Abstract:
A method and apparatus for repairing a submerged pipeline. The method comprises installing a pair of hot tap tees on either side of the damaged section of pipeline, cutting a hole in the pipeline through each hot tap tee, inserting and securing plugging pigs into the pipeline, cutting and removing the damaged section of pipeline, and installing a new section of pipeline. The apparatus includes a plugging pig for use during repair of a damaged pipeline comprising a body shaft, a cup seal secured to the body shaft for engaging an interior wall of the pipeline, a cam attached to the exterior of the body shaft, a slip assembly for sliding on the cam and engaging a slip against the pipeline wall, and a control mechanism for engaging and releasing the slip from the wall.

Description:
This Appln claims the benefit of Provisional No. 60/078,240 filed Mar. 17, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to a method and apparatus for repairing a submerged pipeline. In particular, the invention relates to a method and apparatus for replacing a damaged section of a submerged pipeline without a significant loss of product from or entry of seawater into the undamaged pipeline. 
     2. Description of the Related Art 
     Thousands of miles of pipeline lay on the seabed. Modern pipe laying technology permits the installation of pipelines in a variety of water depths, including water depths of thousands of feet. A serious problem results when damage occurs to a pipeline set in deep water because repair can be cumbersome and difficult. Typically. submerged pipelines are damaged by an object impacting it, by an anchor dragging across it, or by the environment corroding it. If a section of pipeline becomes significantly damaged, it may requirement replacement. 
     In the past, generally two methods were employed to repair a damaged submerged pipeline. One method required the damaged section of the pipeline to be recovered to the water surface for repair on the deck of a large surface vessel. After the repair was completed, the pipeline was lowered and repositioned on the seabed. A disadvantage of this method included the expense associated with recovering the pipeline to the surface. Another disadvantage with this method was that it typically could only be used with small diameter pipelines in relatively shallow water. 
     A second method for repairing a damaged pipeline involved cutting the pipeline into two pieces near the damaged section while the pipeline remained on the seabed. The cut end of each pipe section was pulled to the surface. Once at the surface, the damaged section of pipeline was replaced with a new section. Specially designed connectors gripped the pipe at the cut ends and provided a flange or collet hub that could be used to connect the new section to the pipeline. Often, a third connector was used to make a telescoping spool section that could be expanded between the two cut ends of the pipeline to facilitate the connection. A disadvantage of this method was that product could escape from the pipeline to the subsea environment and seawater could enter into the pipeline. 
     To prevent product from escaping into the environment from the cut pipeline, hot tapping systems were used to plug the pipeline. In general, hot tapping systems use a clamp-on, split tee to provide a fluid tight connection in the pipeline. A special drill was attached to the tee in a manner that prevents loss of pipeline fluid during the tapping operation. The tapping drill was fitted with a special cutter that opened a full diameter hole in the side of the pipeline. After cutting a hole in the pipeline, a stopper machine replaced the drill. The stopper machine installed an elastomeric stopper into the pipeline. After the new pipeline section was attached to the remaining pipeline, the stopper was removed but the tee became a permanent part of the pipeline. 
     A disadvantage of the hot tapping and stopper method was that although the hot tap tee typically could seal under fill pipeline pressure, the tee needed to provide a structural tension capacity to make up for the removal of half of the pipe wall, which was cut away during installation of the stopper. The need for structural capacity demanded that a mechanical connection be made to the pipeline which could withstand full line pressure. 
     Another disadvantage of the hot tapping system was that the opening in the side of the pipe presented a hazard to the passage of a pipeline pig. The opening needed to be protected with bars or plates such that a pipeline pig did not catch on the opening. Eliminating this hazard was typically cumbersome and complicated in deepwater where repairs are usually performed by an ROV. 
