Fabien Ravet

Submillimeter crack detection with brillouin-based fiber-optic sensors

Submillimeter crack is detected with a dedicated fiber-optic strain cable, a 1-m-spatial-resolution (w) distributed Brillouin sensor and an advanced signal processing technique. The signal processing approach consists in spectrum shape analysis and multiple peaks detection.

Challenges, Requirements and Advances for Distributed Fiber Optic Sensors in SURF Structures and Subsea Well Monitoring

During the past 10 years, the oil and gas industry has gained confidence in the capabilities and the reliability of Fiber Optic Distributed Sensing (OFDS). Real world implementation can be found in both downstream and upstream branches. For example, at one end of the industry spectrum, service companies have started to use Distributed Temperature Sensing (DTS) for in-well temperature monitoring to optimize oil recovery processes. At the other end, pipeline operators have installed pipeline integrity monitoring systems based on Distributed Temperature and Strain sensing (DITEST) to fulfill realtime monitoring of soil stability, pipeline 3D deformation and leak detection.The use of OFDS for offshore structure integrity monitoring such as flow lines, risers and umbilicals has been qualified and field tested already whereas other applications such as performance rating of power umbilicals and enhanced flow assurance system using OFDS are being evaluated.Currently, the offshore full scale implementation of OFDS technologies still faces challenges of its own. In particular, sensor integration into the structure to be monitored is a minimum requirement which applies to any OFDS. If fiber optic is a common mean of data transmission, subsea conditions imply the use of specific components such as Wet Mate Connectors (WMC) and Fiber Optic Rotary Joints (FORJ). These components present large insertion and return loss characteristics for which OFDS require special attention to the OFDS system to comply with such characteristics. The effect of these components is twofold. First it impacts the sensor optical budget limiting its measurement range. Second, sensor sections remain completely blind due to the high reflection levels leaving the structure without status information over distances that can be as large as several kilometers.The present works describes how the DITEST based on Stimulated Brillouin Scattering (SBS) can overcome the limitations imposed by both WMC and FORJ components and fully comply with SURF monitoring requirements. The ability of the DITEST is justified theoretically and demonstrated experimentally through qualification trials involving hotspot detection while WMC and FORJ are part of the sensor path. Their effects are quantified through the determination of the measurement dead zone (shorter than 4m), the temperature uncertainty and the resolution. The work also reports the subsequent installation on operational structures as these trials were successful. The DITEST has been installed to continuously monitor the temperature of a 3km long power umbilical and control the heating system of subsea rigid flowlines whose length can be as large a 45km.Copyright

DEH Cable System Preventive Protection With Distributed Temperature and Strain Sensors

Hydrate and wax formation in subsea flowlines is a major cause of production impairment. Among various approaches used to minimize the risk, Direct Electrical Heating (DEH) is being applied. DEH is based on passing a current through the pipe wall to mitigate heat losses from the fluid to the surroundings during events which require flow assurance measures. The Piggyback Cable, a high voltage cable attached to the DEH pipeline, is during operation exposed to thermal and mechanical loads which may be critical for the integrity of the DEH system. The overall safety requirement is that any potential Piggyback Cable fault is detected and disconnected from the power source before damage is caused to the pipeline.Conventional cable fault detection methods based on current measurements give adequate protection for the main part of the pipeline. However, for the far end of the Piggyback Cable complementary fault detection is required. A method based on fiber break monitoring has been qualified for this purpose. The new method is implemented in the North Sea on two DEH pipelines operated by Statoil, 43 and 21 km long respectively. The protection is facilitated by standard single-mode fibers integrated into the DEH cables.Although not basis for the design the integrated fibers open up possibilities for temperature and strain sensing using stimulated Brillouin scattering. Sensing has been performed on a 43 km DEH pipeline using the DITEST AIM (Distributed Temperature and Strain Asset Integrity Monitoring). Analysis of the sensing results reveal that distributed fiber optic sensing is capable of pin-pointing thermal events and strain induced loads for an object of this length.Copyright

Geohazard Prevention and Pipeline Deformation Monitoring Using Distributed Optical Fiber Sensing

The present work introduces the technology background at the origin of FOPIMS (Fiber Optic Pipeline Integrity Monitoring Systems) with an emphasis on geotechnical monitoring. It shows how temperature sensing can be implemented to control soil erosion or dune migration through event localization and spatial quantification. Arctic pipeline monitoring project illustrates the application of soil erosion detection. Direct measurement of strain in soil also enhances environmental threat detection. Combined with temperature sensing, strain sensing composes the geotechnical monitoring system. Transandean pipeline monitoring examples are presented where the DITEST AIM was implemented for geohazard prevention. These study cases concern new pipeline installation as well as retrofit of existing lines. The technique successfully evidenced early events and allowed preventive measures to be taken.In some applications actual pipeline deformation need to be monitored. Such operation is achieved by measuring distributed strain along sensing cables attached to the structure. We show how such measurements complement the geotechnical measurements. We also describe a real implementation in seismic active area.As a whole, the work focuses on the technique principles, the installation and how the system is being implemented for pipeline preventive maintenance. We intend to present a comprehensive set of design guidelines based on real results and lessons learned from the various projects in what concerns geohazard detection and pipeline deformation monitoring.Copyright

Condition monitoring of 88km long Offshore HVDC Power Cable: comparison of DTS and as built data

Distributed temperature sensing (DTS) has been deployed for a field trial on a high voltage direct current (HVDC) cable in the North Sea. Temperature measurements using a fibre optic sensing cable bundled onto the HVDC cables are compared with both the cable characteristics and the cable route demonstrating an excellent agreement. Horizontal Directional Drilling (HDD), land and offshore cable joints, transitions as well as hot spots and excess burial are clearly visible. Thermal data thus helps in assessing the cable condition.

