Abstract:
A subsea multiphase fluid separation system and method are disclosed which provide for efficient and reliable remote separation operation at deep and ultra deep water depths including depths of ten thousand feet or more. The system is preferably of modular construction wherein the modules are secured in a single frame to be lowered as a unit to the seabed. The system utilizes reliable cyclonic operation. The sequence of process steps is designed to make the system more efficient as compared to surface separating systems and thereby permit a more compact size as is desirable for subsea operation. The method of operation includes up to five basic process steps with the initial step in one embodiment including cyclonically separating solids. In a presently preferred embodiment, the cyclonic solids separator is sized to eliminate solids greater than fifty microns. A second stage is directed to cyclonically removing bulk gas from the liquid in either a cyclone or auger separator. A liquid-liquid hydrocyclone for the third stage acts to preseparate the fluid either by separating and/or by coalescing oil droplets in a water continuous stream and/or water droplets in an oil continuous stream. A fourth stage gravity separator is significantly smaller for the flow throughput as compared to surface separating systems due to earlier separation processes and due to the option of subsequent oily-water separation in a deoiling liquid-liquid hydrocyclone.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to a system and method for subsea separation of well fluids and solids and, more particularly, to a compact separating system that may be deployed and operated at deep and ultra-deep water depths for processing well fluids and solids prior to transport to the surface, or disposal either subsea or to the surface. 
     2. Description of the Background 
     In recent years, the oil and gas industry has found significant oil and gas reserves in deepwater offshore locations. Many discoveries have been made in over three thousand feet of water with a number of high potential discoveries being located in over five thousand feet of water. In the not too distant future, discoveries in depths of over ten thousand feet are inevitable. As the production depths increase, the cost of transporting well fluids from the seabed to the surface also increases. Transporting well fluids that contain significant amounts of undesirable solids and water from significant depths limits the flow of the desirable hydrocarbon products to the surface due to significant cost limitations of the internal diameter of the riser pipe from the sea bottom. Not only are there significant inefficiencies associated with transporting multi-phase fluid flow from the seabed but also there are other high costs including production problems such as erosion, corrosion, and hydrate formation. At the present time, technical and economical limitations effectively prevent profitable deepwater production. As the depths reach new levels, there is no current technology that can be used to commercially exploit deep water reserves. On the other hand, the medium to longer term strategic business plans of some oil and gas producers are dependent on the finding of appropriate technology to exploit deep water reserves. 
     The methods currently available to produce and separate the constituent hydrocarbon products from associated solids and produced water utilize production separation equipment that is located topside on fixed or floating facilities. It would be highly desirable if there were means to reduce the cost of these facilities and avoid the other aforementioned costs with a commercially feasible method of separating by-products of oil and gas production at the seabed. Unfortunately, the methods and systems for separating multiphase fluids presently used are not feasible in the remote, hostile, and problematic environment at the seabed in deep waters. Nonetheless, such a highly desirable means would preferably separate some unprofitable by-products, namely water and solids, on the seabed and dispose of them into the reservoir and/or onto the seabed. 
     Numerous speculative subsea separation solutions have been put forward where either one or two such separation processes occur. However, these processes are based on the use of conventional equipment that operates in a manner consistent with topside operations. These solutions fail to address all the issues associated with subsea separation because the adoption of conventional technology is impractical to use as a seabed system. For instance, in one proposed subsea separation method put forward, it is submitted the reliability is highly suspect due to unsolved problems that might be encountered such as erosion, corrosion, hydrate formation, and that furthermore the efficiency of the system is too low such that it would have flow through rates that are unprofitably low. In another proposed subsea system, the size of the system to be placed underwater is so large as to make the system economically unfeasible for use at significant depths. Topside systems cannot simply be lowered onto the seabed because of numerous problems that are encountered thereby. 
     It is submitted that the above discussed proposals for subsea operation do not provide a compact subsea production system that is commercially feasible for producing from significant depths. Consequently, there remains a need for an improved and compact deepwater production separation system that is capable of efficiently separating the constituents of hydrocarbons production, namely oil, gas, water, and solids. It would be desirable to have a system designed to operable in virtually any water depth and having significantly reduced size with respect to production efficiency as compared to existing technology. Thus, such a system would lend itself to be of particular value to production in much greater depths, such as water depths over ten thousand feet deep or more. Those skilled in the art have long sought and will appreciate the present invention which provides solutions to these and other problems. 