     SUMMARY OF THE INVENTION 
     The present invention includes a method and apparatus for replacing a damaged section of a subsea pipeline setting on the seabed. The present invention prevents the product in the pipeline from escaping to the environment and the seawater from entering into the pipeline when the damaged section of pipeline is removed from the pipeline. Additionally, the method and apparatus of the present invention, which uses hot tap tees for the installation of plugging pigs, allows for the removal of the hot tap tees from the repaired pipeline. 
     The method includes installing a pair of hot tap tees on either side of the damaged section of pipeline. An angle-cutting drilling machine attaches to each of the hot tap tees and each drilling machine cuts a hole into the pipeline. After cutting the holes, the drilling machines are removed from the hot tap tees and a pig-set machine is installed on each of the hot tap tees. A plugging pig according to the present invention is inserted by the pig-set machines through each of the hot tap tees and into the pipeline. The plugging pigs are secured in place within the pipeline. A cutting device is lowered to the seabed and the pipeline is cut at two locations, each location being between the hot tap tee and the installed plugging pig. The damaged section of pipeline, which includes the hot tap tees, is removed and replaced by a new section of pipeline. 
     The present invention also includes a method for controlling a plugging pig inserted into a pipeline through a hot tap tee. The method comprises energizing a lock mechanism within the plugging pig to engage a slip on the lock mechanism with the interior wall of the pipeline. The fluid pressure in the pipeline is monitored and the slips are released when the fluid pressure reaches a predetermined pressure. 
     The present invention also includes a plugging pig apparatus inserted through a hot tap tee to prevent the loss of product from a cut pipeline. The apparatus includes a body shaft having an external surface and an internal cavity. A cup seal is mounted to the body shaft and engages an interior wall of the pipeline. A cam is attached to said external surface of the body shaft and a slip assembly slides on the cam to engage a slip with the interior wall. A control mechanism controls the engagement and release of the slip from the interior wall. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A better understanding of the present invention may be had by reference to the following drawings and contained numerals therein of which: 
     FIGS. 1A-1I are side elevational views of the method and apparatus for replacing a damaged section of pipeline according to the present invention; 
     FIG. 2 is a sectional elevational view of a plugging pig assembly according to the present invention; 
     FIGS. 3A-3D are side elevational views, partially in section, of the operation of the upstream plugging pig of the present invention; 
     FIGS. 4A-4D are side elevational views, partially in section, of the operation of the downstream plugging pig of the present invention; and 
     FIG. 5 is an elevational view, partially in section, of a valve assembly used in the plugging pigs of the present invention. 
    
    
     DETAILED DESCRIPTION OF INVENTION 
     The present invention relates to a method and apparatus for repairing a damaged section of a submerged pipeline. Referring to FIG. 1A, once a damaged section D of a pipeline P is located, the pipeline pressure is reduced to less than the ambient seawater pressure at the depth of the repair. The flow in the pipeline P is shut off at the well head or production point (not shown). Using conventional methods, hot tap tees  10  with angled outlets are installed on either side of the damaged section D (FIG.  1 B). The hot tap tees  10  are fitted with simple elastomeric seals  12  rated for the seawater depth of the repair. Structural capacity tees are not required because the tees will not be used at full pipeline pressure. 
     As shown in FIG. 1C, angle-cutting drilling machines  20  are installed on the tees  10  and full diameter holes are cut into the pipeline by a drill bit  22 . Referring to FIG. 1D, the angle-cutting machines  20  are removed from the tees  10  and pig-set machines  30  are installed on the tees  10 . Fluid in the pipeline P is contained during installation of the pig-set machines  30  by containment valves  24  mounted between the tees  10  and the pig-set machines  30 . 
