Page 20 - ITAtube Journal 1-2021

Technical Papers
Donald Gibeaut
Progress in inline seam annealing for small-diameter tube, pipe producers
20
Developments in a critical normalizing application have spillover effects through- out the industry
By Donald Gibeaut
Countless applications use metal tubing, but reusing tubes isn’t a common practice. After it’s affixed, attached, fastened, or installed, that’s usually the end of it. However, a growing practice for a few downhole applica- tions in the oil industry is the use and reuse of small-diameter coiled steel tubes.
The purposes are nothing new. Well operators do frequent visual inspections, dropping a length of tubing outfitted with a camera into the well’s borehole. They also use lengths of tubing to drop tools into wells to carry out various maintenance tasks, inject nitrogen or treatment chemicals to promote flow, and open or close valves to connect or isolate sections of the well. Tubing is also used for cleanout oper- ations and to run electrical cables to machines such as submersible pumps.
Conventional operations involve a series of tubing lengths up to 48 ft. joined with cou- plings. Needless to say, this is cumbersome and labor-intensive, both in the insertion and retraction phases. Much faster is the practice of using an extremely long length of coiled tubing.
Making a long tube and annealing it so that it can withstand repeated coiling and uncoiling is straightforward in principle. However, as with most tube or pipe operations, this sort of thing needs great care. These days, a length of coiled tubing for downhole operations can run nearly seven miles. Nobody wants to make a mile (or six) of tubing destined for the scrap heap because something went wrong.
Making a Supple, Forgiving Material
Uncoil it and use it. Coil it up and move it to the next well. Uncoil it and use it. This can’t last forever. Every time steel changes shape, it undergoes stress. In this case, the metal can undergo only so many cycles of coiling and uncoiling until it becomes too fatigued
to withstand any more deformations; even- tually, splits will appear. Making this tube is an expensive proposition; getting the longest service life from it is a matter of proper heat treating, annealing, or normalizing.
Any sheet, plate, bar, or billet of metal looks like a homogenous, continuous mass. However, it’s not that simple. When steel is heated, the atoms of iron take on a specific structure. If the processing temperature is less than 1,674 degrees F, it takes on a body-centered cubic structure. (Imagine the eight corners of a cube—eight evenly spaced iron atoms—with one more point in the center.) At higher processing temperatures, depending on the percentage of carbon, it undergoes a trans- formation to a face-centered cubic structure, which has 14 iron atoms.
Depending on the temperature, one of these two microstructures appears all throughout the steel, one cubic shape after another, forming a lattice. A vast lattice makes up a grain of the steel. As the metal remains at critical temperature, the grains grow until they come into contact with other grains, which is where grain boundaries form.
Grain growth is halted by cooling the steel rapidly. Cooling it sooner rather than later results in relatively small grains, which are associated with hardness and strength; the downside is that hardness equates to brittle- ness. Cooling it later allows the grains to grow larger, which results in a softer material that forms more easily than a fine-grained material.
So, the steelmaking process is a matter of controlling the processing temperature and the amount of time it spends at that tem- perature, and then cooling it rapidly. These steps, along with adding a bit of carbon while it’s molten—usually less than 2% by weight—and some other elements determine the steel’s properties.
More and More Heat.
When a long, narrow strip of this material is fed into a tube mill, the process starts over
ITAtube Journal 2021 – Special Edition