The paper describes the additional design considerations on top of standard design criteria for casing or tubing strings when subjected to cyclic loads. In addition, this paper chronicles comprehensive laboratory testing programs that have been carried out to analyze and quantify the fatigue limit of various semi-premium and premium connections.

OCTG connections are being increasingly used in drilling environments as an alternative to conventional drill pipe. Furthermore, OCTGs are commonly rotated into place in horizontal and ERD wells. During the drilling and running of OCTG in deviated sections, the combination of the resultant bending of the string and its rotation generate alternating stresses in the pipe body and connections, creating the conditions for potential fatigue damage.

OCTG connections are not as fatigue resistant as tool joints due to its inherent design. Fatigue is a complex process that affects mechanical components that are subjected to cyclic loads. Fatigue failure is a sudden crack, but it comes from a process that involves different development phases until the failure of the component becomes evident; this failure is produced at stress levels far below the static elastic limit of the material. Several factors present in OCTG connections applications including mean stress, stress range, make-up torque, and material grades have been introduced in the full scale testing programs. As a result, robust fatigue life limit curves, used for real cases fatigue analyses, have been developed and are presented in this paper.

Since fatigue damage could lead to the failure of the string based on its weakest link, which in general terms would predominantly be the connections, its resistance to cyclic loading must be thoroughly considered to understand the performance limitations this may impose on the entire drilling assembly.

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