     As shown in FIG. 1E, the pig-set machines  30  insert plugging pigs  40  according to the present invention into the pipeline P. As shown in FIG. 2, and described in greater detail below, the plugging pigs  40  are fitted with locking devices or slips  42  that hold the pigs  40  in position within the pipeline P against the pressure of the surrounding seawater. Referring to FIGS. 1E and 2, slips  42  can be connected by a hydraulic hose  41  extending through the hot tap tee  10  to a pressure connection (not shown) on the outside of the pig-set machine  30 . Hydraulic pressure from an external pump (not shown) is applied to the pressure connection to extend the slips  42  on the pigs  40  such that the slips  42  are forced against an inside wall W of the pipeline P. A check valve  64   a  of a valve circuit  64  holds the slips  42  in engagement with the interior wall W after the external pump is no longer in communication with the pigs  40 . The damaged section D is now isolated from the pipeline P by the plugging pigs  40 . 
     Alternatively, a plugging pig without a locking device or slips may be used in the present invention. If the pipeline fluid conditions are such that the differential pressure between the pipeline fluid and the surrounding seawater can be maintained such that the pig is neither expelled from the pipeline nor driven too far into the pipeline, then non-locking pigs can be used instead of locking pigs  40 . Similarly, if large amounts of seawater, trapped between the pigs during the repair, are acceptable in the pipeline once the repair is completed then non-locking pigs can be secured within the pipeline with the method of the present invention. Large differences in differential pressure or compressible fluids in the pipeline, however, can drive non-locking pigs great distances before equilibrium is established and the pigs become secured within the pipeline. 
     After the plugging pigs  40  are installed in the pipeline P, the pig-set machines  30  can be removed from the hot tap tees  10  (FIG.  1 F). If desired, the damaged section D of the pipeline P can be purged to remove any pipeline fluid trapped between the pigs  40 . This action removes product which might otherwise escape to the environment during the cutting operation. 
     Referring to FIG. 1G, a saw or similar device (not shown) is then used to cut out the damaged section D of the pipeline P between the hot tap tees  10  and the plugging pigs  40 . This action exposes the interior of the damaged section D to ambient seawater pressure. The cutting device also cuts the hoses  41  connecting the plugging pigs  40  to the pig-set machines  30 . When the pipeline P is completely severed, the damaged section D, the hot tap tees  10 , and the pig-set machines  30  may be recovered to the surface. Similarly, if the pipeline P is to be lifted to the surface for installation of pipe end connections (not shown), the plugging pigs  40  prevent the loss of product. Furthermore, because the hot tap tees  10  have been removed, the pipeline&#39;s P cut ends E and can withstand the forces of lifting the pipeline P. 
     Referring to FIG. 1H, if the repair is to be performed subsea, the hydraulic hose  41  can be cut away from the plugging pigs  40  to avoid any interference with installation of a new pipeline section R. If desired, the cut ends E of the pipeline can be cleaned or prepared as necessary to receive pipeline connectors  32 . The cut ends E of the pipeline P are measured and surveyed to determine the proper length and shape of the new section R. On the surface vessel, the new pipeline section R is cut and shaped as needed to match the cut ends E of the pipeline P. The pipeline end connectors  32  are installed on the new section R and the new section R is lowered to the seafloor and installed in the pipeline P. 
     After the new section R is installed, pipeline pressure is restored in the pipeline P. As shown in FIG. 1I, the action of raising the pipeline pressure releases the plugging pigs  40 . The pigs  40  are then pumped through the pipeline P carrying any seawater, trapped between the pigs  40  during installation of the new section R, to the pig receiver (not shown). At the pig receiver, the plugging pigs  40  are removed and the pipeline P is returned to service. 
     In the preferred method of the present invention, the plugging pig  40  shown in FIG. 2 is used to prevent product in the pipeline P from escaping to the underwater environment. The plugging pig  40  includes conventional cup seals  52 , made of an elastomeric material, for sealing the product within the pipe P and for preventing the seawater from entering the pipeline P. The cup seals  52  are attached to a body shaft  54  and fit tightly against the interior wall W of pipeline P. Plate seals  62 , preferably made of an elastomeric material, are connected to the body shaft  54  through a guide  60 . The plate seals  62  also contact the interior wall W and provide a means for centering the pig  40  within the pipeline P. 
