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Multiwall system curbs sour gas corrosion

Reprinted from the 1981 issue of Pipe Line Industry Copyright 1981

Six year leak-free record verifies success of the concept

Ken Tierling, President, Unisert Multiwall Systems, Inc., Conroe, TX
John Baron, Staff Chemical Technologist, Shell Canada Resources, Ltd.

PLASTIC-LINED steel pipe (plastic pipe within steel pipe) has been used in the chemical process industry for many years as an effective means for transporting corrosive chemicals, particularly acids. Since it consists of two very different materials, metal and plastic, joining of pipe sections has generally been achieved with flanges every 20 to 40 feet, the plastic overlapped face compressed between two steel flanges. Although this feature does not present a barrier to process piping, it certainly does in the pipe line industry from the standpoint of both cost and underground corrosion.

Yet the need for such a product definitely exists in the pipe line market where corrosion/erosion resistance and strength are required, i.e., in acid (sour) gas pipe lines, water systems, crude oil lines with a significant water cut, tailings disposal, and slurry lines, and more recently carbon dioxide (CO2) injection and gathering systems. The Unisert process of sliding a long length (1/2 to 1 mile) of plastic piping into a similar segment of steel pipe and then grouting the annular space to lock the two together makes the plastic-lined steel concept applicable for pipe lines, since it eliminates the need for underground flanges. The system is also cost effective, the repair application being 50 to 70 percent of replacement. In new pipeline construction, the costs compare very favorably to high-pressure RTR pipe.

The following discussion concentrates on the materials selection criteria, testing, installation and results relative to the first application of this system to a highly corrosive sour gas pipe line performed for Shell Canada Resources Ltd. at the Hunter Valley field, Alberta, Canada in 1975. The last six-year period allowed time to closely evaluate the success of the project and general concept.

History. The 6-inch sour gas pipe line was originally placed into service in the fall of 1970, transporting sour gas well production at a temperature of 150° F and an operating pressure of about 1,200 psig. The well flowed some 7 million scf/day of natural gas containing 14% H2S and 5% CO2 along with hydrocarbon condensate and several barrels per day of salt water.

After ten months of service, a leak from internal corrosion occurred in a low area some 1,000 feet from the well. The pipeline faces extreme elevation differences, being situated in the Rocky Mountain foothills. A chemical inhibition program was initiated through continuous injection of film forming chemicals supplemented with bi-weekly pigging of the pipeline with slugs of inhibited condensate.

The chemical inhibition program failed to arrest the corrosion and another leak occurred after eleven months. In all probability, adequate filming of the steel pipe by the inhibitor was impaired by heavy sludge material consisting of elemental sulfur and iron sulfide corrosion product which had settled in the pipeline. The pipeline was shut in until a safer method of producing the well could be found.

General Design. In the late sixties, Unisert Systems had initiated development of a close-fitted polyolefin liner expanded against the steel jacket with pressure. However, the experience gathered as well as that observed in the process piping industry suggested that the highest probability of success with a plastic-lined steel concept was to be achieved by not inducing a significant hoop stress on the plastic component during manufacture.

The only way to accomplish that aim without the use of flanges was to fill the annular space between plastic pipe and the steel pipe with a high compressive strength material. The plastic pipe was thus purely used for its long suit, corrosion resistance, without concern for its structural and mechanical integrity over the design life of the pipeline. This procedure, which came to be termed the compressed liner concept, also avoided stress concentration at the end connections since any axial stresses were distributed along the entire length of the pipeline. Unisert Systems received patent rights for the general method in both Canada and the United States and has further developed the system in the last decade.

One huge advantage was that the flexibility in materials selection was greatly enhanced. Any recognized plastic piping system with a sound integral joining system could be used including polyethylene, polyvinyl chloride, polybutylene and fiberglass reinforced thermoset pipe.

The major uses for the polyolefins were seen to be chemical waste water systems and tailings disposal pipelines because of their often superior chemical and abrasion resistance. However, since polyolefin pipe products soften in liquid hydrocarbon service, PVC and RTR pipe has been generally employed in the oil and gas industry.

For the repair of pipe lines with extensive external and internal corrosion, it has since been determined that only the FRP liner increases the failure pressures of defected steel pipe a significant degree, since the combined fracture must shear across the full glass-resin thickness to invite a fluid leak.

Annular material. The annular material is an inorganic Portland cement grout, also to fit several parameters: (1) the required compressive strengths are attained; (2) the viscosities can be easily varied so that the grout can be pumped relatively long distances at minimal pressures; and (3) a set and hardening process that can be achieved without external heat and one that is only influenced by ambient conditions to a minor extent.

