Abstract
In its earliest days Coiled Tubing (CT) suffered from unreliability due to low cycle fatigue and inconsistent material performance. With the introduction of the continuous milling process utilizing the bias weld geometry to join lengths of flat strip before forming the tube and an improved material specification, the industry was able to measure fatigue performance accurately and develop predictive algorithms to avoid over use of the tubing.
Since those early days the reliability of CT has improved and allowed the industry to expand rapidly to the point where there are now approximately 1,500 CT units in operation around the world. This growth has also expanded the range of pipe sizes, wall thicknesses, string lengths, steel grades and types of operations being utilized or undertaken. The operating environment in which CT is routinely used has become more arduous with interventions being performed in high pressure wells, sour gas wells and geothermal wells, for example. The expansion of the industry, especially over the last 5 years, has also led to a dilution of knowledge and experience as companies compete to hire and retain staff.
The expectation might be that all these factors combine to reduce the reliability of the coiled tubing service at the well site. This paper will review a decade of tracking CT failures, their cause and will set the results against the perspective of the changing nature of the business over the same time period. Conclusions will be drawn as to what are the critical factors for obtaining reliable CT service life performance and whether or not reliability is higher today than a decade ago.
