Two-crane tandem lift of an asymmetric vessel
A petrochemical shutdown requires a 34 t pressure vessel to be lifted from a horizontal position on the ground and rotated to vertical for installation. The vessel's centre of gravity is not at its geometric midpoint: it sits 4.2 m from the tail end and 7.8 m from the head end, giving a total length of 12 m. Two cranes are available. Crane A, a 100 t slewing mobile crane, will take the head end at a 14 m radius with a rated capacity of 11.2 t. Crane B, a 60 t crane, will take the tail end at a 9 m radius with a rated capacity of 8.6 t. The rigging plan assigns 60% of the load to Crane A and 40% to Crane B based on the centre of gravity. No lift plan exists. The two operators have not worked together before and there is no agreed communication protocol.
1. Check whether the proposed load split is correct and whether each crane has adequate capacity.
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The load split must be calculated from the centre of gravity, not assumed. With the centre of gravity 4.2 m from the tail end and 7.8 m from the head end, the tail crane carries the fraction of load nearest the centre of gravity. The share taken by Crane A (head end) is the distance from the centre of gravity to the tail end divided by the total span: 4.2 / 12 = 35%. Crane B takes 7.8 / 12 = 65%.
That inverts the plan. At 34 t total, Crane A carries about 11.9 t and Crane B about 22.1 t. Crane A's rated capacity of 11.2 t is exceeded, and Crane B's 8.6 t is exceeded by a wide margin. The proposed 60/40 split is both wrong in principle and unsafe in practice.
The split also changes during the rotation. As the vessel goes from horizontal to vertical, the geometry of the slings and the effective radius change, and the load distribution shifts continuously. A tandem lift plan must show the load share at each stage of the rotation, not just at pick-up. Neither crane is adequate for this lift as configured; the vessel must be re-rigged, a larger tail crane used, or the lift broken into stages.
2. Describe the planning and communication controls that must be in place before a tandem lift of this type is attempted.
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A tandem lift needs a written lift plan prepared by a competent person and signed off before the lift. The plan must include the load's mass and centre of gravity, the rigging arrangement, the calculated load share at each stage of the rotation, the rated capacity of each crane at the actual radius, and the margin. Both cranes should be kept well below rated capacity, because a tandem lift introduces load transfer that a single-crane chart does not model.
There must be a single lift supervisor in charge, with authority over both operators. The operators must be briefed together, not separately, and must agree on a communication method: hand signals, whistle signals, or radio on a dedicated channel. Radio is usually preferred for a two-crane lift because hand signals cannot reach both operators at once. A agreed set of commands and a clear stop command must be rehearsed.
The plan must define the sequence: which crane takes load first, how the rotation is controlled, what happens if one crane loses communication, and the abort criteria. Tag lines must be rigged to control rotation. The area must be excluded to non-essential personnel. Both cranes must be on ground verified for their outrigger loads, and the swing paths must be checked for obstructions. If any of these is missing, the lift does not start.