Determine free point constant FPC
The free point constant can be determined for any type of steel drill pipe if the outside diameter, in., and inside diameter, in., are known:
where: As = pipe wall cross sectional area, sq in.
Example 1: From the drill pipe stretch table: 41/2 in. drill pipe 16.6 lb/ft — ID = 3.826 in.
FPC = (452 — 3.8262 x 0.7854) x 2500 FPC = 4.407 x 2500 FPC = 11,017.5
Example 2: Determine the free point constant and the depth the pipe is stuck using the following data:
23/8 in. tubing — 6.5 lb/ft — ID = 2.441 in. 25 in. of stretch with 20,000 lb of pull force a) Determine free point constant (FPC):
FPC = (2.8752 — 2.4412 x 0.7854) x 2500 FPC = 1.820 x 2500 FPC = 4530
b) Determine the depth of stuck pipe:
20 Feet
Feet of free pipe = 5663 ft Method 2
Free pipe, ft = 735,294 x e x Wdp differential pull, lb where e = pipe stretch, in.
Wdp = drill pipe weight, lb/ft (plain end)
Plain end weight, lb/ft, is the weight of drill pipe excluding tool joints:
Weight, lb/ft = 2.67 x pipe OD, in.2 — pipe; ID, in.2
Example: Determine the feet of free pipe using the following data:
5.0 in. drill pipe; ID — 4.276 in.; 19.5 lb/ft Differential stretch of pipe = 24 in. Differential pull to obtain stretch = 30,000 lb
Weight, lb/ft = 2.67 x (5.02 — 4.2762) Weight = 17.93 lb/ft
Free pipe = 10,547 ft
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