Abstract:
A pressurized slip joint for a marine intervention riser decouples the flowhead assembly in the moon pool of a vessel from the riser string, enabling safe changeover of equipment during workover operations. One part of the slip joint assembly is coupled to the flowhead assembly through a flexible joint assembly. A second part of the slip joint assembly supports the riser string and is coupled to the tensioning mechanism. The first part may be inserted into the second part and locked in place during workover operations except when equipment changeover is taking place. When changeover is being carried out, the first and second parts are unlocked, so that the flowhead assembly does not move relative to the vessel. In the locked position, a metal-to-metal high pressure seal, with a secondary and tertiary seal controls the pressure in the riser. In the unlocked position, a hydraulically operated dynamic low pressure seal is used.

Description:
BACKGROUND OF THE INVENTION 
     1 Field of the Invention 
     This invention relates generally to offshore drilling systems and more particularly to a pressurized slip joint for use with a marine intervention riser system for workover applications after a well has been drilled. The slip joint enables expeditious operations in the moon pool of a vessel in heavy seas. 
     2 Background of the Art 
     Risers for drilling operations typically consist of large diameter pipes extending from the wellhead through an opening in the bottom (“moon pool”) of the vessel. Drilling operations are carried out by means of a drill string within the riser. Drilling mud required for drilling is circulated through the drillstring to the drillbit at the bottom of the drillstring, back up the wellbore and through the annulus between the drillstring and the riser. The riser serves to separate the drilling fluid from the surrounding seawater. When drilling operations are carried out in deep water, the danger of buckling of the riser increases. The reason for this is that the riser has the same buckling characteristics as a vertical column and structural failure under compressive loading may occur. To avoid this structural failure, riser tensioning systems are installed on the vessel for applying a tensile force to the upper end of the riser. A variety of such tensioning systems have been used in prior art, including cables, sheaves and pneumatic cylinder mechanisms connected between the vessel and the upper portions of the riser. 
     Because the riser is fixed at the bottom to the wellhead assembly, wind, wave and tidal action will cause movement of the vessel relative to the top end of the riser. Motion compensating equipment must be incorporated into the tensioning system to maintain the top of the riser within the moon pool. This may include a telescopic coupling arrangement to compensate for heaving motion and a flex joint within the riser to compensate for lateral movement of the vessel. During drilling, pressure inside the riser pipe is comparatively low. However, the pressure may increase if a shallow pocket if gas is encountered and the sliding joint is typically designed to withstand a pressure of 2000 psi or less. 
     In the case of producing wells, however, the pressure inside the riser can easily approach 10000 psi. Fixed production platforms do not require telescopic risers. In deeper waters, tension leg platforms have been used. Such platforms are subject to more motion than fixed platforms and the risers have to be designed accordingly. On marginal fields where the cost of a production platform would be prohibitive, drilling vessels have been used for production. Production riser pipes for mobile production platforms have been constructed as an integrated unit suspended in tension systems and guides, capable of absorbing the necessary telescopic, lateral and angular movements. U.S. Pat. No. 5,069,488 discloses a telescopic device that is volume and pressure balanced for mobile production platforms. Because of the requirement of no relative vertical motion between the riser and the production vessel, the telescopic system has to be designed to withstand the maximum motion expected in heavy seas. 
     Marine intervention riser systems are functionally similar to risers used with mobile production platforms in terms of the pressures that are encountered. However, there is one major difference: workover operations typically require a variety of devices to be inserted into the well. Use of these devices requires a considerable amount of human involvement in the vessel. Any system in which the riser pipes in the moon pool have a large vertical movement with respect to the vessel presents a serious safety hazard when humans are preforming workover operations in the vessel. At these times, it is desirable to have no movement between the top of the riser assembly within the moon pool and the vessel. At other times, when humans are not involved, vertical movement of the riser within the moon pool is acceptable: at such times, a system that allows relative motion between the top of the riser assembly within the moon pool and the vessel is acceptable. The present invention is capable of meeting these requirements. 
     SUMMARY OF THE INVENTION 
     The present invention provides a slip joint assembly for use in a marine intervention riser system. When devices for workover operations are being installed by humans, the invention is configured to act like a low pressure slip joint with the upper end of the assembly fixed relative to the vessel, allowing for safe installation of the devices. Once the workover devices have been installed, the upper end of the assembly is fixed to the riser and is capable of sealing at high pressures. 
