Title:
TRANSFER RISER SYSTEM FOR DEEP SUBOCEANIC OILFIELDS
United States Patent 3602174
Abstract:
A transfer system for offshore petroleum production has a vertically movable riser extending from the ocean surface to the ocean floor and is attachable to a surface vessel during flowing operation. To accommodate the wide range of vertical movement associated with varying wind and the tide effects on the surface vessel, the riser is formed by a three-element linkage, each element being restrained to limit its movement to not more than one dominant direction. The articulation of the riser prevents excessive structural loads from being imposed thereon due to such vertical movement.
US Patent References:
Apparatus for anchoring underwater vessels
Shatto - November 1963 - 3111926
Apparatus for handling liquid cargo
Bily - November 1968 - 3409055
Apparatus for anchoring underwater vessels
Shatto - November 1963 - 3111926
Apparatus for handling liquid cargo
Bily - November 1968 - 3409055
Application Number:
04/837290
Publication Date:
08/31/1971
Filing Date:
06/27/1969
View Patent Images:
Export Citation:
Primary Class:
Other Classes:
441/4
International Classes:
B63B35/44; E21B17/01; E21B17/00; B63B35/00; B63B21/52
Field of Search:
114/.5,.5D,206,230 9/8P 175/7
Primary Examiner:
Blix, Trygve M.
Claims:
I claim
1. In a conduit system for transferring fluids from a first storage facility on an ocean floor to a second storage facility on the ocean surface:
2. The system set forth in claim 1 above, wherein:
3. The system set forth in claim 2 above, wherein;
4. The system set forth in claim 3 above, wherein:
5. The system set forth in claim 4 above, further including:
6. The system set forth in claim 5 above, further including:
7. A spider mooring system for use with a generally vertical riser connected to a floating storage facility, said system comprising:
8. In a conduit system for transferring fluids from a first storage facility on an ocean floor to a second storage facility on the ocean surface:
1. In a conduit system for transferring fluids from a first storage facility on an ocean floor to a second storage facility on the ocean surface:
2. The system set forth in claim 1 above, wherein:
3. The system set forth in claim 2 above, wherein;
4. The system set forth in claim 3 above, wherein:
5. The system set forth in claim 4 above, further including:
6. The system set forth in claim 5 above, further including:
7. A spider mooring system for use with a generally vertical riser connected to a floating storage facility, said system comprising:
8. In a conduit system for transferring fluids from a first storage facility on an ocean floor to a second storage facility on the ocean surface:
Description:
BACKGROUND OF THE INVENTION
Offshore oil and gas production operations necessitate some means of collecting and conveying the mined products obtained from the ocean floor to the ocean surface. Hollow conduits or risers used for the mentioned purpose are required to be strong and impervious to chemical or other attack from the mined products, from sea water, and from oceanic life. Due to their structural complexity and the types of materials necessary to provide the foregoing properties, such risers are relatively massive, somewhat rigid and substantially inelastic. In addition, they are very expensive and difficult to splice or repair in the event of breakage.
Accordingly, prevention of overstress due to excessive loads on risers of the stated type is crucially important, particularly in waters of great depth or in the open sea where winds, tides, and other conditions are widely variant and sometimes severe. Catenary or other curved shapes are completely out of the question in rigging heavy risers of the type contemplated in this case, particularly due to the fact that the minimum radius of curvature permitted in such massive conduits is so great that a riser three or four times longer would be necessary to traverse the considerable distance between ocean floor and ocean surface than a straight riser, and the cost penalty of such increased length, both for the riser itself and the complex mooring system it would require, would be prohibitive.
SUMMARY OF THE INVENTION
The invention comprises a three-link system for a riser adapted to communicate a collection tank situated at extreme suboceanic depths to a storage facility on the ocean surface using an arrangement whereby large variations in the vertical position of the surface facility will not produce substantial changes of unit stress either locally or throughout the total length of the riser. Thus, referring to FIG. 1 of the drawing, a stationary horizontal run of conduit or riser 12 extends between collection tank 10 and neutrally buoyant pivot connection 20 which is substantially fixed in location by suitable anchoring and float means. A second portion or run of riser 14 is connected to run 12 at pivot joint 20 whereby run 14 is pivotally movable in a vertical direction about a pivot point through joint 20. The end of run 14 opposite from its connection with joint 20 is connected to a vertical riser portion 16 through a second pivotal joint 36 which is restrained against lateral movement within narrow limits but is relatively free to move vertically. Due to the articulation of the riser system wherein vertical movement of joint 36 results in substantially constant stress on portion 16 of the riser and substantial avoidance of any resulting stress on portions 12 or 14 of the riser, the risk of overstress or breakage which would otherwise result from wide variation in the vertical position of surface facility 18 is virtually eliminated.
