FIBER SENSING
DISTRIBUTED ACOUSTIC SENSING (DAS)

ASN OPENS A NEW ERA IN SUBSEA INTELLIGENT SENSING BASED ON ADVANCED DAS TECHNOLOGY​

Access to a single optical fibre in a subsea cable can provide :

– Protection of submarine cables and infrastructure
– Seismic measurements and recording of seismic events
– Monitoring of marine life and oceanographic conditions

The OptoDAS interrogator is using a unique interrogation technique providing low-noise and long-range quantitative phase measurements in single mode optical fibers.

OPTODAS​

  • Real-time distortion free measurements of fibre strain modulation over distances in excess of 150 km by having one-way loss budget of 28 dB
  • Extremely low inherent phase noise for fibre lengths up to 70-80 km
  • No practical impact from connector reflections
  • Configurable gauge length with spatial sampling of 1 m
  • The unit can be delivered for permanent installation in a 19″ rack or packaged as a portable unit
  • A dedicated software suite provides extensive functionality and flexibility for instrument control, data management, processing and display

ASN targets applications within the oil and gas industry and the subsea telecom and power cable industry

Offshore-Oil-and-Gas-DAS-Applications

Offshore Oil and Gas DAS Applications

In-well monitoring

Platform and subsea well monitoring
• VSP, flow, sand, valves, integrity, micro-seismic

Seabed monitoring

Pipeline leak detection
• Flexible pipes used in seabed fields

Overburden monitoring
• Low-frequency seabed surface waves

Reservoir monitoring
• PP imaging for 4D seismic

Subsea infrastructure monitoring
• Health monitoring
• Electrical fault monitoring

Applications for offshore oil and gas

A subsea cable das applications

Subsea Cable DAS Applications

Monitoring of integrity threats to subsea cables and associated infrastructure

• Trawl activities
• Anchoring
• Dredging activities
• Vortex induced vibration

Earthquake detection

Measurment of oceanographic conditions

Reservoir monitoring
• Ocean currents
• Seabed rock slides etc

Monitoring of marine mammal activities

Localization of electrical failures
• Partial discharge effects

Applications subsea telecom and power cables

In addition, DAS is currently being used for several terrestrial applications from safety/security to rail and road monitoring.

APPLICATIONS FOR OFFSHORE OIL AND GAS

Borehole monitoring is a mature application of DAS, now entering the offshore production fields. Optical cables are being permanently installed for distributed borehole monitoring. The cables are normally clamped to the outside of production tubing or cemented into the casing annulus.

Distributed acoustic sensing is becoming a recognized monitoring tool for VSP, flow profiling, well integrity, valve control and micro-seismic.

The OptoDAS interrogator has extremely low instrument noise making it very suitable for in-well measurements. The very high optical power budget also facilitates monitoring of subsea wells with long tie-back cables without the need for specialized or engineered in-well fibre.

Seabed monitoring is an application under development. Seabed cables are very sensitive to acoustic seabed surface modes and ground movements. A single fibre in any optical cable installed on the seabed (trenched or un-trenched) can be utilized for various distributed sensing applications with high spatial sampling.

DAS recordings on seabed cables can be utilized for overburden monitoring, pipe leakage detection, time- lapsed seismic reservoir monitoring as well as health monitoring of seabed infrastructure. These applications are currently under development based on access to optical fibres on the seabed through existing riser cable solutions. Dedicated optical fibres in umbilicals, PRM cable networks, DC/FO cables, telecom cables or separate DAS cables may be used in combination with existing cable branching solutions.

The strong benefit of the OptoDAS interrogator for these applications is the low-noise characteristics and the high-power budget capability in combination of high spatial sampling.

APPLICATION FOR SUBSEA TELECOM AND POWER CABLES

A subsea cable das applications

Dark fibre in subsea telecom cables can be used to protect the cables against third party activities such as trawling, anchoring and dredging. Such fibre can also be used for environmental and scientific monitoring such as earthquake and tsunami detection, various oceanographic conditions and mammal monitoring.

On-line DAS monitoring with an optical cable next to an electrical high-power cable might provide an instant localization of partial discharge effects.

The OptoDAS interrogator has an extremely long range (in excess of 150 km with low attenuation fibre) making it very suitable for these seabed measurements. The interrogator has very low frequency capability (detecting signals below 0.01 Hz) and can also be used for temperature rate measurements.

