Heavy-Lift Discharge with a Marginal Shore Crane and Low GM
MV Baltic Handler is a 9,200 t deadweight heavy-lift ship. On arrival at a river berth her displacement is 8,400 t, GM 0.45 m and TPC 19 t/cm. She must discharge a 128 t transformer from the weather deck. The unit sits 13 m to starboard of the centreline on four timber pads, each 1.5 m × 1.5 m, with an overall footprint of 12 m × 4 m. The hatch-cover pontoons are rated 3.5 t/m² uniformly distributed and 12 t per pad locally. The charterer has booked a shore crane working at 22 m radius, where its load chart gives 118 t SWL; at 18 m radius the chart gives 145 t. Spreader and rigging weigh 4.5 t. Wind is 18 knots on the starboard beam and the tide is falling at 0.4 m/hour. The terminal wants the lift started immediately. The ship's own 250 t crane is available but the charterer refuses to pay for it. The master's standing limit is 3° of list during any lift.
1. Check whether the shore crane lift can be made as planned. Show the figures you used and state your decision.
Read the worked answer
Total hook load. 128 t unit + 4.5 t rigging = 132.5 t.
At 22 m radius. SWL 118 t. Utilisation = 132.5 / 118 = 1.12, i.e. 112% of the chart value. This is a hard refusal - the lift cannot be made as booked.
At 18 m radius. SWL 145 t. Static utilisation = 132.5 / 145 = 0.91 (91%). That looks acceptable until a dynamic factor is applied. A factor of 1.1 for a static deck lift gives 132.5 × 1.1 = 145.8 t, which slightly exceeds 145 t (100.6%). With wind on the beam, a long load and tidal rise, I would not sign this off either; a lift this close to the chart line has no margin for snatch or for the load swinging as the ship rises.
Decision. The shore crane lift cannot proceed at 22 m. The realistic options are: reposition the ship so the crane works at a shorter radius with a genuine margin; engage a larger shore or floating crane; or use the ship's own 250 t crane, verifying its load chart at the actual working radius (the 250 t rating will only apply at minimum radius) and its pedestal and foundation loading. I would also check outrigger ground bearing pressure on the berth and the mooring arrangement before agreeing to any configuration, and record the refusal and the reasons in the deck log.
2. Assess the load-spreading problem and state what you require before any lift is attempted.
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Uniform check. 132.5 t over 12 m × 4 m = 48 m² gives 2.76 t/m², which is below the 3.5 t/m² uniform rating - but this is misleading, because a transformer is not a uniformly distributed load and the pontoon rating is a UDL figure.
Point-load check. Each of the four pads carries 132.5 / 4 = 33.1 t over 2.25 m², i.e. 14.7 t/m². Against the stated local limit of 12 t per pad, each pad is carrying 2.75 times the permitted load. The pontoon, its stiffeners and the coaming would be overstressed, and the failure mode is local buckling or collapse under a 33 t point load while the unit is hanging - precisely when the ship cannot take the weight back.
Requirements before lifting. I would require: (1) written confirmation of the pontoon manufacturer's point-load rating, not just the UDL; (2) steel skid beams under the unit, sized to carry 33 t per end and long enough to land on the hatch coaming or a transverse bulkhead, so the load bypasses the pontoon plating; (3) calculation by the ship's technical department or class surveyor for the proposed spreader arrangement; (4) verification that the terminal's spreader and slings bear on the unit's marked lifting points only; and (5) photographic records of the prepared stow before the lift is authorised.
3. Quantify the stability and list implications of the lift and state the conditions you would impose.
Read the worked answer
The transverse moment. When the load comes off the deck it leaves the ship's displacement while still 13 m to starboard, so the remaining ship's centre of gravity shifts to port. Heeling moment = 132.5 × 13 = 1,722.5 t·m.
tan θ = 1,722.5 / (8,400 × 0.45) = 0.4555, giving θ ≈ 24.5° of list to port. That is eight times the master's 3° limit and would put the deck edge under water, shift the grain-free cargo, spill the crane's load and possibly part the moorings.
Allowable offset. To stay within 3°: allowable moment = 8,400 × 0.45 × tan 3° = 198 t·m, so allowable transverse offset = 198 / 132.5 ≈ 1.5 m. The load would have to be within 1.5 m of the centreline at that GM, which is impractical without skidding the unit.
Conditions I would impose. (1) Raise GM first by pressing up low double-bottom tanks, then recompute. (2) Pre-list 2–3° to starboard so the lift-induced shift brings the ship upright rather than over. (3) Have the crane hold the load on the hook while ballast is adjusted, keeping the list within 3° at every instant, with a competent officer calling list and draughts. (4) Verify on the loading computer that the deck edge does not immerse at the maximum list reached. (5) Allow for TPC 19: removing 132.5 t reduces draught about 7 cm, while the tide falls 0.4 m/hour - check sling height, mooring slack and the risk of the load landing back on deck. (6) Stop the lift if wind exceeds the agreed limit or the list exceeds 3°.