As a result of the lateral restraint against movement of pivot joint 36, lateral restraint is also required in respect of surface facility 18 and is provided by a mooring system which includes cables or lines 52, 54, and 56. The three mentioned lines are each connected to riser portion 16 in the manner suggested by FIG. 2 showing the connection between line 56 and the riser, the connections for lines 52 and 54 being identical to that shown for line 56. Thus, weight 64 is suspended from line 56 by two separate and independent chains 64 and 76 and to the riser by a third independent chain 82. Lateral movement of the riser toward the left in the view shown by FIG. 2 results in lifting force applied to weight 64 through chain 82. Such lifting force is naturally resisted by the weight of mass 64 which tends to restore the riser to the original position it had before the mentioned lateral movement occurred. The mooring system thus shown in FIG. 2 is self-correcting and avoids the risk of entanglement between lines 52, 54, and 56 with riser portion 16 which might otherwise result from lateral excursions of storage facility 18.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a general prospective schematic view of the riser and mooring system contemplated in this case, and
FIG. 2 is an isolated elevational view of a portion of the mooring system shown in FIG. 1.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to FIG. 1, tank 10 illustratively represents a collection or storage container for oil or gas accumulated from one or more suboceanic well drillings (not shown). A hollow conduit or riser essentially comprising three separate portions or links 12, 14, and 16 communicates collection tank 10 with a storage facility 18 at the ocean surface. Riser portion 12 is essentially stationary and rigidly connected to tank 10 by any suitable fixed joint (not shown), the details of which may take any convenient form known to the prior art and are not germane to the concept in this case. Portion 12 extends from tank 10 to a pivotal conduit joint 20 which is moored, anchored or otherwise secured by suitable means to prevent any vertical or lateral movement thereof relative to tank 10. Joint 20 may, for example, take the form generally disclosed for joint 17 in U.S. Pat. No. 3,236,266 issued Feb. 22, 1966. Illustratively, the mentioned securing means for joint 20 may include a deadweight 22 lodged on the ocean floor and a line 24 connecting the weight with joint 20 and also to a float 26 which applies tensile force to maintain the joint a constant distance above the weight. In addition, two or more anchors or the like may be connected to joint 20 to restrain the same against horizontal movement, such as anchors 28 and 30 operatively associated with lines 32 and 34, respectively.
Link or section 14 of the riser is connected to portion 12 thereof through joint 20 and is pivotal about an axis coinciding with the center longitudinal axis of portion 12. Link 14 extends between joint 20 and a second pivotal joint 36 which is vertically movable throughout a relatively large range of movement although restraining force biasing joint 36 against vertical movement is provided by suitable anchor means such as suggested by anchors 38, 40, and 42 connected to joint 36 by lines 44, 46, and 48, respectively. Joint 36 may take any detailed form known to the prior art and which permits pivotal movement of the joint about an axis 50 substantially perpendicular to the longitudinal axis of riser section 14 and coplanar therewith. Illustratively, joint 36 may conform generally with pivotal joint 20 taught by mentioned U.S. Pat. No. 3,236,266, but with extraneous portions thereof removed, since joint 36 in this case may be a simple connection between two conduit or riser portions 14 and 16 and need permit pivotal movement between conduit 16 and 14 about a single axis 50 which defines the necessary direction of movement between the riser portions.
Riser link or section 16 extends substantially vertically from joint 36 to surface facility 18 and is secured thereto by any suitable means such as known to the prior art, the details of which are irrelevant to the total system concept disclosed herein. Mooring means are provided as shown in FIG. 1 to stabilize floating facility 18 against lateral movement or excursions in any direction within narrow limits, the stated means including lines 52, 54, and 56 secured at one end thereof to facility 18 and at the lowermost end thereof to separate anchor means of any convenient form (not shown). Means are further provided to stabilize the relative position between riser 16 and each of mooring lines 52, 54, and 56. The stated means include a separate and independent weight or mass suspended from the mentioned mooring lines, as suggested by masses 60, 62, and 64 operatively associated with lines 52, 54, and 56, respectively. Vertical suspension lines for supporting most of the weight of masses 60, 62, and 64 are provided in the form of separate suspension lines 66, 68, and 70, while lateral tether lines 72, 74, and 76 extend between masses 60, 62, and 64 and mooring lines 52, 54, and 56, respectively. Load reaction lines 78, 80, and 82 connect each of the masses 60, 62, and 64, respectively, with riser portion 16 at a location intermediate joint 36 and floating facility 18.
