Introduction to Distributed Temperature Sensing

Distributed Temperature Sensing Systems (DTS) are optoelectronic devices which measure temperatures by means of optical fibres functioning as linear sensors. Temperatures are recorded along the optical sensor cable, thus not at points, but as a continuous profile. A high accuracy of temperature determination is achieved over great distances.

Measuring Principle - Raman Effect
Physical measurement dimensions, such as temperature or pressure and tensile forces, can affect glass fibres and locally change the characteristics of light transmission in the fibre. As a result of the attenuation of the light in the quartz glass fibres through scattering, the location of an external physical effect can be determined so that the optical fibre can be employed as a linear sensor.

Optical fibres are made from doped quartz glass. Quartz glass is a form of silicon dioxide (SiO2) with amorphous solid structure. Thermal effects induce lattice oscillations within the solid. When light falls onto these thermally excited molecular oscillations, an interaction occurs between the light particles (photons) and the electrons of the molecule. Light scattering, also known as Raman scattering, occurs in the optical fibre. Unlike incident light, this scattered light undergoes a spectral shift by an amount equivalent to the resonance frequency of the lattice oscillation.

The light scattered back from the fibre optic therefore contains three different spectral shares:

• the Rayleigh scattering with the wavelength of the laser source used,
• the Stokes line components with the higher wavelength in which photons are generated, and
• the Anti-Stokes line components with a lower wavelength than the Rayleigh scattering, in which photons are destroyed.

The intensity of the so-called Anti-Stokes band is temperature-dependent, while the so-called Stokes band is practically independent of temperature. The local temperature of the optical fibre is derived from the ratio of the Anti-Stokes and Stokes light intensities.

Measuring Principle - OFDR Technology
Latest DTS evaluation units deploy the method of Optical Frequency Domain Reflectometry (OFDR) . The OFDR system provides information on the local characteristic when the backscatter signal detected during the entire measurement time is measured as a function of frequency in a complex fashion, and then subjected to Fourier transformation. The essential benefits of OFDR technology are the quasi continuous wave mode employed by the laser and the narrow-band detection of the optical back scatter signal, whereby a significantly higher signal to noise ratio is achieved than with conventional pulse technology (OTDR). This technical benefit allows the use of affordable semiconductor laser diodes and electronic assemblies for signal averaging.

The optical frequency domain reflectometry has been developed as a high-resolution measurement process for the characterisation of optical wave guides with length dimensions of just a few millimetres. In contrast, its application for the Raman backscatter measurement was introduced and patented by the company LIOS Technology.

Schematic system set up
The temperature measuring system consists of a controller (frequency generator, laser source, optical module, HF mixer, receiver and micro-processor unit) and a quartz glass fibre (fibre optic) as line-shaped temperature sensor.

The design is three-channel, since an additional reference channel is required besides the two measurement channels (Anti-Stokes and Stokes). Corresponding to the OFDR system, the power output of the laser runs through the sinus-shaped frequency starting from a starting frequency in the kilohertz range through the ending frequency in the high megahertz range within a measurement time interval with the help of the High Frequency (HF) modulator. The resulting frequency shift is a direct measurement of the local resolution of the reflectometer. The frequency-modulated laser light is connected to the fibre optic-sensor via the optical module.

The continuously back-scattered Raman light is spectrally filtered in the optical module and converted into electrical signals by means of photo detectors. Then the measurement signals are amplified and mixed in the Low Frequency spectral range (LF range). The Fourier transformation of the averaged LF signals results in the two Raman backscatter curves. The amplitudes of these backscatter curves are proportional to the intensity of the Raman scattering of the viewed location. The fibre temperature along the sensor cable results from the amplitude ratio of the two measurement channels.


High Reliability and Industrial Strength

The semiconductor laser diode has been thoroughly type tested according the Telcordia GR-468 standard. It fulfils telecom standards with a medium lifetime of more than 25 years. The entire system was comprehensively evaluated by various independent international bodies (e.g. the VdS, the association of German asset insurers) including EMC tests as well as endurance tests at accelerated aging environments. Field data of the huge installed base prove the exceptional high reliability of the controllers.

