Patent Publication Number: US-2010116726-A1

Title: Effluent discharge

Description:
The present invention relates generally to effluent discharge, and more particularly to the discharge of concentrated brine from a desalination plant onboard a sea-based vessel. 
     Fresh water is becoming increasingly scarce in many parts of the world for various reasons including global warming, pollution, expanding populations, heavy irrigation and deforestation. Desalination, which is the process of removing salt from salt water to produce fresh water, is becoming an increasingly popular solution to the problem of shortage of fresh water. 
     Large-scale desalination plants, which are located off-shore on ships or other floating vessels, may prove to be an effective way of combating water shortages. Such ‘vessel-based’ desalination plants provide many advantages over land-based desalination plants. For example, whereas the location of land-based desalination plants is fixed, vessel-based desalination plants can be moved and can therefore supply fresh water wherever and whenever it is most needed. 
     In addition to producing fresh water, desalination also produces a waste product, or effluent, which is a concentrated or ‘hypersaline’ brine containing all of the solutes and suspended solids removed from the salt water to produce fresh water. The effluent may also contain water treatment chemicals, and chemicals used for cleaning the desalination equipment. With vessel-based desalination plants, and also with many land-based plants, the effluent is discharged back into the sea. It is important, however, that this is disposed of in such a way that it does not harm the surrounding ecosystems. 
     The present invention seeks to provide an efficient and eco-friendly system for discharging the effluent from a vessel-based desalination plant. The invention also seeks to provide an improved vessel-based desalinating plant incorporating such an effluent discharge system. 
     According to a first aspect of the present invention there is provided an apparatus for discharging effluent from a water-based vessel, the apparatus comprising means for causing the effluent to be introduced into the water at a plurality of discharge locations relative to the vessel, wherein the discharge locations are remote from each other thereby causing the discharged effluent to be dispersed in the surrounding water. 
     The vessel is preferably located at sea. Preferably the effluent is discharged simultaneously at the plurality of discharge locations. 
     In preferred embodiments of the invention, the apparatus comprises conveying means for conveying the effluent to the plurality of discharge locations. The conveying means preferably comprises a branched pipe assembly having one or more inlet pipes connected to a plurality of outlet pipes. Preferably the branched pipe assembly has a plurality of outlets through which, in use, the effluent is discharged. The outlets may be defined by respective open ends of the outlet pipes, or the outlets may be located at other locations along the lengths of the outlet pipes. The outlet pipes are preferably arranged to fan out from a common location, and may extend in directions within all four quadrants of a circle centred on the common location, in other words such that each adjacent pair of radial outlet pipes are oriented at an angle of less than 90 degrees to each other. The outlets may be arranged substantially in a circle. The common location may advantageously be on or directly below the vessel. 
     Preferably a manifold connects the inlet and outlet pipes. The manifold may have one or more manifold inlets and a plurality of manifold outlets. The one or more inlet pipes may be respectively connected to the one or more manifold inlets, and the plurality of outlet pipes may be respectively connected to the plurality of manifold outlets. In other embodiments of the invention, instead of being connected by a manifold, the inlet and outlet pipes may be welded together or joined by any other suitable technique. Moreover, the inlet and outlet pipes could instead form part of a branched pipe assembly having a monolithic structure. 
     The length of each outlet pipe is typically between about 200 m and 10 km, and the length of the or each inlet pipe is typically between about 50 m and 2 km. However, in some embodiments the inlet and/or outlet pipes may have lengths that fall outside these ranges. The water conditions may affect the lengths of pipes that are suitable, for example, shorter pipes may be used when there are relatively strong currents that can disperse the effluent effectively. 
     The inlet and outlet pipes may be formed from high-density polyethylene (HDPE) or other suitable materials. The inlet pipes may have diameters up to 500 mm, and advantageously between about 300 to 350 mm, whereas the outlet pipes may have diameters up to 1500 mm, and advantageously between about 150 to 200 mm. The diameters of the pipes depend on the scale of the water purification plant, and for large plants the pipes may have diameters in the region of 1.5 m. 
     The outlets are preferably located underwater and in preferred embodiments of the invention are located on or close to the seabed. The apparatus may comprise one or more risers for connecting the branched pipe assembly to a source of effluent onboard the vessel. The risers preferably include some slack so that the vessel can move about without stresses arising in the connection to the branched pipe assembly on the seabed. To further prevent or minimise these stresses, the or each riser may be supported part-way along its length by an underwater buoyancy aid. 
