POLYPROPYLENE SYSTEM SCOPES HIGH AS PIPELINE ANTI-CORROSION COATING

Dec. 14, 1992
L.R. Aalund Managing Editor-Technology Polypropylene is attracting increasing attention as a highly desirable component in oil and gas pipeline coating systems. Though long-term in ground or underwater experience is limited with such systems, actual completed projects have been in the field since 1986.
L.R. Aalund
Managing Editor-Technology

Polypropylene is attracting increasing attention as a highly desirable component in oil and gas pipeline coating systems. Though long-term in ground or underwater experience is limited with such systems, actual completed projects have been in the field since 1986.

The earliest anti-corrosion coatings for buried pipelines were bitumen-type coatings: asphalt mastic and enamel, and coal tar enamel. The asphalt coatings absorb water to a greater degree than the coal tar enamel coating, but both are subject to cracking, leading to contact of water with the pipe, and coating disbandment.

Later, epoxy-based coatings were developed. These provided good adhesion to the steel pipe but suffered from poor impact and abrasion resistance. Initially, low density polyethylene was used as an outer coating to protect the epoxy.

Polyethylene is still considered an excellent outer coating but has high temperature limitations.

POLYPROPYLENE

Both polyethylene and polypropylene coatings offer highly desirable qualities, with polypropylene holding, as noted, an advantage at higher temperatures. But neither alone, at present, is the answer to corrosion control. Both are being applied over an initial layer of fusion bonded epoxy (FBE) coating.

MOBIL PROJECT

The reasons were recently detailed by Richard Norsworthy and J.A. Nunn of Mobil Pipe Line Co. in a paper presented at the American Petroleum Institute's pipeline division meeting. Mobil is installing some 140 miles (225 km) of what will be the longest polypropylene coated line laid to date.

The corrosion specialists say that Mobil's efforts to find a coating that will withstand temperatures from 150 to 210 F. (65 to 99 C.) led to the development of the Du-Val coating system. It employs a two-layer coating: a chemically modified polypropylene (CMPP) over FBE.

FBE and the polyolefins have properties that complement each other when used together.

FBE strongly bonds to steel and offers excellent corrosion protection. It allows very little oxygen through the substrate but will absorb small amounts of water. FBE also weathers relatively poorly.

The CMPP, on the other hand, is an excellent water barrier and provides mechanical protection to the FBE during pipe transportation and installation. For instance, special bed preparation in rocky areas wouldn't be necessary to protect the FBE if the pipe were coated with polypropylene, The use of this outer coating also dramatically cuts down on the anodes needed for corrosion protection.

The epoxy/polyethylene system is now being used on numerous installations, but the maximum operating temperature for these systems is in the region of 60-70 C. (140-160 C.). Norsworthy and Nunn report that the development of tests for the new CMPP system have been a challenge.

Twenty-four hr hot water soaks of up to 212 F. (100 C.) have been used with excellent results. Twenty-four hr cathodic disbondments at 180 to 200 F. (82-93 C. and 3.5 v have shown from 0 to 1 mm radius of disbandment.

A weight test has shown that the temperature of the pipe may reach 230 F. or more (110 C.) before the CMPP fails if the bond is good.

A key challenge, however, was to modify the polypropylene so it would bond to the FBE. The Mobil specialists report that Mobil Pipeline Co., Valspar Inc. (Canada), and DuPont Canada Inc. worked several years to develop the coating process, quality control testing, and inspection procedures. Valspar brought its experience with FBE to the program, while DuPont modified the polypropylene.

Mobil Oil first used this dual coating system on pipelines near its Torrance, Calif., refinery (OGJ, Dec. 31, 1990, p. 118). The lines carry heavy San Joaquin Valley crude that requires heating before it will flow. Operating temperatures of about 85 C. (180 F.) make the lines vulnerable to corrosion.

The current project will replace, as noted, 140 miles of an existing crude oil line carrying heavy crude from Lebec in Kern County, Calif., to Mobil's Torrance refinery. The existing 8-in. and 10-in. diameter line is being replaced with 16-in. diameter pipe coated with the DuVal polypropylene system.

To date, 110 miles have been installed. The remainder is scheduled to be laid by the end of this year.

This system has also been applied to a 10 mile section of Mobil's hot oil Oso pipeline offshore Nigeria. It was installed earlier this year.

Mobil's Norsworthy and Nunn describe the coating process in their API paper. In general, after surface preparation, the pipe is heated with heat induction and spiraled through an FBE spray booth where about 22 mils (550 m) of FBE is applied.

Before the extended gel FBE is gelled (12-15 sec), the 22 mils of CMPP are side extruded or flocked on in powder form. The coating is then quenched with water to remove as much heat as possible.

After quenching, the pipe is visually and electrically inspected for holidays and other damage.

FIELD JOINTING

A two-part epoxy patching compound is used when necessary, but the perfect patch is still being sought, the Mobil corrosion specialists say.

For field jointing, the induction, heated joint is first flocked with 22 mils of FBE, which is followed by 22 mils of CMPP. This process, the Mobil specialists say, gives the field joint the same coating as the main pipe.

