Stability factors in Heavy Lifting Vessels

Loading and heavy lifting operations in vessels are not only a matter of how much load a ship can carry but how much does it survive under this load. If you are simply allowed to troll on a walkway with a medium sized brick you may do that easily. But what if the load is being increased persistently and if you are asked to carry a tree trunk ? The probable answer from the majority is that can't. But if momentarily you imagine yourself to be a giant and somehow manage to lift it up, what will be your status? Will you manage to keep yourself upright? And even if so, till how long? Suppose, someone gives you a slight push. Will you be there like before or tumble over? Well same is the case of ships. Stability is an inherent factor for a small boat as well as a heavy lift vessel. In a completely undisturbed condition, or in an equilibrium case, a a given displacement the buoyant force balances the weight acting vertically downwards. Hence in such a case , the centre of gravity(C.G.), the centre of buoyancy(C.B.) and the geometric centroid( which in almost all the normal cases coincides with the centre of gravity) lie on the same line. In such a case, the ship is said to be stable. Considering some external disturbances like wind or some high waves, the stability is said to be lost. Although, by property the C.G. remains intact the position of the C.B. changes. The point of intersection of the original line passing through the C.G and the initial C.B. with that of the line passing through the final position of C.B is termed as the metacentre M.

Fig.5 The position of the various points in an unstable  condition 

In all such heavy lifting errands, like in heavy lift ships, there is a rigorous change in C.G due to loads and also due to operations like lifting through cranes or derricks. This uneven distribution of weight in all such Project Cargoes often cause the C.G. to be on either side of the centreline. Hence the ship has a high probability to heel towards that side. On normal day-to-day ferries or other ships, the stability mainly gets lost due to two reasons; either shift of C.G due to uneven loads or direct and abrupt change of C.B. due to external conditions leading to heeling. However, in heavy lifting operations, mainly, the culprit is the biased CG.

On one side that causes the vessel to list henceforth. As depicted in the above figure, there is a normal GZ, from the initial C.B to the line passing through the final C.B and is called the righting arm. This can cause a righting moment to the ship and may attain back its position. A negative GM is always uncongenial for the ship which may lead to its capsizing of the ships( which is a pertinent disaster !). Also by the virtue of a reverting counter buoyancy, the ship may, in certain chances attain back its stability, i.e, become upright. This tendency of a negative inherent stability is measured in terms of a certain Angle of Loll. It is the state of the ship which is unstable when upright. It has a dramatic property to lurch to another side on the action of external forces producing the same value of Angle of Loll on the opposite side. Remember, do not confuse it with list!! Listing is caused by the sifting of cargo and material grain causing a change in C.G of the body. But this counter event  called Loll is just a reaction force in counter to Rolling. In case of a suspicion that the vessel might be subject to Loll, we must do something which may sound quirky! Ballast or do some weight additions to that side which has the loll or the greater inclination downwards. This dramatically depletes the severity of the Angle of Loll, but provides a boon at the end. The draft increases drastically and the freeboard decreases along with the so-called free-surface effects.  The C.G. immerses further and the greater submersion of weight increases the "Buoyant Reaction", which in turn creates a negative Angle of Loll at that condition. 

Fig.6: Angle of Loll

Another simpler implication is to just undo the cause for this loll. Mainly, the majority of these events are caused by heavy lifting activities on the superficial weather deck that drastically shifts the Centre of Gravity to the heavy load carrying side. Cranes, Derricks or other facilities carrying out these operations are the other culprits, which along with the weights they are carrying act as moment arms, causing a resultant tilt. Maybe, the following image explains it better!

Fig.7.: Heavylifting operations on a ship and the resultant shift of C.G

So, for a heavy lift ship, care must be taken while loading and offloading operations in the deck such that the ship is free of loll. As a Heavy Lifter is designed specifically for loading purposes, it is not as if heavylifting operations should be avoided. But the weight should have even distributions and as it involves weights in the order of 10000 tonnes, things have to be in line! Most of these have modern ballasting technologies,thrusters and Dynamic Positioning Systems. Heavy Lifters, an alternative to the older technology of heavy lifting cranes or projection cargoes straddle the cargo(jackets, rigs, oil wells, semi-submersibles, ships etc.) uniformly on both sides and allow proper suspension of weight in a manner that avoids Loll or list under normal circumstances.