     Examples of the more important features of the invention have been summarized rather broadly in order that the detailed description thereof that follows may be better understood, and in order that the contributions to the art may be appreciated. There are, of course, additional features of the invention that will be described hereinafter and which will form the subject of the claims appended hereto. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For detailed understanding of the present invention, reference should be made to the following detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, in which like elements have been given like numerals: 
     FIG. 1 is an overall elevational view of a riser assembly incorporating the present invention in operation. 
     FIG. 2 is a view of an embodiment of the flexible slip joint 
     FIG. 3 is a sectional view of a flexible slip joint. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 shows a vessel  10  floating at the surface  12  of a body of water  20 . The vessel includes a vertical opening or “moon pool”  14  through its hull. The moon pool is typically located at the center of the vessel in order to avoid destabilizing the vessel due to operations being carried out. The vessel is provided with a support, such as a wireline rig or coiled tubing inserter  16 , that is used for lowering equipment into the well. A riser string  118  carries the wireline or coiled tubing through the wellhead assembly  102  into the borehole (well)  104 . Details of the wellhead assembly and other devices associated with connecting the riser string  118  to the wellhead are not shown. 
     Ocean currents, ocean waves and the like will cause movement of the vessel  10  at the surface  12  relative to the fixed wellhead assembly  102  at the bottom of the body of water. The motion may be vertical (surge or heave), horizontal (drift) or rotational (yaw, pitch and roll). Drillships are usually provided with thrusters to compensate for the drift of the vessel. Additional mechanisms have to be provided for compensate for the other types of motion to avoid damage to the riser that is fixed to the ocean bottom and vessel. At the top of the riser string is a flowhead assembly  32  in the moon pool  14 . A motion compensating system (not shown) compensates for relative motion of the riser string  118  and the vessel  10 . Such motion compensating systems will still result in a relative motion between the flowhead assembly  32  and the vessel. The present invention is part of a decoupling assembly  30  that is adapted to decouple the motion of the flowhead assembly  32  from that of the riser string  118 , so that equipment changes required for workover operations may be safely carried out on the flowhead assembly. 
     Turning now to FIGS. 2 and 3, the main components of the decoupling assembly are shown. Conceptually, it can be considered to have two main components: one component that is fixed to the riser string  118  and a second component that is fixed to the flowhead assembly  32 . The first and second components are designed to move in unison when locked together by a locking mechanism and to be decoupled when unlocked by the locking mechanism. 
     The lower part includes a pressurized slip joint assembly  100  connected at its lower end to the top of the riser string  118 . The top of the slip joint assembly  100  is connected by means of a collet connector and guide funnel  116  to a flexible joint assembly  110 . In a preferred embodiment of the invention, a hydraulic quick connect device is used for coupling the flexible joint assembly to the top end  108  of the slip joint assembly. Such quick connect devices would be known to those versed in the art and are not discussed further. For illustrative purposes, the slip joint assembly  100  and the flexible joint assembly  110  have been shown in a disconnected position. The purpose of the flexible joint assembly is to compensate for the yaw, roll and pitch of the vessel relative to the riser string  118 . The top of the flexible joint assembly  110  is connected to a flowhead assembly (not shown in FIGS. 2 and 3) in the moon pool of the vessel. The flexible joint assembly includes a flex joint and may also include a swivel joint. Flex joints and swivel joints would be known to those versed in the art and are not discussed further. 
     Shown near the top end  108  of slip joint assembly  100  and enclosing it is part of the tension assembly for keeping the riser  18  under tension. A rotational tension ring  112  surrounds the slip joint assembly. The tension ring  112  is provided with lugs  114  through which cables (not shown) are passed. Such tension assemblies for keeping risers under tension would be known to those versed in the art and are not discussed here. 
     FIG. 3 shows a partial sectional view of the slip joint assembly. For clarity, it is shown disengaged from the flexible joint assembly  110 . The rotational tension ring  112  is shown along with the lugs  114 . The rotational tension ring  112  and a downwardly extending cylindrical portion  122  may be considered to define a substantially cylindrical outer housing. Supported inside the rotational tension ring  112  by bearings  119  is an inner housing  120 . This allows rotational movement between the inner housing  120  and the tension ring  112 . The inner housing is of substantially cylindrical shape with a lip  124  at its lower end. Extending circumferentially around the inside wall of the inner housing is a groove  126 . Near the bottom of the cylindrical portion  122  and on its inside is a shoulder  141 . A quick connect device  142  at the bottom of the outer housing is used to connect the slip joint assembly to the riser  118  (not shown in FIG.  3 ). 
