Lowering-In on a Wet 20-Degree Slope
A 16-in. OD, 0.500-in. wall X65 pipeline is being lowered into an 8-ft-deep ditch on a 20° slope. The pipe has an 80-ft joint length and weighs 86 lb/ft with coating. After a rain, groundwater in the ditch rose to 24 in., fully submerging the pipe. The spoil pile is 4 ft back from the trench edge. The crew has three sidebooms, each rated 40,000 lb at an 8-ft radius. The plan is to use three lift points spaced about 60 ft apart and lower the empty pipe downhill. The sideboom operator is concerned about the pipe floating and about the sidebooms sliding on the wet slope. The crew also has 1,000-lb concrete river weights available but no mats. The inspector must decide whether to allow the lowering-in to proceed, require dewatering, or stop work until the slope and trench are stabilized.
1. Calculate the buoyant uplift on one 80-ft joint and determine whether the empty pipe will float in the flooded ditch.
Read the worked answer
The pipe OD is 16 in. = 1.333 ft. Displaced volume per foot is π/4 × (1.333)² = 1.396 ft³/ft. Buoyant force is 1.396 × 62.4 = 87.1 lb/ft. The pipe weighs 86 lb/ft, so the net uplift is only 1.1 lb/ft, or about 88 lb on an 80-ft joint. That means the pipe is slightly buoyant: it will not settle firmly and can float with any disturbance, trapped air, or rise in water level. Applying a 1.25 safety factor gives about 110 lb of required hold-down per joint. One 1,000-lb river weight per joint is more than enough for static flotation if it is properly attached, but the weight does not fix the wet slope or the trench condition. If the coating traps air or the ditch is deeper, recalculate using the actual submerged volume.
2. Evaluate the three-sideboom lift plan and the slope and trench conditions.
Read the worked answer
Each sideboom would carry about 86 × 80 / 3 = 2,293 lb vertically, far below its 40,000-lb rating, so capacity is not the limiting factor. The 20° slope adds an axial component of 6,880 × sin 20° = 2,353 lb that the downhill machine and winch must control. The sidebooms must be set on stable ground with mats, not on the spoil side, and not where the trench edge has cracked. A spoil pile 4 ft from the edge meets the usual 2-ft minimum, but saturated soil and an 8-ft depth increase collapse risk. No one should be in the trench during lowering. A fourth lift point or a spreader beam may give better control of the joint. Stop work if a sideboom starts to slide, the trench edge sloughs, or the pipe begins to move laterally.
3. What decision should the inspector make, and what controls are required?
Read the worked answer
Do not proceed as planned with a flooded ditch and no mats. Require dewatering or control of the water level before lowering, install river weights or anchors that provide at least a 1.25 flotation safety factor, place mats and stabilize the sideboom positions, and inspect the trench and spoil bank immediately before lowering. Use a lift plan with calculated loads, radii, and slope restraint; keep personnel out of the trench. If water cannot be controlled, change the method, such as lowering shorter sections or installing weights before the pipe enters the ditch. Document the decision and the water level at the time of lowering. The inspector should stop the job if the slope shows movement or the sidebooms cannot be set on stable ground.