Leak Detection Systems as a Central Component of Pipeline Safety Design

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Introduction:

Transportation of fluids in pipelines is increasing all over the world, and with good reason: pipelines are among the safest and most economical transportation systems over long routes. To ensure this cost-effectiveness and safety, both new, and especially existing, pipelines must reflect the standard of the most current technology. Leaks pose a potential safety risk. Leaks occur for a wide variety of reasons, from earthquakes, corrosion and material failure to drilling by product thieves. Special leak detection systems are often used to limit these risks. In general, leak detection in pipelines refers to the recognition and quick localization of product leaks.

Reasons to employ leak detection include the following:

  • To minimize the effects of accidents
  • To minimize downtime
  • To minimize product loss
  • Regulatory compliance

Leak detection in pipelines can be performed in various ways, from simple visual controls during inspections to computer-supported systems that monitor conditions, even for underground and undersea pipelines.

Main body:

Selecting a suitable leak detection system is not an easy task for pipeline operators. The system must meet the needs of the particular application and comply with [relevant] regulations. In this article, regulatory requirements will be discussed first. The most important regulations for North America are API RP 1130 and CSA Z662-2011 Annex E.

API RP 1130 is published by the American Petroleum Institute, which is the largest trade association in the oil, gas and petrochemical industries in the USA with influence that reaches far beyond America. API RP (Recommended Practice) 1130 is even more detailed with regard to leak detection systems. Among other items, it includes a collection of general recommendations for operating leak detection systems, such as clear presentation of the results for the operator and for maintenance. It also includes performance criteria for selecting a leak detection system: these criteria are very detailed and explain how leak detection systems work. The criteria are outlined below, and it is easy to see that they are linked and interdependent.

  • Sensitivity: the leak detection system should detect even small leaks within a short period.
  • Precision: the leak detection system should locate leaks precisely. The leakage rate, the quantity of escaped product (leakage rate multiplied by time) and the product that is escaping should all be indicated.
  • Robustness: the leak detection system should continue active monitoring despite unsteady or non-ideal conditions. This includes conditions such as temperature fluctuations, changes in viscosity and sensor failure. It also includes unsteady operating conditions, also known as transient operation, for example due to effects triggered by pumps or valves.
  • Reliability: the leak detection system should not generate false alarms, even though it is highly sensitive.

Canadian Standards Association CSA Z662 Annex E represents recommended practice for liquid hydrocarbon pipeline system leak detection in Canada. It emphasizes the need to establish a procedure for making material balance for the entire product transported: when designing, operators should consider all physical and operational factors that can influence the material balance system and establish tolerances. Under normal operating conditions, the uncertainty in the receipt and delivery values used in the material balance calculation, including uncertainties attributable to processing, transmission, and operational practices, shall not exceed 5% per five minutes, 2% per week, or 1% per month of the sum of the actual receipts or deliveries.

To meet these requirements, the uncertainty in the individual receipt and delivery measurements under installed operating conditions shall not exceed 2% of the actual measurements. Hereby, the CSA Z662 Annex E is the only recommended practice to include precise uncertainties for leak detection systems.

When the operator has clarified relevant requirements of the appropriate regulatory for his application, other characteristics that affect the choice of leak detection system can be considered. These may include technical and environmental parameters such as the length of the pipeline, whether it runs above or below ground, and the volume, type and quantity of different products to be transported. The desired type of monitoring may also be considered: internal (using process measurements), or external (using special measurements).

The next step is to review systems available on the market. Assuming an internal system has been specified, the preferred type all over the world, the options are reduced to a handful of systems based on various mathematical and physical principles. The RTTM (“Real-Time Transient Model”) is the leading technology at the moment. RTTM is a mathematical model that compares measurements taken during the actual operation of a pipeline with those of “virtual pipeline”, or a computer simulation of the pipeline, in real time. KROHNE, a manufacturer of measuring technology and established supplier of systems to the oil and gas industry for more than 30 years, has expanded its product range to include E-RTTM, a leading technology for continuous internal monitoring of pipelines. E-RTTM stands for Extended RTTM, which combines RTTM principle with leak signature analysis using leak pattern detection (see Figure 1: Functional principle of a leak detection system based on E-RTTM).

