Enlightening Subsea Structures
a report by Marc Niklès1
and Fabien Ravet2 1. Chief Executive Officer/Chief Technical Officer; 2. Latin America Area Manager, Omnisens
The number of offshore production facilities continues to grow as the quest for oil extends to deeper-water environments, exposing subsea components and structures to harsh operating conditions. Components such as mooring ropes, submarine cables, umbilicals, risers and flowlines have design limits set by fatigue accumulation, which may be significantly larger than anticipated during installation, commissioning or in operation, reducing their lifetime and putting operations at risk (see Figure 1). Moreover, subsea components are susceptible to corrosion and damage caused by dropped objects, extreme storms, fishing gear and contact from vessels. Periodic remotely operated vehicle (ROV) inspections are used to assess the structural integrity of subsea components over their entire lifetime. However, this method is expensive and unreliable since visual inspection is able to detect only external anomalies, not internal structural deterioration. In recent years, the need for effective tools to manage the safety, environmental and financial risks associated with the operation of offshore production facilities has been emphasised as these tools have become an important part of the subsea structure integrity management plan. To ensure the safety and integrity of new and existing subsea structures, non-destructive testing techniques and improved permanent monitoring solutions have been developed. Among these techniques, fibre-optic-based monitoring systems have been proved to perform effective permanent and realtime structural integrity monitoring. The monitoring technique developed by the company Omnisens, based in Switzerland, has been used in onshore and offshore projects over the past few years and to date has shown unmatched monitoring performance.
The Technology
In 1920, when Léon Brillouin presented his thesis ‘Light and X-ray scattering by a transparent homogeneous body – effect of thermal agitation’, he probably did not suspect that his discovery would be used in the most unexpected places on Earth. His research led him to the conclusion that density fluctuations in the medium can be associated with thermally generated sound waves. That thermal agitation is capable of scattering inelastically any incident lightwave; that is, the scattering products undergo shifts in frequency, which can be seen as the diffraction of light on a dynamic grating generated by an acoustic wave (an acoustic wave is actually a pressure wave that introduces a modulation of the index of refraction through the elasto-optic effect). The diffracted light experiences a Doppler shift since the grating propagates at the acoustic velocity in the fibre. The acoustic velocity is directly related to the medium density, which is temperature- and strain-dependent. As a result, the so-called Brillouin frequency shift carries information about the local temperature and strain of the fibre. The Brillouin frequency shift is an intrinsic parameter of the fibre and its value is independent of the measuring system, ensuring long-term unbiased measurements with no need for periodic recalibration. Furthermore, the perfect linear dependency of the Brillouin shift on temperature and strain allows the accurate and straightforward determination of fibre conditions unaffected by connectors or splice
© TOUCH BRIEFINGS 2011
losses. Localisation of the temperature or strain information along the fibre is possible using a pulsed pump signal. The time of flight of the pump signal and the backscattered component is recorded and converted into distance. Thanks to the high speed of light, fibre lengths of several kilometres can be scanned within a fraction of second, yielding several thousands of measurement points (see Figure 2). The Omnisens solution, the distributed temperature and strain monitoring system (DITEST), is capable of measuring continuous strain and temperature profiles in realtime along tens of kilometres with metre resolution. One of the advantages of the Omnisens DITEST solution is that it is fully compatible with subsea fibre optic components such as fibre optic rotary joints and wet-mate connectors, while the monitoring performance is not affected by optical losses, making the solution robust and reliable for long-term monitoring (see Figures 3 and 4). Fibre optic instrumented structures provide operators with information about any abnormal operational changes, the occurrence and location of damages, leaks and excessive strain, generating alarms and status reports. The complete information about structural conditions is transferred to supervisory control and data acquisition (SCADA) systems and eventually helps operators to make executive decisions based on actual operational and structural conditions.
Submarine Cables, Umbilicals and Risers
Submarine cables, umbilicals and risers must withstand huge mechanical stresses and strains during their installation and operation, especially in
Marc Niklès is the Chief Executive Officer/Chief Technical Officer (CEO/CTO) and co-founder of Omnisens. His previous positions include Programme Manager for ice detection systems in aerospace applications at Vibro-Meter SA. Dr Nickles is a member of the Optical Society of America and the International Society for Optical Engineering (SPIE). He is the author or co-author of over 40 scientific publications and conference papers. He received a PhD in technical
sciences from the Swiss Federal Institute of Technology in Lausanne in 1997 for his research activities in distributed optical fibre sensors. During his PhD research, he studied the properties of Brillouin scattering in optical fibres and its application to distributed fibre optics sensing for strain and temperature monitoring.
Fabien Ravet is Latin America Area Manager, and was formerly Head of Development of Applications and Project Management at Omnisens. His previous positions include Senior Optical Application Designer at Nortel Networks and Optical Technologies Validation Coordinator at GTS Network Services. Dr Ravet is a member of the Institute of Electrical and Electronics Engineers (IEEE) Photonics Society, the IEEE Instrumentation and Measurement Society and the
International Society for Structural Health Monitoring of Intelligent Infrastructure. He is the author or co-author of over 50 scientific publications and conference papers. He received a PhD in physics from the University of Ottawa in 2007, with a doctoral thesis dedicated to the study of stimulated Brillouin scattering and its application to distributed sensing.
E:
fabien.ravet@omnisens.ch
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Subsea & Pipelines
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