How we monitor the safety of underground pipelines

How does this kind of damage occur? Burema explains, ‘Dents and scraping can be caused by excavation works, for example, and corrosion can be caused by bacteria or a defect in the protective coating. The pig pinpoints these damaged areas.’ It works like this. Propelled by the gas flow, the pig travels from the ‘pig launcher’ along the inside of the pipeline to the ‘pig receiver’. The launcher and receiver are also called scraper traps, which are installed for the duration of the pigging. These scraper traps allow the pig to be safely inserted into and removed from the pipeline. Burema is fascinated by the sound of the pig moving through the pipeline: ‘It sounds like the clickity-clack of a train. You can hear the gas flow, and because the pipeline consists of segments welded together, you can hear the pig moving over the welds, just like a train riding over the tracks.’

All sections of pipeline longer than two and a half kilometres and with a diameter of more than 150 millimetres are inspected using a pig. Gasunie only inspects smaller pipelines above ground. It can take anywhere from 6 to 24 hours to inspect a pipeline measuring 40 to 120 kilometres. The time needed depends on the gas flow and the time of year, says Burema. ‘When gas flow is higher, like it is in winter, the pig also moves through the pipeline faster. However, the pig must not move too quickly or too slowly. We have methods to slow down the pig.’ All sorts of people are involved in an inspection. Besides the pigging engineers, this includes field technicians and Gasunie staff involved in gas transmission, stakeholder management, and the Safety and Special Assignments departments, as well as contractors. How safe is an inspection like this? ‘We have many years of experience with pigging. We follow all kinds of procedures, and safety is our top priority. Obstacles in the pipeline may cause a pig to become stuck. When that happens, pigging engineers, together with Special Assignments, locate the pig. After halting pipeline operations, the section containing the pig is removed and replaced, after which operations can start up again and the gas will start to flow once more. For pipelines that are essential for security of supply, we use a special pig with a bypass port through which the gas can flow. If we did not use this, entire areas could be left without gas. Although a pig has never got stuck in all the time I’ve been a project manager, we are, of course, well prepared in case it does happen.'

The data analyst uses the data collected by the inspection pig to pinpoint the location of a defect, dent or corrosion. ‘In the case of corrosion, for example, the pig will measure the decrease in wall thickness and show this as a percentage. Together with staff from the Safety department, the data analyst will then determine the timeframe within which action must be taken.' When a section of pipeline needs to be exposed a separate project is initiated. For safety reasons, we reduce the gas pressure by 10% during excavation works. Burema explains, ‘After excavation, the coating is removed and the pipe is sandblasted. The data analyst and Safety staff then assess the defect. Sometimes it’s not too bad and nothing needs to be done. If there’s minor damage, we weld special sleeves around the section with the defect. And if this is insufficient, Special Assignments will schedule and carry out the replacement of a section of piping.’

As national project manager, Burema is responsible for organising tendering procedures and for the pigging contracts that Gasunie puts out to tender in the EU. Now that we are preparing for the transmission of hydrogen, there are also new requirements for pigging. ‘Existing natural gas pipelines are being prepared for hydrogen transmission. However, the hydrogen molecule is much smaller than the gas molecule and can accumulate in tiny cracks. That’s why we are extra vigilant about safety when it comes to hydrogen transmission. This means that we will also be inspecting the welds in the pipelines for internal cracks.’ We do this using an electro-magnetic acoustic transducer – a slightly different tool – for ‘smart pigging’. ‘The pigging process is essentially the same, but the pipeline needs to be much cleaner to carry this out,’ Burema says. ‘The pipelines occasionally contain a fair amount of contaminants like sand, dirt, metal swarf, welding pins and other debris. Special cleaning pigs are available for this purpose. We just completed a complex tendering procedure for this. This way we continue to develop so that we can be ready for what the future brings.’