Buoyancy tensioning systems for offshore marine risers and methods of use

Abstract

Devices and methods for the enhanced assembly and disassembly of offshore marine risers through the use of a combination of passive and active buoyancy elements in a marine riser. Methods for assembling passive buoyancy joints comprise providing disc handling devices capable of simultaneously manipulating a plurality of buoyancy discs capable of nesting and interlocking with one another. The combination of passive and active buoyancy elements along with a near surface disconnect package allow for quicker upper riser recovery especially in the event of an approaching tropical storm.

1 . A method of retrieving a marine riser system between an offshore rig at the surface of an ocean and a wellhead adjacent the ocean floor, the method comprising the steps of:
 providing a marine riser system comprising:
 a riser comprising an upper riser and a lower riser, 
 a disconnect joint removably attaching the upper riser to the lower riser, and 
 a second buoy having at least one chamber into which buoyancy fluid may be introduced wherein the second buoy is non-integral to the lower riser and removably attached to the lower riser; 
   introducing a buoyancy fluid into the at least one chamber of the second buoy; and   disconnecting the upper riser from the lower riser at the disconnect joint.   
   
   
       2 . The method of  claim 1  wherein the marine riser system further comprises a first buoy attached to the lower riser. 
   
   
       3 . The method of  claim 1  wherein a sufficient quantity of buoyancy fluid is introduced into the second buoy to maintain the lower riser in a substantially vertical position to prevent the lower riser from collapsing. 
   
   
       4 . The method of  claim 1  further comprising:
 retrieving the upper riser; and   leaving the lower riser in place.   
   
   
       5 . The method of  claim 1  further comprising:
 retrieving the upper riser;   retrieving and disassembling the lower riser while simultaneously expelling the buoyancy fluid from the second buoy; and   detaching the second buoy from the lower riser.   
   
   
       6 . The method of  claim 1  further comprising:
 retrieving the upper riser;   disconnecting the lower riser from the wellhead;   displacing the lower riser to a second wellhead location; and   attaching the lower riser to the second wellhead location.   
   
   
       7 . The method of  claim 5  further comprising providing a third stand-alone buoy disposed for receipt of the second buoy when the second buoy is detached from the lower riser. 
   
   
       8 . The method of  claim 5  further comprising storing the second buoy in the third stand-alone buoy. 
   
   
       9 . The method of  claim 8  wherein the third stand-alone buoy comprises a passively buoyant material and wherein the third stand-alone buoy is secured to the ocean floor by a plurality of mooring cables. 
   
   
       10 . The method of  claim 2  wherein the first buoy is attached at substantially the top of the lower riser. 
   
   
       11 . The method of  claim 10  wherein the first buoy is attached at the top of the lower riser. 
   
   
       12 . The method of  claim 2  wherein the first buoy is attached to within about 100 feet of the top of the lower riser. 
   
   
       13 . The method of  claim 10  wherein the second buoy is substantially adjacent the first buoy. 
   
   
       14 . The method of  claim 13  wherein the second buoy is below the first buoy. 
   
   
       15 . The method of  claim 13  wherein the second buoy further comprises a passive buoyancy element. 
   
   
       16 . The method of  claim 13  wherein the first buoy comprises a passive buoy. 
   
   
       17 . The method of  claim 16  wherein the first buoy is a passive buoy and wherein the first buoy is integrally attached to the lower riser. 
   
   
       18 . The method of  claim 1  further comprising providing a landing ring wherein the landing ring is attached to the lower riser, and wherein the second buoy is adapted to seat on the landing ring by virtue of upward buoyancy. 
   
   
       19 . The method of  claim 18  wherein the second buoy comprises a buoyant collar wherein the buoyant collar comprises:
 an upper surface and a lower surface and a bore extending therebetween;   a keyway disposed in the collar, substantially parallel to the bore, the keyway extending from the upper surface to the lower surface.   
   
   
       20 . The method of  claim 19  wherein the buoyancy collar is formed of a buoyant material. 
   
