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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, pipelining, transportation, and refining. This book will discuss upstream operations, with an emphasis on production, and pipelines, which are closely tied to upstream operations. Many "pipelines" could also be termed gathering lines or flowlines, 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 continues as the industry becomes involved in deeper and more aggressive environments [1, 2]. Nonetheless, most metal used in oil and gas production is carbon 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 oilfields that were designed with anticipated operating lives of 20-30 years are still economically viable after more than 50 years. This life extension of oilfields 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 formations. Unfortunately, this tendency to prolong the life of oilfields creates corrosion and reliability problems in older fields 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 tendency 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 reduction in available trained labor for maintenance also drives this trend.
COSTS
A US government report estimated that the cost of corrosion in upstream operations and pipelines was $1372 billion per year, with the largest expenses associated with pipelines followed by downhole tubing and increased capital expenditures (primarily the use of CRAs). 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 oilfields age, they become more corrosive at times when reduced returns on investment are occurring [3]. The 2013 environmental cracking problems with offshore pipelines in the Caspian Sea Kashagan oilfield are estimated to have cost billions of dollars for pipeline replacement costs plus lost production [4]. It is estimated that corrosion costs are approximately equal to mechanical breakdowns in maintenance costs.
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 sulfide, H2S, is a common component of many produced fluids. It is poisonous to humans, and it also causes a variety of environmental cracking problems. The proper selection of H2S-resistant materials is a subject of continuing efforts, and new industrial standards related to defining metals and other materials that can safely be used in H2S-containing (often called "sour") environments are being developed and revised due to research and field investigations [2].
Pipelines and other oilfield equipment frequently operate at high fluid 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 [5]. 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 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, because of the stored energy associated with compression of enclosed fluid.
Figure 1.1 Natural gas pipeline rupture near Carlsbad, New Mexico, in 2000.
Source: From Pipeline Accident Report [5].
ENVIRONMENTAL DAMAGE
Environmental concerns are also a reason for corrosion control [6]. 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 environmental 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 nonadherent protective coatings that shielded the exposed metal surfaces from protective cathodic protection currents.
Figure 1.2 Aboveground leak from an internally corroded crude oil pipeline.
Figure 1.3 An oil containment boom to minimize the spread of crude oil from an external corrosion leak on a submerged pipeline.
In the 1990s, the entire downtown area of Avila Beach, California, was closed because of leaking underground oil pipelines. The cleanup from these corroded pipelines took years and cost millions of dollars.
CORROSION CONTROL
The environmental factors that influence corrosion are:
- CO2 partial pressure
- H2S 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 2½ 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 construction. Fortunately the cathodic protection system was able to provide enough current, even in the fast-flowing 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-five years later oil prices had increased, recovery methods had improved, and the platform was economically profitable. Robust designs, adequate safety margins, and continuous reevaluation of corrosion control methods are important, not just for marine structures but for all oilfield equipment.
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.
While it might seem desirable to stop all corrosion, this is not necessarily cost effective. An 80 : 20 Pareto-type rule probably applies: 80% of corrosion can be prevented for relatively modest cost, but the increased cost of the remaining corrosion would not be justified [7]. The British ALARP (as low as reasonably practicable) terminology is a similar concept discussed in many recent corrosion-related documents and standards [8].
REFERENCES
- 1 Kane, R. (2006). Corrosion in petroleum production operations. In: Metals Handbook, Volume 13C - Corrosion: Corrosion in Specific Industries, 922-966. Materials Park, OH: ASM International.
- 2 Iannuzzi, M. (2011). Chapter 15: Environmentally-assisted cracking in oil and gas production. In: Stress Corrosion Cracking: Theory and Practice (ed. V. Raja and T. Shoji), 570-607. Oxford: Woodhead Publishing, Ltd.
- 3 Ruschau, G. and Al-Anezi, M. (September 2001). Appendix S: Oil and gas exploration and production. In: Corrosion Costs and Preventive Strategies in the United States, Report FHWA-RD-01-156. Washington, DC: US Government Federal Highway Administration.
- 4 Nurshayeva, R. (2014). Update 1 - new pipelines to cost Kashagan oil project up to $3.6 bn. Reuters (10 October)....
