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Apr-2011

Cryogenic monitoring

Fibre optic distributed temperature sensing has several advantages over conventional real-time data sensors in cryogenic leak detection at LNG terminals

Jerry Worsley
Sensornet
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Article Summary
Environmental and safety issues are becoming ever more important in the liquefied natural gas (LNG) process industry. As plants, pipelines and storage facilities become larger in scale and more complex, maximising safety and minimising 
risk requires an absolute understanding.

It is estimated that more than 400 liquefaction, receiving and storage facilities are either operating, under construction or planned today. This effectively equates to more than 500 storage tanks and well in excess of 1000 km of pipeline that requires monitoring.

Recent years have seen significant developments in the technologies available to effectively monitor an ever-increasing number of facilities and associated infrastructures. With conventional technology, there traditionally has been a gap between what is thought to be occurring throughout an installation and what is actually happening. However, technologies are now available that can overcome the limitations of traditional measurement methods to close that monitoring gap.

Fibre optic distributed temperature sensing (DTS) is one such technology that has come to the fore as a realistic alternative monitoring solution. This method of monitoring has several key operational advantages over conventional real-time data (RTD) sensors. One example of the benefits of DTS over traditional monitoring methods is that conventional thermal point sensors are not able to provide full coverage of a pipe or tank and therefore do not have the capability to pinpoint the precise location of any cool-down or leak event. By contrast, using fibre optic DTS, the location of such an incident can be identified almost instantly and remedial action taken. In fibre optic DTS, the fibre is the sensor and the measurement is totally non-
intrusive. No welds are required to the pipe or vessel and the fibre can be easily strapped to the outside of the pipe with no interruption to the operational process. The fibre provides its own communications path and no additional tie cables are required. Most importantly, DTS using optical fibres enables the capture of data every metre at near real-time speeds. It can detect and report cryogenic leaks to the emergency shutdown (ESD) system within 15 seconds. This enables the ESD system to register and close valves within 45 seconds.

A DTS system from Sensornet provides a cryogenic monitoring solution that can instantly locate a temperature event anywhere in a cryogenic process plant such as a LNG terminal. The DTS system can be used in digital leak detection and thermal profiling applications, including LNG, liquefied petroleum gas (LPG), ethylene and CO2 loading and unloading pipelines, storage tanks, spill channels and impounding basins, as well as the cool-down monitoring of pipes and storage tanks.

From an operational perspective, the benefits of a digital cryogenic temperature-monitoring system can be summarised as follows:
• Leaks can be detected rapidly and mitigating action taken immediately, thus minimising the risk to operations personnel and equipment
• Such a system can lead to improved plant operation, allowing the monitoring system maintenance schedule to be aligned to other safety integrity systems in the plant
• The sensing element is an intrinsically safe passive sensing cable with a design life of more than 30 years, resulting in high reliability and low maintenance
• The system is fully automated and so lowers operating costs with less risk of human error. It can interface with existing supervisory control and data acquisition (SCADA) and ESD or fire and gas control systems using standard protocols.
The benefits to an asset manager of such a system are clear. It can improve the safety of both the infrastructure and of personnel, which can reduce insurance and litigation costs, fulfil legal and planning requirements, and enhance a company’s safety reputation. It can also enhance plant reliability through reduced downtime and inspection time, while fewer nuisance and false alarms can result in increased security and improved productivity.

Principle of leak detection using DTS
DTS is based on temperature measurements using fibre optic cables and can be used to detect both liquid and gaseous leaks. The DTS system illuminates the glass core of the optical fibre with a 10-
nanosecond laser pulse, which corresponds to a 1m pulse. As the optical pulse propagates down the fibre, it undergoes scattering even in the absence of impurities and structural defects. Part of this scattered radiation is known as Raman scattering. As 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 fibre at a given point.

DTS can be used on long-distance pipelines, with repeater stations situated every 80 km if required. This can be utilised for send-out lines or carbon capture and storage. More importantly for LNG applications, DTS can be equipped with multiplexing devices (also known as a mux) that can monitor several lines from a single unit. These could include delivery, recirculation and burn-off gas lines, as well as multiple tanks, spillways and impounding pits. This can make DTS technology a versatile and cost-effective option.

Fibre is the sensor
DTS systems consist of a limited number of components, which are the DTS hardware and the sensing cable. The hardware is normally situated within the control room environment. However, a robustly designed data acquisition device such as Sensornet’s Oryx system, which is made for deployment in harsh environments, can be positioned in the field, allowing the monitoring of hard-to-reach places.

The fibre optic cable is almost as important as the DTS itself, because it is the fibre that provides the information. It is important to use the right cable for the proposed scope of work. For leak detection alone, it is possible to install a relatively standard fibre optic cable. However, the user should be prepared for a one-time use only.
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