     SUMMARY OF THE INVENTION 
     Therefore, it is an object of the present invention to provide an improved subsea separation system and method. 
     Another object of the present invention is to provide a subsea system that is suitable for remote operation in that the components efficiently operate with a wide range of multiphase fluids. 
     Yet another object of the present invention is to provide a system that utilizes inherently reliable cyclonic action to provide the system reliability required for remote operation. 
     Yet another object of the present invention is a system design that results in the reduction of mobilization and demobilisation costs. 
     A presently preferred feature of the invention is a process that separates at an initial stage by-products that otherwise may cause damage to downstream production elements such as pipework, valves, and vessels through corrosion, erosion, and hydrate formation. 
     Another presently preferred feature of the invention is a process that separates bulk gas after removal of solids to enhance solids removal, promote operation of subsequent liquid-liquid hydrocyclone operation, and reduce the size required for the gravity separator. 
     A presently preferred advantage of the present invention is that a much smaller gravity separator can be used than would be practical for typical surface separation operation. 
     Another advantage of the present invention is a presently preferred modular construction of the system components for use in a single housing or frame for reliable transport to the ocean floor. 
     These and other objects, features, and advantages of the present invention will become apparent from the drawings, the descriptions given herein, and the appended claims. However, it will be understood that the above discussed presently preferred and/or available objects, features, and advantages of the method and system of the present invention are not all inclusive, represent only a few of the objects, features, and advantages, and are not intended to limit the invention or claims to the invention in any way but are merely intended as an aid to those of skill in the art having interest in practicing the invention for those purposes. 
     Thus, a subsea method for separation of multiphase production fluids from one or more wells drilled below a body of water is disclosed that comprises steps such as positioning separation equipment below a surface of the body of water and in proximity with the one or more wells. The one or more wells are connected to the separation equipment to introduce the multiphase production fluids to the separation equipment. The separation equipment is operated in a definite sequence. In one embodiment of the invention where solids are or may be present in the fluid, the solids are first separated from the multiphase production fluids to produce the solids and a first stage processed fluid. If gas is present in the first stage processed fluid, the bulk gas and liquids are then separated from the first stage processed fluid to produce a gas output and a second stage processed fluid. When water is or may be present, the second stage fluid is preseparated to produce a third stage processed fluid. Oil and water and any residual gas are separated from the third stage processed fluid in the fourth stage to produce an oil output and a fourth stage processed fluid. Finally, cleaning of the fourth stage processed fluid yields a water output and residual oil. 
     A cyclone separator is utilized for the separating of solids from the multiphase production fluids. A cyclone or auger separator is utilized for the separating of gas and liquids from the first stage process fluid. A first liquid-liquid hydrocyclone is utilized for the preseparating of the second stage processed fluid. A gravity separator vessel may be utilized for the separating of oil and water and any residual gas from the third stage processed fluid, and a second liquid-liquid hydrocyclone is utilized for the cleaning of the fourth stage processed fluid. 
     Prior to the step of positioning the separation equipment under the water, a single framework for rigidly supporting the separation equipment as a unit is provided for which the method further comprises steps such as providing a first modularly constructed component comprising the first liquid-liquid hydrocyclone that is rigidly attachable to the single framework. A second modularly constructed component may comprise the second liquid-liquid hydrocyclone and is provided so as to be rigidly attachable to the single framework. A third modularly constructed component may comprise the gravity separator vessel that is provided so as to be rigidly attachable to the single framework. A fourth modularly constructed component may comprise the cyclone or auger separator. A fifth modularly constructed component may comprise the cyclone separator for initially removing solids from the fluids. 
     In a preferred embodiment, the feed composition to the preseparator liquid-liquid hydrocyclone may be monitored, and the flow split across a plurality of outputs from the preseparator liquid-liquid hydrocyclone is adjusted. The step of separating solids from the multiphase production fluids in one preferred embodiment may be designed to remove solids having a particulate size greater than fifty microns due to the significant problems that those size particles tend to create. 
