Developing a new design of welded connector for joining pipe-in-pipe caused problems for one leading oil company. Perhaps the most important was how well it would it perform under fatigue loading.
Such pipe-in-pipe connections are used in production flow lines and risers in deepwater fields. As part of the development, full scale rotating bend fatigue testing was required to characterise the fatigue strength of the connector assembly.
More specifically two connectors were to be tested, one fully assembled and one without the outer pipes. It was important to determine the fatigue strength and failure location in both.
Two test specimens were constructed by the client. One was a complete pipe-in-pipe assembly and the other comprised the forged connector and inner pipes only. They were tested in rotating bending using the resonance technique. Each was internally pressurised to 500 bar for the duration of the test. The tests were deemed complete when a through-wall crack developed in any of the component parts.
From TWI's test results the client was able to conclude that the fatigue strength of the complete pipe-in-pipe connector assembly was governed by the strength of the outer pipe to connector weld. Failure was by fatigue initiating from the weld root toe. The root condition and internal alignment were good and the fatigue strength was in line with the appropriate British Standard design curve.
The fatigue strength of the connector to inner pipe assembly was governed by the strength of the pipe-to-connector weld. Failure was again by fatigue initiating from the weld root toe. The fatigue strength here was found to be significantly better than that of the outer pipe to connector weld. Most importantly the client was able to conclude from TWI's work that there was no evidence of leakage, failure or deterioration of the connector-to-connector weld and the mechanical joint in either test.
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