Fatigue Integrity Analysis of Rotating Coiled Tubing

Abstract

Abstract Conventional coiled tubing endures severe above-ground bending and straightening cycles that cause low-cycle fatigue to be the predominant factor limiting its useful life. For conventional coiled tubing, downhole bending events associated with wellpath curvature are negligible since the strains generated are orders of magnitude less severe than strains casued by above-ground cyclic bending, and are applied with a very low cycle count. A new drilling rig is under development that rotates coiled tubing (CT) downhole, reportedly at rates on the order of 20 RPM, resulting in numerous operational benefits. However, rotation will impose additional rotating-bending events in the high-cycle regime as sections of CT pass through regions of high dog leg severity. This paper presents a study of the influence of downhole rotation on the fatigue durability of CT. In this study, fatigue data taken in the high and low cycle regimes from a variety of sources are used to assess the potential high-cycle fatigue damage that may be caused by downhole rotation. The results indicate that the damage is below the endurance limit and thus negligible relative to fatigue damage accumulated from above-ground bend-straighten cycles for conventional CT. This analysis also took into account another area of potential concern: circumferential abrasion that could occur when rotating coiled tubing contacts with the casing and/or wellbore. Surface roughness factors were estimated and used to modify life estimates. Results still indicate that sub-surface rotation-induced fatigue cycles should not detrimentally influence fatigue life relative to above surface fatigue cycles. Experimental results are presented which validate the conservatism of the assumptions made for surface roughness factors. The results also demonstrate that the abrasion process causes compressive residual surface stresses that somewhat offset the detrimental effect of a rougher surface.