Oil & gas Corrosion Protection – Underground pipeline protection strategies

Oil & gas Corrosion Protection – Underground pipeline protection strategies

9:39 AM, 16th April 2018
Oil & gas Corrosion Protection – Underground pipeline protection strategies

By Professor A S Khanna

Introduction

In today’s world, production of various petroleum products, such as petrol, diesel, compressed gas (CNG), and several other hydrocarbons, decide the economy of a country. Pipelines are the integral part of oil and gas industry. After exploration of crude/gas from sea, it is transported to distribution terminal thru’ under sea pipelines and from this terminal, then the crude is transported to storage tanks, refineries and petrochemical plants, and the gas is transported directly to utilities such as power plants, fertilizer plants, homes, hotels and industries. Pipelines, therefore, are the essential component of the oil and gas production. In order to assure the safety of the people and environment, utmost care is taken, while designing, commissioning and operation of these pipelines.

Safety issues of the pipelines are directly related to the internal or external surface of pipelines. While the internal safety is directly related to the crude and gas quality and on the condition or preparation of internal surface to avoid issues such as drag which affects the flow of the fluid. External surface poses even greater safety issues, mainly related to the corrosion of the pipe from various kinds of terrain. Thus, transportation of crude/gas through pipelines today is a very big issue and has also become multi-disciplinary and with huge expenditure.  In order to make the pipelines safe, metallurgy of steel, design of pipelines, internal flow characteristics of the fluid, health monitoring, maintenance,  corrosion management  and pipeline integrity,  are involved.

Prerequisites to Pipeline Selection

The following are some of the initial requirements that need to be fulfilled before the transporta­tion of crude is actually made operational :

Type of product to be transported: this often sets the parameters for engineering and routing of the proposed pipeline.

Survey of the route through which the pipeline has to pass: for example, details on the route and their locations, dimensions, and so forth; sea; roads (crossing and alongside the route); rivers; drains; pipelines; bridges; rail tracks; 

The materials used for pipelines: Steel is the only material that can be used to construct pipelines. Over the years, several modified steels have been designed to enhance strength as well as specific corrosion problem such as stress cor­rosion cracking, sulfide corrosion, and so forth. A list of various steel formulations is given in Table 1.

Table 1

 

Corrosion Problem in Pipeline

Corrosion cannot be completely avoided but can be reduced considerably and the effective lifetime of the piping system prolonged if its cause and development are known and the piping system is installed and operated with this knowledge. The schematic of pipeline corrosion can be represented by the diagram shown in Figure 1.

Fig. 1 Schematic of an Underground Pipeline

As can be seen, pipelines suffer from both internal and external corrosion. Internal corrosion is due to presence of sulfur-bearing gases such as hydrogen sulfide, carbon dioxide, and moisture, which is entrapped as brine from the sea. The usual rule of thumb is that the internal coatings are applied only if the crude/gas is sour in nature (its hydrogen sulfide concentration is more than 500 ppm). Otherwise, the internal corrosion is usually tackled by addition of inhibitors, either con­tinuously or in a batch process.

The main problem is the protection of external surface, mainly due to the corrosive nature of the soil. Across the pipeline route, the soil can be soft mud, hard mud, rocks, sand, sand with minerals, moisture, and other environments such as marshy lands, brackish water, seawater, and so forth. Ordinary steel with several compositions, shown in Table 1, shows severe corrosion, general corrosion, pit­ting, or stress corrosion. One kind of serious corrosion problem that takes place is microbiological corro­sion, which occurs due to presence of sulfate-reducing bacteria (SRB). They convert sulfate in soil to sulfide, which attacks steel, causing severe pits. Thus, external corrosion is taken care of by a combination of a good coating and cathodic protection. Let us now deal with internal and external corrosion in more detail.

Selection of Protective Coating For Underground Pipelines

Coatings are  natural choice to create a barrier for the corrosive environment in soils. Over the years, there have been different coating materials and formulations used to protect pipelines. For example, in the 1940s and 1950s, coal tar, wax, and vinyl tape were used; in the 1960s, asphalts were used; and in the 1970s to the present day, fusion-bond epoxy (FBE) was and is being used. Polyethylene (PE) tape and extruded PE jacket material also have been used from the early 1950s to the present day.