     A locking mechanism  38  includes the body shaft  54  with a piston cylinder  44  in its interior and a secondary release cam  58  on its exterior. Within the cylinder  44  is a cylinder rod  46  which can be extended and retracted by the cylinder  44 . A cam  56  is secured to the secondary release cam  58  by a shear pin  68 . 
     An actuator plate  48  is attached to the rod  46  and translates along the guides  60  as the rod  46  extends and retracts. As the rod  46  moves, the plate  48  causes an actuator follower  50   b  of an actuator assembly  50  to slide along the cam  56 . The actuator plate  48  moves within a slot  50   c  when the rod  46  translates. An outer surface  50   a  of the actuator assembly  50  includes slips  42 . 
     A valve assembly  64  is also secured within the body shaft  54 . The valve assembly  64  includes the check valve  64   a , a bypass valve  64   b , and a burst disk  64   c.    
     Referring to FIGS. 3A and 4A, the upstream and downstream plugging pigs  40   a  and  40   b , respectively, include the same internal components as shown in FIG.  2 . The engagement with and subsequent release from the interior wall W, however, requires the components to operate differently to resist the seawater pressure against the individual pigs  40   a  and  40   b . Both pigs  40   a  and  40   b  use plate seals  62  and cup seals  52  for containment of the pipeline fluids and for travel through the pipeline P. The locking mechanism  38  that engages the slips  42  against the inside wall W of the pipeline P is the same for both the upstream and downstream pigs. Moreover, both pigs use the hydraulic cylinder  44  and the rod  46  to extend and retract the slips  42 . The slips  42  are curved and arranged to match the inside diameter of the pipeline P. The engagement face of each slip  42  element is selected to match the material and design of the pipeline P. 
     The difference between the upstream and downstream pigs  40   a  and  40   b  is in the control of the locking mechanism  38 . As shown in FIG. 3A, the upstream pig  40   a  is inserted into the pipeline P in the direction of arrow I. Referring to FIG. 3B, a rod end port  45  is connected by the hydraulic hose  41  to a pressure connection on the outside of the pig-set machine  30 . Hydraulic fluid travels through the hose  41  and the port  45  to retract the rod  46  of the cylinder  44  and force the slips  42  to engage the interior wall W of the pipeline P. 
     As described above, the slips  42  are mounted on the actuator assembly  50  which slides along the cam  56 . Thus, movement of the pig  40   a  in the upstream direction (the direction of the arrow I) increases the force of the slips  42  against the interior wall W. The check valve  64   a  in the valve assembly  64  prevents the cylinder  44  from retracting after the hydraulic pressure is removed. Alternately, an overcenter mechanism could be used to prevent the cylinder from retracting. 
     As shown in FIG. 5, the hydraulic hose  41  of the valve assembly  64  is connected hydraulically through a conduit  41   a  to the bypass valve  64   b  which allows pipeline fluid F to pass through a fluid line  39  of the pig  40   a  during installation (FIG.  3 A). The bypass valve  64   b  and the fluid line  39  prevent hydraulic lock from forming in the pipeline P that would otherwise prevent insertion of the plugging pig  40 . The bypass valve  64   b  is closed by hydraulic pressure in the hose  41  and the conduit  41  a at the same time that slips  42  are extended against the wall W. The bypass valve  64   b  remains closed during the repair of the pipeline P. 
