Abstract:
The present invention regards a subsea cooling unit having an inlet for a hot fluid and an outlet for cooled fluid, the cooling unit comprising a number of coils exposed to seawater, and means for generating a flow of seawater past the coils, where the means for generating the flow of seawater comprises a propeller and a rotatable actuator and that the cooler is enclosed in a duct.

Description:
FIELD OF THE INVENTION 
     The following invention relates to a subsea cooler for cooling a hot fluid as a fluid stream produced from one or more subsea wells, flowing through a pipe by using the surrounding seawater as the coolant medium. The invention also relates to a cooling unit comprising at least one coil and means for providing a flow of cooling fluid past the coils. The invention also relates to a method for cooling a hot fluid as a fluid stream produces from one or more subsea wells. 
     BACKGROUND OF THE INVENTION 
     The fluid produced from a hydrocarbon well is at times very hot, sometimes over one hundred degrees centigrade. If the wells are a long distance away from a processing facility it may be necessary to boost the flow by introducing a pump in the flowline. A pump will work better if the fluid is cooled. This is especially important when the fluid is a gas and a compressor is employed. The efficiency of a compressor is very dependent upon the temperature of the gas, i.e. the cooler the gas the more efficient the compressor will be. 
     A well known cooling device is the radiator where a flow of cool air is forced against a piping arrangement that presents a large surface area to the air. 
     SUMMARY AND OBJECTS OF THE INVENTION 
     The present invention regards a cooling unit, a subsea cooling unit and a method for subsea cooling of a fluid as defined in the attached claims. 
     According to the invention there is in one aspect provided a subsea cooling unit having an inlet for a hot fluid stream and an outlet for cooled fluid. The fluid stream will normally be a fluid stream produced from one or more subsea wells. The cooling unit comprising a number of coils exposed to seawater for cooling of the hot fluid, and means for generating a flow of seawater past the coils. According to the invention the means for generating the flow of seawater comprises a propeller and a rotatable actuator. The propeller is arranged such that when the propeller is operated it creates the desired flow of seawater past the coils positioned in the seawater. According to the invention the cooling unit is also enclosed in a duct, or at least the coils of the cooling unit is positioned in the duct. Such a configuration will assist in guiding a flow of seawater past the coils. 
     According to one aspect of the invention the duct may have an inlet with reduced diameter. The inlet may have a reduced inlet compared with the rest of the duct. The propeller may be located in the inlet or in connection with the inlet. The reduced diameter may be formed as a funnel. The smaller end of the funnel may be facing away from the coils in the cooler or possibly be arranged in an opposite manner. The propeller may be arranged by the smallest diameter of the inlet. 
     According to another aspect the cooling unit may comprise a controller. The controller may be connected to the different parts of the cooling unit to regulate the different parts in relation to each other to achieve the desired cooling of the fluid. 
     According to a further aspect the actuator may be an electric motor. In another aspect there may be a power cable extending from a remote location. In another embodiment the power may be a battery pack attached to the cooling unit or the power may be supplied in another manner. The battery pack may be replaceable or attachable or attached to means to periodically or continuously charge the battery pack. 
     According to second aspect of the invention there is provided a cooling unit having an inlet for a hot fluid and an outlet for the cooled fluid. This fluid may be a fluid produced from one or more wells, it may be a lubricant for lubrication of a subsea motor, it may be a gas stream or it may be another fluid needing cooling. The cooling unit may be positioned subsea. According to the invention the cooling unit comprises a number of coils exposed to a cooling fluid for cooling of the hot fluid, and means for generating a flow of cooling fluid past the coils, where the means for generating the flow of cooling fluid comprises a propeller and a rotatable actuator and the cooling unit is enclosed in a duct. With enclosed in a duct, at least the coils of the cooling unit is enclosed in a duct. The power for operation of the actuator is generated from the fluid stream. The cooling fluid may be seawater or it may be a fluid arranged in a closed loop. The fluid in the closed loop may according to one aspect be connected to a cooling unit according to the invention and thereby exposed to the temperature of surrounding seawater if it is a subsea cooling unit, or the closed loop it self may be exposed to the seawater as such, or cooled in a different manner. 
     According to an aspect of this embodiment of the invention a propeller may be located in the hot fluid. This propeller will thereby be positioned within a pipe for the hot fluid. This propeller in the hot fluid may be operatively connected to power generating means located outside of the pipe for the hot fluid. According to one aspect the propeller may be operatively connected with a second propeller located in the cooling fluid stream. In one embodiment the first and second propellers, hence in the cooling fluid and hot fluid, may be mechanically connected, in another embodiment they may be connected by energy lines, with a generator arranged on one propeller an a motor arranged on the other propeller. In another embodiment there first and second propeller may be arranged with a common rotational axis, as ring propellers. The second propeller will thereby act as the rotatable actuator. 