     In an alternative embodiment of the invention, the conveying means may comprise a single pipeline, or at least a non-branched pipeline, which has a plurality of outlets spaced apart from each other along the length of the pipeline. 
     In order to minimise the impact on eco-systems close to the discharge outlets, the apparatus preferably comprises means for diluting the effluent before it is discharged. The diluting means may be arranged to mix the effluent with seawater before it is discharged. This mixing process may take place onboard the vessel in large tanks and can reduce the salinity of the effluent to a salinity close to that of seawater. 
     In order to disperse the effluent most effectively, it is preferable that the discharge locations are spaced apart from each other by distances in excess of about 200 m. However, the relative separation of the discharge outlets may be reduced if the vessel is located in a body of water that has relatively efficient effluent dispersal characteristics, resulting from tidal conditions or strong currents for example. It is also preferable for the discharge locations to be remote from the raw water inlets when the apparatus is used in a desalination or water purification plant respectively so that the effluent is not taken in through the raw water inlets after it has been discharged. The raw water inlets may be close to the vessel, in which case the discharge locations are preferably remote from the vessel. The discharge locations are typically between about 250 m and 12 km from the vessel, depending on the inherent water conditions as discussed previously. In other embodiments of the invention, the raw water inlets may themselves be remote from the vessel. It is preferable that the raw water inlets are within about 0.5 km of the vessel. In either case, the raw water inlets and the discharge outlets are preferably separated by about 250 m to 12 km. 
     According to a second aspect of the present invention, there is provided a method of discharging effluent from a water-based vessel, the method comprising introducing effluent into the water at a plurality of discharge locations relative to the vessel, wherein the discharge locations are remote from each other such that the discharged effluent is dispersed in the surrounding water. 
     Preferably the method is used to discharge effluent from a sea-based vessel. Preferably the effluent is introduced into the water simultaneously at the plurality of discharge locations. 
     The method preferably comprises conveying effluent to the plurality of discharge locations. The effluent may be conveyed through a plurality of pipes. Preferably the effluent is conveyed through a branched pipe assembly. 
     The discharge locations are preferably spaced apart from each other by in excess of about 200 m. However, as discussed above, the relative separation of the discharge outlets may be reduced if the vessel is located in a body of water that has relatively efficient effluent dispersal characteristics, resulting from tidal conditions or strong currents for example. The discharge locations may be located remote from the vessel. The discharge locations are typically between about 250 m and 12 km from the vessel, depending on the inherent water conditions as mentioned above. Preferably the effluent is discharged underwater. The effluent may be discharged close to the surface of the water or closer towards the seabed. Preferably the effluent is discharged substantially on the seabed. 
     The effluent may be pre-mixed with water before it is discharged. When the method is used on a vessel-based desalination plant, the effluent is concentrated brine. The concentrated brine is preferably pre-mixed with seawater before it is discharged in order to reduce its salinity to a level close to that of seawater. 
     When the method is used on a vessel-based water purification plant, for example on a vessel-based desalination plant, purified or desalinated water is preferably pumped to shore via a freshwater pipeline extending between the vessel and the shore. The vessel may discharge at port, or the method may include pumping the purified or desalinated water from the vessel into a container located on a barge or similar vessel, which is then tugged to a port where the purified or desalinated water is then offloaded. 
     The method may further comprise using large containers to store purified or desalinated water offshore, either close to or remote from the vessel. The containers may be manufactured from PVC or fibre, thus making them re-usable. Each container is preferably large enough to store at least 25,000 m 3  of water, and may be capable of storing in excess of 30,000 m 3 . The method may include filling the containers when the containers are in the water. The method may involve pumping water from the vessel, through one or more pipes, and into the or each container. Water stored in the containers may be pumped to shore through a pipeline, or transported to shore on a barge as discussed above. 
     According to a third aspect of the present invention, there is provided a vessel-based water purification plant comprising: a first riser connecting a source of purified water onboard the vessel to a fresh water pipeline disposed substantially on the seabed, and a second riser connecting a source of effluent onboard the vessel to a discharge pipeline disposed substantially on the seabed, the arrangement being such that purified water is pumped to shore through the fresh water pipeline, and effluent, which is produced during purification, is discharged into the sea through the discharge pipeline. 
     Preferably the first and second risers are flexible pipes. Each riser may be supported part-way along its length by a buoyancy device located underwater. The risers may be disposed over separate buoyancy devices or alternatively together over a single buoyancy device. Each buoyancy device may be tethered to a respective base on the seabed. The risers are preferably arranged such that there is a portion of slack between the respective buoyancy devices and the vessel. The vessel may be moored with its own anchor pattern or to a buoy, and is preferably moored to a catenary anchor leg mooring (CALM) buoy. 
