Coiled-Tubing Surface Characteristics and Effects on Fatigue Behavior
- S.M. Tipton (U. of Tulsa)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- March 2000
- Document Type
- Journal Paper
- 63 - 66
- 2000. Society of Petroleum Engineers
- 5.4.2 Gas Injection Methods, 4.2.3 Materials and Corrosion, 4.1.2 Separation and Treating, 4.3.4 Scale, 3 Production and Well Operations, 6.1.5 Human Resources, Competence and Training
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The manufacturing procedure for coiled tubing induces different surface morphologies on the inner and outer surfaces. Since fatigue is a surface phenomenon, this can lead to different fatigue behavior at the two locations. This has been demonstrated in full-scale bending tests and in axial tests conducted on small coupon samples in closed-loop servohydraulic strain control. Furthermore, service environments can cause localized defects on the outer and inner surfaces of coiled tubing, such as grinding marks, corrosion pits, tool marks, or injector block damage, which can have a first order effect on fatigue strength. However, it is demonstrated in this article that there is an inherent tendency for cracks to initiate on the inner surface of coiled tubing. This tendency leads to the hypothesis that a "threshold severity" for external surface defects could exist, below which the fatigue strength of the tubing is not affected. Limited data confirm this hypothesis, and suggest that the severity could depend on the grade of the material.
Effect of Coiled-Tubing Surface Morphologies on Fatigue
Fatigue tests on coiled-tubing (CT) samples routinely produce failure sites that defy conventional engineering intuition. Cracks are expected to initiate at the location experiencing the most severe state of stress and strain. For tubing under flexure, this should be the outer surface of the tensile side of the tubing. However, the majority of fatigue cracks in coiled tubing initiate on the inner surface of the compressive tube wall.1 Plasticity theory has been used to explain that the cyclic states of stress and strain on the compressive tube wall can be more severe than on the tensile wall.2 Radial compression can combine with axial compression to cause a greater rate of circumferential ratcheting and wall thinning at the compressive side of the tubing, compared to at the tensile side. However, arguments based on engineering mechanics have not been able to explain the nucleation of the vast majority of fatigue cracks at the inner surface instead of at the outer surface (which experiences a more severe strain range than the inner surface).
Recently, nearly 90 axial coupon samples of three different coiled-tubing grades of material were tested in strain-controlled low-cycle fatigue tests (described comprehensively in Ref. 3). Coupons, depicted in Fig. 1, were extracted from straight sections of coiled tubing and held in special grip fixtures designed to align the centroid of the gauge section with the axis of the load train, thus assuring pure tensile stress and strain in the gauge section. The edges of the samples were polished longitudinally to a mirror finish, according to ASTM E606 (standard recommended practice for constant-amplitude low-cycle fatigue testing). In 100% of the tests, cracks nucleated on the inner surface of these specimens.
Observing these specimens visually, a dull surface finish is apparent on the inside tube wall, compared to a smoother "glossy" texture on the exterior tube wall. This observation was confirmed using longitudinal surface profilometer readings taken from a single 80 ksi (552 MPa) axial coupon sample. The surface roughness measurements, summarized in Table 1, are averaged from three readings taken on a single axial coupon sample. After the readings were taken, the inner surface of the sample was polished longitudinally. Readings from the polished surface also are presented in Table 1.
The Ra value is the arithmetic mean of the surface height measurements, Ry is the maximum peak to valley measurement, and Rv is the maximum valley depth. The average roughness of the interior surface is nearly three times greater than that of the exterior surface. Scale illustrations of both surface profiles are given in Fig. 2.
The influence of surface roughness on metal fatigue is well documented.4-6 However, conventional treatments of this subject generally conclude that surface roughness has a stronger effect in the high-cycle life regime and is less important in the low-cycle regime. To evaluate the effect in the ultralow-cycle regime associated with coiled-tubing strain ranges, several axial coupon samples were polished longitudinally on the inner surface only. The profilometer measurements in Table 1 indicate that the resulting surfaces were more than an order of magnitude smoother than the as-manufactured outer surfaces. Strain-controlled R=0 axial fatigue data from tests at two strain ranges for polished and as-manufactured surfaces are presented in Table 2.
The limited data in Table 2 suggest that eliminating surface roughness on the inner diameter (ID) has a beneficial effect on fatigue life, nearly doubling life for the 1.6% strain range (a range typically encountered by coiled tubing in the field). But, equally interesting is the fact that the primary cracks that led to the fracture of the polished samples still nucleated at the ID surface in spite of the polishing. (In the remainder of this article "primary crack" refers to the crack that leads to the fatigue failure of a tube or specimen. "Secondary cracks"are cracks that form during the same loading, but do not lead to failure.) This indicates that other factors besides roughness cause the interior surface to be more susceptible to fatigue cracking than the exterior surface.
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