Fatigue Life of Coiled Tubing With External Mechanical Damage

Abstract

Abstract Coiled tubing (CT) is used in a rough and dynamic environment that renders it susceptible to mechanical damage on its outer surface. Such damage can adversely affect the fatigue life of CT by causing localized concentration of stresses and strains, or inducing microcracks from severe localized plastic deformation, that can lead to premature fatigue failure of the pipe. Failure statistics based on detailed failure examinations by BJ Services Company rank mechanical damage as the leading cause of CT failures accounting for 35% of all investigations. Repairs of the damaged pipe by welding or grinding are not always practical or even possible; therefore it is important to be able to estimate the remaining fatigue life of the damaged pipe so that the well servicing job can be completed with minimum risk of failure. Research efforts by the Coiled Tubing Mechanics Research Consortium at Tulsa University to determine the fatigue de-rating factors for damaged CT produced an algorithm ("Flexor TU4") that can predict the remaining fatigue life of the CT based on specific damage geometry, tubing characteristics and operating parameters. However, the majority of the testing for this algorithm has been conducted in air on artificial defects (i.e. machined) introduced on new and pre-fatigued tubing samples. Little previous research has been conducted to compare "Flexor TU4" predictions with the remaining fatigue life measured on CT with real defects (i.e. damage caused during service). Also, the effect of mechanical damage on the fatigue life of coiled tubing when exposed to sour environments (i.e. H2S) has not yet been reported in the literature. This paper describes the effect of different types of damage on the material and fatigue life of CT and presents a comparison between "Flexor TU4" predictions and the fatigue life measured for strings with external mechanical damage incurred during service. Also a comparison between the sour fatigue life and that in air is presented in terms of percentages of sweet life (i.e. non-sour) for coiled tubing fatigue samples containing artificially induced mechanical damage on the external surface prior to being exposed to sour environments. Introduction In normal operations, CT is frequently subjected to mechanical damage on its outer surface in very different forms such as scratches, cuts, dents, impressions and other mishaps (Figure 1). Regardless of shape or cause, this type of damage can adversely affect the CT low cycle fatigue life by creating localized concentration of stresses and strains or, in the worst case, by causing microcracking, both conditions leading to premature fatigue failures. Different studies in the past that included failure statistics [1,2, 3] indicated that, following corrosion as the most frequent, mechanical damage was between the third and fifth most common cause of CT failures accounting for 8% to 11% of the total number of failures. More recent failure statistics based on detailed failure examinations by BJ Services Company between March 2004 and September 2006 indicate a significant shift and rank mechanical damage as the primary cause of CT failure, accounting for 35% of the total number of cases investigated (Figure 2). For those cases where the damage is detected (example: non destructive inspection), the CT can be repaired by cutting out the damaged section and replacing it with a buttweld. Another potential option, proposed by Tipton et al. [4] involves removing the damage by grinding and polishing to a smooth surface finish. However, repairing by welding or grinding is not always practical or even possible (example: defect is detected while executing a job); therefore in some cases it is convenient to be able to have an estimate of the remaining fatigue life of the damaged pipe in order to decide on a sound basis, the feasibility of completing the well servicing job with a minimum risk of failure.