Suspected Angle of Loll After Ballast Exchange
Your 35,000 DWT bulk carrier is completing ballast exchange in port after discharging a heavy cargo. Draft forward 6.2 m, aft 7.0 m. The loading computer shows KG 8.95 m, KM 9.10 m, and a free surface correction of 0.12 m, giving an effective GM of 0.03 m. The vessel has a persistent list of 5° to starboard. When the starboard ballast tank is pressed up, the list increases to 7°; when the port tank is pressed, the list rolls slowly through the upright to 6° to port and then settles back to starboard. The roll period is about 18 seconds, compared with 12 seconds when the vessel sailed fully loaded. There is slack ballast in two centre tanks. The weather forecast is for SW force 6 with 2 m seas. The master wants to sail within two hours.
1. What is your diagnosis of the vessel's condition, and why might the loading computer be misleading?
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The vessel is at or near an angle of loll, not a simple list from off-centre weight. The loading computer's effective GM of 0.03 m is already below the normal minimum and is within the error of the free-surface and KG inputs. The behaviour is decisive: the list changes side when ballast is transferred, the vessel rolls slowly through upright, and the roll period has lengthened from 12 seconds to 18 seconds. A long roll period indicates a small metacentric height; a list that reverses side indicates negative or near-zero GM. The response to pressing up the starboard tank is not consistent with a simple weight list, which also points to loll. The computer may be misleading because it may not include all free surfaces, may have the wrong tank contents, or may use a KG that does not reflect the actual ballast and slack tanks. Treat the computer output as a check, not as proof that the ship is safe. An angle of loll is dangerous because the GZ curve has a negative initial slope and the vessel may flop suddenly to the other side.
2. What immediate actions would you take to make the vessel safe, and what would you tell the master about sailing?
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Do not sail. First reduce free surfaces by pressing up or emptying the slack centre tanks one at a time, keeping each tank either full or empty. Then fill the lowest centre tank to lower G and remove the negative GM; for loll, the correct sequence is to fill the lowest centre tank first, not the high-side wing tank. After the negative GM is removed, correct the list by transferring ballast to the low side or filling low-side tanks. Adding weight to the high side first can increase the loll and lead to capsize. Keep the ship upright, avoid large simultaneous transfers, and calculate each step with the loading computer and a manual check. If the list cannot be controlled, stop, keep the vessel in port and seek shore stability advice. Check for water ingress, slack tanks and any tank left partially filled by the ballast exchange. Tell the master that the vessel should not sail until GM is positive, the list is removed and the stability criteria are satisfied.
3. What stability checks and records would you complete before departure?
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Before sailing, verify the intact stability condition against the IS Code 2008 criteria: initial GM not less than 0.15 m; area under the GZ curve up to 30° not less than 0.055 m·rad; area up to 40° or the angle of flooding not less than 0.09 m·rad; area between 30° and 40° not less than 0.03 m·rad; GZ at 30° not less than 0.20 m; angle of maximum GZ not less than 25°; and adequate freeboard to the deck edge. Check that the loading computer has all tanks correct, free surface included, and KG and displacement correct. Check draft, trim, list, stress and buckling. Check the forecast: SW force 6 and 2 m seas will increase rolling, so if the vessel is still tender, delay. Record the final condition and inform the master. Confirm that the ballast exchange plan is complete, all valves are set correctly, and no tank is pressed up with a slack surface.