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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.
Coiled Tubing (CT) is subjected to plastic mechanical deformation as it is cycled on and off the reel and over the gooseneck. Repeated bending causes CT fatigue,and eventually failure. As fatigue accumulates, CT gradually loses its yield strength. The reduction in yield strength will inevitably degrade CT collapse/burst ratings and lead to CT diametral growth and permanent elongation, especially in high-pressure operations.
The latest theoretical and experimental results relating to CT fatigue are reviewed. The effects of CT yield strength reduction on factors such as collapse/burst, permanent elongation, and diametral growth are discussed, and the limitations of these factors illustrated through examples.
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 A PC computer model has been developed that accurately predicts the effect of floating vessel heave on coiled tubing fatigue life. This model shows that CT fatigue failures can occur in innutes or hours due to vessel heave if not correctly planned for and considered during operations. P. 223
Abstract Coiled tubing is a continuous pipe that, having been coiled around a reel for storage, can be deployed and used as a pipeline or riser. During deployment as a riser, the coiled tubing is unspooled from the reel, run into the water and connected to the wellhead. This process plastically strains the pipe causing plastic, or low cycle, fatigue damage. When the coiled tubing is connected to the wellhead, the environmental loading causes elastic stress cycles, resulting in elastic, or high cycle, fatigue damage. There are numerous methods to determine the fatigue life from either plastic or elastic cycling; however, there is little data within the industry on how the fatigue damage from elastic and plastic cycles combine. This paper presents the experimental work conducted to show the combined fatigue life of coiled tubing that has been plastically and elastically cycled. The data shows that the combined fatigue life can be lower than the summation of the plastic and elastic fatigue damages using Miner's rule. Existing theory suggests that the combined fatigue life could be as low as 10% of the Miner's rule of fatigue damages; however, the experimental data indicates that a more appropriate value is closer to 75% of the Miner's rule fatigue damage.