Temperature monitoring
F
ibre optic-based mon- itoring specialist, Omnisens, has car- ried out a project to
check the temperature of the trace-heated pipe which joins the Islay field to the Forvie sub- sea hub in the North Sea. A world first, the technol- ogy behind this 6km Electri- cally Trace Heated Pipe-in-Pipe (ETH-PiP), was evaluated by Total for two years prior to the project. The seabed survey for the
proposed 6km flowline to join Total’s Islay well to the Forvie Pipeline End Manifold (PLEM) highlighted five de- pression points. It is in these depressions that the risk of hy- drates forming is greatest, es- pecially during longer shut-down and start-up periods when the production fluid tem- perature drops significantly. Since the pipeline design
allows limited opportunity to inject methanol due to the con- straints of the existing subsea system, two alternative steps were taken to prevent hydrates forming.
The first was ‘seabed con- ditioning’ where the undula- tions which caused significant depressions were levelled out over a 2km stretch. The second was to use elec-
trical trace heating (ETH) to ensure the temperature of the pipe did not fall below a speci- fied limit, thus reducing the need for routine chemical in- jection. The ETH-PiP is the re- sult of five years research and development.
Offshore Technology November/December 2012 15 The trace heating was in-
troduced to the Islay flowline to ensure that when the risk of hydrate formation is high, dur- ing longer shut-downs or dur- ing operation, hydrates do not form and block the flowline. Omnisens provided expert- ise on the design and the inte- gration of dedicated fibre optic cable to monitor the electrical heating element inside the pipe-in-pipe structure. The system, an integral part of the pipeline, provides the information required to validate the heating function and also to control it.
The Brillouin based moni- toring system, with an optical sensing budget of 22 dB, pro- vides continuous temperature information all along the pipeline at meter intervals to within 0.1°C.
The data can be viewed as
profiles to see evolutions of temperature change over time, as well as providing an alarm function which can be defined
independently along different pipeline zones.
Its high optical budget en-
sures that the efficiency of the system is maintained despite the losses due to splicing dur- ing pipeline production and in- stallation. This also means that the Omnisens interrogator could be permanently installed on a nearby platform remotely from the Islay field and could continue to provide continuous real time temperature informa- tion for flow assurance pur- poses during daily production. The pipe was assembled in Scotland, where the seven sec- tions of pipe were welded to- gether.
A specially designed ma-
chine deployed the trace heat- ing elements and fibre optic cables around the pipeline. The pipeline installation was com- pleted early in 2012, with first gas produced in April, at a rate of 15 000 barrels of oil equiva- lent per day.
Pipelines
Trace heating has been
introduced to the Islay flowline to counter the risk of hydrate formation
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