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.

Omnisens DITEST interrogators are based on Brillouin sensing using single mode optical fiber as the sensor. The DITEST is a DTS which provides effective monitoring of long umbilicals, flowlines or risers, or where the sensing fiber experiences significant optical loss. It detects temperature and/or strain with consistent spatial resolution.
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.

Omnisens Raman DTS interrogators provide temperature monitoring of subsea assets using multimode optical fiber as the sensor. For many applications, where the distance is shorter and there is normal optical budget in the fiber optic sensor, this is an ideal solution.

One or more dedicated servers are included for data processing and storage.

OMNISENS MAP

An 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:

time before maximum temperature is reached, given a permanent or dynamic load pattern and the maximum conductor temperature limit for the umbilical

temperature of the cable at the end of a given period of (over) load

maximum ampacity of the cable, given the duration (time) and the allowable temperature during an emergency situation.

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. Thanks to its distance range the interrogator can be housed many kilometers from the asset. It is available in a range of cabinets, with battery and UPS and server options. An optical switch provides up to 20 additional channels so that several assets (umbilicals, risers, flowlines) can be monitored from the same interrogator. Alerts can be sent to SCADA via TCP or relays, all standard Communications Protocols are available including IEC 61850. The interrogator is usually housed in the control/ instrument room on- or offshore. Portable versions are available for surveys onshore or from a vessel.

Note: Measurement definitions - Omnisens uses SEAFOM-MSP-01 Measurement Specification for Distributed Temperature Sensing (Download the document here).

Flowlines and pipelines

Flowlines with active heating

Active heating provides an alternative or addition to chemical injection to reduce the risk of wax/hydrate formation. Monitoring the heating mechanism:

ensures that the heating is working

identifies and locates hot spots, which could mean the heating system is overactive (wasting energy) or has a fault

identifies and locates cold spots which should be investigated.

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 bundles

Flowlines 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: trace heating is working along the whole length of the pipe as it heats up. heating power can be optimized with respect to the targeted pipeline temperature.
Showing the key elements of an Electrically Heated Pipe in Pipe (ETH-PiP)

Break Detection

Threats 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.

Risers

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

Risers (flexible, steel catenary (SCR)) are vital to production. Their useful life can be reduced by excessive temperature or strain.

Risks to risers include:

external damage (clashing, fatigue).

corrosion

blockage from wax and hydrate formation

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.

Umbilicals

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:

Bending during reeling, transport, installation and operation due to internal and external forces.

Damage from external forces (crushing or perforation).

Tension forces from the umbilical weight.

Material degradation in response to the hostile environment.

Severe load conditions.

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 umbilical

The 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 umbilical

In 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 validation

By 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

Omnisens Service

APPLICATION ENGINEERING

Specialists are available to ensure that the Omnisens system corresponds to client requirements, via:

Sensing system design, including modeling, dedicated sensing cable design and qualification testing

Sensing cable integration procedures

Configuration optimization, including system redundancy

Design of junction boxes, cabinets, power supplies and other accessories

Alert and communication options

System tests and validation.

Surveys

Flowline and umbilical temperature behavior during start-up, shut-down or in operation, from platform or vessel.

COMMISSIONING AND INSTALLATION

System configuration

On-site acceptance testing

Optimizing alerts and zones

Remote access set-up

Operator training.

Service contracts.

A range of services ensures continuous availability of the monitoring system

These include:

Rapid ‘change-out’

On-site preventative maintenance visit

Refresher training and system re-configuration.

Data interpretation.