An important milestone in offshore oil and gas production was passed on May 17, 2000, when the world's first subsea processing system, the Troll Pilot, came into operation. Subsea processing has long been identified as a key breakthrough in production technology, promising to revitalize offshore economics and unlock fresh reserves.
The event took place on Norsk Hydro's Troll Field, where the wellstream from the S-13 subsea well was directed into the separator of the Troll Pilot. The pilot was supplied by ABB, and is based on its Subsis - subsea processing and injection - system. Hydro reported that the multiphase flow of some 23,900 b/d was being separated into 16,350 b/d of oil and associated gas and 7,550 b/d of water.
The water is disposed into a dedicated injection well. During the first few weeks, when injection occurred using only the water's own pressure, only part of the water could be disposed of in this way. But in late June, the system was completed with the installation of a water injection pump and module, which opened the way to the disposal of all the separated water in the injection well.
The quality of separation was well within the prescribed limits, Hydro said. The proportion of oil in water was about 144 ppm, compared with a design specification of 1,000 ppm. At the moment, about 80% of the water in the oil stream is being removed.
Hydro chose Troll for the subsea processing pilot as early water breakthrough is expected from the thin oil layers that are being produced in the Troll West Gas Province. Once it has gathered operational experience with the pilot, it will consider the deployment of additional systems on the field.
Subsea potential
The industry has long looked forward to subsea processing. Well before it had proved itself in practice, its potential had already been taken on board as a key element in future developments. Hydro's own plans for the Sogn area fields, for example, move from a first phase, in which standard subsea equipment will deliver a multiphase stream to the host platform, to later phases, in which subsea processing systems will export oil and gas to different hosts. Also, subsea processing is a core element in Hydro's preferred plan for developing the deepwater Ormen Lange gas field.
Having set down its marker, ABB is now ready to progress subsea processing technology in a number of directions. In the form used on Troll, Subsis is specialized for shallow water, but with relatively little tweaking, it can be applied in depths down to 1,400 meters, says Ole Berg, Vice President R&D at ABB Offshore Systems. But the major market for the technology will be in deeper waters, he says, and the company is now working to extend its application to water depths of 1,500-2,000 meters.
"We've hit the right time window with subsea processing," says Berg. He sees subsea processing moving through three significant steps which will transform the shape of offshore production. The first, and current, stage entails the reduction of topside facilities through the transference of three-phase separation, produced water re-injection, and pressure boosting to the seabed.
In a second phase, topside facilities will be further reduced or removed, depending on reservoir characteristics and location, as the seabed capabilities expand to include the re-injection of produced gas and long-distance oil transport. In the third stage, gas handling capabilities will be introduced to enable dew point control, gas compression, and long-distance gas transport to be performed subsea.
Semi-cyclone separation
The 350-ton Troll Pilot, which has made the first stage a reality, consists mainly of a separator, which is about 10 meters long 3 meters in diameter, and weighs some 60 tons, and the water injection module and pump, which stand five meters high and weigh 20 tons. Traditional gravity separation is used, but the separator is fitted with a novel inlet device known as a semi-cyclone, which is based on an original idea by Hydro and has been further developed and industrialized by ABB. The device reduces the momentum of the wellstream gradually in such a way that it prevents the formation of small droplets, which would reduce the efficiency of the separation process.
An essential element in assuring separation quality is control of the level of the oil/water interface, which is achieved by adjusting the speed of the injection pump. Feedback on the interface level is provided by a monitoring system based on nucleonic and electric sensors.
The startup of the injection pump also marked an important advance, with the introduction subsea of rotating machinery using a high-voltage power supply, Berg points out. The 2 MW pump was supplied by Framo Engineering, which has worked closely with ABB on the Subsis development. The all-important link between the power and the motor it drives is achieved through a high-voltage connector developed by ABB and known as Mecon - Marine Electra Connector.
Power distribution
The connection is made through a mini-spool mated on one side to the power umbilical termination and on the other to the pump module. Seawater is then excluded from the internal chambers by a series of flushing operations using de-ionized water produced at the seabed and ethanol. The spool is finally filled with dielectric fluid and the power supply can then be switched on.