     The discharge pipeline may comprise a branched pipe assembly. The branched pipe assembly may comprise a plurality of outlets through which the effluent is discharged. The branched pipe assembly preferably comprises a plurality of pipes which fan out from a common point towards open ends that define respective ones of the outlets. The outlets are typically spaced apart from each other by in excess of about 200 m. However, as discussed above, the relative separation of the discharge outlets may be reduced if the vessel is located in a body of water that has relatively efficient effluent dispersal characteristics, resulting from the particular tidal conditions or strong currents for example. The discharge outlets are typically between about 250 m and 12 km from the vessel, depending on the inherent water conditions as mentioned above. 
     In certain embodiments of the invention, the plurality of pipes may fan radially outwards from the common point in a spoke and hub arrangement with the outlets being arranged substantially in a circle. Alternatively, the discharge pipeline may comprise a non-branched pipeline which has a plurality of outlets spaced apart from each other at intervals along its length. 
     The pipelines may be stabilised on the seabed by concrete mattresses positioned on top of the pipelines. A “plough burial” technique may alternatively or additionally be used, whereby the pipelines are laid in furrows on the seabed. 
     The vessel-based water purification plant is preferably arranged to produce between about 5,000 and 150,000 m 3  of purified water per day. However, the invention is of equal application in plants which produce more or less purified water than this. Preferably the water purification plant is a desalination plant, and the effluent is concentrated brine. 
    
    
     
       In order that this invention may be more readily understood, preferred embodiments of the invention will now be described by way of example with reference to the accompanying drawings, in which: 
         FIG. 1  is a plan view of an effluent discharging apparatus in accordance with a preferred embodiment of the present invention; 
         FIG. 2  is a side elevation of the apparatus of  FIG. 1 ; 
         FIG. 3  is a plan view of an effluent discharging apparatus in accordance with an alternative embodiment of the present invention; 
         FIG. 4  is a side elevation of the apparatus of  FIG. 3 ; 
         FIG. 5  is a plan view of an effluent discharging apparatus in accordance with a further embodiment of the present invention; and 
         FIG. 6  is a side elevation of the apparatus of  FIG. 5 . 
     
    
    
       FIG. 1  shows a vessel  10  carrying a large-scale desalination plant  12 . The vessel  10  comprises a ship, such as an oil tanker, or a barge located at sea  14 . The desalination plant  12  utilises a reverse osmosis technique for producing fresh, desalinated water in quantities of between 5,000 to 150,000 m 3  per day. The desalinated water is carried to shore by a freshwater pipeline  16  disposed on the seabed  18  ( FIG. 2 ). A discharge system is provided to discharge the effluent produced during desalination into the sea  14  as will be discussed in more detail below. The vessel  10  is stabilised by a fixed mooring comprising lines  15   a - d  which extend from respective corners of the vessel  10  to four separate locations on the seabed  18  in a catenary, as seen in  FIG. 2 . 
     The discharge system comprises a branched pipeline  22  which is disposed on the seabed  18  ( FIG. 2 ) and which extends away from the vessel  10 . The branched pipeline  22  includes a pair of flexible primary discharge pipes  24  and a dispersal system that comprises four flexible secondary discharge pipes  26   a - d . The primary discharge pipes  24  and the secondary discharge pipes  26   a - d  are formed from high-density polyethylene (HDPE) or other suitable materials. Each secondary discharge pipe  26   a - d  has a respective discharge outlet  28   a - d , defined by an open end which may be raised off the seabed, through which the effluent is discharged into the sea  14 . The four secondary discharge pipes  26   a - d  fan out from a common point  30  to a plurality of relatively remote locations where the effluent is discharged. The double-headed arrows Z on  FIG. 1  indicate the relative separation between adjacent discharge outlets  28   a - d . This separation is typically in the range of about 200 m to about 10 km, and ensures that the effluent is dispersed effectively so that it does not harm the surrounding ecosystems. 
     As can be seen more clearly from  FIG. 2 , one or more flexible pipes, commonly referred to as ‘risers’  32 , connect a number of effluent tanks  34  onboard the vessel  10  to the primary discharge pipes  24  disposed on the seabed  18 . Although not shown in the drawings, in other embodiments of the invention, the effluent tanks  34  may be located in the hull of the vessel  10 . The risers  32  are flexible pipes formed from HDPE or other flexible materials. The risers  32  are sufficiently long to accommodate the vertical and lateral movement of the vessel  10  in the sea  14 . The risers  32  are also suspended, part-way along their length, by an arch-shaped buoyancy aid  36  which is located underwater and tethered to a base  38  on the seabed  18 . 
