In March, the industry saw the federal Pipeline and Hazardous Materials Safety Administration (PHMSA) step forward with a rules proposal that would create new standards to address potential leaks on old pipelines and through rural and moderately populated areas.
Watchdog groups also have been shining a spotlight on the methane emissions that are a result of natural gas pipeline leaks.
These actions, undertaken by public regulators and nonprofit citizens’ organizations, simply underscore the need for natural gas pipeline operators to rigorously safeguard their systems to minimize leaks.
In this article, we will examine some of the special challenges posed in monitoring gas pipelines in comparison with oil pipelines. We also will explain how computational pipeline monitoring (CPM) systems are worth the financial investment to reduce pipeline leaks and potentially deadly explosions.
Liquid vs. Natural Gas Pipelines
In 2014, the Wall Street Journal published a story after reviewing four years of liquid pipeline accident records. The newspaper examined more than 1,400 accident reports provided by PHMSA. “The overwhelming majority of these pipeline spills, ruptures and leaks were discovered by somebody near the accident site,” the newspaper reported.
That finding leads to two critically important conclusions: When there is a liquid spill, such as oil, it’s relatively easy for someone to visually spot a leak. That’s not the case with natural gas. Secondly, it emphasizes the need to carefully select the right CPM system for a given pipeline, whether it is carrying natural gas or oil.
Veteran and novice pipeline operators both recognize that identifying leaks in natural gas pipelines is more problematic than locating spills emanating from liquid pipelines due to the compressibility of natural gas making the monitoring process more complex. This means that there is no one-size-fits-all or perfect approach to detecting or locating a gas leak.
If there is an explosion or fire associated with a gas leak, obviously that level of destruction is going to draw the attention of operators and pipeline neighbors. But slow leaks are nettlesome because the gas emission is rising into the atmosphere as opposed to an oil leak in which the fluid saturates the ground around the pipeline rupture and can therefore easily be seen by people in the vicinity of the pipeline.
Consequently, it’s imperative for operators to have good systems in place to measure the pressure accurately within the natural gas pipeline at multiple locations. If changes are seen in the pressure measurements that are outside of what is expected of the operating conditions, then these irregular pressures could indicate the onset of a leak.
Correctly matching CPM systems to the challenges presented by each individual pipeline can greatly reduce public safety and environmental risks associated with gas leaks.
CPM System Choices
CPM systems are designed to monitor internal pipeline conditions, and the American Petroleum Institute’s Recommended Practice Publication 1130 (API RP 1130) defines the different elements of these systems for liquid pipelines. Field sensor outputs are used to track parameters, such as pressure, temperature, viscosity, density, flow rate, product sonic velocity, product compositions and other measurements and calculations provided by the CPM systems installed. Although API RP 1130 is essentially a standard for liquid pipelines, there are certain aspects of the standard that is equally relevant to natural gas pipelines.
Line balance CPM techniques measure the imbalance between the incoming and outgoing volumes of natural gas. This approach has some limitations because it does not compensate for changes in the pipeline inventory associated with pressure, temperature or composition. This approach can be strengthened by correcting the meter readings to standard conditions and also by compensating for changes in the line pack related to the temperature and pressure in the pipeline.
Real-time transient model CPM features the modeling of fluid dynamic characteristics under all pipeline flow conditions. These include line pack, shut-in and transients. This method employs a detailed configuration and this CPM’s software compares the measured data for a pipeline section with its corresponding modeled conditions.
Statistical analysis CPM looks for patterns linked with a natural gas leak. It does this by statistically evaluating pressure and flow measurements that define the perimeter of the pipeline in real time. A probability value is calculated to assess whether an event is actually a gas leak. It should be noted that this CPM typically would rely on one of the other methods for detecting the leak and as a result the degree of statistical involvement is variable because of the different methods that can be used.
Pressure/flow monitoring CPM searches for anomalies by exploring the relationship between different sensors’ outputs and uses an algorithm to identify them. This computational approach is a supplement to what the human controller does — it searches for unusually large declines in pressure or flow quickly and can be enhanced further with using sophisticated statistics.
Acoustic/negative pressure wave approach is characterized by the two negative pressure waves that surface within the pipeline when the natural gas release happens and the pipeline’s integrity is breached. Under this method, high response rate/moderate accuracy pressure transmitters are installed at various locations along the pipeline. This tactic allows the transmitters to measure fluctuations of the line pressure and transfer statistics to a central location. Data from multiple transmitters are aggregated and calculations are used to ascertain whether a CPM alarm should be issued.
The CPM’s mentioned above and outlined in API RP 1130 rely on creating alarms based on either conservation of mass or signature pattern recognition principles. As indicated above the CPM’s underpinned by conservation of mass principle rely on the calculation of an accurate linepack while the signature recognition evaluate the relationship of system pressures and/or flows and identify irregularities from known patterns in sensor outputs on the pipeline.
Factors for Choosing a CPM System
The overarching consideration for picking a CPM system — on a given stretch of pipeline — is that it enhances the effectiveness of the pipeline controller in the control room. The human controllers and the CPMs need to work hand-in-hand to quickly identify and isolate natural gas leaks.
A leak detection capability analysis should essentially be performed on each individual pipeline, before a decision is taken as to what leak detection system that should be used on the pipeline. Dependent on the need of the customer this could include external based leak detection systems, although they will not be discussed in this article.
Another thing to keep in mind while evaluating CPM systems is whether a CPM system is augmented with more dedicated leak detection capabilities in high consequence areas (HCAs). HCAs bear that classification because a gas leak could trigger a major impact on people, property, the environment or all three.
Speed is critically important because a quick discovery of a leak can lessen the potential harm to humans and
the physical environment. Rapid discoveries and responses not only serve the business goals of the pipeline operators, but they also are compatible with the interests of the many stakeholders who want safe pipelines in
Beyond the possible degradation caused by an explosion incident, federal regulators also are concerned about the chronic harm that leaks can pose for the environment. In a 2014 report by the Office of Inspector General for the Environmental Protection Agency (EPA), it was noted that methane leaks from pipelines in the natural gas distribution sector were responsible for more than 13 million metric tons of carbon dioxide equivalent emissions. “These leaks are comprised of natural gas product, which is almost 100 percent methane, and account for more than 10 percent of total methane emissions from natural gas systems,” the report said.
Pipeline companies are constantly evaluating their business objectives and risk thresholds. Annually, natural gas leaks have been responsible for billions of dollars of lost or unaccounted for product in the United States.
But that lost product is only one huge consideration when it comes to natural gas releases. If there is a major leak or explosion that causes loss of life or serious property damage, there are many other costs that come into play.
Oil and gas companies must deal with clean-up costs and regulatory fines, reputational damage and a potential inability to win regulatory approval for future projects.
These serious public safety, environmental and business factors reinforce the importance of effective CPM systems in detecting leaks. As natural gas use increases, pipeline operators are poised to act as good business people and environmental stewards by deploying the right CPM systems.
Lars Larsson is senior product manager at Schneider Electric. He has more than 22 years of oil and gas pipeline industry experience from all phases of a project, from system design to commissioning and support.