Ballast Exchange, Free Surface and List
Your 25,000 t displacement bulk carrier is in ballast and conducting ballast-water exchange in a moderate swell. The vessel has developed a 4° starboard list. Soundings show No.2 port double-bottom tank 60% full (300 t) and No.2 starboard tank 95% full (475 t). Each No.2 tank is 20 m long and 12 m wide, with its centre 8 m from the centreline. The stability booklet gives KM 8.5 m and the calculated KG is 7.6 m. No.3 hold is empty and No.3 ballast tanks are pressed full. The ballast exchange plan requires both No.2 tanks to be partially filled at the same time. The master wants to continue the exchange to meet the D-2 standard before entering the next port. The weather forecast is worsening. You are the duty engineer working with the chief officer.
1. Calculate the solid GM, the fluid GM and the approximate list. State your assumptions.
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Solid GM = KM - KG = 8.5 - 7.6 = 0.90 m.
For each rectangular tank, free-surface moment FSM = ρ × L × B³ / 12. With seawater density 1.025 t/m³, L = 20 m and B = 12 m: FSM = 1.025 × 20 × 12³ / 12 = 1.025 × 20 × 144 = 2,952 t·m per tank. Two slack tanks give 5,904 t·m. Free-surface correction FSC = FSM / displacement = 5,904 / 25,000 = 0.236 m. Fluid GM = 0.90 - 0.236 = 0.664 m.
The list is caused by the transverse weight difference: 475 - 300 = 175 t, acting 8 m from the centreline, giving a healing moment of 175 × 8 = 1,400 t·m. For small angles, tan θ = healing moment / (displacement × fluid GM) = 1,400 / (25,000 × 0.664) = 1,400 / 16,600 = 0.0843, so θ ≈ 4.8°. This agrees with the observed 4° list. Assumptions are rectangular tanks, small list, no significant trim, constant KM, and free surface in both No.2 tanks.
2. Decide whether to continue the exchange and state the immediate corrective actions.
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I would not continue the exchange with both No.2 tanks slack in a worsening swell. The vessel still has positive fluid GM, but 0.664 m is reduced by 0.236 m of free surface, and a 4.8° list in a moderate swell is an unnecessary risk. The exchange plan itself is poor because it requires two large tanks to be partially filled at the same time.
Immediate actions are to inform the master and chief officer, stop the exchange, reduce speed and alter course to ease rolling, and close unnecessary cross-connections. The list should be corrected in small controlled steps, preferably by transferring ballast from the starboard tank to the port tank or by pressing up the port tank while monitoring soundings, list and GM. One tank should be pressed full or emptied before another is opened; avoid leaving both No.2 tanks slack. The stability instrument should be updated with actual soundings. If the list cannot be controlled, the vessel should proceed to sheltered water and complete the exchange later or use an approved alternative management method.
3. Explain the operational and construction lessons from this event.
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The main lesson is that ballast-water exchange must be planned with stability, not only with the D-2 standard, in mind. Large double-bottom tanks produce significant free-surface moments when slack; filling or emptying one tank at a time, or using smaller tanks and a sequenced plan, reduces the risk. The plan should specify maximum allowable simultaneous slack tanks and require soundings and list checks at each step.
Operationally, the crew should compare actual soundings with the stability instrument, monitor list and roll period, and slow down or change course when free surface is high. The chief officer and duty engineer should understand how FSC reduces GM and how a transverse weight difference creates list. The SMS and exchange plan should be revised, and the crew briefed, so that commercial pressure to meet the D-2 deadline does not override intact-stability limits.