Coiled tubing (CT) bending fatigue has been the subject of much research. Slickline (SL) fatigue is the subject of more recent, ongoing research. Fatigue life tracking and prediction systems are available for both CT and SL. However, these systems do not adequately include the reduction in life due to corrosion. This paper discusses the CT and SL fatigue, and presents a new approach to handling the life reduction due to corrosion.
CT fatigue studies were first conducted in the 1980's. CT fatigue life tracking systems 1 became standard in the industry during the 1990's, replacing the old "running feet" method of quantifying pipe life. Research and validation of CT fatigue is ongoing 2,3,4. Due to the amount of work that has been previously documented, this paper will not discuss CT fatigue in detail.
Unlike CT fatigue, SL fatigue is a relatively new field of study. The first SL fatigue life tracking system has recently been introduced to the field. Since it is new it will be discussed more extensively in this paper.
For both CT and SL, the affect of corrosion on the fatigue life is not, in general, well understood. Significant work has been done by Luft 4 and others on some types of corrosion, but the affect of the many types of corrosion on the life of the CT is a very complex problem. These types of corrosion are difficult to track and quantify with regard to their impact on remaining life.
This paper presents a new approach which involves performing fatigue tests on samples from the downhole end of the CT or SL string. The results of these fatigue tests are then used to estimate the reduction in fatigue life (if any) due to corrosion. This method is somewhat crude in that it assumes that the corrosion at the downhole end is the maximum corrosion in the string, and applies the corrosion reduction to the entire string. The advantages of this method are that it doesn't require any additional tracking or measurements during field operations (other than the tracking already performed for fatigue life monitoring), and the methodology is easy to understand and implement.
SL diameters have evolved to larger sizes as a result of the strength required to perform more challenging field operations, with 0.125" slickline now being very common and 0.16" being used in some places. However, the plastic bending fatigue increases as wire diameter increases. Also, the cost of SL has increased, making it desirable to use a spool of SL for as long as possible without a fatigue failure.
A joint industry project was launched in 2002, in which several of the major oil companies funded the development of a SL fatigue tracking system. The first step in this project was to build a SL fatigue test machine which simulated actual SL service conditions. Figure 1 shows the machine that was built. This machine uses a 100' sample of SL. The SL is placed on a two-sided storage drum, seen on the right side of the machine. The SL then passes from the storage drum around the lower sheave on the left, around the center sheave, around the upper sheave and then back to the other side of the storage drum. An electric motor rotates the storage drum, moving the SL sample through the sheaves. An air cylinder is used to apply constant tension (at predetermined levels) to the SL via lateral movement of the center sheave. A Linear Variable Differential Transformer is used to measure lateral movement of the center sheave, and thus measures stretch in the SL sample during testing.
Figure 2 contains a sketch of the machine and a plot of the fluctuating strain history for one SL "trip". A trip is defined as a portion of the SL sample moving off of the storage drum, through the sheaves to the other side of the storage drum, and then back through the sheaves to its original position. If the bending strain is defined as positive when the sample is on the storage drum, the bending on the top and bottom sheaves is also in the positive direction. However, the bending on the center sheave is in the opposite, or negative direction. Thus in one trip there are two downward spikes in the bending strain, both associated with point 5 which is the center sheave. These negative bends are defined as "reverse bends", and are the most detrimental bends in terms of fatigue.