As we already had a brief insight from one of our previous articles, 'Stability' is defined as the phenomenon of a ship to resist external or internal loads on it and to acquire its original upright state on removal of the external or internal loads. Stability is a crucial phenomenon governing ship design and seakeeping performance of a vessel.
Moreover they can be classified into two types:
· Intact stability
|Figure 1: (Copyright: Wordpress)|
- Collision: This is a very common reason often leading to adverse effects. Collision may be with another vessel (remember Titanic?) or some landmass like harbor, port, reef or island. Most of them are caused by compounded operator error, carelessness, technical flaws, machinery and equipment failure, problems in maneuverability, accidents or sometimes unavoidable circumstances leading to damage.
- Grounding: Grounding is often caused by improper draft considerations in water bodies, excessive trim or in shallow draft conditions.
- Structural problems: Sometimes there is lack of structural soundness due to manufacturing defects, improper behaviour, lack of maintenance, fatigue or unprecedented loading.
- Environmental Vagaries like rogue waves, cyclones, sea-storms, heavy rainfall or sometimes cold weather conditions leading to ice accretions. It may be worthwhile to mention that icebergs which are very much prevalent in northern seas are very big problems for navigation which can often lead to precarious collisions just like in case of Titanic.
CLASSIFYING DAMAGE STABILITY
- Deterministic Damage Stability
- Probabilistic Damage Stability
In intact stability, the preliminary condition is that the ship remains 'undamaged'. Now what could be the cause for a vessel to capsize without suffering physical damage? The answer is simple. As the loss of stability is solely caused by the loss of equilibrium of forces, the most occurrent cause is due to shifting of cargo or injudicious distribution of weights (cargo, ballast, machinery etc.) which triggers of the ship to heel to one side in a local phenomenon termed as Listing. However, in dealing with damage stability problems, we isolate the causes caused due to internal effects and merely concentrate on the damaged aftermath stability conditions.
In damage stability criterion, capsizing is caused due to the breaching of hull after suffering damage sideways which can cause water flooding in that region only (tanks, cargo spaces etc.). This sudden flooding of water causes the ship to heel to one side accounting to its loss in equilibrium.
As illustrated in the figure, unwanted flooding of a compartment or space leads to drastically altered buoyancy forces along with their lines of action. This difference in the line of action of the overall buoyancy of the damaged ship with respect to the weight still acting through its center of gravity creates a large heeling moment causing it to topple and finally capsize. As a result, it heels over large angle such that its righting moment is insufficient and it topples over. This happened in the disaster of ship COSTA CONCORDIA due to the collision.
|Figure 3: The capsizing of ship due to heeling moment|
EFFECT OF FLOODING ON STABILITY
|Figure 6: Conditions of Transverse Statical Stability|
|Figure 7: Division into watertight bulkheads (Image Courtesy: NEEC)|
|Figure 8: Watertight Subdivision of a ship (Courtesy: Wikipedia)|
|Figure 12: Image Courtesy:NEEC|
PROBABILISTIC DAMAGE STABILITY
- How ships are damaged?
- How often is a part damaged?
- What is the chance of survival if that part of ship is damaged?
ATTAINED SUBDIVISION INDEX