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RAWWATER INJECTION: |
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Offshore Magazine Article - January, 2000 Authored by: Tony Cousins, Eamon Connor & David Eggers - Mentor Subsea Technology Services Mentor Subsea Technology Services leads the industry in understanding the rawwater injection issues associated with reservoir pressure maintenance. The Combined Filtration And Seawater Treatment (C-FAST) system has been developed by Mentor Subsea and CAPCIS. C-Fast will provide substantial cost savings to operators, by eliminating water injection flowlines and topsides treatment equipment. Mentor Subsea represents a consolidation of J. Ray McDermott's worldwide subsea and deepwater expertise into a single organization. The goal is to apply state-of-the-art technology in order to maximize the value of otherwise marginal hydrocarbon resources found in deep or shallow water. Our knowledge base and experience in the global subsea market allows us to identify industry needs and trends that lead to innovative design of new products such as the C-FAST system. Mentor subsea offers a complete range of products and services required for various aspects of subsea system design to full field EPCI developments. The core engineering and project management personnel for Mentor Subsea are located in Houston and London and have been providing subsea services since 1987. CAPCIS was established in 1973 by the University of Manchester Institute of Science and Technology and is an independent company providing consultancy, specialist testing, research and technology development and failure investigation services in all aspects of corrosion control, materials reliability and environmental investigations to industry worldwide. CAPCIS provides it's clients with leading services by providing over sixty highly qualified engineers, technologists and senior industrially experienced consultants. This powerful resource is further reinforced by links with the academic Corrosion and Protection Center. What is C-FAST? The Combined Filtration And Seawater Treatment System:
The C-FAST system is suitable for both shallow and deepwater developments. This combined effort represents a 10 year investment program partly funded by the offshore oil and gas industry. During this period, over $3M has been extended in the development of this technology. The C-FAST rawwater injection system is a low-cost subsea alternative to existing topsides water injection facilities. Installed on the seabed, C-FAST eliminates the need for topsides seawater treatment facilities and the costly risers and flowline systems that currently supply water injection to reservoir well sites. As part of a Joint Industry Project (JIP), the C-FAST technology has undergone a rigorous concept- proving program, overseen by a JIP Steering Committee. After successful completion of all tests, the subsea water injection module is now a proven system. Delivered water quality meets or exceeds that delivered by conventional means and the performance reliability is high. The Operational Expenditures (OPEX) costs required to maintain the facility throughout its field life are similar to conventional systems. 44% Cost Savings Costs for C-FAST depend on the exact nature of the development scheme, but are potentially much lower than the equivalent topsides facilities. This is principally due to elimination of process risers and flowlines for the water injection process. A detailed costing analysis has been undertaken on a multi-well development producing back to a floating production facility from two equidistant drilling centers, each 2.2 miles from the vessel, located in 1,640' of water. The water injection facility was based on 6 wells located at one drilling center delivering a peak injection rate of 180,000 to 200,000 bbl/d at an injection pressure 3,000psig. The analysis demonstrated that a 44% cost savings could be achieved by using a subsea rawwater injection system instead of a conventional topsides water treatment and injection facility. C-FAST: System Components consists of:
Tube Settler The tube settler is located upstream of the injection pump and was developed specifically for C-FAST. The unit has no moving parts and is a proven separation process utilizing a simple glass reinforced plastic construction. The main objective of the tube settler is the separation of sand and other larger particles at the intake of seawater. The separated products settle out into a contra-flow stream and are dispersed by the prevailing current or by the gravity to the seafloor. Well over 99% of the particles greater than 22 microns are removed at this initial stage. Bernoulli Strainer The Bernoulli strainer is located downstream of the injection pump and limits the ingestion of neutrally buoyant matter greater than 200 microns. This unit is commonly utilized with the fish farming industry for water cleansing and is ideally suited for subsea applications. The design involves only one moving part, the cleaning actuator. The insert cartridge strainer is diver or ROV retrievable, providing both reliance and easy access. Electro-Chlorinator Chlorine can be supplied either from the host facility via the control umbilical or from a chlorinator unit, that is integral with the subsea facility. The design of the electro-chlorinator is based on the marinization of a standard topsides unit as an insert cartridge concept. This is ideal for subsea maintenance and/or replacement. A nominal power input is required to polarize the electrodes and a switching circuit for electrode descaling by polarity reversal. Injection Pump/Motor The pump modular assembly consists of a pump motor cartridge and an electric/hydraulic wet mateable connection assembly. The assembly is installed into a receiver barrel that forms an integral part of the support structure/protection frame. This configuration allows easy vertical installation and retrieval operations. The current power requirement of the subsea electrical motors is a maximum of 2MW. This unit can supply two water injection wells with up to 30,000 bbl/d. The electric motor design is based on a standard range of high voltage (6kV) electric submersible pumps, providing high reliability of both motor and electrical connectors. Motor internals are protected from over-temperature and ingress of seawater by dielectric oil. It is then maintained at an over-pressure and supplied via the power cable or control umbilical from a barrier fluid (mineral oil) power pack located at the host facility. Power Distribution System and Speed Control The control of pump supply pressure and flow can be achieved either by a variable speed drive or by a minimum flow by-pass loop around the pump set. Factors that determine the exact configuration depends on the injection characteristics, distance from the host facility, motor and power requirements. Emerging subsea power distribution technologies designed for at least a 5MW-power supply will ultimately be integrated into the overall injection facility architecture as a stand-alone unit. C-FAST Buoy Concept In order to provide a power distribution system for C-FAST today we propose to use a buoy. The patented C-FAST buoy concept, jointly developed by Mentor Subsea, CAPCIS and Ocean Resource Ltd., provides a low cost field water injection system specifically targeted at remote satellite wells to be tied-back to a host facility. This concept is a development of the C-FAST technology with the Ocean Resource power and control buoy technology. The buoy is based on the successfully operating 'Sea Commander Buoy' that significantly reduces the cost of power transmission, control and data acquisition of the sea bed facilities and incorporates a telecommunications link with either the host facility and or shorebase. Control and Data Acquisition It is the intention to integrate the control communication and data acquisition system into a standard water injection control module. The base solution for the control and data acquisition of the C-FAST system is to tie into the multiplexed electro-hydraulic unit used to control the respective water injection trees. Typically for a given development, the design, function and operation of the subsea control modules (SCM) are dictated by the production well requirements for maximum interchangeability. Given the reduced number of functions to control a water injection tree, spare capacity exists for the additional control elements. Diver/Diverless Systems The overall system configuration presented in the schematics is for a deep water installation. It is designed for ROV installation, inspection, maintenance and repair (IMR). However, for a shallow water application, the option exists to simplify some of the interfaces for a diver assist work platform for installation and IMR activities. Some CAPEX reduction would result from the diver assist option. Further CAPEX Reductions are Planned Future developments of the C-FAST technology are planned that will reduce the system CAPEX even more. The following ideas are also being considered for inclusion in the basic design:
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