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The CT injector is the equipment component used to grip the continuous-length tubing and provide the forces needed for deployment and retrieval of the tube into and out of the wellbore. Figure 1.5--CT injector and typical well-control stack rig-up (courtesy of SAS Industries Inc.). The tubing guide arch assembly may incorporate a series of rollers along the arch to support the tubing or may be equipped with a fluoropolymer-type slide pad run along the length of the arch. The tubing guide arch should also include a series of secondary rollers mounted above the CT to center the tubing as it travels over the guide arch. The number, size, material, and spacing of the rollers can vary significantly with different tubing guide arch designs. For CT used repeatedly in well intervention and drilling applications, the radius of the tubing guide arch should be at least 30 times the specified OD of the CT in service. This factor may be less for CT that will be bend-cycled only a few times, such ...
Coiled tubing (CT) well intervention and drilling operations require that the continuous-length tube be subjected to repeated deployment and retrieval cycles during its working life. The tubing stored on a service reel is deployed into the wellbore to the designated depth and then retrieved back onto the service reel. All of the aforementioned items act on the tube body to some degree during any CT service and contribute to the eventual mechanical failure of the tubing. To ensure safe and reliable well intervention and drilling operations, the user must understand the unique behavior of CT to minimize the possibility of tubing failure. Numerous decisions must be made throughout the working life of a CT string to maximize the remaining life. From this approach, the decision to retire the tubing must be made on the basis of current tube conditions, service history, and the anticipated service loading. Fatigue is generally considered to be the single major factor in determining the working life of CT. The deployment and retrieval of the continuous-length tubing string require that the tube be subjected to repeated bending and straightening events, commonly referred to as "bend-cycling." The amount of strain imposed upon the tube body during the bend-cycling process is considered to be enormous, in many cases on the order of 2 to 3%. When subjecting the CT to this type of fatigue cycling, the stress and/or strain fluctuations to failure may be estimated using conventional axial fatigue life prediction approaches.
Abstract Coiled tubing (CT) monitoring tools are being utilized on a larger percentage of field jobs now as compared to the past. They are being used on both CT intervention and CT drilling operations. The objective of this paper is to demonstrate how a wall thickness measurement device can enhance and improve the calculations made by a CT fatigue algorithm. Experimental work was done which shows how the wall thickness can vary depending on the pump rates, the fluid being pumped and the amount of tubing that is spooled on the reel. Reference four provides detailed information related to wall reduction in CT during pumping operations. Typically, fatigue models will utilize an estimated, nominal or minimal wall thickness to perform the fatigue calculations. The amount of wall reduction in CT can vary greatly and as a CT string acquires fatigue, there is a chance that the estimated wall thickness may not reflect the actual wall thickness of the pipe. Recent modifications to fatigue tracking software allow the user to incorporate the real-time (or recorded) measured wall thickness into the fatigue calculations. This paper will discuss the experimental work, the modifications to the model, and case histories in which the wall thickness measurement device was used. Calculating the CT fatigue using the measured wall thickness will increase the accuracy of the CT fatigue profile. This will allow CT service companies to operate at an increased efficiency. Operating companies will also benefit from this technology because there will be fewer fatigue failures at the well site due to the increased accuracy in the fatigue calculations.
Abstract Tracking the bending and internal pressure history of coiled tubing (CT) at the reel and guide arch, as it is run in and out of wells during field operations, is accepted throughout the industry as the preferred method for predicting the fatigue damage distribution along the string throughout its life. A mathematical model, relating bending strain and hoop stress (from internal pressure) to cumulative fatigue damage can be applied to this recorded field loading history. Such models determine when tubing should be retired or when tubing cuts should be made in order to maximize the utilization of the pipe while minimizing the chance of pipe failure. The problem of CT fatigue has been studied extensively in the lab, using constant pressure bend cycling and/or axial coupon testing. However, models resulting from experimental programs have seldom been correlated in the literature with CT behavior under field loading conditions. This paper presents such validation by correlating the experimentally determined cycles to failure of CT samples which have had extensive use in field operations, to predictions from an advanced predictive fatigue algorithm. The string, from which the samples were taken, had been removed from service based on criteria specified by the operating company for which the CT work was being performed. This stated that strings were to be retired after a given value of "running meters" had been reached, rather than on the actual bending and internal pressure history. Excellent agreement is demonstrated between the actual remaining fatigue life of the tubing, determined experimentally, and the estimated cycles to failure predicted by the fatigue model. Therefore, it is shown that the string could have been safely utilized much longer than the current string retirement specification allows. Based on the results of this testing, the operating company has changed its criterion for CT string retirement to one that relies on tracking of bending and internal pressure history and subsequent predictive fatigue modeling based on this history. Introduction As the performance envelope for CT well service continues to expand, the importance of being able to accurately predict fatigue damage to the pipe, induced by the tremendous bending strain the pipe is subjected to, becomes increasingly important. Fatigue damage is the primary factor used to determine when a CT string should be retired from service. There are essentially no other applications where steel alloys are subjected to cyclic loading at such high strain values. Thus, the development of mathematical models used to specifically predict fatigue damage under the conditions encountered in field operations has taken place only in the last 10 to 12 years, making it a relatively new field of study. A considerable amount of laboratory-scale testing, and full-scale testing under controlled conditions, have been conducted in conjunction with the development of existing predictive fatigue models. However, there has been very little correlation between the data generated by these fatigue models and the actual fatigue damage observed in pipe that has seen extensive use in field operations. Lack of such validation still leads some service companies and operating companies to base CT string retirement criteria on the concept of "running feet/meters". This is defined as the distance traveled by the pipe as it is run off the reel and over the guide arch, typically measured in one direction only. In other words, a single trip in and out of a 10,000 ft well would be interpreted as 10,000 running feet. While this concept does give a measure of the utilization of the CT, it ignores the effects of internal pressure, the effects of localized repetitive cycling (which can lead to untracked fatigue "spikes"), variations in wall thickness of tapered strings, and the severity of the bending strain the CT has been subjected to.
Abstract Coiled tubing endures unique cyclic stress and strain histories. The loading imposed on pressurized coiled tubing can trigger deformation mechanisms resulting in incremental plastic diametral growth and elongation. This growth occurs in spite of the fact that both the hoop stress and net axial stress from tension are well below the material yield stress. The most dominant factors controlling the deformation behavior of coiled tubing are the bending-straightening cycles associated with the spool and gooseneck. It is the interaction of the bending stresses and strains with those from axial loading and pressure that result in plastic elongation and diametral growth. Severe cyclic plasticity imposed by these events actually changes the structure of the coiled tubing material, causing a corresponding change in mechanical properties. The material properties in a section of coiled tubing along a string are thus dependent upon the localized service loading history. The operating parameters that control the loading on a section of coiled tubing are discussed in terms of the tubing geometry, the above-surface deployment equipment and the sub-surface environment. With the load history characterized in terms of imposed stresses and strains, mechanisms that lead to diametral growth and elongation are demonstrated from the standpoint of simple material plasticity models. Refined models are described, based on more sophisticated plasticity relations, capable of characterizing transient behavior and multilateral effects. Implications from these results for depth calculation are discussed.