     The sliding member  128  of the slip joint assembly has a head  132  and a downwardly extending cylindrical body  134 . The head is sized to fit on the inside of the inner housing  120  while the body (a liner)  134  is sized to fit inside the outer housing. The head is provided with a lockdown ring (or segments of a lockdown ring)  130  that is designed to engage the cylindrical groove  126  of the inner housing in a locked position and to allow slidable movement (in a vertical direction) of the sliding member in an unlocked position. The sliding member is provided with a number of hydraulic leads to control its operation. These are labeled  148 ,  150 ,  152 , and  154  and are discussed below. 
     When the sliding member  128  is in the locked position, the bottom end  135  of the body  134  forms a metal-to-metal seal  146  against the shoulder  141  on the outer housing. This seal  146  forms the primary high pressure seal when sliding member  128  is in the locked position. Secondary  140  and tertiary  138  high pressure seals are also provided between the body  134  of the sliding member and the outer housing  122  as a backup to the primary high pressure seal  146 . The secondary and tertiary seals are preferably made of elastomeric material. In addition, a dynamic low pressure seal  136  is also provided for the annulus between the body  134  of the sliding member and the outer housing  122 . 
     A plurality of hydraulic leads that perform various functions lead into the head  132  of the sliding member. Leads  148   a,    148   b  and  150   a,    150   b  activate the latch/unlatch and the lock/unlock mechanism of the lockdown ring  130 . Lead  152  activates the dynamic low pressure seal  136 . Lead  154  is provided to monitor the pressure in the space  144  between the primary  146  and secondary  140  seals. A pressure monitor  149  is used for the purpose. This may also be used to monitor the position of the sliding member  128  relative to the outer housing and hence the integrity of the primary metal-to-metal seal. 
     The operation of the slip joint is now discussed. Under normal conditions, wellhead assembly is in the open position and the inside of the riser  118  would be at high pressure. The riser string  118 , the rotational tension ring  112 , the flexible joint assembly  110  ( and the flowhead assembly in the moon pool of the vessel, not shown) move in unison, so that there may be relative motion between the flowhead assembly and the vessel. The dynamic low pressure seal  136  may be inoperative at this time. When it is desired to perform workover operations, e.g., run a wireline, the wellhead assembly is closed so that there is no direct communication between the inside of riser string  118  and the well  104 . The pressure inside the riser assembly is bled down and the locking ring  130  is disengaged. This allows relative motion between the body  134  of the sliding member and the outer housing  122 . The low pressure dynamic seal is activated. In this configuration, the flowhead assembly (not shown) above the sliding member  128  and the flexible joint assembly  110  is decoupled from the riser string  118 . Tool changeover may safely be performed by humans in the moon pool. Once the new tools have been inserted into the flowhead assembly and lowered to the well head, the lockdown ring  130  is engaged, and the wellhead opened up. In this manner, the invention makes it possible to decouple relative motion of the upper end of the riser assembly from the lower end of the riser assembly. 
     To connect the slip joint, the slip joint is closed by stroking the inner liner  134  fully into the outer housing item  122 . Pressure is applied down a hydraulic line  148   a  to activate the lockdown ring or collet mechanism  130 . The lockdown ring  130  engages the groove  126  to lock the inner liner  134  and outer housing together and providing the force to seal the metal—metal seal  146 . Pressure is then applied down line  150   a  to lock the lockdown ring  130  in place preventing accidental unlatching of lockdown ring  130  from the groove  126 . To monitor the status of the primary seal during well operations line  154  is used as a monitor line from the pressure monitor  149 . 
     To disconnect the slip joint, pressure is applied down line  150   b  to unlock the lockdown ring. Pressure is applied down line  148   b  to unlatch ring  130  from groove  126 . The slip joint is then free to move with vessel motion. Line  152  provides a positive LP dynamic seal (air or hydraulic fluid) to prevent loss of wellbore fluids to the environment and may also provide lubrication for the slip joint during movement of the inner to the outer barrel (although lubrication may come from an alternative source). The sliding members (inner barrel and outer housing) are not controlled by hydraulic lines. The lifting and lowering of inner barrel to outer housing is provided by means of a external lifting device on the vessel. Motion between these items  122  and  134  is the motion of the vessel relative to the seabed during the unlatched state of the lockdown ring. 
     While the foregoing disclosure is directed to the preferred embodiments of the invention, various modifications will be apparent to those skilled in the art. It is intended that all variations within the scope and spirit of the appended claims be embraced by the foregoing disclosure.