As a result of many industry efforts, the premature fatigue failure of undamaged coiled tubing (CT) strings is almost negligible. However, despite the current understanding and control of low-cycle fatigue, CT string failures remain present in the industry. Several prior technical publications reviewed the causes and trends of CT string failures that occurred within the period of 1994 to 2005. This paper will review CT failures mechanisms and trends as observed over the last twelve years and compare them to the prior ten years period. It will also review the new failure mechanisms that have appeared with more challenging operational conditions and the associated actions taken to reduce their influence.
Within one major service company, all failures that occur are analyzed for the root cause(s) of failure. This results in the identification of corrective actions to avoid their recurrence. Statistical data is kept to observe trends on failure causes.
Several technical publications show that approximately 80% – 90% of CT string failures within the period of 1994 to 2005 were associated with corrosion, mechanical damage, human error, and string manufacturing problems. Actions taken in the last two decades by the CT services companies, and constant improvement implemented by CT manufacturers have reduced the influence of some of these causes. However, work in ever-more challenging well conditions (such as higher pressures, temperatures, and depths), the need to use larger-diameter and higher-strength CT, and the use of recycled fluids for the interventions, have created new issues and introduced new CT failure mechanisms. The new mechanisms within the industry include: microbiologically influenced corrosion (MIC), premature fatigue failures on bias welds of high-strength grades, and mechanical damage associated with pipe slippages.
This paper will compare the failure trends reported for the period before 2005 with the trends observed for this service company within the period of 12 years after 2005 (i.e., from 2006 to 2017). The changes in the failures trends are analyzed, and examples of the newer CT failure mechanisms and the mitigating actions taken are presented.