Damage Stability (Fundamentals)

Q. What is stability of a ship?
Ans. Stability is ability of a ship to come back to upright equilibrium, when disturbed.

Q. What do you mean by upright equilibrium?
Ans. Equilibrium means state of rest. It is defined in physics as a state in which the algebraic sum of all the forces and moments acting is zero. Algebraic sum of forces becoming zero results in object not having a linear motion. Also algebraic sum of moments acting upon body, becoming zero results in body not making any rotary motion.

In respect of ship, equilibrium means COG and COB are in one vertical line. On the GZ curve, at equilibrium GZ = 0. Ship may have different angles at which this may happen. Thus, a ship can be at state of rest at angle of vanishing stability. But since the ship is meant to be at upright condition the ability of ship to come back to this equilibrium is referred to as ship’s stability.

Q. If you want to judge your stability in damaged condition or for that matter in any condition, what is that you need for quick assessment?
Ans. I need the GZ curve in that condition and the damage stability criteria from relevant IMO instrument with the present values of stability parameters to know how well their criteria are satisfied.

Q. What stability information must be supplied to ships?
Ans. Regulation 5-1 Stability information to be supplied to the Master, deals with this. The Master shall be supplied with such information to the satisfaction of the Administration as is necessary to enable him by rapid and simple processes to obtain accurate guidance as to the stability of the ship under varying conditions of service. A copy of the stability information shall be furnished to the Administration.

The information should include:
a. curves or tables of minimum operational metacentric height (GM) and maximum permissible trim versus draught which assures compliance with the intact and damage stability requirements where applicable, alternatively corresponding curves or tables of the maximum allowable vertical centre of gravity (KG) and maximum permissible trim versus draught, or with the equivalents of either of these curves or tables;
b. instructions concerning the operation of cross-flooding arrangements; and
c. all other data and aids which might be necessary to maintain the required intact stability and stability after damage.

The intact and damage stability information thus required, shall be presented as consolidated data and encompass the full operating range of draught and trim. Applied trim values shall coincide in all stability information intended for use on board. Information not required for determination of stability and trim limits should be excluded from this information.

If the damage stability is calculated in accordance with the probabilistic concept, and, if applicable, with regulations regarding:

  • Special requirements concerning passenger ship stability;
  • System capabilities and operational information; and
  • regulation regarding DBs on non tankers;

a stability limit curve is to be determined using linear interpolation between the minimum required GM assumed for each of the three draughts ds, dp and dl. When additional subdivision indices are calculated for different trims, a single envelope curve based on the minimum values from these calculations shall be presented. Maximum permissible KG curves will be made with similar considerations. Alternative to a single envelope curve, additional calculations are prescribed. When curves or tables of minimum operational metacentric height (GM) or maximum allowable KG versus draught are not provided, the Master shall ensure that the operating condition does not deviate from approved loading conditions, or verify by calculation that the stability requirements are satisfied for this loading condition.

Q. Won’t the external assistance serve the purpose?
Ans. Post damage changes can be very fast. Master being a person at site should be competent to take immediate action. Hence, the guidance source must be onboard. Role of external agencies like Emergency Response Services should actually be limited to a guiding agency outside the ship who knows ship’s problem. To depend on directions for every action is not wise. A few reasons to justify this are as follows:

  1. In certain situations as probably been the case in case of bulk carriers, just 2 or 3 minutes may be available to take decision to abandon the ship. Thus, with hold number one damaged, with the cargo of iron ore, bulkhead abaft starts giving signs of failure.
  2. External agencies would guide on the basis of precise information, which they have received. By the time they convey their message, situation at site probably would have changed.
  3. Appropriate heading, speed, no list, smooth trim, minimum stresses, weights at bottom, buoyancy at top are a few general rules, for which Master should not have to wait for someone from outside to instruct him.

Company can arrange a ship specific damage stability course for the Master before he joins a ship.

Q. What are the initial steps you will take as soon as the damage to the hull is suspected?
Ans. Any breach of hull can be very dangerous Hence, the very immediate steps would be:

  1. General alarm must be sounded at once. Head count and a quick briefing is done with the crew.

2. At the same time, the ship’s speed can be reduced and vessel headed in the direction in which minimum stress or pitching is experienced.
3. All watertight doors are closed, sounding pipes in engine room are shut and subdivision is well maintained.
4. Soundings are taken for all tanks, spaces, bilges, etc.
5. Emergency response services, charters owners, etc. are informed.
6. A meeting of senior officers is held and further course of action is decided.

Q. How will you find loss in freeboard?
Ans. Sinkage subsequent to, damage to an empty compartment, engine room or compartment with dry rigid cargo is found by a simple but accurate formula from constant displacement method principle. The formula is:

Q. What are the salient features of Constant Displacement Method of calculation?
Ans. If breach occurs to an empty compartment or a compartment void of liquid, the displacement, KG and LCG do not change after bilging. Loss of buoyancy, loss of water plane area and loss of moment of inertia of ship’s waterplane are the three losses (in respect of stability). The loss of water plane area & moment of inertia of waterplane are accounted for, if the damage reaches the final waterline. The free surface correction is not accounted because the water in compartment belongs to sea rather than to the ship. It is in a state of free flow with outside water. Change of position of COB is accounted owing to the loss in buoyancy from below and replenishment of reserve buoyancy from above. Since the displacement is constant, the intact underwater volume will also be constant. The centre of floatation moves away from the damaged part of waterplane area. It is because the sinkage is due to the replenishment of reserve buoyancy to compensate the lost buoyancy. This method is also called Lost Buoyancy Method.

