Fatigue life improvement of threaded connections by cold rolling

Abstract

Failures in drillstring threaded connections continue to afflict the oil industry at a cost of millions of dollars per year. In an attempt to reduce these failures the industry has generally adopted the fatigue-life-enhancing technique of cold rolling the threaded connections which act as stress concentrations. However, this beneficial process is only specified as optional in the industry standards, and where it is performed there is very little detail provided about this important process. This paper describes full-scale fatigue tests that investigate the cold-rolling process. From the development of cold-rolling equipment and the gathering of fatigue experimental data, several key factors have been identified that influence the operational lifetime of drillstring threaded connections.

Controlled failure design of drillstring threaded connections

“…The fatigue crack growth ratio or crack aspect ratio for the stitch rolled specimens remained relatively constant throughout the section thickness. The aspect ratio at failure (through‐thickness) was significantly less than the previously tested specimens 12 Figure 13. illustrates the relationship between crack length and crack depth for the pin connectors.…”

“…Results from earlier tests 12 had indicated that surface crack growth may be influenced by the levels of residual compressive surface stress introduced from cold rolling. Fig.…”

“…This paper presents an investigation into the significance of compressive residual stress and its effect on controlling crack shape, and continues the initial work 12 on the fatigue life improvement of drillstring threaded connections. It has been demonstrated that the fatigue process can be controlled by the controlled application of surface compressive residual stress.…”

“…Manufacturers do not currently cold roll connectors below inner diameter, 13 this tenuous dimension is based solely upon a lack of equipment able to access the bores of the smaller diameter connectors. However, it has been demonstrated that smaller connectors can be successfully cold rolled with significant improvements in fatigue life 12 . A rolling procedure has evolved from the development of this ‘miniaturized’ cold rolling equipment, which uses multiple passes and an oversized roller.…”

“…However, it has been demonstrated that smaller connectors can be successfully cold rolled with significant improvements in fatigue life. 12 A rolling procedure has evolved from the development of this 'miniaturized' cold rolling equipment, which uses multiple passes and an oversized roller. A rolling force of between 3 and 8 kN was found to cause a measurable deformation of the thread root of between 0.0254 and 0.254 mm.…”