Patent Publication Number: US-7721807-B2

Title: Method for managing hydrates in subsea production line

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is the National Stage of International Application No. PCT/US2005/28485 filed 11 Aug. 2005, which claims the benefit of U.S. Provisional Patent Application No. 60/609,422 filed on Sep. 13, 2004. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   Embodiments of the present invention generally relate to subsea production systems. Embodiments of the present invention further pertain to methods for managing hydrate formation in subsea equipment such as production lines. 
   2. Description of the Related Art 
   Over the last thirty years, the search for oil and gas offshore has moved into progressively deeper waters. Wells are now commonly drilled at depths of several hundred feet and even several thousand feet below the surface of the ocean. In addition, wells are now being drilled in more remote offshore locations. 
   The drilling and maintenance of deep and remote offshore wells is expensive. In an effort to reduce drilling and maintenance expenses, remote offshore wells are oftentimes drilled in clusters. A grouping of wells in a clustered subsea arrangement is sometimes referred to as a “subsea well-site.” A subsea well-site typically includes producing wells completed for production at one and oftentimes more pay zones. In addition, a well-site will oftentimes include one or more injection wells to aid in maintaining in-situ pressure for water drive and gas expansion drive reservoirs. 
   The grouping of subsea wells facilitates the gathering of production fluids into a local production manifold. Fluids from the clustered wells are delivered to the manifold through flowlines called “jumpers.” From the manifold, production fluids may be delivered together to a gathering and separating facility through a production line, or “riser.” For well-sites that are in deeper waters, the gathering facility is typically a floating production storage and offloading vessel, or “FPSO.” 
   The clustering of wells also allows for multiple control lines and chemical treatment lines to be run from the ocean surface, downward to the clustered wells. These lines are commonly bundled into one or more “umbilicals.” The umbilical terminates at an “umbilical termination assembly,” or “UTA,” at the ocean floor. A control line may carry hydraulic fluid used for controlling items of subsea equipment such as subsea distribution units (“SDU&#39;s”), manifolds and trees. Such control lines allow the actuation of valves, chokes, downhole safety valves and other subsea components from the surface. In addition, the umbilical may transmit chemical inhibitors to the ocean floor and then to equipment of the subsea processing system. The inhibitors are designed and provided in order to ensure that flow from the wells is not affected by the formation of solids in the flow stream such as hydrates, waxes and scale. Electrical lines may also be included in an umbilical for monitoring or control of subsea functions. 
   In cold water production environments, the management of hydrates in subsea equipment is important. Those of ordinary skill in the art will understand that hydrates may form along subsea wellheads and risers, restricting the flow of production fluids to the gathering facility. Hydrates are crystals consisting of water and gas molecules. The water molecules in produced fluid form a lattice structure into which many types of gas molecules may fit. Examples of such gas molecules include H 2 S, CO 2  and CH 4 . Hydrates that form as a result of H 2 S, CO 2  and non-hydrocarbon gases are generically referred to as “gas hydrates.” Hydrates that form as a result of natural gas (such as CH 4 ) in the production fluids may be more specifically referred to as “natural gas hydrates.” Natural gas hydrates may form by water entrapping natural gases and associated liquids in a ratio of 85 mole % water to 15% hydrocarbons. Thus, when production fluids include water and gas molecules, and when such production fluids are at low temperatures and high pressures, the formation of hydrates in subsea equipment may restrict the flow of production fluids to a gathering facility. 
   In a production line, hydrate masses tend to form at the hydrocarbon-water interface. The hydrates may accumulate as fluid flow pushes the hydrate masses downstream. The hydrate mass can grow to a size that creates a “plug” or restriction to fluid flow. The resulting porous hydrate plugs have the unusual ability to transmit some degree of gas pressure, while acting as a liquid flow hindrance. 
   In order to manage hydrate formation, the operator may use jumpers and production lines that are insulated. In addition, the operator may inject chemical “inhibitors” at or near the subsea wellhead, such as into the manifold. Gas hydrates may be thermodynamically suppressed by adding materials such as salts or glycols, which operate as “antifreeze.” Commonly, methanol or methyl ethylene glycol (MEG) may be injected at the subsea tree as the antifreeze material. Inhibitors are oftentimes introduced during well startup. The inhibitor will continue to be injected until the subsea equipment is sufficiently warmed by the produced fluids such that the risk of hydrate formation is abated. Inhibitors may also be introduced prior to a planned shut-in of a wellhead. In that instance, the injected methanol will commingle with the produced fluids before shut-in so that hydrate formation is avoided during the subsequent cooldown. 
   The management of hydrates becomes more difficult when production is shut in unplanned. In this instance, the operator may not have time to inject an inhibitor so as to “inhibit” produced fluids resident in the production line. This may occur, for example, where a gas compressor suddenly goes down. To prevent hydrate formation in the production line in this instance, it is known to provide a second alternate production line. A displacement fluid is injected into the second production line so as to circulate out the uninhibited produced fluids before hydrate formation occurs. Displacement is commonly accomplished by pushing a pig through the line. The pig is launched into the second production line and may be driven by a dehydrated crude out to the production manifold. The pig is then pumped through the production manifold and returned to the gathering facility through the first, or “live,” production line. Displacement is completed before the uninhibited production fluids cool down below the hydrate formation temperature, thereby preventing the creation of a hydrate blockage in the line. 
   For relatively small offshore developments, the cost of a second production line can be prohibitive. Therefore, there is a need for an alternate method of displacing production fluids from a production line in order to manage hydrate formation. 
   SUMMARY OF THE INVENTION 
   A method for managing hydrates in a subsea production system is provided. The system has at least one producing subsea well, a jumper for delivering produced fluids from the subsea well to a manifold, a production line for delivering produced fluids to a production gathering facility, and an umbilical for delivering chemicals to the manifold. The producing well typically has at least some uninhibited produced fluids therein. The method includes the steps of shutting in the flow of produced fluids from the subsea well and through the production line; pumping a displacement fluid into the umbilical through a chemical injection tubing; pumping the displacement fluid through the chemical injection tubing, through the manifold, and into the production line; and pumping the displacement fluid through the production line so as to displace the produced fluids before hydrate formation begins. 
   The chemical injection tubing is preferably tied back to the gathering facility. Preferably, the umbilical defines a first umbilical portion that connects the gathering facility with an umbilical termination assembly, and a second umbilical portion that connects the umbilical termination assembly with the manifold. In one embodiment, the method further includes the step of pumping a chemical inhibitor into the chemical injection tubing before pumping the displacement fluid into the chemical injection line. 
   The gathering facility may be a floating production, storage and offloading vessel (FPSO), it may be a ship-shaped vessel, or it may be a facility located on shore or near shore. 
   In one aspect, the method employs a pig. The pig is placed in the chemical injection tubing ahead of the displacement fluid to aid in the displacing of produced fluids in the production line. In one embodiment, the pig is pumped through the chemical injection tubing, through the manifold, and through the production line using diesel. 
   A method for transporting hydrocarbons from an offshore production facility is also provided herein. In this method, the production facility receives produced hydrocarbons from one or more subsea wells, and from a production line associated with the one or more subsea wells. The subsea well and production line are associated with a subsea production system. The method generally comprises the steps of shutting in the flow of produced fluids from the subsea well and the production line; pumping a displacement fluid from the production facility into a chemical injection tubing, the chemical injection tubing being within an umbilical; further pumping the displacement fluid into the chemical injection tubing so that displacement fluid is urged through a subsea manifold and into the production line; further pumping the displacement fluid through the production line so as to displace the produced fluids before hydrate formation begins; re-initiating the flow of produced fluids from the subsea wells and through the production line to the production facility; and transporting the produced fluids from the offshore production facility. 
   In one aspect, the step of transporting the produced fluids from the offshore production facility comprises offloading the produced fluids from the offshore production facility onto a tanker; and transporting the produced fluids to an onshore terminal. 
   The subsea production system further comprises a jumper for delivering produced fluids from the subsea well to a manifold, and a valve for selectively placing the chemical injection tubing in fluid communication with the manifold. The umbilical further comprises a first umbilical portion that connects the gathering facility with an umbilical termination assembly, and a second umbilical portion that connects the umbilical termination assembly with the manifold. The production line comprises a production riser in fluid communication with the production facility, and a flowline for placing the manifold in fluid communication with the production riser. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     As an aid in understanding certain embodiments of the inventions herein, drawings, tables and charts are provided. Appended drawings include: 
       FIG. 1  is a plan view of a subsea cluster production system, or well site. The illustrative cluster production system includes multiple producing wells, with flowline jumpers delivering produced fluids into a manifold. An umbilical deliver(s) fluids such as hydraulic control fluids or chemical inhibitors to the individual wells through a central distribution unit. 
       FIG. 2  provides a plan view of a more modest subsea cluster production system. A production gathering system is shown at an ocean surface. A single production line connects the manifold of the subsea production system to the gathering facility. 
       FIG. 3  provides a somewhat schematic side view of a production line and an umbilical as part of a subsea production system. The production line and umbilical each tie into a manifold at one end, and to an FPSO at the other end. In this view, production is being obtained through the production line. 
       FIG. 4  shows the production line and umbilical of  FIG. 3 . Production from the production line has been shut-in. A displacing fluid is being pumped through the umbilical and through the manifold, and into the production line. 
       FIG. 5  shows the production line and umbilical of  FIG. 3 . Here, produced fluids in the production line and chemical inhibitor have been substantially displaced from both the umbilical and the production line. 
       FIG. 6  again presents the production line and umbilical of  FIG. 3 . Here, a chemical inhibitor has been pumped into the umbilical for future use in the event of an unplanned production shut-in. 
       FIG. 7  is a chart showing 8-inch flowline water content during displacement as a function of dead crude injection rate. 
       FIG. 8  is a chart providing a demonstration of flow rate during fluid displacement. 
       FIG. 9  is a chart presenting water displacement for a 10-inch line. 
       FIG. 10  is a chart demonstrating water displacement using diesel as the displacement fluid. 
       FIG. 11  is a profile plot of the aqueous phase content in an 8-inch line. 
   