Combining two principles has several advantages. In 2012, the U.S. Department of Transportation Pipelines and Hazardous Materials Safety Administration published a Leak Detection Study, DTPH56-11-D-000001, which states: “The leak detection system itself should always be redundant, by using multiple techniques that differ from each other and therefore compensate for any inherent weaknesses they do not share.” It also describes the benefits of combined leak detection methods: “There is no reason why several different internal leak detection methods should not be implemented at the same time. In fact, a basic engineering robustness principle calls for at least two methods that rely on entirely separate physical principles. As an example, the Extended-RTTM system trademarked by KROHNE uses an RTTM in conjunction with several other API 1130 techniques”.

Figure 1: Functional principle of a leak detection system based on E-RTTM

An E-RTTM leak detection system creates a virtual image of a pipeline based on real measured data. Measurement values from flow, temperature and pressure sensors installed at the inlet and outlet of the pipeline and along the pipeline in places such as pump and valve stations are crucial. The flow, pressure, temperature and density at each point along the virtual pipeline are calculated from the measured pressure and temperature values. The model compares the calculated flow values with the actual values from the flow meters. If the model detects a flow discrepancy, the leak signature analysis module then determines whether it was caused by an instrument error, a gradual leak or a sudden leak.

The increased capacities of modern computers allow leak signature analysis to apply powerful statistical hypothesis testing, outperforming the detection times of A. Wald’s old fashioned SPRT algorithm from 1942, while still providing the same confidence into the results. Based on modern statistical tests, the signature analysis decides, whether the pipeline is affected by a leak or not. Signature analysis is critical to reliability because it provides a high degree of protection from false alarms.

The performance criteria from API RP 1130 provide a useful guide to the detailed functions of the E-RTTM. It provides a high degree of sensitivity and quick leak detection with real-time comparison of existing measuring results against leak signatures, which are stored in a database. Comparison of measurement values with the leak signatures is also critical to reliability because it provides a high degree of protection from false alarms. E-RTTM-based leak detection systems are able to handle changing or transient operating conditions that are not recognized by less sophisticated internal leak detection systems. An E-RTTM-based leak detection system works with dynamic values, which also affects robustness: the system can adapt automatically and very quickly to changes in the operating conditions such as sensor failure, communications failure, a valve closing or a product change in the pipeline.

The precision of the E-RTTM is based on three different methods of leak localisation: the gradient intersection method, the wave propagation method and the extended wave propagation method. The leak detection system calculates the most probable leak location(s) by comparing the results of these methods. The gradient intersection method is based on the pressure profile of a pipeline: the occurrence of a leak changes the pressure gradient along the pipeline in characteristic manner (see Figure 2: Leak localisation by the gradient intersection method). Without a leak, the drop in pressure in a liquid pipeline is linear (blue line). When there is a leak, the pressure gradient changes and two linear segments appear with different slopes (orange). The leak position can be determined by calculating the intersection point.

Figure 2: Leak localisation by the gradient intersection method

The second option for leak localisation is the wave propagation method, which analyzes the pressure waves that result from a leak. If a sufficiently large enough leak occurs suddenly, for example if the pipeline is damaged by an excavator, a negative pressure wave spreads at the speed of sound in both directions along the pipeline. The leak position can be calculated by comparing the arrival time of the pressure wave at the pipeline inlet and outlet pressure sensors (see Figure 3: Leak localisation by the wave propagation method).

Figure 3: Leak localisation by the wave propagation method

The extended wave propagation method is based on the same physical principle as the wave propagation method. It takes into account additional values from pressure sensors installed in measuring and control stations along the pipeline, for example, and speed of sound data for the current product. This enables more precise localisation of the leak by reducing errors due to delayed sensor reaction or slow signal transfer, (see Figure 4: Leak localisation by the extended wave propagation method).

