Two-crane tilt-up of a derrick mast near an exclusion zone
A 10 t pedestal derrick mast is being erected in a plant yard. The mast is 24 m long and is being raised from horizontal by two mobile cranes: Crane A at the base end and Crane B at the top end. The lift plan assigns 60 percent of the weight to Crane A and 40 percent to Crane B, and both cranes are rated for that share at the planned radii. The tailing crane B will travel slowly forward as the mast comes up. A dogger is positioned at the base to control a tag line. During the trial lift, the mast rises 300 mm and the load cell on Crane B reads 5.6 t against a planned 4.0 t. The dogger reports the tag line is slack and the mast is swinging slightly. The supervisor wants to continue because the mast is only 300 mm up and the cranes are well within their charts.
1. What does the load cell reading tell you, and what should happen next?
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The reading is a warning that the load share is not as planned. Crane B is carrying 5.6 t instead of 4.0 t, a 40 percent increase over the plan. That shift means Crane A is carrying less than assumed, and the centre of gravity is closer to Crane B than the plan predicted. Possible causes include rigging geometry, sling length, hook positions, or an error in the assumed centre of gravity. The swing and slack tag line show the mast is not under control.
Stop the lift and lower the mast to the ground. Re-check the rigging geometry, the actual centre of gravity, and the load share calculation. Re-plan the lift so both cranes have adequate margin for the actual share, not the assumed one. Do not continue on the basis that both cranes are within their charts, because the chart margin was calculated for a share that is not occurring. Resume only after the trial lift reproduces the planned shares within an agreed tolerance, and with the tag line tensioned and controlled.
2. Explain the specific danger of continuing a two-crane lift when one crane is overloaded relative to plan but still inside its chart.
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A crane chart is a limit for a single crane in a defined configuration. In a two-crane lift, the real risk is the interaction: if one crane takes more load than planned, the other takes less, and the load's orientation and the forces in the rigging change. The overloaded crane may still be inside its chart, but its margin for dynamic effects, wind, and slight movement is now much smaller. If it reaches its limit first, it will not simply stop; the load will transfer abruptly to the other crane, which may then be overloaded. That transfer can shock-load the rigging and cause the mast to swing or drop.
The controlling principle is that each crane must be able to carry its actual share with a margin, and the share must be verified, not assumed. Continuing because the chart is not exceeded confuses a single-crane limit with a system check.