The first step in the construction of a gas transmission pipeline is surveying the route. Transmission pipelines can be hundreds of kilometres in length and they can pass through a wide variety of terrain with different geological and topographical features, as well as State and Territory borders. Choosing the final route depends on a number of variables, including soil types, topographical features that pose additional challenges such as river crossings and cliffs, the location of flora and fauna habitat, the presence of other utilities such as high-tension power lines, population centres, heritage sites, and land acquisition and other legislative requirements. Once the route has been selected, the process of securing all the necessary approvals begins. This is likely to include applications involving several processes in different government departments and agencies – in more than one jurisdiction if the pipeline is crossing borders – and working with local governments. While the approvals process is under way, the design engineers and community liaison officers start work.
Designing the pipeline
Transmission pipelines operate at high pressures, typically between 10,000 and 15,000 kilopascals which is up to 150 times atmospheric pressure at sea level, and this means they must be carefully designed and constructed so they are safe and reliable to operate. Australian Standard 2885 applies to high-pressure hydrocarbon pipelines and engineers in the design phase will take into account the relevant requirements of the Standard which as a significant focus on public safety. During design, engineers undertake modelling and analysis on a range of issues. The route of the pipeline will require particular designs for crossing each water course and for other obstacles, as well as for managing risk in areas where there may be traffic or similar considerations. When designing a pipeline, engineers have to consider aspects of the pipeline’s operation, such as its maximum allowable operating pressure, fatigue and fracture analysis, mitigating corrosion, modelling pressure control and over-pressure protection, the pipe wall thickness, soil mechanics and electrical influences and earthing. The designers also have to provide for compressor stations and other structures required by the pipeline and for elements related to its operation such as control systems.
Working with the community
Constructing a pipeline can bring a boost in economic activity to regional towns, but it can also mean some temporary disruption to normal life during the construction phase. Community liaison officers start working with local communities and individual land owners as soon as the pipeline’s route is decided so that everyone will understand what will happen while the pipeline is being built and how the impact on local communities can be minimised.
Making the pipe
Once the route and design are finalised, the pipe can be ordered and manufactured. Most pipe used for transmission pipelines begins as a flat sheet of steel. The sheet plate is heated and then moulded into the pipe shape, often in a two-step process: from flat to U-shape and then into a cylindrical shape. The pipe is then welded along the seam, expanded to its final size and then tested. Once it has passed the test, it is sent for coating. Before coating, rust and scale is removed, the pipe is heated and coating is applied to both the outside and inside of the pipe. Then the pipe is ready to be transported to the construction site.
Clearing the site and laying the pipe
As the pipe is being shipped to the construction site, the crews begin working at the site. Cultural heritage and environmental surveys are carried out at the site before any clearance, and then the pipeline easement is pegged out by the surveyors. Vegetation is cleared along the pipeline route and the easement is levelled. This is done carefully so that the rehabilitation of the site can be undertaken more efficiently once the pipeline has been laid. The pipeline is constructed in sections. When the pipe is delivered to the site, it is laid out end to end along the right-of-way in a practice called stringing. Then the bending crew comes along to shape the pipes to ensure they fit the terrain as required. After the bending has been completed, the pipes are welded together into a pipeline that can be up to a kilometre or so in length. The welding is a significant operation carried out by highly skilled technicians with the joins welded on both the inside and outside. The welds are thoroughly tested with ultrasonic and radiographic testers, then the outside weld is sandblasted and coated. The trench is dug, the pipeline is lowered in, and then the pipeline is covered over, with the topsoil the final layer. After rehabilitation, which usually takes little time, the land can be used again, for instance for grazing cattle.
In some situations, such as river crossings, a process called horizontal directional drilling is used to minimise the impact on the environment. In this case a hole is drilled from one side of the river to the other and then a pipe casing is pulled through the hole to the other side.
Operating the pipeline
Before the pipeline can be used for transporting gas, it must be tested to ensure it is safe to operate. The pipeline is tested by filling it with water which is then pressurised to detect any leaks. The water is pressurised to a level higher than that which will be used for transporting gas. Once all the testing is complete, gas can flow.
Testing continues once the pipeline is in operation to ensure it remains safe and reliable. Pipelines are tested for corrosion and other defects by pushing intelligent robotic devices known as smart pigs through sections of pipeline to test for thickness, roundness, corrosions, small leaks and any defects. The gas is regularly sampled and pipelines are also routinely maintained. They are clearly marked to reduce the likelihood of interference. The Dial Before You Dig program provides information on pipeline location so that excavation activities can be carried out safely.
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