Stresses experienced by ship

 SHIP STRESSES

What is Stress and Strain ?

Stress is a load or force acting per unit area ( in kg/mm² ). They are maily three types –

1.      Tensile Stress : Force causing tension to increase the length ( elongation force ). Now a days, Tensile steel is used in ship building to counter this which also reduces weight of vessel, hence more cargo.

2.      Compression :  Forces acting in such a direction as to decrease the length.

3.      Shear :  the effect of two forces acting in opposite directions and along parallel lines so as to cause a tearing effect or to cause various parts of a section to slide over one another.

Strain is the distortion in a material due to stress.

Forces to which ship is subjected:

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.

Dynamic forces

They result from

• The ship’s motion at sea.

• The action of wind and waves.

• The effects of operating machinery.

These 👆Forces produce stresses in ship structure which may be divided into two categories:

(1) Global stress - those affecting the whole ship.

(2) Local stress - those affecting a particular part of the ship.

Global stresses:

• Shearing stress
• Longitudinal stresses in still water
• Longitudinal stresses in seaways
• Racking
• Torsion
• Water pressure
• Drydocking

Local stresses:

• Panting
• Pounding
• Localized loading
• Vibration stress
• Stress set up in the vicinity of hawse pipe, windlass and winches
• Deck openings (Hatch ways)

Shearing stresses:

     The longitudinal stresses imposed by the weight and buoyancy distribution can give rise to longitudinal shearing stresses. The maximum longitudinal shearing stress occurs at the neutral axis and decreases to a minimum at the deck and keel. Vertical shearing stresses also occur as the result of the non-uniform longitudinal distribution of weight and buoyancy.

Longitudinal stresses in still water:

Hog : When weights are placed at more at the ends as compared to amidship, the force of buoyancy amidship will exceed force of gravity amidship – causing, vessel to be hogged (midship draft less than fore and aft draft ). As a result, the deck of vessel will be under tension and bottom plating under compression.

Sag : When weights are placed more amidship as compared to, at  the ends., the force of buoyancy amidship will be less than the force of gravity amidship – causing, vessel to be Sagged (midship draft more than fore and aft draft ). As a result, the deck of vessel will be under compression and bottom plating under tension.

Longitudinal stresses in seaways

Hog: When the wave crest is in midship and trough is at ends of the vessel, there is more force of buoyancy at midship than the force of gravity, therefore the vessel Hogs.

Sag: When the wave trough is in midship and crest is at ends of the vessel, there is more force of gravity at midship than the force of buoyancy, therefore the vessel Sags.

SAG                                   HOG

Racking stress:

     When vessel is moving through cross seas, i.e, with the sea crossing from port to starboard or vice-versa.when crest is hitting one side of the ship, it tends to get deformed away from the crest, towards the trough. This process alternates and is called Racking forces

Compensation for racking stress: 

• Brackets
• Beam Knees
• Transverse Bulkhead
• Web Frames
• Floor
• Shell plating
• Pillars

Torsional stress:

     When vessel is riding on a wave at an angle, it will be subjected to a twisting moment ( torque ) and the structure in under torsion. The greatest effect occurs with decks having large openings.

Compensation for Torsional stress: Box shape girder ( Torsion Box )

Water pressure : 

     When vessel is partly submerged, water pressure acts at its bottom as well as on its sides. Pressure increases with depth. Hence, bottom part of the ship must be extra strengthened to counter this.

Compensation for Water pressure: 

• Bulkhead
• Longitudinal Girders
• Frames
• Floors
• Beam
• Shell Plating

Drydocking : 

        In docking, the whole length of ship touches to the blocks is called Grounding. When the vessel is docked on keel blocks only there is a tendency to sag transversely. This is reduced by including additional rows of blocks outward of this.

Compensation for Drydocking stress: 

• Floors
• Additional rows of blocks 

Panting stress:

     The movement of waves along a ship causes fluctuations in water pressure on the plating. This creates an in-and-out movement of the shell plating, known as panting. 

Compensation for Panting stress/ Panting arrangements on forepeak:

• In the forepeak tank, side stringers(Panting stringer) are fitted to the shell at intervals of 2 meters and supported by web frames. 
• Side stringer (Panting stringer) fitted at intervals of 2 meters and supported by Panting beams fitted at alternate frames.
• Panting beams are connected to frames by brackets and if long panting beams are supported by Wash plate.
• Perforated plate spaced not more than 2.5 meters apart. The area of perforations being not more than 10% of the total area of the flat.

Pounding stress:

     When a vessel is pitching heavily, there will be times when the bow is completely lifted out of water and slammed back into the water causing tremendous stresses on the forwards part of the vessel.

Compensation for Pounding stress:

• Pounding stresses are to be expected in the ship’s bottom between points 5% of the ship’s length abaft the stem and 25% of the length abaft the stem; or 30% in some cases. This is often called the “Pounding Region”.
• This "Pounding region" bottom plating must be additionally thickend.
• In longitudinally-framed bottoms: Plate floors are fitted at alternate frames, longitudinals may have to be stronger than normal, and side girders must be not more than 2.1 metres apart.
• In transversely-framed bottoms: Plate floors are fitted at every frame space and Extra intercostal side girders are to be fitted, so that the distance between side girders does not exceed 2.2 metres.
  

Localized loading : 

          Often in case of loading, High density cargo, load may be placed in only one hold. This will cause the force of gravity exceed the force of buoyancy in that local region. The bottom structure will tend to sag outward, as in the figure.

These stresses can be overcome by evenly spreading the cargo or by using the dunage.

Vibration stress:

      Experienced mainly in areas of Machinery spaces, pump room and propeller etc. adequately strengthened to dissipate the vibration and avoid structural damage.

Stress set up in the vicinity of hawse pipe, windlass and winches:

        Hawse pipe is fitted to enable a smooth run of the anchor cable to the windlass and to maintain the water tight integrity of the forecastle. It should be of ample size to pass the cable which is snagging when raising or lowering the anchor the anchor. construction is usually of thick plating which is attached to a Doubling plate at the forecastle deck and a reinforced stake of plating at the side shell. A rubbing or Chaffing ring is also fitted at the ship's hull at the entry to hawse pipe for protection against anchor hitting.

Deck openings (Hatch ways):

          Due to the nature of general and bulk cargo, it becomes necessary for a solid  cargo ship (general cargo or bulk carrier) to have large openings cut in her decks to enable the cargo to be loaded and discharged in an efficient and quick manner.                        Whenever large openings are cut in the strength deck the hull is tremendously weakened for two main reasons;

     i).The continuity of the dock structure is broken repeatedly from forward to aft to accommodate the hatchways, and stress concentration builds up at the hatch corners.

     ii).The transverse and longitudinal framing system under the deck is also cut.

Compensation for strength due to discontinuity:

In order to counteract for this weakening, several methods are adopted in way of hatchways.

i) At the forward and aft ends of the hatchway, deep hatch end beams are fitted;

ii) At the sides of the hatchway, deep hatch side girders are fitted;

iii) The intersection of the hatch end beams and the hatch side girders are also reinforced with the fitting of a gusset plate;

iv) The deck plating in the vicinity of the hatch way is usually doubled;

v) Hatch corners are always rounded (radiused i.e. part of a circle, or elliptical i.e. Part of a ellipse);

vi) Above the deck, stiffened Hatch coamings are fitted around the hatchway for:

1. Additional strength;
2. Safety of personnel when working near the hatchway;
3. To provide efficient means of supporting the hatch covers to close the hatch opening.