Q. What is added weight method?
Ans. It assumes that the mass of seawater added to or removed from the ship, after bilging can be treated as added / removed mass on intact ship causing increase or decrease of draft, displacement, etc. Thus, the displacement, KG and LCG change after bilging. Loss of moment of inertia is not accounted for, even if the damage reaches the final waterline. The free surface correction is accounted. Thus, the calculations are similar to intact stability calculation and are appropriate for finding the list and trim.

Q. How will you find lost buoyancy?
Ans. In case of damage to a compartment void of liquid the volume of compartment below initial waterline at mid hold multiplied by permeability gives lost buoyancy. For lost buoyancy the damaged height till initial level at middle of compartment damaged is taken.

Q. What is permeability?
Ans. Permeability is the percentage or proportion of space that will be taken over by water after bilging, accounting for loss of buoyancy.

Q. What other parameters are affected?
Ans. Other losses are loss of water plane area and loss of moment of inertia of W.P.A. if damage reaches waterline.

Q. What kind of cargoes are good in damaged condition?
Ans. The cargo that is stowed compactly within a hold, heavier than seawater and is not porus is best because such cargoes will not allow water to take its position and will act like buoyant chambers. Being heavier is not a matter of worry as cargo’s weight is already accounted for in displacement.

Q. What is the relationship in permeability and survivability of ship after damage?
Ans. Theoretically, if permeability is zero the stability losses are zero. So even if 4, 5 or more compartments are damaged buoyancy and stability losses are not there. A ship with zero permeability can withstand any number of compartments being damaged.

Q. What is intact buoyancy?
Ans. In the context of damaged stability, the term ‘intact buoyancy’ is used to refer to a still intact or undamaged compartment within the damaged length of the ship. Thus, if hold no.3 is damaged but no.3 DB is still intact, then no.3 DB’s buoyancy is referred to as intact buoyancy.

Q. How helpful is this buoyancy in a damaged condition?
Ans. If freeboard is critical but GM of the ship is very high then this buoyancy is helpful for maintaining the reserve buoyancy. But if the post damage GM is very low then the DB must be filled up. As a general rule buoyancy is welcome but above water line. This intact buoyancy at DB level, in fact is detrimental for low GM ships.

Q. Can the GM increase after damage?
Ans. The GM may in fact increase in most cases of damage where water plane area is not damaged.

Q. Can you tell me how GM can increase using the principle of constant displacement method?
Ans. Under constant displacement principle Displacement, KG and LCG remain unchanged. KB always must increase after the damage. This is because buoyancy is always lost from under water portion and recreated at higher areas or is replenished from the areas of reserve buoyancy.

We know that GM = KB+BM-KG After damage KB always increases, BM depends on two things: one, the waterplane area and two, the intact underwater volume. Since the intact underwater volume remains constant, BM purely depends on residual water plane area, if the water plane area is not damaged the BM either will remain same or may even increase if the water plane area is more at final draft. Thus, we may conclude that in all cases of damage where final waterplane area is not damaged GM increases, provided the free surface effect does not cause a virtual loss of GM.

Q. How will one know if the final water plane area is undamaged?
Ans. If there is an undamaged water-tight flat between the damage and final water line, we may say that the flat is not allowing the damage to reach the final water line and hence the waterplane area is still intact.
Hence the ship, stability wise is well secured, provided free surface effect is curtailed.

Q. If FSE is curtailed and final water plane area is undamaged then what are the other problem areas to concentrate on?
Ans. Effect of waves and loss of strength are always the concern and are the variable unknowns. Additionally the dangerous effect on cargo, transient flooding, damage extending to adjacent compartment and loss of freeboard must also be watched.

Q. Earlier, you said that free surface effect is not accounted in constant displacement methods. However, now you claim that free surface effect may cause problem. Why so?
Ans. Constant displacement method assumes free flow of liquid. This means, the damaged aperture is so big that the outside water does not cause its weight on ship and maintains a steady level with sea, outside.

Effectively however, if the aperture is small and the sloshing water causes a shift of mass to and fro, the righting level is reduced due to the shift of COG to and fro on either side of CL.

This amounts to a virtual loss of GM and hence reduction of righting lever. In any case FSE must be accounted for the free liquid in the sound compartments. An off-centre compartment bilged near water line can cause reduction of righting lever and hence virtual loss of GM due free communication of sea water. A list is caused due water, which remains in and exerts weight on ship all the times. Moves G to G1, reducing stability to listed side. During the roll G1 moves to G2, further reducing the righting lever to a size merely equal to G2Z. This amounts to a virtual rise of G to G3.

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