   
       21 . The method of  claim 19  wherein the bore has a diameter and said keyway has a width and the bore diameter is equal to the keyway width. 
   
   
       22 . The method of  claim 19  wherein the bore has a diameter and the keyway has a width and the bore diameter is larger than the keyway width. 
   
   
       23 . The method of  claim 19  wherein the at least one chamber is open to the lower surface. 
   
   
       24 . The method of  claim 19  further comprising a recess disposed in the upper surface of the buoyant collar. 
   
   
       25 . The method of  claim 19  further comprising an orientation notch disposed in the upper surface of the buoyant collar. 
   
   
       26 . The method of  claim 19  wherein the buoyant collar is disc shaped. 
   
   
       27 . The method of  claim 19  wherein the buoyant collar is spherical in shape. 
   
   
       28 . The method of  claim 19  wherein the buoyant collar is cylindrical in shape. 
   
   
       29 . The method of  claim 19  wherein the buoyant collar is asymmetrical in shape. 
   
   
       30 . The method of  claim 19  further comprising providing a centralizer element wherein the centralizer element is attached to the lower riser. 
   
   
       31 . The method of  claim 30  wherein the centralizer element is a centralizer ring. 
   
   
       32 . The method of  claim 1  wherein the buoyancy fluid comprises a gas wherein the gas is air or nitrogen. 
   
   
       33 . The method of  claim 30  wherein the disconnect joint comprises an upper disconnect joint and a lower disconnect joint and further comprising providing a blow out preventer stack wherein the blow out preventer stack is attached to the lower riser. 
   
   
       34 . The method of  claim 3  further comprising:
 wherein the marine riser system further comprises a first buoy attached to the lower riser;   providing a landing ring wherein the landing ring is attached to the lower riser, and wherein the second buoy is adapted to mate with and removably attach to the landing ring so as to enable the second buoy to removably affix to the lower riser;   wherein the second buoy is non-integral to the lower riser and wherein the second buoy is removably attached to the lower riser;   wherein the second buoy is substantially adjacent the first buoy;   wherein the first buoy comprises a passive buoy;   wherein the first buoy is attached at substantially the top of the lower riser retrieving the upper riser;   wherein the disconnect joint comprises an upper disconnect joint and a lower disconnect joint and further comprising providing a blow out preventer stack wherein the blow out preventer stack is attached to the lower riser;   detaching the second buoy from the lower riser;   providing a third stand-alone buoy disposed for receipt of the second buoy when the second buoy is detached from the lower riser wherein the third stand-alone buoy comprises a passively buoyant material and wherein stand-alone buoy is secured to the ocean floor by a plurality of mooring cables;   retrieving and disassembling the lower riser;   storing the second buoy in the third stand-alone buoy;   wherein the second buoy comprises a buoyant collar that is formed of buoyant material wherein the buoyant collar comprises:
 an upper surface and a lower surface and a bore extending therebetween; and 
 a keyway disposed in the collar, substantially parallel to the bore, the keyway extending from the upper surface to the lower surface. 
   
   
   
       35 . The method of  claim 1  wherein the lower riser and any other devices connected thereto are together substantially neutrally buoyant before the step of disconnecting the upper riser from the lower riser so as to prevent the lower riser from forcefully rising up upon disconnecting the upper riser from the lower riser. 
   
   
       36 . A buoyancy tensioning system for a marine riser, said buoyancy system comprising:
 a riser string defined by a first end and a second end;   a disconnect joint disposed in said riser string;   a blow out preventer stack attached to said second end of said riser string;   a first buoy attached to said riser string adjacent to said disconnect joint;   a landing ring attached to said riser string between said first buoy and said second end; and   a second buoy seated on said landing ring is seated, said second buoy comprising at least one chamber for receipt of a buoyancy fluid.   
   
   
       37 . The system of  claim 36 , further comprising a third stand alone buoy disposed for receipt of said second buoy when said second buoy is not mounted on by said landing ring. 
   
   
       38 . The system of  claim 36 , further comprising a centralizer mounted on said riser string between said landing ring and said second end of said riser string. 
   
