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Our Arctic Neighbors: VetcoGray brings umbilical subsea technology to life Offshore Arctic projects require complex subsea systems and the GE subsidiary is already breaking records at Snohvit while looking to the future Sarah Hurst For Petroleum News
VetcoGray, the GE subsidiary that provides the subsea equipment for the Snohvit natural gas project in the Barents Sea, believes technology could be developed to extend the length of subsea tie-backs to a record-breaking 600 kilometers (373 miles). In a presentation given recently at the Russian Offshore Oil and Gas Development conference in Moscow, VetcoGray’s Fredrik Witting explained what the challenges would be.
Snohvit is currently the world’s longest step-out subsea-to-shore development, with VetcoGray’s system connecting the gas field to the onshore Melkoya liquefied natural gas plant 170 kilometers (106 miles) away, Witting said. The system provides 3 kilovolts alternating current of power from shore to the subsea station, technology to avoid hydrates in the pipeline, carbon dioxide reinjection and fiber optic communication that can cope with high data rate requirements for heavily instrumented wells, with a bandwidth of 10 megabytes per second.
If the main umbilical connection fails, there is a backup intervention control system at Snohvit. The main umbilical is 143 kilometers (89 miles) long with a diameter of 90 millimeters (3.5 inches) and a total dry weight of 2,000 metric tons. There are also four infield umbilicals with a total length of 30 kilometers (19 miles). The umbilicals are supplied by cable company Nexans. The hydraulic lines, chemical injection line, optical fibers and electrical power conductors are all inside the umbilicals.
Most of area could be covered If step-out distance could be increased to 600 kilometers, this would theoretically cover most of the Norwegian Sea and Barents Sea areas with exploration and production activities, Witting said. One of the problems would be that the longer pipeline length would give a higher pressure drop for gas tie-backs. Increased pipeline diameter would not solve the problem, but subsea wet gas compression might boost the pressure, according to Witting.
Manufacturing and installation of a 600-kilometer single-length umbilical would raise other issues. Manufacturing capacity would be stretched — upgrading of plant facilities would be required. Also, only a handful of vessels worldwide are capable of installing the Snohvit umbilical, and a considerably longer umbilical would require vessel upgrades or modifications. Several separate umbilical lengths with field splices might be a more viable alternative, Witting said.
A longer step-out would also place burdens on the communications technology. Snohvit’s fiber optic technology is “generally deemed OK for at least up to 230 kilometers,” Witting said. For longer distances regenerators or optical amplifiers of the same type used in the telecoms industry would be required. Secondary satellite link back-up systems are independent of step-out distance, but with limited bandwidth, he added.
As for power supply, high power levels at long and ultra-long distances will require use of direct-current-based technology. Direct current can be used for the main step-out distance, with a subsea DC-alternating-current inverter and local AC network to consumers. DC is not sensitive to transmission distances, and length-incurred power loss can be compensated in the receiving DC-AC inverter.
A 600-kilometer step-out subsea-to-shore gas development would be technically feasible, Witting concluded, and Norway’s Ormen Lange, Asgard and Snohvit projects will serve as reference subsea-to-shore developments for the future. Offshore Russia projects such as Shtokman and Ob Taz will offer opportunities to take the subsea-to-shore technology to the next level, Witting said.
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