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Technical Information

The IT3 System: A Tertiary Materials Concept

Compressed Liner Classification

Typical Applications

Economic Advantages

Design Levels

Quality Control Measures

Liner Material

Outer Case Material

Annular Material

Centralization

End Connections

Termination & Transition



The IT3 System: A Tertiary Materials Concept:

The concept which Unisert focused on proposed amalgamation of three materials, a cementious structure sandwiched between a plastic inner layer and steel or metal outer layer. The patented design attacked for the most part the growing concern for internal corrosion control in oilfield pipelines, which is one of the more involved problems to define and solve due to intrinsic difficulties in accessibility, distance and monitoring uncertainty. The design’s great advantage, however, is that the implementation method eliminates short-length joints, a troublesome area for many multi-wall piping systems. Essentially speaking, a pipeline is slid into a pipeline, after which a cement slurry is pumped through predetermined lengths to completely fill the annulus between the inner plastic tube and outer steel case. The major art and science of the procedure involves controlling the flow characteristics and properties of a non-Newtonian fluid, the cement slurry, to assure complete continuity of the stress transfer mechanism. Our patented techniques, based on decades of experience, are the only ones known to render an extremely high rate of success.

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Compressed Liner Classification

In terms of the plastic inner tube, the design is classified as a compressed liner concept since the plastic-steel connection is not achieved through deformation of the polymer component. Rather, the design inserts an incompressible material under pressure at the interface thereby totally limiting radial strain of the inner polymer tube. The cement layer serves to transfer the majority of the stress to the outer metal component
as well as acting as a continuous enclosing shield to perfectly anchor the plastic line.

Because of the extremely low stress in the tensile mode, initiations of microfissures in the plastic liner are entirely avoided. This allows numerous polymeric materials to be successfully implemented without endangering their structural integrity.

Very few problems have been experienced with this concept, which can be partly attributed to the large safety factor that characterizes the compressed liner concept. Another contributing factor is that the concept has built into it a secondary corrosion defense system, the intermediate structure essentially forming a cement-lined pipe system so that the basic pH of the lime also serves to inhibit the steel.

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Typical Applications

The objective of most multi-layer systems is to combine a number of properties that are difficult to obtain in a single material. One of the major purposes of the IT3 multiwall system is to combine the corrosion resistance of polymeric materials with the mechanical stability of metal pipe.

Typical applications for the Unisert IT3 multiwall piping system include:

  1. New pipelines where internal corrosion-erosion resistance is required. Within the oil industry, this normally refers to fluids having a high pitting potential generated by the chloride ion and corrosive gases such as O2, CO2 & H2S. Much of Unisert’s work has revolved around produced water gathering and injection/disposal lines and multiphase oil/gas lines with rising water cuts.

  2. In-situ repair/rejuvenation projects where the existing pipeline has experienced a corrosion problem and/or accessibility to the existing pipe is hampered. Other in-situ rejuvenation candidates include pipelines located in congested pipeline alleys, in rocky or swampy terrain suffering high excavation costs, or pipelines buried underneath gas plants, buildings, highways, built-up, and environmentally sensitive areas. In all of these cases it becomes more economical to expose both ends of an existing pipeline, insert the plastic liner and grout the annular space in-situ.

  3. Offshore pipelines, to which the IT3 system is particularly well suited. The weight of the internal grout layer replaces or supplements the requirement for conventional weighting designs such as thicker wall steel, external concrete coating or external point anchoring. Hence, a completely corrosion resistant pipeline can be installed at initial capital costs competitive to conventional practices but with the subsequent elimination of long term expense items such as repair and inhibitor treatment. In 1999 the S-SK Collar joining mechanism was developed to allow for connecting the internal steel carrier pipe without the need for welding.

  4. Double-wall containment systems. In cases where the DOT and other governmental entities have required double-wall containment, the new IT3 system has been approved for this type of service. The IT3 system can be used in place of typical steel-inside-steel pipelines.

  5. Special, non-traditional applications. The IT3 system has been configured to address external corrosion caused by unique conditions such as a high water table and freezing ground. Insulation properties can be built directly into the annular space, thereby avoiding the use of external insulation, which can shield the cathodic protection system and render it ineffective.

