A 24-tonne lift with the ship's own crane and two slack tanks
MV RIVER OGOJA, a 12,000 t deadweight multipurpose ship, is alongside at Onne loading a skid-mounted gas compressor of 24 t into the lower hold. Present condition: displacement 8,900 t, KG 6.10 m, solid GM 0.45 m. The starboard deck crane is rated 40 t SWL and will work at 20 m radius, where its capacity is 26 t. The compressor sits on the quay 18 m from the ship's centreline. Two double-bottom tanks are slack: No. 2 port holds 60 t and No. 3 starboard 40 t, giving a combined free surface moment of 1,800 t·m. The Chief Officer has planned the lift on the crane alone, with no ballast change. The crane manual states the maximum permitted list during a lift is 5°.
1. Is the lift as planned acceptable? Show the reasoning, including the estimated list.
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No. The lift must not proceed as planned.
Stability available. The starting GM of 0.45 m is not the GM that resists the lift. Free surface correction = free surface moment ÷ displacement = 1,800 ÷ 8,900 = 0.20 m, so the effective GM is 0.45 − 0.20 = 0.25 m. That is a thin margin before anything is lifted.
Heeling moment. Once the load comes off the quay it is a suspended weight acting at the crane radius from the centreline: 24 t × 20 m = 480 t·m to starboard.
Righting moment. Using RM = W × GM × sin θ at small angles with W = 8,900 t: 8,900 × 0.25 = 2,225 t·m per unit sin θ. Equilibrium: sin θ = 480 ÷ 2,225 = 0.216, giving θ ≈ 12°.
That is more than double the 5° limit in the crane manual, and it is the best case. The calculation ignores that: the 24 t is added to the displacement when it leaves the quay and its centre of gravity is high, raising the ship's KG further; the list changes the immersion and may bring the deck edge or a freeing port close to the water; and a real lift has dynamic effects as the load swings. A ship at 12° alongside with moorings still on is also a mooring and fender problem.
Conclusion. The plan is unsafe as written. The two slack tanks are a double error: they cost 0.20 m of GM and they give no counter-heeling moment precisely when one is needed.
2. Set out the corrective actions before the lift, and what must be verified before the load leaves the quay.
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Ballast first, lift second.
- Eliminate the free surface. Either press No. 2 port and No. 3 starboard full, or empty them completely. A part-filled tank is the worst of both worlds. Filling the 60 t in No. 2 port low down both removes its free surface and provides a counter-heeling moment to port.
- Recompute the condition. New displacement, new KG, new GM, and re-run the list calculation with the load at 20 m radius. I want a predicted list on the starboard side of no more than 2–3° with the load suspended, so that trim and moorings stay sane.
- Pre-list to port. It is standard practice to give the ship a small list away from the load - 1–2° - before breaking the load out, so the lift brings her upright rather than over.
- Check the crane, not just the load. The 26 t capacity at 20 m must cover the load plus the block, slings, spreader and any dynamic factor. The load is 24 t, so the margin is 2 t - barely adequate. I would rather work at a shorter radius or use a shore crane or tandem lift. Verify the SWL plaque, the recent load test certificate, the wire ropes, the limit switches and the outrigger/pedestal condition.
- Check the structure. Tank top loading for a 24 t skid, deck load limits, the hold's point loading, and the effect on still-water bending moment of the ballast transfer.
- Check the environment. Slack the moorings so she can list freely, confirm the fenders will not catch, check the tide and the under-keel clearance as she lists, and confirm the wharf can take the load reaction.
- Brief the team. Crane driver, riggers, banksman, and a man on the list indicator and the telltale. Signals agreed and radio checked.
3. Once the unit is stowed, what must be confirmed and recorded before the ship sails, and which intact stability criteria apply?
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Stability criteria (2008 Intact Stability Code, Part A, 2.2). The departure condition must show: initial GM not less than 0.15 m; area under the righting lever curve up to 30° not less than 0.055 m·rad; area up to 40° (or the flooding angle if less) not less than 0.09 m·rad; area between 30° and 40° not less than 0.03 m·rad; and maximum GZ of at least 0.15 m occurring at an angle of 15° or more. A GM of 0.25 m before the lift is marginal, so the departure condition has to be recalculated from scratch after the ballast transfer and the stowage - not assumed to have improved.
Before sailing I want confirmed:
- The final departure condition computed from the actual ballast and cargo figures, checked against the criteria above and against the loading manual's allowable still-water shear force and bending moment, particularly if a heavy item is concentrated in one hold.
- The unit's centre of gravity as stowed, the securings and lashings per the Cargo Securing Manual, and the sea-fastening inspected by the Chief Officer and, ideally, a surveyor or the shipper's representative.
- The hold tank top and structure not overstressed; the hatch covers and coamings clear; the hold ladder and bilge wells unobstructed.
- Draughts read fore and aft and amidships, air draught, and under-keel clearance for the channel.
- The crane stowed and secured, the jib in its cradle, and the wire ropes on the drum.
- Ballast Water Record Book entries for any transfer or discharge, with the management plan requirement met.
The Chief Officer's loading plan, the signed stability calculation, the lashing certificate and the draught survey all go to the master, who signs them off before the pilot boards. Any change to the plan after signature means the calculation is redone.