The largely unseen optical fibre that we take so much for granted today is a marvel of science. Normally consisting mostly of pure silica, it has a diameter of just a few millionths of a metre and levels of impurity in the glass of a few parts per billion – leading to optical losses close to the theoretical limit. Optical fibre is also extremely strong and, when deployed correctly, has a lifetime of more than 25 years.
Optical fibre sensors can be used to measure a variety of things – from temperature, pressure and strain to acoustics, rotation, and electric and magnetic fields. They have a number of key advantages over conventional electronic sensors that make them suitable for use in complex applications and harsh environments. These include high sensitivity and resistance to electromagnetic interference. They are also suitable for explosive environments as they generally use no electrical power at the measurement location. And they are reliable in harsh environments – with only a handful of components in the deployed part of the sensor, the probability of failure is very low.
So where are you likely to find these marvels of science? If you’ve flown in an Airbus A380 or Boeing 787 aircraft, you’ve relied on a fibre optic gyro to get you safely to your destination – they’re used to provide very precise navigation information. Your journey to work through road or rail tunnels – and, in fact, your office building itself – are likely to be protected by a distributed temperature sensing (DTS) system that uses optical fibres to detect and locate the rapid rise in temperature arising from a fire.
Power cable monitoring is another use of optical fibres. DTS can detect and locate the appearance of hot spots arising from failing insulation in things like power transmission networks and subsea cables – potentially allowing an operator to perform preventive maintenance rather than simply reacting to failures. In addition, the use of dynamic cable rating software in conjunction with DTS allows an operator to run power cables at their maximum capacity, providing a real economic benefit.
Because of its continuous sensitivity along a cable, DTS can also be used as a leak detection system in situations where a leak generates a localised temperature anomaly – as is the case with liquefied natural gas or insulated steam pipes. DTS can rapidly and reliably detect and locate any anomaly – activating alarms and emergency shutdown systems automatically, if required. In long, buried pipelines, the monitoring of temperature can be combined with a distributed acoustic sensor (DAS) system to rapidly detect both leaks and unauthorised activities along the pipeline route.
Fibre sensors can also be found embedded in the offshore risers that convey oil and gas from the seabed to the surface in oil and gas fields. The risers range in length from a few hundred to a few thousand metres – and monitoring their integrity is essential for safety and environmental protection. Fibre sensors can monitor both the temperature of the fluid being conveyed – for flow assurance purposes – and the mechanical strain (and hence fatigue) of the riser. These measurements enable an operator to optimise the amount of electrical heating – or chemical injection – needed to maintain flow, and to track the wear and tear of the riser. This in turn offers the potential to reduce running costs, perform proactive maintenance and extend the economic lifetime of the riser.
As the oil and gas industry searches ever further from shore for oil reserves – and produced oil tends towards being heavier and more viscous – challenges relating to flow assurance are becoming increasingly significant. The use of DTS to monitor temperature continuously along subsea flowlines and pipelines enables operators to optimise the use of electrical heating techniques to maintain flow – avoiding waste by focusing heating energy only on pipeline sections prone to cold spots.
Optical fibre sensors are also beginning to replace traditional electronic downhole pressure/temperature gauges in more extreme applications – and adding new levels of functionality and understanding of well performance, with a growing track record of in-well measurements of distributed temperature and acoustics, permanent seismic sensor systems and novel fibre-based flow meters.
The oil and gas industry is making increasing use of optical fibre sensors for seismic monitoring. Offering improved performance – such as wider bandwidth and lower noise – and reliability over conventional geophone devices, these are typically deployed permanently in oil and gas wells to monitor the movement of reservoir fluids and so aid productivity, as well as monitoring the fracturing process. Their ability to detect microseismic events also enables an operator to monitor the stability of the formation surrounding the well. Systems deployed permanently on the seabed are also used for monitoring larger areas such as an entire oilfield.
Security is another area where optical fibre can come to the rescue – the use of distributed acoustic measurements can detect and locate human and other activity along a single buried optical cable. This type of technology is being adopted increasingly in areas such as sensitive borders, long pipelines and secure installations.
Meanwhile, fibre-based strain sensors can be embedded in civil engineering structures and alongside longer assets, such as oil and gas transmission pipelines, to monitor small ground movements or frost heave which, over time, can lead to structural failure. And low-cost versions woven into fabric are opening the door to a new generation of truly wearable technology.
But even though optical fibre sensors offer many advances over conventional electronic sensors, they do have limitations. Electronic sensors will often be cheaper, unless very long distances need to be monitored – or the environment is harsh and they will therefore need regular replacement. Distributed fibre optic sensors also have limits to their spatial resolution.
Design and implementation of effective fibre sensing systems can also be challenging. Some types of fibre sensor are sensitive to the quality of optical connectors used – the performance of telecoms-grade connector designs is sometimes not good enough. Fibre sensors can produce large amounts of real-time data, and solution designers may need to consider ways of reducing, storing, transmitting, interpreting and visualising data so as to be able to extract useful, actionable information. And, for some applications, optical fibres require specialist installation teams to ensure success.
But when a fibre sensor system is implemented well, new users are astounded at the levels of performance that can be achieved – and they join the growing group of converts to this fascinating technology.
Read more articles from our Interface magazine about current and emerging technologies and markets.
Andrew is going to be attending Offshore Europe 2015 at the AECC in Aberdeen from 8-11 September 2015 – why not come by our stand 3A42.