Calculation of Collapse Pressure According to Clinedinst (1977)
Clinedinst (1977) conducted 2,777 collapse pressure tests on casing lengths between 14 in. (355 mm) and 33 in. (850 mm) from six manufacturers and found the following: 49 test results indicated that the use of Barlow’s formula (Eq. 2.118) to calculate the collapse pressure for yield range provided better agreement with the experimental results than did the use of the Lame formula (Eq.
|
/ n i / t .0<08 P(L/do) — P(L/do=8) (8 d0/L) |
Table 2.4: EXAMPLE 2-10: Failure Model and the d0/1 range for which it is valid.
Failure model Applicable d0/t range
1. Elastic
TOC o "1-5" h z pe =————————— ll,-‘)~l x 1Qb? d0/1 > 31.03
20.392 (20.392 — l)[1] ‘ ~
= 6,123 psi
2. Transition
pt = — 0.0434) 80.000 22.48 < d0/t < 31.03
= 4.366 psi
3. Plastic
pp = 80.000 (0.0839) — 1.955 13.39 < d0/t < 22.48
— 4.754 psi
4. Yield
Py = 2 x 80,000^^ djt< 13.39
= 7,461 psi
As is the case with the corresponding API equations, the effect of the out-of — roundness is implicitly contained in the empirical formula. Introducing the effect of length on the test results, Clinedinst found the following solution for determining the average collapse strength in the plastic range:
pPav = 1.672 x 10e (t/do)(2 096-iAi2x 10-6 Yp) (r/f,/I)0 0708 ( 2.144)
where:
L = length of the test specimen, in.
The minimum for the collapse strength has been specified at 77.8 % of the average collapse strength provided that no more than 0.5 (/c of the test results are less than this limit. Hence,
pPmin = 1.301 X 10й (f/rfo)H-°96-3.432X10-‘ Vp) 0.0708 (2Л45)
For test specimens having L/d0 = 8. the collapse resistance in the plastic range is given by:
_ 1.123 x 106
Pp ~ (^o/^(2.096-3.432xl0-6 Vp) (-.14 >)
The results obtained using the equations for calculating the critical collapse pressure for the four collapse ranges are presented in Fig. 2.15. The solid line indicates the methods used by the API. whereas the dashed line indicates the method used by Clinedinst.