     The buoyancy aid  36  prevents the risers  32  from dragging on the seabed  18  when the vessel  10  is directly above the base  38 . The base  38  is formed from concrete or steel. Each riser  32  is arranged with respect to the buoyancy aid  36  so that there is a portion of slack  40  between the buoyancy aid  36  and the vessel  10 . This arrangement allows each riser  32  to move with respect to the buoyancy aid  36  as the vessel  10  moves, thereby substantially preventing stress at the connection to the primary discharge pipes  24  on the seabed  18 . Risers  32  arranged in this way are hereinafter also referred to as ‘dynamic risers’. 
     A first pipeline end manifold (PLEM)  42  is located adjacent to the base  38  and connects the dynamic risers  32  to the primary discharge pipes  24 . The primary discharge pipes  24  extend from respective outlets of the first PLEM  42 , to respective inlets of a second PLEM  44 . The second PLEM  44  is also disposed on the seabed  18 , and is located approximately 0.5 km from the first PLEM  42 . The four secondary discharge pipes  26   a - d  extend from respective outlets of the second PLEM  44 . 
     Before the effluent is discharged, it is mixed with a quantity of seawater in the effluent tanks  34 . This dilutes the effluent to a salinity that is close to the salinity of seawater. The resulting dilute effluent is then pumped from the effluent tanks  34 , down through the dynamic risers  32 , along the primary discharge pipes  24  and out through the discharge outlets  28   a - d  at the ends of the secondary discharge pipes  28   a - d . The effluent is discharged on, or close to, the seabed  18 . In addition to being dispersed effectively, the effluent is also discharged far enough away from the vessel  10  in order for it not to affect the continued operation of the desalination plant  12 . The double-headed arrow Y in  FIG. 2  indicates the separation between the discharge outlets  28   a - d  and seawater inlets  46  which extend downwards from the stern  48  of the vessel  10 . The separation Y is between about 250 m and 12 km. 
     The system for transporting fresh, desalinated water to shore will now be described briefly. Freshwater tanks  50  onboard the vessel  10  are used to store the desalinated water produced by the desalination plant  12 . Although not shown in the drawings, in other embodiments of the invention, the freshwater tanks  50  may be located in the hull of the vessel  10 . The freshwater tanks  50  are connected to the freshwater pipeline  16 , which is disposed on the seabed  18 , by one or more dynamic risers  52  which are arranged over a second arch-shape buoyancy aid  54  tethered to a second base  56  on the seabed  18 . A third PLEM  58  located adjacent to the second base  56  connects the dynamic risers  52  to the freshwater pipeline  16 . The freshwater pipeline  16  extends along the seabed  18  to an onshore processing or distribution plant (not shown). In use, desalinated water is pumped from the freshwater tanks  50 , down through the dynamic risers  52  and along the freshwater pipeline  16  to the onshore processing or distribution plant. 
     Referring now to  FIG. 3 , which shows the vessel  10  moored to a Catenary Anchor Leg Mooring (CALM) buoy  60 . The CALM buoy  60  replaces the fixed mooring  15   a - d  shown in  FIGS. 1 and 2 , and allows the vessel  10  to rotate freely about the CALM buoy  60  in order to adjust to the tide and/or prevailing weather conditions. The vessel  10  is attached to the CALM buoy  60  by a number of wires and/or ropes (not shown in  FIG. 3 ) which extend from the bow  62  of the vessel  10  to the CALM buoy  60 . The CALM buoy  60  is, in turn, anchored to the seabed  18  by tethers  64 . 
     In this example, a first pair of pipes  65  connect the effluent tanks  34  onboard the vessel  10  to a manifold  66  at the bow of vessel  10 . The freshwater tanks  50  are also connected to the manifold  66  by a second pair of pipes  67 . A pair of flexible connecting pipes  68  extend from the manifold  66  to the CALM buoy  60 , with effluent being channeled through one of these connecting pipes  68 , and freshwater being channeled through the other connecting pipe  68 . There is slack  69  in the connecting pipes  68  so that the vessel  10  can move apart from the CALM buoy  60  in accordance with the tide and weather conditions. 
