Past and Present Coiled Tubing String Failures - History and Recent New Failures Mechanisms

Abstract

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.

Mechanical integrity of tubing string in high‐temperature and high‐pressure ultra‐deep gas wells based on uncertainty theory

“…Simultaneously, the triaxial equivalent stress is adopted to analyze the stress of the tubing string under the complex stress state 28 . According to von Mises equivalent stress calculation method, the triaxial equivalent stress is calculated from radial stress σ r , axial stress σ Z , and circumferential stress σ θ according to the equation below as follows: $$${F}_{4}=\frac{1}{\sqrt{2}}\sqrt{{({\sigma }_{z}-{\sigma }_{\theta })}^{2}+{({\sigma }_{\theta }-{\sigma }_{r})}^{2}+{({\sigma }_{r}-{\sigma }_{z})}^{2}},$$$where σ z denotes the axial stress of tubing string (MPa), σ r denotes the radial stress of tubing string (MPa); and σ θ denotes the circumferential stress of tubing string (MPa).…”

“…They also include China's Keshen, Shunnan, and Sichuan blocks. Failures of tubing strings occur in complex downhole conditions owing to the influence of high‐temperature, high‐pressure, and ultra‐deep environments, and it was determined in a market survey conducted by Archer in 2010 wherein 41% of well integrity failure cases were due to tubing string failures 1,2 . On a certain platform in the North Sea of Norway, there are 407 wells in total.…”

“…Failures of tubing strings occur in complex downhole conditions owing to the influence of high-temperature, high-pressure, and ultra-deep environments, and it was determined in a market survey conducted by Archer in 2010 wherein 41% of well integrity failure cases were due to tubing string failures. 1,2 On a certain platform in the North Sea of Norway, there are 407 wells in total. Among these wells, 75 wells failed in well integrity and 30 wells failed in tubing string integrity, accounting for 40% of the total number of failed wells.…”

Corrosion resistance of steel for coiled tubing units

“…However, a special place among the above methods is occupied by "coiled tubing" (CT) technologies, which are based on the use of a very long metal pipe. They have high economic efficiency, are in a state of continuous development and improvement [1,2], and there exists a certain international association to coordinate these efforts (Intervention & Coiled Tubing Association (ICoTA) https://www.icota.com/).…”

Improvement and determination of the influencing factors of coiled tubing fatigue life prediction

“…In fact, CT has various failure modes, and most of the failures are caused by mechanical damage, corrosion (storage corrosion, downhole corrosion, microbial corrosion), manufacturing defects, erosion, and so on, which cause a sharp decline in life. 6 The size parameters of CT often change with the use of CT. These changes will affect their fatigue life, but the changes in these parameters cannot be accurately predicted.…”