European program studies ways to fight internal pressure losses in gas lines.
One way to reduce the friction factor significantly is to coat a pipeline internally in order to obtain a relatively smooth surface. Several other techniques may be employed to further reduce the drag caused by the pipe wall even below those corresponding to an ideally smooth surface. The following advanced techniques are among those that could be used:
* Structured surfaces (riblets),
* Porous/permeable coatings,
* Specific coating materials,
* Lubricated walls,
* Compliant coatings,
* Pulsed flows,
* Re-laminarization, and
* Particle injection.
The IFP is evaluating and working on the improvement of the aerodynamic performance of some of these technologies.
Flow Drag Measurement
Conventionally, the performance of an internal coating is evaluated directly in a section of pipe. However, this test procedure requires a long length of pipe for reaching full flow development, large flowrate and volume storage of the test fluid, a boosting station and a long time for stabilizing the test loop parameters. These constraints are particularly costly when it is required to test coatings at high shear stress conditions (high fluid velocity and pressure).
To overcome these difficulties, IFP has designed and patented an apparatus working on a rotating principle requiring a small volume of gas providing almost instant equilibrium test conditions (Figure 4-the Rotating Cylinder Unit). As an example, 100 test points may be obtained in a few hours and a test section mounted in less than one hour. The apparatus may be operated at conditions simulating natural gas transport up to 250 bar and 10 m/s.
[FIGURE 4 OMITTED]
At an equilibrium condition, the drag resistance is measured either by a torque meter mounted on the shaft driving the rotating device or by a Pitot tube mounted in the test cell in between the rotating device and a fixed tube section coated internally. Tests are usually carried out, at first, with uncoated pipe sections made of steel with a broad range in wall roughness (0.1 to 16 microns in Ra) to get performance reference. To date, more than 40 different types of coated pipe sections have been tested. Figure 1 shows some results. Tests of pipe internal coatings have provided useful information, for instance:
* Pipe internal coatings present generally lower friction factor than steel based on the same wall roughness;
* Solvent-based, solvent-free, water-based and fusion-bonding coatings present different friction factor values based on the same wall roughness;
* The friction factor of a specific coating depends not only on the wall surface roughness but also on other surface texture parameters. As a consequence, it depends not only on the film thickness but also on the method used to apply the coating; and
* Internally coated pipes should not be sized on the physical roughness of the internal coating but on the equivalent steel hydraulic roughness in view of saving capital costs.
[FIGURE 1 OMITTED]
The benefit of pipe internal coatings and also pipes provided internally with structured surfaces has been evaluated economically for several pipeline configurations, including pipe with compressors at inlet and/or outlet, and several pipe design pressure and diameter cases.
Herein, the case of a 170-km pipe supplied with gas at a pressure of 150 bar absolute is considered, (Figure 2). Two parameters may be adjusted for minimizing the cost of the production system--the pipe diameter (from 32 to 40 inches) and the discharge pressure of a dowrtstream boosting station (120 or 140 bar abs). The results of an economical evaluation taking into account the cost of pipe steel, the fuel gas consumption over a 30-year period at the downstream compressor and the application of an internal coating, are presented below.
[FIGURE 2 OMITTED]
Case of a discharge pressure of 120 bar abs. When the hydraulic wall roughness is greater than 20 microns (case of a corroded pipe), the larger diameter pipe is more economical due to relatively small fuel consumption (small pressure loss) compared to the two smaller diameter pipes (Figure 3.a).
[FIGURE 3 OMITTED]
At 20 microns (slightly corroded pipe), the 36-inch pipe turns out to be the most economical solution. Applying internal coatings into these pipes provides different trends: the production system gets less economical in the case of the 40-inch pipe, rapidly less expensive in the case of the 32-inch pipe and very economical in the case of the 36-inch pipe (the optimum case).
Reducing the wall roughness from 20 to 2 microns provides a cost saving of 12.5%. Using structured surfaces (represented here by negative roughness) provides 17% cost saving (extra 4.5% gain relative to a coating with 2 micron roughness).
Case of a discharge pressure of 140 bar abs. When the hydraulic wall roughness is greater than 20 microns (case of a corroded pipe), the larger diameter pipe is the optimum choice (figure 3.b). It is only when coating the pipes internally that the 36-inch pipe becomes the most economical solution (below 8 microns hydraulic roughness). Contrary to the previous case, applying internal coatings reduces the overall cost of the three diameter cases. Reducing the wall roughness from 20 to 2 microns permits a cost reduction of 11% in the 36-inch case. Using structured surfaces provides 15.5% and 17% cost saving, respectively, for structured surfaces designed for 5% and 10% drag reduction relative to a smooth surface.
On the basis of these economic considerations, IFP has decided to undergo a program aiming at providing operating and services companies data to choose the right coating in order to reduce pressure losses in gas pipelines. This program--named ICARE (Internal CoAting for pressure loss REduction)--includes the study of conventional coatings, their evolution at the production stage and also new technologies. At the present time three companies are participating in this: BP, Total and Saipem. The program was launched in December 2003.
Following discussions with coating manufacturers and pipeline operators--and also based on IFP experience--an aging test program has been established. The program includes the evaluation of thermal aging, gas pressure blistering and the resistance to various chemicals, for instance, distilled water, hydrocarbons, tri-ethylene glycol, lubricating oil and pure methanol. These tests have been selected to evaluate coating performance under harsh environmental and process conditions. IFP will perform aging tests on:
* Coated coupons for analyzing the aging effects and comparison purposes between the coatings tested;
* Internally coated cylinders for further aerodynamic tests in the Rotating Cylinder Unit dedicated to the measurement of gas flow drag (Figure 4). By comparison with a brand new coating. the variation in flow drag will provide an indication of the aging effect (possible change in roughness and hardness).
