For its client on the Isle of Grain in Kent, U.K., Sensornet installed a digital fibre-optic

For its client on the Isle of Grain in Kent, U.K., Sensornet installed a digital fibre-optic leak detection system on the National Grid LNG import terminal.

The fibre-optic monitoring industry has come a long way from the days of delivering vast amounts of unmanageable temperature arrays. Continuous research and the development of distributed-temperature sensing (DTS) using fibre-optic cable over the past decade means the real-time and highly accurate monitoring technology is now delivering meaningful results and is deployed globally across a range of processes in the oil and gas industry.

With conventional technology, there is a gap between what is thought to be occurring throughout an installation and what is actually happening. Sensornet, a global provider of advanced digital-monitoring solutions and part of Tendeka, uses reach, precision and speed to prevent costly and damaging leaks. The company’s revolutionary DTS system overcomes the limitations of measurement technologies to close the monitoring gap.

For floating- and shore-based liquefied natural gas (LNG) facilities, piping system performance and integrity is crucial to safety and reliability. Apart from the temperature profiling of pipe and containment, fibre-optic sensors also offer an intrinsically safe and non-intrusive method of monitoring pipeline and storage tank cool down, the strain on LNG pipe, integrity of the pipe’s insulation and carrying out piping and LNG storage-tank health checks.

Industrial applications of fibre-optic DTS include using the system as an early warning system, allowing operators to act swiftly in the event of an LNG containment breach as a part of the safety management system and providing a real-time temperature profile during loading, offloading and cool down cycles.

The physics of fibre-optic DTS

A technician uses LN2 to simulate LNG leak during the site acceptance test.

A technician uses LN2 to simulate LNG leak during the site acceptance test.

Distributed fibre-optic sensing systems are unique in that the fibre-optic cable is the sensor. By using sensing cables that are based on standard fibre-optic cables, a measurement profile along the entire length of the sensing cable is obtained at intervals. The distance between measurement points along the sensing fibre is referred to as the spatial resolution of the system. Today, DTS systems offer a wide range of spatial resolutions monitored from a few centimetres to several metres to suit the specific process to be monitored. For example, in an installation requiring an optical length of 10 kilometers, it is possible to obtain 10,000 individual sensing points at a spatial resolution of one meter. There is no specialized sensing point required, making it a very simple system for providing total monitoring protection over large distances or around several processes within an installation.

The fibre-optic cable is almost as important as the DTS itself because it provides information. Tests have shown that standard cable and fibre types will detect a leak at its initial stage, but when subjected to temperatures of less than -40°C for even the shortest time, the fibre and jacket construction will deteriorate and become unusable in the future. It may be acceptable in certain applications for these cables to be sacrificial.

For long term use and cool down, monitoring cable and fibres with specialist coatings will be required. Sensornet has tested a number of solutions and has developed cables that are capable of consistently measuring cables down to -175°C.

Leak-detection principle using DTS
The DTS system is based on temperature measurements using distributed fibre-optic sensing technology and can be used to detect both liquid and gaseous leaks. It illuminates the glass core of the optical fiber with a laser pulse with a 10-nanosecond duration, which corresponds to a one-meter pulse. As the optical pulse propagates down the fiber, it undergoes scattering even in the absence of impurities and structural defects. Part of this scattered radiation is known as Raman scattering. Because this vibration energy is a well-defined function of temperature, the ratio of the signals, in conjunction with the time of flight of an optical pulse, is used to determine the temperature of the fiber at any given point.

Due to the nature of the liquefaction process, LNG has a thermal signature that gives rise to a significant difference in temperature from the immediate environment in the event of a leak. However, products in pipelines are often under significant pressure and pipeline-leak detection based on temperature differentials is equally achieved in most products as long as the product is compressed under pressure within the pipe. The resulting leak would lead to a drop in temperature as the effluent exits the pipe.

DTS tests on an LNG piping system
Prior to full-scale installation, a system test is installed by R&M Ship Tec Group to perform suitability-tests for a fibre-optic DTS monitoring system on an insulated test pipe by means of controlled cryogenic cool-down cycles.
These tests include:

• The examination of the fibre-optic DTS monitoring system under cryogenic conditions.
• The durability of the fibre-optic sensing cable against thermal and mechanical loads.
• The installation process and the functionality of fibre-optic DTS system under cryogenic cooling-down cycles.
• The differentiation of installation possibilities of fibre-optic systems on the cryogenic pipe surface as well as between insulation layers of the pipe.
• Determining several options of
sensor locations and zonings.

For these tests, the following systems are used:

• Sensornet’s 4 channel HALO DTS system with a spatial resolution of 2 meters and temperature accuracy of better than 0.1°C.
• 0.8 millimeter-diameter single-fiber core cable, which is highly flexible and chosen for its size and ease of installation between the insulation layers.
• Two types of four-core-cables of 3.5-millimeter outer diameter and with Kevlar yarn strengthening and polyethylene sheath that are less flexible, but sufficiently robust for handling during the installation process.

