Abstract:
A system for storing pigs and launching them into a pipeline comprises a pig storage barrel having an inlet end and an outlet end and an inside diameter sized to accommodate the pigs, the storage barrel including a pig parking chamber for controlling release of the pigs, and a pig launching chamber downstream of the storage barrel. The launching chamber is connected to the pipeline so as to be able to launch a pig into the pipeline. The apparatus also includes a hydraulic pig advancing system in fluid communication with the storage barrel and the launching chamber and a fluid flush system in fluid communication with the storage barrel chamber and the launching chamber. The method includes flushing the storage barrel and the launching chamber after each launching cycle so that between launchings the system is filled with hydraulic fluid.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims benefit of provisional application Ser. No. 60/246,769, filed Nov. 8, 2000 and entitled “Subsea Pig Launcher,” which is incorporated herein by reference. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     TECHNICAL FIELD OF THE INVENTION 
     The present invention relates generally to subsea pig launching, and more particularly to a method and apparatus for storing and sequentially launching multiple pigs from a remote location. Still more particularly, the present invention is a method and apparatus for launching a series of pigs over a relatively long period of time. 
     BACKGROUND OF THE INVENTION 
     In the oil and gas industry, it is common to remove petroleum deposits and other debris from pipelines by pushing a scraper, referred to as a “pig,” through the lines using fluid pressure. The pig is introduced into the pipeline from a launching system that typically includes a releasable retainer for retaining the unlaunched pig and a source of fluid pressure connected to the housing behind the pig. 
     Subsea satellite wells, manifolds or templates in deepwater are typically connected to a host platform via subsea flowlines that transport the produced hydrocarbon fluid along the sea floor. Such systems are often referred to as “tie-back” systems. As with other types of pipelines, flowlines in tie-back systems need to be pigged periodically during their operation to remove paraffin deposits, displace liquids, etc. The traditional method of pigging such flowlines has been to use “round trip pigging” which requires a pair of parallel flowlines between the host platform and the manifold or the wellhead. Thus, pigs are launched from and received at the host platform, traveling outward through one flowline and returning through the other. 
     A more economical option for deepwater flowlines is to use “single trip pigging” using a single flowline between the manifold and the host platform. In single trip pigging, the pigs are launched from a pig launcher mounted on the subsea manifold and received on the host platform. The pig launcher can also be mounted on wellhead or pipeline end manifold (PLEM) sled which is connected to production manifold/wellhead via jumpers. The economic advantages of single trip pigging over round trip pigging become even greater in “extended tie-back” systems, where the distance between the manifold and the platform can be as much as 50 to 60 miles. 
     In order to maximize the advantage of a single trip pigging system, the cost of offshore intervention in the system should be minimized. Such intervention is required either to replace the pigs in the launcher or to retrieve the launcher and recharge it with a new set of pigs after the previous set has been launched into the flowline. This implies that the pig launcher should be able to hold large number of pigs and, for deepwater application, all pigging operations should be performed using an ROV or remotely from the host platform. Furthermore, depending on the pigging operation frequency, the pigs are likely to be left in the launcher over a long period, as much as one to two years. However, since conventional pigs tend to degrade when exposed to hydrocarbons or methanol for a long period, conventional pigs cannot be left exposed to hydrocarbons or methanol while they are stored in the pig launcher. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides an apparatus for launching pigs into a subsea flowline that connects a subsea manifold, template or a wellhead to host platform. The subsea pig launcher described herein addresses the issues identified above. Whereas in the following discussion, the subsea pig launcher is always assumed to have been installed on a manifold, the apparatus can be easily adapted for other applications such as pigging from a subsea wellhead, template or a pipeline end manifold (PLEM) sled at the end of flowline. 
     More particularly, a method and apparatus have been developed for loading multiple pigs into a subsea pig launcher barrel, storing them inside the barrel over an extended period of time and then releasing the pigs remotely, one at a time, into a subsea flowline or pipeline as needed. The inside diameter of the launcher barrel is preferably slightly larger than the outside diameter of the pigs, so as to allow easy movement of pigs inside the barrel. Since the pigs are pushed inside the barrel by a positive fluid pressure, the system can operate even with the barrel made to fit the pig outside diameter. The top or rearmost pig is constructed so that its outside diameter seals against the inside wall of the barrel, and is therefore referred to as the “piston pig.” 