In the case of the Troll Pilot, there is only one power consumer. But if subsea processing is to deliver the benefits claimed for it, it must be possible to provide power and control for complex subsea systems involving multiple consumers. This is a capability, which ABB, again, working closely with Framo, is developing through the Sepdis - subsea electrical power distribution system - joint industry project. ABB is responsible for the electronics and Framo for the cooling system.
Sepdis, which is receiving funding from the Demo 2000 program and from the Ormen Lange license, is now about to undergo an extensive qualification program at Framo's test facility at Fusa. Most parts of the system will undergo wet testing. The program is due to be completed by the first quarter of next year.
The separation technology employed in the Troll Pilot is relatively straightforward, but interesting challenges start to arise in the context of deepwater application. A gravity separator of the kind used in the pilot could prove to be impractically large and heavy for waters of 1,500 meters or more. One solution could be the addition of pre-processing and post-processing to reduce retention time in the tank, and hence tank size, says Berg.
Another solution, which the company is working on, is using cyclone technology as the basis for a compact separator. The cyclone technology is conventional, but is being used in conjunction with advanced computerized flow dynamics.
The separation of the different phases takes place in a vortex, which is created by utilizing the inlet energy. High-pressure tests have so far returned very good results, indicating a gas/liquid separation efficiency of 99.98% and sand separation efficiency of 50-90%, Berg says. Control and instrumentation is, however, a more complex issue than in the case of gravity separation.
Sand handling is a problem at any water depth. At present, a remotely operated vehicle has to visit the subsea installation and bring sand that has been separated out back to the topside, an operation, which adds to costs and is only practical if the amount of produced sand is small. In a scenario, where there are no topsides facilities, a way will probably have to be found of cleaning the sand and returning it downhole or to the sea. The alternative would be the unsatisfactory solution of sending it to shore with the oil and gas.
Stranded gas
Berg admits that the third stage of development of subsea processing is at the moment rather visionary. But the sooner the gas issue can be solved, the better, for there is already plenty of stranded gas in mature areas like the North Sea. As exploration moves into deeper water and more remote areas, the challenge of dealing with associated gas will increase. At the same time, governments are becoming less willing to see gas wasted through flaring or even being returned to the reservoir.
If gas, once separated from the liquids, is to be transported over long distances, it will have to be conditioned for transport by controlling the dew point. In other words, gas will have to be dried to a degree, Berg says. Whether it is to be transported or re-injected, it may also have to be compressed, in which case, subsea compression systems will have to be developed. One way of easing this task will be if separation can be performed at higher pressures than occurs now, as less work will then be required for compression.
These are real challenges, Berg says - the equipment for performing such functions will be costly. But they are not just technical challenges, they also concern the work culture of the oil industry. In the past, the time taken from starting a new technology development to getting it accepted in the market as qualified has probably been in the order of 10 years. But now, oil companies need much more rapid technology development, and the key to this is greater cooperation.
On the one hand, vendors must also learn to work together where the technology in question requires their complementary expertise. When several vendors cooperate, it creates a larger competence and financial base for the development, with beneficial effects for the reliability and availability of the technology.
But there is an important role for oil companies, too, and that is to offer real test sites, or in other words, reservoirs. Field testing is much more valuable than generic technology development and trials in artificial conditions. This realization is beginning to spread, and a number of international oil companies are now becoming more willing to make this contribution.
The systems that are developed must also be flexible. At the pre-production stage of reservoir development, there is considerable uncertainty as to how the reservoir will behave. If there is no flexibility in the production scheme, there is a big risk that it may turn out to be less than optimally suited to the reservoir as production goes on. So, oil companies want a simple and flexible system at startup, which can be developed as the specific circumstances require. Production could start with a simple subsea solution, to which more advanced processing can be added as required without expensive modifications having to be made. It's not such a difficult problem, but oil companies and technology providers have to be aware of it and cater to it, Berg says.
"Even though subsea processing is not the answer to all challenges and to all types of reservoirs, we expect the technology to play an important and sometimes decisive role in making deepwater developments profitable and possible in the years to come," says Berg. At the moment the industry is investing heavily in the technology, both as technology developments and in order to establish the financial and operational aspects of using the technology. The speed with which the technology is introduced and accepted into the market will be wholly dependent on the degree of cooperation, he concludes.
