Motion compensator

A motion compensator is used on a floating vessel servicing a subsea well. The motion compensator includes a first frame assembly adapted to be connected to a cable extending from a lifting structure. When connected to the cable, the first frame assembly extends longitudinally along an axis substantially parallel with that of the cable. The motion compensator also includes a second frame assembly connected to the first frame assembly. The second frame assembly overlaps a longitudinal portion of the first frame assembly. The first and second frame assemblies are moveable relative to each other and define an expanded position and a contracted position. The motion compensator further includes a piston assembly positioned between the first and second frame assemblies. The piston assembly has a piston chamber and a piston that slidingly engages the piston chamber when the first and second rod assemblies move relative to each other.

BACKGROUND OF INVENTION

1. Field of the Invention

The present invention relates generally to offshore platforms, and more specifically to an assembly for compensating for motion.

2. Background of the Invention

When servicing a subsea well from a floating vessel, tidal variations cause the vessel, as well as surface wellhead assemblies connected an upper end of a riser from the subsea well location, to drift. This phenomenon is commonly known as “tidal drift.” When servicing the well through the surface wellhead assembly, the servicing equipment is typically suspended above the surface wellhead assembly. The typical servicing equipment can be the equipment commonly known and associated in the art for coiled tubing, wireline, and snubbing well intervention work. The tidal drift can cause excessive forces to be experienced on the equipment that can damage or break the servicing equipment and the surface wellhead assembly.

Conventional devices used for accommodating for such movements are large and bulky in size. These devices are so large that they cannot be used within a drilling rig. Moreover, the conventional devices are not responsive to the tidal drift. Rather, the operator has to monitor the status of the equipment in response to tidal drift, and then manually adjust the device as needed. This process can be costly and dangerous, because it is desirous to keep the line supporting the servicing equipment taught so that as little weight as possible is supported by the surface wellhead assembly.

SUMMARY OF THE INVENTION

An offshore assembly is associated with an offshore well. The offshore assembly includes a floating vessel upon which operations for a subsea well are performed. The floating vessel is responsive to tidal movements of water upon which the vessel floats. The tidal movements include the movements that are associated with tidal drift of the vessel. The offshore assembly also includes a surface wellhead assembly in fluid communication with the subsea well. The wellhead assembly is supported on a riser extending up to the surface wellhead assembly from a subsea location. The floating vessel is moveable relative to the wellhead assembly while the wellhead assembly is in communication with the subsea well. The offshore assembly further includes a lifting apparatus for lifting and supporting an interface device connecting to the wellhead assembly. The lifting apparatus has a cable extending therefrom and being positioned on the floating vessel. The lifting apparatus moves with the floating vessel. The offshore assembly also includes a motion compensator positioned between the surface wellhead assembly and the cable. The motion compensator is moveable between an expanded position and a contracted position in order to compensate for movement of the floating vessel and the lifting apparatus responsive to the tidal movement of the water.

The present invention also provides a motion compensator for use on a floating vessel servicing a subsea well. The motion compensator includes a first frame assembly adapted to be connected to a cable extending from a lifting structure. When connected to the cable, the first frame assembly extends longitudinally along an axis substantially parallel with that of the cable. The motion compensator also includes a second frame assembly connected to the first frame assembly. The second frame assembly overlaps a longitudinal portion of the first frame assembly. The first and second frame assemblies are moveable relative to each other and define an expanded position and a contracted position. The motion compensator further includes a piston assembly positioned between the first and second frame assemblies. The piston assembly has a piston chamber and a piston that slidingly engages the piston chamber when the first and second rod assemblies move relative to each other.

