Effect of External Mechanical Damage on the Fatigue Life of Coiled Tubing Exposed to Sour Environments

Abstract

Abstract In normal operations, coiled tubing (CT) is frequently subjected to mechanical damage on its outer surface that could adversely affect its fatigue life. The effect of transverse external mechanical damage, with depths in the range of 10% to 20% of the nominal wall thickness, on the fatigue life of grades 90 and 110 CT samples exposed to sour environments (i.e. solution NACE A saturated with 100% H2S) was reported in a previous paper (SPE 107113). The results showed that the fatigue life of the pipe was reduced significantly when the samples were pre-exposed to the sour environment used for these experiments. Additional testing was subsequently performed to asses less severe damage depths (i.e. < 10%) and orientations (i.e. longitudinal), less severe environmental conditions (i.e. H2S concentrations lower than 100%), application of anti-cracking inhibitors, and the response of other CT grades (example: grade 100). The present paper summarizes the results from the fatigue testing of more than 100 CT samples (grades 90, 100 and 110) with artificial mechanical damage in the transverse and longitudinal orientations, and depths from 5% to 20% of the nominal wall thickness, after being exposed to different sour environments (H2S concentrations from 1% to 100%) and including three different anti-cracking inhibitors. A comparison between the sour fatigue life of the samples containing external mechanical damage and that of samples without damage is presented in terms of percentages of sweet life (i.e. fatigue life measured in air). Also, the effects of the damage depth and orientation, H2S concentration, and anti-cracking inhibitors on the fatigue life are analyzed. Finally, an inspection criterion for mechanical damage on strings to be used in sour environments is proposed.