girders are used to support deck transverse.
•  Transverse frames along with side longitudinals used to support the ship side plating.
•  Floor plates with longitudinals and girders are used to strengthen the double bottle space. Brackets and knees are used to connecting members.
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