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.
Introduction
Since the beginning of 1996 all early failures of coiled tubing strings experienced in our operations have been forensically examined to identify the cause of the failure. The results of these investigations are recorded in a database to enable historical tracking in order that trends might be observed and appropriate policy or procedural changes made. This paper presents 11 years of data captured from 1997 to 2007, inclusive. Early failures are those which are experienced before the safe working life 1 of a string of tubing has been fully consumed. For practical purposes, very few strings are actually used until, or beyond, they reach their maximum allowable safe working life and, therefore, this database realistically captures all string failure data.
During this 11 year time span the database contains information on over 250 failure investigations. During the same period; over 2,200 CT strings were in service, performing close to 60,000 jobs, using an average of 138 coiled tubing units, covering both land and offshore operating locations.
Research into the low cycle fatigue performance of CT has been conducted since the early 1990's from which predictive algorithms and fatigue management software have been developed 1–5. This, coupled with improved string manufacturing techniques of the same era, has all but eradicated occurrence of most pure cyclic strain induced fatigue failures, i.e. fatigue not associated with secondary mechanisms such as corrosion 6. Today, the causes of early string failure are dominated by other phenomena. These have been classified within the database. The number of classifications has been deliberately limited so that trends may be more readily discernable.