   DETAILED DESCRIPTION 
   Definitions 
   The following words and phrases are specifically defined for purposes of the descriptions and claims herein. To the extent that a term has not been defined, it should be given its broadest definition that persons in the pertinent art have given that term as reflected in printed publications, dictionaries or issued patents. 
   “Gathering facility” means any facility for receiving produced hydrocarbons. The gathering facility may be a ship-shaped vessel located over a subsea well site, an FPSO vessel located over or near a subsea well site, a near-shore separation facility, or an onshore separation facility. 
   The terms “tieback,” “tieback line,” “riser” and “production line” are used interchangeably herein, and are intended to be synonymous. These terms mean any tubular structure for transporting produced hydrocarbons to a gathering facility. “Tied back” means to place a line (such as a production line or umbilical) in fluid communication. 
   “Subsea production system” means an assembly of production equipment placed in a marine body. The marine body may be an ocean environment, or it may be, for example, a fresh water lake. Similarly, “subsea” includes both an ocean body and a deepwater lake. 
   “Subsea equipment” means any item of equipment placed proximate the bottom of a marine body as part of a subsea production system. 
   “Subsea well” means a well that has a tree proximate the marine body bottom, such as an ocean bottom. “Subsea tree,” in turn, means any collection of valves disposed over a wellhead in a water body. 
   “Umbilical termination assembly” means any item of subsea equipment that provides a termination point for one or more umbilical lines. The umbilical termination assembly, or “UTA,” may be placed on an ocean bottom, a mud mat, a manifold, a suction pile, or any other position proximate to the sea floor. 
   “Subsea distribution unit” means any item of subsea equipment that provides at least hydraulic and/or chemical distribution in a subsea production system. “Subsea distribution unit” may be abbreviated as “SDU.” 
   “Manifold” means any item of subsea equipment that gathers produced fluids from one or more subsea trees, and delivers those fluids to a production line, either directly or through a jumper line. 
   “Pig” means any device used to provide a fluid barrier between two different types of fluids within a flow line. The term may include a mechanical fluid displacement device, or it may include another fluid, such as an expandable foam plug or a gel. 
   “Jumper” means any flowline for connecting items of subsea equipment. 
   “Inhibited” means that produced fluids have been mixed with or otherwise been exposed to a chemical inhibitor for inhibiting formation of gas hydrates including natural gas hydrates. “Uninhibited” means that produced fluids have not been mixed with or otherwise been exposed to a chemical inhibitor for inhibiting formation of gas hydrates. 
   DESCRIPTION OF SPECIFIC EMBODIMENTS 
   The following provides a description of certain specific embodiments of the present invention: 
     FIG. 1  presents a plan view of a subsea cluster production system, or well site  10 . The illustrative subsea well-site  10  includes four wells  12 ,  14 ,  16 ,  18 . The illustrated wells  12 ,  14 ,  16 ,  18  represent producing wells. Flow lines, or “tree jumpers,”  22  deliver produced fluids from the individual wells  12 ,  14 ,  16 ,  18  to a manifold  20 . The manifold  20  collects the produced fluids from the individual wells  12 ,  14 ,  16 ,  18 . In one arrangement, production collected from jumpers  22  may be commingled, and then delivered to a first production sled  34 ′. Production is delivered to the sled  34 ′ via jumper  24 . From the sled  34 ′, produced fluids are transported up to a gathering facility (not shown in  FIG. 1 ) through a production line  38 . 
   In the arrangement shown in  FIG. 1 , production is commingled and delivered from the manifold  20  to the first production sled  34 ′. A second production sled  34 ″ is provided that is also connected to the manifold  20  by a jumper  24 . The second production sled  34 ″ also has a production line  38  connected to a gathering facility. The second sled  34 ″ is used to produce fluid from the wells, and is used when production in the subsea wells  12 ,  14 ,  16 ,  18  is shut-in unexpectedly. The second sled  34 ″ then receives a displacing fluid which is circulated through the manifold  20  and into the primary sled  34 ′ and the connected production line  38 . 
   It is desirable for the operator of the subsea production system  10  to be able to remotely control valves at the manifold  20 . It is also desirable that the operator be able to monitor subsea conditions such as fluid temperature within the manifold  20 . Those of ordinary skill in the art will understand that manifold and sled designs vary in sophistication and complexity, and may include complex control and distribution systems, sometimes known as “control pods” or subsea control modules (SCM). Control pods are modules that contain electro-hydraulic controls, logic software, and communication signal devices. A master computer in a host platform control room (not shown) communicates with the subsea control pods to operate the valves and other functions on the manifold to increase or reduce flow rates, or to shut in the flow entirely, if needed. 
   It is desirable that the operator also be able to inject chemicals into the manifold and the individual wellheads to maintain flow assurance. As noted above, water present in the produced fluids can form natural gas hydrates. In addition, at low temperatures the waxy paraffins in some crude oils deposit on pipeline walls, constricting flows. To overcome these conditions, the operator may inject paraffin inhibitors to keep paraffins and waxes from solidifying or depositing in the flow streams. In addition, the operator may inject methanol or glycol to serve as a form of “antifreeze,” preventing hydrates from forming. Further, the operator may inject scale inhibitors and corrosion inhibitors through flowline jumpers and subsea equipment. 
     FIG. 1  shows line  42 ′ delivered from the host platform or other source to an umbilical termination assembly (“UTA”)  40 ′. Line  42 ′ represents an integrated electrical/hydraulic umbilical. Line  42 ′ provides conductive wires for providing power to subsea equipment, and also provides hydraulic fluid needed to power subsea functions. Finally, line  42 ′ provides chemicals to be distributed through the system  10 . The various sublines within line  42 ′ are typically bundled together, such as in a thermoplastic sheath. Line  42 ′ terminates at the umbilical termination assembly  40 ′. From the umbilical termination assembly  40 ′, umbilical line  44 ′ is provided, and connects to a subsea distribution unit (“SDU”)  50 . Line  44 ′ may be a flying lead line for delivery of fluids and signals from line  42 ′. From the SDU  50 , flying leads  52 ,  54 ,  56 ,  58  connect to the individual wells  12 ,  14 ,  16 ,  18 , respectively. In addition, flying lead  55  may be installed to connect to the manifold  20  so as to deliver chemicals and to provide power or control to the manifold  20 , as desired by the operator. 
