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Oil & gas


Seismic monitoring for critical infrastructure Hamble Terminal Operated by BP. Ian_Stewart / Shutterstock.com


Many nuclear power stations, oil and gas terminals and critical facilities are located on the coast. The risk of flooding from seismic events and severe weather has been put in to particular focus following the Fukushima incident in 2011. Here, Russell King, Sensonics, explains why seismic monitoring is essential for critical infrastructure


caused by nature. The various regulatory bodies around the world have gone back to basics to consider not only the design basis for operational sites but also revisit the risk analysis, the accident management strategy and the periodic safety review policy. Considering that many nuclear power stations, oil and gas terminals and critical facilities are located on the coast, particularly true for the UK, the risk of flooding from both seismic events and severe weather has been put in to particular focus. Of significant importance has been the review of methodology used to derive the seismic hazard and how that hazard has been mitigated through the design process and present day operational systems. The operational monitoring of seismic vibration for both structures and equipment plays an important role in providing automatic shutdown protection and the recording of seismic events for post analysis. This article discusses the latest trends in seismic monitoring and protection systems, technologies adopted and current best practice driven by these enhanced risk management demands.


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Condition monitoring specialists Sensonics experience suggests no two facilities are the same in terms of their approach to seismic


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ollowing the Fukushima incident in 2011 industry has placed a renewed focus on risk mitigation from extraordinary events


monitoring and protection. Some sites utilise data from the national network of geophysical instruments, whilst others implement independent monitoring and shutdown on each critical plant item. In each case, the derivation of the required seismic monitoring and protection strategy must meet with the safety case and provide appropriate risk mitigation. 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 of time (for example OBE may be specified within 100 years). Secondary response spectra are derived from the ground accelerations through modelling to predict the response of each structure and each level within that structure. Typically a plant will allocate several seismic categories for specifying the design requirements and assess according to the safety class. For example the highest or most stringent category will demand the equipment or process be tested to the DBE level plus a margin (for example +40 per cent is recommended in IEEE- 344,


Standard for Seismic Qualification of Equipment for Nuclear power generating Stations), since the process must still remain operable to the design basis even if other less critical plant processes may have failed above the OBE level. Any earthquake above the OBE level may result in the plant being shutdown and to remain shutdown until post analysis / inspection has determined the plant is safe to continue operations. The challenge is to design and construct in a


cost effective manner to meet with the seismic categorisation and to provide sufficient design margin. It may not be possible for all equipment or processes through either analysis or testing to meet with its categorisation fully and this is where independent seismic monitoring systems can be utilised to provide detection of the OBE event and to bring the process to a safe state. Not only must these monitoring systems be robust to seismic events but they also need to exhibit high levels of availability beyond the DBE magnitude event to maintain a valid alarm function. In combination with the seismic requirements 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 at the same time providing an understanding of the systematic failures and ensuring compliance with the 61508 life cycle model.


March 2019 Instrumentation Monthly


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