It is most important to note that the coating alone is not a permanent solution for the corrosion protec­tion of pipelines as the coating deteriorates with time while interacting with environment. Hence, to take care of this degradation in the underground pipelines, an additional protection using cathodic protection is given. A total pipeline protection system thus includes consideration of steel quality, coating applica­tion, surface condition and treatments, design of coating, and cathodic protection system.

Pipeline Coatings

Though there are many types of coatings that have been applied on buried pipelines, the three main coatings commonly used for pipelines are coal tar, FBE coating, and three-layer PE (3LPE) coat­ings. It is important to discuss the salient features of these coatings, while extensive information on the same can be found elsewhere.

Coal Tar Coatings

Coal tar enamel (CTE) protective coating systems have been used to protect underground and subsea pipelines from corrosion for decades. Its ease of application, low cost, compatibility with cathodic protection, and proven performance in the field for over 80 years make it a popular choice of pipeline companies worldwide. Its resistance to water absorption, hydrocarbons, soil chemicals, and bacteria is excellent. The CTE coating system has evolved into a sophisticated application of primers, multiple grades of plasticized enamel, and high-strength-resin-bonded glass fiber wraps. These improvements have resulted in a CTE coating system with greater bendability, improved handling characteristics, an increase in the temperature exposure range from −28°C to 80°C, and a lower safe handling temperature of −10°C [1]. The most serious drawback of CTE coatings is the emission of carcinogenic vapors during its applications, which not only threatens the workers carrying out the application but also pollutes the environment. Many Western countries have completely banned the use of this system. In India, CTE is still being used sparingly by some oil companies. The most commonly adopted CTE coating system, in use within the country, is CTE applied often over a primer and at times with two or three layers of fiberglass reinforcements.

FBE Coatings

Out of various organic coatings, epoxies have the strongest resistance to oxygen, moisture, and chlorides, which are important constituents of soil. Further, they are highly insulating with very low conductivity and high dielectric resistance. That is why epoxies are the preferred choice where strong corrosion resistance is the main requirement. There are many ways by which the epoxy coat­ings can be applied: brush, spray, using liquid epoxies, or electrostatically spraying the fine epoxy powder on a heated pipe, which immediately melts it and fuses instantly.

The coating has good cathodic disbondment resistance, hot water resistance, and good flexibility (5° of pipe diameter) at −50°C. FBE coatings are thermosetting compounds, which, once set, cannot be remelted. The most important requirements of the coating are surface cleanliness, proper heat­ing, and sufficient cure. The first step is the blast cleaning of the pipe to Sa 21/2, followed by heating the pipe uniformly using an induction furnace [2]. This is followed by electrostatic spraying of FBE powder, which immediately melts and fuses. The hot coated pipe is quenched immediately. The temperature at the pipe surface usually ranges from 180°C to 210°C. The coating thickness depends upon the pressure of the FBE powder, electrostatic voltage, and conveyor belt speed. A 350–500 μm coating is required from a pipe diameter of 8 to 36 in.

Multilayer Coatings

One of the drawbacks of such a thin-coated pipe is its damage during transportation and handling. In the mid-1980s and in the beginning of the 1990s, two additional coating systems were discovered. The first was the 3LPE system and the other was the dual-layer FBE system. The purpose of both these systems was to enhance the damage resistance of the single-layer FBE coating, described above. In 3LPE, it was achieved by the application of an extruded PE coating of 1500 to 3000 μm over the FBE primer layer of 100–150 μm. Since it is not possible to directly coat a PE layer over an already-coated FBE layer, an intermediate adhesive coat of polyolefin is made, which adheres primer to the FBE layer through its polar functional groups and to PE by its hydrocarbon groups [3]. The temperatures required to coat an adhesive layer and PE layers are, respectively, 220°C and 238°C, and the two coatings must be applied within a small time interval of 13–25 s (depending upon the pipe surface temperature).