     The pigs are advanced inside the barrel by providing hydraulic fluid under pressure behind the piston pig. This advances the stored pigs until the foremost pig is adjacent to a chamber called “pig parking chamber” at the bottom of the barrel. The entrance of the pig parking chamber can be opened to allow the foremost pig to enter the chamber. In the chamber, the foremost pig is held between two pig stops. A kicker line connected to parking chamber is provided for pumping hydraulic fluid behind the parked pig. By releasing the lower pig stop and pumping hydraulic fluid under pressure through the kicker line, the foremost pig can be launched into another chamber called the “pig launching chamber.” The pig launching chamber preferably has an isolation valve at each end, which isolates it from the production header and flowline on one side and the pig parking chamber on the other side. A production kicker line connected to the launching chamber permits introduction of production fluids (hydrocarbons) behind the pig inside the launching chamber. 
     From the launching chamber, the pig is pushed into the production header by opening the isolation valve between the production header and the launching chamber. Once the pig travels from the production header into the flowline, the isolation valve between the launching chamber and the production header is closed. The launching chamber is preferably then flushed with methanol and then with hydraulic fluid to wash out the residual hydrocarbon fluids. 
     After all pigs have been launched, the closure at the top of launcher barrel is opened by ROV (remotely operated vehicle) and a new set of pigs is stacked inside the barrel. The apparatus of the present invention, and in particular the pig storage barrel and pig parking chamber, are novel features. By separating the pig launching from the pig storage, the present system allows the pigs to be stored in hydraulic or control fluid, i.e. fluid that will not degrade the pigs, and also ensures that sufficient fluid volume is available to launch the pigs. 
     The hydraulic fluid to be used for pushing pigs can be similar to the control fluids conventionally used for subsea systems, or can be any other suitable fluid that has a density greater than seawater, is environmentally acceptable, and is chemically non-reactive with the pig material. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more detailed understanding of the preferred embodiments, reference is made to the accompanying Figures, wherein: 
     FIG. 1 is a schematic illustration of an apparatus constructed in accordance with a preferred embodiment of the present invention; 
     FIG. 2 is a side view, partially in cross-section, of a preferred pig configuration suitable for use in the apparatus of FIG. 1; 
     FIG. 3 is a side view, partially in cross-section, of a preferred piston pig configuration suitable for use in the apparatus of FIG. 1; 
     FIG. 4 is a schematic side elevation of a preferred embodiment of pig transfer equipment used in conjunction with the apparatus of FIG. 1; and 
     FIG. 5 is a schematic plan view of the pig transfer equipment of FIG.  4 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring initially to FIG. 1, a preferred embodiment of the present subsea pig launcher assembly  10  includes a pig storage barrel  20 , which is preferably sized and configured to receive a ten to twelve pigs  22  stacked end to end. Although the discussion that follows and the attached Figures show barrel  20  installed on the manifold in vertical orientation, it will be understood that the system can be adapted for horizontal or inclined orientation of barrel  20 . The inside diameter of barrel  20  is preferably slightly greater than the outside diameter of the pigs. This facilitates easy movement of pigs inside the barrel. 
     Referring briefly to FIGS. 2 and 3, in a preferred embodiment, pigs  22  each have an extending nose section  24 , which facilitates stacking them directly and maintains a space between their radially extending fins  25 . Each pig  22  may be fitted with any of various pipeline-cleaning or other special devices around its circumference, such as are known in the art. 
     Referring now to FIGS. 1 and 3, the last pig near the top of the barrel has a larger diameter fin  27  than the other pigs  22  and fits snugly and sealingly inside barrel  20  and is thus referred to as a piston pig  26 . At the top of barrel  20  is a valve  28 , which closes the inlet end  21  of barrel  20 . Other types of closures can be used in place of valve  28 . When valve  28  is open, pigs  22  can be loaded into barrel  20 . A debris cap (not shown) above the valve is used to protect the valve inlet. It is preferred that valve  28  be operated “manually,” although it is also contemplated that valve  28  could be remotely actuable. 
     If desired, a magnetic sensor  38  mounted on the barrel detects the passage of the piston pig, which includes a preinstalled magnet. Sensor  38  can be used to send a signal to inform the operator to reload a new stack of pigs into the barrel. It will be understood that other types of sensing devices can be used to sense the passage of the piston pig and that sensor  38  can alternatively be mounted at other suitable points along the launching apparatus  10 . 
     Below pig storage barrel  20  is the pig parking chamber  40 . Parking chamber  40  is defined by an upper pig stop  42  and a lower pig stop  44 . Pig stops  42 ,  44  are preferably spaced one pig length apart. A spare set of pig stops  46 ,  48  is preferably provided as a backup. All pig stops are preferably provided with actuators for remote operation. The inside diameter of parking chamber is preferably selected to substantially fit the pig outside diameter. A parking chamber kicker valve  35  allows hydraulic fluid under pressure to flow behind the pig inside the parking chamber so as to push it out of the chamber when lower pig stop  44  is released. 
