subsea flowline and pipeline monitoring
TRANSCRIPT
Why monitor subsea flowlines and pipelines using fiber optic distributed sensing ?
Long tiebacks, ageing reservoirs, deep water and arctic production increase flow assurance challenges. To ans-wer these challenges engineers have a range of tools and techniques. Having selected the techniques, how does the flow assurance team ensure that these tech-niques are performing? Distributed temperature sen-sing (DTS) brings a new level of monitoring to optimize
the flow assurance technique and ensure that any pro-blems are seen before they become catastrophic.
For many installations, models and discrete sensors are satisfactory. However, when active heating is used and where flowline bundles allow the installation of a fiberoptic cable, this can be used to provide leak detec-tion, condition monitoring and heating system control.
Subsea Flowline and Pipeline temperature monitoring> Leak detection and condition monitoring from topside-controlled system.> Control and performance monitoring of electric heating systems.> Distributed temperature monitoring along the length of the asset, continuously withno‘deadzones’forflowassurance.>Automatic,real-timefiberopticbreakdetection,inferringassetdamage.
SUBSEA
Application Note1
Based on Brillouin sensing, the Omnisens system comprises an interrogator and a single-mode (com-munications grade) fiber optic sensor. The sensor, im-mune to EMI, is hermetically sealed, light, pliable, and has a tiny footprint. Since it can monitor very long dis-tances, a single Omnisens interrogator can be housed on the platform to monitor up to 70 km depending on the losses from connections and splices. Repeaters can be used to extend this to more than 300 km, mea-ning all but the longest flowlines can be monitored.
Omnisens fiber optic-based distributed temperature sensing (DTS) is ideal for sensing the temperature of the flowline or pipeline all along its length. Using a standard Omnisens solution the operator knows the temperature of the asset to within less than 2ºC, with a spatial resolution of 4 m and a measurement time of 10 minutes over a distance of 70 km. Customizable solutions are available for situations requiring perfor-mance parameters outside this range.
For electrically heated flowlines the Omnisens system can also provide a closed-loop real time feedback control to ensure only the energy required to reach the
desired temperature is used, reducing energy wasted by overheating. Post installation, the flowline tempe-rature response can be characterized and verified.
The system also provides leak (cold or hot spot) detection all along the flowline. During times of heigh-tened risk (e.g. shutdowns) any cold spots are identi-fied along the flowline enabling decisions to be taken regarding heating, chemical injection or other mitiga-ting action.
“ During times of heightened risk any coldspots
are identified along the flowline enabling decisions to be taken
regarding mitigation. ”
Temperaturemonitoringsoftware
(intelligent dataprocessing)
Leak detection• alert with location
Flow assurance• alert with location• historic temperature evolution
Heating system control• closed-loop real time feedback• overheat detection• heating performance
Damage detection• fiber optic break detection alert within 50 ms
Condition monitoring• alert with location• historic temperature evolution• actual temperature versus model prediction
Eventmonitoringsoftware
Fibe
r opt
ic s
ensi
ng c
able
Ass
et
Ope
rato
r
Flowline and pipeline integrity monitoring
Sensor Data ActionInformation
Measure Monitor Manage
DITEST Brillouin by Omnisens
POWER O
N
LASER O
N
STATU
S
SUBSEA
Application Note2
Flowlines with active heating:
Direct electrically heated flowlines (DEH)
During Direct Electrical Heating, an induced AC elec-trical 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 shut-downs, DEH can be operated semi-continuously. As water-cut in-creases, 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 and conti-nues to be developed.
Distributed temperature sensing is the only monito-ring tool able to locate and detect local events such as thermal bottlenecks, wherever they arise. The Omnisens system provides continuous temperature monitoring for fast detection of developing hotspots. Alerts and zones are user-definable.
Break Detection
An important safety requirement for the DEH flow-line is that a power cable failure be detected and located quickly to avoid damage to the pipe-line. Threats exist from trawling, anchor drag and dropped objects, which could rupture the cable. 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) is avai-lable for fast detection of broken optical fibers. The BDS is designed to constantly monitor the integrity of optical fibers (which are the sensors). When one or more fiber breaks are detected the BDS provides alerts via relays in less than 50 ms which means the heating system can be shut down rapidly to avoid fur-ther damage.
In the majority of DEH installations the optional Om-nisens BDS is included for fast optical fiber break detection and localization. For inclusion in a SCADA system, alerts are available via TCP and relays.
Example
The monitored cable is composed of three sections: DEH riser, feeder cable and piggyback cable. Each has a stainless steel tube containing six single-mode optical fibers for sensing, integrated into the semi-conductive insulation screen layer. These six sensing fibers are spliced at the farthest point to provide the 3 loops required for this installation.
Since the DEH cables are not energized during pro-duction, the measurements show ambient tempera-ture.
