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PIPELINE HAZARD MITIGATION USING FIBER OPTIC MONITORING SYSTEM

General description

Fiber-optic geotechnical monitoring system (FOGMS) includes 3 basic components: analyzer (or interrogator), distributed (uninterrupted) fiber-optic sensor and a dedicated software (Fig.1). The analyzer is a powerful diagnostic instrument for distributed measurement of strain and temperature over 65 km per channel, allowing measurement of thousands of locations by means of a single sensing cable. Use of internal optical switch featuring 2 measuring channels allows uninterrupted measurement of up to 135 km in two opposite directions from the analyzer. The number of measurement channels can be further extended by using an external multiple optical switch module and star topology. Once an external switch is connected to the analyzer, up to 21 sensors can be used for distributed sensing.

Distributed sensor is an uninterrupted fiber-optic cable that is often custom-designed for a specific application and environment. Each millimeter of sensor is used as a sensitive element offering a great alternative to numerous point sensors. Taking into account high spatial resolution of the analyzer each 50 centimeters of sensor can be considered as an individual point sensor, therefore 50 centimeter sensor section is equivalent to 100 000 point sensors. Fiber-optic distributed sensors are fully passive and do not require connection to power supply. One sensor is usually connected to the analyzer from one or two ends and is adjacent to the entire length of a monitoring facility. Optical fiber integrated in sensor design is sensitive to various external parameters (temperature, elongation/compression, acoustic pressure, etc.) by changing its optical properties. Thus, fiber-optic geotechnical monitoring system offers a wide range of performances and suitability for different applications.

The system automatically controls an extended facility in every section with an installed sensor providing accurate measurement capabilities in real time. The superior sensing technique and operation flexibility makes FOGMS a unique and unrivalled solution to most demanding applications. For the details of system’s operation principles and advantages of fiber-optic sensors, please refer to Section “Resources”.


A)  

B)

 

C)

Fig. 1. FOSGTM includes a set of components which consist of:

A) Analyzer unit and supporting equipment mounted in a 19” rack cabinet; B) Fiber-optic sensor; C) Dedicated software to be installed in a server room.

 

Ground control at zero temperatures

Knowing ground temperatures for pipelines crossing permafrost areas is the key to managing major risks to pipeline integrity. Controlling temperature range between -3°C to 0°C is of particular importance because thawing and further transition of soil from permafrost to thermokarst take place within this interval. The process of ground transition from permafrost to thermokarst is usually associated with sudden decrease in bearing capacity of soil. Opposite seasonal transition to a frozen ground may induce frost heave. Thus control of ground temperature allows to make an early assessment (early forecast) of structural behavior of pipeline supporting grounds ensuring accurate and timely prediction and mitigation of hazardous processes.

Commonly the addressed tasks are solved by visual inspection methods, for example, using thermometric wells located within 0.5 km interval along the pipeline route. The method involves periodic visual checks made by on-site team members. This method has two main disadvantages: point-to-point location of the wells which is ineffective for control of all pipelines sections; and manual data acquisition technique featuring long time intervals between measurements.

The proposed alternative to traditional method is free of these disadvantages. The complete monitoring solution provides uninterrupted control of the entire pipeline section where fiber-optic sensor is installed. Automatic measurement settings maintain highest measurement performance and can be adjusted to specific customer’s requirements (each 5 min., each 1 hour, each 24 hours, etc.). High spatial resolution measurements offer accurate localization of the critical event along 1-2 m pipeline section with temperature resolution of ~ 1°С.



Leakage

 

Leakage detection solution provides protection over the entire length of the pipeline enabling to dramatically minimize operating costs and environmental risks. Leak detection is based on Joules-Thompson effect, which is identified by the development of hot spot.

In case of liquid pipelines, leaking fluid induces the local increase of the surrounding soil temperature as shown in Fig.2. Leakage detection system continuously monitors the entire pipeline detecting threats to pipeline integrity by analyzing soil temperature distribution. A temperature gradient developing along the pipeline is used by the monitoring system to identify size and location of the leak. The extremely low detection limit guarantees the early detection of even small leaks (~ 0.1% of the transported volume) with 1-2 meter accuracy before they develop into large leaks. Compared to conventional leakage detection techniques, the proposed monitoring system offers best performance without the risk of false alarms.

If the pipeline carries natural gas the surrounding of the pipe will be cooled due to gas pressure release. Typically gas cooling effects are ~4°C per each MPa of the excessive pressure caused by a leak. The monitoring system demonstrates temperature sensitivity ranging from 0.2°C-0.5°C. Apparently a high temperature resolution offered by the leakage monitoring system meets the pressure requirements of urban gas pipelines (0.2- 1.2 MPa) and transmission gas pipelines particularly.