     A hydraulic line  30  enters barrel  20  near the top of barrel  20  allows hydraulic fluid or control fluid to be introduced under pressure behind the piston pig  26 . Hydraulic line  30  is controlled by a remotely actuated valve  32 . Providing hydraulic or control fluid under pressure behind piston pig  26  advances piston pig  26  and the other pigs down the barrel. A branch line  33  controlled by a valve  35  connects hydraulic line  30  to pig parking chamber  40 . At its other end, hydraulic line  30  connects to vent line  31 , which is controlled by valve  34 , and to a fluid supply system that includes fluid accumulators  65 . A valve  39  control fluid flow from accumulators to line  30 . Thus, hydraulic line  30  can also be used as a vent line for discharging the seawater from barrel  20  during loading of pigs into the barrel. As mentioned above, the hydraulic or control fluid to be used for pushing pigs can be similar to the control fluids conventionally used for subsea systems, or can be any suitable fluid that has a density greater than seawater, is environmentally acceptable, and is chemically non-reactive with the polyurethane used in pigs. 
     At the bottom of the barrel is the hub of a dual bore mechanical connector  50  (collet type or alternate), which attaches the barrel to the system downstream on the manifold. Docking guides a soft landing system (not shown) and an alignment funnel (not shown) are preferably included to ensure proper orientation of the respective hubs during the connector makeup. Connector  50  is hydraulically operated to make-up and break the connection between storage barrel  20  and the manifold. The dual bore connector  50  provides a connection between barrel  20  and manifold pipe  52  connected to the flowline header for the pig transfer and also between the hydraulic line  30  on the manifold pipe  52  and on the launcher. 
     A vent line  53  is preferably provided on manifold pipe  52  and is controlled by valve  36 . Hence, valves  34  and  36  allow venting of fluids from the launcher barrel from the bottom and the top, respectively, as explained in detail below. Similarly, a fluid supply line  55  provides hydraulic or control fluid under pressure from accumulators  65  to manifold pipe  52 . As disclosed above, direct hydraulic supply from hoses connected to the host platform can provide adequate fluid supply in some cases. Flow through line  55  is controlled by valve  37 . 
     Downstream of manifold pipe  52 , is a pig launching chamber  60 . Upstream and downstream isolation valves  62 ,  64 , respectively, enclose the ends of the launching chamber  60 . A flush line  63 , which is controlled by valve  69 , connects the upstream end of launching chamber  60  to the supply of control or hydraulic fluid  65 . The downstream end of launching chamber  60  is preferably connected via valve  64  to the manifold production header  100 , which in turn connects to the subsea flowline  102 . A plurality of production lines  71 ,  75  feed produced hydrocarbons from wells into production header  100  downstream of valve  64 . Fluid flow through lines  71 ,  75  into production header  100  is controlled by valves  70 ,  74 , respectively. In a preferred embodiment, each production line also supports a branch line that feeds produced fluids into a kicker line  66 , which communicates with the upstream end of launching chamber  60 . These branch lines are controlled by branch valves  72 ,  76 , respectively. Production fluids from the wells can be introduced into pig launching chamber  60  via kicker line  66 . If desired, a methanol feed line  67  can also be connected to kicker  66 , with fluid flow being controlled by a valve  73 . A remotely activated valve  68  controls fluid flow through kicker line  66 . A crossover connection  80  between launching chamber  60  and production header  100  via a remotely operated valve  82  and a check valve  84  allows fluids to flow from launcher barrel  60  into the production header  100  while bypassing valve  64 . 
     Upstream of valve  62 , hydraulic fluid under pressure from accumulators  65  or hydraulic line  30  can be introduced into pig storage barrel  20 . Hydraulic line  31  and vent valve  34  permit draining of fluids from the bottom of the barrel. 
     In one preferred embodiment all valves except valve  28  are remotely operated and valve  28  is operated by remotely operated vehicle. In an alternative embodiment, all valves are ROV operable. 
     Operation 
     Installing the Launcher: The pig delivery barrel  20  is installed on manifold pipe  52  using multi-bore hydraulic connector  50 . The connector is preferably configured such that the connection can be made using a remotely operated vehicle (ROV), such as that shown in FIG. 4, or other suitable ROV. The ROV preferably connects hydraulic control lines and electrical signal lines on pig storage barrel  20  to a control pod (not shown) on manifold pipe  52  using flying leads. 
     Pig storage barrel  20  is filled with seawater after installation on the manifold. All valves except branch valves  70 ,  74  etc., which connect the individual wells to header  100 , are closed. 
     Loading Pigs: Referring to FIGS. 4 and 5, an ROV  200  carrying a predetermined number of pigs  22  lands on or connects to a platform at the top of storage barrel  20 . A preferred pig transportation and loading mechanism used by ROV  200  is described in provisional application Ser. No. 06/246,769, referenced above. The debris cap is opened by the ROV  200 . The loading valve  28  at the top of the barrel is opened. Also, valve  32  and vent valves  34  and  36  are opened. ROV  200  preferably loads the pigs  22  into the barrel one at a time. The last pig to be loaded is piston pig  26 , after which loading valve  28  is closed. 
