Influence of a Straightener on Coiled Tubing Fatigue

Abstract

Abstract When coiled tubing exits the injector on its way into the wellbore, it contains residual curvature as a result of prior bending events (i.e. being wrapped around the coiled tubing reel). Coiled tubing straighteners are being used increasingly during field operations to remove this residual curvature in order to help delay the onset of coiled tubing lockup and successfully perform work in extended reach wellbores containing long horizontal intervals. To straighten the coiled tubing, rollers are used above the injector to apply bending in the reverse direction of the residual curvature. After the tubing exits the straightener assembly, the residual bend of the tubing will have been mitigated and the tubing will spring back into a predominantly straight line. However, the reverse bend applied during the straightening process increases the overall strain range experienced by the tubing and should adversely shorten its fatigue life. Accurate estimation of cumulative fatigue damage along the tubing is critical for making decisions about retiring strings prior to tubing failure, which can result in injury to personnel, equipment damage, unplanned release of fluids at surface, as well as costly fishing operations. This paper presents a methodology to quantify the incremental fatigue damage imparted on the tubing as a result of coiled tubing straightener use. An analytical technique is presented to compute the reverse bending radius required for tubing straightening and to quantify its subsequent influence on incremental fatigue experienced by the tubing. The analysis shows that only the radius associated with the tubing reel affects the residual curvature of the tubing under normal field operating conditions. It is also shown that it is not necessary to release the straightener during POOH operations to avoid incremental fatigue damage from straightener use. A case study is presented and fatigue life reduction is estimated on the order of 30% for the example tubing size and grade and the geometry of the deployment hardware. A test program is discussed to explore the influence of these parameters.