     More generally, the subsea method for separation of multiphase production fluids from one or more wells drilled below a body of water comprises operating the separation equipment for separating phases of the multiphase production fluids such that at least two of the following five processes are provided: separating solids from the multiphase fluid, separating a gas phase and a liquid phase, liquid-liquid preseparating to enhance separation of an oil phase and a water phase, gravity separating the oil phase and the water phase, and cleaning oily water to produce water and residual oil. Each of the at least two of the five processes, when used, is operated in an order of operation as follows: the step of separating solids from the multiphase fluid, the step of separating the gas phase and the liquid phase, the liquid-liquid preseparating, the step of gravity separating, and the step of cleaning oily water to produce water and residual oil. 
     As discussed above, a cyclone separator is used for the step of separating of solids from the multiphase production fluids. A cyclone or auger is used for the step of separating the gas phase and the liquid phase. A liquid-liquid hydrocyclone is used for the step of liquid-liquid preseparating. A gravity separator is used for the step of gravity separating, and a liquid-liquid deoiling hydrocyclone for the step of cleaning oily water to produce water and residual oil. Preferably a single frame is provided and each of the components is rigidly secured to the single frame to form the separation equipment. 
     Thus, a subsea system is disclosed for separating a multiphase fluid from one or more subsea wells. The subsea system comprises a separator system frame for rigidly supporting the subsea system so as to be suitable for operation in a subsea environment. In general terms, at least two of the following five components are rigidly secured to the separator system frame consisting of the following: a cyclone separator operable for removing solids from the multiphase fluid, a cyclone or auger separator operable for removing a gas phase from a liquid phase, a preseparator liquid-liquid hydrocyclone operable for enhancing separation of an oil phase and a water phase, a gravity separator operable for separating the oil phase and the water phase, and a deoiling liquid-liquid hydrocyclone operable for deoiling oily water. 
     The at least two of the five components are interconnected with each other such that when used each of the at least two of the following five components are arranged in the following sequence of operation for processing the multiphase fluid: the cyclone separator for separating solids and fluid, the cyclone or auger separator for separating bulk gas and fluid, the preseparator liquid-liquid hydrocyclone for preseparating oil and water, the gravity separator for separating oil, water, and any residual gas, and the deoiling liquid-liquid hydrocyclone. 
     In one embodiment used where sand may occur in the production fluid, the system comprises the cyclone separator operable for removing solids from the multiphase fluid and at least two of the five components other than the cyclone separator. 
     In another embodiment where water and oil may be included in the production fluid, the system comprises the gravity separator and the at least two of the five components other than the gravity separator. 
     The claims to the invention appended to this specification are intended to be given a full breath of interpretation consistent with the language used in the claims and suitable equivalents thereof especially in light of the specification and drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic view of components for a separating system that operates in accord with the method of the present invention. 
     While the present invention will be described in connection with presently preferred embodiments, it will be understood that it is not intended to limit the invention to those embodiments. On the contrary, it is intended to cover all alternatives, modifications, and equivalents included within the spirit of the invention and as defined in th e appended claims. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention is a modular, multi-component system that utilizes predominately cyclonic technology. The present invention provides a unique and integrated approach to packaging of separation systems. The present invention is designed to meet the problems of operating in a new, isolated, and hostile subsea environment. The performance of the system is superior to previous attempts to provide subsea separation processing in terms of increased system reliability as well as overall higher separation efficiencies that allows for a smaller system package. The system of the present invention has high operational flexibility for separation processing of oil, water, gas, and solids. The composition and/or flow rate of each component can vary significantly without detrimental effect on system performance. In a subsea environment, this is especially important in situations where fluid property information is poor, misunderstood, or completely lacking. 
     Referring now to FIG. 1, a presently preferred embodiment of subsea separator system  10  in accord with the present invention, is illustrated. For reasons discussed subsequently, the order of operation of the steps of separation processing in system  10  is important. The benefits obtained from processing in the order of operation discussed include a reduction in overall system size. The large gravity separator normally used on the surface is not economically feasible for deep subsea operation. The present invention substantially reduces the size needed for the gravity separator to otherwise obtain the same throughput. Therefore system  10  offers a great improvement over previous attempts of subsea separation which have attempted to use large gravity separators at relatively shallow depths. The smaller size of the system reduces maintenance costs, mobilization costs, and demobilization costs. The lower capital and operating costs are particularly significant for use in deepwater operation. 
