SYSTEM AND METHOD FOR DESALINATION OF WATER

Abstract

A system for producing desalinated water from seawater or other saline water, wherein the system comprises the following equipment features or equipment items, in a typical direction of flow: a subsea seawater inlet, a subsea pretreatment stage, at least comprising one or more of a subsea filter and/or a subsea seawater pre-treatment unit and/or a subsea coarse screen unit, and/or a hydrocyclone, a subsea feed pump, fluidly coupled to, a subsea RO unit, a subsea product pump fluidly coupled to a RO unit product outlet, a product line fluidly coupled to the subsea product pump, and a RO unit reject outlet, preferably coupled to a reject line. The system is distinctive in that: the subsea seawater inlet is at a depth below or within the thermocline layer of the seawater, that is below the epipelagic zone, or as close as feasible if the local depth is insufficient to reach the thermocline layer, and wherein the system preferably is without any barrier fluid supply to the subsea pumps.

Claims

1-13. (canceled)

14. A system for producing desalinated water from seawater or other saline water, the system comprising: a subsea seawater inlet; a subsea pretreatment stage, at least comprising one or more of a subsea filter, a subsea seawater pre-treatment unit, a subsea coarse screen unit, and a hydrocyclone; a subsea feed pump fluidly coupled to a subsea RO unit; a subsea product pump fluidly coupled to a RO unit product outlet; a product line fluidly coupled to the subsea product pump; and a RO unit reject outlet, preferably coupled to a reject line; wherein the subsea seawater inlet is at a depth below or within the thermocline layer of the seawater, below the epipelagic zone, or as close as feasible if the local depth is insufficient to reach the thermocline layer; one or more subsea pods, retrievable and installable in one operation; and wherein at least one of the subsea pods comprises structure for retrieval and installation, structure protecting the equipment through splash zone during deployment and retrieval.

15. The system according to claim 14, comprising a single subsea pod, retrievable and installable in one operation, including all the specified equipment up to and including the subsea product pump and product outlet, wherein the couplings to the single subsea pod comprise power and control in addition to desalinated water out.

16. The system according to claim 14, comprising a plurality of subsea pods, comprising at least one of at least one subsea pod with a subsea product pump, at least one subsea treatment pod with seawater inlet and pre-treatment stage, at least one subsea treatment pod with seawater inlet, pre-treatment stage, and feed pump, at least one subsea treatment pod with seawater inlet, pre-treatment stage, feed pump and subsea RO unit, at least one subsea treatment pod with feed pump and subsea RO unit, and at least one subsea pretreatment-RO pod combined with a smaller number of product pumps arranged on separate or combined subsea pods.

17. The system according to claim 14, wherein at least one of the subsea pods comprises tanks for ballasting and de-ballasting and/or foam-based buoyancy, for facilitating transport, installation and retrieval allowing use of smaller vessels and lifting equipment.

18. The system according to claim 14, wherein the couplings to the system, as located on the seabed to shore or surface, comprise power and control in addition to desalinated water out.

19. The system according to claim 1 wherein the subsea pods comprise individually retrievable equipment items operatively arranged.

20. The system according to claim 14, comprising subsea mateable fluid connections, comprising subsea mateable flowline couplings, with or without clamps, comprising one or two flowline coupling parts and one coupling part for power or combined power and control, for each subsea pod.

21. The system according to claim 20, comprising ROV operable subsea clamp connectors for mating and un-mating flowline couplings.

22. The system according to claim 14, comprising stab in couplings between the subsea prefiltration unit equipment features and the subsea treatment pods.

23. The system according to claim 14, comprising transponders for positioning, arranged on at least one of guideposts, coupling parts, and equipment features.

24. The system according to claim 14, comprising cameras on at least one of equipment items and subsea pods, coupled to a control system, monitorable from at least one of an installation vessel and a maintenance vessel.

