Jonathan Bowman

Sour Service Corrosion Fatigue Testing of Flowline Welds

An examination of the corrosion-fatigue behavior of production quality welds in X65-type pipes was performed. Due to the low cycle operational nature of the production flowline system, the fatigue test frequency was substantially lower (0.01Hz vs. 0.33Hz) than typically utilized during corrosion-fatigue testing. Also the tests were performed at higher stress ranges than previous sour service fatigue tests, which to date have targeted riser fatigue loading regimes. Stress-life (S-N) samples were removed from segments of pipe with outside diameters of 10.75 inch (wall thickness of 1.30 inch) and 9.625 inch (wall thickness of 1.26 inch) containing fully inspected, production-quality circumferential welds. Environments examined included laboratory air conditions as well as deoxygenated brine supplemented by a gas mix of H2 S and N2 . For all environmental tests performed, the dissolved oxygen levels were maintained at less than 10 ppb during all testing. The measured fatigue life decrease in the curved pipe segments was in the range of 8–110 times due to the combined effect of the material and fluid property variables examined. The results of this work clearly illustrated the impact of sour-service corrosion fatigue, in welded carbon steel pipes, to the multitude of variables involved. Nevertheless, the foregoing experimental work clearly demonstrated the importance of performing environmental relevant testing when considering material and process selection for offshore applications.© 2007 ASME

Sour Service Corrosion Fatigue Testing of Flowline and Riser Welds

Standard X-65 carbon steel pipe material is to be utilized for a flowline and riser system for an on-going project within the Gulf of Mexico. The project-produced fluids are corrosive with CO2 and produced water chemistry being the primary corrosive driving forces. The oil field will utilize water injection for reservoir pressure maintenance and as a consequence souring of the field in the later stages of life could occur. An experimental program of work was undertaken to measure the fatigue “knock-down” factor of sour/brine environments compared to that of laboratory air. Stress-life (S-N) samples were removed from segments of pipe with an outside diameter of 9.625 inch (wall thickness of 1.26 inch) containing fully inspected, production-quality circumferential welds. The environment examined included laboratory air conditions as well as deoxygenated brine supplemented by a gas mix of H2 S and CO2 . The primary variable changed during the test program was the test frequency. Fatigue testing in the high stress, or flowline, region was performed at 0.01Hz compared to a test frequency of 0.33Hz, utilized for the lower stress region, indicative of riser conditions. For all environmental tests performed, the dissolved oxygen levels were maintained at less than 10 ppb during testing. The measured fatigue life decrease in the curved pipe segments was in the range of 3–12 times when tested under sour brine environmental conditions compared to lab air. The results of this work clearly illustrates the importance of performing environmentally relevant fatigue testing when considering material selection for offshore applications that may contain a sour environment.Copyright

Consistent Endurance Fatigue Knockdown Factors for Sour Service From Industry-Wide Database

The embrittlement of steel in the presence of water and hydrogen sulfide is a well known phenomenon. For the resulting degradation in fatigue performance, the industry today applies a knock-down factor onto in-air S-N endurance curve that relates the sour-service life to the in-air performance. Several published results are available, and most of these, although rigorous in test approach, report knock-down factors that include unspecified levels of conservatism, consistent with that particular author’s engineering intuition. (For example, typically a lower bound S-N curve in sour-service testing is related to a mean S-N curve in-air). This paper summarizes all of the published small scale sour service testing results, and applies a consistent knock-down factor calculation. Analyses of the data show that sour degradation is highly correlated to H2 S concentration and solution pH value. In practice frequency scanning test is highly recommended since sour fatigue test results are highly dependent on loading frequency. Although the database is small, some trends are discernable. In particular, observations indicate that sour service exposure may act as an equalizer, removing the initiation life associated with the time for initial micro-defects at the weld toes to become macro-cracks and leaving only aggravated propagation due to sour service. In this paper, we use initiation life (for welds) to describe the life for the micro-defects (∼0.1mm height) to become macro-cracks (∼1.0 mm height), and a postulation is made that associates the fatigue performance of girth welds (F2, E, D, etc.) with the size and magnitude presence of these micro-defects. The metal surface attack of the sour environment is postulated to provide pitting-like initiation sites for the macro-crack for fatigue propagation. As a base we can use the F2 level performance as the performance due to presence of macro-cracks, and any margin for the D and E level fatigue performances then is associated with more benign initial micro-defects. Once we remove the differences in initiation life; all of the sour performance converges on a single lower performance curve. In this scenario, the knockdown factor is more consistently computed from a standard performance S-N curve rather than the same girth weld’s in-air performance since the in-air data may include significant initiation life. Furthermore, if project sour condition is less severe than NACE TM0177, Solution B with pH = 3.5 and H2 S partial pressure = 70mbar, a knock down factor of 45 indexed to BS7608 E design curve is supported from the current database. This provides a design recommendation which can be used for preliminary design in sour environments.Copyright

Inspection Considerations for Deepwater Thick-Walled Riser Systems

The fatigue loading on deepwater risers results in the need to impose very tight weld acceptance criteria for pipe alignment and flaw sizes. The production of high-pressure, high-temperature reservoirs calls for increasingly thick-walled riser systems. The combination of thicker wall pipe and the maintenance of tight flaw acceptance criteria challenges automated ultrasonic testing (AUT) which is the primary method of riser weld inspection. An understanding of the limitations of the weld inspection system must be determined and accommodated as part of the engineering process and in conjunction with adequate knowledge of the pipe end dimensions can help optimize the inspection process. This paper discusses the challenges associated with the preparation for and inspection of thick-walled riser system welds and the impact this can have on the system design and engineering requirement. In support of the discussion an overview of a recent study to determine the detect-ability and sizing accuracy of an AUT system with thick-wall pipe girth welds is presented. The qualification program is based upon the AUT inspection of seeded defect welds which are subsequently cut into weld rings, re-inspected with an immersion scan and sectioned to determine the size of the flaws present. All AUT inspections are performed under similar conditions to the inspection of the production welds (i.e., no prior knowledge of the nature of the seeded welds).Copyright ?? 2008 by ASME