     Referring to FIGS. 3C and 5, after the new pipeline section R is installed, pressure in a monitor line  43  on the upstream side of the pig  40   a  is increased to release the slips  42 . Alternatively, a remote (acoustic or other) signal and an energy storage device on the pig could be used to release the slips. To release the slips  42 , a piston end port  47  is hydraulically connected to the burst disk  64   c  through a hydraulic line  43   a  and when the upstream pressure in the pipeline P and the monitor line  43  exceeds the burst pressure of the burst disk  64   c , the disk  64   c  breaks and allows the pipeline pressure to extend the rod  46  and release the slips  42 . Fluid from the pipeline P flows through the hydraulic line  43   a  and the port  47  into the cylinder  44  causing the rod  46  to translate and the actuator follower  50   b  to slide along the cam  56 . The fluid pressure now in line  43   a  communicates with the check valve  64   a  through a conduit  43   b  to open the check valve  64   a  Once the check valve  64   a  is opened, the hydraulic fluid that was pumped into the cylinder  44  through the port  45  and the hose  41  can exit through the port  45  and the hose  41 . 
     The difference in forces between the rod end and the piston end of the cylinder  44  keeps the slips  42  released as the pig travels through the pipeline P. The pig  40   a  is now free to travel with the pipeline fluid flow. As the pig moves forward, it pressurizes the seawater in the new pipeline section. This action raises the pressure on the upstream side of the downstream pig  40   b  (FIGS. 1H,  1 I, and  4 A). The ratio of the cylinder rod to the piston diameter is selected such that the pressure in the new pipeline section R, created by upstream pressure on the upstream pig  40   a , is sufficient to release the downstream pig  40   b.    
     Referring to FIGS. 4A-4C, the downstream pig  40   b  control system is configured slightly differently from the upstream pig  40   a . As shown in FIG. 4A, the downstream pig  40   b  is inserted into the pipeline P in the direction of arrow I. The direction of the arrow I is downstream. In the downstream pig  40   b , the hydraulic hose  41  on the upstream side of the pig  40   b  is connected to the rod end port  45  and as hydraulic pressure increases in the hose  41  the rod  46  retracts into the cylinder  44  and the slips  42  engage the wall W of the pipeline P. The slips  42  are mounted on the actuator assembly  50  which slides along the cam  56 . Thus, the movement of the pig in the downstream direction increases the force of the slips  42  against the pipeline wall W. The bypass valve  64   b  prevents a hydraulic lock during installation by permitting fluid F to escape from the pipeline P. 
     The burst disk  64   c  is connected to the piston end port  47  on the upstream side of the pig  40   b . When fluid pressure D in the new pipeline section R is raised by the movement and pressure driving the upstream pig  40   a , the burst disk  64   c  breaks and allows pipeline fluid pressure D to operate the cylinder  44  and release the slips  42 . The hydraulic fluid within the cylinder  44  escapes through port  45  while fluid enters into port  47 . The burst pressure of the disk is significantly higher than ambient seawater pressure to prevent inadvertent release of the pig slips  42 . As the pigs  40   a  and  40   b  are driven into the pipeline P, the downstream pressure increases and the pressure differential across the pig decreases to that required to drive the pig. The differential areas of the cylinder piston and rod are therefore selected to maintain retraction of the slips during this condition. 
     As a contingency against failure of the cylinder  44  to release the slips  42 , a secondary slip release mechanism can be incorporated into each pig  40   a  and  40   b . Referring to FIGS. 3D and 4D, the secondary release mechanism can be operated by pressurizing the upstream side of the upstream pig  40   a  with the fluid F and pressurizing the downstream side of the downstream pig  40   b  with the fluid F. The operation of the secondary release is the same in either case. If the plugging pig slips  42  fail to release, the pressure in the pipeline P is increased to a point where the pigs  40  will begin to move. Prior to reaching that pressure point, however, the shear pins  68  break and allow the release cam follower  56   a  of the cam  56  to slide along the release cam  58 . The cam follower  56   a  eventually collapses into notches of the secondary release cam  58 . Once the cam  56  has collapsed within release cam  58 , the slips  42  are no longer in contact with the wall W and the pigs  40  are free to move within the pipeline P. 
     The foregoing disclosure and description of the invention are illustrative and explanatory thereof, and various changes in the details of the illustrated apparatus and construction and method of operation may be made without departing from the spirit of the invention.