Leak detection

Rules and Regulations

Some countries formally regulate pipeline operation.

API RP 1130 “Computational Pipeline Monitoring for Liquids” (USA)

This recommended practice (RP) focuses on the design, implementation, testing and operation of LDS that use an algorithmic approach. The purpose of this recommended practice is to assist the Pipeline Operator in identifying issues relevant to the selection, implementation, testing, and operation of an LDS. LDS are classified into internally based and externally based. Internally based systems utilize field instrumentation (e.g. for flow, pressure and fluid temperature) to monitor internal pipeline parameters; these pipeline parameters are subsequently used for inferring a leak. Externally based systems use local, dedicated sensors.

TRFL (Germany)

TRFL is the abbreviation for “Technische Regel für Fernleitungsanlagen” (Technical Rule for Pipeline Systems). The TRFL summarizes requirements for pipelines being subject of official regulations. It covers pipelines transporting flammable liquids, pipelines transporting liquids that are dangerous for water, and most of the pipelines transporting gas. Five different kinds of LDS or LDS functions are required:

Requirements

API 1155(replaced by API RP 1130) defines the following important requirements for an LDS:

Steady-state and transient conditions

During steady-state conditions, the flow, pressures, etc. in the pipeline are (more or less) constant over time. During transient conditions, these variables may change rapidly. The changes propagate like waves through the pipeline with the speed of sound of the fluid. Transient conditions occur in a pipeline for example at start-up, if the pressure at inlet or outlet changes (even if the change is small), and when a batch changes, or when multiple products are in the pipeline. Gas pipelines are almost always in transient conditions, because gases are very compressible. Even in liquid pipelines, transient effects cannot be disregarded most of the time. LDS should allow for detection of leaks for both conditions to provide leak detection during the entire operating time of the pipeline.

Internally based LDS

Internally based systems utilize field instrumentation (e.g. for flow, pressure and fluid temperature) to monitor internal pipeline parameters; these pipeline parameters are subsequently used for inferring a leak. System cost and complexity of internally based LDS are moderate because they use existing field instrumentation. This kind of LDS is used for standard safety requirements.

Pressure/Flow monitoring

A leak changes the hydraulics of the pipeline, and therefore changes the pressure or flow readings after some time. Local monitoring of pressure or flow at only one point can therefore provide simple leak detection. As it is done locally it requires in principle no telemetry. It is only useful in steady-state conditions, however, and its ability to deal with gas pipelines is limited.

Acoustic Pressure Waves

The acoustic pressure wave method analyses the rarefaction waves produced when a leak occurs. When a pipeline wall breakdown occurs, fluid or gas escapes in the form of a high velocity jet. This produces negative pressure waves which propagate in both directions within the pipeline and can be detected and analyzed. The operating principles of the method are based on the very important characteristic of pressure waves to travel over long distances at the speed of sound guided by the pipeline walls. The amplitude of a pressure wave increases with the leak size. A complex mathematical algorithm analyzes data from pressure sensors and is able in a matter of seconds to point to the location of the leakage with accuracy less than 50 m (164 ft). Experimental data has shown the method's ability to detect leaks less than 3mm (0.1 inch) in diameter and operate with the lowest false alarm rate in the industry – less than 1 false alarm per year.

However, the method is unable to detect an ongoing leak after the initial event: after the pipeline wall breakdown (or rupture), the initial pressure waves subside and no subsequent pressure waves are generated. Therefore, if the system fails to detect the leak (for instance, because the pressure waves were masked by transient pressure waves caused by an operational event such as a change in pumping pressure or valve switching), the system will not detect the ongoing leak.

Balancing methods

These methods base on the principle of conservation of mass. In the steady state, the mass flow M ˙ I entering a leak-free pipeline will balance the mass flow M ˙ O leaving it; any drop in mass leaving the pipeline (mass imbalance M ˙ I − M ˙ O ) indicates a leak. Balancing methods measure M ˙ I and M ˙ O using flowmeters and finally compute the imbalance which is an estimate of the unknown, true leak flow. Comparing this imbalance (typically monitored over a number of periods) against a leak alarm threshold γ generates an alarm if this monitored imbalance. Enhanced balancing methods additionally take into account the change rate of the mass inventory of the pipeline. Names that are used for enhanced line balancing techniques are volume balance, modified volume balance, and compensated mass balance.

Statistical methods

Statistical LDS use statistical methods (e.g. from the field of decision theory) to analyse pressure/flow at only one point or the imbalance in order to detect a leak. This leads to the opportunity to optimise the leak decision if some statistical assumptions hold. A common approach is the use of the hypothesis test procedure

This is a classical detection problem, and there are various solutions known from statistics.

RTTM methods

RTTM means “Real-Time Transient Model”. RTTM LDS use mathematical models of the flow within a pipeline using basic physical laws such as conservation of mass, conservation of momentum, and conservation of energy. RTTM methods can be seen as an enhancement of balancing methods as they additionally use the conservation principle of momentum and energy. An RTTM makes it possible to calculate mass flow, pressure, density and temperature at every point along the pipeline in real-time with the help of mathematical algorithms. RTTM LDS can easily model steady-state and transient flow in a pipeline. Using RTTM technology, leaks can be detected during steady-state and transient conditions. With proper functioning instrumentation, leak rates may be functionally estimated using available formulas.

E-RTTM methods

E-RTTM stands for “Extended Real-Time Transient Model”, using RTTM technology with statistical methods. So, leak detection is possible during steady-state and transient condition with high sensitivity, and false alarms will be avoided using statistical methods.

For the residual method, an RTTM module calculates estimates M ˙ ^ I , M ˙ ^ O for MASS FLOW at inlet and outlet, respectively. This can be done using measurements for pressure and temperature at inlet ( p I , T I ) and outlet ( p O , T O ). These estimated mass flows are compared with the measured mass flows M ˙ I , M ˙ O , yielding the residuals x = M ˙ I − M ˙ ^ I and y = M ˙ O − M ˙ ^ O . These residuals are close to zero if there is no leak; otherwise the residuals show a characteristic signature. In a next step, the residuals are subject of a leak signature analysis. This module analyses their temporal behaviour by extracting and comparing the leak signature with leak signatures in a database (“fingerprint”). Leak alarm is declared if the extracted leak signature matches the fingerprint.