Capsizing of a ship refers to a situation where in the vessel at sea list to one side to such an extent that it’s not able to upright or regain its original position, leading to tipping over of the vessel in water and making it unsafe for both crew and machinery onboard. In this article, we will discuss various reasons leading to capsize of the ship and precaution to avoid the same.
Free surface effect:
Large free surface, a result of slack tanks and improper subdivisions of tanks (no longitudinal divisions) results in reduction of metacentric height and increases the possibility of capsizing, especially if GM (metacentric) is less( Ro Ro vessel).
Certain tanker vessels designed with no longitudinal bulkhead in cargo tank are more prone to capsizing because of large free surface effect, especially when the vessel is heavily loaded in harsh weather, which allows large amount of water to come on deck. In such cases if adequate drainage is not provided, the possibility of capsizing increases drastically.
Shifting of Cargo:
Shifting of cargo on the ship can result in heavy listing of the ship, which increases progressive rolling and thus possibility of capsize of vessel.
Proper securing of cargo is necessary. This can also be avoided by good design of ship, for e g. Provision of hopper tanks in bulk carrier minimise the shift of cargo.
Nature of Cargo:
Some types of bulk cargo are susceptible to absorbing moisture (hydroscopic nature). When moisture content rises above a certain limit, the dry cargo behaves like liquid cargo giving rise to high free surface effect or shift of cargo, giving dangerous list to the vessel and possible capsize.
If the vessel runs aground, specifically at a spot off the central line and subsequently if the water level drops, there is a virtual rise in the centre of gravity.
This may make metacentric height negative, resulting in vessel capsizing. For this reason adequate vigilance is required to ensure that no grounding occurs.
Any type of flooding due to external or internal reasons will result in loss of buoyancy of the ship. If this loss is greater than the reserve buoyancy, the vessel will capsize. Hence hull, water tight and weather tight integrity and proper leak proof piping systems should be maintained at all times.
Heavy Weather Damage:
Any kind of heavy weather damage that leads to progressive flooding will cause loss of buoyancy. If this loss is greater than reserve buoyancy, the vessel will capsize.
Proper weather routing is to be done to avoid heavy weather.
While fighting fire using fire pumps on ship, especially at higher decks, there might be a substantial addition of weight raising ‘G’ centre of gravity and reduction of GM leading to possible capsize. This point is to be noted while fighting a fire.
Every vessel has a natural rolling period which is inversely proportional to the square root of the metacentric height and directly proportional to the beam of the ship.
If the vessel encounters a series of swells in such a manner that the wave period matches the rolling, the vessel will have no time righting itself before the next wave strikes. This situation, if not corrected, can result in capsizing of the ship. The speed and direction of ship can be checked and altered as necessary to avoid ill effect of synchronous rolling.