     The components of system  10  are modular and interact and fit like small interchangeable building blocks. This gives significant economic flexibility due to ease of system expandability. The separation modules are used only when required and are then preferably incorporated into a unitary design such as a frame or housing indicated at  12 . The single frame or housing  12  affixes the modules together into unitary form so as to be durable and sturdy for lowering onto and operation at the seabed. 
     As discussed in greater detail hereinafter, system  10  provides an effective method of handling solids. The problem of solids removal has been largely ignored with respect to prior attempts to provide a subsea separation system. The solution to solids removal provided by system  10  reduces the frequency of intervention and lowers operating costs. 
     A connection to subsea wells that produce multiphase fluids is indicated at  14  of module  16  for stage  1  operation that is preferably a cyclone separator as indicated by dashed internal cyclone  18 . The majority of produced solids tend to be greater than 50 microns in size. Moreover, it is this size range of solids that is believed by the inventors to contribute most to erosion and corrosion in process equipment. The preferred cyclone separator module  16  for remote subsea separation processing therefore takes advantage of this fact and includes cyclone  18  sized such that solids greater than approximately 50 microns will travel down the length of cyclone  18  to solids output  20  whereas gasses and liquids flow to outlet  22 . Solids leave system  10  as indicated at  21  as discussed subsequently. When more information is known about the well production fluids, other size cyclone separator modules could be used alternatively or in addition that would handle smaller size solids as desired. Thus, cyclone separator module  16  is preferably sized to remove especially those solids of fifty micron diameter and greater that otherwise typically cause blockages of equipment, including but not limited to pipework, valves and vessels. Moreover, the solids separated at this stage tend to be water wet, and as such do not yet tend to form problematic agglomerated masses as may occur, for instance, in gravity separators or other downstream separation equipment, where the solids may be in contact with oil for prolonged periods of time. 
     Another advantage of module  16  is that the removal of solids, especially in this size range, acts to maintain more efficient operation of other downstream separating equipment and especially the smaller size downstream equipment that is more desirable for subsea operation. This is because such solids present in liquid emulsions have a stabilizing effect that inhibits coalescence and separation of oil and water. The removal of the solids prevents the stabilizing effect. 
     Module  16  also takes advantage of the fact that the presence of gas in the multiphase fluids optimizes the performance of cyclone type solid separators by reducing the viscosity and density of the multiphase fluids thereby aiding the removal of solids prior to removing the gas phase from the multiphase fluid. Other proposed attempts at subsea separation processes have been designed to remove gases from the multiphase fluid first. However, those designs make solids removal less efficient than that of system  10 . Solids removed by module  16  include sand and other particulate matter. Unless it is known that little or no solids are present in the production fluids, then it is highly desirable to include solid removal module  16  at the location indicated in system  10  at the beginning of the separation process. Solid removal module  16  will typically strongly influence the mechanical integrity, the process performance, and reliability of system  10 . 
     In situations where it is known that little or no solids will be present in initial and/or subsequent multiphase fluid flow, then the modular construction of system  10  permits easy construction of system  10  without module  16  whereby the next desired stage, as the stages are discussed subsequently, can be the input. It will be noted that if any modules are left out of system  10  design, then the utilized modules are still preferably ordered as shown by the arrows of FIG.  1  and as discussed herein. 
