> Flowlines and pipelines
Monitoring with distributed fiber optic sensing provides continuous real time information about temperature and/or strain events along the length of the asset, helping to detect possible failures at the earliest stage, giving the operator time to take action. This monitoring can be used alone or included in an asset integrity management program, alongside in-line inspections and/or as a feedback loop for models.
An international joint industry project promoting the growth of fiber optic sensing in subsea applications
The Society for Underwater Technology www.sut.org.uk
Omnisens systems, based on DTS and DTSS monitor:
Flowlines, pipelines, subsea tiebacks
Fatigue and elongation detection and location.
Omnisens supplies both Distributed Temperature Sensing (DTS, uses multimode fiber) and Distributed Temperature and Strain Sensing (DTSS, uses single mode fiber).
What is Omnisens Subsea?
Omnisens subsea asset monitoring system is a modular fiber optic distributed monitoring solution for condition monitoring and performance optimization.
The system comprises:
OMNISENS HIGH PERFORMANCE DISTRIBUTED SENSING (DTS)Based on Brillouin or Raman scattering depending on the project requirements, the system detects, locates and profiles small temperature and/or strain changes along the entire length of the asset continuously, in real time.
Due to manufacturing, transport and installation challenges of long cables (umbilicals and piggy-back cables), the sensing fiber may be placed under tension and strained. The affected cable sections can be successfully monitored for temperature using compensation techniques developed by Omnisens. Whether unintentionally detected or actively monitored for using an appropriate fiber optic sensor design, strain provides another insight into the cable's condition. Knowing where strain exists or is developing, the operator can mitigate against the effects this may have. Should the fiber optic sensor break, then the system will alarm, locate the break and measure up to the break.
One or more dedicated servers are included for data processing and storage.
OMNISENS MAPAn optional customizable graphical user interface which provides a simple ‘see at a glance’ visualization of the entire asset, showing alerts and events.
Omnisens Break Detection Module (BDS)This option provides fast detection of broken optical fibers, ideal for use with a Direct Electrical Heating (DEH) monitoring system. In less than 50 ms an alert is sent to the SCADA.
Dynamic Cable Rating for optimizing power umbilical performance (optional module)Dynamic Cable Rating (DCR) establishes the relationship between load and temperature for the power cable and uses this to manage the performance of the cable and provide ratings in emergency situations.
Dynamic Cable Rating is based on IEC 60287 and 60853. The DCR calculates the conductor temperature based on the cable construction, electrical design, laying configuration and environment.
Knowing the temperature of the conductor and the load, the following predictions can be made:
During design studies for new cables, Dynamic Cable Rating informs decisions on specifications, e.g.: conductor diameter.
(Note: DCR is also known as RTTR and DRS)
Omnisens systems monitor temperature all along the cable.
Note: Measurement definitions - Omnisens uses SEAFOM-MSP-01 Measurement Specification for Distributed Temperature Sensing (Download the document here).
Flowlines and pipelines
Flowlines with active heatingActive heating provides an alternative or addition to chemical injection to reduce the risk of wax/hydrate formation. Monitoring the heating mechanism:
Direct electrically heated flowlines (DEH)
|During Direct Electrical Heating, an induced AC electrical current passes through the steel pipeline from electrodes attached to the (piggyback) power cable, warming up the pipeline. Heat generated along the length of the pipe ensures the well fluid temperature remains above that at which hydrates or waxes would form.
Most commonly used during transient phases (shut-downs and re-starts), DEH can be operated semi-continuously. As water-cut increases, pressure falls and shut-downs become more frequent, DEH can help extend the productive life of the well. Since 2000 the technique has offered a cost effective alternative to chemical injection.
An example of a Direct Electrically Heated Flowline (DEH)
Temperature excursions on a DEH flowline can be detected and located by Omnisens DTS. In this example two 'hotspots' are seen, one in the area of the cable splice the other under the rock dump.Distributed temperature sensing (DTS) is the only monitoring tool able to locate and detect temperature events, wherever they arise, as well as showing the temperature profile along the cable. The Omnisens system provides continuous temperature monitoring for fast detection of developing hotspots and cold spots. Alerts and zones are user-definable.
Flowline bundlesFlowlines may be incorporated into bundles (any combination of flowlines, water injection, gas lift, chemical injection and control systems, p-i-p or integrated production bundle (IPB)). They may include active heating (warm water or electrical heating) and/or passive (wet or dry) insulation of the product lines to ensure flow.
Fiber optic cables already in the bundle can be used as the sensor, or a dedicated fiber optic cable can be introduced pre-installation to provide leak detection and condition (via temperature) monitoring.