   
       39 . A buoy system for an oil and gas marine riser string, said buoy system comprising:
 a first housing comprised of buoyant material, said first housing defined by a top wall, a bottom wall and a side wall joining said top and bottom walls, said walls defining an interior compartment within said housing, wherein said top wall is provided with an aperture and an opening is provided in at least a side wall or the bottom wall; and   a second housing comprised of buoyant material, said second housing sized to pass through said opening in a wall of said first housing, said second housing having an aperture passing axially therethrough and at least one chamber for receipt of a buoyancy fluid.   
   
   
       40 . A buoyancy system for use with an offshore oil and gas riser string, said buoyancy system comprising:
 a buoyant collar comprising:   an upper surface and a lower surface and a bore extending therebetween;   a keyway disposed in said collar, substantially parallel to said bore, said keyway extending from said upper surface to said lower surface; and   at least one chamber into which buoyancy fluid can be pumped.   
   
   
       41 . The system of  claim 40  wherein said buoyant collar is formed of buoyant material. 
   
   
       42 . The system of  claim 40  wherein said bore has a diameter and said keyway has a width and said bore diameter is equal to said keyway width. 
   
   
       43 . The system of  claim 40  wherein said bore has a diameter and said keyway has a width and said bore diameter is larger than said keyway width. 
   
   
       44 . The system of  claim 40  wherein said chamber is open to said lower surface. 
   
   
       45 . The system of  claim 40  further comprising a seat disposed in the top surface of said collar. 
   
   
       46 . The system of  claim 40  further comprising an orientation notch disposed in the surface of said collar. 
   
   
       47 . The system of  claim 40  wherein said buoyant collar is disc shaped. 
   
   
       48 . The system of  claim 40  wherein said buoyant collar is spherical in shape. 
   
   
       49 . The system of  claim 40  wherein said buoyant collar is cylindrical in shape. 
   
   
       50 . The system of  claim 40  wherein said buoyant collar is asymmetrical in shape. 
   
   
       51 . A method for installing an offshore oil and gas riser string between a rig at the surface of the ocean and a wellhead adjacent the ocean floor, said method comprising the steps of:
 securing a first buoy having a second buoy removably disposed therein adjacent the ocean floor;   attaching a blowout preventer to the lower end of a riser string;   attaching a landing ring on said riser string above said blowout preventer;   attaching a third buoy on said riser string above said landing ring;   attaching a riser disconnect joint in said riser string above said third buoy;   attaching riser joints above said riser disconnect joint;   positioning said riser string adjacent said first buoy;   moving said riser string so as to engage the second buoy;   moving said riser string with the engaged second buoy away from said first buoy; and   attaching said blowout preventer adjacent a wellhead on the ocean floor.   
   
   
       52 . The system of  claim 51  wherein the step of moving the riser string to engage the second buoy comprises passing the riser string through a slot in the first buoy, axially aligning said riser string with a bore defined in said first buoy and causing said riser string to move axially downward until said second buoy seats against said landing ring. 
   
   
       53 . A method for moving an offshore oil and gas rig attached to a riser extending down to the ocean floor and attached to a blowout preventer, said method comprising the steps of:
 providing a riser with a first buoy secured adjacent a disconnect joint and a second buoy secured to said riser below said first buoy, wherein said riser system has an upper riser portion above said disconnect joint and a lower riser portion below said disconnect joint;   pumping a fluid into said second buoy so as to increase the buoyancy of said second buoy and thereby the tension on the lower riser portion of said riser string; and   disconnecting said rig and upper riser portion from said lower riser portion at said disconnect joint.   
   
   
       54 . A passive buoyancy system for attaching to and providing buoyancy for a marine riser comprising:
 a support mandrel wherein the support mandrel is capable of integrally attaching to a marine riser;   a plurality of stackable elements stacked on the support mandrel;   wherein each stackable element comprises a buoyant material; and   wherein each stackable element is substantially in the shape of a disc with a bore therethrough and a keyway for allowing each stackable element to slide onto the support mandrel to affix to the support mandrel.   
   
