ABB's INSUBSEA® Long step-out system provides a low-cost solution to brown-field extension and a cost-effective solution to green-field developments.
If you had to select the biggest challenge facing the offshore oil and gas industry in the 21st century, you might pick out the need to keep the exploration and production segment profitable. The reasons are pretty familiar, of course. The price of hydrocarbons still at low level; meanwhile, the fields beneath our continental shelves are depleting and the new ones we find are often in deeper water in more remote locations. In these circumstances, the industry has to look for new tools to do the job.
I thought it would be interesting to tell you a little bit about “Step-out” for subsea production: the need to create transmission and distribution systems for underwater equipment. This was something we began to think about in the 1990s. The issue was how to deliver high quality alternating current with a variable frequency to an installation several kilometers away – not a straightforward task in the best of circumstances.
In early 2000s we were commissioned to install a system with subsea transformers at the Topacio and Ceiba fields in the Gulf of Guinea, after which we faced the Tyrihans project in 2006, off the coast of Trondheim. Before that we hadn’t tackled a step-out much longer than 9 km and power transmissions greater than 2 MVA. This would require us to deliver 4MVA over 31 km.
However, Tyrihans was but the prelude to the toughest challenge of all: bringing power to the immense compressors of the Åsgard field. In 2008 we were awarded a contract to develop and qualify a variable speed drive power system for Åsgard. This was a massive challenge, because the power requirements, the operating speed range and the distance were a level of difficulty beyond anything. The system was to deliver 20 MVA over 47 km, and it had to do so with frequencies as high as 200Hz – compared with the usual 65-70Hz range. This led us into a new category of challenge, both on the system and the components side.
Despite that, Åsgard was, in principle, the same as the other jobs – just much more difficult. We tackled it in with our customer, Statoil, with whom we built a full-scale testing station at its laboratory in Kårstø, where we performed many, many tests to prove the system’s operation and its robustness.
Three key lessons we learned as we solved this challenge were:
- Pre-study and qualification test before project execution help closing technical gap and providing reliable and cost effective solution.
- Never underestimate the challenge of driving and controlling compressors sit on seabed through a long-distance cable.
- Transmit high-voltage power with variable frequency over long distance enable subsea compression.
In the end, ABB completed the qualification project with great success and we were awarded the contract to deliver the final step-out solution. Åsgard was the world’s first subsea gas compression project, and it has now been in operation for around a year and everything is still working perfectly.
For more information on long step out, you can see some of our additional resources here: INSUBSEA® Long Step out system.