For several years, fiber optic sensors, and in particular, DTS systems, where an optical fiber is used as sensing medium based, have provided higher bandwidth, inherently safe operation (no generation of electric sparks), and immunity from EMI (Electromagnetic Interference) for parameter measurements. DTS systems are used in many industries, including, the oil and natural gas industry, electrical power cable industry, process control industry, and many other industrial applications where distributed asset monitoring is required. Generally, DTS systems use spontaneous Raman scattering as an underlying principle. A light source, typically a laser, launches a primary laser pulse that gives rise to two spectral components namely Stokes, which has a lower frequency and higher wavelength than launch laser pulse, and anti-Stokes, which has higher frequency and lower wavelength than the launch laser pulse. The anti-Stokes signal is usually about an order of magnitude weaker than the Stokes signal at room temperature and is typically a temperature sensitive signal while the Stokes signal is weakly temperature dependent. The ratio between the anti-Stokes and Stokes signals may be used to determine the temperature of the optical fiber.
One challenge with current systems and techniques is the ability to measure temperature profiles with a rapid response to temperature change. A DTS system uses a sensing cable consisting of an optical fiber encased in suitable materials to provide the ruggedness required for the deployment and operating environment. This can mean that the optical fiber is too well insulated from the temperature to be measured fast and accurately. But there are applications in which the response time is critical. One application is linear heat detection systems for use in for example fire detection in structures.
Man-made structures of all kinds often require linear heat detection systems. Early detection of abnormal build-up of heat is critical to prevent large-scale fire damage. Many systems have been used commercially to meet this need. These range from simple smoke detectors to thermocouple systems that provide single point detection to distributed systems that may provide continuous temperature readings along an extended distance.
Linear heat detection systems are required in several areas like tunnels and subways. Early detection of abnormal heat sources may be used to prevent fires before a fire occurs. In the event of fire, the systems provide an indication of a fire. There are many different systems available on the market ranging from simple on/off alarms like smoke detectors to thermo-couples that provide single point temperatures to distributed sensing systems that may provide continuous temperature readings along the full length of the sensing cable.
Electric cabling systems that can extend over long distances in tunnels are an example application in which linear heat detection is important. Current approaches can be divided into single point sensors of various kinds and digital linear heat detection systems that provide indication of hot spots along the length of a cable.
Another application for linear heat detection is the need to track fluid flow in oil and gas wells by monitoring differences in fluid temperature. Monitoring the movement of a fluid front or slug of chemicals rapidly can indicate which perforated zones are being treated by chemicals and allow the operator to divert the flow in real time to the zones that did not get fluids/chemicals.
A related application is the monitoring of certain kinds of pipelines such as sulfur pipelines. One of the critical aspects of the system performance criteria for a sulfur pipeline is the ability to re-melt solidified sulfur in the pipeline. Under adverse conditions, there is a possibility that power to the pipe heating system could be cut-off. If a prolonged power outage occurs, the down time may be long enough to result in sulfur solidification inside the pipeline. Liquid sulfur shrinks in its volume by approximately 10% as it changes from liquid to solid. The reduction in volume will create voids at various locations and packed sulfur in other locations. When the sulfur pipeline is re-heated, excessive pressure generated by uneven melting of sulfur (expansion) in pockets could burst the pipe due to localized high pressure.
The use of fiber optic temperature monitoring and control systems offers incredible insight into the temperature profile of the pipeline, especially during re-melt conditions.
Single point temperature sensors may be based on e.g. thermistors where the resistance changes with temperature. These devices require two electrical leads per sensor and the number of electrical leads will therefore increase proportionally with the number of sensing points required. The drawbacks of electrical single point sensors are:                (1) The number of leads required grows proportionally with the number of sensing points.        (2) The location and spacing of single point sensors are critical and a fire may start at a location between sensors and this may increase the time before a fire is detected and preventive actions can be taken.        
To overcome the second drawback, digital Linear Heat Detection Sensing (LHDS) systems have been brought to the market, especially for fire detection. A distributed Digital Linear Heat Detector sensing cable often comprises a twisted pair core cable. Each core has a carefully selected metal to add tensile strength, good conductivity while providing good corrosion resistance. This twisted pair core cable will then be deployed under tension. The conductor has special heat reactive polymer insulation. The cable has an overall protective sheath.
The primary mechanism of heat detection in such a system is that the inner core insulating polymer is specially formulated such that it plasticizes at a specific temperature. The cable is constructed such that the twisted steel cores are in tension, and at the polymer trigger temperature the conductors connect. This provides an alarm signal to any associated monitoring device. The system will be able to detect a single point fire or a heat source that exceeds the polymer trigger temperature. The drawbacks with this type of Linear Heat Detection Systems are:                (1) The fact that only a single point fire can be detected as the sensing cable will be in electrical contact (short circuit) at the point of the closest fire and the system cannot detect another event further down the cable.        (2) The fact that no advance warning is available before the cable polymer coating reaches the set temperature and melts. There may be a gradual increase in temperature over a long time that may be detected using a well placed temperature measuring device like e.g. a thermo-couple.        
To overcome the drawbacks of electrical sensors, Distributed Temperature Sensing (DTS) systems based on fiber optics has been introduced to the Linear Heat Detection System market. Fiber optic based DTS systems provide near real-time temperature measurements along the complete length of an optical fiber. A DTS system uses a sensing cable consisting of an optical fiber encased in suitable materials to provide the ruggedness required for the deployment and operating environment.
The DTS system transmits a laser pulse down the optical fiber. As the laser pulse travels down the optical fiber, it interacts with the molecular structure of the fused silica in the optical fiber core. These interactions cause a fraction of the light to scatter back towards the DTS system. The most common back scattered signals are Rayleigh, Brillouin Stokes and Brillouin anti-Stoke components as well as Raman Stokes and Raman anti-Stoke components. Filtering out and measuring selected components of the back-scattered light allows calculation the temperature along the optical fiber.
The ratio of the intensities of the Raman Stokes to Raman anti-Stokes components can be used to calculate the temperature at the point where the scattering event took place. The refractive index of the optical fiber is well known, and the speed of light is well known. This allows a time-of-flight calculation to be done by measuring the time between launch of the laser pulse and the return of the backscattered light.
The drawback of existing DTS is that the Raman Stokes and anti-Stokes signals are very weak and a fair number of averages must be done to achieve a good temperature resolution. This has in the past limited the response time of DTS based LHDS systems. Recent advances in DTS technology, has improved the performance of the systems to a point where the cable response time to thermal events is a significant portion of the overall system response time.
This invention disclosure outlines a system and method to significantly improve the overall system response time.