In dual-layer FBE coating, an outer layer of FBE powder of different composition is applied over the primer layer [4,5] The purpose of the inner layer is to have strong adhesion to the pipe, while the outer layer is expected to be very tough to have high impact resistance. 3M made a coating system where the outer layer achieves maximum toughness because of the cellular structure of the coating. The method of application is very similar to that of a single FBE coating, and coating can be carried out in a single booth, with some guns carrying inner-layer powders and others carrying outer-layer powders, placed in such a way that the outer powder sprays after the inner powder is sprayed and is still not gelled. The coating of the inner layer is generally fixed at 250 μm, while the thickness of the outer layer varies with the diameter of the pipe, 250 μm for 8–22 in. pipe diameter and 350 μm for a pipe of diameter above 22 in. An approximate comparison of various properties for the above three coatings is given in Table 8.3.

In addition to these three main coating systems, several other coatings are also applied on pipe­lines. For example, for high-temperature fluids, polypropylene coating instead of PE is used. Other coatings such as liquid epoxy, polyurethane, tapes and wrappers, and now polyurea are being used for underground CCP coatings. All these four coatings mentioned, though, are commonly used for rehabilitation jobs and girth weld rather than for new pipelines.

Protection of Pipeline by Cathodic Protection

As discussed above, coating a pipeline does not finish the protection. Complete protection requires cathodic protection (CP) also. For a CTE-coated pipe, one needs a CP station after every 10–20 km; for an FBE-coated pipe, the requirement is one CP station after every 100 km; while for 3LPE, one CP station after 200–300 km is sufficient. Also, the current requirement goes on decreasing with an increase in the quality of coating. Thus, the best coating requires low electric current to sustain effective CP. In order to maintain CP, one needs a temporary power station near CP stations. The CP current requirement cannot be dependent on the grid power, especially for a country like India. In addition, regular monitoring of the CP station is required to know the health of the pipeline [6]. This makes CCP corrosion protection a very interesting and challenging subject.

Field Joint Coatings

The field joint coatings are the coatings that are used to join individual pipes, coated in the yard, just before laying them in the ditch. Such coatings are also called girth weld coatings as these are applied on the weld joint between the two coatings [7]. Since welds are vulnerable areas, prone to be attacked by corrosive species, they need to be coated with special care to enhance their resistance to corrosion while at the same time maintaining their compatibility with the main pipeline coating. Though there are many coatings that can be applied at field joints, such as liquid epoxy, elastomeric polyurethane, cold-applied tapes, and viscoelastic coatings [8], the most common and acceptable field joint coating is the heat shrink coating. These are basically the rubberized coatings that, on application and after heating shrink, hold the pipe. The first step is full cleaning of the girth weld area, followed by application of a primer epoxy coat. Uniform heating is important to make an excellent coating.

References

  1. W.R. Roder. Coal tar enamel the protective pipeline coating of the past, present and what’s new. Proceedings of the PIC on Conference, May 20, 2000, Canada.
  2. Performance of FBE on pipelines at operating temperatures above 120°F. White paper prepared for Joint Industry Project (JIP) Committee on Alternate Design with Life Cycle Management, May 16, 2008. www.regulations.gov/search.
  3. J.J.W.B. Cox. Three layer HDPE exterior pipelines coatings: job reference and case histories. Proceedings of the 14th International Conference on Pipeline Protection, Oct. 29–31, 2001, Barcelona, Spain.
  4. A. Kehr, M. Dabiri and R. Hislop. Dual layer FBE coating for pipeline. http://alankehr-anti-corrosion.com/Technical%20Papers/Dual layer%20fusionbonded%20epoxy %20(FBE) %20coatings%20protect%20pipelines.pdf
  5. A.S. Khanna. Dual fusion bond epoxy coatings—versatile solution for underground pipelines. Chemical World, 2010, 52–54.
  6. D. Wessling. Capabilities and limitations of techniques for assessing coating quality and cathodic protec­tion on buried pipelines. Proceedings of the Cathodic Protection Theory and Practice Conference, 2002, Sopot, Poland.
  7. J.F. Doddema. Self healing visco-elastic anti-corrosion system for critical infrastructural objects such as oil and gas pipelines. Paint Coating News, 2011, 7, 74–78.
  8. J. Duncan and G. Friberg. The definitive field joint coating—is there or can there be such a thing. Proceedings of the 14th International Conference on Pipeline Protection, Oct. 29–31, 2001, Barcelona, Spain, pp. 129–138.

Author: By Professor A S Khanna is from the Department of Metallurgical Engineering & Materials Science, Indian Institute of Technology (IIT) Bombay.

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