A ship’s stability can be defined as its ability and tendency to return to its original state as it completes its tasks and occasionally used to extremes of its capabilities. The stability factor plays a part when a vessel interacts with external forces when they are applied and removed; making the vessel, at first sustaining the application of a certain force and returning/adjusting itself to the initial state upon removal of the force. Stability also focuses on a vessel’s behavior at sea by calculations concerning its center of gravity, buoyancy and the relationships between the two factors.
There are two types of stability calculations; damage and intact stability where the main difference lies in the focus and the difficulty in generating data. Intact stability involves a straightforward calculation between all center masses of objects on the vessel as well as the vessel itself; computed and calculated to identify the center of gravity and buoyancy of the hull. Cargo arrangement+loading , crane operation and design sea is also considered.
Damage stability is more challenging due to the long time to gather the necessary variables with the latter also changing due to the situation at hand. What are the potential hazards at sea? Flooding is usually common and there lies the first threat to stability. However since a vessel consists of many interlinked parts which maintain stability with their own characteristics without which a ship will behave differently. Loss of stability during flooding may also occur due to the free surface effect; water accumulating in the hull usually drains to the bilges which lowers its center of gravity; however if a ship is inclined to any degree (e.g a wave strike) fluids in bilges move and make the ship list.
A listing ship
A ship is at equilibrium when its weight acts down through a center of gravity, equal to the up-thrust force of water acting on the center of buoyancy with both forces being at a vertical line.
Stability in a flood is also lost due to seawater filling up the empty tank which takes its buoyancy resulting in that section of the ship lowering into water; creating a list unless the tank is on the center-line of the vessel. Stability calculations show that when a tank is filled the contents are assumed to be lost and replaced by seawater. However if the contents are lighter than seawater, (e.g light oil) then buoyancy is lost and a list happens.
Calculations, the challenging aspect of stability is of two approaches; deterministic and probabilistic. Deterministic approach bases the ship’s subdivisions/ division of the ship into smaller compartments on theoretical principles. Standard dimensions of damage used in the deterministic approach extending anywhere along the ship’s length and/ or between transverse bulkheads; depending on requirements. The probabilistic approach applies to cargo ships of length above 80M which in-turn evade deterministic methods. This principle works by a pattern of accidents whose consequences are used to improve ship designs. Probabilistic approach is also based on statistical evidences relating to ship design choices, situations at sea (collisions, weather conditions.)
Probabilistic method is made from three probabilities that relate to subdivision and damage stability requirements; probability of occurrence/ damage to the ship, probability of damage in a certain location and property of flooding and assessing the on-deck situation.
So which is the most beneficial attitude towards stability of your vessel? The best answer lies within the degree of negativity and the management of factors that go with that method as negativity persists with some factors in maritime; evaded by some methods and maximised by others. Starting from the deterministic method, the consequences of taking such approach are the creation of damage cases the number of which along with the number of compartments/sub-divisions involved depends on the ship’s dimensions. Each loading condition has to be considered along with compiling all applicable criteria.
Probabilistic approach is more realistic and practical as all ideas are based on real incidents where design choices of similar vessels can easily be fixed and future casualties avoided. However, this method is considerably old (devised in 1973) and with new vessel builds being designed and produced, with emphasis on safety therefore this method is mute regarding builds that never saw accidents.
While theoretical methods composed of ideas that may share similar characteristics to newer vessel builds, (which maybe constructed on older principles) and thus be valid when considering the stability issue aboard newer vessels; the fact remains that the ideas are still theories and yet to have seen practical application. Along with this, the theories have a chance to be wrong as new technology, though tested before implementation may still respond and work differently to the theory. Whereas the probabilistic approach which was based on real-life accidents where technology was, possibly modern to the times of the incident therefore making the conclusions drawn more accurate. Regardless, this leaves the question which attitude to adopt regarding stability of a vessel.
However, in the maritime industry the stakes are always high and the margin for error is low which warrants an experienced approach of aspects that were tested and have a higher guarantee of working. Experience is key to avoiding errors in maritime where errors and mishaps are easier to detect prior to conducting operations at sea.