     The present invention also relates to a method for subsea cooling of at least a part of a fluid stream produced from one or more subsea wells, where at least a part of the fluid is guided into an inlet and through a number of coils arranged in a duct, and then through an outlet, where the coils are exposed to seawater for heat exchanging with the fluid, where the seawater is driven past the coils arranged in the duct by a propeller. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be described with reference to the accompanying drawing where 
         FIG. 1  is a drawing showing the principle of the invention 
         FIG. 2  is a detail showing an alternative power generating device 
         FIG. 3  is a drawing showing an embodiment of the invention, 
         FIG. 4  is a detail drawing of  FIG. 3 , 
         FIG. 5  is a drawing showing a second embodiment of the invention, 
         FIG. 6  is a detail drawing of  FIG. 5 , 
         FIG. 7  is a drawing showing a third embodiment of the invention, 
         FIG. 8  is a detail drawing of  FIG. 7 , 
         FIG. 9  is a schematic of a subsea separation system, and 
         FIG. 10  is a drawing of an alternative embodiment of that shown on  FIGS. 4 and 8 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In  FIG. 1  there is shown a cooling unit, or called a cooler, in the form of a piping arrangement  10  which may consist of one or more pipes that may be arranged as a number of individual coils to achieve the greatest possible surface area. The piping arrangement is connected to an inlet pipe  18  and an outlet pipe  20 . When the cooler is made up in more than one coil, the inlet pipe is connected to a distribution unit  22  that distributes the flow from the inlet pipe into an individual coil of the cooler. Likewise, as the fluid leaves the coils each flow is gathered in a unit  24  at the outlet pipe  20 . The piping arrangement of the cooler is not shown in detail since such coil systems are well known to those skilled in the art and such persons will be able to determine the number and size of pipes necessary for maximum efficiency, i.e. the amount of cooling desired. In a subsea system the inlet pipe  18  will be connected to a flowline  19  that transports a hot hydrocarbon fluid from one or more subsea wells  1  and into the cooler. The purpose of the cooler is to cool the hot fluid by utilizing the cold seawater surrounding the cooler as the cooling medium. Seawater at depth is quite cold, close to zero centigrade. 
     The free flow of seawater may be too slow to enable efficient cooling of the hot fluid. The invention therefore proposes to include means to increase the flow of the seawater past the coils  10 . To this end a propeller  26  is located in front of the cooler. The propeller is rotated by a rotating actuator or motor  30  via a shaft  28 . The motor is supplied with power (electric or hydraulic) through a line  32 . A controller  34  receives signals and power through umbilical  36  that in turn extends to a remote control station. The remote control station may be located on a floating production unit or a land station. When the propeller is rotated it will force a stream of seawater past the coils of the cooler  10 . The propeller may as an alternative be arranged downstream of the coils, and thereby draw seawater past the coils. 
     To further enhance the cooling effect the cooler is enclosed by an open-ended duct  12 . The duct is at one side connected to a funnel  13 . The funnel has at its other side an inlet  11  with an opening diameter that is substantially of the same size as the propeller  26 , as shown in  FIG. 1 . The cooling medium, i.e. sea water, is by the propeller  26  forced to flow through the cooler as shown by arrows  14  and  15 , respectively. In another embodiment of the invention the duct may form part of a closed system for the cooling fluid. The cooling fluid may thereby be another fluid than seawater. 
     In the piping inlet  18  there is arranged a valve  37  which is controlled by the controller  34 . Also in the inlet  18  and the outlet  20  there are pressure and temperature transmitters  38 ,  39  respectively, also connected to the controller  34 . 
     The positions of the piping inlet and outlet may be reversed such that the inlet is closest to the propeller. 
     In the controller  34  there may be arranged an electrical storage device such as a battery (not shown) to enable the motor  30  to be powered even in the event that the power supply from the control station fails. 
     The temperature transmitters  38  and  39  measure the temperatures and pressures of the fluid at the piping inlet  18  and outlet  20 . This enables the control of the temperature of the fluid at the outlet and to regulate the temperature to achieve a desired level and to maintain a constant outlet temperature. Also by measuring the pressure at the outlet and inlet it is possible to gain information about the flow of fluid and to calculate the amount of flow. 
     In the event that the fluid is a gas the subsea system will generally include a gas compressor to boost the gas flow. In this case it is important that the gas compressor is fed the gas at a uniform temperature as this increases the efficiency of the compressor. With the temperature data the controller  34  may regulate the speed of the motor  30  so that the desired temperature in the gas fed to the compressor is uniform at all times. 