     As indicated by the flow arrows of FIG. 1, processed fluid from solids removal module  16  flows through modular interconnections as indicated at  24  to input  26  of bulk gas-liquid separation module  28  for stage  2  operation in accord with a presently preferred embodiment of the method of the present invention. Placement of module  28  directly after solids removal and before oil and water separation is especially useful for a subsea separation system in accord with the present invention. Placement of module  28  at this position reduces significantly the size and weight of the gravity separator, discussed subsequently, and in turn further reduces the size and weight of overall system  10  in accord with the present invention. This is because the gravity separator then requires less volume to process and separate gas from liquid. Moreover, in the design of system  10 , bulk gas removal must occur for the satisfactory operation of stage  3 , discussed hereinafter, due to the detrimental effects of gas in a pre-separator liquid-liquid hydrocyclone. A reduction of gas in the fluids substantially lowers fluid velocity to thereby reduce emulsion forming characteristics and enhance oil and water separation in the downstream equipment. The reduction in gas in the fluids also results in a substantial reduction of foam formation that further enhances oil and water separation in the downstream equipment of system  10 . The reduction in emulsions and foam/spume formation reduces or eliminates the need for costly chemical injection facilities. As with solids cyclone remover module  16 , cyclone or auger gas removal module  28  may be eliminated from system  10  if little or no gas is known to be in the production fluid either during initial flow or subsequent flow of the one or more wells that may be drilled. Gas separation is preferably accomplished with either a cyclone or auger type gas liquid separator. Tangential forces created by blades  30  and otherwise by a cyclone portion of separator  28  causes liquids to flow towards the outer wall due to the difference in phase density so that the processed liquid leaves by tangential flow path from bulk gas-liquid separation module  28  at liquid outlet  32  for stage  3  processing as discussed subsequently. The majority of gas is removed at this time and exits at gas outlet  34  and leaves system  10  at a gas outlet connection as indicated at  36 . Valve  38  controls gas flow and valve  40  controls liquid flow from module  28 . As the cyclone or auger type separator operates efficiently over a wide range of gas void fractions, the cyclonic type operation of either the cyclone or auger type bulk gas separator is ideal for subsea remote operation where the gas void fraction is unknown or may vary as the field is produced. 
     The reduction in the gas permits use of a simplified oil/water composition monitor for the control of downstream oil and water separation equipment so as to eliminate the need for expensive and often unreliable multiphase flow meters. Thus, oil/water composition may be monitored, such as at point  42 , for oil/water composition. 
     Stage  3  preferably includes the oil/water conditioning or separation stage and is referred to here as a preseparating stage that is intended to include both the process of coalescence of oil and/or water droplets fluid conditioning and/or initial oil/water separation. Module  44  is preferably a preseparator liquid-liquid hydrocyclone with input at  46 . 
     Separated bulk oil phase output is at  50  and separated bulk water phase output is at  48 . Preconditioned fluid output is the combination of  48  and  50 . Valves  52 ,  54 , and  56  are used to control flow split across preseparator liquid-liquid hydrocyclone module  44  while monitoring feed composition such as at  42 . Preseparator module  44  ensures that the gravity separator handles only simple to separate water continuous or oil continuous streams and shifts the mixture composition away from the phase inversion point. The result is that system  10  of the present invention requires a greatly reduced size and weight gravity separator. Module  44  may be used to separate the bulk water from the liquid stream. Moreover, by conditioning the fluid stream by coalescence of oil and water at stage  3 , the size and weight of the gravity separator is reduced. A significant growth through coalescence in water and/or oil drop size ensures the gravity separator will perform more effectively and efficiently and require less residence time to process and separate the oil from the water. Where it is known that water cut is not problematic, this stage may be left out of system  10  due to the modular construction of the stages. 
     Stage  4  is a conventional gravity separator that is greatly reduced in size with respect to flow rate due to the highly efficient operation of system  10 . Thus stage  4  preferably incorporates a gravity separator for oil/water/residual gas separation vessel as module  58  of the present invention. To the extent preseparator module  44  is used to produce bulk water, water flows from output  48 , through control valve  52 , and to bulk water input  57  of gravity separator module  58 . Conditioned oil/water flow with enlarged droplet size flows from the combination of outputs  50  and  48  of preseparator module  44  through valve  52  and  54  to input  57 . Flow split across the two outputs of preseparator  44  is additionally controlled by valves  52  and  56 . Valve  54  is used to alternate between the separation mode of operation or conditioning or combination mode of operation. The appropriate mode of operation will depend on the composition of the stream at  46 . Input  60  is where the separated oil from module  44  is directed to the gravity separator module  58  in the separation mode of operation, entering either the oil zone or bucket location. 
