Protecting petroleum pipelines

Working with academia and industry, the Australasian Corrosion Association (ACA) supports research into all aspects of corrosion in order to provide an extensive knowledge base of the latest technologies and best practices in corrosion management.

As the global economy continues to grow, the markets for petroleum products continue to expand. To meet increasing demand, more and more oil and gas infrastructure is being built. Corrosion poses a threat to all infrastructure and the economic impact of all types of corrosion and its degradation of infrastructure, such as pipelines, oil rigs and towers, represents an annual cost of many millions of dollars to the industry.

Pipelines vary from simple steel tubes to state-of-the-art spiral-wound, flexible lines, with diameters ranging from 50 mm to 2 m. Pipelines are integral to the oil and gas industry, where they form the gathering systems joining wells to process facilities and the distribution system delivering product to refineries and markets. While non-ferrous materials such as fibreglass and polypropylene can be used in non-critical, low-pressure applications, the overwhelming majority of petroleum pipelines are constructed from metal. Whether buried or on the surface, all metal pipelines are exposed to a range of physical, climatic and chemical environments that can cause corrosion.

Ageing or damaged infrastructure presents many challenges to the oil and gas industry and regulators worldwide. There are thousands of kilometres of pipelines associated with the oil and gas wells and platforms operating in more than 50 countries around the world. These facilities vary in size, shape and degree of complexity.

Much of this infrastructure was built in the 1950s and designed in accordance with lower standards than are currently prescribed. Some facilities are operating well beyond their intended service life and others have suffered damage as a result of storms or accidents or, because of the lack of active maintenance programs, have deteriorated to the extent that there is now doubt as to their continued structural integrity.

External protection

Oil and gas pipelines are often coated with several layers of protective material and fitted with cathodic protection devices that inhibit corrosion. Internal pipe maintenance and cleaning is usually conducted by sending a scrubbing device or ‘pig’ (originally named because of the squealing noise early versions made as they traversed the line) through the pipeline at regular intervals. Other, more sophisticated pigs, fitted with cameras and sensors, are able to inspect the integrity of welds and the internal condition of the pipe as they move along.

Achieving the most effective corrosion control strategies is likely to require changes in industry management and government policies. Industry must take advantage of future developments in protective coatings technology in order to reduce the overall cost of corrosion. Advances in corrosion protection will include coatings that are both physical barriers and contain corrosion inhibitors that are released when a coating becomes damaged or in the presence of a corrosive environment.

Working with academia and industry, the Australasian Corrosion Association (ACA) supports research into all aspects of corrosion in order to provide an extensive knowledge base of the latest technologies and best practices in corrosion management. The organisation aims to ensure all impacts of corrosion are responsibly managed, the environment is protected, public safety is enhanced and economies are improved.

Historically, metallic zinc and primers containing chromate have provided excellent corrosion protection. These materials have properties that allow coatings containing them to actively respond to a corrosive environment while maintaining a barrier to that environment.

Advances in coating technology can offer significant cost savings if developed and successfully demonstrated. Zinc, polyurethane and powder-coating technologies make them a superior alternative to epoxy resin technology for longer-term service life. Zinc gives a very basic cathodic protection effect as a thin coating; polyurethane is effective and aesthetically appealing; and powder coatings can meet the environmental and regulatory challenges.

All companies are striving to reduce maintenance budgets for their infrastructure while optimising performance, so new corrosion protection materials must be cost effective and non-hazardous. Some of the latest advances in coating technology have included the development of protective coatings that can respond to damage and changes in the external environment. However, such coatings must not be a threat to the environment and maintenance personnel and ideally must be applied using conventional methods currently used to coat structures for environmental protection. New materials, such as nanostructured materials and organic metals, may be appropriate as the basis for developing damage-responsive coatings and structures.

Internal protection

Internal corrosion controls for gas pipelines includes reducing the water content of the gas and adding corrosion inhibitors to the fluid flow. For oil pipelines, internal corrosion is mitigated by reducing the water content then adding corrosion inhibitors, scale inhibitors and biological controls. Pipeline operators must continually monitor the effectiveness of their chosen corrosion controls. Erosion in the internal pipeline wall can be controlled by removing solids from the stream and by the mechanical design of the layout, while corrosion caused by moisture in a gas stream can be controlled by decreasing the dew point of the gas to a temperature below the lowest operating temperature likely to be encountered in the pipeline. One way to allow for corrosion is to make the pipe wall thicker to provide additional metal for corrosion loss. The corrosion allowance should anticipate the maximum metal loss over the life of the pipeline and ensure that sufficient wall thickness remains to enable the pipeline to operate safely. A corrosion allowance should not be a substitute for other corrosion protection measures, however, since actual corrosion rates in practice can be much higher than those used in the estimation of the corrosion allowance.

Pipeline supports

The oil and gas industry invests large sums of money in the design, laying and protection of pipelines. In comparison, far less attention is paid to the mounting and bracing structures that support and guide a pipeline.

One of the most common support methods is to lay the pipe onto a standard structural element such as an I-beam or metal channel and secure it in place with a stabilising U-bolt. A similar method is to use a saddle clamp, where the pipe is clamped between two rolled plates, with one of the plates welded to a structural element. These two categories account for more than 95% of support points on a typical structure.

One alternative is to weld a part of the pipe, which is usually free to move, directly to the support structure. This is a common approach for insulated piping systems. There are a number of other alternative pipe supports, such as flange bolt supports, various type of pipe hangers and other specialty-type supports.

Not surprisingly, it is the beam supports and the saddle clamps that cause the majority of problems. Visual inspection and other non-destructive testing is often difficult and it is virtually impossible to paint or otherwise maintain some areas of the pipe at the support. Some of these support types may even develop bimetallic contact. Despite both the pipe and support being steel, the metallurgical differences can still provide a small potential difference to create a corrosion cell.

The shape of a cylindrical pipe on a flat surface also forms a crevice where moisture gathers and evaporation is restricted. The moisture softens the paint, which fails and exposes bare metal which is then in constant contact with water. Once corrosion starts, there can be rapid wall loss leading to eventual failure of the pipe.

An effective way to reduce corrosion risk is to minimise the contact point between the support and pipe so that no crevice is formed. Water cannot be trapped, so corrosion no longer occurs. With minimal contact, air can also circulate and evaporate moisture beneath pipes, and it’s far easier to inspect the contact area. If the material of the support is non-metallic, the pipe can be electrically isolated so there is no contact between dissimilar metals.

About the ACA

The ACA is a not-for-profit industry association established in 1955 to service the needs of Australian and New Zealand companies, organisations and individuals involved in the fight against corrosion. The vision of the organisation is to reduce the impact of corrosion.

Throughout the year, the ACA also conducts educational activities such as seminars and training courses across Australia and New Zealand to inform and guide organisations and practitioners about corrosion topics. Corrosion specialists certified by the ACA and other organisations have the experience and understanding of corrosion causes and solutions that allow them to recommend mechanisms and procedures to consultants and asset owners.