Two-Leg Lift of an Offset Vessel
A 12.5 t horizontal vessel, 9.0 m long, is to be lifted from its saddles onto a foundation. The centre of gravity is 3.6 m from the tail end. Two lifting lugs are already welded on the shell at 1.5 m and 7.5 m from the tail end. The rigging plan shows a single two-leg sling from one crane hook, with each leg at 60 degrees to the horizontal. The available shackles at the lugs are rated 8.5 t WLL; the slings are 10 t WLL each, and the hook block is rated 20 t. The crane is positioned so the hook is directly above the midpoint between the lugs. A tag line is attached at the tail. The vessel has a 1.2 m diameter and no internal trays. The lift area is level but has a 2 percent cross slope. The crew plans to lift, boom out, and set the vessel on anchor bolts without a tailing crane. The site supervisor asks for a lift plan review before the crane is rigged.
1. Calculate the vertical reactions at each lug and the sling tensions. Compare them with the rigging gear ratings.
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Take moments about lug A at 1.5 m from the tail. The centre of gravity is 2.1 m from A and 3.9 m from B, with a lug span of 6.0 m. Reaction at A = 12.5 x 3.9 / 6.0 = 8.13 t. Reaction at B = 12.5 - 8.13 = 4.37 t. If each sling is at 60 degrees to the horizontal, the vertical component is T sin 60, so T_A = 8.13 / 0.866 = 9.39 t and T_B = 4.37 / 0.866 = 5.05 t. The 8.5 t shackle at A is overloaded by about 10 percent in static conditions, before any dynamic or wind allowance. The 10 t slings are nominally adequate, but the A sling has little margin. The 20 t hook block is adequate for the 12.5 t load. The lift should not proceed as planned without changing the A lug gear or reducing the load at A.
2. The hook is positioned above the midpoint between the lugs and both legs are specified at 60 degrees. Is that geometry consistent with the calculated reactions? What will the vessel do when lifted?
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It is not consistent. For a suspended load, the horizontal components of the two sling tensions must balance. If both legs are at 60 degrees to the horizontal, the horizontal components are T_A cos 60 and T_B cos 60. With unequal vertical reactions, the horizontal components are also unequal, so the vessel will swing until the centre of gravity is below the hook and the sling geometry changes. The hook is above the midpoint at 4.5 m from the tail, while the centre of gravity is at 3.6 m, so the vessel will rotate tail-down until the CG aligns under the hook. A single hook with both legs at equal angles cannot hold this offset load level. The plan should position the hook over the CG, use different sling lengths, or use an equalizing beam. Without a tailing crane, the rotation is uncontrolled and the vessel can swing into the anchor bolts or the crane.
3. Recommend changes to the lift plan, including gear, hook position, and load control.
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Reject the plan as drawn. Replace the 8.5 t shackle at lug A with a shackle rated above the calculated tension with a suitable dynamic factor, or relocate the lug to reduce the A reaction. Recalculate the sling lengths so the hook is over the centre of gravity and the horizontal components balance. Use a spreader or equalizing beam if the lugs must remain in place. Add a tailing crane or a controlled tailing line and use tag lines at both ends; the 2 percent cross slope and anchor bolts make uncontrolled rotation unacceptable. Confirm the lugs and their welds are rated for the calculated loads, check ground bearing under the outriggers, and have a competent person approve the revised lift plan before rigging.