     Flushing Pig Delivery Barrel: Valves  32  and  34  remain open. Valve  36  is closed. Hydraulic fluid under pressure is introduced at the bottom of the barrel by opening valve  37 . The hydraulic fluid preferably has a higher density than seawater; hence it flushes the seawater from barrel  20  as it fills barrel  20  from the bottom. The seawater leaving barrel  20  exits through valves  32  and  34 . Once barrel  20  is full of hydraulic fluid, which can be detected by observing color of fluid at the vent port by the ROV  200 , then valves  37 ,  32 ,  34  and  36  will be closed. 
     Loading Pig into Parking Chamber: Upper pig stop  42  is opened, while lower pig stop remains closed. Fluid pressure across valve  62  is equalized by opening valves  37  and  69 . Then valve  62  is opened, valves  37  and  69  are closed, and crossover valve  82  is opened. By opening valves  39  and  32 , hydraulic fluid pressure can be applied behind piston pig  26 . This advances one pig into the pig parking chamber  40 , where it is stopped by lower pig stop  44 . Once a pig is received in parking chamber  40 , upper pig stop  42  and valves  39  and  32  are closed. Upper pig stop  42  prevents any additional pigs from entering chamber  40  until it is desired to launch another pig. 
     Loading Pig into Launching Chamber: Lower pig stop  44  and valves  39  and  35  are opened. This introduces hydraulic fluid under pressure behind the pig in parking chamber  40 , causing it to advance into launching chamber  60 . The fluid in front of the pig is dumped into production header  100  via crossover valve  82 . After the pig is inside launching chamber  60 , valves  39 ,  35 ,  62  and  82  are closed. 
     Launching Pig: Valves  76  and  68  are opened to equalize pressure inside launching chamber  60 . Valve  64  is opened. The trees are choked down as necessary. Branch valves  70 ,  74  on some or all wells may be closed as necessary so as to divert production flow behind the pig and launch the pig into production header  100 . After the pig is detected leaving production header  100 , valve  64  is closed and the wells are brought back to full production. 
     Flushing Kicker Line and Pig Launching Chamber: All production kicker valves  72 ,  76  etc. between the wells and kicker line  66  are closed. Launching chamber isolation valve  64  is closed. Valve  68  remains open. Methanol injection valve  73  and bypass valve  82  are opened. This permits flushing of kicker line  66  and launching chamber  60  with methanol so as to displace any residual hydrocarbon fluid. After flushing with methanol, valves  82 ,  68 , and  73  are closed. To flush launching chamber  60  with hydraulic fluid, valves  82  and  69  are opened and high pressure control fluid is allowed to flush launching chamber  60 . The excess control fluid is pushed into production header  100  via valve cross-over  82 . After a sufficient time to ensure that chamber  60  is filled with control fluid, valves  82  and  69  are closed. 
     At this point, the pig launcher is ready to launch another pig into the flowline. Each launch cycle includes the steps of flushing the components of the apparatus with a hydraulic or control fluid. The time period between launchings will vary, depending on the rate at which the downstream flowline accumulates buildup, and may be as long as two to three years. The last pig i.e., piston pig  26  is launched in the same manner as the other pigs. 
     It is preferred that each valve in the present system be remotely actuable. The valves can be hardwired to a controller, or can be configured to respond to radio, acoustic, electric, hydraulic or other types of signal. In addition, a microprocessor (not shown) can be provided for operating the various valves in sequence. 
     Although the invention is intended for deepwater pigging operations, it can be also used for pigging of flowlines in shallow water with the benefit of eliminating any diver intervention during such operations and also reducing the frequency of offshore intervention. The method and apparatus of the present invention make it possible to launch as many pigs as may be needed from a remote pig launcher without disconnecting the pig storage barrel from the launching apparatus. Also, because it separates pig storage and parking from pig launching, the present invention makes it possible to store the pigs in a hydraulic or control fluid that will not degrade the material from which they are made, and yet does not require the large volume of hydraulic or control fluid that would be needed if the same fluid were used to launch the pigs. 
     While a preferred embodiment of the present invention has be described, it will be understood that various modifications thereto can be made without departing from the scope of the present invention. For example, the precise positioning of various lines and valves can be altered, the number of various components, including barrels, chambers, accumulators and lines, can be altered, and the orientation of various components can be modified without affecting the operation of the present system. Likewise, while various steps may be disclosed or claimed in a particular order herein, it is not intended that any particular order be required unless such order is inherent in the carrying-out of the claim or explicitly recited.