   The subsea cluster  10  of  FIG. 1  represents a relatively complex and expensive production system. For smaller developments or fields, the cost of a second sled  34 ″ and production line can be prohibitive. At the same time, the need remains to displace produced fluids from the primary sled  34 ′ and production line  38  in the event production is shut in so as to prevent the formation of hydrates in the line  38 . Accordingly, methods are provided herein for displacing fluids through a production line  38  without pumping a displacing fluid through a second production line. 
     FIG. 2  provides a plan view of a more modest subsea cluster production system  11  which may be used to produce from a smaller field. In this view, three wells  12 ,  16  and  18  are shown. The wells  12 ,  16 ,  18  have subsea trees on a marine floor  85 . One of the wells, e.g., well  16 , may be a water injection well. Jumper lines  22  are again shown delivering produced fluids from the wells  12 ,  16   18  to a manifold  20 . The second production sled ( 34 ″ from  FIG. 1 ) has been eliminated. Produced fluids are commingled at the manifold  20 , and exported from the well-site through a single production line  38 . 
   It can be seen in  FIG. 2  that the production line  38  ties back to the gathering facility. An FPSO is illustrated at  70  as the gathering facility. However, it is understood that the gathering facility may alternatively be a ship-shaped vessel capable of self-propulsion. The gathering facility  70  is shown positioned in a marine body  80 , such as an ocean. The marine body has a surface  82  and a bottom  85 . 
   A utility umbilical  42  is again used. Line  42  represents an integrated electrical/hydraulic umbilical. Line  42  provides conductive wires for providing power to subsea equipment, and also provides hydraulic fluid needed to power subsea functions. Line  42  also provides chemicals to be distributed through the system  11 . Preferably, the line  42  is tied back to the host platform or gathering facility. The umbilical  42  again connects to an umbilical termination assembly (“UTA”)  40 . From the umbilical termination assembly  40 , line  44  is provided, and connects to a subsea distribution unit (“SDU”)  50 . From the SDU  50 , flying leads  52 ,  56 ,  58  connect to the individual wells  12 ,  16 ,  18 , respectively. 
   In addition to these lines, which are common with the architecture of  FIG. 1 , a separate umbilical line  51  is directed from the UTA  40  directly to the manifold  20 . A chemical injection tubing (not seen in  FIG. 2 ) is placed in both of service umbilical lines  42  and  51 . The chemical injection tubing is sized for the pumping of a fluid inhibitor followed by a displacement fluid. The displacing fluid is pumped through the chemical tubing, through the manifold  20 , and into the production line  38  in order to displace produced fluids from the production line  38  before hydrate formation begins. The chemical inhibitor may be injected through the same chemical injection tubing prior to pumping of the displacement fluid to partially displace and at least partially inhibit the uninhibited produced fluids in the single production line  38 . 
   The displacing fluid may be dehydrated and degassed crude oil. Alternatively, the displacing fluid may be diesel. In either instance, it is preferred that the injection of the displacement fluid into the chemical tubing be preceded by the chemical inhibitor to serve as an inhibitor “pill.” The “pill” may be methanol, glycol, MEG or other inhibitor fluid. Preferably, the inhibitor fluid is retained within the chemical injection tubing during times of production. In this aspect, the inhibitor fluid would be held in reserve pending an unexpected production shut-in. A valve (shown at  37  in  FIG. 3 ) may be placed in-line between the chemical tubing and the manifold  20  to provide selective fluid communication with the production line  38 . 
   It is understood that the architecture of system  11  shown in  FIG. 2  is illustrative, and that other arrangements may be employed for practicing the methods disclosed herein. For example, the gathering facility  70  may be a separation facility on land or near shore. 
   The process of displacing uninhibited production fluids using a tubing in the service umbilical line  42  is illustrated in the following figures: 
     FIG. 3  provides a side view of a production line  38  and a utility umbilical. The umbilical represents both a primary umbilical line  42  and a manifold umbilical line  51 . The umbilicals  42 ,  51  are connected to each other at a UTA  40 . The utility umbilicals  42 ,  51  again represent integrated umbilicals where control lines, conductive power lines, and/or chemical lines are bundled together for delivery of hydraulic fluid, electrical power, chemical inhibitors or other components to other subsea equipment and lines. The bundled umbilical lines  42 ,  51  may be made up of thermoplastic hoses of various sizes and configurations. In one known arrangement, a nylon “Type 11” internal pressure sheath is utilized as the inner layer. A reinforcement layer is provided around the internal pressure sheath. A polyurethane outer sheath may be provided for water proofing. Where additional collapse resistance is needed, a stainless steel internal carcass may be disposed within the internal pressure sheath. An example of such an internal carcass is a spiral wound interlocked  316  stainless steel carcass. Where colder temperatures and higher pressures are encountered, the umbilicals  42 ,  51  may be comprised of a collection of separate steel tubes bundled within a flexible vented plastic tube. The use of steel tubes, however, reduces line flexibility. It is understood that the methods of the present invention are not limited by any particular umbilical arrangements so long as the utility umbilicals  42 ,  51  each include a chemical injection tubing  41 . The chemical tubing  41  is sized to accommodate the pumping of a displacement fluid. In one embodiment, the chemical tubing within the umbilical  51  is a 3-inch line, while the chemical tubing in the umbilical  42  is 3½-inches ID. 
   The production line  38  ties into a manifold  20  at one end, and to an FPSO  70  at the other end. An intermediate sled and jumper line (not shown) may be used. The production line  38  may be, in one aspect, an 8-inch line. Alternatively, the production line  38  may be a 10-inch line. Preferably, the production line  38  is insulated with an outer and, possibly, an inner layer of thermally insulative material. The subsea umbilical  51  is fluidly connected to the manifold  20 , while the utility umbilical  42  preferably ties back to the FPSO  70 . The two umbilicals  42 / 51  are preferably connected via a UTA  40 . A valve  37  is provided at or near the junction between the subsea umbilical  51  and the manifold  20 . The valve  37  allows selective fluid communication between the chemical tubing  41  within the umbilicals  42 / 51  and the manifold  20 . In the view of  FIG. 3 , the valve  37  is closed. 