25. A method employing the system of claim 14, the method comprising the following steps, in any combination: wet-towing subsea pods to location or transport otherwise, installing the subsea pods with tanks for ballasting filled with air, whereby the ballast tanks are emptied successively until negative buoyancy reach a feasible level, after which the subsea pod is deployed to location, preferably using transponders and/or cameras for position control, using ROV free coupling or couplings mateable by a light work class ROV when connecting the subsea pod to more permanent structure on the seabed, and using ROV free coupling or couplings mateable by a light work class ROV when disconnecting the subsea pod from more permanent structure on the seabed.

26. The method according to claim 25, comprising using a feed pump differential pressure lower than normal practice when operation for reverse osmosis, while the differential pressure is increased periodically, and the reject thereby having significant pressure is directed for backflushing of filters and hydrocyclones of the pre-treatment unit.

27. The method according to claim 25, comprising operating the system at a low recovery rate of 15-70% of maximum recommended recovery rate on land of the RO units.

28. The system according to claim 14, wherein system components up to and including the RO unit and outlets therefrom are at a depth below or within the thermocline layer of the seawater, below the epipelagic zone, or as close as feasible if the local depth is insufficient to reach the thermocline layer.

29. The system according to claim 14, wherein the thermocline layer of the seawater extends from 200 m to 1000 m depth and the epipelagic zone extends down to 200 m below surface.

30. The system according to claim 14, wherein at least one of the subsea pods comprises structure for trawler protection when on the seabed.

31. The system according to claim 21, wherein the ROV operable subsea clamp connectors are by a light work-class ROV.

32. The method according to claim 25, comprising using ROV free coupling and disconnecting when installing or retrieving pre-treatment unit equipment items.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0085] FIG. 1 illustrates an embodiment of the system of the invention.

[0086] FIG. 2 illustrates how a subsea treatment pod of a system of the invention can be lifted in one operation.

[0087] FIG. 3 illustrates a further embodiment of a system of the invention.

[0088] FIG. 4 illustrates a further embodiment of a system of the invention.

[0089] FIGS. 5 and 6 illustrate preferable features of many embodiments of the system and the method of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0090] Reference is made to FIG. 1, illustrating a preferable embodiment of a system 1 of the invention. The system 1 of the invention for producing desalinated water from seawater or other saline or contaminated water comprises, in the direction of flow: a subsea seawater inlet 2; a subsea pretreatment stage 3 at least comprising one or more of a subsea filter and/or a subsea seawater pre-treatment unit and/or a subsea coarse screen unit, and/or a hydrocyclone; a subsea feed pump 4, fluidly coupled to; a subsea RO unit 5; a subsea product pump 6 fluidly coupled to a RO unit product outlet 7; a product line 8 fluidly coupled to the subsea product pump; and a RO unit reject outlet 9, coupled to a reject line 10. The subsea seawater inlet is at a depth 11 (static head) below or within the thermocline layer of the seawater, that is below the epipelagic zone, or as close as feasible if the local depth is insufficient to reach the thermocline layer. The umbilical, or more correctly the power and control line 12 operatively coupled to the subsea pumps or subsea treatment pods for power and control, extending from a location above the seawater surface, consist of one or two lines or cables without any barrier fluid supply line to the pumps. For operating the system of the invention, it is sufficient to connect power and control only, which apparently is a unique feature saving equipment and infrastructure subsea and on shore or topsides, while simplifying installation, retrieval, maintenance and operation.

[0091] The illustrated equipment items are arranged in and on a subsea treatment pod 13, and the treatment pod is retrievable in a single operation and is installable or deployable in a single operation. The number of coupling operations can be as low as one or two, one combined for power and control and one additional or combined for connecting to the product pipeline. Such simplicity apparently is unique.