DAS is currently widely used for monitoring of onshore pipelines including leak detection, external interference and ground movement monitoring.

DAS is also an attractive solution for perimeter intrusion detection targeted at the protection of critical assets in different market segments. With the long- range capability, it also provides an effective solution for border control.

By using existing optical cable networks in urban areas, DAS can provide sensing grids for high resolution traffic monitoring (including vehicle and road condition).

Railway monitoring is also an emerging application of DAS. With an optical cable along the track, DAS can provide information about moving trains, detect third party activity and locate rocks and landslide events.

The strong benefit of the OptoDAS interrogator for most of these applications is the low-noise characteristics and the high-power budget capability.

Need more information ?

The performance of the OptoDAS unit has been verified by a complete SEAFOM testing and the key results are included in the datasheet. You can request a copy of the OptoDAS datasheet by using the form below.

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What is DAS?

If a section of the optical fibre is subjected to strain, the propagating light will experience an optical phase delay.

By analyzing the back-reflected signal, one can extract the optical phase modulations induced along the optical fibre. This is done with a coherent OTDR technique where the phase between two adjacent scattering regions is taken to be proportional to strain. The distance between the centers of the two scattering regions is known as the gauge length.

Any measure impacting the cable strain condition can, in principle, be recorded.

Pipeline monitoring

Flow in a subsea pipeline (bringing the resource to shore) or in flowlines (connecting wellheads to the platform) will generate acoustic noise that can be picked up by an optical cable installed on the seabed nearby. Such noise will generate seabed surface waves imposing strain modulations on the cable observable by DAS measurements.

The picture shows recordings along a section of a subsea cable crossing the flowline (water injection) at the 24 km cable position. The surface waves are visible more than one km away from the crossing point.

By utilizing a fibre in an optical seabed cable, health monitoring (flow rate, leakages) of subsea flowlines can be performed with DAS interrogation.

Pipeline monitoring

Trawl monitoring

When a trawl hits the sea floor, acoustic waves (propagating along the seabed) will be excited. Such waves will induce strain in an optical cable installed on the seabed. Detectable strain is distributed over several kilometers along the cable (depending on the distance between the trawl and cable).

Data have been recorded from trawl activities in a fishing area in the North Sea at cable positions 55 – 60 km from the shore end. Trawls on the seabed are recorded within 1-2 km distances from the cable.

Real-time processing software provides continuous localization and tracking of trawl activities by using the optical cable as a coherent DAS antenna. The software issues warnings to the telecom operator when trawls are approaching the cable. Vessel identification can be made through the use of AIS data.

Trawl monitoring

Localisation of electrical failure

Some of the electrical energy released in a partial discharge (PD) transforms to acoustic energy. The generated acoustic emission will depend on surrounding materials of the void discharge. The main acoustic energy is at ultrasonic frequencies, but some energy is also emitted at much lower frequencies (suitable for long-range DAS monitoring).

DAS can be used for on-line monitoring of partial discharge in HVDC cables providing an instant location of the failure position. Although partial discharge effects occur quite seldom, an on-line OptoDAS system enables significantly faster repair and reduced period of lost revenue when a failure occurs.

OptoDAS can monitor power cables in excess of 300 km length (by monitoring from both ends). The figure illustrates background noise level measured over more than 160 km in an optical cable attached to an HVDC subsea cable. The figure also includes estimated DAS signals in case of a partial discharge. These signal levels are based on experimental results.

Localization of electrical failures

Contact us

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Alain Biston

Alain Biston

President & CEO

Alain has been working for more than 25 years in telecoms, with Nortel, then Alcatel-Lucent / Nokia, holding management and leadership positions in R&D, Product Line, Industrial Operations, Sales & Marketing, Business Unit P&L accountability.

He brings to ASN his thorough knowledge of the telecoms industry, his extensive international management background with several postings overseas, and his field-proven customer-facing acumen.

Since 2016, as a Nokia executive, Alain has been Senior VP Customer Operations End2End and until now Senior Vice President in charge of Mobile Network business management.

Alain holds a degree in Information Technology from INSA, Rennes, France. He was also honored with the National Order of Merit in 2006 from the French Minister of Industry.