Crane Lift of a Fluid End: Sling Angle and Off-Centre Centre of Gravity
At 14:00 a 4.8 t mud pump fluid end is to be lifted from the pipe rack to the pump skid, a 9 m lift at 18 m radius. The crane is a 25 t truck crane with current certification. The rigging crew proposes two-leg wire rope slings, each with a vertical WLL of 3.0 t, choked around the fluid end. The included angle between the legs will be 90°, and the centre of gravity sits 300 mm off the midpoint between the two attachment points, which are 3 m apart. The landing area is 4 m from the edge of the mud pit. The tag line is missing, so two floorhands plan to guide the load by hand. Wind is 8 m/s and the banksman is also acting as the slinger because the regular slinger is at lunch.
1. Calculate the load in each sling leg and compare it with the sling capacity. Is the proposed lift acceptable?
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With a 90° included angle, each leg is 45° from vertical, so the vertical share must be divided by cos 45° (0.707).
Ignoring the off-centre centre of gravity first: Each leg carries 4.8 ÷ 2 = 2.4 t vertical. Sling tension = 2.4 ÷ 0.707 = 3.39 t per leg. That already exceeds the 3.0 t vertical WLL. The allowable vertical share at 45° is 3.0 × 0.707 = 2.12 t, so the lift is overloaded even before the centre of gravity is considered.
Now include the 300 mm offset. The centre of gravity is 1.2 m from one attachment and 1.8 m from the other, over a 3 m span. Taking moments: R1 = 4.8 × 1.8 ÷ 3 = 2.88 t R2 = 4.8 × 1.2 ÷ 3 = 1.92 t Sling tensions: 2.88 ÷ 0.707 = 4.07 t and 1.92 ÷ 0.707 = 2.72 t.
Both legs exceed the 2.12 t allowable at this angle, and the heavily loaded leg exceeds the 3.0 t vertical WLL by about 36%. A choked hitch on a fabricated component may reduce capacity further and can damage the sling on sharp edges. The lift is not acceptable. The crane's capacity at 18 m radius must also be checked against its load chart before any alternative rigging is approved.
2. Identify the unsafe practices in the proposed lift and set out a safer rigging and lift plan.
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Unsafe practices:
- Sling legs at 90° included angle are overloaded, and the off-centre centre of gravity makes one leg carry much more than the other.
- No tag line, with two people hand-guiding a 4.8 t load.
- The banksman is also slinging, so no one is solely controlling the lift.
- No defined exclusion zone around the landing area or the pit edge.
- The load is being lifted at 18 m radius without a check of the crane's rated capacity at that radius.
- No pre-lift meeting or written lift plan for a load near the edge of an excavation.
Safer plan: Use a spreader beam so the legs stay vertical, or use four legs of adequate capacity with certified shackles. If a spreader is used, each vertical leg load becomes 2.88 t and 1.92 t, within the 3.0 t WLL. Fit softeners or edge protectors at the choke points, mark the centre of gravity, and use two tag lines with people outside the fall zone. Appoint a dedicated banksman and a separate slinger, agree radio or hand signals, barricade the landing area and the pit edge, and hold a toolbox talk. Check the crane load chart for 18 m radius and the wind limit before lifting.
3. Who has authority to stop this lift, and what communication and documentation should accompany the revised lift?
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Any crew member who sees the overload or the uncontrolled load has stop-work authority, and the banksman, slinger, crane operator and driller all have a duty to stop it. The lifting supervisor or rig manager should formally halt the job until the plan is revised.
Communication:
- Hold a pre-lift meeting with the crane operator, slinger, banksman and floor crew.
- Confirm the load weight, centre of gravity, rigging arrangement, crane radius, wind limit and exclusion zone.
- Agree hand signals and radio channel, and confirm that only the banksman directs the crane operator.
- Confirm the emergency stop signal and the route to the landing area.
Documentation:
- A written lift plan or permit to work for the lift, including the rigging calculation and crane capacity check.
- Sling, shackle and crane certificates with WLLs recorded.
- A toolbox talk record and the names of the appointed banksman and slinger.
- An inspection record for the slings and a note of any defect found.
If the lift cannot be made safely with the available equipment, the correct decision is to stop and obtain larger slings, a spreader beam or a different crane position - not to proceed because the pump is needed.