   In one illustrative embodiment, the umbilical lines  42 ,  51  together are 10.3 km, and the production line  38  is 10.5 km. A 3-inch ID chemical tubing  41  of that length may receive 300 to 375 barrels of fluid. The 8-inch production line holds approximately 1,885 barrels of fluid. Of course, other lengths and diameters for the lines  41 ,  38  may be provided. For example, the chemical tubing  41  may have an inner diameter of 3½-inches, and the production line may have an inner diameter of 10-inches. 
   In  FIG. 3 , production is being obtained through the production line  38 . More specifically, oil, water and gas (“live fluids”) are being produced from a subsurface formation (not shown) through the manifold  20  and through the production line  38 . Again, the line  38  is preferably insulated in such as way that the produced fluids retain their heat and arrive at a separator (not shown) on the gathering facility  70  at a temperature higher than the hydrate formation temperature. The insulation quality of the production line  38  should be such that the uninhibited production fluids in the line  38  remain above the hydrate formation temperature for a period of time defined as the cool down time, which is the time where no action is required by the operator to prevent hydrate formation in an essentially static condition, plus the time it takes to displace the production fluids to the gathering facility  70 . 
   As noted, during normal production the chemical tubing  41  is preferably filled with an inhibitor fluid such as methanol. The displacement fluid is optionally maintained in the chemical tubing  41  for reserve in the event the production line  38  is shut in. In the view of  FIG. 3 , the chemical tubing  41  is filled with methanol. The valve  37  remains closed, with the methanol in reserve. 
     FIG. 4  shows the production line  38  and umbilicals  42 ,  51  of  FIG. 3 . Flow of produced fluids from the wells  12 ,  16 ,  18  and through the production line  38  has now been shut-in. It is thus desirable to displace the produced fluids from the production line  38  before hydrate formation begins to occur. To this end, the inhibitor “pill” is pumped through the umbilicals  42 ,  51 . More specifically, the inhibitor is pumped through the chemical tubing  41 , through the valve  37 , through the manifold  20  and into the production line  38 . No pig is required. In  FIG. 4 , methanol is beginning to invade the production line  38 . 
   It is acknowledged that initial displacement of the produced fluids by pumping of the inhibitor and without a pig is inefficient. This is particularly true where pumping is at a relatively low velocity. Movement of the inhibitor fluid into the production line  38  allows some bypassing of fluids by the methanol. Further, the methanol in the tubing  41  will be at ambient sea temperature, which is below the hydrate formation temperature of the uninhibited production fluids in the production line  38 . The cold methanol will cool the production fluids to temperatures below the uninhibited hydrate formation temperature. Thus, displacement without a pig and with fluids that are below the hydrate formation temperature is counter-intuitive. However, methanol is a thermodynamic inhibiting chemical and will depress hydrate formation temperature in production fluids, thereby preventing hydrate formation. Displacing methanol out of the service tubing  41  and into the production line  38  ahead of a displacement fluid such as dead crude oil or diesel will ensure that all uninhibited production fluids in the production line  38 , which is not displaced out of the line  38 , will be inhibited. Where a pig is not used for displacement it is important that a sufficient quantity of hydrate inhibiting chemical be used so as to ensure that all production fluids which are not displaced are hydrate inhibited. 
   The methanol (or other hydrate inhibitor) is pumped using the primary displacement fluid. As noted, the displacement fluid is preferably either a dehydrated crude oil or diesel. The methanol generally isolates the live fluids in the production line  38  from the cold dead crude or other displacement fluid. Preferably, the production line  38  will be depressurized after the methanol is moved through the chemical tubing  41  but before the displacement fluid reaches the manifold  20 . This further reduces the risk of hydrate formation. In one embodiment, the line is depressurized for a period of one hour. In one aspect, the depressurization is conducted during the cool down period. In another aspect, the depressurization is conducted after the cool down time period. 
   Next, dead crude or diesel is further pumped into the chemical tubing  41  to continue to displace fluids out of the production line  38 . Pumping should preferably take place at a high rate. For example, dead crude may be injected at a rate of 5 to 8 kbpd to achieve desired displacement of live fluids. The injection rate may be limited to 8 kbpd if necessary for FPSO processes. 
   It is noted from  FIGS. 2-4  that the production line  38  runs “uphill” from the well manifold  20  to the FPSO  70 . If a well is shut in for 8 hours, the produced fluids in the production line  38  will largely segregate into layers of water, live oil and gas. Variable terrain, emulsions or foaming will restrict segregation. When displacement begins, the methanol pill enters the well manifold end of the production line  38 , which is followed by the dead crude. The behavior of the interfaces between these layers is noted as follows: 
   Live oil and gas interface. Due to the uphill geometry and the lower density of gas as compared to the live oil, most gas naturally flows towards the FPSO  70 . Some gas is trapped at high points in the system. However, the methanol pill will treat this gas. Also, the dead crude or diesel may absorb the gas and transport it to the FPSO  70 . 
   Water and live oil interface. Due to the uphill geometry and the lower density of live oil/diesel as compared to the water, most live oil naturally flows towards the FPSO  70 . 
   Methanol and water interface. Due to the uphill geometry and the lower density of methanol as compared to the water, the methanol could overrun and bypass the water if the flow rate is too low. In one embodiment where displacement is pumped at a rate of 5.0 kbpd into a 10-inch production line  38 , the methanol/water interface Reynolds number is 44,000, which indicates turbulent flow. Also, methanol is miscible in water. Therefore, there should be good mixing and sweep of the water by methanol. The volume and behavior of the methanol is a function of various factors, such as injection tubing ID and flowline ID. The chemical injection tubing preferably has an inner diameter of 3 and ½ inches, though this may be adjusted. Subsea flowlines typically have an inner diameter of 4 inches to 10 inches. The pump rate will also vary depending upon line capacity, line ID, fluid viscosity, and so forth. 
