Two-Leg Bridle on an Offset Pipe Spool
A 4,800-lb, 24-ft pipe spool is being lifted from a laydown yard to a barge deck. The center of gravity is 3 ft from the left end. The planned pick points are at the two ends, using two 5/8-in wire-rope slings, each with a single-leg vertical WLL of 3,300 lb. With the hook at the planned height, each sling will be 40° above horizontal. The crane is set at an 18-ft radius, where its load chart allows 6,200 lb; rigging and hardware weigh 250 lb. A tag line is rigged. The rigger says, "Two legs share the load, so each takes 2,400 lb; we are fine." The lift director notices the spool is not level in the trial lift and that one sling is tighter. The barge deck has a 12-in camber, and the landing area is clear. Wind is 8 mph.
1. Calculate the approximate vertical reaction at each end and the tension in the more heavily loaded sling at 40° above horizontal. Does the planned lift exceed the sling WLL?
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Treat the spool as a simply supported beam for a first check. With the CG 3 ft from the left end of a 24-ft spool, the left reaction is 4,800 × (24 − 3) / 24 = 4,200 lb; the right reaction is 600 lb. The more heavily loaded sling therefore carries about 4,200 lb vertical. At 40° above horizontal, tension T = 4,200 / sin 40° ≈ 6,530 lb. That is about 198% of the 3,300-lb WLL, before adding the 250-lb rigging/hardware and dynamic effects. The rigger's "each takes 2,400 lb" assumes a centered load and equal leg loading, neither of which is true. The crane's 6,200-lb chart capacity at 18 ft is not the governing problem; the sling is.
2. Should the lift proceed? Identify the load-control, rigging, and crane-capacity checks that must be resolved before any further trial lift.
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No. The trial lift already shows the spool is not level and one sling is tighter, which means the assumed load distribution is wrong. Before another lift: confirm actual weight and CG; inspect sling tags, WLL, and damage; verify the crane radius, boom configuration, outrigger ground, and capacity with the 250-lb rigging; check edge protection and tag line; calculate tension using the actual geometry; and require an engineered lift plan if the CG or pick points cannot be verified. Do not use a trial lift to prove an overloaded sling. The load must be landed and the rigging changed.
3. Recommend a rigging change that would make the lift acceptable, and explain how you would verify it.
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Use a spreader beam with lower legs vertical at the ends, or a four-point equalizing rig approved by a competent person. If the lower legs are vertical, the left leg still sees about 4,200 lb, so the two 3,300-lb slings are still inadequate; select lower slings with a rated WLL above the calculated tension plus the site design factor (for example, a 6,000-lb WLL class or stronger, subject to the lift plan). Reposition pick points to reduce the offset only if the resulting overhang and sling geometry are engineered. Verify by recalculating reactions, checking the actual hook height/angle, using a load indicator if available, and repeating a controlled trial lift with the load just off the deck.