Stability When Grounded

?The problem of the stability of grounded ships is limited to the dry docking of ships of relatively small GM and to salvage operations

?Stability During Dry Docking:

–When a vessel enters dry dock it generally has a trim and hence the keel makes an angle with the keel blocks

–As the water level falls, due to the pumping out of the dry dock, the keel of the ship comes into contact with the keel blocks.

–Vessels are usually trimmed by the stern, in which case the after part of the keel will ordinarily touch first.

–The weight that is supported by the keel blocks at any subsequent time is the difference between the displacement when fully water borne and the displacement to the waterline in the aground condition.

– As the water continues to recede, the slope of the keel gradually approaches the slope of the keel blocks. 

?When a ship has just landed on the keel blocks, part of its weight is borne by the blocks and part of it is water borne. 

?P = upward force exerted by keel blocks

?M1 = Metacenter at waterline W1L1

?G = center of gravity of the water-borne ship

?W = displacement of the water-borne ship

?W - P = water-borne displacement  at  waterline 

?GM1= virtual metacentric height of ship at water- line W1L1 

?If the force P is considered as a weight removed from the ship at point K, the center of gravity of the ship rises to point Gv

?This point is the virtual center of gravity of the partially water-borne ship.

?The virtual rise of the center of gravity is

?The virtual height of the center of gravity will be 

?The virtual metacentric height of the grounded ship is

?When a vessel enters dry dock with a trim, generally the most critical stage of docking is at the time when the vessel's keel comes in contact with the keel blocks throughout its length. 

?Subdivision and Damage  Stability

?All types of ships and boats are subject to the risk of sinking if they lose their watertight integrity, whether by collision, grounding or internal accident such as an explosion.

?Exceptions, of course, are vessels constructed entirely of buoyant materials and having mostly buoyant contents.

?Such accidents are frequent enough in practice that some degree of protection against the effects of accidental flooding is an essential feature of the design of any water craft.

?The most effective protection is provided by internal subdivision by means of watertight transverse and/or longitudinal bulkheads, and by some horizontal subdivision double bottoms in commercial ships and watertight flats in naval vessels. 

?The flooding of a ship's hull can have one or the other of two principal consequences.

?One is loss of buoyancy and change of trim, which if unchecked will lead to sinking by foundering.

?The other is loss of transverse stability or build-up of such an upsetting moment that capsizing takes place.

?There are many uncertainties in providing adequate subdivision.

?First of all, the location and extent of damage to be protected against is unknown in advance.

?Second, the amount, type and location of cargo and liquids in the ship varies both during and between voyages.

?Finally, the designer cannot be sure that corrective measures that might be followed by the ship's officers in an emergency will be taken or that hazardous steps might be adopted by mistake.

?Furthermore, subdivision inevitably adds to the cost of the ship and may interfere with its ability to perform its function economically.

?In fact, a ship so ideally compartmentalized as to be virtually unsinkable might be of no economic or military value whatsoever.

?Consequently, the subdivision of ships inevitably involves a compromise between safety and cost.

? This dilemma has been partially resolved for passenger ships by the development of national and international standards of what is considered acceptable, considering the size and type of ship, the number of passengers carried, the nature of the service, etc

?It will be seen that for cargo ships subdivision standards have been minimal.

?However, for these vessels, the above dilemma of cost versus safety can be resolved more scientifically, provided that loss of human life can be ruled out by virtue of reliable provisions for lifesaving—with time to use them provided by some subdivision.

Extent of Damage and Location and Number of Bulkheads

?The length and depth of damage, and its location relative to transverse bulkheads, has a strong influence on probability of survival. In general, it might be expected that the more bulkheads the safer the ship. But damage may occur entirely between adjacent bulkheads or may involve one or more bulkheads. Hence, for a given length of damage, any increase in the number of bulkheads may actually increase the likelihood of bulkhead damage, which would reduce rather than increase the chances of survival

?An international SOLAS working group assembled and analyzed statistical data on collisions, particularly of cargo and passenger ships, to determine location and extent of damage

?Effects of Flooding,  

–Change of draft.

–The draft will change so that the displacement of the remaining unflooded part of the ship is equal to the displacement of the ship before damage less the weight of any liquids which were in the space opened to the sea.

–Change of trim

–The ship will trim until the center of buoyancy of the remaining unflooded part of the ship lies in a transverse plane through the ship's center of gravity and perpendicular to the equilibrium waterline.

–Heel

–If the flooded space is unsymmetrical with respect to the centerline, the ship will heel until the center of buoyancy of the remaining unflooded part of the ship lies in a fore-and-aft plane through the ship's center of gravity and perpendicular to the equilibrium waterline

–Change of Stability

–Flooding changes both the transverse and the longitudinal stability

–Change of Freeboard

–The increase in draft after flooding results in a decrease in the amount of free board. Even though the residual GM may be positive, if the freeboard is minimal and the waterline is close to the deck edge, submerging the deck edge at small angles of heel greatly reduces the range of positive righting arm, and leaves the vessel vulnerable to the forces of wind and sea

?Loss of Ship.

?Where changes in draft, trim and/ or heel necessary to attain stable equilibrium are such as to immerse non-watertight portions of a ship, equilibrium will not be reached because of progressive flooding and the ship will sink either with or without capsizing.

?Where the loss in GM is such that the remaining maximum righting arm is less than any existing heeling arm, capsizing will occur.

?Even if there were no heeling arm, capsizing could be expected if the GM in the damaged condition were negative and if the maximum righting arm were so small as to result in negative dynamical stability.

?Practically, even for symmetrical flooding, there is always some heeling arm due to unsymmetrical weights and/or wind. 

I would request others to please let me know about more about this topic and how can we automatize the process and create an unsinkable ship and turn fiction into facts.

Looking forward for your support and guidance Oleg Vishnepolsky

Thank You,

Diganta Ghosh

Indian Maritime University , Visakhapatnam Campus.