Traditionally, coiled tubing (CT) string life has been determined by the amount of low-cycle fatigue accumulated resulting from plastic deformation during successive operations. However, with more operations being performed using larger outer-diameter (OD) pipe, it has also become necessary to account for diameter growth when predicting CT life.
When predicting CT longevity, low-cycle fatigue is determined using software to simulate planned operations. This helps minimize the risk of a specific CT string experiencing fatigue failure during the planned process. The software then tracks added fatigue using sensor measurements during actual operations. The CT string can then be retired from service as it approaches its useful life before failures occur. In recent years, some locations have observed a trend toward higher-pressure operations with larger OD pipe. These conditions have led to OD growth becoming a greater issue at these locations than in previous years.
Diameter growth can be sufficiently severe to halt operations before the accumulated low-cycle fatigue failure prediction. This paper shares details of one such operation, which was stopped prematurely because of excessive OD growth before any fatigue failure was predicted. Changes in the design process are described where, in addition to fatigue accumulation, OD growth is predicted to become a limiting factor in the life of a CT string. This paper also highlights changes made to the software to help ensure any potential problems are identified during the operations design phase to help avoid any unplanned events in the field.
CT engineering design software is now a key element to predicting any type of CT limitation and failure mechanism during operations. Updating the simulation software to consider both diameter growth and low-cycle fatigue effects helps operators better predict potential problems and avoid operational interruptions.