On energization, the Omnisens system revealed seve-ral hot spots. Reviewing the hot spots against route survey and flowline configuration information, the rea-sons could be explained and the operator was able to understand where action had to be taken and where potential issues may arise. The first hotspot was identified on the DEH riser before it entered the water. To overcome this, seawater was injected along the DEH riser during energization. Two further hotspots were found at the connections between the cable sections.
Gravel had been laid over the power cable and flow-line, further upstream, limiting local heat dissipa-tion, which showed as a temperature hotspot on the Omnisens system.
An example of a Direct Electrically Heated Flowline (DEH)
“ The Omnisens system provides continuous temperature monitoring
for fast detection of developing hotspots. Alerts and zones are
user-definable. ”
SUBSEA
Application Note3
100°
80°
60°
40°
20°
0°
Power Cable
Optical fibers
Straps
Power cable Rock dump
Temperatureevent at splice
Thermally insulated wellstream pipe/flowline
Hotspot at KP28 - Gravel dumping prevents heat dissipaton
Field
DEH Topsidetransformer
DITEST Brillouin by Omnisens
POWER O
N
LASER O
N
STATU
S
Areas which could reach the maximum or minimum temperature soonest can be set as individual zones within the Omnisens system. Multiple absolute or relative temperature alerts are possible for each user-definable zone (any number of zones can be set up) in the system.
Electrically heated Pipe-in-Pipe (ETH-PiP)
The electrically heated pipe-in-pipe flowline com-bines 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 is used to ensure that the trace heating is working along the whole length of the pipe as it heats up. In addition, the heating power can be optimized with respect to the targeted pipeline temperature.
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.
Reel lay installation of Electrically Heated Pipe-in-Pipe (ETH-PiP).
SUBSEA
Application Note4
To reach the installed pipeline, the sensing signal-passes through switches and connectors at points between the various structures, likely to result in significant optical loss. The Omnisens DTS, based on stimulated Brillouin scattering, is ideal for situa-tions where the optical loss is high due to, e.g.: fiber optic rotary joints, wet-mate connectors and multiple splices. In addition, thanks to its ability to monitor distances of over 70 km from a single interrogator, it can be mounted on-board ship for surveying or on a nearby platform. An optional switch means that several assets (flowlines, umbilicals, risers) can be monitored from a single platform-mounted DITEST interrogator.
Example
To monitor 7 km of ETH-PiP the Omnisens DITEST was configured from on-board the pipe-lay vessel to measure temperature (from 0 to 90°C) with a 2 m spatial resolution in less than 5 minutes. The sensing signals were routed through standard single-mode fibers in an umbilical to the subsea umbilical termi-nation assembly (SUTA) manifold and connected to the flowline’s optical sensing cables with a wet mate connector (WMC).
The seabed profiles were correlated successfully to the corresponding distributed temperature measurements. The temperature response to energization was uni-formly distributed with the exception of 3 short sec-tions, which appeared as cold spots and could be associated to lowest point in the line near injection points where methanol accumulates. The system being empty elsewhere, these short sections filled with methanol appeared as cold spots.
Thanks to the quality of the temperature information from the Omnisens system, the thermal response time of the ETH-PiP could be fully qualified in different operational conditions.
Flowline bundles
Since the 1980s, flowlines have been incorporated into bundles (any combination of flowlines, water injection, gas lift, chemical injection and control sys-tems). They may include active heating (warm water or electrical heating) and/or passive (wet or dry) insu-lation of the product lines to ensure flow.
Fiber optic cables already in the bundle can be used as the sensor, or a suitable dedicated fiber optic cable can be introduced pre-installation to provide leak de-tection and condition (temperature) monitoring.
Showing the key elements of an Electrically Heated Pipe in Pipe (ETH-PiP)
“ Based on stimulated Brillouin scattering, the system is ideal for situations where the
optical loss is high. ”
Optical fiber sensing cable
Electricaltrace heating elements
Insulation layer
Outer pipe
6” flowline
Spacer
Chemical injection locations appear as “cold spots” on flowline.
SUBSEA
Application Note5
With time, corrosion of pipes and break-down of insu-lation materials can occur, out of sight. 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, from which a leak can be inferred. This change in tempera-ture can be programmed to signal an alert via TCP-IP or relays.
Example
The value of using the Omnisens temperature detec-tion system in a flowline bundle was demonstrated in Alaska. To transport fluids from the offshore well to the inland tie-in pad a flowline was built above ground (on-shore) and buried 2 m below the seabed in shallow water for the 9 km offshore section. In the subsea sec-tion the flowline was threatened by: strudel scour, sub-sea permafrost thaw settlement, upheaval buckling, river channel migration and seabed ice gauging.
The fiber optic sensing cable was strapped to the bundle, fiber loops were created by splicing the fibers at the cable end in a termination box at the drill site.