Fig. 2.  Temperature effects caused by leakages

 

To optimize the detection of temperature events resulting from the leaking fluid in buried pipelines it is recommended that temperature sensor is placed on top of the pipeline.


Ground movement

Any thawing, soil heave, landslide and other soil-related geologic processes are recognized as major threats to an engineering construction. Additional elongation/compression induced on the sensing cable is usually caused by geohazards or ground movements and can be used for calculation and localization of strain events. As shown in Fig.3 below, one sees that the original section d of sensing cable is submitted to a constant strain ɛ, whereas the rest of the cable remains strain-free. The cable elongationd depends on the lateral displacement L and the strain ɛ is simply given by:

 

 

By transforming the formula, we obtain:

 

where L / d ratio provides information about the magnitude of sensing cable displacement.

 

 

Fig. 3.  Schematic representation of elongation/compression induced on fiber-optic sensing cable caused by a landslide or other geohazard.

 

To avoid the slippage of strain sensor in the soil, a combination of a corrugated cable, strain sensor and ground anchors are used (please see Fig.4 below). In the case of large ground movements, when excessive loads close to the tensile limit of strain sensor are generated, the ground anchor (represented at Fig. 4 (B)) snaps off from the strain sensor protecting it from breakup. In more detail, this construction features the detachment of anchor flange and not the entire anchor. Eventually, the ground monitoring in this section will become less sensitive, yet its integrity and functionality are maintained.

 
   

A)

B)

Fig. 4. The classical version of ground anchors rigidly fastened to strain sensor (A)

Anchor with a latch mechanism designed by «Laser Solutions», CJSC (patent pending) (B).

 

Pipeline deformation

When pipelines are crossing critical unstable grounds due to seismic activity, permafrost or other harsh environmental conditions the direct pipeline strain measurements are required. Distributed pipeline deformation monitoring system is designed to control these critical pipeline regions. To achieve deformation measurements strain sensing cables must be directly attached to the pipeline structure (as illustrated in Fig.5). For best measurement performance at different pipeline sections strain sensing cables are installed to the pipe in three positions along its length. Such configuration enables to control the stressed-deformed state of the pipeline and compute pipeline deformation repositioning in space, i.e. 3-D modeling. The system features strain range of 10 000 microstrain (10mm/m) combined with sensitivity from 25 microstrain (25 um/m), which depend on the project conditions (such as number and location of sensors, sensor bonding method chosen for the project, opportunity of temperature compensation, length of the monitored structure, acquisition frequency, mathematical algorithm, etc.).

 
   

A

B

Fig. 5. Sensing cables (three strain sensors and one temperature sensor) installed on Sakhalin-Khabarovsk-Vladivostok gas transmission pipeline. Systems integrator – “Laser Solutions”, CJSC.  

 


It should be noted that ground movements are recognized as the ultimate cause that submits a transmission pipeline to large strain. Therefore, ground movement monitoring enables to record the detrimental effects of ground movement at an early stage in order to take preventative measures related to ground reinforcement and pipeline strain relief.
Fig. 6 illustrates typical positioning of sensors on the pipeline. Several methods were developed by Laser Solutions engineers to perform the installation of sensors on the pipeline. A fully automated gluing station depicted in Fig.7 was designed for large scale installations. The automated gluing station enables the simultaneous installation of 3 strain sensors set on the pipeline at the 9, 12 and 3 o’clock positions. The developed device reaches an installation speed of 5m of pipe per minute including pipe surface cleaning treatment, pipe warm-up, hot-melt-adhesion, attachment of strain sensor with controllable tension, attachment of the protective polyethylene tape and rolling of the protective tape. Follow this link to see demonstration of automated gluing station capabilities:  Automated gluing station-video (47,7 Мб).

 

Fig. 6. Typical layout of sensor installation on the pipe (three strain sensors with a protective tape and one temperature sensor).


Fig. 7  Fully automated sensor gluing station developed by «Laser Solutions», CJSC . Automated gluing station enables the simultaneous installation of 3 strain sensors set on the pipeline in different positions. The developed device reaches an installation speed of 5m of pipe per minute including pipe surface cleaning treatment.

 

Intrusion

Among other threats leading to potential buried pipeline failure are various unauthorized interferences, such as intrusion, hot taps, thieves, unintentional work in construction area, digging, human, animal, vehicle activity etc. Those threats can be mitigated with use of complete vibro-acoustic monitoring system.


