Probe into pipe integrity
NEL delves into the key role that monitoring and leakage detection systems play in minimising the impact of leaks in pipelines
The visible evidence of a leak is often the first sign that the integrity of a pipeline has been breached. In an ideal world, one in which cost did not matter, we would design our pipelines to have guaranteed integrity, but in the real world we have to accept that leaks will occur.
Whether carrying water, gas or oil, there are financial, political, regulatory, safety and environmental issues that must be addressed when designing and operating pipelines. Pipe-line integrity monitoring and leakage detection systems therefore have a key role to play in minimising the occurrence of leaks and their impact, and NEL's UK National Measurement Standards laboratory in East Kilbride is heavily involved in this activity.
Specific instrumentation and methodologies are routinely used to reduce the likelihood of pipeline failure. An ideal system is one which gives before-the-event warning of impending loss of integrity. For instance, the gas distribution industry observational surveys are made by air, road vehicle and by foot in order to detect the presence of excavating activities close to the pipeline, this helps prevent failure due to impact. The pipeline is also inspected in-line using an 'intelligent pig' to detect the presence of metal loss defects, such as those caused be corrosion and external impact.
Pressure cycling is monitored and controlled to prevent the growth of construction defects due to fatigue. While specific instrumentation and methods are used to reduce the risk of pipeline failure, flowmetering by difference is also used to identify actual leaks. This method is most commonly used in the water industry to detect burst mains but has also been applied in the oil sector, both on and off-shore. Although flowmetering by difference is an after-the-event system, it has the appeal that, in principle, it can be applied over long pipe runs, even when the pipeline is inaccessible except at its ends.
Furthermore, it is a non-intrusive technique that can be integrated into the normal operational regime of the pipeline. Despite these obvious advantages, however, there are a number of practical difficulties which have limited its uptake in certain sectors.
One key issue is the acceptable size of a leak before it is detected. Depending on the location of the pipeline and the fluid it is carrying, the rate of leakage or the total volume lost over a given period may be more significant. For example, a very slow leak from an oil pipeline may go undetected for a long period. While this is unlikely to have a major financial impact on the operation of the pipeline, the environmental impact may be significant. On the other hand, the environmental impact of a long-term slow leak from a gas pipeline would probably be insignificant but the safety implications huge - in the case of an underground gas main, the gas could remain trapped around the pipe, building to a volume which, if ignited, could cause serious damage.
At its most basic, flowmetering by difference is based on simple conservation of mass. This raises an immediate issue - most flowmeters are volumetric devices and so it is necessary to know the density of the fluid throughout the section of the pipeline being monitored. For gas, the temperature and pressure in the pipeline will have a significant effect on the density. Furthermore, the composition of the gas (and hence the effect of temperature and pressure on its density) between two flowmeters within any section of pipe can change significantly over comparatively short periods of time, as gas from alternative sources is introduced into the distribution system. Although the effects of temperature and pressure on the density of oils are lower than for gases, they must still be taken into account. On oil pipelines, flowmeters for fiscal and custody transfer applications generally have an uncertainty of 0.25% and, depending on the operational regime of the pipeline, it may be possible to resolve imbalances (and hence leaks) of the order of 0.1%. Although the safety and environmental implications of water leaks are much less severe than gas or oil leaks, there are still significant financial, political and regulatory pressures, which should drive water companies to adopt good pipeline integrity and leakage detection systems.
In principle, flowmetering by difference should be much easier for water - the composition is essentially constant and its variation of density with temperature and pressure is small across the normal range of operating conditions. However, the complex nature of water distribution (networks rather than simple point-to-point pipelines) and the typical meter uncertainties (from 1-5%) make the task far from trivial. The requirement to monitor the integrity of extensive and inaccessible pipelines is clearly widespread and there is a belief some of the issues discussed previously can be addressed by sharing experiences and practices across sectoral boundaries.
As part of DTI's National Measurement System Directorate's 2002-2005 Flow Programme, NEL (the main contractor) is undertaking a project to review state-of-the-art monitoring systems for pipeline integrity across all relevant industrial sectors. The review will cover leakage detection in general, with a focus on documenting flowmetering-based methodologies. Although, as noted previously, such techniques are used across a range of industries, the different operational and safety requirements have lead to a variety of implementations. By NEL identifying best practice in each industrial sector and promulgating it across all sectors, the project will help companies realise the benefits of efficient and cost-effective leakage detection. NEL is a leading international technology services organisation with a successful track record of more than five decades, delivering cutting-edge research and world-class innovative solutions to difficult problems.
NEL's customers include government departments and many of the major world businesses. The National Measurement System (NMS) is the UK's national infrastructure of measurement laboratories, which delivers world-class measurement science and technology and provides traceable and increasingly accurate standards of measurement for use in trade, industry, academia and government. The NMS ensures users can be confident their measurements, and those of their customers and suppliers, are consistently traceable back to nationally and internationally accepted primary reference standards - and are therefore both valid and fit for purpose