Other than practicality, the cement grout also provides a second defense system against corrosion of the steel component. It is well known that the basic nature of cement (pH of 11 to 13) passivates steel beyond its corrosive range. However, cement structures exposed directly to the environment deteriorate over time since the fluid reacts with the lime and clinker material in the cement grout in the form of: (1) leaching from reaction with water, (2) ion exchange with magnesium and sodium chloride found in salt waters and also with carbonic or hydrochloric acid, and (3) the formation of expansive salt reaction product with the sulfate ion. The time period after which these interactions are felt becomes a direct function of the velocity and aggressiveness of the fluid.

Since the Unisert process separates the fluid from the cementatious structure by an inner plastic liner, the annular material is spared these corrosion mechanisms. If, however, the inner plastic liner suffers a microcrack or pinhole failure the grout would then become exposed to the fluids, but in a stagnant velocity situation so that the corrosive elements would not be continuously replenished. Hence, the selection of inorganic cement grout as the annular material was designed to cost-effectively increase the design life of the plastic lined concept - particularly since cement is 1/10 to 1/20 of the cost of most plastic materials.

Design for gas systems. A gas system involves an additional design constraint over and above that common to all systems: that of external collapse of the inner plastic.

wall crushing action on plastic liner

In any system that places a non-metallic material inside a metallic one, the difference in gas permeability rates between the two basic material types leads to gas entrapment at the interface. This phenomenon occurs regardless of whether this interface is a micro-area as with thin film coatings or a macro area as with plastic-lined steel, the only difference being the volume. The smaller the volume, the faster the pressure develops at this interface.

Interfacial gas pressure becomes of consequence only when the pipe line system is quickly depressured creating a situation in which the internal pressure exceeds the internal pressure. Thin film coatings then tend to spall or blister at localized areas, and in a pipe-soil envelope, the mode of external collapse transfers to what has been called a wall crushing action which requires a considerably higher pressure than what is normally expected of plastic pipe. (Fig. 1)

The calculation method utilizes the familiar Barlow equation, but with the application of compressive strengths. Therefore, because the reinforced thermoset materials retain a much greater strength, particularly in compression, it becomes mandatory to use them in high pressure gas systems.

Testing. Shell Canada initiated an experiment to evaluate lining materials for high pressure H2S gas service in relation to the afore-mentioned design constraint.

The test samples included PVC lined steel and RTR lined steel along with various thin film coatings (epoxy, phenolic, cement mortar, etc.). The various suppliers were asked to submit lined four-foot tubing pups which were subsequently screwed together in a test loop and installed in a high pressure (1,200psi) sour gas stream at 140° F. The gas was composed of 5.8% H2S, 5.74% CO2 and 81.5% methane, the liquids 13 and .3 barrels/MMscfd of condensate and salt water respectively.

The test lasted one year. The first phase involved a static test with rapid depressuring every three days for one month. Following this phase, the gas stream was followed through the surviving systems for 11-months with monthly depressuring and inspection.

Several of the above systems survived the test with differing degrees of deterioration. Based on visual examination, the only system to pass the test program unscathed was to be the RTR lined steel pipe. The PVC liner deformed due to a gas pocket formed in the upper quadrant of the circumference, indicating that the wall crushing strength had been exceeded.

Subsequent testing independent of these showed that the short term compressive strength for plastic pipe could be used to determine its collapse resistance, since depressuring is a relatively instantaneous procedure. Typical calculations for 4-inch plastic pipe, for example, are: (1) 500 psi for PE SDR 15.5; (2) 950 psi for PVC SDR 17; and (3) 1,850 psi for standard thin wall RTR pipe - which lends further credence to the nature of the results in the Shell Canada tests.

Installation. It was decided in 1974 to install the RTR lined steel concept in a critical 1.1-mile portion of the Hunter Valley pipeline as a repair system.

Line graph: Pipe line profile and layout

A nominal 4-inch RTR pipe was chosen which had a rating of 300 psi in a free- standing system. It was constructed with anhydride-cured bisphenol A epoxy resin reinforced with continuous winding of glass strands and a plasticized inner resin-rich layer.

Unisert Systems installed the RTR lined steel pipe in the summer of 1975. The RTR pipe was completely joined on surface and pressure tested to 500 psi. It was then winched into the existing steel case with a steel cable as is common in a slip-lining technique. The annular space was then packed off at the ends to provide a defined volume. The cementing ports, 500 to 1,000 feet apart, were situated to act as air escape vents. (Fig. 3) The grout was pumped from one port to next port in succession. The final system was then pressure tested at 2,200 psi.

