Introduction

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.

  • Onshore workover, no surface pressure, competitive market with low margins. The tubing is run close to the end of its expected life. Both service company and operator recognize that some well site failures are inevitable, even using the best fatigue model, but the economic consequences of such a failure for these types of operations are relatively limited and the risk is considered acceptable in many cases.

  • Offshore exploration, surface pressure, environmentally sensitive area. The consequences of a tubing failure at the wellsite may far exceed any economic benefit to be gained from squeezing a few extra jobs from the string.

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