PASTE & FILTERED TAILINGS pipeline owners is that existing non destruction test methods for HDPE pipes are effectively limited to visual inspection , meaning that flaws internal to the pipeline cannot be detected .
Managing the problem
Due to the significant repercussions of pipeline failure , various operations have attempted to automate leakage detection by more traditional means . In the oil and gas industry , it is possible to monitor pressure drop at various locations along the pipe or use a process called mass balance , which measures the total mass ( or volume ) of material moving past a measured point .
However , due to the variable density and hydraulic nature of tailings , fluid pressures are not transmitted well along the pipeline . When looking at mass balance , the variances in density pressure and flow rate found in tailings plus the abrasive nature of the material seriously hamper accuracy and leak detection . Any detection of a leak whose flow is below five percent of total flow is not realistically achievable in the field .
A common thread to both of these methods is that neither technology will identify the location of a leakage or rupture , only that it has occurred between two sampling points .
Subsequently , leakage detection on tailing pipelines , although proving to be a large risk to operations , is conducted via routine visual inspections , meaning leaks can go undiscovered for long periods risking regulatory enquiry , occupational health and safety risks , and potential loss of license to operate .
A better solution
The ideal solution for this problem is a system that detects physical effects created by leaks in real time and identifies the precise location of a leak . Fortunately , due to recent advances in modern computing and electronics , this ideal solution is now a reality in the form of distributed fibre optic sensors .
Fibre Optic Sensors are systems which are able to determine physical effects occurring to a fibre optic cable over vast distances . These devices operate by connecting to a fibre optic cable and using focused lasers that excite the fibre , so that returning light wavelengths can shift in response to changes in vibration , temperature and strain in the cable itself . These shifts are detected and their origin located and are analysed to give a simple digital output that a leak has occurred . It is then handed across to the plants existing Distributed Control System ( DCS ) or Supervisory Control and Data Acquisition System ( SCADA ) alerting the plant operator . All in real time .
One of the biggest advantages of fibre optic sensors is that the only in-field component is the cable itself ( usually a tight buffered configuration ), which will have a number of ( 6 or more ) optical fibres contained within the cable itself .
How does a system find a leak ?
There are a two main ways that leakages can be detected through distributed sensors . The two methods are called Distributed Temperature Sensing ( DTS ) and Distributed Acoustic Sensing ( DAS ) and they respond to changes in temperature and vibration respectively .
Distributed Acoustic Sensor ( DAS ) systems look for the noise generated by the fluid escaping ( under pressure ) from existing leaks , which then vibrates the pipe wall that the fibre is connected to . This vibration can detect leaks down to half a litre ( 0.13 gallons ) per minute in water pipelines and the abrasive nature and high density of tailings mean that the noise generated is increased over water .
“ For the detection of new leaks a phenomenon called Negative Pressure Pulse is utilised . This occurs when an opening appears in a pipeline , a near instantaneous pressure localised pressure drop occurs at the location of the leak . A negative pressure shockwave then moves out from the leak point through the fluid at the fluids speed of sound in both directions and creates a tell-tale V shaped acoustic signal which is able to be captured by signal analysis software set up to monitor specifically for this signal . Followed by an orifice noise from escaping fluid and then by temperature change due to fluid loss it provides three separate detection methods that can be used to confirm leakages .”
Distributed Temperature Sensor ( DTS ) systems rely on a physical principle called the Thompson-Joule effect , which is attributed to a localised drop in temperature that occurs when any fluid is moved from high to low pressure . An example of this is aerosol cans getting cold with extended use or a barbeque gas cylinder freezing on a hot day . This effect is more prevalent in gases but does occur in liquids and slurries . This localised drop can be detected and subsequently the presence of a leak can be inferred .
Some systems are , based on the Raman principle , utilising a low cost LED ( light emitting diode ) or a simple fibre optic data laser in order to transmit the light required . As these systems rely on less stable light sources , they are only able to use the most basic return signal ( Raman principle ), which limits their range significantly and limits them to temperature sensing ( DTS ) only .
Due to the typically above ground installation of HDPE tailings piles and direct exposure to the elements , the temperature only systems are not recommended for these applications .
When selecting a suitable system for tailings pipelines monitoring , the best choice is a Multivariable Fibre Optic Sensor ( M-FOS ). The main advantage of these sensors is the resistance to false reporting . Typically , these M-FOS systems are configured in such a way that means that both an appropriate DAS and DTS response should be seen at the same location . In the loosest of terms at a given location for a signal to be confirmed to be a leak , an increase in acoustic signal must be present at the same place as a localised decrease in temperature .
This built in “ self-checking ” method means that not only is the system highly accurate but also immune to almost all environmental effects ( such as rain , snow , wind , etc .) and is all but tamper proof .
Due to the optical nature of the signal captured by the device , the systems are immune to all electrical and environmental interference , unless the cable is crushed or has severed the system .
What are the technology limitations ?
Like all sensors , there are a number of limitations that distributed fibre optic sensors have , however , these limitations are based around the difficulty of measurement at extreme range (> 40 km pipe length ) and the difficulty of cable retrofit in underground applications .
In HAWK ’ s experience almost all of the limitations of fibre optic sensing are only encountered when attempting to retrofit to existing infrastructure . Whereas in new installations , these issues are often handled at the design stage .
The Praetorian Fibre Optic Sensing Systems also use backscatter components that are incompatible with mutli-mode fibre .
Multiple-Variable Fibre Optic Sensing ( M-FOS ) systems also use backscatter components that are incompatible with multi-mode fibre . Due to the requirement for the fibre to be on good contact , or “ coupled ” to the application , existing fibre is also not always ideally mounted for sensing through a distributed sensor . Once buried , it is often not economical to retrofit sensing suitable fibre .
Some existing fibres , due to their mechanical design are , although perfect for data transmission , not ideal for sensing due to their physical properties .
Fortunately , tailings dam pipelines are typically shorter lengths ( 10 km or less ) and are usually above ground . Meaning that technology and logistical limitations of the technology are rarely encountered . Because of the ease of application , this technology is often a low cost per metre and is easy to install , compared to alternative traditional methods of detection that can be rapidly designed and implemented without interruption to production . This makes tailings pipelines the perfect application for Multi-variable Fibre Optic Sensors such as the Praetorian Leak Detection System from HAWK .( LDS ) from HAWK . IM
JULY 2022 | International Mining SUPPLEMENT P1