CT begins its life plastically deformed, because it is wound on a reel. Moreover, each round trip into the well and back plastically deforms (bends) the tubing six times. These bending events are:

RIH - unwind and straighten from the reel.

RIH - bend across the guide arch.

RIH - straighten in the injector.

POOH - bend onto the guide arch.

POOH - straighten from the guide arch.

POOH - wind back onto the reel.

 

As Figure 1 shows, two-thirds of the bending cycles a CT segment experiences during a round trip are due to the guide arch. This does not mean that two-thirds of the fatigue damage occurs at the guide arch, because the reel typically has a smaller bending radius than the guide arch.  Note that all of the fatigue damage occurs in the surface equipment; none occurs in the well.

 

The magnitude of plastic deformation with each bending cycle depends on the bending radius, CT dimensions, and material yield strength. Thus, the radius of the guide arch and the diameter of the reel have a profound effect on CT fatigue life. Internal pressure during plastic deformation amplifies the fatigue damage. In order to estimate the accumulated fatigue damage for a segment of CT, one must know the number of bending cycles and magnitude of plastic deformation and pressure at each cycle. Figure 2 depicts the working life of a CT segment, its ability to resist failure, as a pie. The whole pie is the working life for a given set of conditions.

 

The different size wedges indicate the variability of fatigue damage accumulated in a CT segment during successive CT operations. The larger "slices" of damage correspond to more bending cycles, higher pressure, higher plastic stress, or some combination of these factors. The total fatigue damage accumulated in the CT segment is the sum of the damage for operations 1-14. The balance of the pie is the remaining working life or resistance to failure. Therefore, estimating the remaining working life for a CT segment is impossible without a complete and accurate history of the accumulated fatigue damage. The number of trips corresponding to the remaining life depends on the operating conditions for each trip.

 

NOV CTES uses a fatigue model developed by Professor Steve Tipton at the University of Tulsa.  Steve developed the initial fatigue model in 1990, and has continued working with NOV CTES to improve it through the years. It incorporates thousands of test data points as well as new CT materials that have been developed in recent years. NOV CTES uses this model in the CerberusT Reel-Trak ^TM fatigue tracking software that it markets to the majority of the CT service suppliers. The objective of the fatigue model is to predict the remaining working life for a segment of CT:

- аt any location in a string of varying properties;

- аfter a complex sequence of bending events;

- subjected to changing pressure conditions.

 

Numerous factors affect the ability of the model to accomplish this objective, including the:

- аccuracy of the input data;

- сompleteness of the operational history for the CT string;

- аccounting (tracking) method used to apply the fatigue model to a CT string.

 

The input to these models includes:

- initial accumulated fatigue damage;

- locations of physical damage (dents, corrosion, scrapes, etc.);

- CT diameter;

- CT wall thickness;

- locations of welds (bias and butt);

- bending radius for each bending event;

- pressure inside the CT at each bending event;

- CT material properties;

- number of bending events.

 

Figure 3 is an example produced from Professor Tipton's fatigue model showing the effect of CT diameter on CT fatigue life in terms of trips to failure. A trip means the three bending events for RIH plus the three bending events for POOH.

 

Advanced fatigue modeling software like CerberusT from NOV CTES divides a CT string into manageable segments (about 3 m long) and applies the fatigue model to each segment in order to determine the distribution of fatigue along the string. Based on the geometry of the surface equipment, the location of the segment in the CT string, and the current CT depth, the model tracks the position of each segment relative to the bending locations. The software activates the fatigue model whenever the segment is between the reel and the stripper and increments the fatigue damage to the segment. Figure 4 is a schematic of a fatigue model in operation. The figure indicates the information describing the geometry of the surface equipment.

 

The software follows each CT segment from the reel through the injector and back again. Most fatigue modeling software operates off-line with data input manually. Advanced fatigue monitoring software like Reel-TrakT can operate on-line with a data acquisition system like OrionT to provide real-time information about accumulated fatigue in a CT string.

 

As was mentioned above, accurate input data is required for fatigue modeling to be accurate.  During a CT operation the depth and pumping pressure must be recorded by a data acquisition system.  This recorded data is then used by the fatigue model to calculate the percentage of the fatigue life used in the string.

 

NOV CTES has developed the Orion data acquisition system that work with the Reel-Trak ^TM modeling software to calculate the fatigue along a string. About 400 of these systems are now working around the world.

A CT fatigue tracking system, including the data acquisition system and modeling software, saves significant amounts of money for 2 reasons:

1) Often this system allows the CT to be used longer than it would have been used previously using the total meters run method.

2) Intelligent decisions can be made about how to improve the fatigue life such as when to remove a portion of the string or when to turn the string around so the bottom becomes the top.

Ken Newman. How can CT fatigue be reduced? // Coiled Tubing Times - 2005. - №12.

Ken Newman. Saving money by fatigue modeling and tracking // Coiled Tubing Times - 2005. - №11.

Ken Newman. CT working life // Coiled Tubing Times - 2004. - №10.