     Stage  4  is necessary to supply a final separation stage for oil, gas, and water in which a residence time in the system is provided for slug ingestion. Stage  4  also provides a buffer tank from which produced water re-injection pumps, for example, can operate. The upstream separation stages discussed hereinbefore act to reduce the size, weight, and cost of the gravity separator module which is typically rather large in surface and in proposed subsea separation systems. The upstream deployment stages also optimize operation efficiency and reliability of gravity separator module  58  as is especially important for subsea processing. Residual gas from module  58  leaves output  62  through valve  64  to connect to output gas line  36 . Separated oil flows out at outlet  66  through valve  68  for connection to oil outlet line  70 . Water and oily water that need further processing leaves gravity separator module  58  at outlet  72  for input to deoiling liquid-liquid hydrocyclone module  74 . As with other modules, if there is little or no problem with water cut, then gravity separator module can be easily eliminated from system  10  due to a preferably modular construction of the stages of operation. However, as noted earlier, if a module is removed then the remaining modules are maintained in the same order previously described. 
     Stage  5  provides oily water clean-up so that water separated in module  58  of stage  4  may be cleaned prior to disposal. Preferably, module  74  is a deoiling liquid-liquid hydrocyclone that utilizes cyclonic operation to further separate the residual oil from the water. In situations where water is simply reinjected to the well, water clean-up may not be required subject to the re-injection water quality specification. However, another benefit in deploying module  74  as the final stage allows gravity separator to operate at a higher throughput. This is because module  74  treats the resulting higher oil-in-water concentrations resulting is a lower oil/water interface, and a therefore lower residence time in gravity separator module  58 . Residual oil flows through output  76  and preferably valve  78  to output oil line. Cleaned water exits system  10  at water connection line  80  from outlet  82  and valve  84 . 
     It will be noted that control valves have been listed herein to indicate control elements over flow operation of system  10  and may be removed and/or additional valves added as necessary. The valves also permit standard modules to be used with valves being opened/closed as necessary where a module is not used. 
     In summary of operation, system  10  utilizes up to five distinct process steps. The preferred order of the process steps is important because each step compliments and improves the action of subsequent and/or previous process steps. While the number of process steps varies with the configuration of the system due to the type of multiphase fluid to be processed, the modular construction of system  10  permits configuration of the system as desired. Therefore, in generally described terms, multiphase fluid enters system  10  as indicated in FIG.  1  and is directed into cyclone separator  16  where any solids including produced sand or other particulate matter are separated. The initially processed fluid then enters gas-liquid separator  28 , that may be of either a cyclone or auger type design as indicated in FIG. 1, where the bulk of free gas is separated. The remaining liquid is directed to preseparator liquid-liquid hydrocyclone  44 . Module  44  is operated to remove some produced water from the crude and/or operates to condition the production fluid by growing and coalescing oil in water droplets for a water-continuous stream from outlet  48 , or by growing and coalescing water droplets in an oil-continuous stream from outlet  50 . The resultant fluid is then directed to separation container or vessel  58  where gravitational separation occurs. Oil is separated from water, and any residual gas not removed from the upstream gas-liquid separator is removed and exported as wet gas. This vessel may contain internal devices to enhance this gravitational separation, however the size of vessel  58  will be significantly smaller than that of conventional systems due to the separating/conditioning effects of upstream devices and downstream separation of deoiler liquid-liquid hydrocyclone  74 . The final step includes directing produced water from separator vessel  58  to deoiling liquid-liquid hydrocyclones where oil is removed from the processed water for water reinjection or disposal. 
     The processed products, wet oil and wet gas may be re-combined and transported, or transported individually to the host facility. Wet gas can be re-injected for pressure maintenance or used to lift low pressure wells. The by-products, water, and solids, can be disposed of locally by discharge to the environment or by reinjection, or be transported back to the host facility for further treatment and disposal. 
     System  10  preferably consists of five main elements or modules that interact in such a way as to ensure utilization of standard components and that the components interact and fit like small interchangeable building blocks. Only the components required to meet separation specifications need to be provided in system  10  to the preferred modular construction without having to invest in extra equipment that may not be needed. Therefore, system  10  combines the design flexibility of custom equipment, with the simplicity of pre-engineered modules. 
     The foregoing disclosure and description of the invention is illustrative and explanatory thereof, and it will appreciated by those skilled in the art, that various changes in the organization, general type and shape, and specific materials as well as the details of the illustrated construction or combinations of features of the components and/or process functions may be made without departing from the spirit of the invention.