   Displacement fluid/methanol interface. Displacement fluid should not overrun methanol in uphill flow due to (1) the gravity effects of the higher density of dead crude (900 kg/m3) as compared to methanol (797 kg/m3), and (2) the higher viscosity of dead crude (199 cp) than methanol (0.5 cp), which makes the dead crude more resistant to flow than methanol. At an average rate of 5.0 kbpd in a 10-inch line, the dead crude Reynolds number is 327, which indicates laminar flow. Therefore, there should be very little mixing of dead crude and methanol. It is understood that these numbers are merely for illustration. The volume and behavior of the displacement fluid is also a function of various factors, such as flowline ID. The pump rate will also vary depending upon line capacity, line ID, fluid viscosity, and so forth. 
   The operator may choose to periodically monitor the displacement efficiency of the displacement fluid. For example, the fluids recovered at the FPSO  70  may be sampled every two hours and analyzed for water and methanol content. The dead oil (or diesel) injection rate during displacement might be compared to predicted values. It has been observed that higher pump rates will improve the displacement efficiency, while lower rates will lower the displacement efficiency. At the time when the predicted remaining aqueous phase volume equals the methanol pill volume, the methanol content in the sampled aqueous phase should be rapidly increasing. For example, after 12 or 16 hours of displacement for 8-inch and 10-inch lines, respectively, the sampled aqueous phase should have a high methanol concentration. 
   If after 12 or 16 hours of displacement for 8-inch and 10-inch lines, respectively, the sampled aqueous phase does not have a substantial methanol concentration, e.g., 1.0 bbl methanol per 1.0 bbl water, then it is recommended that future displacements utilize additional methanol injection. For example, the volume of the methanol pill could be increased from 400 to 500 barrels by injecting methanol at the well manifold via umbilical methanol supply lines (not separately shown) while injecting dead crude into the chemical tubing  41 . 
   Moving now to the next drawing,  FIG. 5  depicts the state of the system  11  after the production line fluids are substantially inhibited and substantially displaced. The gate  37  remains open. Both the chemical tubing  41  and the production line  38  are now substantially filled with displacement fluids, though the production line  38  may have some remaining methanol and water. To the extent any water remains in the production line  38  due to pumping bypass, such will now be inhibited from hydrate formation due to the prior injection of and mixing with methanol. In one embodiment of the hydrate management methods herein, the operator may continue to inject dead crude or diesel into the chemical tubing  41  for an additional length of time, such as four hours, to ensure that water has been displaced from the production line  38  and that methanol has treated the entire length of the line  38 . In this instance, the total duration of displacement fluid injection is increased to 16 and 20 hours for 8-inch and 10-inch lines, respectively, for example. 
   Referring now to  FIG. 7 , a chart  700  is provided showing 8-inch flowline  38  water content during displacement as a function of dead crude injection rate. The production line  38  was producing wells with a 72% watercut at the time of shut-in, and had been shut-in for 8 hours. Time  0  on the plot  700  represents the beginning of the displacement process. 
   As shown in  FIG. 5 , the dead crude should be circulated at the highest possible rate to achieve the best sweep of live fluids. Preferably, the pump rate should be greater than 5 kbpd, and more preferably 5 to 9 kbpd. 
     FIG. 6  depicts the state of the system  20  after expected full aqueous phase displacement/inhibition and prior to well restart. Methanol or other inhibitor has been reinjected into the chemical tubing  41 . The displacement fluid has been pushed by the methanol through the valve  37 , into the manifold  20 , and into the production line  38 . The displacement fluid, in turn, has displaced the methanol and produced fluids that were ahead of it. The produced fluids are received at the gathering facility  70 . Fluids are preferably received into a high pressure separator, or they can be routed to a flare scrubber. Liquids are stored preferably in an “off-spec” tank, while gas may be routed to flare. 
     FIGS. 3-6  depict the displacement of fluids without a pig. It is preferred that a pig not be employed, as the substantial difference in diameter between the chemical injection tubing  41  and the production line  38  creates difficult design issues. However, the methods may also be conducted with a pig between an inhibitor “pill” and the displacement fluid. In either option, the current methods provide a lower volume of chemical inhibitor, thereby saving the operator money. 
   In order to displace the uninhibited production fluids from the production line  38  using a pig, a pig would be placed in the chemical injection tubing  41  of the umbilical line  42 . The pig is pumped through the umbilical line  42  using a displacing fluid, such as diesel. In one aspect, the pig is pumped from the FASO  75 , through the chemical tubing  41 , and to the manifold  20 . Valves (not shown) on the manifold  20  are controlled so that the pig and displacing fluid move through the manifold and into the production line  38 . The pig and displacing fluid are then pumped through the production line  38  and to the gathering facility  70 . In this way, hydrate blockage during a production shut-in is avoided. 
   Before production from the subsea system  11  is resumed, the chemical tubing  41  should preferably be refilled with methanol or other inhibitor of choice. A complete sweep of the displacement fluid from the tubing  41  is desired. If a gel or foam pig is used to isolate methanol from displacement fluid, filling the tubing  41  at 3.4 kbpd rate for a 3 and ½ inch tubing ID will yield a 1.0 m/s velocity in the tubing  41 . In one instance, flowing about 410 bbl of methanol provides a 10% margin for the tubing  41  with a 375 bbl volume. If no pig is used to isolate methanol from displacement fluid, the chemical tubing  41  should preferably be filled at the fastest rate possible (4.2 kbpd rate, for example). The methanol may overrun the displacement fluid some, since the methanol has a lower viscosity (0.5 cp) than the displacement fluid (dead oil, for example, has a viscosity of 199 cp). The lighter density of methanol (797 kg/m3) than dead oil (900 kg/m3) will tend to reduce methanol overrun of dead oil in downhill flow. Flowing about 450 bbl of methanol provides a 20% margin for a tubing  41  with a 375 bbl volume. 
   As noted, the preferred displacement fluid is either dehydrated and degassed crude oil or diesel. Different design considerations come into play, depending upon which displacement fluid is used. The following tables (Tables 1-4) provide volumetric comparisons when using either dehydrated crude oil or diesel. In Tables 1 and 2, produced fluids are displaced through 8- and 10-inch lines, respectively, using methanol followed by dead crude. In Tables 3 and 4, produced fluids are displaced through 8- and 10-inch lines, respectively, using methanol followed by diesel. 
   