   
       55 . The passive buoyancy system of  claim 54  wherein each stackable element has an upper surface and a lower surface and wherein each upper surface comprises a raised section for interfacing and interlocking with an adjacent disc. 
   
   
       56 . The passive buoyancy system of  claim 55  wherein each stackable element comprises a mesh netting having a buoyant material within the mesh netting. 
   
   
       57 . The passive buoyancy system of  claim 55  wherein the plurality of stackable elements comprise:
 a plurality of clockwise stackable elements;   a plurality of counterclockwise stackable elements.   
   
   
       58 . The passive buoyancy system of  claim 57  wherein each of the clockwise stackable elements are adapted to mate with each of the counterclockwise stackable elements when each of the clockwise stackable elements is stacked adjacent to each of the counterclockwise stackable elements and when each of the clockwise stackable elements is rotated to mate and lock with each of the counterclockwise stackable elements. 
   
   
       59 . The passive buoyancy system of  claim 55  wherein each stackable disc further comprises a plurality of lifting pockets for receiving a lifting arm for manipulating and orientating each stackable disc onto the support mandrel. 
   
   
       60 . A disc handling device for assembling a passive buoyancy system for integration of the passive buoyancy system into a marine riser
 a frame; and   a plurality of lifting arms attached to the frame with a means for interfacing with, retrieving, lifting, and rotating a plurality of stackable elements from a transport spool and a means for rotating and lowering the stackable elements onto a support mandrel.   
   
   
       61 . A method for assembling a passive buoyancy system for integration of the passive buoyancy system into a marine riser comprising:
 providing a disc handling device wherein the disc handling device comprises a frame, and a plurality of lifting arms attached to the frame with a means for interfacing with, retrieving, lifting, and rotating a plurality of stackable elements from a support spool and a means for rotating and lowering the stackable elements onto a support mandrel;   providing a transport spool;   providing a support mandrel wherein the support mandrel is capable of integrally attaching to a marine riser;   providing a plurality of stackable elements stacked on the transport spool wherein each stackable element comprises a buoyant material wherein each stackable element is substantially in the shape of a disc with a bore therethrough and a keyway for allowing each stackable element to slide onto a shaft wherein each stackable element comprises an orientation element for interfacing and locking with an adjacently stacked stackable element;   interfacing each stackable element with the plurality of lifting arms;   lifting each stackable element a distance apart from each adjacently stacked stackable element;   rotating each stackable element so as to configure each keyway into alignment with one another;   sliding the support mandrel through the aligned keyways of each stackable element so as to dispose the support mandrel substantially within the bore of each stackable element;   rotating each stackable element to an angle that allows the orientation notch of each stackable disc to interface and lock with each adjacently stacked stackable element; and   lowering and loading each stackable element onto the support mandrel.   
   
   
       62 . The method of  claim 61  further comprising integrally attaching the lower end of the support mandrel to the marine riser. 
   
   
       63 . The method of  claim 62  further comprising integrally attaching the upper end of the support mandrel to a disconnect joint. 
   
   
       64 . A method of assembling a marine riser system between an offshore rig at the surface of an ocean and a wellhead adjacent the ocean floor, the method comprising the steps of:
 providing a marine riser system comprising:
 a riser comprising an upper riser and a lower riser, 
 a disconnect joint removably attaching the upper riser to the lower riser, and 
 a hang-off ring attached to the lower riser; 
   providing a second buoy having at least one chamber into which buoyancy fluid may be introduced wherein the second buoy is moored to the seabed by one or more mooring lines wherein the second buoy is configured to mate with the hang-off ring of the lower riser;   mating the marine riser system to the second buoy via the hang-off ring;   introducing a buoyancy fluid into the at least one chamber of the second buoy; and   disconnecting the upper riser from the lower riser at the disconnect joint.   
   
   
       65 . The method of  claim 64  further comprising a spring buoy attached to at least one of the mooring lines.