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№ 11 (November 2009)

DuPont Advanced Technology in Pipeline Performance

   Specific features of Russian upstream include chemical composition of the produced oil and its watercut, which results in rapid corrosion of on-site steel pipelines – on some fields, up to several millimeters per annum. Naturally, the sheer quantity of leaks forces oil professionals to develop efficient rust protection methods.

By A. Ioffe, marketing representative, DuPont Science and Technologies, A. Sazonov, project manager, POLIPLASTIC Group

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   The type and rate of corrosion are influenced by the properties linked both to the nature of the produced medium and operating conditions. Most important of these include composition and properties of oil and co-produced associated gas and water, content of oxygen, carbon dioxide and hydrogen disulfide, presence of sludge-forming and abrasive particles in field-produced mediums, oil/water phase relation and hydrodynamic parameters of gas-water flow, contamination of field-produced mediums with corrosive organisms.

   Currently there are several options for reducing the accident rate of industrial pipelines:
– Inhibition, when water-dispersible, water- and hydrocarbon-soluble inhibitors are introduced into the transported medium; this protects only internal surface of the pipe and requires enormous quantities of inhibitor.
– Coating with corrosion-proof inner and outer coatings. In this case there is no protection for welded joint, lower maintainability and the looming danger of collapse of the inner polyethylene pipe during exploitation of the coated pipes.
– Usage of non-metallic pipes.

   Investigating flexible plastic pipes, as the means of corrosion protection at oil production sites, one must consider that in parallel to being resistant to chemicals, such pipes must endure high pressure used for reservoir-pressure maintenance and transfer water-in-oil emulsion and sewage water. This resistance level is achieved by reinforcing the polymer pipe with high-duty reinforcing element. Specific feature of reinforced plastic pipes exploitation is that the main load falls on the reinforced layer, so slight soaking of internal layer won’t violate structural integrity of the pipe. When choosing the type of reinforcement, due consideration must be paid to wall erosion depth by the aggressive agents, as in this case the reinforcing layer could be open for corrosion and disintegration. Metal-free system of pipes and joints reinforced with aramid fiber (such as Kevlar®) presents the most safe and reliable structure. The Kevlar® fiber is resistant to virtually any type of organic chemicals and to high temperatures, its strength properties are similar to those of steel, but while having same tensile strength, the fiber is five times lighter than steel.

   Experts recognize as optimal the construction of the pipe consisting of inner airproof polymer coating, outer surface of which is coated with two layers of reinforcing fiber under the angle ±α to the pipe axis (Fig. 1). The reinforcing layer is covered with another polymer layer, which protects reinforcement system from mechanical damage while ensuring overall integrity of the construction.

   Used as reinforcement system, high-duty high-modulus material such as Kevlar® would absorb all stress-load coming from internal pressure. Due to low deformation of the system, the load on polymeric layers is insignificant. Due to this the strength properties (both long- and short-term) are defined primarily by the respective fiber properties, while the parameters for the reinforcement system (fiber durability and quantity) may be selected without consideration of strength and deformational properties of the polymeric layers.

   Pressure exerted by the reinforcing layer onto the pipe is governed by the equation: where N =  – number of fiber turns per unit of pipe length, dвнутр – inner diameter of the pipe, s – thickness of the inner layer, q – the number of fibers.
Hence: fiber durability.

   As known, on the condition of tg(α) =√ 2 and α = 54°44’, two layers of fiber arranged in mutually antithetic direction ensure strength balance of the reinforcing system regarding internal pressure in both axial and radial directions. Here, to improve durability of reinforcing, makes sense to increase the angle of fiber laying, that is, to choose α > 54°44’. In this case, radial component of the durability of reinforcing increases while axial – goes down. To compensate reduction of axial component of the durability of reinforcing, extra fibers laid parallel to longitudinal axis are required.

   The shown method of analyzing the reinforced pipes allows calculation of “disintegration” pressure, i.e. the rate evaluated at testing the sample with pressure which grows at constant rate.

   As known, for pipelines based on polymer materials, required level of continuous durability is calculated taking into account temporal factor of tensional impact from internal pressure. There is globally standardized method of defining operating pressure by strength properties of pipeline material (MRS), pipe geometry and exploitation temperature.

   One of the leading producers of engineering plastic and polymer pipes is POLIPLASTIC Group – long-standing strategic partner of DuPont in Russia.
The range of polymeric pipes produced by the Group covers practically every pipe type for outer networks of utility piping: pressure pipe for gas and water supply, free-flow contoured pipes for water disposal and sewage, flexible heat-insulated pipes for heat supply, as well as various types of pipes tailored to difficult construction environments.

   Technology for reinforcing pipes with synthetic fiber has been used as a basis for creating unique pipeline system ISOPROFLEX® А (Fig. 2), designed for outside heating systems. As a result, we created multilayered pressure pipe on the base of cross-linked polyethylene РЕХ-а with reinforcing system from Kevlar® fiber, which has good durability and heat resistance. This construction has no analogues anywhere in the world, neither by the production range (pressure pipe diameter up to 160 mm) nor by operating parameters (operating pressure 1.0 MPa at the temperature of up to 125 С).

   Design and development of new polymeric pipes, as well as expansion of application range, are the key tasks of the pipe department at the POLIPLASTIC Group. The company views oil&gas industry as highly prospective market for these innovative products.

   The company has developed full range of piping for oil&gas segment (supplied with all required profiles and, if needed, installation equipment), namely, for developing shift camps and partly production sites – installation of gas, water and heat supply systems, sewage and water drainage works. As practice shows, installation of polymer-based pipeline systems guarantees significant – several-fold – cut of construction time for utility networks, in parallel improving their reliability and cutting installation cost.

   For pipelines used on the production site, the company selected РЕХ-а cross-linked polyethylene as the base-layer material ensuring structural integrity of the pipe and providing better than standard polyethylene chemical and temperature resistance. Calculation for reinforcement systems is done using the above method, basing on temperature-time strength curve of Kevlar® fiber.

   Already received results indicate that in the nearest future oil professionals will be offered flexible pipes for oilfield systems – these will be resistant to formation fluid and rated for 4.0 MPa operating pressure. The company invests much in developing high-pressure pipes rated for operating pressure of 10 MPa and higher, which will be used in reservoir-pressure maintenance systems. Tests have been completed on usage of cross-linked polyethylene pipes reinforced with Kevlar® fiber for high pressure gas pipelines (2.5 MPa pressure, up to 160 mm diameter); relevant approval documentation is being obtained.

   In parallel to being used in flexible reinforced pipelines, high-duty Kevlar® fiber is also widely used in self-supporting fiber-optic cables for reliable communications throughout oil&gas industry facilities, as well as in flexible logging cables. Such cables aren’t armored with steel mesh coat and usually come as flexible sealed cables with varied set of power and signal connectors, hydrolysis-resistant PBT fiberglass module Crastin® and load-carrying element based on high-duty aramid fiber Kevlar®. The overall product range is extensive, while area of application includes ground and offshore physics, aerophysics, geoelectric prospecting, shelf exploration, robotics, towage of under- and over-water equipment, etc. Of special importance are the cables with neutral or positive floatability, used for offshore geoelectric prospecting. The listed areas of application require such cable properties as high-speed information transfer, including video etc., flexibility, wear resistance of the cable coat to frequent reeling of the cables, low unit weight, presence of load-bearing element and, usually, axial hermetic sealing. The combination of high-level strength properties, resistance to cyclic loads, flexibility, and low weight of Kevlar® aramide fiber magnify manyfold the functional quality and service life of the cables.

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