Referring particularly to FIG. 2, the connections between riser length 16 and line 56 are shown in isolated schematic form, it being understood that the connections between lines 52 and 54 with riser 16 are identical to those shown with respect to line 56. Thus, mass 64 is nonbuoyant and subject to the pull of gravity in a vertically downward direction as viewed in FIG. 2, the mentioned gravitational force being resisted primarily by suspension line 70 which may take the form of a chain where desirable or convenient. Line 70 is connected to line 56 in any convenient manner such as the ring connection 32 shown in FIG. 3 of U.S. Pat. No. 3,138,135 issued June 23, 1964. Line 56, being connected to buoyant vessel or facility 18 naturally aids in resisting the gravitational pull on weight 64 by reason of its connection between line 70 and facility 18. Lines 76 and 82 are also connected between mass 64 and lines 56 and riser 16, respectively, and thus transmit lateral forces between mass 64 and each of the mentioned items to which the mass is thus connected. From the foregoing arrangements, it will be understood that as riser section 16 attempts to move toward the left, for example, in the view shown by FIG. 2, force will be applied through connection 82 resulting in a slight lifting of mass 64 above the position which the mass would normally have when riser 16 is centered equidistantly between lines 52, 54, and 56 This lifting of weight 64 is accompanied by slight lateral movement thereof whereby lines 76 and 82 tend to become longitudinally aligned with each other instead of in the slight angular relationship shown by FIG. 2. The lifting movement of mass 64 also results in a slackening of line or chain 70, the gravitational force on mass 64 being resisted primarily by lines 76 and 82 in the disturbed state of unbalance resulting from lateral movement of riser section 16. The stated gravitational force naturally continues with regard to mass 64 during all conditions of movement or disturbance, and results in a counteracting force applied to line 82 tending to pull riser 16 towards the right in the view shown by FIG. 2. The mentioned self-correcting action achieved by masses 60, 62, and 64 in combination with the mooring arrangement shown by FIG. 1, may be seen to result in continuous automatic self-correction of any external loads applied to riser section 16 or facility 18 which would otherwise tend to move the riser laterally toward any of the mooring lines 52, 54, and 56. The suspension system thus shown in FIG. 1 and particularly in FIG. 2 is most effective in preventing entanglement between riser section 16 and the mentioned mooring lines.
From the discussion set forth above, it will be understood that riser section 12 should have sufficient length so that any binding or other failure of joint 20 to pivot in a substantially vertical plane will not produce damaging torsional stresses or severe angular displacement of joint 20. Joint 36 heaves vertically over a wide range as required to accommodate different vertical positions of facility 18 relative to the ocean floor. Riser length 14 is subjected to hydrodynamic stresses due to its relative movement through the water during heaving of joint 36, the mentioned stresses being naturally greater at the end of portion 14 closest to joint 36. One or more weights or nonbouyant masses may be attached to or otherwise secured to riser portion 16 proximate the lower end thereof as suggested by masses 84 and 86 in FIG. 1 to minimize bending stresses in the riser due to the resulting increase in tensile force throughout link 16 between its upper attachment point with facility 18 and its lower connection with the mentioned weights. It may further be noticed that elongate riser links 12, 14, and 16 are each maintained in straight condition, also that links 12 and 14 define a substantially horizontal plane while links 14 and 16 define a substantially vertical plane. It is the foregoing relationship which results in a minimum of stress in links 12 and 14, and substantially pure and constant tensile stress in link 16, all of which minimizes the risk of riser damage or breakage.
Offshore oil and gas production operations necessitate some means of collecting and conveying the mined products obtained from the ocean floor to the ocean surface. Hollow conduits or risers used for the mentioned purpose are required to be strong and impervious to chemical or other attack from the mined products, from sea water, and from oceanic life. Due to their structural complexity and the types of materials necessary to provide the foregoing properties, such risers are relatively massive, somewhat rigid and substantially inelastic. In addition, they are very expensive and difficult to splice or repair in the event of breakage.
Accordingly, prevention of overstress due to excessive loads on risers of the stated type is crucially important, particularly in waters of great depth or in the open sea where winds, tides, and other conditions are widely variant and sometimes severe. Catenary or other curved shapes are completely out of the question in rigging heavy risers of the type contemplated in this case, particularly due to the fact that the minimum radius of curvature permitted in such massive conduits is so great that a riser three or four times longer would be necessary to traverse the considerable distance between ocean floor and ocean surface than a straight riser, and the cost penalty of such increased length, both for the riser itself and the complex mooring system it would require, would be prohibitive.