Fluids may leak into or out of, and water may leak into, flowline bundles. Small changes in temperature may indicate this at the earliest stage. Here, the temperature of a flowline bundle is monitored in Alaska.With time, corrosion of pipes and break-down of insulation materials can occur. A leak, into or out of the asset, or failure of the heating or insulation is usually accompanied by a temperature change. The fiber optic sensor rapidly detects and locates any change in the temperature all along the flowline.
Electrically heated Pipe-in-Pipe (ETH-PiP)The electrically heated pipe-in-pipe flow line combines active heating with dry insulation to ensure flow. Electrical trace heating is applied along the length of the flowline. The heating elements and fiber optic monitoring cables are applied by a specially designed machine and held in place by spacers; both are then secured by the insulation material. Its low power requirement means ETH-PiP requires minimal topside infrastructure.
The Omnisens system ensures that the:
Showing the key elements of an Electrically Heated Pipe in Pipe (ETH-PiP)
Break DetectionThreats exist from trawling, anchor drag and dropped objects, which could rupture the cable and damage the pipeline. Breaks in one or more of the fibers monitoring the cable may indicate damage to the power cable or the flowline. An Omnisens Break Detection Module (BDS) provides fast detection of broken optical fibers. An alert is sent in less than 50 ms, via relays or TCP-IP to the SCADA, so that the heating system can be shut down rapidly to avoid further damage.
Subsea umbilicals and risers benefit from continuous monitoring of fatigue and condition.Using fiber optic fibers integrated into the riser, Omnisens systems monitor strain and/or temperature along the length of that asset, continuously and in real time. Small changes in temperature (± 0.1°C) or strain (0.002%) are detected. This continuous monitoring compliments in-line inspections.
Sensor position in a riser
Sensor position(s) in a riser bundle
Risks to risers include:
Damage or corrosion can lead to a crack, breakdown or delamination of materials, resulting in possible seawater ingress or leaks.
Risers are also at risk from blockage when the temperature falls below a certain level, depending on the composition of the well stream. During shut-downs and as wells near depletion (increasing water cut, decreasing pressure), the risk increases and the requirements for flow assurance increases.
Monitoring temperature in real time within 0.1°C with meter accuracy along the length of the riser provides the operator with an early warning of developing cold or hot spots, before the failure becomes catastrophic.
Bending and elongation lead to fatigue accumulation. Capable of measuring strain with 1 Hz acquisition rate, fast enough to detect low frequency vibration, Omnisens systems can monitor fatigue accumulation.
Knowing where failures are most likely to occur, the operator can act preemptively to avoid further damage.
Photo courtesy of Aker Solutions
Umbilicals are increasing in length, weight, complexity and power transmission ability in response to deepwater production and subsea processing demand.
They are subject to:
Fiber optic distributed sensing offers valuable information to manufacturers from design to installation and to operators throughout the service life of the umbilical cable. Using optical fiber cable integrated into the umbilical or riser, Omnisens systems monitor strain and/or temperature along the length of that asset, continuously and in real time. Small changes in temperature (± 0.1°C) or strain (0.002%) are detected. Details of the temperature or strain event’s size and location are logged or sent as an alarm to the asset’s control system, via SCADA, e-mail or SMS. Strain profiles are available to compute fatigue accumulation at every point along the structure. Temperature behavior can be seen under at different electrical loads or under bend stiffeners, for example.
Sensor integration into the umbilicalThe fiber optic sensor, correctly integrated during manufacture, retains its accuracy, speed of response and sensitivity not only during reeling and installation but also throughout the operational life of the umbilical. Within the sensing cable the temperature sensors can be single mode fibers or multimode, while the strain sensing cables are tightly embedded, hermetically-sealed single mode fiber. The strain sensing cables have a uniform diameter and are jacketed in a way to ensure a consistent friction fit within the structure.
Often fibers selected from an existing optical fiber cable serves as a sensor, if none is available, an optical fiber sensor (standard telecom single mode fiber), can be integrated during manufacturing.
Theoretical sensing cable positions in an umbilical cable. This configuration would measure bend and cyclical strain (both elongation and compression are measured by the Omnisens system.)
Sensor integration into the umbilicalIn operation the effects of gravity, bending and creep may cause the umbilical to lengthen. An Omnisens system provides a continuous monitor as to whether and where elongation is taking place. The optical fiber sensor has an amount of elasticity which allows it to detect both elongation and compression.
Design validationBy providing temperature and strain information during qualification and performance testing, Omnisens systems help validate the umbilical design, delivering actual rather than ‘sum of the parts’ information on strain and temperature, guiding the design of the final product.
Umbilcals benefit from continuous condition and performance monitoring
Specialists are available to ensure that the Omnisens system corresponds to client requirements, via:
SurveysFlowline and umbilical temperature behavior during start-up, shut-down or in operation, from platform or vessel.
COMMISSIONING AND INSTALLATION
A range of services ensures continuous availability of the monitoring systemThese include: