Abstract:
Water that is produced during offshore hydrocarbon processing, such as hot produced water accompanying hydrocarbons taken from subsea reservoirs, or cold water resulting from heating LNG (liquified natural gas) to convert it to gas, is changed in temperature to be closer to that of the surrounding sea using apparatus of minimal cost. The apparatus includes a mixer tube ( 52 ) that lies totally submerged in the sea and a nozzle ( 54 ) that receives the produced water and that has a nozzle end ( 76 ) lying in a middle portion of the mixer tube. A location of the mixer tube middle portion at the nozzle end has an inside diameter (A) much larger than the nozzle end outside diameter (B) to induce the through flow of sea water from the surrounding sea through the mixer tube. The produced water is pumped to a high enough pressure to create turbulence in the mixer tube immediately downstream of the nozzle end to better mix the produced and sea waters.

Description:
CROSS-REFERENCE 
   Applicant claims priority from U.S. provisional application No. 60/517,295 filed Nov. 03, 2003 and U.S. provisional application No. 60/493,056 filed Aug. 05, 2003. 

   BACKGROUND OF THE INVENTION 
   Large quantities of water are produced during the processing of hydrocarbons in offshore facilities. One example is in the production (removal) of hydrocarbons from subsea reservoirs by flowing the hydrocarbons up to a structure at the sea surface such as a floating vessel, a spar or floating tension leg platform (TPL), or a platform. Processing equipment on the sea surface structure separates the hydrocarbons from other material, which commonly consists primarily of water, and may include sand, etc. The large quantities of such produced water must be disposed of, either by injection into the reservoir (which is undesirable and costly) or by discharge into the environment. The produced water may be at an elevated temperature that is viewed by many as potentially detrimental to normal marine flora and fauna. Local regulations commonly require that large quantities of water such as the quantities commonly produced from undersea reservoirs, be cooled to a certain temperature before release into the sea. 
   In one example, water accompanying hydrocarbons from an undersea reservoir is at a temperature such as 90° C. (194° F.) and local regulations require that the temperature of discharged water be no greater than 40° C. (104° F.). Since the temperature of the sea is below that of hot water from the reservoir and the facility has ready access to sea water, it is logical to use sea water to cool the water from the reservoir. However, because of the large quantities of water that are produced (e.g. 1000 gallons per minute), the cost of conventional temperature-reduction heat equipment comprising sea water lift pumps, filters, heat-exchangers, etc. can be considerable. A cooling system with a minimal number of parts, which effectively cooled large quantities of produced water, would be of value. 
   There is a need for systems in the regassification of transported LNG (liquified natural gas), to heat cold water prior to its discharge into the sea. Gaseous hydrocarbons are commonly transported as LNG at −160° C. (−320° F.) if it contains methane, as LPG (propane and butane) at −50° C., or as hydrates (gas trapped in ice crystals) at −40° C., all at atmospheric pressure. Such gaseous hydrocarbons are offloaded, as directly into a gas pipeline whose outer end is located on a fixed or floating structure, so the gas can flow to shore and/or to an underground (under sea or shore) storage cavern for later use. The liquified gas is heated, as to 5° C. to avoid very cold pipes on which moisture condenses and to avoid cracking of walls of a salt dome cavern in which gas is stored. In this application it also is logical to use sea water to warm the very cold liquid to regas it. Local regulations may require that the temperature of large quantities of discharged water be at least 10° C. (50° F.). 
   In both the heating and cooling of produced water, local regulations require avoidance of “hot spots” or “cold spots” where marine life may be subjected to extreme temperatures. For examples, sea animals may be attracted to warm discharged water, and they must be protected from being burned as a result of a close approach to the location(s) where warm water is discharged into the sea. A system that changed the temperature of large quantities of discharged water to be closer to the temperature of the ambient or surrounding sea while avoiding “hot” or “cold” spots, and which used a low cost and effective system to accomplish this, would be of value. 
   SUMMARY OF THE INVENTION 
   In accordance with one embodiment of the present invention, a compact, low cost and efficient apparatus and method are provided for use in an offshore hydrocarbon processing facility that is located in a surrounding sea, that brings the temperature of produced water closer to the temperature of the surrounding sea while avoiding “hot” or “cold” spots. The apparatus includes a mixer tube that has input and output ends and a middle portion, and that is immersed in the sea. Produced water that is much hotter or colder than the sea, is flowed through a conduit down to a nozzle that has a nozzle end lying in the middle portion of the mixer tube and pointed toward the output, or downstream end, of the mixer tube. The downstream flow of produced water out of the nozzle induces the flow of sea water into the input end, or upstream end, of the mixer tube. The sea water that is induced to flow through the mixer tube, mixes with the produced water, and water that exits through the downstream end of the mixer tube is at a temperature much closer to that of the sea than the original produced water. 