The success of the cementing process is attributed to the rising action of the liner during grout injection. This eliminates any stagnant areas in the annular space and ensures a complete fill with solid material.

The plastic pipe positions itself in the following manner after being inserted into the existing metal carrier pipe: Figure 5A - after pulling, the plastic may be positioned eccentrically within the outer pipe, particularly around side bends. Figure 5B - the plastic pipe is filed with water, the weight of which relaxes any slight residual tension from the pull and forces the inner pipe to be cradled at the bottom of the outer pipe as in two concentric circles. The pipe is internally pressured at this time to prevent its collapse during the cement process. In the case of the 4-inch nominal diameter RTR, the pipe itself weighs approximately .80 lb./ft.

Positions that liner assumes after insertion and during cementing process

Figure 5C - the grout initially starts flowing in the upper zone between plastic pipe and metal pipe.

Figure 5D - the buoyancy action form the plastic pipe encased in fluid cement (5.5 lb./ft. for 4") forces the pipe to rise, thereby eliminating any stagnant areas at the bottom of the pipe circumference.

Results. A leak-free record in the last six years verifies the success of the project. To date, the RTR lined system has outlasted the longest previous run life by a factor of six while allowing uninterrupted production of the well. The aggressiveness of the fluid stream has not decreased over time as borne out by further corrosion studies.

Pressure gages had been placed at the cementing ports to monitor the annular pressure. It has been found that the annular pressure is generally 200 psi lower than the system pressure while in operation. They also indicate that the RTR liner had been subject to microcracking, likely from two factors: first, the freestanding liner was internally over-pressured during the cementing operation due to incorrect design specification; second, carbon disulfide solvent known to be very aggressive to the epoxy resin had been inadvertently pumped through the line following well stimulation, this practice being subsequently discontinued.

A TV camera inspection survey performed by Shell Canada in 1980 showed that the inner resin-rich layer of the RTR pipe had deteriorated at localized areas, but also showed that the RTR tube had not collapsed in any section from lime depressure. Radiographic and ultrasonic inspection of the steel outer case at those areas that the TV survey indicated liner deterioration has shown no metal loss. The cementing ports have also been monitored by gas detectors to determine the amount of H2S flowing through the annular space and to date, not even trace amounts have been detected.

Subsequent developments. Since that first RTR lined steel installation in 1975, several improvements on the design and installation practice have been instituted:

Materials selection. Various RTR products presently show marked differences in flexibility and corrosion resistance, a direct function of the materials and procedures used in the manufacturing process.

In the same nominal diameter size, some, for example, show twice the flexibility in bending radius than others. Recent studies in CO2 systems have also proven that while some RTR products blister land fail within short periods of time in this environment, others are resistant even at higher temperatures. The inner plastic liner must therefore be carefully chosen to match the design parameters.

Subsequent developments. Since that first RTR lined steel installation in 1975, several improvements on the design and installation practice have been instituted:

Joining techniques. The tapered bell and spigot RTR joint has been verified as a sound integral connection.

Upon the application of epoxy to the tapered surfaces, the spigot end can hydraulic out of the bell end. Therefore, hydraulic come-alongs, which force the adjoining tapers together to a specified pressure, have been used to produce high quality joints. This procedure has been used by Unisert Systems up to diameters of 18 inches.

In terms of the final IT3 Multi-layer connections, joints have been eliminated even between the one-half to 1-mile segments with an overwrapped steel sleeve as illustrated in Fig. 6. These have been successfully installed in gas systems containing up to 26 percent H2S. Hence, cross-country pipelining without interruption of the RTR liner has been made possible.

Quality Control. For repair applications of existing systems, an elongated steel barrel to specified dimensions is initially pumped through the line to verify that the liner can be slipped in without exceeding its bending radius.

The plastic liner is generally subject to a three-step hydrostatic pressure test, on surface, after insertion and within the final system. Batch mixing of the grout has always been viewed as essential to the process to eliminate any variations in the properties from that tested. The grout flow cone test is correlated with more sensitive laboratory measurements of viscosity to provide a method of checking the grout properties under field conditions.

The acoustic cement bond log has also been tested as a method to verify that the annular space is completely filled with solid material. Electromagnetic techniques have been applied to locate any microcrazed areas in specifically the RTR liner. The combination of all these methods then ensures that the plastic pipe is constructed as a sealed unit within Unisert's IT3 plastic-lined steel system.

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