Figure 4: Leak localisation by the extended wave propagation method

The E-RTTM introduced here is the basis of the PipePatrol leak detection system by KROHNE. The company, which supplies systems to the oil and gas industry, also provides pipeline monitoring technology. The system is suitable for monitoring liquid and gas pipelines (including liquefied gas and supercritical products) and meets all the requirements of API RP 1130 and CSA Z662 Annex E. PipePatrol is very easy to use: it is installed on a dedicated server and operates completely autonomously. The user interface can runs on a separate workstation, or be integrated into an existing control system (see Figure 5: PipePatrol leak detection system, basic topology. The user interface features intuitive operation: only the information that the current user needs for his scope of work is displayed.

Figure 5: PipePatrol leak detection system, basic topology

In principle, PipePatrol can be integrated into any new or existing infrastructure; this applies to both the control system and the measuring technology used. An example of a more complex application is shown in Figure 6: PipePatrol leak detection system, extended topology.

Figure 6: PipePatrol leak detection system, extended topology

Operators can learn to use the system in just a few hours. In addition to the visualization of the pipeline operating conditions, PipePatrol can indicate leak positions on a map, which simplifies and speeds up a service technician’s work. Ethernet and serial interfaces support protocols such as OPC, Modbus TCP/lP, Modbus Serial, HART and PROFIBUS.

Application note

In addition to standard applications (liquid, gas, liquefied gas and supercritcial products), PipePatrol is also capable of monitoring pipeline networks. Based on KROHNE’s many years of experience in the area of pipeline monitoring, the system offers outstanding performance and adapts quickly to changing operating conditions. An example application in Canada demonstrates how quickly and precisely leak detection functions in practice. A major oil & gas company operates several pipelines for which CSA Z662 Annex E applies. Following thorough consultation, the company opted for the PipePatrol leak detection system. KROHNE commissioned, configured and tuned the system on site. During the project execution according to ISO 9001 standards, the documentation to fullfil CSA Z662 Annex E requirements was created and handed to the customer. PipePatrol used the measurement values provided by the process control system and was integrated into the pipeline monitoring system at the customer’s request. The leak tests performed for the site acceptance tests were conducted  using valves in the pipeline to simulate leaks by real fluid withdrawal into a vacuum truck. The following results were achieved:

The first pipeline is a condensate pipeline with a length of 24690 m / 15.34 mi. The location of the fluid withdrawal was close to the outlet flow meter of the pipeline. The detection threshold for leaks is set to 1.1 m³/h / 4.84 gal (US)/min . After starting the leak test with a leak rate of 5 m³/h / 22.01 gal (US)/min, the system recognized the signature of the leak within 55 sec and went to “Leak Signature Detected” state. After 2:30 min, it reached the desired statistical confidence level and raised the leak alarm. At this point, only 208 l / 54.95 US gal were spilled. Gradient intersection method calculated a leak position of 24689 m / 15.34 mi, while Wave propagation method calculated a leak position of 24677 m / 15.33 mi, both less than 0.1% of pipeline length away from the real leak position.

The second pipeline is a sales oil pipeline with a length of 59700 m / 37.1 mi. The location of the fluid withdrawal was close to the inlet flow meter of the pipeline. The detection threshold for leaks is set to 3 m³/h / 13.21 gal (US)/min. After starting the leak test with a leak rate of 3.5 m³/h / 15.41 gal (US)/min, the system recognized the signature of the leak within 50 sec and went to “Leak Signature Detected” state. After 6:40 min, it reached the desired statistical confidence level and raised the leak alarm. At this point, only 380 l / 100.39 US gal were spilled. Gradient intersection method calculated a leak position of 0 m / 0 mi, less than 0.1% of pipeline length away from the actual leak position. Wave propagation method was disabled for this test because time stamping was temporarily not available for the measurements, but has been activated in the meantime.

Conclusion:

Modern leak detection systems are based on various mathematical and physical models. The pipeline properties as well as regulatory requirements must be taken into account when selecting a system. The most advanced technology currently available is the E-RTTM model. E-RTTM-based leak detection systems guarantee reliable leak monitoring for various types and lengths of pipelines, even under transient operating conditions. KROHNE supplies the PipePatrol E-RTTM-based leak detection system either installed on separate hardware or for integration into an existing control system and measurement installation.