Multi-phase pumping up the volume
Increasing number of projects bring technique into industry's mainstream and provide important lessons for future schemes by ADRIAN COTTRILL, Amsterdam
The technology for pumping a varying mix of unseparated oil, gas and water immediately upon raw wellstream reaching the seabed is now becoming well established at the "oily" end of the spectrum.
A growing list of commercial projects are bringing subsea multiphase pumping into the mainstream and yielding useful feedback for the future.
At the "gassy" end of the same spectrum in the related field of wet gas compression the technology is more complex and has further to go before it sees commercial adoption.
However, specialist manufacturers are working hard on the challenge and some forward-looking operators now have an impressive pilot investigation or two in their sights.
The ability to boost a multiphase mix from an offshore oil or gas field as soon as it reaches the subsea tree has huge attractions in a business where both water depth and tie-back distance to host platforms are continually extending.
Indeed the entire viability of one project Total's upcoming Pazflor scheme off Angola is already predicated on the use of such technology right from the start.
The related refinement of separating out and re-injecting a good proportion of produced water at the seabed is also very useful in removing that job from the topside of a host platform.
This is what Norway's Tordis project, on stream just two months ago and designed to handle 200,000 barrels per day of liquids, is pioneering at a commercial level.
One of the earliest major multiphase projects, the Hess-operated Ceiba scheme off West Africa, is now five years old. Pump run times of up to 18 months have been achieved here through proactive monitoring, and gas volume fractions exceeding 70% have been handled.
Among the high-profile projects at present, BP's King system over in US waters started to pump oil in the last few days and should be fully-commissioned in a matter of weeks.
However, the pair of stations at that operator's Schiehallion system, west of Shetland despite being installed on the seabed and originally scheduled to start up at least a year ago is still waiting for the asset team to install the necessary umbilical.
The much-anticipated and somewhat delayed contract award for the Pazflor subsea processing system is still "imminent". Also, Brazil's long-delayed SBMS-500 prototype test is now predicted to go into the water early next year.
Certainly the select group of experts at the latest multiphase pump user roundtable (MPUR) just held in Amsterdam had plenty to chew over in the way of case histories as they debated lessons learned and the next advances needed.
The roundtable was founded in 1999 by Stuart Scott, who runs a research group on the subject at Texas A&M university, where he is an associate professor.
On the subsea front, he and his team have scored in Europe. The Amsterdam meeting attended by close to 70 delegates, including a gratifying proportion of operator personnel was the third in what is becoming an annual series in this region. Next year's is to be held in France.
Much emphasis is presently falling on "wet gas", particularly against the background of the very large scale development work in this area being carried out by StatoilHydro for the Ormen Lange and Aasgard fields.
Although the definition of wet gas is open to varying interpretation, broadly it is accepted as a gas stream with a liquid content that falls between 1% and 5% by volume. As delegates pointed out, the challenges of tackling this can be solved from either of two directions.
One of these is via the liquids pump route extending the ability of multiphase pumps to handle very high and fluctuating gas fractions. The other is from the gas compressor direction extending the ability of these single phase machines to tolerate a certain amount of liquid slugging. Among multiphase pumps, Scott noted the helico-axial concept is now considered proven technology for subsea. Twin-screw pumps are proven onshore and moving into subsea use. Other technologies such as Hydril's piston pump are at the conceptual stage.
In wet gas compression which some Amsterdam delegates suggested might more meaningfully be renamed as "multiphase compression" major challenges identified included tolerance of liquids and getting rid of the heat generated by motors.
Central issues for subsea boosting and processing in general continue to be the need to maximise reliability, redundancy, ease of intervention, monitoring and power generation not least penetrators that are currently at a 2.5 megawatt limit.
In the area of protection against erosion by produced sand, Tordis is the leader, with much investigation carried out into the performance of harder steels and optimising of fluid routing to avoid sand traps.
Future challenges for the industry include an increase in the pressure boost that multiphase pumps can provide, said Scott.
"For ultra-deep water and long tiebacks we are looking for a 'delta P' in excess of 150 bar."
Also, there is a lack of testing facilities, particularly hyperbaric.