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Containment Integrity Monitoring Solutions for LNG Carriers

LNG Carrier

LIOS Technology distributed temperature measurement system (DTS-System) is suited ideally to detect reliably and to locate precisely any temperature anomaly caused by an accidental release of LNG. Our innovative and patented OFDR evaluation method enables the determination of a continuous temperature profile along the entire length of an optical fibre offering thousand of data points at a single measurement. Based on our expert knowledge and our impressive record of safety installations we have developed suitable alarm processing algorithm to detect temperature events fast, reliably and precisely. Also multiple temperature events are identified and pinpointed simultaneously allowing the operator to initiate proper counter measures rapidly.
With the capability to use multiple optical channels and a measurement range of  several kilometres per channel the LIOS Technology DTS system is a cost - sensitive solution for an entire cryogenic supervision including containments, foundations, pumps and piping.  The system supplies precise temperature information which facilitates the control of cool-down operations like load- and unload procedures. Additionally an optional loop set-up of the sensor cable enables a redundant operation of the system. Consequently the entire measurement length keeps maintained in case of single fibre break.  Summarized the operator takes the following benefit from using LIOS Technology DTS system:

• Full information of containment integrity – everywhere at any time
• Significantly reduced off-times due to exact pinpointing of leaks
• Enabling effective and automatic counter measures
• Fast detection of slow leaks as well as sudden leaks
• Better control of load and unload procedures (cool-down operations)
• Maximum safety of people, assets and environment
• Redundant operation thanks to loop set-up of the sensor cable
• Reduced installation efforts – additional wiring is not required
• Low maintenance efforts and cost
• Long lifetime

  Read more:

	LNG and LPG Containment Integrity Monitoring Solutions
	Information centre for LNG operations

Condition Monitoring of high voltage cables: Turning information into extra capacity

Increasing demand for power is forcing power utilities to load power cables to their physical limit, and safety and efficiency concerns are making it more and more critical for operators to understand what is happening thermally both inside the cables themselves, and along the cable route.
Real Time Determination of Thermal Conditions along HV Power Cable Systems
During high-load conditions and under emergency circumstances (such as when a failure occurs in a segment of the grid and power must be shifted to other sections to compensate) it often becomes necessary to load cables right up to the limits permitted by the relevant regulations. In these operational situations, it must be ensured that maximum temperature limits are not exceeded. Due to the increasing complexity of the thermal relationships along cable routes, the ability to continuously measure the temperatures along the cable has proven invaluable, providing critical operational data to engineers, especially in the case of a system faults such as a hot spots that could result in cable failure if they are not corrected.
The intrinsic temperature measuring system DTS based on LIOS Technology’s design, with fibre optic sensors either installed within the power cable’s cross section (FIMT – Fibre in metal tube ) or attached to the exterior of the cable, makes it possible to record the temperature profile along an entire cable route continuously, and to pinpoint the exact location of hot spots within a metre. Since the measuring principle employed is purely optical, the presence of electromagnetic influences, which can result in false sensor signals in other technologies, does not affect the DTS unit.
Distributed temperature sensing is a powerful tool that allows the accurate rating of high voltage power cables in real time, and provides the following operating benefits:

• Reduce power outages or blackouts
• Ensure continuity of supply
• Activate hidden capacity reserves of existing assets
• React quickly to overload conditions
• Conduct precisely and in real-time load predictions as new sources of energy are added to the grid

The temperature profile data, in any desired form or format, can be transmitted via standard interfaces from systems of the OTS product series as required, and be either displayed or further processed by PC, PLC or SCADA systems. As a result of the increasing demands placed on the ability to network and integrate measuring systems into management systems, LIOS offers network components that summarise DTS data from several DTS units and transport or convert it to the required network standards and protocols like Modbus, DNP3, IEC60870-5 or XML based data interfaces.
Dynamic Cable Rating
LIOS also provides an integrated Real Time Thermal Rating (RTTR) package via a well defined interface between its DTS data visualisation software and a commercialised – the industry’s de facto standard – cable ampacity program based on IEC standardised methods (mainly IEC 60287 and IEC 60853). Dynamic rating software couples the accurate temperature data provided by the DTS unit to predictive functions of cable ratings, providing engineers with even more information upon which they can base distribution and load decisions. The result is dramatically more efficient transmission of power, fewer “brown-outs”, and decreased operating costs.

