Effects of various stresses in ships structures and reinforcement required
Ships are affected by movement in six degrees of freedom: rolling, pitching, heaving, swaying, surging and yawing. Of these, rolling, pitching and heaving generate the greatest forces in heavy weather.
Stresses caused by movement of the vessel through water as fluctuations in the water pressure upon shell plating results in `in-out' movement of the plates. Shell plating constantly subjected to panting stresses may fracture due to fatigue.
Stress location: Generally for 15% of the ship's length from stem and abaft of the aft peak bulkhead.
Reinforcement required : Panting beams, panting stringers, wash bulkhead, additional stiffeners and reduced frame spacing in the panting region reinforce the structure against panting.
When a vessel slams and pitches heavily in heavy seas, pounding stresses occur. These stresses are rigorous when the vessel is in light condition.
Stress location: Severe effects in the region between 5% and 25% of the ship's length from stem.
Reinforcement required : Shell plating in the pounding region is thicker, and plate floors are fitted at alternate frames (for longitudinally framed ships) and at every frame space (for transversely framed ships).
Hogging & sagging :
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. Hogging stresses cause tensile stress in the deck area and compression stress in the bottom area in the midships region.
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. Sagging stresses cause tensile stress in the bottom of the ship and compression stress in the deck area in the midships region.
Reinforcement required :
Hogging and sagging are both counteracted by reinforcement of the mid length of the vessel through a centre girder, intercostal side girders, and increased thickness of deck, side and bottom plating.
Additionally, the prudent watchkeeping officers must ensure that the weights are distributed evenly throughout the vessel's sea passage and during loading and discharging. They must also be vigilant about the vessel being placed on the crest or trough of the wave to cause hogging or sagging when the wavelength is equal to the ship's length.
Torsion stresses occur when the ship's hull is twisted due to the effect of oblique sea or unequal transverse distribution of weights. Excessive torsion stresses can cause watertight partitions to buckle and develop leaks through cracks.
Stress location: Generally between any two compartments, ie near the transverse bulkheads if caused by oblique seas, and at the partition between any two compartments if caused by unequal distribution of weights about the centreline of the ship.
Reinforcement required : The corners of the deck and DB tanks are reinforced by the use of beam knees and brackets respectively.
Racking: Racking stresses occur when a ship's hull is distorted transversely due to the effect of rolling.
Stress location: The effect is predominantly seen at the joint of the deck stringer and sheer strake, as well as at the turn of the bilge.
Drydocking stress :
This is the stress that occurs when a vessel takes up bottom blocks in the drydock.
Stress location: Along the full length of the vessel where blocks are placed under the bottom of the ship. If shores are used to support the vessel on the sides (eg if a loaded vessel enters drydock), then the stresses occur where support is provided by shores.
Reinforcement required : Strengthening of the bottom of the vessel for drydocking. Also, the bottom blocks are positioned under the transverse bulkheads, plate floors, girders, etc.
Panting stresses,Pounding, hogging,sagging,Torsion,Racking, Drydocking stress
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