In one version of motion compensator for use on a floating vessel servicing a subsea well, the motion compensator includes a first frame assembly adapted to be connected to a cable extending from a lifting structure. The first frame assembly extends longitudinally along an axis substantially parallel with that of the cable when connected. The first frame assembly has a first end plate and a first medial plate that are fixedly connected to each other by a plurality of first rods. The motion compensator also includes a second frame assembly connected to the first frame assembly such that the second frame assembly overlaps a longitudinal portion of the first frame assembly. The second frame assembly has a second end plate and a second medial plate that are fixedly connected to each other by a plurality of second rods. The first and second frame assemblies being moveable relative to each other to define an expanded position and a contracted position. The motion compensator further includes a piston assembly positioned between the first and second frame assemblies. The piston assembly has a piston chamber and a piston that slidingly engages the piston chamber when the first and second rod assemblies move relative to each other.

Each of the plurality of second rods preferably extend through and slidingly engage the first medial plate when the motion compensator moves between the expanded and contracted positions. Each of the plurality of first rods also preferably extend through and slidingly engage the second medial plate when the motion compensator moves between the expanded and contracted positions.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring toFIG. 1, a crane11is shown on top of a platform13. Platform13is typically a platform associated with an offshore facility for oil wells. A surface wellhead assembly17rests atop of a distal end of casing that extends through a deck12of the platform to a subsea well (not shown) positioned below platform13. A coiled tubing injector15is suspended from crane11for connection with wellhead17. Coiled tubing injector15can be used in a manner known in the art for injecting coiled tubing in order to perform intervention on the well. A coiled tubing blowout preventer system19is preferably located between coiled tubing injector15and wellhead17in order to control possible blowouts from a well during operations.

A motion compensator21is also suspended from crane11in a position above coiled tubing injector15. Motion compensator21advantageously compensates for motions of platform13relative to wellhead17due to tidal variations of the water below. A hydraulic power pack23is located on platform13for supplying hydraulic fluid and power to motion compensator21. Hydraulic power pack23also controls the hydraulic fluid injected and removed from motion compensator21. A hydraulic control hose25extends from hydraulic power pack23to motion compensator21suspended from crane11for the transfer of hydraulic fluid between hydraulic power pack23and motion compensator21. An upper connector27connects motion compensator21to a cable extending from crane11, while a lower connector29connects motion compensator21to a cable extending to coiled tubing injector15.

Referring toFIGS. 2 and 3, motion compensator21preferably includes end plates31connected to upper connector and lower connector27,29. For ease of reference, end plate31connected to upper connector27is upper end plate31A, and end plate31connected to lower connector29is lower connector31B. A plurality of upper guide rods33extend downward from end plate31A, and a plurality of lower guide rods35extend upward from end plate31B. A plurality of middle plates37are positioned between end plates31A,31B. An upper middle plate37A is positioned adjacent upper end plate31A. Likewise, a lower middle plate37B is positioned adjacent lower end plate31B. Upper guide rods33extend downward through upper middle plate37A and connect to lower middle plate37B. Upper guide rods35extend upward from end plate31B through middle plate37B and connect to middle plate37A. Fasteners39connect to ends of upper and lower guide rods33,35in order to hold upper and lower guide rods33,35relative to end plates31A,31B and middle plates37A,37B. A guide sleeve41is positioned around each upper and lower guide rod33,35extending through middle plates37. In the preferred embodiment, guide sleeves41allow upper and lower guide rods33,35to slide relative the middle plates37A,37B that upper and lower guide rods33,35are passing through. In the preferred embodiment, a plurality of openings43(FIGS. 4 and 5) allow upper and lower guide rods33,35to pass through middle plates37A,37B and end plates31A,31B.

Referring toFIGS. 4 and 5, middle plates37are preferably octagonal or square shaped, while end plates31are preferably rectangular in shape. End plates31preferably include openings43located adjacent each of the corners of rectangular shaped end plate31. End plates31are preferably offset by 90 degrees so that end plate31A extends in a direction generally perpendicular to the direction that end plate31B extends. The result of the 90 degree offset is best shown inFIGS. 2 and 3wherein connector plate31A connected to upper connector27is shown along its narrow side inFIG. 2and along its wider side inFIG. 3. Connector plate31B connected to lower connector29however is shown inFIG. 2along its wider side and along its narrow side inFIG. 3. Due to this configuration inFIG. 2upper connector rods33are shown within lower connector rods35inFIG. 2but are shown outside of lower connector rods35inFIG. 3when viewed from a different direction.

