June 17, 2008
The energy sector is hot right now, and so is pipe production. Finding the optimum material for making pipe for this industry is tricky. Low-alloy carbon steels tend to be strong, but lack corrosion resistance. Stainless steels resist corrosion but lack strength. Cladding low-alloy carbon steel with a thin layer of a corrosion-resistant alloy is a suitable process, one that AWS Schaefer has devised for manufacturing such pipes.
It's no secret that the energy sector is hot right now. To say that fuel prices have been rising unpredictably during the past few years would be an understatement. According to the Department of Energy, the wellhead price for natural gas in the U.S. more than doubled from $5.06 per thousand cubic feet to $10.33 per thousand cu. ft. in just 14 months (September 2004 to October 2005). The spot price for a barrel of West Texas Intermediate crude oil was in the low $30s in 2004 and doubled the following year. It didn't stop there, increasing another 75 percent from April 2007 to a lofty $109.35 per barrel in March 2008.
But that's only part of the story. The entire energy industry is generating new infrastructure at a breakneck pace. The worldwide rotary rig count nearly doubled from 560 in January 2000 to 1,053 in January 2008.
Of course, pipe for the energy sector is in high demand. Pipe for transporting crude oil and crude gas must meet several criteria. The material must have sufficient durability, corrosion resistance, and strength, and the size must be large enough to transport the desired volume.
Corrosion resistance is necessary to prevent erosion damage from pollutants in the oil or gas, which include hydrogen sulfide, chlorides, and water. A traditional method for preventing corrosion is to introduce a corrosion inhibitor, such as glycerin. Drawbacks to this method include the costs associated with an injection plant to inject the inhibitor; waste disposal; continuous breakdown susceptibility; and maintenance costs. This type of system also presents environmental hazards associated with the byproducts generated by the corrosion inhibitor removal process.
An alternative is to produce the pipe from a corrosion-resistant material, such as a stainless steel or a nickel alloy. The drawback is that most of these materials lack the strength of low-alloy steels. The cost of producing such pipe thick enough to withstand normal transportation pressures is prohibitive.
Yet another alternative, one that makes the best use of corrosion-resistant alloys and low-alloy steels, is clad pipe. Such pipe typically is made from a strong, low-alloy carbon steel and lined with a sleeve made from a corrosion-resistant material approximately 0.19 inch thick.
Pipe producers provide two types of clad pipe: metallurgically clad and mechanically clad.
Metallurgically Clad. In manufacturing metallurgically clad pipes, pipe producers use one of several proven bonding processes to join the corrosion-resistant internal sleeve to the strong external low-alloy carbon steel:
This process has two limitations. First, because a single heat treatment is used for both the ferritic external layer and the austenitic internal layer, some material combinations are not technically feasible. Second, it isn't an efficient method for small-diameter pipe; the economics involved make this process suitable only for pipe at least 12 in. OD.
Regardless of the process, metallurgically clad pipes start as two materials that become a single mass. In various tests—load, hydrostatic, butt welding, and corrosion resistance—they behave the same as pipes made from a single material.
Mechanically Clad. The simplest mechanically clad pipe consists of a corrosion-resistant liner inserted into a low-alloy external carbon steel pipe. A more sophisticated mechanically clad pipe is produced by shrinking the external pipe or rolling one pipe inside the other. The nature of the mechanical bond depends on the process. Regardless of the method, the bond is purely mechanical. The two distinct materials remain two distinct materials—they do not fuse together to become a single mass as metallurgically bonded pipes do.
These processes generally aren't suitable for mass production of pipelines; they provide marginal contact between the two metals and they lack the necessary uniformity and reliability.
Finding a Practical Mechanical Alternative. A practical alternative to these methods would be a process with the simplicity of mechanical cladding that produces the performance of a metallurgically clad pipe. Indeed, such a process exists, and many tube fabricators already are familiar with it. It's hydromechanical forming, also known as hydroforming.
Nippon Steel developed a pipe cladding process approximately 30 years ago for manufacturing C II pipe. The process used hydraulic pressure on the inner pipe and induction heating on the outer pipe. The hydraulic pressure caused the inner pipe to expand; removing the heat caused the outer pipe to shrink as it cooled.
A modern improvement to this process uses a hydraulic pipe calibration and lining machine equipped with an additional water system as well as sophisticated controls. It uses a process similar to automotive parts hydroforming machines to attain a high degree of compressive contact between the two pipes.