One reason for selecting Omnisens was its ability to monitor despite significant loss of optical budget in
the sensing fiber. The fiber optic sensing cable (13.14 km) in this case had a total attenuation of 3.02 dB (0.23dB/km max rated attenuation). Additional optical budget was required for the additional 20 splices per fiber. Omnisens stimulated Brillouin sensing was able to monitor temperature reliably, despite this loss.
A power cable was buried parallel to the flowline, 15 m distant. One of the two measurement channels avai-lable on the Omnisens DITEST was connected to the monitoring cable attached to the flowline; the other-monitored the power cable.
Two particular risks to the flowline were identified as erosion and water penetration of the bundle’s insula-tion. Either would result in temperature events. In case of erosion, colder water eroding the relatively warm soil surrounding the flowline would result in a cold spot as would water penetration of the insulation.
During the initial warm-up, flowline temperatures were monitored to ensure a slow increase to minimize the-risk of buckling.
In the first year of operation 33 erosion events were identified by the Omnisens system, spatially and di-mensionally. These were confirmed by sonar and 6 were mitigated. In subsequent years, a smaller num-ber of erosion events were identified. No Leak events were recorded.
Oil
Drill site
Temperaturemonitoring cable
Water
Gas
Pipelinetie-in pad
Flowline bundle, burial depth 2.5 mwith temperature monitoring. Water depth 3-4 m
DITEST Brillouin by Omnisens
POWER ON
LASER O
N
STATU
S
Fluids may leak into or out of, and water may leak into, flow line bundles. Small changes in temperature may indicate this at the earliest stage. Here, the temperature of a flowline bundle is monitored in Alaska.
SUBSEA
Application Note6
Omnisens Service
Systems with a range of performances designed for pipeline and flowline monitoring are supported with application engineering, commissioning and service contracts.
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, testing and validation
> Configuration optimization, including system redundancy
> Design of junction boxes, cabinets, power supplies and other accessories > Alert and communication options
> System tests and validation.
Surveys
> the temperature of a flowline 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’ in case of equipment failure or damage
> On-site service preventive maintenance visit
> Refresher training and system re-configuration
> Data interpretation.
DITEST interrogator measures temperature along the flowline. Thanks to its distance range the interrogator can be housed many kilometers from the asset being monitored. An optical switch provides extra channels so that several assets (not only flowlines, but also risers and umbilicals), can be monitored from the same interrogator, minimizing space and weight on the plat-form.Alerts can be sent to SCADA via TCP or relays. Versions of the DITEST interrogator are available for permanent installation or survey, on platform or vessel.
SUBSEA
Application Note7
REF:
AN
-006
(Sub
sea
flow
line
and
pipe
line
mon
itorin
g) e
n-01
Applications Benefits
Leak detection Alert and location of event allows the operator to take rapid action to avoid further damage.
Control and monitoring of flowline electric heating
Early warning and location of over- or under- heating so that the operator can take appropriate action, resulting in optimized flow assurance and energy consumption.
Flow assuranceLocation and evolution of ‘cold’ areas along the flowline where waxes and hydrates my form. Chemical injection can be targeted to only these areas- significantly reducing opex.
Condition monitoring
Temperature events are located and highlighted so that targeted ILI or other action can be taken to avoid loss of fluid, production and their associated dangers.
Detection and location of pipeline sections exposed to seawater due to erosion or landslide.
Damage detectionRapid (50 ms) indication of a fiber optic sensing cable break may indicate that the piggyback (power) cable or even the flowline is damaged. Action can be taken to investigate, shut down or otherwise avoid further damage.
References:
Gyger, F. Rochat, E. Chin, S. Ravet, F. Nikles. M. 2014, “Ultra Long Range DTS (>300km) to Support Deep Off-shore and Long Tieback Developments,” 33rd International Conference on Ocean, Offshore and Arctic Engineering, San Francisco, OMAE-24019.
Ravet, F. Børnes, A. Borda, C. Tjåland, E. Hilde, H. and Nikles, M. 2012, “DEH Cable System preventive protection with distributed temperature and strain sensors.” 9th International Pipeline Conference, Volume 4: Calgary Paper No. IPC2012-90274, pp. 21-30.
Fisher, R.C. Hall, S. Cam, J-F. and Delaporte, D. 2012,“Field Deployment of the World's First Electrically Trace Heated Pipe in Pipe,” Offshore Technology Conference, Houston, OTC-23108.
Decrin, M.K. Nebell, F. de Naurois, H. and Parenteau, T. 2013, “Flow Assurance Modelling Using An Electrical Trace Heated Pipe-In-Pipe: From Qualification To Offshore Testing.” Offshore Technology Conference Houston, OTC-24060.
Lanan, G.A. Cowin, T. Hazen, B. Maguire, D.H. Hall, J.D. and Perry, C.J. 2008, “Oooguruk Offshore Arctic Flowline Design and Construction.” Offshore Technology Conference, Houston, OTC-19353.
DITEST is a trademark of Omnisens.
SUBSEA
Application Note8