Marginal GM with slack tanks and a deck heavy lift
A 12,000 t displacement general cargo ship is completing loading at a river berth. The approved loading instrument shows displacement 12,000 t, KG 6.85 m, KM 7.20 m, GM 0.35 m, draughts 7.62 m forward and 7.85 m aft. The stability booklet requires a minimum initial GM of 0.15 m, and the IS Code 2008 intact stability criteria apply. Two heavy lift packages, each 90 t with their centre of gravity 12.5 m above the keel, are still to be loaded on the No. 2 hatch cover. To trim the ship for the river passage, the chief officer intends to leave No. 3 and No. 4 double bottom tanks slack: each is 20 m long and 12 m wide with 3.2 m of salt water in it. No. 2 double bottom tanks are full. The second mate estimates the rolling period alongside at about 18 s. The river bar has a maximum permitted draught of 8.10 m and the berth has a minimum under-keel clearance rule.
1. State the sequence of actions to bring the ship to a compliant condition before departure, including the constraints on draught and on stiffness.
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1. Verify the input before acting. Sound every tank and compare with the loading instrument. The rolling period of about 18 s is inconsistent with a GM of 0.35 m - for this hull it suggests a GM nearer 0.6 m. Either the instrument has a tank wrongly entered, or the roll estimate is unreliable. Resolve this before making any change, because the two possibilities lead to different corrective action.
2. Deal with the free surface first - it is the dominant term. Either press the tanks full or pump them empty; a partly filled 20 × 12 m tank costs about 0.24 m of GM each. Pressing both full would add about 1,570 t of ballast, roughly 0.3 m of draught: 7.85 + 0.3 m exceeds the 8.10 m bar limit and eats the under-keel clearance. Pumping both empty removes about 1,570 t and brings the ship up bodily, which is compatible with the bar but changes trim. The practical answer is usually to empty one and press the other full, or to press both full and adjust the cargo, but the arithmetic must be run on the instrument with the actual sounding figures.
3. Re-run the instrument and confirm: GM ≥ 0.15 m with the free surface correction applied; the full IS Code 2008 criteria on the GZ curve; draught within 8.10 m and within the berth's under-keel clearance rule; trim by the stern as required for the river.
4. Check for over-stiffness. Do not chase a large GM to fix the loll problem: a stiff ship rolls quickly and sharply, and the two 90 t packages on the hatch cover will impose heavy racking and lashing loads. A GM of roughly 0.3–0.6 m for this size of ship gives an acceptable compromise.
5. Consider the deck cargo. If the packages can be stowed lower, in a hold, the KG penalty disappears and the problem is solved at source. Otherwise, secure them to the approved cargo securing manual and re-check the lashings after the first day at sea.
2. The ship sails with a negative GM and takes up an angle of loll. Explain what loll is, why it differs from a list, and the correct and incorrect ways to recover.
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What loll is. With a negative metacentric height, G is above M, so at the upright there is no righting lever; the ship is in unstable equilibrium. She rolls off to one side until the righting lever rises to zero again - the angle of loll - where she hesitates, and then flops to the same angle on the other side. The GZ curve starts negative at zero degrees, crosses zero at the angle of loll, rises to a small positive maximum and then falls again.
Why it is not a list. A list is caused by an off-centre weight or an asymmetric flooding; the ship has positive GM and settles at a definite angle on one side only, and the GZ curve is normal. A lolled ship has no stable upright, swings from side to side, and cannot be corrected by transferring weight across the deck.
Correct recovery
- Reduce free surface. Press up or empty the slack tanks, starting with the lowest tanks, doing it in small steps and watching the inclinometer after each one. This is the single most effective action because it lowers G without adding an off-centre moment.
- Add weight low down, and prefer the low side rather than the high side. Adding to the high side can drive the ship through the upright into an even larger loll on the other side.
- Keep the ship upright with minimum heel, slack off on the ballast transfer, and avoid any simultaneous free surface in two tanks.
Incorrect actions
- Counter-ballasting to the high side to 'level' her, which will simply flop her over.
- Assuming the heel is a list and shifting cargo or ballast across.
- Increasing speed to 'roll her upright' - the synchronous roll and the slack tanks may make it worse.
Afterwards: the vessel must be re-assessed by the loading instrument before any further cargo work, and the incident recorded and reported as a near-miss under the SMS.