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Offshore Magazine: Subsea processing advances to meet industry needs
12.04.2011 + + + The offshore industry, in particular the Gulf of Mexico, is still in a retrospective mode, assessing the impact from the subsea blowout in early 2010 and the ongoing global economic crisis. However, with signs of economic recovery and the potential for higher oil prices, the level of confidence for deployment of subsea processing equipment shows signs of revival.
Subsea processing generally is considered to encompass pumps, separators, power distribution systems, and compressors installed on the seafloor. For oil systems, this equipment provides pressure boosting, processing, bulk water separation, sales-quality oil polishing, and raw seawater injection for reservoir pressure support. For gas systems, processing includes subsea gas compression and subsea gas dew point control/dehydration for flow assurance and sales quality.
Current status as of March 2011 for subsea processing projects worldwide.
There is some element of technical maturity involved in the increased application of the technology, but there is little doubt that higher commodity prices are the main drivers for the upswing in subsea processing applications.
Subsea boosting
Subsea boosting reduces or eliminates backpressure on the wells from the riser hydrostatic head, and, secondarily, the riser and flowline viscous pressure drop. Despite the associated cost and technical challenges, boosting is planned in deepwater and ultra deepwater projects well beyond the water depth ratings of existing seafloor boosting applications.
Subsea separation
Subsea separation can be grouped into three technology areas:
Gravity separation system
Caisson separation systems
Compact/dynamic separation systems.
Each technology group is progressing and being implemented in the field at different paces. Caisson separation systems are the most affordable and have been used by both the majors and independent operators. This technology is proving to be more acceptable to the industry because of projects like BC-10 and Perdido.
Gravity separation systems will be used by Total on Pazflor and have been used by Statoil on Tordis and on Troll C.
Alternatively, compact/dynamic separation systems using cyclone separation are being developed to minimize the size of the separator for deepwater applications.
Bulk subsea water separation and re-injection can be used to relieve the topsides water processing capacity in situations where topside water handling is a bottleneck, enabling additional production to an existing host. Currently, there is a pilot program under way which uses subsea seawater chemical treatment to minimize reservoir souring potential by improving the quality of the injection water.
Subsea gas compression
Subsea gas compression competes with onshore compression for shorter offsets and also with floating compression for longer offsets. Economies of scale are substantial, so subsea compression is likely to be favored in large fields at moderately long offsets. At shorter offsets, subsea compression may add value where liquid holdup is substantial and, consequently, topside compression is ineffective.
Improvements needed
Anticipated developments in the Gulf of Mexico and offshore Brazil, West Africa and Southeast Asia all need improved subsea processing capabilities. The qualification for some of these capabilities is expected to be completed by year end 2011. The world map represents the current status of subsea processing projects around the world.
In the Gulf of Mexico, applications exist for both deep and ultra deepwater light-oil boosting. In Brazil, the higher viscosity heavy oil exploration prospects require boosting beyond the capability of existing twin screw pump technology. In West Africa, typical water depths are more moderate and available multi-phase pumps can be used, but are limited in ultimate recovery by volumetric inefficiency at low suction pressures. In the North Sea, water depths generally are shallow and existing multi-phase pumping is adequate.
Based on the potential applications and the technology limitations of available solutions, it is desirable to achieve the boost and gas tolerance of the ESP/caisson along with the high-power and low intervention cost of seafloor multi-phase pump technology.
Two general directions of the industry for improved subsea oil boosting technology are increased boosting capability and power for seafloor pumps, and bulk gas separation upstream of a gas-tolerant pump to avoid loss in volumetric efficiency at low suction pressure or high gas-to-oil ratios.
Moving forward
According to Eric Hansen of Saipem, “each operator has a different approach” to subsea processing. Some operators are more prescriptive and others rely on the system packager to integrate hardware as a system, a sort of “wait and see” approach. However, it is becoming apparent that research and development has shifted from general R & D for subsea processing to project-specific work.
In comparison with project lifecycle, the qualification of the boosting, separation, and compression technology takes a long time. Only the majors and the large NOCs can afford to take a long-term view of using the technology to increase reservoir recovery or to accelerate production rates. However, these companies are answering the industry’s call.
Among NOCs, the leader in subsea processing technology is unquestionably Statoil, with Petrobras a close second. Among the majors, Total and Shell are on the front lines with the use of subsea boosting and separation technology on multiple projects.
The worldwide oil and gas industry continues to climb slowly out of the economic downturn. With the increasing demand for oil in China, Iraq emerging onto the world stage as an oil exporter, and conflicting opinions concerning peak oil, the industry looks forward with healthy optimism. Offshore oil and gas exploration will continue to advance into deeper and more challenging waters as new technology and increasing demand allow.
K Janardhanan - Mac McKee - INTECSEA
Source: Offshore Magazine, March Issue
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