     
       
         
             
           
             
               TABLE 1 
             
           
          
             
                 
             
             
               Methanol/Dead Crude Displacement - 8″ Line 
             
          
         
         
             
             
             
             
          
             
                 
               3.5″ ID 
               8″ NPS 
               Loop 
             
             
               Scenario 
               US Line 
               Line 
               Total 
             
             
                 
             
          
         
         
             
             
             
             
             
          
             
               Flowline ID 
               (in) 
               3.50 
               7.50 
                 
             
             
               Flowline Length 
               (m) 
               8,306 
               8,819 
             
             
               Flowline Volume 
               (bbl) 
               324 
               1,581 
               1,905 
             
             
               Riser ID 
               (in) 
               3.50 
               7.50 
             
             
               Riser Length 
               (m) 
               1,222 
               1,078 
             
             
               Riser Volume 
               (bbl) 
               48 
               193 
               241 
             
             
               Flexible 
               (in) 
               3.50 
               7.50 
             
             
               Flexible Length 
               (m) 
               — 
               548 
             
             
               Flexible 
               (bbl) 
               — 
               98 
               98 
             
             
               Topsides Line 
               (in) 
               3.50 
               7.50 
             
             
               Topsides Line 
               (m) 
               70 
               70 
             
             
               Topsides Line 
               (bbl) 
               3 
               13 
               15 
             
             
               Total Length 
               (m) 
               9,598 
               10,515 
               20,113 
             
             
               Total Volume 
               (bbl) 
               375 
               1,885 
               2,260 
             
             
               Total Volume 
               (m3) 
               60 
               300 
               359 
             
             
               Dry Oil Storage 
               (bbl) 
               — 
               — 
               5,333 
             
             
               Wet Liquids Storage 
               (bbl) 
               — 
               — 
               5,333 
             
             
               Injection 
               (stbpd) 
               5,250 
               5,250 
               5,250 
             
             
                 
               (m3/h) 
               35 
               35 
               35 
             
             
               Average Velocity 
               (m/s) 
               1.56 
               0.34 
               na 
             
             
               Duration 
               (hrs) 
               1.7 
               8.6 
               10.3 
             
             
               Start Time 
               (h:mm) 
               8:00 
               9:42 
               8:00 
             
             
               Finish 
               (h:mm) 
               9:42 
               18:19 
               18:19 
             
             
               Cool Down 
               (hrs) 
               na 
               20.0 
               na 
             
             
                 
             
          
         
       
     
   
   
     
       
         
             
           
             
               TABLE 2 
             
           
          
             
                 
             
             
               Methanol/Dead Crude Displacement - 10″ Line 
             
          
         
         
             
             
             
             
          
             
                 
               3.5″ ID 
               10″ NPS 
               Loop 
             
             
               Scenario 
               US Line 
               Line 
               Total 
             
             
                 
             
          
         
         
             
             
             
             
             
          
             
               Flowline ID 
               (in) 
               3.50 
               9.50 
                 
             
             
               Flowline Length 
               (m) 
               8,306 
               8,819 
             
             
               Flowline Volume 
               (bbl) 
               324 
               2,537 
               2,861 
             
             
               Riser ID 
               (in) 
               3.50 
               9.50 
             
             
               Riser Length 
               (m) 
               1,222 
               1,078 
             
             
               Riser Volume 
               (bbl) 
               48 
               310 
               358 
             
             
               Flexible 
               (in) 
               3.50 
               9.50 
             
             
               Flexible Length 
               (m) 
               — 
               548 
             
             
               Flexible 
               (bbl) 
               — 
               158 
               158 
             
             
               Topsides Line 
               (in) 
               3.50 
               9.50 
             
             
               Topsides Line 
               (m) 
               70 
               70 
             
             
               Topsides Line 
               (bbl) 
               3 
               20 
               23 
             
             
               Total Length 
               (m) 
               9,598 
               10,515 
               20,113 
             
             
               Total Volume 
               (bbl) 
               375 
               3,025 
               3,399 
             
             
               Total Volume 
               (m3) 
               60 
               481 
               540 
             
             
               Dry Oil Storage 
               (bbl) 
               — 
               — 
               6,667 
             
             
               Wet Liquids Storage 
               (bbl) 
               — 
               — 
               6,667 
             
             
               Injection 
               (stbpd) 
               5,250 
               5,250 
               5,250 
             
             
                 
               (m3/h) 
               35 
               35 
               35 
             
             
               Average Velocity 
               (m/s) 
               1.56 
               0.21 
               na 
             
             
               Duration 
               (hrs) 
               1.7 
               13.8 
               15.5 
             
             
               Start Time 
               (h:mm) 
               8:00 
               9:42 
               8:00 
             
             
               Finish 
               (h:mm) 
               9:42 
               23:31 
               23:31 
             
             
               Cool Down 
               (hrs) 
               na 
               24.0 
               na 
             
             
                 
             
          
         
       
     
   
