The past few years have seen several important developments in the related areas of coiled tubing (CT) fatigue prediction and life monitoring [1-8]. These include:

  • A move away from the Running Feet method, particularly for complex and high risk applications, in favor of theoretical or statistical fatigue models.

  • Calculating and maintaining fatigue records for shorter tubing increments, down from typically 500 feet when depth and pressure data are logged manually, to as small as 5 feet through use of built-for-purpose tracking software and databases.

  • Accounting for factors affecting the working life other than fatigue, such as diameter growth, ovality, welds, surface damage and corrosion.

  • Use of data acquisition systems, providing more frequent and accurate data plus the option of real-time fatigue calculations (especially valuable for CT drilling).

The purpose of any CT life monitoring method is presumed to be to maximize the working life of the pipe while minimizing tubing failures in the field. All of the above developments contribute towards that goal. However, the problem is usually more complicated than this suggests because the criteria vary with the circumstances. Consider two quite different scenarios.

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