Effect of External Mechanical Damage on the Fatigue Life of Coiled Tubing Exposed to Sour Environments: Criteria for CT100 Grade

Abstract

Abstract BJ Services has successfully used grades 70 and 80 coiled tubing (CT) in sour environments. Recent laboratory studies have shown the feasibility (with some restrictions) of using high-strength CT grades (i.e. 90 and 100) in sour wells (SPE 107014). However, in normal operations, CT can be subjected to mechanical damage on its outer surface that adversely affects its fatigue life, so it is important to know the performance of these CT grades in sour environments when external mechanical damage is present. In a previous paper (SPE 113149) it was concluded that for grade 90 the presence of external mechanical damage with a transverse orientation and depth equivalent or higher than 10% of the nominal wall thickness should not be acceptable for sour service. This conclusion was based on the observation that for these damage characteristics the "damage effect" overshadowed the effect of hydrogen embrittlement (i.e. H2S effect) on the fatigue life reduction. On the other hand, this type of behaviour was not observed when the damage was longitudinal with depths up to 10% of the nominal wall thickness or transverse with depths up to 5%, so these limits were proposed as potential criteria regarding mechanical damage in CT90 grade intended for sour service. However, additional testing was needed in order to propose a set of criteria for CT100 grade. The present paper summarizes the results from the fatigue testing of CT100 samples with artificial external mechanical damage of different orientations and depths and compares their performance with that for CT90 and CT80 grade samples, in order to define a set of criteria regarding mechanical damage and sour service for CT100 grade. Introduction External mechanical damage on CT strings is not an uncommon issue during normal operations and it is well known that the presence of this damage can accelerate the onset of fatigue by constituting localized points of stress and strain concentration [1], or by originating micro-cracking or high hardness spots due to the severe plastic deformation associated with it [2]. Different inspection criteria can be found that impose limits to the size of an "acceptable" external mechanical damage for CT operations under sweet conditions and some of them were mentioned in a previous paper [3]; as a rule of thumb, for sweet conditions any mechanical damage with a depth greater than 10% of the nominal wall thickness requires a close assessment or repair. Experience has shown that CT grades 70 and 80 can be used successfully in sour environments [4], however in many applications higher CT strengths (i.e. 90 grade and up) are required to complete jobs in sour environments. Recent laboratory studies have shown the feasibility, with some restrictions, of using high-strength CT grades (i.e. 90 and 100) in sour wells [5].