SUMMARY OF THE INVENTION
The invention comprises a three-link system for a riser adapted to communicate a collection tank situated at extreme suboceanic depths to a storage facility on the ocean surface using an arrangement whereby large variations in the vertical position of the surface facility will not produce substantial changes of unit stress either locally or throughout the total length of the riser. Thus, referring to FIG. 1 of the drawing, a stationary horizontal run of conduit or riser 12 extends between collection tank 10 and neutrally buoyant pivot connection 20 which is substantially fixed in location by suitable anchoring and float means. A second portion or run of riser 14 is connected to run 12 at pivot joint 20 whereby run 14 is pivotally movable in a vertical direction about a pivot point through joint 20. The end of run 14 opposite from its connection with joint 20 is connected to a vertical riser portion 16 through a second pivotal joint 36 which is restrained against lateral movement within narrow limits but is relatively free to move vertically. Due to the articulation of the riser system wherein vertical movement of joint 36 results in substantially constant stress on portion 16 of the riser and substantial avoidance of any resulting stress on portions 12 or 14 of the riser, the risk of overstress or breakage which would otherwise result from wide variation in the vertical position of surface facility 18 is virtually eliminated.
As a result of the lateral restraint against movement of pivot joint 36, lateral restraint is also required in respect of surface facility 18 and is provided by a mooring system which includes cables or lines 52, 54, and 56. The three mentioned lines are each connected to riser portion 16 in the manner suggested by FIG. 2 showing the connection between line 56 and the riser, the connections for lines 52 and 54 being identical to that shown for line 56. Thus, weight 64 is suspended from line 56 by two separate and independent chains 64 and 76 and to the riser by a third independent chain 82. Lateral movement of the riser toward the left in the view shown by FIG. 2 results in lifting force applied to weight 64 through chain 82. Such lifting force is naturally resisted by the weight of mass 64 which tends to restore the riser to the original position it had before the mentioned lateral movement occurred. The mooring system thus shown in FIG. 2 is self-correcting and avoids the risk of entanglement between lines 52, 54, and 56 with riser portion 16 which might otherwise result from lateral excursions of storage facility 18.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a general prospective schematic view of the riser and mooring system contemplated in this case, and
FIG. 2 is an isolated elevational view of a portion of the mooring system shown in FIG. 1.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to FIG. 1, tank 10 illustratively represents a collection or storage container for oil or gas accumulated from one or more suboceanic well drillings (not shown). A hollow conduit or riser essentially comprising three separate portions or links 12, 14, and 16 communicates collection tank 10 with a storage facility 18 at the ocean surface. Riser portion 12 is essentially stationary and rigidly connected to tank 10 by any suitable fixed joint (not shown), the details of which may take any convenient form known to the prior art and are not germane to the concept in this case. Portion 12 extends from tank 10 to a pivotal conduit joint 20 which is moored, anchored or otherwise secured by suitable means to prevent any vertical or lateral movement thereof relative to tank 10. Joint 20 may, for example, take the form generally disclosed for joint 17 in U.S. Pat. No. 3,236,266 issued Feb. 22, 1966. Illustratively, the mentioned securing means for joint 20 may include a deadweight 22 lodged on the ocean floor and a line 24 connecting the weight with joint 20 and also to a float 26 which applies tensile force to maintain the joint a constant distance above the weight. In addition, two or more anchors or the like may be connected to joint 20 to restrain the same against horizontal movement, such as anchors 28 and 30 operatively associated with lines 32 and 34, respectively.
Link or section 14 of the riser is connected to portion 12 thereof through joint 20 and is pivotal about an axis coinciding with the center longitudinal axis of portion 12. Link 14 extends between joint 20 and a second pivotal joint 36 which is vertically movable throughout a relatively large range of movement although restraining force biasing joint 36 against vertical movement is provided by suitable anchor means such as suggested by anchors 38, 40, and 42 connected to joint 36 by lines 44, 46, and 48, respectively. Joint 36 may take any detailed form known to the prior art and which permits pivotal movement of the joint about an axis 50 substantially perpendicular to the longitudinal axis of riser section 14 and coplanar therewith. Illustratively, joint 36 may conform generally with pivotal joint 20 taught by mentioned U.S. Pat. No. 3,236,266, but with extraneous portions thereof removed, since joint 36 in this case may be a simple connection between two conduit or riser portions 14 and 16 and need permit pivotal movement between conduit 16 and 14 about a single axis 50 which defines the necessary direction of movement between the riser portions.
