Oil & gas The starting point with any seismic


monitoring design is the sensor. And it is important to recognise there is a clear technical difference between the types of sensors that are used for seismic protection and those used for geophysical earthquake monitoring. Geophysical seismic monitoring utilise


broadband magnet and moving coil (electrodynamic) sensor arrangements capable of measuring micro g acceleration events with sinusoidal periods of over 100 seconds. Strong motion sensors on the other hand for seismic protection applications only need to provide a resolution down to 1mg and a response to 10 seconds; whilst historically electrodynamic sensors have been used, nowadays for these applications piezoelectric based accelerometers are preferred as they match the technical requirement closely and provide higher reliability as they have no moving parts. A trend in vibration monitoring is the adoption of MEMS (Micro Electro-Mechanical Systems) devices in a wide range of sensing applications. These devices offer an excellent low frequency response and exhibit the required dynamic range for strong motion seismic monitoring. MEMS devices have been widely used in civil engineering applications since the 1990s, their relative low cost and small size suite applications where many measurement points are required on structures for a limited period of time. However, adoption of this technology has been slow for the seismic protection market, where stated reliability and maintainability are the key requirements. MEMS are a fast moving technology and with limited application experience, selection of a specific sensor may prove difficult to maintain in future years as a result of obsolescence. Significant earthquake events are few and


far between, so how do we verify an installed strong motion sensor is working correctly, when for most of the time there is nothing to measure? This situation is exacerbated by the sensor installation which is normally difficult to access. With broadband seismometers it is common to have a secondary coil arrangement which can be excited and therefore stimulate movement of the mass to verify calibration without physical shaking. Sensonics have incorporated a similar mechanism in to its piezo electric based seismic sensors to ensure the measuring element is operating to the correct sensitivity. This self-test feature is a critical requirement which will become apparent when we look at the overall system design. It is common to utilise redundant sensor configurations in the overall monitoring system concept (please refer to the detailed functional block diagram of a modern day seismic monitoring and protection system). Three separate physical locations are


monitored with triaxial sensors capable of measuring acceleration in the three orthogonal axes. The acceleration of each sensor is


Instrumentation Monthly March 2019


Seismic system configuration diagram


processed by a trip amplifier with the overall triaxial unit performing a one out of three (1oo3) logic operation to derive the location OBE alarm. The trip alarms from each location are fed back to the central control panel which performs a subsequent two out of three logic operation to determine the final trip result. In this example the voting logic is also redundant to enhance reliability and maintainability. The final element of the system is connected to the specific plant circuit breakers or to the emergency shutdown system to complete the safety loop.


making the 2oo3 system the norm for critical protection applications.


Combine these channels with dual voting


arrangements and the sensor inbuilt test function results in a system design that can be fully proof tested whilst on line, maximising the availability of the system. Each voting circuit can be isolated and tested in turn through signal injection of each sensor, a critical aspect of the system performance being the sensor will still respond to an real seismic event even whilst under test. The avoidance of SMART devices within the


protection loop also eases the analysis burden to meet with the safety requirements and is the preferred solution for most clients. Separating the protection and event recording functions is a logical step which enables the latest technologies and features to be utilised for the seismic waveform recording without impacting on the protection safety case. Use of proven technologies in combination with measurement redundancy tends to be the industrial norm for modern day applications; with self-testing features and spurious trip performance being of particular importance in relation to the automatic shutdown systems. Adopting this best practice has become


For redundancy, a simple one out of two (1oo2) system will usually meet with


the reliability requirements, in fact demonstrating a higher reliability than the 2oo3 system. However, this system configuration offers no protection against spurious trips which can result from mechanical interference or sensor failure. Two out of two (2oo2) is an alternative option that can be considered, however on failure of a channel the system defaults to a 1oo1 system, whilst the 2oo3 option on channel failure can revert to either 2oo2 or 1oo2, both of which are preferred over 1oo1,


standard for new installations and should also be considered for obsolete seismic monitoring equipment on existing sites. A stated and demonstrated reliability, minimal spurious trip occurrence, full measurement loop proof testing, maximum design life and maintainability combined with a low demand and high integrity shutdown system is now the expected norm. Typical nuclear industry applications include, fuel handling, reactor structural monitoring, waste processing and the monitoring of crane equipment, while for the oil and gas industry applications include LNG processing, extraction platforms and gas shut-off valves.


Sensonics www.sensonics.co.uk 39


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