The Effect of Surface Enhancement on Improving the Fatigue and Sour Service Performance of Downhole Tubular Components

Abstract

Sulfide stress cracking (SSC) and hydrogen embrittlement (HE) prevent the use of high strength carbon steel alloys in the recovery of fossil fuels in H2S containing ‘sour’ environments commonly experienced in deep well fossil fuel recovery efforts. Couplings are a common weak point in casing strings as high magnitude mean tensile stresses are generated by connection interferences created during power make-up of downhole tubular components. When subject to service loads both mean and alternating stresses are increased further providing the high tensile stresses necessary for SSC initiation. Since high strength carbon steel alloys are not typically suited for sour service environments, the current solution is to use or develop much more expensive alloys with increased corrosion-cracking resistance, or limit their use to significantly weaker sour environments, or higher operating temperatures. Failure due to fatigue is another major problem in downhole tubular components. Likelihood of fatigue failure is further exacerbated in corrosive environments (such as H2S and NaCl), commonly encountered in service. The cost for detecting the impending failure before final separation is dramatic at a factor 10X. A cost effective method of mitigating failure from SSC and corrosion fatigue would greatly reduce operational costs and extend component life. Introduction of stable, high magnitude compressive residual stresses into less expensive carbon steel alloys alleviates the tensile stresses, and mitigates SSC, while also improving fatigue performance. Low plasticity burnishing (LPB) is an advanced surface enhancement process providing a means of introducing compressive residual stresses into metallic components for enhanced fatigue, damage tolerance, and SCC performance. The effects of LPB on high cycle fatigue (HCF) and SSC were evaluated on quench and tempered API P110 grade steel. LPB processed specimens had an increase in fatigue life greater than an order of magnitude over untreated specimens. LPB was successful in completely mitigating SSC in all test specimens at tensile stresses up to 90% specified minimum yield strength (SMYS). The initial results indicate that LPB processing of P110 steel provides an economical means of SSC mitigation and fatigue strength improvement in sour environments.