Riser link or section 16 extends substantially vertically from joint 36 to surface facility 18 and is secured thereto by any suitable means such as known to the prior art, the details of which are irrelevant to the total system concept disclosed herein. Mooring means are provided as shown in FIG. 1 to stabilize floating facility 18 against lateral movement or excursions in any direction within narrow limits, the stated means including lines 52, 54, and 56 secured at one end thereof to facility 18 and at the lowermost end thereof to separate anchor means of any convenient form (not shown). Means are further provided to stabilize the relative position between riser 16 and each of mooring lines 52, 54, and 56. The stated means include a separate and independent weight or mass suspended from the mentioned mooring lines, as suggested by masses 60, 62, and 64 operatively associated with lines 52, 54, and 56, respectively. Vertical suspension lines for supporting most of the weight of masses 60, 62, and 64 are provided in the form of separate suspension lines 66, 68, and 70, while lateral tether lines 72, 74, and 76 extend between masses 60, 62, and 64 and mooring lines 52, 54, and 56, respectively. Load reaction lines 78, 80, and 82 connect each of the masses 60, 62, and 64, respectively, with riser portion 16 at a location intermediate joint 36 and floating facility 18.
Referring particularly to FIG. 2, the connections between riser length 16 and line 56 are shown in isolated schematic form, it being understood that the connections between lines 52 and 54 with riser 16 are identical to those shown with respect to line 56. Thus, mass 64 is nonbuoyant and subject to the pull of gravity in a vertically downward direction as viewed in FIG. 2, the mentioned gravitational force being resisted primarily by suspension line 70 which may take the form of a chain where desirable or convenient. Line 70 is connected to line 56 in any convenient manner such as the ring connection 32 shown in FIG. 3 of U.S. Pat. No. 3,138,135 issued June 23, 1964. Line 56, being connected to buoyant vessel or facility 18 naturally aids in resisting the gravitational pull on weight 64 by reason of its connection between line 70 and facility 18. Lines 76 and 82 are also connected between mass 64 and lines 56 and riser 16, respectively, and thus transmit lateral forces between mass 64 and each of the mentioned items to which the mass is thus connected. From the foregoing arrangements, it will be understood that as riser section 16 attempts to move toward the left, for example, in the view shown by FIG. 2, force will be applied through connection 82 resulting in a slight lifting of mass 64 above the position which the mass would normally have when riser 16 is centered equidistantly between lines 52, 54, and 56 This lifting of weight 64 is accompanied by slight lateral movement thereof whereby lines 76 and 82 tend to become longitudinally aligned with each other instead of in the slight angular relationship shown by FIG. 2. The lifting movement of mass 64 also results in a slackening of line or chain 70, the gravitational force on mass 64 being resisted primarily by lines 76 and 82 in the disturbed state of unbalance resulting from lateral movement of riser section 16. The stated gravitational force naturally continues with regard to mass 64 during all conditions of movement or disturbance, and results in a counteracting force applied to line 82 tending to pull riser 16 towards the right in the view shown by FIG. 2. The mentioned self-correcting action achieved by masses 60, 62, and 64 in combination with the mooring arrangement shown by FIG. 1, may be seen to result in continuous automatic self-correction of any external loads applied to riser section 16 or facility 18 which would otherwise tend to move the riser laterally toward any of the mooring lines 52, 54, and 56. The suspension system thus shown in FIG. 1 and particularly in FIG. 2 is most effective in preventing entanglement between riser section 16 and the mentioned mooring lines.
From the discussion set forth above, it will be understood that riser section 12 should have sufficient length so that any binding or other failure of joint 20 to pivot in a substantially vertical plane will not produce damaging torsional stresses or severe angular displacement of joint 20. Joint 36 heaves vertically over a wide range as required to accommodate different vertical positions of facility 18 relative to the ocean floor. Riser length 14 is subjected to hydrodynamic stresses due to its relative movement through the water during heaving of joint 36, the mentioned stresses being naturally greater at the end of portion 14 closest to joint 36. One or more weights or nonbouyant masses may be attached to or otherwise secured to riser portion 16 proximate the lower end thereof as suggested by masses 84 and 86 in FIG. 1 to minimize bending stresses in the riser due to the resulting increase in tensile force throughout link 16 between its upper attachment point with facility 18 and its lower connection with the mentioned weights. It may further be noticed that elongate riser links 12, 14, and 16 are each maintained in straight condition, also that links 12 and 14 define a substantially horizontal plane while links 14 and 16 define a substantially vertical plane. It is the foregoing relationship which results in a minimum of stress in links 12 and 14, and substantially pure and constant tensile stress in link 16, all of which minimizes the risk of riser damage or breakage.