   The nozzle end has a diameter that is no more than one half the diameter A of the middle portion of the mixer tube at the location of the nozzle end. This leaves a large area around the nozzle through which sea water can flow, to mix with the produced water. The mixer tube has a length of more than twice the mixer tube inside diameter A at the nozzle end, to provide time for the produced and sea water to mix. Input and output portions of the mixer tube are tapered in diameter, with the mixer tube ends having at least twice as great a diameter as the diameter A at the nozzle end, to induce the large flow of sea water through the mixer tube. The produced water is pressurized to flow sufficiently rapidly through the nozzle end to create turbulent flow through the mixer tube downstream portion, to better mix the produced and sea water. 
   The novel features of the invention are set forth with particularity in the appended claims. The invention will be best understood from the following description when read in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an isometric view of a facility of one embodiment of the present invention that produces hydrocarbons and large amounts of hot water from an undersea reservoir, and that efficiently cools the hot water before releasing it into the surrounding sea. 
       FIG. 2  is a sectional view of mixer apparatus of the facility of  FIG. 1  for cooling the produced water. 
       FIG. 3  is a sectional view of the sea surface structure of the facility of  FIG. 1 . 
       FIG. 4  is a sectional view of a structure similar to that of  FIG. 3 , but modified to enable the mixer tube to be lifted. 
       FIG. 5  is a sectional view of a facility that uses sea water to heat LNG (liquified natural gas) offloaded from a tanker, and that warms the sea water produced by the warming of LNG before discharging the produced water into the sea. 
       FIG. 6  is a sectional view of a portion of a mixer apparatus of another embodiment of the invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  illustrates a hydrocarbon production system  10  which includes a structure  12  in the form of a vessel that floats at the sea surface  16  and that supports a turret  20  that is anchored to the sea floor  22  by catenary chains  24 . Risers  30  (only one is shown) extend from a pipe  32  that connects to a subsea reservoir  34 , and carry fluid from the reservoir to a fluid swivel  36  at the top of the turret. The riser carries large quantities of water in addition to large quantities of hydrocarbons, and both may be at an elevated temperature. The fluid swivel connects to processing equipment  40  on the vessel hull  42  that separates the hydrocarbons from the hot water, any sand, etc. The hydrocarbons may be temporarily stored in the vessel hull and later offloaded to a tanker at intervals. Large quantities of hot produced water must be released from the processing equipment  40  and disposed of. Local regulations commonly require that any water discharged into the sea must not be so hot as to endanger flora and fauna in the sea. 
   In one example, hot water from the undersea reservoir is at a temperature such as 90° C. (194° F.) and local regulations require that the temperature of discharged water be no greater than 40° C. (104° F.). The regulations require that there be no “hot spots” of over 40° C. that might burn sea animals that closely approach the warm water. The surrounding sea may have a temperature such as 15° C. (59° F.) and it is logical to use the surrounding sea water to cool the hot water to the required release temperature or below it. Because of the large amount of hot produced water that must be released, it is important to use equipment of low cost and easy maintenance to cool the hot water. 
   In accordance with the invention, applicant cools the hot produces water by the use of apparatus  50  that comprises a mixer tube  52  that is submerged in the sea and a nozzle  54  that lies at least partially in the mixer tube. A conduit  56  carries the hot produced water from the processing equipment  40 , though a pump  60  to the nozzle  54 . The top of conduit  56  is a plurality of meters above the sea surface, so produced water pressure increases as the produced water moves down toward the nozzle. As shown in  FIG. 2 , the mixer tube  52  has an upstream or input end  70 , a downstream or output end  72 , and a middle portion  74 . Both ends are open to the sea, except for a screen at each end. The nozzle  54  has a nozzle output end  76  that lies within the middle portion of the mixer tube. The nozzle end is directed towards the downstream end of the mixer tube. The nozzle has a reduced diameter at its end  76  which creates a high velocity stream of produced water. The mixer pipe has tapered end portions  80 ,  82  that are of progressively increasing diameters near the ends, leaving a constriction at the middle portion  74 . 
   When the hot produced water is passed at a high pressure through the nozzle, high velocity produced water emerges at the nozzle end  76 . The high velocity stream of produced water from the nozzle induces a large flow of sea water past the nozzle, resulting in a large flow of sea water into the mixer tube input end and out of the mixer tube output end. The sea water mixes with the hot produced water, resulting in the water emerging from the mixer tube output end having a temperature only moderately above the temperature of the surrounding sea. 
   It is important to avoid “hot spots”, where water emerging from the mixer tube output end  72  might have a temperature much hotter than the average temperature of the water emerging from the mixer tube. Such “hot spots” are a result of incomplete mixing of the hot produced water with the cooler sea water. Applicant creates thorough mixing of the produced water and sea water by creating a turbulent flow of water along the downstream end portion  82  of the mixer tube. Such turbulent flow can be induced by several factors, including a sharp-edged obstacle downstream of the nozzle end, a rough mixer tube inside surface, etc. A major factor in creating turbulence is the difference in velocities between produced water exiting the nozzle end and sea water induced to flow downstream through the mixer tube. Applicant pumps the produced water to a high pressure before it passes through the nozzle to create a large velocity difference between produced and sea water to create such turbulence and consequent mixing. This usually requires that the velocity of produced water from the nozzle be at least 3 meters per second (10 feet per second). 
