The third-generation subsea processing system as envisaged by partners Aker Maritime and Kongsberg Offshore.
One of the key technologies for the offshore industry in this century, in the opinion of Aker Maritime (AMA) and Kongsberg Offshore (KOS), will be subsea processing. The two Norwegian contractors have joined forces to stake out their place in this emerging market, with AMA contributing its process and flow assurance expertise and KOS its expertise in developing and integrating subsea systems and tooling.
At the end of last year, the AMA/KOS alliance was due to submit a concept evaluation study to Norsk Hydro for its proposed Sogn area development. It has also performed studies for other operators such as Statoil, Shell, Mobil, BP Amoco and Elf relating to fields in the North and Norwegian Seas, West Africa, and the Gulf of Mexico.
The policy of working as closely as possible with potential customers is a deliberate one. "We're not primarily interested in doing our own in-house technology development," says Svein Olaf Klemetsen, Technology Manager, Business Development, Subsea Technology at Aker Engineering, who heads Aker Maritime's participation in the alliance. "We want as much input from the oil companies as possible."
The Sogn area project, which embraces the Fram and Gjøa Fields and several gas finds in Norwegian quadrant 35, is an example of how the offshore industry is changing its approach to field development. Hydro has already discarded proposals for developing Fram and Gjøa with separate, stand-alone floaters as too expensive.
Instead, it is looking at a phased subsea development of the area starting in 2003 with a few wells on Fram tied back to the Troll C floater. This would involve a relatively small up-front investment, and would allow a cash flow to be generated to help finance the further stages of development. Subsea processing would be called for in a second stage of development of Fram, starting in 2005, and for the subsequent development of Gjøa. The AMA/KOS study is aimed at identifying the technology gaps which need to be made good if such a scheme is to be feasible.
Subsea processing
How AMA/KOS's subsea processing plant is likely to look on the seabed, connected to a KOS production template.
The world's first subsea processing operation will get underway in spring when the Troll Pilot supplied by ABB to Hydro is due to come on stream, separating water from hydrocarbons and re-injecting it beneath the seabed.
AMA and KOS are already thinking ahead in terms of what they call third generation subsea processing. While the first generation of this technology is taken as multiphase pumping, which involves separating the wellstream components before recombining them, and the second generation is water removal as represented by the Troll Pilot, the third generation involves a wellstream de-sander. Sand is a big problem where many reservoirs are concerned, Klemetsen says, and an efficient means of removing it from the wellstream and disposing of it is required.
One of the basic building blocks in the work is KOS' highly successful HOST subsea production system, or rather the modular philosophy that underlies it. "We are moving on from the production HOST to a template-based processing system with various modules that can be independently changed out," says Erlend Fjøsne, KOS project manager for processing technology who is heading the work on the Sogn study for his company.
Wellstream de-sanding
Put briefly, the third-generation subsea processing system involves de-sanding the wellstream and then separating it into its component parts - oil and gas, which are then dispatched down their respective export pipelines, and water, which is re-injected or discharged into the sea. The main technologies required are desanding, separation, metering, pumping, and power supply. The alliance is working closely with a number of leading suppliers in the various fields.
De-sanding technology and sand disposal are being developed. The sand slurry which is left after de-sanding cannot be discharged into the sea as it is contaminated with oil. Disposal options include re-injection, export along with the oil if the quantity of sand is small enough, or storage in seabed containers.
The partner for separation technology is Proser-Nat, which supplied the internals for the Troll Pilot separator. Here, the work focuses on traditional gravity separators and cyclone separators, Klemetsen says, rather than centrifuges which have the drawback, as far as operation on the seabed is concerned, of involving rotating machinery.
Metering, monitoring
Metering and monitoring are essential activities, firstly as part of the system for controlling the level of liquids in the separator, which is crucial if efficient separation is to be achieved. Then, when they leave the separator, the oil and water have to be monitored for their water and oil contents respectively. At this stage it may be beneficial to pass the oil through a high-efficiency separator, Klemetsen says, to reduce the water content to less than 0.5% and thus drastically reduce the danger of hydrate formation.
If the produced water separated out in the first-stage separator is to be re-injected, it needs to be monitored to ensure that its oil content is no more than 1,000 parts per million. A second option, discharge into the sea, is available if the oil content is less than 40 ppm. Means of monitoring such a small oil content are being studied in conjunction with cleaning methods which could also achieve it.
Subsea pumping is involved at several points in the process, from controlling the level of liquids in the separator to exporting the oil and gas, and perhaps sand, and re-injecting the water (and perhaps sand). The technology for subsea oil and water pumps already exists. For wet gas boosting, however, it is only at an early stage - the alliance has some ideas on how to develop it but does not yet wish to disclose them, Klemetsen says.
Subsea power
Another key area has been identified as power related issues such as subsea power distribution to multiple consumers and variable speed drive capability. Here, the alliance is working together with Alstom, which has demonstrated subsea power distribution under the Speed joint industry project, and Tronic, which has developed a 36 kV wet mateable connector. Expertise on umbilicals and power cables is being supplied by Alcatel.
Each consumer requires 3-5 MW, which has not yet been demonstrated. So application has been made to the European Union's fifth research and development framework program for funds to support the development of a 5 MW frequency converter, plus the associated switchgear and wet mateable connector.
Looking to the future, the alliance is working together with Natec to develop the concept of an electromagnetic frequency converter. This is a new application of an old principle, according to Fj sne, which could provide a simpler and more reliable solution for high voltages than the semiconductor-based frequency converter, and might be the basis for a compact system for use in deepwater.
The AMA/KOS alliance has developed a system for analyzing the cost of different subsea processing scenarios. This followed initial work on a subsea processing system for Statoil's Statfjord North and Sygna development, involving two processing templates, which proved to be too costly to be competitive. A second scheme with just one template reduced the cost by more than two thirds.
"At this point, we decided we needed to develop a cost model to be better able to determine the dominating cost elements," says Klemetsen. A life-cycle recovery/cost model has now been developed which is sufficiently accurate to make it possible to rank alternative scenarios, and thus to focus from an early stage on those which offer the most cost-effective outcome.
The AMA/KOS cooperation has proved successful, and two years after signing their original agreement, the partners are now preparing to revise it for continued cooperation. "We will also be looking in greater depth into EPC contract formats to ensure that we can deliver in the proper manner," says Klemetsen. "With our agreement and contract philosophy in place we can show clients that we operate as one team."
