1 INTRODUCTION TO OILFIELD METALLURGY AND CORROSION CONTROL

The American Petroleum Institute (API) divides the petroleum industry into the following categories:

• Upstream • Downstream • Pipelines

Other organizations use terms like production, pipelin-ing, transportation, and refi ning. This book will discuss upstream operations, with an emphasis on production, and pipelines, which are closely tied to upstream opera-tions. Many “ pipelines ” could also be termed gathering lines or fl owlines, and the technologies involved in materials selection and corrosion control are similar for all three categories of equipment.

Until the 1980s, metals used in upstream production operations were primarily carbon steels. Developments of deep, hot gas wells in the 1980s led to the use of corrosion - resistant alloys (CRAs), and this trend con-tinues as the industry becomes involved in deeper and more aggressive environments. 1 Nonetheless, the most used metal in oil and gas production is carbon steel or low - alloy steel , and nonmetallic materials are used much less than metals.

Increased emphasis on reliability also contributes to the use of newer or more corrosion - resistant materials. Many oil fi elds that were designed with anticipated operating lives of 20 – 30 years are still economically viable after more than 50 years. This life extension of oil fi elds is the result of increases in the market value of

petroleum products and the development of enhanced recovery techniques that make possible the recovery of larger fractions of the hydrocarbons in downhole for-mations. Unfortunately, this tendency to prolong the life of oil fi elds creates corrosion and reliability prob-lems in older oil fi elds when reductions in production and return on investment cause management to become reluctant to spend additional resources on maintenance and inspection.

These trends have all led to an industry that tends to design for much longer production lives and tries to use more reliable designs and materials. The previous ten-dency to rely on maintenance is being replaced by the trend to design more robust and reliable systems instead of relying on inspection and maintenance. The reduc-tion in available trained labor for maintenance also drives this trend .

COSTS

A U.S. government report estimated that the cost of corrosion in upstream operations and pipelines was $1372 billion per year, with the largest expenses associ-ated with pipelines followed by downhole tubing and increased capital expenditures (CRAs, etc .). The most important opportunity for savings is the prevention of failures that lead to lost production. The same report suggested that the lack of corrosion problems in existing systems does not justify reduced maintenance budgets, which is a recognition that as oil fi elds age, they become more corrosive at times when reduced returns on invest-ment are occurring. 2 It is estimated that corrosion costs are approximately equal to mechanical breakdowns in maintenance costs.

1

Metallurgy and Corrosion Control in Oil and Gas Production, by Robert Heidersbach Copyright © 2011 John Wiley & Sons, Inc.

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2 METALLURGY AND CORROSION CONTROL IN OIL AND GAS PRODUCTION

ENVIRONMENTAL DAMAGE

Environmental concerns are also a reason for corrosion control. Figure 1.2 shows oil leaking from a pipeline that suffered internal corrosion followed by subsequent splitting along a longitudinal weld seam. The damages due to this leak are minimal compared with the envi-ronmental damages that would have resulted if the leak had been on a submerged pipeline. Figure 1.3 shows an oil containment boom on a river where a submerged crude oil pipeline was leaking due to external corrosion caused by non - adherent protective coatings that shielded the exposed metal surfaces from protective cathodic protection currents.

SAFETY

While proper equipment design, materials selection, and corrosion control can result in monetary savings, a perhaps more important reason for corrosion control is safety. Hydrogen sulfi de, H 2 S, is a common component of many produced fl uids. It is poisonous to humans, and it also causes a variety of environmental cracking prob-lems. The proper selection of H 2 S - resistant materials is a subject of continuing efforts, and new industrial stan-dards related to defi ning metals and other materials that can safely be used in H 2 S - containing (often called “ sour ” ) environments are being developed and revised due to research and fi eld investigations.

Pipelines and other oilfi eld equipment frequently operate at high fl uid pressures. Crude oil pipelines can leak and cause environmental damage, but natural gas pipeline leaks, like the corrosion - related rupture in Carlsbad, New Mexico, shown in Figure 1.1 , can lead to explosions and are sometimes fatal. 3 High - pressure gas releases can also cause expansive cooling leading to brittle behavior on otherwise ductile pipelines. API standards for line pipe were revised in 2000 to recognize this possibility. Older pipelines, constructed before the implementation of these revised standards, are usually made from steel with no controls on low - temperature brittle behavior and may develop brittle problems if they leak. Gas pipelines are more dangerous than liquid pipelines.

Figure 1.1 Natural gas pipeline rupture near Carlsbad, New Mexico, in 2000. 3

Figure 1.2 Above - ground leak from an internally corroded crude oil pipeline.

Figure 1.3 An oil containment boom to minimize the spread of crude oil from a corroded pipeline .

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INTRODUCTION TO OILFIELD METALLURGY AND CORROSION CONTROL 3

later, oil prices had increased, recovery methods had improved, and the platform was economically profi t-able. Robust designs, adequate safety margins, and con-tinuous re - evaluation of corrosion control methods are important, not just for marine structures, but for all oilfi eld equipment.

REFERENCES

1 R. Kane . 2006 . Corrosion in Petroleum Production Operations, Metals Handbook, Vol. 13C — Corrosion: Corrosion in Specifi c Industries . Metals Park, OH : ASM International . 922 – 966 .

2 G. Ruschau and M. Al - Anezi . 2001 . Oil and gas explora-tion and production . Appendix S to Corrosion Costs and Preventive Strategies in the United States, Report FHWA - RD - 01 - 156 , September 2001.

3 NTSB. 2003 . Pipeline Accident Report, Natural gas pipe-line rupture and fi re near Carlsbad, New Mexico, August 19, 2000 . National Transportation Safety Board, NTSB/PAR - 03/01 , February 11, 2003.

4 Iranian Petroleum Standard, 1997 . IPS - E - TP - 760 . Corro-sion control in design. http://igs.nigc.ir/IPS/tp/e - tp - 760.pdf (accessed May 1, 2010).

CORROSION CONTROL

The environmental factors that infl uence corrosion are: 4

• CO 2 partial pressure • H 2 S partial pressure • Fluid temperature • Water salinity • Water cut • Fluid dynamics • pH

Corrosion is normally controlled by one or more of the following:

• Material choice • Protective coatings • Cathodic protection • Inhibition • Treatment of environment • Structural design including corrosion allowances • Scheduled maintenance and inspection

Figure 1.4 shows an offshore platform leg in relatively shallow water, approximately 30 m (100 ft) deep, in Cook Inlet, Alaska. The leg is made from carbon steel, which would corrode in this service. Corrosion control is provided by an impressed current cathodic protection system. The bottom of the leg is 2.5 cm (1 in.) thicker than the rest of the leg, and this is intended as a corrosion allowance for the submerged portions of the platform legs. Note that the water level goes above the corrosion allowance twice a day during high tides, because the platform is located in water 3 m (10 ft) deeper than was intended during design and construc-tion. Fortunately, the cathodic protection system was able to provide enough current, even in the fast - fl owing abrasive tidal waters of Cook Inlet, to control corrosion. This platform was obsolete when the picture was taken, but it was less expensive to operate and maintain the platform than it was to remove it. Thirty - fi ve years

Figure 1.4 Offshore platform leg in Cook Inlet, Alaska. The extra metal for the corrosion allowance is submerged twice a day during high tides.

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