Internally coated pipe presents little risk for corrosion except at non-coated welds. Corrosion products, gas impurities and, in a general manner, pipeline debris entrained by a high pressure gas with high velocity, may cause mechanical damage to an internal pipe coating.
IFP will build a test bench to simulate the erosion of small particles on plates coated with pipe internal coatings. Structured surfaces will be also tested in terms of resistance to erosion.
Traveling pigs are often used inside pipelines either for cleaning, corrosion inhibitor injection or for pipe inspection. In the latter case, the purpose of the pig inspection may be to evaluate the degree of corrosion, to measure the wall thickness or to detect dents along the pipe. Pig features depend on their application providing a large variety of pig types differing by their geometry, length, design and also the material used. Among this large variety of pigs some may induce scratches on the internal coating. While some scratches may have minor effects (small depth), others may not. IFP will simulate pigging operations with existing equipment now used for studies on tribology (solid friction analysis).
The abrasion will be simulated in a sequence of successive backward and forward movements with a constant load being applied between the abrasive element and the coating to be tested. Several coating types will be tested on the same basis, providing some means of comparison between them. IFP will also test coatings after aging in methanol and exposure to methane. Thus the resistance to abrasion will be evaluated for new and for aged coatings.
Structured surfaces will be also tested in terms of resistance to abrasion.
Tests in the Rotating Cylinder Unit of steel and coated walls have shown that the friction factor of a pipe was greatly reduced not only by smoothing the surface of a pipe wall but also by interfacing the fluid with some drag reduction material. Several thousand kilometers of existing gas pipelines, not presently internally coated, could transmit a larger gas flow (reducing operating cost) if a coating were applied inside the pipeline. There are a few cases where coatings have already been applied on installed pipelines. However, the coating application reasons were mostly for corrosion protection purposes. Coating applications on installed pipelines involved relatively short pipelines.
As a consequence, a general survey of the literature will be performed considering, for instance, the technological problems posed by the cleaning of old pipelines and the need for rapid curing of the paint for a prompt return to service.
This task focuses on the aerodynamic performance (further drag reduction) of existing structured surfaces. A state-of-the-art review on structured surfaces has shown that a drag reduction on the order of 7-12% relative to a smooth surface could be obtained depending on the cross-section shape of these structures. This performance improvement, obtained with conventional shapes (two-dimensional surfaces), is considered very reliable due to extensive testing over the last 20 years. However, the drag reduction could still be improved by bringing some modifications to the existing configurations, for instance:
* In associating 2-D structures with different dimensions to provide attenuation of energy dissipation corresponding to turbulence eddies of different sizes,
* Or in designing 3-D structures to reinforce the turbulence control not only in the transverse direction but also in normal and longitudinal directions.
To optimize the shape of structured surfaces, three main types of computational fluid dynamics (CFD) codes are available--Direct Numerical Simulation (DNS), Large Eddy Simulation (LES) and Reynolds Average Navier Stokes (RANS) codes. IFP has been carrying out studies aiming at validating one RANS commercial code on the basis of previous experimental works. The flow simulations have shown that some turbulence models are suitable for predicting the main characteristics of structured surfaces (drag reduction variable with the flow) and also induced secondary flows (Figure 5). IFP will then test LES codes due to the inherent limitation of RANS codes for more complex structured shapes. Optimization tools are also considered to get maximum performance for some specific shapes.
Manufacturing of micro structures. A significant drag reduction may he obtained by providing micro structures with suitable shape and dimension on the internal surface of a gas pipe. As such, work has been carried out on the manufacturing of micro structures for gas pipes with pulsed lasers, (Figure 6). Because of the numerous technical issues to overcome and the relatively high cost corresponding to the development of an industrial tool suitable for printing micro structures inside a gas pipe. IFP is proceeding in successive steps. In the framework of the ICARE program, it is intended to demonstrate the printing feasibility of micro structures on flat and circular plates. In the future, IFP will focus on mold replication, then on the design of a tool suitable for manufacturing structures in a 12-meter-long pipe.
[FIGURE 6 OMITTED]
Other advanced surfaces. This task includes theoretical studies aiming at determining the characteristics of a porous/permeable coating or of a pulsed flow in view of further wall drag reduction and some experimental works aiming at evaluating the aerodynamic performance of special surfaces--low friction materials and lubricated walls or of flow re-laminarization. Other items may be studied in the framework of the ICARE program, such as:
* Tests of pipe internal coatings with different types of hydrocarbon gases (methane for natural gas transport and ethane for simulating dense phase production);
* Tests on industrial sites of selected pipe internal coatings following testing in the Rotating Cylinder Unit:
* Improvement of the aerodynamic performance of internal coatings:
* Effect and characterization of internal coating undulations: and
* Pig development for the measurement of the internal hydraulic roughness of a pipe.
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|Title Annotation:||Internal Roughness|
|Comment:||European program studies ways to fight internal pressure losses in gas lines.(Internal Roughness)|
|Author:||Charron, Yves; Mabile, Claude|
|Publication:||Pipeline & Gas Journal|
|Date:||Jun 1, 2004|
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