Sulfide Stress Cracking (SSC) performance before and after expansion of expandable tubulars for API L80 class was investigated with C-ring test and tensile test methods which follow NACE Standard TM0177-96. The expansion ratio was 20%. Mechanical properties before and after expansion were also investigated. The expandable tubulars were quenched and tempered HF-ERW pipes and consisted of tempered martensite microstructure. In both of the C-ring tests and tensile tests, SSC did not occurred under 0.01MPa partial pressure of H2S at pH of 3.5, which is a rather severe sour environment, for the post-expanded pipes. These results suggest that expansion does not deteriorate SSC resistance under 0.01MPa partial pressure of H2S at pH of 3.5.
Recently expandable tubular technology which is able to drill and complete a slim hole well without increasing the costs was developed. There are slender type, mono diameter type and several other types of expandable tubular technology. In order to realize optimum cost reduction, high expandability as material properties of expandable tubular is essential. Because API L80 class that is often employed as expandable tubulars could be used in slightly soured environments, SSC resistance of the post-expanded pipe should be evaluated. Furthermore, to know the SSC resistance of post-expanded tubulars is important for increasing the window of application of expandable technology. It is generally admitted that cold working reduces the SSC resistance of steels, probably because of the strength increase induced by cold working. For example, it was reported that the SSC resistance of API L80 steel decreased expansion. On the other hand, Asahi reported that SSC resistance for tempered martensitic steel was not so deteriorated by cold-working compared in the case where the same strength increase was achieved by lowering tempering temperature. In this report, we investigated SSC properties after expansion with a tempered martensitic steel pipe in some sour environments.
EXPERIMENTAL PROCEDURE
Materials
Pipes used for this study were made of low alloy steel. The pipes were quenched and tempered (QT) after pipe manufacturing in HF-ERW (High-Frequency Electric Resistance Welding) process. The sizes are 7-5/8 and 9-5/8 inches in outer diameter. Table 1 shows their chemical composition. Quenching temperature and tempering temperature were 950°C and 700°C, respectively. It was an API L80 type of grade.
Microstructures
Figures 1 shows Scanning Electron Micrographs of the test pipes before and after expansion. When ERW portion was taken to be 0° position, each of the test specimens was taken from 90° position. These steels were etched by nital etchant (3 mass% nitric acid + ethanol). QT type steels consist of tempered martensite structures.
Expansion Methods
Test pipes with 7-5/8 and 9-5/8 inches in outer diameter were expanded by inserting a cone into a pipe. The expansion rate was 17%, which was calculated based on change in the internal diameter.
Mechanical Testing
Tensile testing. Tensile test specimens were taken from the center of thickness in the longitudinal (L-) and circumferential (C-) directions, and taken at 0°, 90° and 180° positions from the ERW seam. In the case of C-direction, the specimens include the weld portion. The gage section of tensile specimens had a length of 24mm and a diameter of 6mm.
Charpy impact testing. V-notch Charpy impact test specimens with the 5mm in thickness were machined from 90° position in the C-direction. The test temperature was -20°C.
Sulfide Stress Cracking Test
Test solution. SSC properties were evaluated using tensile test method and
