An angle of loll situation can arise in a ship for a number of reasons:
- Ships carrying timber deck cargoes
(Deck cargoes will absorb moisture that will cause G to rise. An allowance of 15% of the weight of timber carried on deck should be made when conducting calculations and determining GZ values for the condition in question.)
- Consumption of fuel and water and introduction of free surfaces
(Fuel and water will be consumed from low down in the ship raising G due to the removal of weight from low down in the ship and the introduction of free surfaces in tanks that may have been initially full. Even ballast tanks that are initially full may become slack as a voyage progresses due to the constant rolling and pitching of the ship at sea causing water to escape from tank air pipes on deck. Tanks may require to be topped up occasionally, particularly when stability is considered marginal at any stage during the voyage. Poor cargo and ballast tank management resulting in excessive free surface moments is the most common cause of loll situations excepting damage scenarios.)
- Heavy lift operations using ship’s lifting gear
An instantaneous loss of GM will take place immediately that a weight is lifted either off the deck or from a position over the side. Such large increases in KG must be considered prior to any heavy lift operations and calculations should be conducted prior to any such operation to ensure that:
- The ship has adequate stability at all stages during the lift, and;
- Maximum list is restricted to an acceptable limit.
- Shift of bulk cargo
The vertical component of a shift of solid bulk cargo may be sufficient to reduce GM sufficiently to cause a loll situation, particularly if accompanied by one of the above. The larger horizontal component will also create listing moments compounding the situation further.
The following procedures should be carefully observed taken:
- Alter course to put the ship’s head into the predominant waves.
(If the ship is in a lolled situation it is essential that the ship stays lolled to the same side. Wave action may cause the ship to roll through the vertical to loll on the other side. This is a dangerous situation since the ship will heel from the vertical of it’s own accord and the momentum it will have in lolling over to the other side may be sufficient to capsize it. In any event, the ship will initially heel beyond the angle of loll before settling at the angle of loll whereby cargo shift may result, which will worsen the situation further.)
- Check that port and starboard listing moments are the same.
(By verifying tank soundings and checking for cargo shift it should be possible to account for any listing moments that may cause the ship to be in a listed situation. If it is calculated that there are no net listing moments then a case of instability may be assumed and the ship will be lying at an angle of loll. Recalculation of the ship’s effective KG should also be undertaken to verify the ship’s GM.)
- Check for slack tanks
(In carrying out (2) above it should also be evident if there are excessive free surface moments causing a loss of GM sufficient enough to make the ship unstable. In this case a loll situation may be confirmed.)
If a loll situation is confirmed:
- Take action to lower G (reduce KG)
(It would be impractical to consider shifting weights on board using ship’s lifting equipment at sea. If the ship has high ballast tanks that are full then these may be emptied, discharging ballast from the high side tank first – the greater vertical distance between G of the ship and g of the weight being discharged will ensure that the greatest lowering of G will take place in the first instance. Once the high side tank is empty the one on the low side may then be emptied.)
Empty high up ballast tank on the high side first to ensure greatest lowering of G
- Minimise Free surfaces
(Having sounded all the tanks any that are slack will be identified. Minimise the loss of GM due to free surface effect by topping up low down ballast tanks and transferring fuel as necessary. This action alone may remedy the situation.)
- Ballast tanks low down in the ship
(Select a set of suitably subdivided double bottom tanks to ballast. ideally start with tanks that have the smallest free surface areas to minimise the effects of free surface whilst filling. The order of filling is as follows and must be strictly adhered to:
- Start by filling the tank on the low side (No. 1) as shown. Because of the introduction of more free surfaces whilst filling the first tank the situation will initially worsen.
- When the first tank is completely full, fill the centre tank (No. 2).
- When the centre tank is full, fill the final tank (No. 3)
- If G is lowered sufficiently then the ship should complete in an upright condition (having initially verified that the port and starboard moments were the same).
The movement of G after completely filling each of the tanks is as depicted in figure below (ignoring the upward movement of G which arises as a result of the introduced free surface at intermediate stages of filling the tanks)
If the situation is not remedied then a second set of tanks must be chosen for filling, the process is repeated.
Once a loll situation is confirmed only ever fill one tank at a time. Always start by filling the low side tank first.
The angle of loll may initially worsen because the introduced free surfaces when initially starting to fill the tank may cause a greater rise of G than the fall of G caused by the added bottom weight. Hence the importance of filling small tanks first. If there is any doubt as to whether the ship is lolled or listed, always initially assume it is a lolled situation and take appropriate action, monitoring the situation carefully.
When correcting a list it is sufficient to shift a weight to the high side. This may be Achieved by shifting weights on deck or by transferring ballast from a listed side tank to a high side tank. Alternatively, excess ballast from the listed side (possibly low down in the ship) may be discharged or a low tank on the high side should be filled.
To treat a loll situation in the same way as for list would have disastrous consequences for reasons already explained!