Progressive List After Ballast Transfer
A 28,000 GT bulk carrier is alongside loading grain. The chief officer reports that the vessel has developed a 4-degree list to port over the last two hours. The list began after the crew transferred ballast from the port double-bottom tank to the starboard heeling tank to correct an earlier 2-degree starboard list. The vessel is in a load condition with a calculated KG of 8.2 m and a KM of 8.9 m. Free surface in the partially filled heeling tank is estimated to reduce GM by 0.12 m. The loading plan requires the vessel to sail with a GM of at least 0.30 m. Cargo operations are continuing at 400 tonnes per hour. The weather forecast indicates 25-knot beam winds for departure.
1. Calculate the effective GM and assess whether the vessel meets the departure criterion, showing your reasoning.
Read the worked answer
GM (solid) = KM − KG = 8.9 − 8.2 = 0.70 m.
Free surface correction = 0.12 m, so effective GM = 0.70 − 0.12 = 0.58 m.
This exceeds the required 0.30 m minimum, so on the figures given the vessel meets the departure criterion. However, the list indicates an off-centre centre of gravity, not necessarily a GM deficiency. The list is caused by a transverse shift of weight, so the righting lever at zero heel is reduced on the listed side.
The judgement is that a 4-degree list with 25-knot beam winds is not acceptable for departure even though GM is adequate. The list must be corrected before sailing, and the effective GM should be recalculated after correction because the free surface condition will change.
2. Explain the operational actions you would take to correct the list safely, and the risks of continuing cargo operations in the current condition.
Read the worked answer
Actions:
- Stop or slow cargo operations until the list trend is understood, because continuing to load at 400 t/h changes the KG and the list simultaneously.
- Identify the cause. Check tank soundings and the loading computer; the transfer may have overfilled the heeling tank or left a large free surface. Verify against the plan.
- Correct the list by transferring ballast in small steps, monitoring the inclinometer and the loading computer after each step. Avoid large simultaneous transfers that could create a dangerous free surface or a sudden shift.
- Recalculate stability after correction, including free surface, and confirm effective GM remains above 0.30 m for the actual sailing condition.
- Assess wind heel. With 25-knot beam winds, calculate the heel angle under wind pressure and confirm it is acceptable against the vessel's criteria and the grain stability requirements.
Risks of continuing: progressive list can lead to cargo shift, especially grain, which is prone to shifting; a list reduces freeboard on the low side and can ship water; and the loading computer model may no longer represent the actual condition. The master should not sail until the list is corrected and stability is re-verified.