SEISMIC MONITORING

INFRASTRUCTURE MUST BE PROTECTED AGAINST DISASTER

BY RUSSELL KING

Following the Fukushima incident in 2011, industry placed a renewed focus on risk mitigation from extraordinary events caused by nature. Various regulatory bodies around the world revisited the design basis as well as risk analysis, the accident management strategy and the periodic safety review policy related to facility location. Many nuclear power stations, oil and gas terminals and power plants are located on the coast where there is higher risk of flooding from seismic events and severe weather.

The operational monitoring of seismic vibration for structures and equipment plays an important role in providing automatic shutdown protection and the recording of seismic events for post analysis. No two facilities are the same in terms of their approach to seismic monitoring and protection. Some sites utilize data from the national network of geophysical instruments, whilst others implement independent monitoring and shutdown on each critical plant item.

The structural effects to be expected at a site from an earthquake result from the vibration induced by the event, classified in terms of seismic response spectra. This defines the ground acceleration magnitude versus frequency, typically over a range of 0.1Hz to 100Hz. Two such types of spectra are specified, the Operational Basis Earthquake (OBE) and the Design Basis Earthquake (DBE), based on a predicted worst case seismic event within a specified period (for example, OBE may be specified within 100 years).

Secondary response spectra are derived from ground accelerations through modelling to predict the response of each structure and each level within that structure. Typically, a plant allocates several seismic categories for specifying design requirements according to the safety class. The highest category demands the equipment or process be tested to the DBE level plus a margin (+40% is recommended in IEEE- 344, Standard for Seismic Qualification of Equipment for Nuclear Power Generating Stations).

Any earthquake above the OBE level may result in a plant shutdown until post analysis and inspection determine that the plant can safely operate. The challenge is to design and construct cost-effectively to meet the seismic categorization and provide sufficient margin. It may not be possible for all equipment or processes to meet its categorization fully.

This is where seismic monitoring systems can provide detection of the OBE event and bring the process to a safe state. Not only must these monitoring systems be robust, they also need to exhibit high levels of availability beyond the DBE magnitude event to maintain a valid alarm function. Various safety standards are applied to obtain a stated availability, with EN IEC 61508 being the most common approach. Adherence to such a standard provides a stated system reliability and availability whilst providing an understanding of the systematic failures and ensuring compliance with the 61508 lifecycle model.

Sensor differences

The starting point with any seismic monitoring design is the sensor. There is a clear technical difference between the types of sensors used for seismic protection and those used for geophysical earthquake monitoring. Geophysical seismic monitors utilize 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 for seismic protection applications only need to provide a resolution down to 1mg and a response to 10 seconds.

While electrodynamic sensors have been used historically, piezoelectric-based accelerometers are preferred as they match the technical requirement and provide higher reliability as they have no moving parts. A trend in vibration monitoring is the adoption of Micro Electro-Mechanical Systems (MEMS) devices for sensing applications. They offer excellent low-frequency response and exhibit the required dynamic range for strong motion seismic monitoring. The relatively low cost and small size of MEMS devices suit applications where many measurement points are required on structures for a limited time. However, adoption has been slow for the seismic protection market where reliability and maintainability are key.

Significant earthquake events are few and far between. Therefore, it is necessary to verify an installed strong motion sensor is working correctly. It is common to have a secondary coil arrangement which can be excited and stimulate movement of the mass to verify calibration without physical shaking. This self-test feature is a critical requirement. It is common to utilize redundant sensor configurations in the overall monitoring system concept.

Three physical locations are monitored with triaxial sensors capable of measuring acceleration in the three orthogonal axes. The acceleration of each sensor is processed by a trip amplifier with the overall triaxial unit performing a one out of three (1oo3) logic operation to derive the OBE alarm. Alarms from each location are fed to the control panel which determines the final trip result. The avoidance of SMART devices within the protection loop eases the analysis burden to meet safety requirements. It is the solution preferred by most users. Separating the protection and event recording functions enables the latest technologies and features to be utilized for seismic waveform recording without impacting protection safety.

■ Russell King is Management Director at Sensonics, a UK-based company specializing in vibration, displacement and speed monitoring. For more information, visit sensonics.co.uk