[0092] The illustrated system embodiment makes actively use of the benefits of the natural hydrostatic head at depth to achieve the osmotic pressure differential. In this system embodiment, the subsea feed (seawater) pump only needs to provide sufficient flow to ensure cross-flow across the RO membranes but does not need to generate a high differential pressure to overcome the osmotic pressure of seawater. The Product pump, being downstream of the membranes would in this system experience suction pressures lower than the ambient pressure and down toward 1 bar absolute, or lower, however, pump cavitation must be avoided. The Product pump creates sufficient differential pressure to lift the flow to the surface with sufficient residual pressure for the receiving end, which could be a storage tank or further processing facilities. Since the Product pump, being downstream to the RO membrane, now is generating most of the differential pressure, and this pump is only handling a subset, typically around 50%, preferably less of the total feedwater flowrate (feedwater minus the rejected concentrate), for prolonged operation while minimizing requirement for expensive intervention, the system also becomes more energy efficient. As the inherent osmotic pressure of seawater is 25-33 bar (https://www.sciencedirect.com/topics/chemistry/osmotic-pressure), such system embodiments would generally need to be placed at 300 meter water depth or below, such as at or below 330 m, to work at optimal efficiencies. However, the feed pump can be configured to deliver higher differential pressures, as required according to the location depth and the requirement to exceed the osmotic pressure of seawater over freshwater on the R0 membrane. For example, the shallower the water depth, the higher the differential pressure of the feed pump, to ensure sufficient pressure to exceed the osmotic pressure while also avoiding cavitation in the product pump. The system embodiment illustrated in FIG. 1 includes embodiments spanning from full active use of the pressure head, at locations at about 300 or 330 m depth or deeper, to partial active use of the pressure head, at locations from about 300 to about 25 m depth, still with all process equipment of the full embodiment arranged at a subsea location, preferably arranged as a subsea RO plant at the level of the seawater inlet. As the feed pump is barrier-fluid less it is not contaminating the downstream membranes with hydraulic fluid, ensuring longer life for the membranes and better-quality product water.

[0093] FIG. 2 illustrates how a subsea treatment pod of a system of the invention can be lifted in one operation. More specifically, the subsea treatment pod frame is also a lifting frame, in addition to protecting the structure and equipment items inside from impact through the splash zone during installation and retrieval and functioning as a trawling protection when on the seabed. Coupling operations on the seabed are limited to water out from and for some embodiments also water into the subsea treatment pod, as one or two coupling operations, and power and control, as one or two coupling operations.

[0094] The raw seawater inlet can be arranged elsewhere than illustrated, for example further elevated from the seabed, especially with soft, silty seabed conditions.

[0095] FIG. 3 illustrates the principle, where a number of pretreatmentRO treatment pods are coupled to a smaller number of product pumps, arranged on separate or combined treatment pods on more permanent structure on the seabed, termed flow base. A common product flowline directs the product freshwater to a location above seawater surface.

[0096] FIG. 4 illustrates a further embodiment of a system of the invention, arrange as a train of equipment items on a treatment pod, with horizontally oriented flowline couplings 14 for subsea connection-disconnection of the equipment items together.

[0097] FIGS. 5 and 6 illustrate preferable features of many embodiment of the system of the invention, wherein deballasting-ballasting allows simpler and less expensive installation and operation than typical for operations subsea. Evacuating air from integrated tanks provides negative buoyancy, at a level feasible for installation. These features allow a low overall cost of installation and retrieval since this can be 20-40% of CapEx in a subsea project. This includes, but is not limited to avoiding expensive Heave-compensated crane-vessels. Rather use methods that can be achieved with available workboats that are common in the Middle-Eastern and Mediterranean region. Installation should ideally be based on wet-tow from shore and ballast/de-ballasting operations using compressed air, as illustrated or vacuum and/or buoyancy elements. If needed, a larger vessel may be used for CapEx (installation) but is preferably avoided for OpEx (maintenance). The seabed structure will differ depending on local soil conditions and other factors, so requirement or not for mudmats, slabs, skirts, suction anchors, grouting or similar must be clarified in either case. Also, the pretreatment requirement vary and must be decided for each case, depending on water quality, life-time expectations etc.