Upending a Vessel: Load Share Changes With the Stage
A 47,000 lb horizontal vessel, rigging included, is to be upended and set on a foundation. The vessel is 58 ft long. The tailing lug is 5 ft from the tail end, the main lug is 53 ft from the tail end, and the center of gravity is 33 ft from the tail end. The tailing crane is a 60-ton AT at a 22 ft radius, chart capacity 34,000 lb. The main crane is a 200-ton AT with 120 ft of boom at a 44 ft radius, chart capacity 41,000 lb. The plan is for the tailing crane to hold the tail while the main crane booms up, then to release the tailing crane once the vessel is vertical. Both operators are certified, rigging has been inspected, and the yard is paved and level. Wind is 8 mph. The superintendent wants the vessel set before the afternoon shift comes on.
1. Compute each crane's initial load share and its percentage of chart capacity.
Read the worked answer
The share is a statics problem, not a guess. The lugs are 48 ft apart. Taking moments about the main lug: tail share x 48 = 47,000 x (48 - 28), where 28 ft is the distance from the tail lug to the CG (33 - 5).
Tail = 47,000 x 20 / 48 = 19,583 lb. Main = 47,000 x 28 / 48 = 27,417 lb. Check: the two add to 47,000 lb.
Utilization at pick-up:
- Tailing crane: 19,583 / 34,000 = 58 percent.
- Main crane: 27,417 / 41,000 = 67 percent.
Both look comfortable, and this is exactly where a plan built only on the pick-up condition goes wrong. The tail share is a snapshot of one geometry. As the vessel rotates toward vertical, the horizontal distance from the main lug to the CG shrinks, so the tail load falls and the main load climbs. The tailing crane gets stronger as its radius follows the tail in; the main crane is headed the other way.
2. Identify the stage at which this plan fails and explain why the numbers at pick-up do not reveal it.
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The plan fails at the end of the upend. When the tailing crane goes slack, the main crane carries the entire 47,000 lb. At the planned 44 ft radius the chart gives 41,000 lb, so the main crane would be at 47,000 / 41,000 = 115 percent of rated capacity - over the chart, not near it.
The pick-up numbers miss this because load share is a function of geometry, and the geometry changes through the lift. Each crane's utilization curve has to be plotted against rotation angle, and the governing case for the main crane is the moment of tail release, not the moment of pick.
Two more things belong in the same analysis. If company policy caps lifts at 75 percent, the main crane needs a chart capacity of at least 62,700 lb at whatever radius it will be at when it holds full weight. And the tail release itself must be a controlled hold point, not a call made in the moment: confirm the vessel is plumb, confirm the main crane is actually holding the full weight, then release.
3. What changes to the plan would you require before approving it?
Read the worked answer
Change the end condition, not the procedure.
- Reconfigure the main crane so that full weight falls well inside the chart. Options are a larger crane, a shorter boom, or a shorter radius; whichever is chosen, verify against the manufacturer's chart for the exact boom length, counterweight, and outrigger configuration, with margin for the 75 percent policy if it applies.
- Check the tailing crane at its minimum radius during follow-in and confirm it has capacity with the load descending under control; also confirm the tail hook can be released without shock loading the main.
- Mark the tail-release point as a hold point in the plan, with the lift director in control and agreed hand signals or radio protocol.
- Add tag lines from two directions so the vessel cannot spin, and confirm the radius the main crane will actually be at, measured, not assumed.
- Write it up as a critical lift with stage-by-stage capacity for both cranes, a pre-lift meeting with both operators and the rigger, and a barricaded drop zone.