Ultraviolet rays will deteriorate the polypropylene and cause the surface to crack and discolor. Some of the pipe has been in storage for over a year with very little damage, Norsworthy and Nunn report. But Mobil pipeline believes that long-term storage could be a problem if the coating is not protected properly. This problem is being studied.

HIMONT

Himont Italia, an operating subsidiary of Himont Inc., the world's largest producer of polypropylene, has done pioneering work on three-layer polypropylene coating systems.

This system has been used on the projects shown in Table 1. The company has also developed a two-layer system.

Himont says that the polyolefins, in contrast to FBE, bond poorly to steel because of their non-polar nature. Himont has therefore developed a range of polypropylene-based copolymers modified by the addition of polar monomers onto their " backbones."

The functional polar groups in this intermediate layer, or bonding resin, trademarked Questron by Himont, are able to bond with the free epoxy within the FBE primer layer, while the non-polar backbone of this intermediate layer can be easily welded to the polypropylene outer coat, or third layer, trademarked Moplen by Himont.

ZEEPIPE

As an example of polypropylene required, a three-layer polypropylene system of 4 mm thickness is applied to the final 6 km onshore section of the Zeepipe North Sea gas line on the approach to the Statoil terminal at Zeebrugge, Belgium. A 40 ft (12.2 m) long joint of the 40-in. (1,031 mm) diameter pipe requires, depending on the polypropylene thickness, some 100-125 kg (220-275 lb) of polypropylene (Fig. 1).

The offshore anti-corrosion coating for Zeepipe is asphalt enamel surrounded by concrete. But had the entire 820 km long pipeline been coated with polypropylene, up to 18 million lb of polymer could have been required. For comparisons sake, this much polypropylene would represent less than 4% of the 500 million lb of polypropylene capacity added at Antwerp this spring by North Sea Petrochemicals, a joint venture of Statoil and Himont.

RISER

The Himont coating system was used to coat a 10-in. diameter, 480 m long gas line riser in the North Sea earlier this year (Table 1). This is believed to be the first time a polypropylene coating has been used to protect a riser,

The thicker than usual coating (Fig. 2) furnishes both mechanical and corrosion protection. G. Pietro Guidetti, who heads up applications and market development of the polypropylene coating system for Himont Italia, says the splash zone is a critical area with a combination of factors that stress the coating system. It is exposed to large amounts of oxygen and sunlight, large temperature changes, and mechanical forces from wave action. He believes this system is more economical than traditional solutions.

Fig. 3 gives the recommended thickness for low density polyethylene and polypropylene as set out by the Ceocor norm. Figs. 4a, 4b, 4c, and 4d compare some of the characteristics of low density polyethylene (LDPE) and Himont's polypropylene, Moplen.

Himont's ranking of various coatings in several categories is given in Table 2.

THREE, TWO LAYERS

The Himont three-layer mill coating procedure is basically similar to that described by Mobil. However, as noted, the epoxy primer is followed by the intermediate layer that is applied to a thickness of about 300 m.

The polypropylene outer coat can then be applied with a normal lateral or cross heat extruder. A pressure roller is used to eliminate air bubbles and ensure good contact between the layers.

At an extrusion temperature of 240 C., normal line speeds of 3.6 m/min can be achieved, depending on the pipe diameter.

Himont has developed a two-coat epoxy/polypropylene system. The company says it offers the same high level of performance as a three-layer system but is easier to install and permits a reduced coating thickness (less than 1.5 mm compared with 3 mm for traditional polyethylene systems).

The Himont-recommended field jointing process is similar to that of Mobil's, employing induction heating and powdered polypropylene.

However, Guidetti says there are systems developed by others for "cold" field jointing at ambient temperature. One involves a three ply with a core of polypropylene within inside and outside layers of butyl rubber adhesive.

This tape is wrapped with overlaps at the weld joint. Then this layer is covered with a shrinkable polypropylene sheet. When this sheet is shrunk by a flame, it and the three-ply tape bond.

ENVIRONMENT

In a presentation last March at the International Conference for Corrosion Prevention of the European Gas Grid, Gunnar Friberg, Norwegian Pipeline A/S, and Peter A. Blome, Blome GmbH & Co. KG, covered the environmental and economical aspects of multilayer coatings.

They compared coal tar, asphalt, FBE, and combinations of FBE/polyethylene, and FBE/polypropylene. They designated the latter two respectively as System 1 (30-70 m of FBIF as the corrosion coating with a top layer of 2-3 mm of polyethylene) and System 2 (250-400 m FBF and 2-4 mm of polypropylene),

They note that traditional asphalt and coal tar coatings require solvent-based primers containing volatiles and polyaromatic hydrocarbons. Such coatings can pose health risks to workers during the application process. They also add hydrocarbons to the atmosphere,

Friberg and Blome find few if any health or environmental risks associated with FBE or the multilayer polyolefin systems.

ECONOMICS

Table 3 shows a cost comparison developed by Friberg and Blome for the various systems. The cost of cathodic protection (i.e., cost of anodes) is strikingly lower for the polyolefin systems when compared to the asphalt and coal tar coatings. They conclude that the use of multilayer coatings reduces costs.

DuPont Canada says based on an expectancy of 30 years of operations that the ratio of the costs of pipeline cathodic protection for the various coating systems is 10 for coal tar, 5 for FBE, and 2.5 for the Du-Vall system. ,

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