Reliable Energy Supply for the city of Graz, Austria

In order to improve the energy supply for the city of Graz, CCC GmbH Berlin installed new cable connections across the town between the transformer stations of Graz-South and Graz-West. The entire underground engineering, i.e. the joint base installation mand complete trenching construction was built by STEWEAG-STEG GmbH. The new cable installation is one of the city’s main transmission lines and is heavily loaded. In order to optimize the transmission performance and to guarantee its secure reliability, the cable installation has been equipped with large scale cross-bonding systems at its sheath. In addition, by the help of the LIOS Distributed Temperature Sensing (DTS) system, the level of backup capacity can be assessed. These optical fibres can monitor the cable temperature along the trench and have been integrated into the sheath area of the cable.

Highlights

• 110 kV high-voltage power cables between transformer stations Graz-South and Graz-West
• 3 cable lines with a length of 3,2 km each (total 9,6 km cable),one of which is equipped for the entire distance of 3,2 km with integrated optical fibres in a steel tube integrated in the copper screen of the HV-cable for temperature monitoring (DTS)
• 18 sets of accessories
• Cross-bonding system in order to maximize transmission performance
• During the planning period numerous calculations for currentcarrying capacity were carried out in order to judge the influenceof crossing long-distance heating lines.
• Direct DTS data interface for temperature and operational data into the overall SCADA system of STEWEAG-STEG (Fernwirkanlage STEWEAG-STEG) using protocol IEC 60870-5-104

NKT group to decrease emission of CO2 by 12 percent over three years

“Recycling and the environment have been an integral part of NKT’s working agenda for many years. But our ambition is to be even greener,” says NKT’s CTO Søren Isaksen.

Over the next three years NKT will cut corporate carbon emission by a further 12%. The announcement of this specific target is a direct result of NKT’s decision last year to join the Carbon Disclosure Project.

“When submitting our first report to the Carbon Disclosure Project we could see that we had actually exceeded our own expectations. In the period 2001 to 2008, in our cable business, we reduced CO2 emission relative to output by 48%. So we feel we are already well on the way,” says Søren Isaksen, Chief Technology Officer for the NKT Group.

“However, this does not prevent NKT from setting new targets,” stresses Søren Isaksen. “In future, the ability to document carbon dioxide emission will be an important competitive parameter. Just as we impose demands on our suppliers, our customers impose demands on us. In this way, all players in the supply chain influence the environmental and carbon balance sheets.”

For more information on the Carbon Disclosure Project see:

• Carbon Disclosure Project to assess world business CO2 footprint


• Carbon Disclosure Project


• NKT lowers CO2 emission

Setting Standards for Highly Reliable DTS Performance

LIOS Technology GmbH is the leading manufacturer of industrial DTS systems with an impressive track record of more than 2000 installations in permanent operation. Its current DTS product series was carefully designed and thoroughly tested targeting reliable performance and smooth operation in industrial environments.
This was imposingly endorsed by the latest statistical field analysis in respect of a mean time between failures (MTBF) evaluation. Adapted from field data of our installed base of the current DTS product series an excellent MTBF figure of 28 years was reached.

High reliability is also a result of the unique optical frequency-domain reflectometry (OFDR) technology of the LIOS DTS systems. In contrast to time-domain technology using pulsed lasers with high peak powers, the LIOS OFDR DTS uses a quasi-continuous laser with low peak power resulting in a nonexistent wear out of the laser unit or any other fatigue of other optical components. The exceptional reliability favors the deployment of the LIOS DTS systems in all remote, safety-relevant and industrial applications.

Offshore Europe 2009 - Meet us in Aberdeen - 8-11 September 2009

LIOS Technology will be exhibiting at Offshore Europe 2009, from 8 till 11 September in Aberdeen, German Pavilion. In this show, we proudly present our full range and latest Distributed Temperature Sensing (DTS) systems for Oil & Gas applications.

See the latest information about our new HT SAGD application (high temperature steam assisted gravity drainage) for Oil & Gas including our new hydrogen resistant fibre rated 300 degree C and our latest auto calibration system.

Please contact us directly at oil@downhole-monitoring.com for more information or to make an appointment.

Visitors are invited to stop by the LIOS stand  No. 1397 b.
LIOS staff will be on hand to answer questions and discuss particular challenges.

Please register for free admission at  http://www.offshore-europe.co.uk/LIOSTechnology

Read more at our  Downhole Monitoring website.