Coiled Tubing String Retirement
Fatigue life modeling has extended the economic life of Coiled Tubing (CT), making the ability to predict the safe working life of CT string an essential part of coiled tubing operations. New applications like drilling and fracturing can now be undertaken with confidence and success, revolutionizing the CT industry.
A coiled tubing string becomes unsuitable for continued use if the accumulated fatigue exceeds the limits set by the operating company. There are several methods utilized to predict the service life of the coiled tubing string. Some are quite simple; some can be more complex. The simplest approach that is easy to implement would be a “running-feet method”. A total footage for the CT string run into wells is recorded, when a specified limit is reached, a decision to retire the string is made. Typical length would depend on the CT string properties, and can vary from 250,000 ft up to 750,000 ft. With new coiled tubing developments, this range limit can now be increased. However, the range limit is based on the previous experience with similar tubing performing similar operations under similar well conditions. A company cannot simply extend the same limits to a different size string or different well operating conditions as the fatigue life will be different.