Analysis and Testing of Factors Affecting Collapse Performance of Casing

Abstract

This paper describes testing and analysis which demonstrates that collapse specimens tested in short testers with rigid end seals give artificially high collapse values. It also compares the API collapse formulas with conventional strength of materials collapse criteria which incorporate ovality and finds the two approaches give very similar results. A statistical analysis indicates that ovality, while not nearly as important as yield strength and D/t, is important to collapse. Finally, it shows that pipe with lower ovality exhibits better collapse resistance.

Collapse Behavior of Casings: Measurement Techniques, Numerical Analyses and Full Scale Testing

“…Comparison with results from the database. The above results are coincident with most of what is already known in collapse of pipes [6][7][8][9] . However, even if the results have an important qualitative significance, a case by case comparison shows quantitative differences.…”

“…• The experimental set-up does not impose any axial restrain on the pipes. • The length of the sample being tested fulfills in all cases the relation 6 : L/D ≥ 10.…”

“…• In developing the 2D models the measured ovality is entirely assigned to the first elastic buckling mode and this conservative approach partially accounts for the fact that the numerical values are lower than the actual ones. • The collapse chamber imposes on the samples unilateral radial restraints at both ends 6 , these restraints are not described by the 2D models; the difference between the numerical and actual boundary conditions also partially accounts for the fact that the numerical values are in general lower than the actual ones.…”

“…The external collapse pressure of very thin steel pipes is governed by classical elastic buckling formulas 1,2 ; however, for thicker pipes more involved elasto-plastic considerations have to be taken into account. There are many factors that have some degree of influence on the external pressure that produce the collapse of a steel pipe, among them [3][4][5][6][7][8][9] :…”

Computing Collapse Pressure for Coiled Tubing

“…7125. With all these corrections, critical external pressure becomes Pcr=P oKEKL (9) Combined Loading Factor Ky 1. Holmquist, and A. Nadai 8 have discussed CP in the presence of axial stress.…”