Detecting and working with stress-induced deformation
August 5, 2009
High production threading of tube and pipe for use by the petroleum industry is accomplished using either mechanically actuated or digitally controlled, carbide tooled machines. The most effective way to deal with residual stress is to full body anneal each tube prior to machining; however, this may not be economically possible. One alternative is to limit the amount of energy that the tube is subjected to during machining. An examination of the cutting tools that are used to produce threaded product will reveal how the shape of the insert may affect the deformation of the tube.
Producing threaded connections that comply with the standards set forth by the American Petroleum Institute (API) requires precise control of each element of the thread form.
High-production threading of oil country tubular goods (OCTG) is accomplished using carbide-tooled machines that may be either mechanically actuated or digitally controlled. Whether mechanically actuated or digitally controlled, high-production machines may be the rotating-tool type or the rotating-product type (see Figure 1). The highest production rates are achieved using machines that are capable of producing pipe in a single threading pass.
The digital equipment provides a level of flexibility that is unavailable on mechanically actuated machines. A new mechanically actuated machine can produce threaded pipe joints consistently within one-half of the allowable API tolerance; however, the challenge is trying to maintain this tolerance in an economical manner over time. When the machine no longer can be adjusted to compensate for wear, it must be rebuilt to bring it back to its "as-built" tolerances.
A look at the tolerances set by API reveals why it is desirable to be able to produce threaded product to one-half of the allowable tolerance. The typical standoff tolerance placed on OCTG products by API is plus or minus one pitch for the threaded pipe (pin) end and plus or minus one pitch for the coupling (box end of the connection). The standoff is measured using a solid ring gauge for the pin end and a plug gauge for the box end or coupling.
Because the pipe and couplings are manufactured during separate processes, each component may be inspected using API tolerances without consideration of the size of the mating element in the final joint assembly. Using the full API tolerance, it is possible to produce a pin end that measures plus one pitch standoff and a box end that measures minus one pitch standoff. Each of these individual elements is within API specifications for size; however, when they are joined, the combined tolerance yields a variation of two thread pitches from the API-specified "J" dimension.
The J dimension is defined as the distance from the center of the coupling to the end of the pipe in the assembled joint. API places a tolerance of plus or minus one pitch on the J dimension. This example illustrates that, short of trying to match the sizes of the pipe and couplings, the only way to ensure that each assembled joint falls within the J dimension tolerance is to manufacture the components to one-half of the allowable API tolerance for standoff.
Equipment that utilizes either CNC or digital motion controllers allows incremental adjustments to be made to the taper, lead, and pitch diameter elements of the thread in axial distances as fine as each pitch. This means that if the material being cut produces a taper that measures in tolerance at each end but out of tolerance in the center, a taper adjustment can be made directly to the center area of the thread.
The process of producing tubular products imparts a tremendous amount of energy into the pipe. Seamless pipe is hot-worked to its final shape and then passed over a cooling bed before being cut into working lengths. Electric resistance-welded (ERW) pipe is cold-formed, rolled to its final shape, welded, and cut to working lengths. Then both types of product are straightened. Further processing may be required to heat-treat the pipe to its final hardness.
As the pipe is presented to the machining station for threading, it contains residual internal stresses that act in a manner that allows it to maintain a stabilized shape. Even though the stresses may not be equally distributed throughout the pipe, they will have reached equilibrium. The removal of material and the reaction to the cutting energy created during threading may disturb the equilibrium of these stresses.
Whether the threading operation is done on a rotating-product machine or a rotating-tool machine, the pipe retains its shape as long as it is securely held for machining. It is only after the pipe is removed from the machine that the effects of changes in pipe stresses are evident. The deformation that results from the internal residual stresses that occur during the pipe production and the cutting force-induced stresses that occur during threading may be great enough to exceed the tolerance window for the finished product.
Stress-induced deformation often escapes detection during inspection of the threaded product because of the tools available to the inspector. Because the thread being applied to the pipe is tapered, there is no simple way to inspect the pipe's OD or its pitch diameter for ovality caused by deformation (see Figure 2).
Typically, an inspector uses one of four types of measuring gauges—a lead gauge, a taper gauge, a thread-height gauge, and a ring-type standoff gauge (see introductory photo). In its mildest form, tube deformation is measured as an increase in standoff as the thread is being checked with a ring gauge. This is because the ring gauge contacts the high points of the out-of-round-tube, yielding an apparent oversize thread. It is almost impossible for an inspector to differentiate between a deformation-caused standoff increase and a tool-wear standoff increase.
One indication that a deformation problem may be present is the apparent instability in standoff when measurements are compared over several sequentially produced parts. In cases of minor pin-end deformation, the tube simply conforms to the shape of the coupling during power tight makeup, resulting in an acceptable joint.
In cases of severe deformation, the contact stress of the mating surfaces of the threaded components may result in galling during power tight makeup. In these cases, it is critical that the power tight equipment being used can detect minute variations in the joint's makeup profile.
In the most severe cases of tube deformation, a change in the thread taper can be detected as it is measured in multiple positions around the tube. Because machines in good operating condition produce threads that gradually change in standoff as the threading inserts wear and the taper of the cut thread usually remains constant, any instability in these measurements can be a good indication that stress-induced deformation is occurring.
Although the most effective way to deal with residual stress is to thermal stress-relieve each tube prior to machining, it may not be economically possible. An alternative is to limit the amount of energy that the tube is subjected to during machining. One way to accomplish this is to remove material in small amounts. This requires the capability to perform multiple threading passes. Although multiple threading passes extend cutting time, which results in fewer pieces per hour, it still may be more economical than cutting off a rejected pipe end and rethreading it—and possibly producing another rejected end.
Another method of reducing the amount of energy that the tube is subjected to during machining is to use cutting tools chosen for their free-cutting characteristics rather than their tool life.
One last method of dealing with residual stress has been proven to be effective on rotating-tool equipment. This method uses a machining sequence that removes the majority of material from the tube but does not finish the thread form. The machine components holding the tube then are released individually, allowing the tube to deform.
This requires the capability to release the inboard pipe chuck while the outboard pipe chuck and inside pipe support (IPS) securely grip the tube. The inboard pipe chuck then regrips the OD of the tube. The IPS releases and regrips the tube on its ID. Once the tube has been allowed to deform and assume a shape that is stable with equalized stresses, a final threading pass finishes the thread form, producing a round, correctly sized end.