Over the years, attempts have been made to track the working history of coiled tubing strings in service to allow for maximum utility with minimal fatigue failures. The result of these attempts has yielded three commonly used methodologies for predicting the fatigue condition of the coiled tubing.

RUNNING FEET METHOD

An all but obsolete method, yet relatively simplistic approach used to predict the working life of coiled tubing is commonly described as the running-feet method, in which the footage of coiled tubing deployed into a wellbore is recorded for each job performed. This deployed footage is then added to the existing record of footage deployed in service for any given string.

Depending upon the service environment, type of commonly performed services and local field history, the coiled tubing string is retired when the total running-feet reaches a predetermined amount. Values quoted for some service operators range from 250,000’ to 750,000’ (76196 to 228589 m), depending upon the type of service and working conditions.

The running-feet method offers the service vendor relative simplicity of use, requiring only that the maximum depth of the coiled tubing deployed into the wellbore is recorded.
However, there are numerous limitations to use of this fatigue tracking method as a reliable means of determining ultimate working life of a coiled tubing service string. Several of these limitations are described below.

-The value of maximum footage to retirement for any coiled tubing string is based on the service vendors previous experience with the same type of tubing, performing wellsite operations with similar well depths and types of service. There is generally no consideration given to duration of corrosive services performed.

-The running-feet method typically focuses on the specified outside diameter of the coiled tubing string in service, with minimal consideration for tubing wall thickness, tube material and yield strength.

-The running-feet method does not have a means of accounting for variations in tubing guide arch radius, service reel core radius, internal pressure loading, or identifying specific tube segments where additional bending cycles are applied.

-The working life derating method used in the running-feet approach cannot be extended to different tubing sizes or operating conditions. This method can be used only where working history for the specific tube material, geometry and surface handling equipment has been gathered and analyzed to arrive at the prescribed maximum running-feet value.