  6. Leak detection for the Unisert Multiwall Systems, Inc. (Unisert) IT3 system is very simple. The cement grout in the annular space is permeable. This allows the annulus to be pressurized along the entire pipeline from a single point. Tapping the steel case provides access to the annular space as points along the pipeline provide locations for installing pressure transducers.

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Economic Advantages

The economic advantage of polymeric materials in oilfield applications have long been recognized; the point is that the IT3 system considerably reduces some disadvantages, chief among which rank their somewhat lower mechanical strength and their viscoelastic nature. In other words, the IT3 multiwall system enables utilization of polymer materials at pressures as high as 3500psi as well as with other forms of more severe mechanical loading. The resistance of the IT3 multiwall system to the electrochemical corrosion mechanism unquestionably offers an extended life in many applications which implies not only a savings in equipment cost but also in the expense associated with environmental cleanup.

Moreover, the system inherits the benefit of energy conservation from the smooth, friction reducing plastic surface. Manifestly, there is also the more intangible benefit of increased reliability and hence greater production output in the customers’ operations. Situations where the IT3 system out-performs bare steel pipelines (even aided by various other corrosion control methods) by two or three times have not been uncommon.

One of the major uses of the concept has been the rejuvenation of bare steel pipelines. Since most pipeline failures result from localized corrosion with perhaps only a moderate degree of general corrosion being present, it still becomes technically feasible to utilize the steel pipe. Once any leaks have been repaired, as the outer case of the IT3 multiwall system instead of discarding the used pipeline altogether. Under such conditions, the IT3 system provides a 2-defense corrosion control program over and above that. Although it is economically invalid to compare the bare steel replacement alternative with the IT3 system because the end results and benefits cannot at all be treated the same; nevertheless experience testifies that the concept still competes favorably with bare steel replacement cost. This is largely due to two factors: since the end points and cementing locations need only be accessed, the formidable expense of reditching and reclamation are almost entirely avoided. Pipe material and joining costs are considerably reduced since only a plastic pipeline need be purchased in a repair installation.

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Design Levels

Unisert evaluates each piping project individually and employs one of three Design Levels based on the project parameters.

I The liner is designed to handle the system operating pressure and criteria without relying on the existing outer case pipe for pressure containment. The outer case pipe is used primarily as a conduit for the liner and to provide structural support and impact protection.

II Operating the liner up to its short term burst pressure capabilities. The primary reasoning behind this design level is that in FRP pipe, there is a 5-1 to 7-1 safety factor designed into the piping system to compensate for cyclic loading. The failure mode in this case is the shear between the glass and the resin due to yield of the pipe, which creates the shear. Therefore, the liner being in a compressed mode in the multiwall structure does not see that strain, therefore negating that particular failure mode.

III Operating the liner above its short term burst pressure capabilities. This also refers to the level of quality control measures taken during installation. The outer case pipe (steel in this case) is used to handle the pressure containment for the system.

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Quality Control Measures

Unisert employs strict quality control measures in all phases of the process, from design evaluation and review to materials acquisition, fabrication and installation. All liner materials and fittings are inspected and pressure tested according to manufacturer recommendations and Unisert standards prior to installation. These materials are then pressure tested again in accordance with local, state and/or federal requirements before a new system is brought online or an existing system is placed back into service.

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Liner Material

Selection of the appropriate liner material is determined based on specific project parameters, taking into account the type of gas or fluid stream, the structural integrity and condition of the outer case pipe, temperature and pressure requirements, and the need for abrasion or chemical resistance. While selected liner materials generally include FRP (fiberglass reinforced plastic), PE, PB and PVC, the liner material most often specified is fiberglass, especially for in-situ installations where the steel outer case pipe is corroded.

With a new and ever-expanding range of thermoplastics available, temperatures up to 350° F can now be achieved.

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Outer Case Material

Any material that has a higher rigidity (function of modulus of elasticity and thickness) than the inner liner can be successfully used as the outer case for a pressure multiwall pipeline. The intent is to force the more flexible liner against a more rigid case from internal pressure thereby enhancing the lock between the two pipes. Corrugated steel, Steel, wrought iron, concrete cylinder, aluminum and fiberglass pipe have all been successfully lined.