   In Tables 1 and 2, produced fluids are displaced using methanol and “dead crude.” The “flowline length” and “riser length” together provide a total length of line originally having uninhibited produced fluids. A 3½-inch chemical injection tubing  41  is used for fluid displacement. During normal operations, the chemical injection tubing  41  of the utility lines  42 ,  51  is preferably left full of roughly 375 bbl of methanol. During displacement, this methanol forms a pill in the flowline  38  that isolates the live fluids from the cold dead crude. The pill is 2.1 and 1.3 km long in the 10.5 km 8-inch and 10-inch lines, respectively. 
   The dead crude displacement fluid should be injected into the chemical tubing  41  at the maximum allowable pressure. The maximum allowable dead crude pumping system discharge pressure is estimated to be 191 bara, atm. in one pumping system. Injection rates also affect displacement time requirements. It is noted that the preferred minimum displacement time requirements for 8-inch and 10-inch lines in the above test are 10 and 15 hours, respectively. Adding in 6 hours of cool down time, 2 hours of light touch time, and 1 to 2 hours of contingency time yields a total cool down time requirement of 20 and 24 hours for 8-inch and 10-inch lines, respectively. These times will vary depending upon injection rates and the use of other flowline geometries. 
   To further reduce the risk of hydrate formation, the arrival pressure may be reduced. This, in turn, increases the displacement efficiency rate. In addition, the viscosity of the dead oil displacement fluid may be reduced by using a warmer fluid. This can be achieved by utilizing the warmest dead crude from the most recently filled cargo tank, and/or by slightly insulating the utility line  42 . Alternatively, a more durable chemical injection tubing could be used, thereby permitting more vigorous injection rates. For instance, increasing the flowline rating from 301 bara to 351 bara atm. increases the water displacement efficiency rate by an estimated 26%. Finally, a viscosity reducing agent may be injected into the circulated dead oil. Reducing the dead oil viscosity from 125 to 10 cp increases the displacement efficiency rate by an estimated 41%. 
   Simulations have been conducted for displacing produced fluids from an 8-inch production line using dehydrated crude oil as the displacement fluid. It was found in one model that optimum fluid displacement was realized using a methanol “pill” of 375 to 404 barrels, pumped for 12 hours. The dead crude rate ranged from 2.0 to 7.7 kbpd. The aqueous phase (water plus methanol) content after 12 hours of displacement was 41 bbl. It is therefore expected that the remaining aqueous phase in the line will be nearly pure methanol. 
   Simulations were also conducted for displacing produced fluids from a 10-inch production line using dehydrated crude oil as the displacement fluid. It was again found that optimum fluid displacement was realized using a methanol “pill” of 375 to 404 barrels, but pumped for 16 hours. The dead crude rate ranged from 4.9 to 8.1 kbpd. 
   Referring now to  FIG. 8 , a chart  800  provides a demonstration of flow rate during fluid displacement. The early peak is due to the low viscosity of the methanol originally in the chemical injection tubing  41 . After the dead crude fills the chemical injection tubing  41 , the flow rate reaches a minimum of 4.9 kbpd. As the 38° C. dead crude warms the tubing  41 , the dead crude viscosity decreases, which allows the dead crude flow rate to increase to 8.1 kbpd. 
     FIG. 9  provides a chart  900  presenting water displacement for a 10-inch line. Note that the aqueous phase first increases while the methanol flows from the chemical injection tubing  41  into the production line  38 , and then decreases as the dead crude displaces the aqueous phase to the FPSO  70 . 
   Simulations were also conducted for displacing produced fluids from an 8-inch production line using diesel as the displacement fluid. It was found in one test that diesel should be pumped into the chemical injection tubing  41  at a rate of 8.0 kbpd. For 8 hours in order to obtain optimum displacement. A methanol pill of 275 barrels was used to partially displace and partially inhibit produced fluids from the production line  38  ahead of the diesel. A total diesel volume of 2,700 barrels was injected to then displace the methanol and remaining produced fluids. 
   A similar volume for recovered live fluid storage is also required, and can be broken down as follows. A 50% watercut is used as an example: 
   Recovered live crude is equivalent to chemical line volume×0.95×(1−watercut)=1,885 bbl×(1-0.50)=895 bbl crude; 
   Recovered water is equivalent to chemical line volume×0.95×(watercut)=1,885 bbl×(0.50)=895 bbl water; 
   Recovered methanol=chemical line volume+injected methanol=275+0 bbl=275 bbl methanol; and 
   Recovered diesel is equivalent to injected−line−chemical line volume=(2,700−1,885−275 bbl)=540 bbl diesel. 
   Total liquids recovered during displacement therefore are 2,605 bbl. 
   If a higher injection rate for the methanol and the diesel can be achieved, then the injection time period can be reduced. The total injected diesel volume is still 2,700 barrels. The volume of the methanol pill can be increased from 275 barrels up to a maximum of 980 barrels by injecting methanol at the well manifold via a separate chemical injection line while injecting diesel into the chemical tubing  41 . Increasing the methanol pill size allows the operator to reduce the diesel injection duration and total injection volume. Since the methanol resides mainly in the aqueous phase with water, adding methanol will hasten the displacement of water from the lines. 
   After the diesel front reaches the well manifold, the cold diesel (approximately 5 cp) in the 8-inch line will have a Reynolds number of 15000, which indicates turbulent flow. There would be good mixing and contact of diesel and methanol with any remaining water. It is therefore acceptable to continue any additional methanol injection at the well manifold beyond the time the diesel front reaches the well manifold. If methanol is injected at a 14 m 3 /hr during an 8 hour displacement period, then 704 barrels of methanol would be added. 
   The following tables (Tables 3 and 4) show the remaining production line  38  aqueous phase content over time for a range of diesel injections. The amount of methanol required to treat the remaining aqueous phase assumes a factor of two error in aqueous phase volume prediction. Note that the methanol volume may not be less than the 275 to 287 barrel volume of the 3.0-inch ID line  41 . The diesel, methanol and total costs of the displacement are calculated, assuming the displacement is halted at the tabulated time. Displacement for 7 hours at an 8.0 kbpd rate for an 8-inch line minimizes total cost and methanol consumption. An additional hour of displacement is recommended. 
   
     
       
         
             
           
             
               TABLE 3 
             
           
          
             
                 
             
             
               Comparing Diesel and Methanol Volumes and Costs 8″ Line Methanol and Diesel Displacement 
             
          
         
         
             
             
             
             
             
             
             
             
             
          
             
               Displace- 
                 
                 
               Diesel 
               Aqueous 
               Methanol 
                 
                 
                 
             
             
               ment 
               Diesel 
               Diesel 
               Cost, k$ (1 
               Content At 
               Volume, 
               Methanol 
               Displaced 
             
             
               Time, 
               Rate, 
               Volume, 
               $/gal - 15 
               This Time, 
               bbl 
               Cost, k$ 
               Volume, 
               Total 
             
             
               hours 
               kbpd 
               bbl 
               $/bbl) 
               bbl 
               (+100%) 
               (1 $/gal) 
               bbl 
               Cost, k$ 
             
             
                 
             
          
         
         
             
             
             
             
             
             
             
             
             
          
             
               4 
               8.0 
               1333 
               36 
               824 
               1648 
               69 
               2681 
               105 
             
             
               5 
               8.0 
               1667 
               45 
               496 
               992 
               42 
               2359 
               87 
             
             
               6 
               8.0 
               2000 
               54 
               235 
               470 
               20 
               2170 
               74 
             
             
               7 
               8.0 
               2333 
               63 
               117 
               275 
               12 
               2308 
               75 
             
             
               8 
               8.0 
               2667 
               72 
               70 
               275 
               12 
               2642 
               84 
             
             
               9 
               8.0 
               3000 
               81 
               47 
               275 
               12 
               2975 
               93 
             
             
               10 
               8.0 
               3333 
               90 
               32 
               275 
               12 
               3308 
               102 
             
             
               12 
               8.0 
               4000 
               108 
               12 
               275 
               12 
               3975 
               120 
             
             
               14 
               8.0 
               4667 
               126 
               3 
               275 
               12 
               4642 
               138 
             
             
               16 
               8.0 
               5333 
               144 
               0 
               275 
               12 
               5308 
               156 
             
             
               4 
               7.0 
               1167 
               32 
               974 
               1948 
               82 
               2815 
               113 
             
             
               5 
               7.0 
               1458 
               39 
               691 
               1382 
               58 
               2540 
               97 
             
             
               6 
               7.0 
               1750 
               47 
               417 
               834 
               35 
               2284 
               82 
             
             
               7 
               7.0 
               2042 
               55 
               229 
               458 
               19 
               2200 
               74 
             
             
               8 
               7.0 
               2333 
               63 
               139 
               278 
               12 
               2311 
               75 
             
             
               9 
               7.0 
               2625 
               71 
               93 
               275 
               12 
               2600 
               82 
             
             
               10 
               7.0 
               2917 
               79 
               67 
               275 
               12 
               2892 
               90 
             
             
               12 
               7.0 
               3500 
               95 
               39 
               275 
               12 
               3475 
               106 
             
             
               14 
               7.0 
               4083 
               110 
               21 
               275 
               12 
               4058 
               122 
             
             
               16 
               7.0 
               4667 
               126 
               10 
               275 
               12 
               4642 
               138 
             