The HALO system allows the user to take temperature measurements in both single-ended and double-ended modes. The use of the DTS in double-end-mode ensures a more enhanced temperature resolution by the internal calibration of the system than in the single-ended-mode.

The DTS system is qualified to monitor the cool-down process at LNG pipelines and to control the integrity of the pipe insulation. Temperature differences are caused by the sensor location on the pipe surface, the sensor type and the sensor fixation on the pipe surface.

The temperature profile over the circumference of the test pipe is different. The recorded results of the fibre-optic sensing cables are average temperature values from 58 intervals over the circumference of the pipe, each of them with the Halo system set at 2-meter spatial resolution.

A further influencing factor of the temperature accuracy is the fixation of the sensor at the pipe surface. An air gap between the pipe surface and the sensor is comparable to a thin insulation layer. Temperature differences can be expected between sensing cables fixed at the pipe surface and sensing cables not fixed at the pipe surface. Temperature errors should be prevented by the application of the double-end-mode of the fibre-optical sensing system.

Industrial case study of fibre-optic DTS leak detection

The Sentinel DTS data shows a temperature drop during leak detection.

The Sentinel DTS data shows a temperature drop during leak detection.

For its client on the Isle of Grain in Kent, U.K., Sensornet installed a digital fibre-optic leak-detection system on the National Grid LNG import terminal. Based on the Health & Safety Executive (HSE) requirement, the fibre-optic sensing cable was retrofitted in an operational plant without having to shut down or stop operations.

The independent fibre-optic sensing cable loops covering both pipelines provide added resilience to the installation. Additional multiplexer channels also ensure future expansion and flexibility is possible. The system includes self-contained test points at regular intervals on the cable loops, which can also be used after the site acceptance tests (SAT) for periodic testing.

Sensornet’s SentinelDTS XR 20 installation is optimized to provide temperature resolution of 0.1ºC in 10 seconds along the entire length of the pipeline at 1.5-meter intervals. This high specification allows the plant to monitor the pipeline with high levels of accuracy and integrity.

The fibre-optic leak-detection system was designed with two cable loops and two instrumentation units, capable of 1 out of 2 voting (1oo2). Each DTS system provides bi-directional monitoring from either side of the loop. The 24-way relay modules provide hard-wire contact relays to the client SCADA system in the following instances:

• System error.
• Rate of temperature change exceeds a set temperature.
• Minimum temperature exceeded.
• Fiber break.
• Instrument cabinet overheating.

During the SAT, small quantities of LN2 were injected into the test loops to simulate LNG leakage. The temperature drop triggered alarms on the affected channels of the DTS system. The alarm output to the client SCADA was accomplished.
For further development of the use of distributed fibre-optic sensors in LNG applications, fibre-optic strain sensors could be combined with temperature sensors to monitor piping for strain during cool-down cycles and normal operations.

Today, laboratory work is ongoing to test the combination of fibre-optic point sensors or fiber-bragg gratings with fibre-optic DTS, which will minimize the instrumentation needed to completely monitor LNG piping systems with the use of fibre-optic sensors.

Conclusion
The DTS system is qualified to monitor the cool-down process of LNG pipelines and control the integrity of the pipe insulation. The sensing cable should also be sufficiently robust against mechanical impact during the installation process. A robust four-core-sensing cable ensures the measurement system has long durability and ensures redundancy of the fiber. The longitudinal laying and the crosswise laying of the sensing cable are suitable for field application. The zoning of the sensing cable allows a precise observation of local high-risk areas of the respective LNG pipe.

The leak detection tests in a semi-scale test set-up and finite-element analyses performed by R&M Ship Tec Group verified the suitability of fibre-optic DTS monitoring systems for the identification of leaks in insulated cryogenic LNG piping in a defined reporting frame. To achieve a measurable temperature differential for leak-detection, the fibre-optic sensing cable must be installed between the first and the second insulation layer. For crucial and sensitive areas of LNG-pipes with an insulation thickness of less than 120-millimeters, the sensor-cables must be installed in two directions or wrapped around the piping in very close loops. The final design is subject to individual engineering parameters, taking into account the pipe-diameter, insulation thickness, economical aspects and other requirements.

The testing results recommend that a fibre-optic sensing monitoring system is deployed for on-condition surveillance of the integrity of LNG pipe systems in operation.

In its industrial applications, the fibre-optic DTS method has been proven to provide several advantages over conventional sensors because the fiber cable is the sensor. This intrinsically safe cable can be installed in potentially hazardous areas. The ability to interface with existing client-data communication systems and SCADA systems also provides automated input from fibre-optic monitoring systems where product egress or loss of containment of LNG requires prompt intervention.