     At the CALM buoy  60 , the connecting pipes  68  are respectively connected to a pair of dynamic risers  32 ,  52 . In this example, both of the dynamic risers  32 ,  52  are disposed over a single underwater arch-shaped buoyancy aid  54  which is tethered to a concrete base  56  on the seabed  18 . However, in other embodiments, the dynamic risers  32 ,  52  could be disposed over separate underwater buoyancy aids in a similar arrangement to that shown in  FIG. 2 . The dynamic risers  52  are connected to respective PLEMS  42 ,  58  located on either side of the base  56 . The primary discharge pipe  24  and freshwater pipeline  16  extend from these PLEMs  42 ,  58  in much the same way as that described with reference to  FIGS. 1 and 2 . 
     The combination of a CALM buoy  60  and the dynamic risers  32 ,  52  described above, allows the vessel  10  to move about without stresses arising in respective connections to the primary discharge pipes  24  and the freshwater pipeline  16  on the seabed  18 . This allows the desalination plant  12  to operate effectively in all weather conditions. 
       FIGS. 5 and 6  show an alternative embodiment of the invention, in which the vessel  10  incorporating the desalination plant  12  employs a turret mooring assembly  70  in the bow  62  of the vessel  10  instead of the CALM buoy  60  described with reference to  FIGS. 3 and 4 .  FIG. 5  is a plan view of the vessel  10  in which it can be seen that a cylindrical bore  72  extends through the bow  62  of the vessel  10 . The cylindrical bore  72  accommodates the turret mooring assembly  70 . A freshwater pipe  67  and an effluent pipe  65  extend from tanks  50 ,  34  on the vessel  10  to the turret mooring assembly  70  where they are connected to a pair of dynamic risers  32 ,  52  which are surrounded by a single flexible outer sleeve  74  as best seen in the schematic elevation of  FIG. 6 . 
     The dynamic risers  32 ,  52  surrounded by the outer sleeve  74  extend from the vessel  10  to the seabed  18 , where they are respectively connected to the freshwater pipeline  16  and the primary discharge pipeline  24  by PLEMs  58 ,  42 . The dynamic risers  32 ,  52 , within the outer sleeve  74 , are disposed over a single underwater buoyancy aid  54 , although other arrangements are possible as discussed above with reference to  FIGS. 1 to 4 . 
     The turret mooring assembly  70  enables the vessel  10  to rotate about variable bearings. In this embodiment the turret mooring assembly  70  comprises a split-cylinder configuration in which an upper cylinder  76  and a lower cylinder  78  are separated by a bearing assembly  80 . The upper cylinder  76  is fixed to the vessel  10 , and the lower cylinder  78  is anchored to the seabed  18  by mooring lines  82   a - d . The bearing assembly  80  allows the upper cylinder  76  to rotate relative to the lower cylinder  78 , the lower cylinder  78  having a substantially fixed orientation by virtue of the mooring lines  82   a - d  to the seabed  18 . The turret mooring assembly  70  allows the vessel  10  to rotate in the water relative to the dynamic risers  32 ,  52 . In other embodiments of the invention, different designs of turret mooring assembly  70  may be employed. 
     The dynamic risers  32 ,  52  remain substantially unaffected by rotation of the vessel  10  such that stresses in the connections to the respective pipelines  16 ,  24  on the seabed  18  are minimised. The provision of the turret mooring assembly  70  enables the vessel  10  to be moved between locations more readily than with the CALM buoy  60 , since the mooring system is integral with the vessel  10 . 
     The examples described above are for illustrative purposes only and many modifications or variations may be made to these systems within the general ambit of the invention. For example, the specific pipe arrangements described in the examples above may be varied depending on, amongst other factors, the scale of the desalination plant  12 . In other embodiments of the invention, there may be more than, or fewer than four secondary discharge pipes  26   a - d.    
     Whereas the freshwater pipeline  16  and the discharge pipes  24 ,  26   a - d  are disposed on the seabed  18  in the examples described above, in other embodiments some, or all of these pipes may float on the surface  71  of the sea  14 . In such systems, the effluent may be discharged closer to, or on the surface  71  of the sea  14 . 
     Although the reverse osmosis technique is mentioned specifically, the invention is not limited to this method of desalination, and indeed any other suitable desalination technique may be employed. Furthermore, although the term ‘sea’ has been used throughout the description, this is not intended to limit the scope of the invention, which is equally suitable for use in any other such body of salt or brackish water. Further still, the invention may also be put to effect in a body of fresh water, such as a lake, in which case the plant would be a water purification plant rather than a desalination plant.