Liquid purification or separation – Structural installation – Geographic
Reexamination Certificate
1998-10-22
2001-01-09
Upton, Christopher (Department: 1724)
Liquid purification or separation
Structural installation
Geographic
C210S258000, C210S305000, C210S312000, C210S521000, C210S532100
Reexamination Certificate
active
06171483
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a subsea raw water injection facility for injecting sea water into an oil bearing formation.
There is a demand for new techniques to reduce the cost of exploration and production activity in off-shore oil fields such as the North Sea. As oil is discovered in ever deeper water, the costs of and risks associated with fixed platforms supporting production equipment increase. Thus anything which can be done to reduce the amount of equipment that has to be supported above the surface of the sea is useful.
2. Discussion of the Related Art
It is conventional practice to inject sea water into subsea oil bearing formations to assist with the process of sweeping oil from the formation and maintaining the pressure of the formation. Generally an oil production platform is provided with a processing plant to which sea water gathered from close to the sea surface is delivered. The processing plant typically comprises in series a coarse filter in the form of a screen, a fine filter, typically a body of sand, a chlorination unit, an oxygen scavenging unit and a de-oxygenation unit. The water is chlorinated to avoid biological activity and de-aerated to prevent rapid corrosion of pipes used to deliver processed water to an injection string leading to the subsea formation. The various sea water processing units are heavy and bulky and therefore supporting them above the surface of the sea is a significant problem, particularly in deep water.
International Patent Specification No. WO 94/29222 describes a sea water injection system in which the relevant processing units are mounted on the seabed. In the described system, a pump connected to an injection string is located within an enclosure the walls of which are porous. The pump is energized to draw sea water through the porous wall and appropriate arrangements are made to chlorinate the water as it passes through the wall. Particulates separated from the sea water passing through the wall will build up on the outside of that wall but it is believed that turbulence within the sea water will be sufficient to prevent the filter defined by the wall from being blinded.
The arrangement described in the published patent specification has its attractions in terms of simplicity but the concentration of particulates in sea water adjacent the seabed will be sufficiently high during periods of turbulence to make it difficult to predict the performance of the filter. Given the cost implications of an installed system failing there is a preference for using tried and test filtration systems which have a positive mechanism for discharging particulates separated from the pumped sea water.
A filter system is available which efficiently separates both mineral particulates such as sand and organic particulates which have neutral buoyancy. The known system incorporates an ejection mechanism which is periodically actuated so as to discharge separated particulates from the system. The use of such a system in the seabed environment is considered to be acceptable in terms of performance, but there is great concern about the long term viability of the particulate ejection system given that it is expected that large quantities of particulates will be separated and therefore the ejection system will have to be operated at regular intervals, for example many times each day.
SUMMARY OF THE PREFERRED EMBODIMENTS
It is an object of the present invention to provide a subsea injector system incorporating a filter which obviates or mitigates the problems outlined above.
According to the present invention, there is provided a subsea raw water injection facility comprising a pump which in use is positioned on the seabed and connected to an injector string of an oil bearing formation, and a filter connected to an inlet of the pump, the filter being open to the sea and being arranged to remove particulates from sea water drawn through it by the pump, wherein the filter comprises an inclined tube settler disposed such that particulates separated from sea water flowing through it are discharged to the seabed.
A large array of individual settling tubes can be supported on an assembly also incorporating the pump and ancillary filtration equipment designed to removed particulates of neutral buoyancy. The flow rate through each tube can be sufficiently low to ensure a very high percentage of mineral particulates are discharged from the tube and therefore do not contribute to the load on the ancilliary filtration equipment which is primarily provided to remove neutral buoyancy particulates.
Inclined tube settlers are well known, having been first introduced in the 1940s. They are used in for example water treatment plants to separate coagulated/flocculated material from processed water. In the known tube settlers, separated particulates are delivered to a discharge mechanism. In the subsea environment, such a discharge mechanism would carry with it the potential problems referred to above with regard to the available filtration and discharge mechanisms. The present invention is based firstly on the realization that tube settlers are well suited to separating out the highly variable concentrations of mineral particulates which occur in sea water adjacent the seabed, and secondly on the realization that, providing the tubes are appropriately positioned, separated particulates can be allowed to accumulate beneath the tubes until turbulent conditions arise which result in the dispersal of those particulates. Simply by ensuring that the seawater inlet/particulate discharge ends of the tubes are at a sufficient height above the seabed to prevent those ends becoming buried in accumulating sand ensures reliable continuous operation.
Preferably the tubes of the tube settler are formed from an array of superimposed corrugated sheets with the corrugations of adjacent sheets offset and secured together.
The injection facility may comprise a frame which supports the pump and defines a cover to protect the facility against dropped objects, the tube settler being in the form of arrays of tubes distributed beneath the edges of the cover.
The downstream ends of the tubes may be connected to a manifold which is connected to the pump inlet, means being provided to deter marine life from entering the manifold. Marine life may be deterred by chlorinating water within the manifold. The slow current flow through each tube avoids the irreversible ingestion of marine animals capable of swimming against that current and out of the facility through the seawater inlet/particulate discharge end of the tube. A simple device such as a net may be disposed across the upstream ends of the tubes to prevent the entry of all but small marine animals. In addition or as an alternative the tubes may be vibrated, for example by coupling the tubes to a vibrating component such as the pump. Such vibration assists in the self-cleaning of particulates from the tubes.
Preferably an ancillary filter is provided between the pump and the tube settler to remove neutral buoyancy particulates, the ancillary filter being provided with means for periodically discharging accumulations of particulates to the surrounding sea water.
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Matthews, et al., 16th Annual offshore Technology Conference May 7-9 1984, Houst
Cousins Anthony Robert
Eden Robert David
Saputra Budiono Kurniawan
Capcis Limited
Hogan & Hartson LLP
Upton Christopher
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