Lifting a Temporary Casing With the Rig's Certified Lifting Mode
A drill rig with a manufacturer-certified lifting mode is to lift a 6.0 m long, 1.0 m diameter temporary casing weighing 4.2 t. The rigging weighs 0.25 t. The load chart for the lifting mode shows: 4.0 m radius - 6.0 t; 5.0 m - 4.6 t; 6.0 m - 3.2 t. The proposed pick point is at 5.5 m radius, and the chart is based on a freely suspended load with the boom over the end. You plan a two-leg bridle with a 60° included angle between the legs. The available shackles are rated 3.25 t each; the slings are rated 5 t vertical each. The casing has no certified lifting points; you plan to choke the slings around the casing.
1. Calculate the net load and the chart capacity at 5.5 m. Is the lift acceptable?
Read the worked answer
Net load = casing 4.2 t + rigging 0.25 t = 4.45 t.
The chart values are linear between the listed radii. At 5.5 m, halfway between 5.0 m (4.6 t) and 6.0 m (3.2 t), capacity = 4.6 − 0.5 × (4.6 − 3.2) = 4.6 − 0.7 = 3.9 t.
4.45 t demanded > 3.9 t capacity, so the lift is not acceptable at 5.5 m. The overload is 0.55 t, about 12% above chart capacity. The chart is already based on a freely suspended load with the boom over the end; do not apply an additional deduction and then call it acceptable. To make the lift legal, reduce the radius to about 5.1 m or less where the chart reaches 4.45 t, but that leaves no margin. A better plan is to reposition so the pick is at 4.5 m or less and verify the chart, or use a crane with adequate capacity.
2. Calculate the sling leg tension and check the shackles and slings. What additional rigging concerns arise from choking the casing?
Read the worked answer
For a 60° included angle, each leg is 30° from vertical. The vertical share per leg is half the load, and the leg tension is:
T = W ÷ (2 cos 30°) = 4.45 ÷ (2 × 0.866) = 4.45 ÷ 1.732 = 2.57 t per leg.
The 3.25 t shackles are loaded to 2.57 ÷ 3.25 = 79% of rating, which is acceptable for a static, symmetrical lift. The 5 t vertical slings are also acceptable in straight tension, but a choke hitch reduces sling capacity, often to about 75–80% of the vertical rating. At 75%, capacity is 3.75 t per leg, still above 2.57 t, but the efficiency must be confirmed from the sling manufacturer's data.
The bigger concerns are the load and the hitch. The casing has no certified lifting points, and a choke hitch can slip or crush a thin-wall casing. Use softeners, verify the casing wall and weight, and have a competent rigger confirm the choke will not slide toward the casing center. The center of gravity must be below the pick point and the load must be balanced. Add tag lines and keep personnel out from under the load.
3. What changes would you require in the lift plan before the pick is made?
Read the worked answer
Require a revised lift plan that:
- Repositions the rig or load so the pick radius is within chart capacity with a documented margin, preferably at 4.5 m or less.
- Uses the actual rigging weight and confirms the chart's configuration, boom position, and outrigger/crawler setup.
- Identifies the sling type, choke efficiency, shackle ratings, and softeners; verifies the casing's weight and wall condition.
- Shows the center of gravity, pick points, tag line locations, and the exclusion zone under the load.
- Includes a pre-lift meeting, a dry run at low height, and a stop signal.
- Confirms the operator is trained for the lifting mode and that the load chart in the cab matches the configuration.
If the casing cannot be choked safely, fabricate or use a certified lifting yoke or lifting points. Do not use the drilling winch as a crane unless the manufacturer's lifting mode and chart specifically permit it.