In the petroleum industry, lengths of pipe are threaded together in order to form a drill string, which is a series of connected pipes. Drill strings are employed in harsh application environments, prompting the need for stronger, more durable threaded connections; industry professionals utilize a process known as cold root rolling to achieve these enhanced characteristics.
Cold root rolling involves burnishing the root radius of a thread that has been freshly cut or previously cut in a rotary shouldered connection. A hardened roll is forced into contact with the thread’s root radius, and then pressure is applied so that the hardened roll penetrates the surface of the root radius.
Cold root rolled connections are primarily used in offshore drilling applications. However these connections have also proven to be advantageous for onshore drilling applications—especially with the rise of extended-reach drilling, multi-lateral wells, and horizontal well applications, which can significantly increase the stress placed on threaded connections in these drill strings.
Cold root rolling can substantially boost the return on investment (ROI) of your efforts by greatly prolonging the service life of your equipment—by three to five times, in fact. Improved product lifespan translates into reduced equipment costs, which in turn, leads your company to a better bottom line. Cold root rolling can also lower thread root stress, reduce the rate of chemical erosion, and help you keep your workforce employed by moving work in-house.
Let’s take a look an in-depth look at the various benefits of the cold root rolling process.
Cold root rolling works to increase the fatigue life of each rotary-shouldered connection by creating a much stronger connection. This enhanced strength is a result of the process’ hardening effect, which essentially displaces the steel grain structure from a uniform structure to a structure with numerous dislocations.
During this process, the structure of the steel material changes on an atomic level; in a typical steel pattern, the crystal structure has a uniform, lattice kind of pattern. As a result of the compression in the rolling process, this structure changes, causing many different dislocations in the steel. These dislocations cause the crystal structures to interlock, thus making the finished product much stronger.
These dislocations help prevent damage when the tool is in use and prevent small cracks from developing into wider structural flaws that could necessitate costly repairs or drastically reduce the lifespan of your product.
Lower Thread Root Stress
The weakest point of a threaded connection is near its root. The amount of stress placed on the root will depend on the specific type of thread being used; coarse threads will reduce the stress, while fine threads increase it. The cold root rolling process can eliminate tensile stress by creating a small zone of residual compressive stress in the thread’s root region—this will offset the tensile stress produced during operations and in other critical regions of the root thread.
Research has shown that cracks occurring in cold-rolled joints exhibit a considerably low crack aspect ratio, typically between 30 and 50 percent. Dislocations of the steel material prevent cracks from growing lengthwise, and diminishing the length of cracks is critical for getting the most from your initial investment. Long and shallow cracks can lead to a joint’s structural failure, potentially forcing a sudden halt in operations. In contrast, deep cracks are more easily detected and more easily repaired, ensuring that a damaged drill string can be repaired or recovered prior to complete structural failure. This will prevent costly shutdowns and extensive maintenance.
Reduce Chemical Erosion
In the cold root rolling process, the root of a thread that has been previously cut in a rotary shoulder connection is burnished. Overall, this process deforms and cold-works the material and improves the thread’s overall surface finish. This works the material on the thread profile into a more even, uniform surface, reducing scratches and imperfections. This not only reduces stress and improves strength, but it also reduces chemical erosion.
Any imperfections on the material, especially scratches, accelerate the rate of chemical erosion, as they are jagged and porous, and also tend to harbor the corrosive chemicals commonly used in drilling environments. When imperfections are smoothed out in the burnishing process, there are fewer nooks and crannies for these corrosive chemicals to take hold, slowing down the rate of chemical erosion. This improves overall performance and reduces the need for costly repairs, which saves money in the long run.
Move Work In-House
With cold root rolling, larger companies can bring work in-house, simultaneously keeping their workforce employed and improving the economy of the market. When you decide to implement this process into your company’s operations, make sure to choose a versatile tool—such as the CJWinter Cold Root Rolling (CRR) Tool.
The CRR Tool can be easily integrated into several types of modern industrial lathes, including those supplied from companies such as Weiler, Fanuc, and Mazak—this integration will not strain your company’s budget. The CRR Tool is error-proof and highly intuitive for machinists; the tool cannot be loaded backwards, and it is fitted with a 1:1 pressure ratio that effortlessly converts pounds per square inch (psi) to pounds of force (lbf). These features improve the overall safety of current employees, while simplifying the training process for future employees.
Through the use of cold root rolling, your employees will be kept busy (even during periods of fluctuation in the industry) as they will be better equipped to manufacture superior threaded connections at much higher volumes.
Invest in Cold Root Rolling Today
Cold root rolling provides so many notable benefits, easily making the investment worthwhile for your company in the long run. Contact CJWinter to learn how to integrate cold root rolling into your capabilities.