             
               4 
               6.0 
               1000 
               27 
               1120 
               2240 
               94 
               2940 
               121 
             
             
               5 
               6.0 
               1250 
               34 
               881 
               1762 
               74 
               2712 
               108 
             
             
               6 
               6.0 
               1500 
               41 
               642 
               1284 
               54 
               2484 
               94 
             
             
               7 
               6.0 
               1750 
               47 
               410 
               820 
               34 
               2270 
               82 
             
             
               8 
               6.0 
               2000 
               54 
               263 
               526 
               22 
               2226 
               76 
             
             
               9 
               6.0 
               2250 
               61 
               179 
               358 
               15 
               2308 
               76 
             
             
               10 
               6.0 
               2500 
               68 
               132 
               275 
               12 
               2475 
               79 
             
             
               12 
               6.0 
               3000 
               81 
               83 
               275 
               12 
               2975 
               93 
             
             
               14 
               6.0 
               3500 
               95 
               59 
               275 
               12 
               3475 
               106 
             
             
               16 
               6.0 
               4000 
               108 
               44 
               275 
               12 
               3975 
               120 
             
             
                 
             
          
         
       
     
   
   
     
       
         
             
           
             
               TABLE 4 
             
           
          
             
                 
             
             
               (Comparing Diesel and Methanol Volumes Using an 8-Inch Production Line) 
             
             
               Comparing Diesel and Methanol Volumes and Costs 10″ Line Methanol and Diesel Displacement 
             
          
         
         
             
             
             
             
             
             
             
             
             
          
             
               Displace- 
                 
                 
               Diesel 
               Aqueous 
               Methanol 
                 
                 
                 
             
             
               ment 
               Diesel 
               Diesel 
               Cost, k$ (1 
               Content At 
               Volume, 
               Methanol 
               Displaced 
             
             
               Time, 
               Rate, 
               Volume, 
               $/gal - 15 
               This Time, 
               bbl 
               Cost, k$ 
               Volume, 
               Total 
             
             
               hours 
               kbpd 
               bbl 
               $/bbl) 
               bbl 
               (+100%) 
               (1 $/gal) 
               bbl 
               Cost, k$ 
             
             
                 
             
          
         
         
             
             
             
             
             
             
             
             
             
          
             
               4 
               8.9 
               1485 
               40 
               1755 
               3510 
               147 
               4695 
               188 
             
             
               6 
               8.9 
               2228 
               60 
               1116 
               2232 
               94 
               4160 
               154 
             
             
               8 
               8.9 
               2970 
               80 
               573 
               1146 
               48 
               3816 
               128 
             
             
               10 
               8.9 
               3713 
               100 
               344 
               688 
               29 
               4101 
               129 
             
             
               12 
               8.9 
               4455 
               120 
               241 
               482 
               20 
               4637 
               141 
             
             
               14 
               8.9 
               5198 
               140 
               189 
               378 
               16 
               5276 
               156 
             
             
               16 
               8.9 
               5940 
               160 
               158 
               316 
               13 
               5956 
               174 
             
             
               4 
               8.0 
               1333 
               36 
               1880 
               3760 
               158 
               4793 
               194 
             
             
               6 
               8.0 
               2000 
               54 
               1243 
               2486 
               104 
               4186 
               158 
             
             
               8 
               8.0 
               2667 
               72 
               664 
               1328 
               56 
               3695 
               128 
             
             
               10 
               8.0 
               3333 
               90 
               402 
               804 
               34 
               3837 
               124 
             
             
               12 
               8.0 
               4000 
               108 
               287 
               574 
               24 
               4274 
               132 
             
             
               14 
               8.0 
               4667 
               126 
               230 
               460 
               19 
               4827 
               145 
             
             
               16 
               8.0 
               5333 
               144 
               197 
               394 
               17 
               5427 
               161 
             
             
               4 
               7.0 
               1167 
               32 
               2020 
               4040 
               170 
               4907 
               201 
             
             
               6 
               7.0 
               1750 
               47 
               1480 
               2960 
               124 
               4410 
               172 
             
             
               8 
               7.0 
               2333 
               63 
               921 
               1842 
               77 
               3875 
               140 
             
             
               10 
               7.0 
               2917 
               79 
               576 
               1152 
               48 
               3769 
               127 
             
             
               12 
               7.0 
               3500 
               95 
               418 
               836 
               35 
               4036 
               130 
             
             
               14 
               7.0 
               4083 
               110 
               336 
               672 
               28 
               4455 
               138 
             
             
               16 
               7.0 
               4667 
               126 
               291 
               582 
               24 
               4949 
               150 
             
             
                 
             
          
         
       
     
   
   It is noted that diesel should preferably not overrun the methanol for the following reasons: 
   the gravity effects of the higher density diesel (818 kg/m3) as compared to methanol (797 kg/m3) in uphill flow; and 
   the higher viscosity of diesel (5 cp) as compared to methanol (0.5 cp), which makes the diesel more resistant to flow than methanol. 
   The chart  1000  of  FIG. 10  demonstrates water displacement using diesel as the displacement fluid. Note that the aqueous phase first increases while the methanol flows from the chemical injection tubing  41  into the production line  38 , and then decreases as the diesel displaces the aqueous phase to the FPSO  70 . 
   A profile plot of the aqueous phase content in the 8-inch line is shown in the chart  1100  of  FIG. 11 . The solid black curve  1102  shows water holdup volume fraction after 8 hours of shut-in time. The other curves show the water holdup fraction in one hour increments. After 8 hours of displacement, the line aqueous phase content is 70 bbl. 
   A method for transporting hydrocarbons from an offshore production facility is also provided herein. In this method, the production facility receives produced hydrocarbons from one or more subsea wells, and from a production line associated with the one or more subsea wells. The subsea wells and production line are associated with a subsea production system. The method generally comprises the steps of shutting in the flow of produced fluids from the subsea well and the production line; pumping a displacement fluid from the production facility into a chemical injection tubing, the chemical injection tubing being within an umbilical; further pumping the displacement fluid into the chemical injection tubing so that displacement fluid is urged through a subsea manifold and into the production line; further pumping the displacement fluid through the production line so as to displace the produced fluids before hydrate formation begins; re-initiating the flow of produced fluids from the subsea wells and through the production line to the production facility; and transporting the produced fluids from the offshore production facility. 
   In one aspect, the step of transporting the produced fluids from the offshore production facility comprises offloading the produced fluids from the offshore production facility onto a tanker; and transporting the produced fluids to an onshore terminal. 
   The subsea production system further comprises a jumper for delivering produced fluids from the subsea well to a manifold, and a valve for selectively placing the chemical injection tubing in fluid communication with the manifold. The umbilical further comprises a first umbilical portion that connects the gathering facility with an umbilical termination assembly, and a second umbilical portion that connects the umbilical termination assembly with the manifold. The production line comprises a production riser in fluid communication with the production facility, and a flowline for placing the manifold in fluid communication with the production riser. 
   While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.