A winning combination: leak-detection22 June 2015
A new leak-detection methodology combining various tried and tested techniques promises to improve performance, robustness and applicability, as Elly Earls finds out from the man behind the research, Dr Gerhard Geiger.
Leak detection for pipelines is a crucial part of the inspection, maintenance and repair regime in the oil and gas sector. Not only do operators need to be able to detect leaks when pipelines begin to fail towards the end of their allotted lifespans; accidents, terrorism and sabotage can also lead to leaks, which, if left undetected, could potentially result in catastrophe. It's certainly not a field that's been standing still over recent years; in fact, far from it, with some of the latest research involving combinations of various tried and tested methods, with the ultimate aim of creating a system far superior to those that already exist in terms of performance, robustness and applicability.
Pipeline networks are the most economic - and the safest - method of transporting mineral oil, gases and other fluid products from A to B, and are required to meet high levels of safety, reliability and efficiency. Nonetheless, when a pipeline's lifespan, which is usually around 25 years, starts nearing its end, it will begin to fail, albeit slowly, beginning with leaks at poor construction joints, corrosion points and small structural material cracks, and gradually progressing to catastrophic failure. Of course, accidents, terrorism, sabotage or theft can have the same ultimate result.
Leak-detection systems (LDSs), therefore, were created to help pipeline controllers detect and localise leaks by providing an alarm and displaying other relevant data. They can also enhance productivity and system reliability thanks to reduced downtime and inspection time.
LDSs: the basics
So how do these crucial systems - in all their forms - actually work? There are two different types of LDSs: internal and external, both with their advantages and drawbacks, explains Dr Gerhard Geiger, leak-detection specialist at the Westphalian Energy Institute, Germany.
"Internal systems use existing measurement sensors for flow, pressure, temperature and so on, and usually run continuously," he says, "Sensitivity is slightly lower than external-based detection systems, but so are investment and operations costs, and for this reason, internal systems are very common and required by law for some countries."
Internal systems include rarefaction wave methods (where negative pressure waves caused by leaks can be recognised using installed pressure transmitters, which then sound a leak alarm); balancing methods (where mass or volume flow imbalances indicate a leak); and real-time transient-model (RTTM)-based methods, which make it possible to calculate mass flow, pressure, density and temperature at every point along a pipeline in real time - with the help of mathematical algorithms - and provide a fast and sensitive leak-alarm declaration. According to Geiger, when it comes to internal methods, RTTM-based methods have been the key advancement in the leak-detection sector over the past ten years.
External systems, on the other hand, use dedicated measurement equipment such as probes and sensor cables, and, while they do provide very good performance, also boast corresponding investment and operations costs.
"They need dedicated measurement equipment such as sensor cables that must be laid along the pipeline," Geiger explains. "Furthermore, in many cases, it is impossible to retrofit existing pipelines with this type of LDS. Therefore, external systems will only be used in critical applications such as pipelines crossing high-consequence areas."
Geiger's latest research
Because of this, Geiger's research has been focused on internal LDSs, more specifically RTTM-based methods - and he's certainly not stopping at what's been done before. "External systems will only be used in critical applications; therefore, in most cases, internal LDSs will be installed, so I decided to focus my research work on sophisticated internal LDSs," he stresses. "Classical RTTM-based LDSs use mathematical models of the flow in pipelines together with digital computers to provide fast leak-alarm declaration with low alarm thresholds, but they also show many false alarms. Therefore, I realised that it would be very advantageous to combine RTTM-based technology with statistical classification methods in order to form a pattern-recognition scheme for leak detection."
This is precisely what he did back in 2003 when he proposed a new system - E-RTTM (or extended RTTM). "E-RTTM-based LDSs provide a leak signature analysis to reduce the number of false alarms without losing the advantages of RTTM-based LDSs," he says. "This offers improved sensitivity (lower leak-alarm threshold), faster leak-alarm declaration and improved reliability (a lower number of false alarms)."