Motion compensator21preferably includes a piston housing45located between middle plates37. Piston housing45is preferably connected to middle plate37A by upper piston support47. A piston49ends from lower middle plate37B into piston housing45. Piston housing45and piston49define a piston chamber51that changes in size as piston49strokes within piston chamber45. As shown inFIG. 2, piston45is fully stroked to its compressed state. However, piston49is stroked to its expanded state inFIG. 3. A bracket53extends from lower middle plate37B and connects to a piston connector55. Piston49is fixedly connected to lower middle plate37B via piston connector55and bracket53. Therefore, as upper and lower middle plates37A,37B move relative to each other piston49strokes relative to piston housing45.

In operation, upper connector27connects to a cable suspended from crane11located on platform13. Lower connector29connects to a cable extending below and connecting to coiled tubing injector15which in turn supports coiled tubing blowout preventers19and wellhead17. Typically, coiled tubing is rigid in an axial direction such that the coiled tubing does not compress or lengthen due to upward and downward movement of platform13. Therefore, any upward and downward movement of platform13relative to the sea floor is transferred through coiled tubing injector15to motion compensator21.

Any upward movements of platform13relative to the sea floor, causes end plates31on motion compensator21to separate to the position shown inFIG. 2. Increasing the distance between end plates31A,31B causes lower guide rods35to pull downward against upper middle plate37A and upper guide rods33to pull upward on lower middle plate37B. Accordingly, the separation of end plates31A,31B causes upper and lower middle plates37A,37B to compress toward each other, which in turn causes piston49to stroke inward relative to piston housing45. Any hydraulic fluid, which can be oil and/or nitrogen gas located within chamber51, provides resistance to piston49stroking within piston chamber45. As piston49strokes inward and compresses piston chamber51, hydraulic fluid is transferred out of piston chamber45through control hose25to hydraulic power pack23. Hydraulic power pack23stores the hydraulic fluid for injection into chamber51when piston49strokes axially downward to its extended state shown inFIG. 3. Hydraulic power pack23preferably also includes an accumulator system for storing hydraulic energy from the hydraulic fluid. In the preferred embodiment, hydraulic power pack23also dampens shock forces experienced through motion compensator21.

When the tides of the sea cause platform13to lower relative to sea floor, the cable from crane11and between motion compensator21will no longer be in tension. Hydraulic power pack23preferably supplies hydraulic fluid into piston chamber51via hydraulic control hose25in order to stroke piston49to its extended state as shown inFIG. 3. Forcing piston49to its extended state by injecting the hydraulic fluid within piston chamber45pushes upper and lower middle plates37A,37B apart. By separating upper and lower middle plates37A,37B, upper and lower guide rods33,35pull end plates31A,31B toward each other. By decreasing the distance between end plates31A,31B, the tension between crane11and coiled tubing blowout preventers19is maintained even while platform13has lowered relative to the sea floor.

Motion compensator21is small enough to be suspended from a variety of lifting devices11.FIG. 1illustrates a crane, but lifting device11for suspending motion compensator21can also be a derrick, an A-frame or another temporary support assembly. Motion compensator21helps to automatically respond to tidal variations in order to keep cable27taught so that as little weight of the servicing equipment as possible is transferred or carried by surface wellhead assembly17.

While the invention has been shown in only one of its forms, it should be apparent to those skilled in the art that it is not so limited, but is susceptible to various changes without departing from the scope of the invention. For example, middle and end plates37,31can be designed with different geometries than shown inFIGS. 4 and 5while performing substantially the same functions. Moreover, while the invention has only been shown and described for use with coiled tubing, motion compensator21can also be useful for invention during utilizing wireline, electric-line, and snubbing operations.