METALLURGY SELECTION
Three metallurgies were selected based on their mechanical properties, finite element analysis results, fatigue testing, a tiered costing approach and their availability in strip form. The alloys selected had to meet the yield strength requirements to support the weight of the cable and the ESP string. They also had to have superior resistance to extreme sour conditions. Some information was found on the corrosion resistance of these selected alloys, but this information did not cover the material condition and well environments that the project was targeting. The alloys were chosen based on topical knowledge, but how they would perform given their unannealed welds in extreme sour environments was unknown.
NONSTANDARD NACE
The alloys were good selections for testing, except they were nonstandard NACE materials due to their unannealed weld condition. This was the driving force behind completing the extensive and costly qualification testing. The process used in producing the cable included starting with a flat strip, slowly forming the strip into a tube, adding the cable with armor inside the unwelded tubing, then welding the strip with the cable inside.
H2S TESTING
The H2S testing allowed us to select the best candidates for further electrical and mechanical testing. The testing procedure was as follows.
Purpose
The purpose of the H2S study was to evaluate test samples of different corrosion resistant alloys having unannealed laser welds as they were subjected to corrosive environments. The sample welds were longitudinal seam welds representative of the strip splice (bias) welds that would be in the finished tubular product. Because the final tubular product had to be provided in an “as welded” and “cold worked” condition, stress corrosion cracking due to H2S attack on the unannealed alloys was of concern. The corrosion testing would evaluate the different alloy candidates to ensure proper grade selection was made depending on the well environment.
Testing Setup and Procedure
The alloys selected for H2S testing were: UNS N08825 (825) — nickel alloy, UNS S32205 (2205) — duplex stainless steel, and UNS S34565 (4565) — austenitic stainless steel. Sample coupons were prepared that simulated the actual unannealed conditions in both longitudinal and transverse welds. The samples were then bent in a way to induce stress levels greater than actual field tube conditions. Setup followed best practices, including special precautions to prevent any possible galvanic reaction. Figures 3a and 3b show the test coupon cut and configuration.
The test coupons were welded and bent around a mandrel to the first position with holes drilled in each leg for inserting a bolt to provide additional stress, Fig. 4. Titanium bolts added to the test coupons were then tightened to a calculated stress more severe than in situ stress levels. The titanium bolts were wrapped with Teflon tape at all contact points with the test coupons to prevent galvanic corrosion between the bolts and coupons. Pretest micro-hardness values were taken in the base metal and in the weld section, and this was repeated at three elevations across the coupon thickness.
Four different autoclaves — test vessels — were set up for testing the coupons. Three autoclaves, each with a different level of H2S at 1%, 5% and 15%, were used for 30-day tests of the 2205 and 4565 alloys. The fourth autoclave had 15% H2S and was planned for a 90-day test of the 825 alloy. The remaining test parameters were the same for all four autoclaves: 150,000 mg/l chlorine, 5% CO2, a total pressure of 300 psi and a temperature of 275 °F (135 °C). Because the 825 alloy had the highest confidence of passing all the testing, it was put in a 90-day test. The 825 test samples consisted of U-bend specimens and the actual welded tubing with both longitudinal and bias welds. Three samples of each alloy were placed into each autoclave test vessel to provide multiple test point confirmation.
After the full duration of the test, the samples were removed and a post-test evaluation was performed. Posttesting consisted of a visual examination and a dye penetrant test, after which the samples were mounted for microscopic work. Hardness values before and after testing were compared along with the microstructures of the base and weld.
H2S Test Results
The lab report noted, “Cracking has been observed for all 2205 U-bend specimens by the visual examination.” All nine samples of that alloy (three samples each from the 1%, 5% and 15% H2S tests) were returned for further examination. Evaluations indicated that the cracks were in seam weld areas; however, there was no sign of defective welds upon micro-examination. There were no significant differences in crack frequency and crack depth among the autoclave sample lots.
The lab report noted similar results for the other alloy: “Cracking has been observed for all 4565 U-bend specimens by the visual examination.” All nine samples of that alloy (three samples each from the 1%, 5% and 15% H2S tests) were returned for further examination. Additional evaluations indicated that the cracks were in the base metal area. There were no significant differences in crack frequency and crack depth among the autoclave sample lots.
The 825 tube sample, originally planned for a 90-day test, was visually inspected at 35 days after the 2205 and 4565 alloys failed their 30-day tests. After a good visual inspection of the 825 tube, the sample was placed back into the charged autoclave to complete the remainder of the 90-day test. According to the lab report, “No cracking has been observed for all 825 U-bend specimens and 825 tube specimens after a check with dye penetrant and visual examination”.
The 825 alloy was preliminarily qualified for use at H2S levels up to 15% based upon the autoclave testing. The other two alloys, 2205 and 4565, were disqualified from use based upon their failed test results.
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