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Annular Material

A neat organic Portland cement grout comprises the annular material. It was selected for the following reasons:

1. High modulus of elasticity in compression (2.0 - 3.0 x 106 psi) and overall high compressive strength (8,000 - 10,000 psi). Deformation of the annular material translates into a hoop strain on the liner and relatively high compression values greatly restrict the magnitude of that stress.
2. Low shrinkage value - drying shrinkage of grout trapped between two impermeable forms is negligible. If required, additives can be mixed with the grout to expand its volume.
3. Hydraulic set - external heat is not required to cure grout, a factor especially advantageous for field installed repair projects.
4. Similar thermal expansion to steel.
5. Optimum flow properties.
6. 2nd corrosion defense system - the high pH of the grout passivates steel and therefore acts as an inhibitor between the plastic liner and steel.
7. Economical and readily available - inorganic cement is still one of the least expensive materials in the market.

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Centralization

One of the questions that had to be considered in the design of the IT3 system was "Did the liner need to be centralized to maintain effectiveness of the concept?"

Without centralizers, a buoyancy effect, arising from a plastic tube being immersed in a cement slurry (specific gravity 1.8), tends to float the liner upward just after the liner has been grouted in. In regards to rejuvenation applications, however, most of the localized corrosion occurs at the bottom of the pipeline since aqueous media tends to concentrate in this area and also due to the very mechanism of pit propagation. Hence, it very often becomes an advantage to retain a greater thickness of cement grout on the bottom of the pipeline.

Moreover, the cement thickness affects the stress state of the plastic liner very little. It was found that, with a uniform cement thickness of .75", a PVC pipe bears a stress of 160 psi, which is well below its plastic limit of 2660 psi. If now the liner is not centralized even in the extreme fashion of the corresponding diagram, the stress in the plastic liner varies insignificantly from 175 psi at the top to 150 psi at the bottom. In these situations, therefore, it even becomes preferable to avoid the use of centralizers. However, with other more peculiar conditions, centralizing methods are utilized.

Mechanical centralizers are used in the fabrication of new IT3 plant manufactured pipe. Centralizers are primarily used so that double random length joints of pipe can be field cut to length, thus insuring that all pipe can be matched and lined up properly during the installation process.

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End Connections

Five types of IT3 pipe end connections are available: the SK, SL, SF, S-SK and S-CF. The S-SK, unique to other connectors, is available for joining steel carrier pipes in doublewall steel systems and is used primarily for offshore piping systems. The S-CF, developed for wastewater and storm sewer applications, is a coupler for gravity flow systems.

SK - series quick-coupling - features a single butt weld and male nipple. The male nipple consists of a fiberglass collar supported with a steel collar. This interference fitted connection joins random lengths of pipe. Its main property is efficiency and speed.

SL - series in-line Features a steel split sleeve or slide-over sleeve around the plastic pipe. The annular space is then grouted to produce a continuous plastic lined steel system. This connection is used primarily to join the long lengths of existing pipe that have been lined. Its main property is that it does not require stringent dimensional control and that the steel pipe need not be moved to effect the joint.

SF - series flanged packer Consists of a mini-flange joined to the liner and inset within the steel flange. The mini-flange acts as the new face but the steel flange provides the strength for pressure rating and bolt torque. The mini-flange can be machined from plastic or stainless steel stock. Its main property is that it’s the simplest connection to other piping systems.

Unisert IT3 S-SK Bonded Steel Joint

S-SK series quick coupling bonded steel joint Constructed of steel and featuring a locking-thread serration and slight taper, this interference fit joint is bonded with a specially formulated epoxy compound. The "locking-thread" serration provides minimum resistance during insertion and excellent resistance to separation. This serration design also provides excellent geometry for epoxy fill, sealing and lap shear resistance. An O-ring provides an additional measure of protection.

Unisert IT3 S-CF Corrugated Steel and Fiberglass Joint

S-CF - series corrugated steel and fiberglass joint Consisits of a bell and spigot configuration featuring a rubber gasket seal. A guide ring is affixed to the exterior of the corrugated steel pipe on the fiberglass pipe bell end of the joint. The guide ring assists the final alignment during insertion/coupling and provides continuous support and protection at each joint.

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Termination and Transition Connections

The illustration shows how a newly lined segment can be transitioned back to an unlined segment or connection point on an existing line. Other termination connections, including both welded and flanged, are use to tie-in lines.

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