     In an embodiment of the invention the power to drive the propeller  26  is derived from the energy in the fluid stream. This is shown in  FIG. 3  and  FIG. 4 . The outlet pipe  20  for the hot fluid has a bend  62 . In the straight part of the bend there is arranged a propeller  64 . The propeller  64  is attached to a shaft  66  that extends through the wall of the pipe bend and is at its other end connected to the rotor (not shown) of a generator  68 . An electric cable  76  connects the generator  68  with the controller  34  and hence the motor  30 . When the gas flows through the pipe, as shown by arrows  65 , it will cause the propeller  64  to rotate which in turn generates electrical power in generator  68 . The power is passed through cable  76  to controller  34  which in turn feeds power as necessary to the electric motor  30 . When motor  30  is powered it will cause the propeller  26  to rotate, thus increasing the flow of coolant medium past the cooler unit  10 . 
     Alternatively the propeller may be in the form of a ring propeller that induces a current in coils located around the outer periphery of the pipe  20 . This is shown in  FIG. 2 . A propeller  54  includes an outer ring  56  which is supported by bearings (not shown) so that it will rotate when fluid flows past the propeller. In the ring there is a number of magnets  57 . Around the outer periphery of the pipe  20  there is another ring  58  with magnetic coils  59 . The outer magnetic ring generates electrical current when the propeller ring rotates, as is well known in the art. The current is passed through cable  76  to the controller  34  which in turn controls the feed of power to the electric motor  30 . 
     Preferably the controller  34  includes one or more electrical storage devices such as batteries (not shown) to act as a buffer between the generator and the motor. This enables the propeller  26  to be rotated as needed and act as a power reserve when the generator is not running, because there is no flow past propeller  64 . the batteries may also be charged by the propeller. 
     In yet another embodiment of the invention the propeller  26  is directly connected to a second propeller located in either the fluid inlet or outlet pipe. In a first alternative of this embodiment shown in  FIGS. 5 and 6  the first propeller  27  is a ring propeller, similar to the one shown in  FIG. 2 . The fluid outlet pipe  40  is in this case is located centrally in the funnel  13 . When a propeller  42  is rotated by the flow of fluid, as indicated by arrow  52 , the propeller  27  will also be forced to rotate, in a similar manner as described with relation to  FIG. 2 . 
     In an alternative of the above embodiment shown in  FIGS. 7 and 8  a propeller  29  is mechanically connected with a second propeller  44 . This is in principle similar to the embodiment shown in  FIG. 3 . The propeller  29  is located in a bend  33  of an outlet pipe  50 . The propeller  26  is fastened to a shaft  28  which extends through the wall of the pipe  50  at the bend  33  and is at its other end connected to the second propeller  44  which is located in the inlet of funnel  13 . 
     When the hot fluid is pumped through the outlet pipe  50 , as shown by arrows  46 , it will cause the propeller  29  to rotate which in turn causes the propeller  44  to rotate. The rotation of propeller  44  will propagate a flow of cold seawater past the cooler  10   
     In an alternative design of the shaft  28  shown in  FIG. 10  the shaft is enclosed in a pipe that is welded or otherwise fixed to the bend. The shaft rotates on bearings inside the pipe. The advantage with this design is that grease can be supplied to the annulus between the shaft and the pipe to protect the bearings and to avoid hydrocarbons leaking out to the environment. The supply of grease is controlled by a valve as shown. This design may also be used in the embodiment shown in  FIG. 4 . 
     The invention is intended for use with a subsea separation system where cooling of the produced hydrocarbons gas is an advantage for increasing the efficiency of a gas compressor. The efficiency of a compressor is related to the temperature of the fluid and it is desirable to lower this temperature as far as possible. 
     In  FIG. 9  there is shown a subsea separation and boosting system where the invention may find particular use. In a gas separation and compression system with rotating machinery there is a need for a safety system that can recirculate the fluid to ensure a minimum volume stream through the compressor at all times. This is especially necessary at start-up or if there are disturbances in the process that creates a lower fluid flow trough the compressor. If this persists there is also a potential for a temperature rise in the fluid that may limit the operations or even create a dangerous situation. To reduce this risk a cooler should be included in the recirculation circuit. 
     A special condition exists when the need for cooling comes suddenly, as in an anti-surge situation. 
     To this end  FIG. 9  shows a subsea process system for hydrocarbons produced by one or more wells. The system comprises a separator  102  being fed from a flowline  104 . 
     The separated gas is conveyed through pipe  106  to a compressor  108  which in turn is connected to an export flowline  110 . Liquids separated from the gas in the separator  102  are conveyed through pipe  112  to a pump  114  and thence to flowline  116 . Flowline  116  may connect to flowline  110  or be a separate flowline to a process facility. A liquid bypass  118  having a valve  119  may form a reverse circuit between flowline  116  and separator  102 . An anti-surge bypass  120  connects the compressor  108  outlet with the flowline  104 . In the bypass  120  there is located an anti-surge valve  122  and a cooler  124 . The cooler may be any of the kinds previously described or according to the attached claims. If so desired a cooler may also be incorporated into liquid bypass  118 . 
     The invention has now been explained with different embodiments. A skilled person will understand that there may be made several alterations and modifications to the embodiments within the scope of the invention as defined in the attached claims.