Pipeline protection and the options available

An Ask the Expert feature with David Neill of Proclad

Proclad’s David Neill looks at what can companies do to safeguard their pipeline equipment against long-term corrosion and what technologies are available to provide the best solutions.

The ability to safely transport produced hydrocarbons is an important factor for any operating company in the development of an oil and gas field – and poor pipeline maintenance can lead to extensive corrosion and leaks. One of the recent examples of an oil leakage was Alaska’s Prudhoe Bay which caused the operating company to shut down a field due to “unexpectedly severe corrosion”. This leakage led the operating company to shut down a field producing 400,000 barrels per day (bpd), this shut down effected eight percent of US oil production pushing petrol prices to an all time high.

For the oil and gas industry, specific condition of operating variables need constant monitoring if the asset is required to operate for the design lifetime of the oilfield. Material selection must be tailored to the service conditions with specific attention to correct fabrication procedures and maintenance schedules. There are many guidelines available for material selection, and corrosion testing of material.

To ensure of correct materials selection to protect for corrosion some of the main variables that have to be considered are:
• Operating pressure and temperature
• CO2 content 
• H2S content
• Oxygen and oxidising agent’s content.

If the corrosivity of the hydrocarbon requires the use of corrosion resistant alloys (CRA) the capital cost, particularly in large diameter pipelines, can become prohibitively high. However, if consideration is given to the purpose of the corrosion resistant alloy in providing a protective barrier from the well service, consideration should then be given to providing a protective barrier of CRA, but combining this protective barrier with a less noble material.

In selecting materials that are a combination of CRA and carbon steel, the same process variables listed above have to be considered. The outcome of the selection process will yield the same results in alloy selection, but in the case of a 14-inch Schedule 80 pipe, 16mm of a 19mm wall thickness pipe will be carbon steel reducing the capital cost significantly. This technology is widely known within the oil and gas industry as cladding. The clad layer, generally 3mm thick can be applied in a variety of forms, and covers most pipeline components including fittings, flanges, and valves. There are two types of clad layer:
Metallurgically bonded, which includes process such as weld overlay, explosive bonding, roll bonding, and co-extrusion. Mechanically bonded, which involves using a mandrel or using hydrostatic pressure to expand the CRA inner pipe material beyond its plastic limit, whilst elastically deforming the carbon steel outer pipe. Relaxation of pressure produces a mechanical bond. 

There have been many concerns over the jointing of these various components since it consists to two dissimilar materials. The objective in this case is to ensure the corrosion resistance is maintained; therefore the materials used for the jointing process must also be a corrosion resistant alloy. In consideration of the pipeline design, the strength is also a factor. For example if the base material grade is API 5LX 65 which has a yield value of 448MPA the CRA jointing material must at least match this value. In which case UNS 006625, with an as welded yield strength of 460MPA would be the correct material selection.

David Neill has been with Proclad since its inception in 1983. As a welding engineer, Neill worked with several large engineering companies and has several years’ experience working in the North Sea operations with a major oil producer. Educated as a welding engineer in Central Scotland, his passion for research and development has served him well having developed many new techniques and processes for the benefit of the oil and gas industry.