BOTDA-based DTS robustness demonstration for subsea structure monitoring applications

We demonstrate that BOTDA is the equipment of choice for Distributed Temperature Sensing (DTS) when challenging components such as Wet Mate Connectors (WMC) and Fiber Optic Rotary Joints (FORJ) are inserted in the sensing line. The parameter of Temperature Measurement Dead Zone (TMDZ) is introduced to support the discussion and quantify possible impairments. A laboratory qualification test with real FORJ and WMC is conducted confirming the theoretical expectations. The system is then implemented offshore demonstrating that FORJ and WMC do not affect the sensing operation in field environment.

Analysis of Strain Sensor Cable Models and Effective Deployments for Distributed Fiber Optical Geotechnical Monitoring System

Long range, distributed fiber optic sensing systems have been an available tool for more than a decade to monitor pipeline subsidence integrity challenges. Effective deployment scenarios are an important decision to be factored into the selection of this monitoring equipment and typologies relative to specific project needs. In an effort to analyze the effectiveness of various fiber optic deployment conditions, a controlled field experiment was conducted. Within this field experiment, a variety of distributed fiber optic sensors and point sensors were deployed in predefined positions. These positions relative to the pipeline were selected to support a range of deployment needs including new construction or retrofitting of existing pipelines. A 16-inch diameter by 60-meter long epoxy coated pipeline that was capable of being pressurized to mimic operating conditions was utilized. This test pipe was installed in a typical trench setting. Conventional point gauges were installed at key locations on the pipeline. Fiber optic sensor cables were installed at key locations providing 14 alternative scenarios in terms of sensitivity, accuracy, and cost.After construction of the test pipeline, real time continuous monitoring via the array of conventional and fiber optic sensors commenced. A deep trench was excavated adjacent and parallel to the central portion of the pipeline which began to induce subsidence in the test pipeline. Continued monitoring of the various sensors produced real time visualization of the evolving subsidence. A comparison of the reaction of the sensors is compiled to provide an intelligent selection criteria for integrity managers in terms of accuracy, deployment, and costs for pipeline subsidence monitoring projects. In addition, further analysis of this sensor data should provide more insight into pipeline/soil interaction models and behaviors.Copyright

Ultra Long Range DTS (>300km) to Support Deep Offshore and Long Tieback Developments

The continuous demand for energy supplies leads operators to explore and exploit more remote offshore oil fields characterized by deeper deployments, longer tiebacks and subsea completion. These offshore developments either lie in tropical regions subject to stormy weather or in extreme arctic conditions. They imply new requirements in terms of operational efficiency and predictive maintenance for cost optimization as well as environmental footprint minimization.Consequently, leak detection and flow assurance are required over distances exceeding hundreds of kilometers. And the increasing trend of building all electric completion and heated flowlines impose the use of equally long high voltage power cables.Distributed temperature sensing (DTS) is a widely adopted technology for leak detection and localization, for heated flow line monitoring and for power cable thermal rating. Optical fiber cables are installed alongside the structure providing detection with meter accuracy localization and degree thermal variation sensitivity. However, there is currently no commercially available DTS instrument capable of reaching a 100km single span without any means of repeating the signal, let alone longer range. Thus, a 300km pipeline would normally require marinized equipment, which is often not a practical solution.The Omnisens DITEST, a DTS instrument based on Stimulated Brillouin Scattering (SBS) is intrinsically capable of the longest available sensing length, now reaching a sensing distance of around 70km at once; in addition, it is compatible with optical amplifier technology as found in any submarine network. In the present work, we show that, by combining 5 spans of slightly above 65km and 4 optical repeaters together, it is possible to achieve a total of 330km sensing from a single DITEST. Measurement time over the full distance was typically 100minutes, providing a temperature resolution of typically 1K for a spatial resolution of 3m at the end of the 330km long sensing fiber. Shorter measurement time, more suitable with the application could be achieved using optical coding.As the optical repeater technology is similar to that of telecommunication, efficient cost reduction could be achieved by sharing the repeaters between the sensing system and the needed communication system. At the end of each sensing span, the sensing cable is connected to the repeater for amplification purposes and then goes back to the pipeline for monitoring the next span. The repeaters themselves are electrically powered by conductors embedded in the communication infrastructure.A viable and cost effective solution to meet the requirements of subsea long distance leak monitoring is thus demonstrated.Copyright