The corrosion-resistant pipe is inserted into the outer low-alloy carbon steel pipe in a semiautomated operation and is then placed into the calibration machine's open tool form. The tool closes and axial cylinders seal each of the pipe ends. Hydraulic fluid under high pressure expands the inner tube. A firm compressive contact is achieved by the elastic and plastic behaviors of the outer pipe and the inner pipe. The elastic springback of the outer pipe is greater than the plastic expansion of the inner pipe; the resulting residual pressure stress of the inner pipe is in the region of 7,250 to 14,500 pounds per square inch (PSI).
This provides a homogenous contact along the pipe's entire length.
A closed outer tool determines the size, roundness, and straightness of the finished product.
Materials. Any standard pipeline steel that complies with American Petroleum Institute specifications can be used for the outer pipe. A corrosion-resistant metal (a stainless steel such as 316L or 317L, or a nickel alloy such as INCOLOY® 825 or INCOLOY 625) is used as the lining material. The liner alloy depends on the specific corrosion requirements.
The pipes can be produced by either the longitudinal seam-welded or seamless method.
Quality Assurance. Because clad pipe gets its strength from the outer pipe and its corrosion resistance from the inner pipe, it follows that strength testing is necessary on the outer pipe only and chemical or corrosion resistance testing is necessary for the inner pipe only.
For strength and safety verification, it is necessary to determine the characteristic values (yield strength and notched impact strength) of the outer pipe. Typically, these values are measured before the cladding process begins; although the process changes these values, the difference is negligible.
Various compressive contact and pressure tests can be carried out on the outer and inner pipes. Certifying the pipe usually encompasses a pressure test of the final product as well as a control of the respective pipe identity number, which ensures that each pipe is traceable. The required pressure test can be performed immediately after the pipe is expanded. If the hydroforming press is properly equipped, the test can be done in the press.
Additional nondestructive tests, such as ultrasonic or X-ray, can be performed on the end connection profile, before or after the cladding process is completed.
Pipe End Preparation. Following calibration, the pipe ends must be milled to a predetermined form for weld setup. To facilitate welding mechanically clad pipe, seal-welding the theoretical ring gap between the inner and outer pipe is recommended.
For joining metallurgically clad pipes, an overlay weld is suggested. This requires removing up to 2 in. of the inner surface at each end of the pipe and using gas tungsten arc welding to seal-weld the corrosion-resistant material to the low-alloy carbon steel layer. The welding then continues outward to the end of the pipe.
One of the chief advantages of using a hydroforming process to manufacture mechanically clad pipe is simple economics. Compared to producing a nonclad or a metallurgically clad pipe, manufacturing clad pipe with this method represents a significant cost reduction.
Economic Comparison. An economic comparison (based on the prevailing prices as of January 2007) reveals the price differential. This scenario involves pipe for transporting natural gas. The OD is 12 in.; the working pressure is 150 bar (2,176 PSI); the contaminants are hydrogen sulfide and chlorides. The three cases are:
Case 1. The wall is approximately 0.59 in. thick and the pipe weighs about 81 lbs./ft. This is the reference case. The cost is 100 units per meter.
Case 2. The outer pipe is approximately 0.39 in. thick; the liner is about 0.12 in thick. The pipe weight is 71 lbs./ft. Relative to Case 1, Case 2 costs 80 units per meter.
Case 3. As in Case 2, the outer pipe is 0.39 in. thick and the liner is 0.12 in. thick. Compared to Case 1, the cost for this pipe is 46 units per meter.
Therefore, mechanically clad pipe represents a cost reduction of 54 percent compared with a homogenous pipe and a cost reduction of 42.5 percent compared with a metallurgically clad pipe.
Another area of potential cost reduction is in welding, because clad pipe has thinner walls than homogenous pipe, and so requires less welding time. In this scenario, the clad pipes are 0.39 in. thick, whereas the homogenous pipe is 0.59 in. thick, a 13 percent difference.
Other Advantages and Disadvantages. During the continuous spool laying process of mechanically clad pipe, it is critical not to subject the pipe to a bending radius smaller than 40 x D. For example, the minimum bending radius for a 12-in.-OD pipe is 40 ft. Smaller bending radii can result in wrinkling and cause the liner to detach from the outer pipe.
For field inspections, the ultrasonic measuring method cannot measure the wall thickness of the liner from outside the pipe. This sort of inspection must be carried out on the inner surface of the pipe.
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