Study Explores Mechanisms of Surface Damage on Coiled Tubing
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Although several kinds of mechanical damage cause coiled tubing (CT) failure, longitudinal plowing marks (LPMs) account for a large portion of such damage. Understanding the mechanisms of LPM damage on CT surfaces will help reduce the occurrence of this type of damage. In this paper, the authors study the stress status of CT string held by gripper blocks, the axial force distribution of gripper blocks, and the longitudinal resonance of CT strings in vertical wells.
The overwhelming majority (greater than 80%) of CT failures have been classified into four categories: corrosion, mechanical damage, string-manufacturing issues, and human error. Of these, failures caused by mechanical damage constitute almost 30% of all CT failures. Further study of these types of mechanical damage reached the following two important conclusions:
- LPMs, which are injector-related, account for 46% of mechanical damage observed on CT strings from 2006 to 2017.
- Recent periods that have seen dramatic increases in mechanical damage coincide with plug milling and higher-grade-CT operations.
The authors propose that the pressure in the CT string plays a role in mechanical damage.
Contact Status Between CT and Gripper Blocks
The gripper blocks carried by chains are used to hold CT strings in place. The gripper blocks are divided into single-pitch and two-pitch categories. The engagements between the CT string and the gripper blocks are sometimes suboptimal. The contact area between the gripper and CT string is only at the endpoint of the arc, and the contact stress is much higher there than at the point at which the gripper blocks grip the original CT string. According to the von Mises yield criterion, if the von Mises stress is larger than the yield stress at any point, the material at that point will yield and plastic strain will be produced.
Distribution of Gripper-Block Lifting Forces in the Injector
Commonly, several pairs of gripper blocks engage the CT string to provide the friction forces needed for deployment and retrieval of the tubing into and out of the wellbore. How the axial loads of these pairs of gripper blocks are distributed is essential to safe operation and avoidance of CT damage.
To obtain the axial load distribution of all pairs of gripper blocks, a mechanical model is created by the authors and discussed in the complete paper.
The mechanical model demonstrates that the distribution of lifting forces of the gripper blocks is not uniform. As the spring constant linking the gripper block and the chain becomes harder, the axial-force distribution of the gripper block becomes more uneven. These axial forces are increased from the bottom to the top. Because the axial force of the top gripper block is the maximum axial force, when it disengages from the CT string, a sudden displacement change for the CT string beneath the injector may increase vibration.
In the process of CT injection, the gripper block immediately above the contact section between the gripper blocks and the CT enters into contact with the CT, and the gripper block at the bottom of the contact section is disengaged. In the initial stage of injection, the gravity of the CT is insufficient and the downward injection must be forced by the injection head. The load in this stage is small and usually not considered. When injected to a certain depth, the CT drives the gripper-block chain movement and the hydraulic motor turns to provide drag force. The injection head of the hydraulic motor plays a braking role; at this point, the static load on the injector caused by the weight of the CT string is significant.
Now, from top to bottom, the axial force of the gripper block increases and the axial force of the bottom gripper block is the largest. As with the lifting process, the bottom gripper block bearing the greatest axial force disengages with the CT string. The CT will be sunk, along with other grippers in contact with the CT string, to balance the axial force of the gripper and the gravity of the CT.
The authors also devote a subsection of the complete paper to discussion of longitudinal resonance of CT on vertical wells, an observation supported by field data.
Mechanism of LPM Damage to CT Strings
Typically, LPM damage to CT resembles fish scaling and is formed gradually. The fish scaling indicates that the LPM is not formed only by one slippage between the CT string and gripper block. Vibration is one reason for LPMs; another is CT-diameter growth as the CT is bent and straightened with high internal pressure.
An image of the process of LPM formation can be constructed that provides a reasonable explanation for earlier results provided in the literature. When a CT string is used through many cycles of bending and straightening with high internal pressure, the diameter of the CT will grow. The radius of the section arc of the gripping surface of the gripper block will be smaller than the radius of the CT string. The gripping force of the gripper block on the CT string will cause large bending moment in a section of the CT string, which, in turn, will produce a large first principal stress on the surface of the CT. At this moment, if rock debris becomes stuck in a groove of a gripper block, the resulting pressure on the CT string will result in an equivalent stress that exceeds the yield stress of the material of the CT string and will produce permanent damage on its surface. If there is no slip between the gripper block and the CT string, an indentation on the surface will form. However, if CT-string resonance occurs, and the amplitude of resonance is large enough, the results are noteworthy. Suppose that the total tension is the sum of the gravity of the CT string in the well and the dynamic load from vibration of the CT string. When this tension peaks, it may exceed the friction between a gripper block and the CT string. Slippage between the gripper block and CT string will occur, and the rock debris will scrape the surface of the CT string, forming a plastic metal tumor at the end of the scrape. At the trough of the cycle, slipping will stop and some elastic recovery will occur. At the next peak, the tumor or the debris will produce a larger scrape and the tumor will grow. As it does, fish-scaling damage becomes more extensive.
As the pressure increases in CT-milling-plug operations, the diameter of the CT string grows quickly and the percentage of the LPM damage increases. With high-strength CT string, the diameter of the CT string grows more slowly, so that the LPM damage tends to decrease.
How To Reduce LPM Damage?
To meet this goal, two issues must be addressed. The first is the diameter growth of CT string, and the second is the resonance of the CT system excited by the injector head. To solve the first problem, increasing the strength of CT material or decreasing the operation pressure can reduce the speed of the CT-diameter growth and thus reduce the possibility of LPM damage. To solve the second problem, the best means is to cut down the possibility of CT-string resonance. Lowering the speed at which the CT string is run into or withdrawn from the well can reduce the growth of stress amplitude of the CT string, thus reducing the possibility of resonance. In order to avoid resonance totally, natural frequency and excitation frequency cannot be equal for too long (e.g., over 1 minute). The natural frequencies of the CT string at a certain depth are fixed, but the excitation frequency is related to the speed of lifting or running down the CT.
- When CT string with a grown diameter is held by gripper blocks, soft material, such as rock debris, can carve the CT surface easily.
- CT-string-lifting systems will result in nonuniform distribution of axial forces among the gripper blocks and produce a period excitation while the motor of the injector runs at a constant velocity.
- The longitudinal resonance of the CT string will result in large stress amplitude if the time of resonance is long enough. This conclusion is supported by field data.
Study Explores Mechanisms of Surface Damage on Coiled Tubing
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03 July 2019