But Geiger hasn't stopped there; his latest paper 'A Combined Leak Detection Method Using Pattern Recognition Techniques', which was published in 2014, outlining the new leak-detection methodology he is currently working on, which uses pattern-recognition techniques to combine two or more internal methods (such as RTTM-based methods, mass-balancing methods, volume-balancing methods and rarefaction-wave methods) seamlessly into one scheme, thereby improving performance, robustness and applicability.
When it comes to performance, the use of different features at the same time provides more information about the pipeline, which can be used for pipeline state evaluation (is there a leak - yes or no?) as well as offering better sensitivity (lower alarm thresholds and shorter detection times, for example). Meanwhile, robustness is also improved because more methods are being used (even if one fails, there is at least one back-up), while applicability is also much wider - depending on which sensors are available, one, two or more methods will be used.
So does the new methodology actually work? According to the first test case carried out and documented in Geiger's latest paper - at a liquid multiproduct pipeline in Germany - where RTTM-based mass balancing and volume balancing were combined: yes. During start-up, there was no single false-alarm declaration and, for leak tests with two different products, all leaks were detected in less than a minute with only 70L lost between leak occurrence and leak detection.
Looking forward, Geiger only hopes to further advance his research. "My vision is to realise (together with industrial partners like KROHNE Oil & Gas) a pattern-recognition-based LDS combining RTTM-based technology, uncompensated mass balancing, uncompensated volume balancing and rarefaction-wave technology seamlessly into one scheme, hence [further] improving performance, robustness and applicability," he concludes.
Rules and regulations
According to Geiger, research work in the leak-detection field must take into account the existing regulations across the sector. "Safe pipeline transport of liquids is important due to the high consequences that could be associated with a leak," he stresses. "[And] industrialised countries like Canada, the US and Germany, therefore, regulate the design, construction, operation and maintenance of pipelines, with part of these regulations addressing LDSs. Successful and reasonable research work has to consider these regulations."
For Geiger, some of the key regulations and recommendations that must be considered are the following:
- CSA Z661-11 (Canada): The sixth edition of Canadian Standards Association (CSA) Standard Z661-11 'Oil and gas pipeline systems' covers the design, construction, operation, and maintenance of oil and gas industry pipeline systems that convey liquid hydrocarbons, oilfield water and steam, CO2 used in oilfield enhanced recovery schemes and gas, he explains, adding that the Canadian standard is intended to establish recommendations, essential requirements and minimum standards. "Following this standard, 'operating companies shall make periodic line balance measurements for system integrity' for liquid hydrocarbon pipeline systems," Geiger continues. "CSA Z662 defines material balance as 'a mathematical procedure, based upon the laws of conservation of matter and fluid mechanics that is used to determine whether a leak has developed in a pipeline segment'. The standard also includes model-based leak-detection methods."
- API RP 1130 (US): The American Petroleum Institute (API) Recommended Practice (RP) 1130 'Computational Pipeline Monitoring for Liquid Pipelines' does not directly impose legal requirements but gives a technical overview of leak-detection technology, describes infrastructure support for LDSs, and discusses LDS operation, maintenance and testing, according to Geiger. "It provides the necessary technical information for conscientious operators and pipeline controllers to manage their pipelines safely and covers liquid pipelines only," he summarises.
- TRFL (Germany): The Technische Regeln für Rohrfernleitungsanlagen (TRFL) provides technical rules for pipeline systems that apply to most pipelines transporting liquids or gases in Germany. "It requires two autonomous, continuously operating LDSs that can detect leaks in the steady state," notes Geiger. "Either of these systems, or both, or a third one must be able to detect leaks in transient conditions. Special attention should be paid to the difference between the steady state and the transient state." Moreover, the TRFL also requires that each pipeline has a system to detect leaks in paused flow conditions, while in addition, it addresses gradual leaks (for example, caused by corrosion), which have two important characteristics (leak flow usually is very small and it develops slowly). "For that reason, TRFL requires a dedicated LDS for this purpose," Geiger notes. Last, the TRFL requires a system (or other procedure) to locate leaks rapidly, enabling targeted actions for repair and re-establishing safety.