Conductor Pipe (26-in. pipe)
Conductor pipe is set to a depth of 350 ft and cemented back to the surface. In addition to the principal loads of collapse, burst, and tension, it is also subjected to a compression load, because it carries the weight of the other pipes. Thus, the conductor pipe must be checked for compression load as well.
In the design of collapse load, the following assumptions are made (refer to Fig. 3.15):
1. Complete loss of fluid inside the pipe.
2. Specific weight of the fluid outside the pipe is that of the drilling fluid in
the well when the pipe was run.
TOC o "1-5" h z Collapse pressure at the surface = 0
Collapse pressure at the casing shoe = 9.5 x 0.052 x 320 — 0
— 158 psi
Fig. 3.15: Collap se and burst loads on conductor.
In calculating the burst load, it is assumed that no gas exists at shallow depths and a saturated salt water kick is encountered in drilling the next interval. Hence, in calculating the burst pressure, the following assumptions are made (refer to Fig. 3.13):
1. Casing is filled with saturated salt water.
2. Saturated salt water is present outside the casing.
SHAPE * MERGEFORMAT
Burst pressure at casing shoe Internal pressure at casing shoe
= internal pressure — external pressure
= formation pressure at 5.000 ft
— hydrostatic pressure due to the salt water between 350 and 5,000 ft = 0.465 x 5.000 — [(5.000 — 350) x 0.465] = 162.75 psi
162.75 0 psi
|
Burst pressure at surface = |
formation pressure at 5,000 ft — hydrostatic pressure at the fluid column — external pressure
5. (0.465 — 0.465) — 0 0 psi
Selection based on collapse and burst
As shown in Table 3.3, both available grades have collapse and burst resistance values well in excess of those calculated above. Conductor pipe will, however, be subjected to a compression load resulting from the weight of casing-head housing and subsequent casing strings. Taking this factor into consideration, grade K-55 (133 lb/ft) with regular buttress coupling can be selected.
In checking for compression load, it is assumed that the tensile strength is equal to the compressive strength of casing. A safety factor of greater than 1.1 is desired.
Compressive load carried by the conductor pipe is equal to the total buoyant weight, Wbu- of the subsequent casing strings.
Compressive load = Hq, u of surface pipe
+ TTTu °f intermediate pipe + Vf, u of liner + И’би of production pipe = 390.093 + 863.242 + 134.928 + 588,430 = 1.976.990 lbf
Safety Factor, SF = Vr of K-oo (133 lb/ft) Total buoyant weight
2.125.000′ _
= 1.08
1,976.990
This suggests that the steel grade K-55. 133 satisfies the requirement for compression load.