Combined Axial Tension/Compression and Internal Pressure Testing of Mini-Pipe Specimens in H2S Environment to Determine Three Dimensional (Triaxial) Stress States which Produce Crack Initiation Failure: Explanation of the New Test Fixture, Mini-Pipe Specimen, and Preliminary Test Results

Abstract

Abstract The oil and gas industry is heading toward development of Extreme-HPHT wells which can expose low alloy, carbon steel production casing to shut-in pressures over 18,000 psi, 50-450°F changing temperatures, and a sour environment containing H2S, CO2, and water. These conditions represent a step increase in the severity of the service conditions acting on well casing. The completion of such wells requires a new, comprehensive understanding of the relationship between three dimensional service stresses (caused by load and pressure) and the capability of the pipe material to resist crack initiation. This step in understanding is aided by an evolution from the one-dimensional, pass/fail, NACE-A tensile test to an environmental test of a hollow pipe specimen loaded with a three dimensional stress state. The same type of test is able to calibrate the accuracy of a formula for pipe performance based on fracture propagation in a sour environment. This paper reports development of a compact (less than 2 ft x 1 ft), inexpensive test frame and hollow specimen geometry to address this testing need; and the paper reports initial results from such testing. Combined-load testing specimens in this frame provides a means to quantitatively determine the way that principal stresses in the well combine to cause (1) crack initiation and (2) crack propagation in low alloy carbon steel pipe in a sour environment. The testing is able to simulate the pipe combined loading which can occur in the well. Part I of the paper explains the key features for the new test frame and specimens. The paper is intended to help enable the reader to build similar frames and specimens. The frame is able to passively apply axial tension or compression combined with internal pressure; internal H2S environment; and/or external H2S environment. The low cost and passive mechanism enable several frames to be run at the same time. The mini-pipe specimens are cut from the wall of coupling stock or heavy walled casing. The paper explains the specimen geometry, threading, end caps, H2S containment, and surface finish; the methodology for controlling loading of the coupon; and the low stiffness designed into the frame Part II of the paper provides preliminary results of seventeen tests with combinations of tension, compression, and internal pressure; and internal/external H2S exposure. Additional tests are planned for 2005. The tests are intended to determine a critical combination of combined axial-pressure loading stresses, H2S corrosion exposure, and pipe material toughness which leads to crack initiation and failure in pressurized pipes. The role of the initiation failure formula is shown to be separate from a different formula for pipe failure due to crack propagation.