Any cases of unauthorized access to the pipeline are leading to ambient vibrations in the ground and acoustic noises. All of them can be identified and localized by the analyzer connected to special optical sensing cable embed in the ground.  Sensibility to the detected even and time of an advanced warning sent to an operator depends on project conditions, such as sensor laying depth, soil type, snow pack, type and specifications of the controlled facility, remote location of the facility from sensor, etc.

 

Event

Distance from the cable

Human activity

Digging activity (shovel)

15 m

Light equipment

30 m

Heavy equipment

Excavating works

50 m

 

 

 
 

A)

B)

Fig. 8. The correspondence between pipeline interference and systems distance detection range (A),(B).

 

Intellectual safety system uses dedicated algorithms for a precise event detection and classification. The accurate assessment of alarms and events is crucial for correct responsiveness of the system to true events. The vibro-acoustic system features the following benefits:
- alarm threshold that can be associated to ambient noises to reduce false events (as illustrated in Fig. 9 A);
- ability to distinguish moving objects from footsteps (Fig. 9 B);
- compatibility with video cameras focusing at event localization (can be used for target tracking and localization);
- tracking a certain type of event by analyzing amplitude-frequency images of the events that caused vibration. The system can provide early warning of an event before the activity gets within damaging range of the pipeline. System algorithm recognizes the events by classifying them in a database and filtering out non-threatening activity. Example: tapping – pedestrian – digging - damage to block valve station - leakage.
- GIS option used to display structure on the map if an optimized performance is required (Fig.9 B).

   

А)

B)

C)

Fig. 9. GUI of the complete vibro-acoustic monitoring system: reflectogram of vibro-acoustic influence (A); 3-D diagram «coordinate»- «time»- «influence of amplitude» (horizontal, vertical and colored axis (B); system mapping (C).

 

Dynamic strain

Additional benefits of using FOGMS and an optical sensing cable installed on the pipeline can be obtained by controlling dynamic strains induced to the optical cable at frequencies between 1 HZ- 5 kHz (applicable for linear and areal facilities). Dynamic strain is controlled by registration of the acoustic pressure signals emitted by the pipeline. Acoustic pressure signals reaching the fiber inside an optical cable causes change of its optical properties. The negative impact to the pipeline can be thus controlled by FOGMS offering a range of benefits for various applications:

Ensuring pipeline integrity in high seismic activity regions (unlike indirect methods of seismic identification, such as using data registered by seismic stations and recorded by seismic sensors, that are generally located in remote areas from the pipeline routes, FOGMS provides direct monitoring of certain exposure induced to the pipeline or areal facility during seismic event); 

Real-time monitoring of pipeline pigs (internal cleaning, inspection of pipeline condition by flaw detection devices); identifying anomalies in pipeline behavior; localization of the pigging equipment stuck in the pipe within ~ 10 m distance accuracy.

 

Fig. 10. Illustration of a pipeline pig that can be localized with a complete vibroacoustic monitoring system.

 

Telecommunications

Fiber optic sensor is a part of complete geotechnical pipeline monitoring system that can be both used for sensing and telecommunications.
Usually it includes an excess number of single mode fibers that can be used telecommunications (ITU G.652 or G.655 standards). First of all, these fibers are intended to provide data transmission from single analyzers to a central control room. Additionally to monitoring these spare fibers can be used for telecommunication purposes of the Customer

Fig. 11. Data transmission channels in optical fibers (picture is the courtesy of Cisco)

 

“Intellectual pipeline” concept

The complete geotechnical monitoring system for pipeline hazard mitigation presented in this article can be used as one of the major system for developing the concept of “Intellectual pipeline” addressing the advanced safety and operational issues of 21st century. It’s not just about its wide range of functionality, data acquisition capacity and unrivalled accuracy. The main opportunity offered by the system is the possibility to use uninterrupted fiber optic sensor of the monitoring system for advanced communication services to central control room. Fiber optic sensor appears as a transmission line interpreting and correlating data that comes from numerous sources. The only effective way to deliver reliable, stable and fast communication today is with fiber optic based infrastructure. Overall framework of transport system as part of “Intellectual pipeline” concept is shown in Fig.12.

Fig. 12. Overal framework of gas transmission pipeline with an installed fiber optic geotechnical monitoring system (CS-compressor station, BVS- block valve station, RTU – remote terminal unit).

 

 

 

 

   

 

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