   The inside diameter A of the mixer tube at the nozzle end should be at least twice as large as the diameter B of the outside of the nozzle, so the area of the space  90  between them [π(A 2 –B 2 )] is not so small that it creates a major constriction that greatly limits the flow rate of sea water. That is, the area of the space  90  between them should be a plurality of times the area of the nozzle end. However, the space  90  should not be too large (e.g., A should not be more than about 10 times B) or else produced water emitted from the nozzle will not induce a large sea water flow through the mixer tube. The input and output end portions of the mixer tube are tapered so the middle of the mixer tube is of a small diameter while the tube end portions are large enough to enable sea water flow with minimum resistance. The length C of the mixer tube downstream from the nozzle end should be at least twice and preferably at least three times the diameter A at the nozzle end to provide time and distance for the flowing produced and sea waters to mix. The input end portion  80  is similarly long and tapered to facilitate the flow of sea water to the tube middle portion. The mixer tube output end diameter D is at least twice the diameter A. Applicant prefers that the mixer tube lie under the bottom  92  of the vessel hull, and preferably at the rear of the vessel, so the warmed water emerging from the mixer tube does not tend to warm the vessel. 
   A variety of mixer tube-nozzle apparatuses can be designed, such as ones with more than one nozzle in a mixer tube.  FIG. 6  illustrates a modified apparatus  50 A which includes a plurality of nozzles  54 A that lie around the periphery of the inside of the mixer tube  52 A. An obstruction  94  with holes  96  lies downstream of the nozzles and there is a rough inside surface area  98  to help mix the produced and sea waters. 
   In one system that applicant has designed, of the type shown in  FIG. 2 , the mixer tube  52  has a length of one meter and has opposite ends  70 ,  72  that are each of 10 inches (25 cm) diameter. The middle has an inside diameter A of 4.5 inches (11.5 cm). The nozzle end  76  has an outside diameter of 1.2 inch (3 cm).  FIG. 2  shows, in phantom lines, a submerged pump at  100  that can be connected to the input end  70  of the mixer tube to increase the inflow of sea water. In many facilities a larger mixer apparatus  50  is used to enable the discharge of larger flow rates of produced water. 
   The vessel of  FIG. 1  may move in shallow water prior to attachment of the mooring chains and sometimes afterwards.  FIG. 4  shows a system  110  in which the conduit  112  that extends from the pump  60  to the mixer tube, extends outside a side of the vessel hull, and has a pivot joint  114 . The pivot joint allows the mixer assembly  116  and much of the length of the conduit to be lifted in shallow water. 
     FIG. 5  illustrates a tanker  120  that carries LNG (liquified natural gas)  122  at a temperature such as −160° C. The LNG is offloaded through a cryogenic pipe or hose  124  to an offshore processing station  126 , with a fixed platform being shown although a dedicated moored vessel could be used. The processing station includes a regas unit  130  that heats the LNG. The LNG is heated to turn it into a gas, and to a high enough temperature that when it is pumped through pipes  132 ,  134 , to a shore station  136  and/or to a storage cavern  138 , a lot of moisture will not condense on the pipes and the cavern will not crack. 
   The regas unit  130  uses sea water to heat the LNG, usually with an intermediate fluid for initial heating at low temperatures. The regas unit has a sea water inlet pipe  140  that takes in seawater and an outlet conduit  142  that disposes of the cooled seawater. In one example, the ambient sea is at 15° C. (59° F.) and the water flowing through the outlet conduit  142  is at 1° C. Also, local regulations require that discharged water be at at least 10° C. (50° F.). Thus, the produced water has to be heated only several degrees centigrade. 
   The outlet conduit  142  leads to a mixer assembly  150  of the same construction as shown in  FIG. 2 , although the dimensions can be varied because the temperature of the cold (1° C.) water in the outlet conduit does not have to be changed as much (e.g., by only 9° C. instead of 40° C.). 
   It should be noted that there are other applications where large amounts of water must be changed in temperature before being discharged into the sea. One of them is in the cooling of natural gas to produce LNG for transport in a tanker. 
   Thus, the invention provides an apparatus and method for use in an offshore hydrocarbon processing facility that produces large quantities of produced water, and which uses sea water to alter the temperature of the produced water before it is discharged into the open sea, in a low cost, compact and efficient manner. The apparatus includes a mixer tube that is immersed in the sea and that has upstream and downstream ends open to the sea and a middle portion. The apparatus also includes a nozzle that discharges the produced water within the middle portion of the mixer tube. The nozzle discharges the produced water at at least a moderate velocity to induce the flow of larger quantities of seawater through the mixer tube to mix with the produced water before exiting the downstream end of the mixer tube. The produced water is pressurized prior to exiting the nozzle to create rapid flow such as above 10 feet per second (3 meters per second) to create turbulent flow downstream of the nozzle so as to better mix the produced water with the sea water. 
   Although particular embodiments of the invention have been described and illustrated herein, it is recognized that modifications and variations may readily occur to those skilled in the art, and consequently, it is intended that the claims be interpreted to cover such modifications and equivalents.