The impact of an operational failure during a coiled tubing (CT) intervention is typically more severe than that of other failures because of the nature of the activity. Failure of the tubing or any component of the well intervention process in a live well scenario can compromise well control and/or the safety of personnel. Statistics on causes of CT operational failures (OFs) indicate that a majority of these failures can be attributed to human error. Incorrect actions, or the lack of action, are very difficult to predict and therefore a major challenge to control. Running CT in and out of the well involves a high degree of human interaction and human fatigue, and short periods of inattention during this process are not uncommon.
During such activities, inattention can lead to actions that damage, kink or part the CT, with potentially disastrous results. Other causes for OFs include unintentional tensile overloads, overpressuring, runaways and other such events.
An electric over-ride device, developed for installation in the hydraulic circuitry of a CT unit, allows setting of limits on all pertinent operating parameters of the injector head. Setting equipment limits for weight, velocity and pressure gives the operator an extra set of eyes, greatly increasing operational safety and efficiency of the treatment.
This paper discusses OFs caused by human error and presents case histories that contributed to the conclusion of which parameters require control. The over-ride device used in the control process is discussed in technical detail, and case histories demonstrate the impact of its use on overall safety and service quality in the CT industry.
CT material failures have been an industry focus for some time. Comprehensive research in the failure mechanisms of the tubing steel has made the behavior of low-carbon steel fairly well known, and this behavior is well documented. CT failures are typically very serious, but they are not the only failures to consider. Total system improvement through better service quality is obtainable through an investigation of all failures associated with CT well interventions, including those caused by equipment failure or human error.
A database of companywide, in-house service quality statistics on OFs is used to identify problem areas. Data are drawn from worldwide CT operations for 2000 that represent a cross section of CT activites in the industry; shallow to deep land operations, arctic operations, offshore platform operations and deepwater work.
Coiled Tubing Failures
OFs that do not lead to injuries are not systematically or consistently tracked and reported across the industry. Unlike safety statistics, which are readily available in a standard format, service quality statistics are still very much organization specific and therefore do not allow for industry benchmarks.
Schlumberger has defined and adapted mandatory service quality indicators for all product lines. These are grouped in the following categories:
Severity of the OF based on nonproductive time (NPT) and financial loss
Catastrophic operational failure (COF): NPT >48 hr and/or loss >$500,000
Major operational failure (MOF): 12 hr <NPT<48 hr and/or loss between $50,000 and $500,000
Serious operational failure (SOF): 4 hr<NPT<12 hr and/or loss between $500 and $50,000
Frequency of OFs defined as the sum of the Catastrophic, Major and Serious OFs per 1000 jobs (CMS/1000)
Impact of an OF on the treatment execution, determined by calculating NPT as a percentage of operational time.