It is well known that residual stresses in a structure can have a detrimental effect on its flaw cracking process. Through a three-dimensional (3D) finite element analysis (FEA) on a six-roll cold straightening process, it was found that seamless oil country tubular goods (OCTG) pipe has a unique residual stresses pattern due to its manufacturing process. An X-Ray diffraction through wall residual stress measurement shows that even with hot straightening, there is still a significant amount residual stresses existing in the pipe body with a similar pattern found in the cold straightened pipe from FEA. The residual stresses were found to have a significant effect on the pipe sour cracking resistance through an internal pressure burst test of a full-size pipe with a pre-existing flaw in sour environment in an American Petroleum Institute (API) study. A formula was proposed to include the effect of residual stress on determining the sour cracking (sulfide stress cracking) resistance on OCTG tubulars.
The higher and higher material strength requirements in deepwater drilling and sour wells have pushed pipe materials to their performance limits. Understanding the materials’ actual performance properties is very important for material selection in well design. The residual stresses in OCTG (oil country tubular goods) were previously identified as having a significant impact on pipe performance properties, such as collapse and sour cracking.1,2 Based on the past investigations on seamless pipes, it was found that the cold straightened pipes and hot straightened pipes can have compressive hoop residual stress on the inner surface fibers up to 40% and 30% of the materials yield strength, respectively.3 Residual stresses higher than 50% of material yield strength have also been reported in cold straightened pipes.1
During the seamless pipe manufacturing process, after the pipe is pierced, elongated and heat treated, it normally goes through a rotary straightening process, either six-roll, seven-roll or other types. The primary purpose of straightening process is to make pipe straight enough to meet the API requirements, as shown in Figure 1. But some system, such as six-roll, can also help make pipe rounder. The straightening mechanisms include squeezing/crushing effects as the gap between the roll pair is smaller than the pipe outside diameter as shown in Figure 1 (a), and bending effects as the middle roll pair is slightly offset from the end pairs as shown in Figure 1 (b). The amount of squeezing and bending depends on the rolls setup.