Steel for pipelines: delivering under pressure
Stuart Laird, Senior Metallurgist, BlueScope Steel
Steel is the ideal and proven choice for high pressure gas transmission pipelines, in an age when communities depend more than ever upon a continual, economic supply of energy sources.
Pipelines need to be able to consistently deliver product year in and year out under a range of operating conditions. The pipes need to be handled, transported and installed with minimal risk of damage. They also need to be resistant to long term loss of strength or damage through corrosion, ageing and other external effects.
Whilst other materials are utilised in lower pressure distribution networks, steel remains the material of choice for high pressure gas transportation for a number of reasons.
Strength to weight ratio
Steel offers a very high strength to weight ratio. Of all commercial piping materials, steel has the greatest strength in proportion to pipe wall thickness. This attribute has weight saving implications due to the reduction in pipe wall thicknesses or through increases in gas supply via higher operating pressures. The less steel that is utilised in a pipeline, the less it costs.
Longevity, durability and property stability
As the physical properties of steel do not appreciably degrade over time, under pipeline operating conditions it will reliably perform over many generations. There are numerous examples of steel pipelines in Australia with long service periods. The longevity of a pipeline has considerable – and positive – technical, maintenance and financial ramifications.
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Proof of durability: The durability of steel used in pipelines is best illustrated in operational pipelines in the USA that date from the 1930s and 40s. In Australia, BlueScope Steel provided steel for the 440 km Roma to Brisbane pipeline, constructed in 1969, which has now been operating for over 40 years. |
Sustainability
Globally, over 80 per cent of all scrap steel is captured and either reused or recycled . Steel has one of the highest recycling rates of solid waste in Australia, so it is a viable option for any project with sustainability imperatives. If recovered at the end of each use phase, the lifecycle of steel is potentially endless. All steel has recycled content, with BlueScope Steel’s steel making process utilising between 17 and 20% of recovered or reutilised scrap steel.
Corrosion control
Through use of modern coatings and well-established cathodic protection techniques, steel pipe is resistant to corrosion, allowing many decades of service.
Ability to absorb deformation
The ductility of steel allows the pipe to deform locally under extreme loads such as those from vehicles, as well as earth movements and soil erosion, whilst maintaining resistance to the load and ensuring pipeline integrity. A prime example of the latter is the Darwin to Alice Springs pipeline. This pipeline endured an earthquake measuring 6.8 on the Richter scale near Tennant Creek in 1988 and, although being forced to buckle in upon itself, the pipe did not leak.
Steel also possesses a work hardening (or strain hardening) characteristic. This strengthening occurs because of complex dislocation interactions within the crystal structure of the steel. The benefit of this property is that it provides a buffer against fluctuations in operating loads.
Response to static and variable internal pressures
How a pipe material responds over time to the pressures created by the material it is transporting is vital. Steel is an elastic material. When subjected to a load less than the yield stress, steel will bear the load irrespective of the time the load is applied – it has a time-independent response to loads applied for sustained durations.
Unlike other materials, steel exhibits a defined fatigue strength (or endurance limit) below which the number of stress cycles required to cause failure increases to tens of millions, which can imply infinite endurance.
Ease in laying
For buried pipelines, the strength of steel allows less demanding compaction requirements than pipes with low ring stiffness (the ability of pipe to retain its shape). Steel thus provides the greatest amount of resistance, compared to other available options, to excessive distortion, which could restrict gas flows or result in pipe failure.
The higher beam strength (stiffness) of steel also has the advantage of providing protection against poor bedding and settlement. Often steel pipes can be laid without the need for imported bedding material, thereby lowering project costs.
Joinability
Whilst welding is by far the most common method used for joining high pressure gas transmission pipe, steel can be joined using a number of mechanical joining techniques including ball and socket joints, threaded joints, clamping and bolted flanges. For high pressure gas transmission applications, welding eliminates the need for complicated joint designs and special threading equipment. It produces a high strength joint that does not compromise operating conditions and the pipe has less flow restriction compared to mechanical connections. Overall, the installation cost of welded pipe is lower than that of other connection methods.
Steel grades have been designed to be easily weldable by commonly employed welding processes. This in turn allows for more robust welding procedures and higher welding rates which, in turn, will help lower construction costs.
Well known material properties
The underpinning metallurgical principles for achieving the necessary strength, toughness and weldability of steel are well known and highly developed through technical and scientific research. Steel has been produced since at least the late middle ages and there has been over a century of the metallurgical study of steel.
Research into steel is conducted throughout the world by steel manufacturers, universities and numerous research institutes, using highly advanced techniques that can analyse steel down to the atomic level. It is now one of the most common materials in the world. It is a material that behaves safely and predictably.
Conclusion: technical excellence in Australia
When all relevant characteristics are considered, the properties of steel make it the material of choice in the economic construction and reliability in service for high pressure gas pipelines. Though its properties and relative advantages are well known, research into the use of steel in gas transmission pipelines continues. Significant breakthroughs in steel design and serviceability continue to be made both internationally and by internationally recognised experts based here in Australia, which continue to help deliver a best practice outcome for the use of steel in pipelines.
BlueScope Steel produces hot rolled coil feed for oil, gas and water pipelines, as well as products such as XLERPLATE® steel, COLORBOND® steel, ZINCALUME® steel and many other products consumed in a vast range of applications. BlueScope Steel has been part of the Australian pipeline industry since 1968. Over this period it has supplied around 90%, or about 1.25 million tonnes, of the steel for over 150 oil and gas linepipe projects.
Corus Ltd. (2006) Sustainable steel construction: The design and construction of sustainable buildings. 48pp.
