US3602174A

US3602174A - Transfer riser system for deep suboceanic oilfields

Info

Publication number: US3602174A
Application number: US837290A
Authority: US
Inventors: Francis T Gorman
Original assignee: North American Rockwell Corp
Current assignee: Boeing North American Inc
Priority date: 1969-06-27
Filing date: 1969-06-27
Publication date: 1971-08-31
Anticipated expiration: 1988-08-31

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.

Description

United States Patent Inventor Francis T. Gorman Lomlta, Calif. Appl. No. 837,290 Filed v June 27, 1969 Patented. Aug. 31, 1971 Assignee North Amerimn Rockwell Corporation TRANSFER RISER SYSTEM FOR DEEP SUBOCEANIC OILFIELDS 8 Claims, 2 Drawing Figs.

US. Cl 114/05, 9/8

Int. Cl ..B63b 35/00, B63b 21/52 Field ofSearch ..'...ll4/0.5,0.5

[56] References Cited UNITED STATES PATENTS 3,111,926 11/1963 Shatto 114/206 3,409,055 1 1/1968 Bily 9/8 (0?) X Primary ExaminerTrygve M. Blix Attorneys-William R. Lane, Allan Rothenberg and Harold H.

Card, Jr.

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.

PATENTEDAUBB] I971 3.602.174

INVENTOR. FRANCIS T. GORMAN k xm A T TORNE Y TRANSFER RISER SYSTEM FOR DEEP SUBOCEANIC OILFIELDS 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. I-Iollow 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 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 US Pat. No. 3,236,266 issued Feb. 22, I966. 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 l4-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 US. 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 oneend 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 l6 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 ofU.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 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 I2, 14, and 16 are each maintained in straight condition, also that links 12 and I4 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.

I claim:

I. 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:

a first elongate and straight riser link extending horizontally from said first storage facility to a relatively fixed anchored joint in fixed relationship with said first storage facility to maintain said first link substantially stationary,

a second elongate and straight riser link extending from said anchored joint to a movable joint disposed to move vertically and restrained from moving horizontally and communicating with said first riser link, and

a third elongate and straight riser link extending substantially vertically from said vertically movable joint to said second storage facility and communicating with said second riser link,

said first and second riser links being substantially normal to each other and defining a substantially horizontal plane, and

said second and third riser links defining a substantially vertical plane.

2. The system set forth in claim 1 above, wherein:

said anchored joint is adapted for pivotal movement between said first and second riser links about a substantially horizontal axis defined by the longitudinal center axis of said elongate first riser link.

3. The system set forth in claim 2 above, wherein;

said vertically movable joint is adapted for pivotal movement between said second and third riser links about a horizontal axis normal to the longitudinal center axis of said elongate second riser link.

4. The system set forth in claim 3 above, wherein:

said third riser link is secured to said second storage facility at the uppermost end of said third riser link, and

said system further includes at least one weight secured to said third riser link proximate the lowermost end thereof to maintain said third riser link under substantially continuous tensile stress.

5. The system set forth in claim 4 above, further including:

a plurality of mooring lines secured at one end thereof to said second storage facility and at the other end thereof to a corresponding plurality of anchors for limiting excursions of said second storage facility.

6. The system set forth in claim 5 above, further including:

a weight suspended from each of said mooring lines proximate said one end thereof,

a tether line extending from each of said weights to each of said mooring lines,

a lateral load-reaction line connected between each of said weights and said third riser link so that lateral movement of said third riser link away from any one of said mooring lines causes the lateral load-reaction line connected to the particular weight suspended from said any one mooring line to apply lifting force to said particular weight.

7. A spider mooring system for use with a generally vertical riser connected to a floating storage facility, said system com-- prising:

at least three generally downwardly depending mooring lines secured to said storage facility and defining a corresponding number of substantially equal angles in plan view with said riser located at the apices of said angles,

anchor means connected to the lower end of each of said mooring lines, said anchor means being spaced apart substantially equidistantly,

weight means suspended from each of said three mooring lines, and

lateral connection means between said weight.

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:

a first elongate and straight riser link extending from said first storage facility to an anchored joint in fixed relationship with said first storage facility,

a second elongate and straight riser link extending from said anchored joint to a vertically movable joint and communicating with said first riser link, and

a third elongate and straight riser link extending substan tially vertically from said vertically movable joint to said second storage facility and communicating with said second riser link,

said first and second riser links being substantially horizontal plane, and

said second and third riser links defining a substantially vertical plane,

said anchored joint being adapted for pivotal movement between said first and second riser links about a substantially horizontal axis defined by the longitudinal center axis of said elongate first riser link,

said vertically movable joint being adapted for pivotal movement between said second and third riser links about a horizontal axis normal to the longitudinal center axis of said elongate second riser link,

said third riser link being secured to said second storage facility at the uppermost end of said third riser link, and

at least one weight secured to said third riser link proximate a tether line extending from each of said weights to each of said mooring lines,

zg g UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,602,17h a d Angust 31, 1971 Inventofla) Francis T. German It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as lhown below:

Column Claim 7 Delete the period at end of Claim 7 and insert the following lines which were omitted in error.

"means and said riser so that some of the weight of said weight means is carried by the riser to cause the lifting of said weight means against the force of gravity when said riser moves laterally away from any of said mooring lines.

Signed and sealed this 21 st day of March 1972.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTISCHALK Attesting Officer Commissioner of Patents

Claims

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: a first elongate and straight riser link extending horizontally from said first storage facility to a relatively fixed anchored joint in fixed relationship with said first storage facility to maintain said first link substantially stationary, a second elongate and straight riser link extending from said anchored joint to a movable joint disposed to move vertically and restrained from moving horizontally and communicating with said first riser link, and a third elongate and straight riser link extending substantially vertically from said vertically movable joint to said second storage facility and communicating with said second riser link, said first and second riser links being substantially normal to each other and defining a substantially horizontal plane, and said second and third riser links defining a substantially vertical plane.

2. The system set forth in claim 1 above, wherein: said anchored joint is adapted for pivotal movement between said first and second riser links about a substantially horizontal axis defined by the longitudinal center axis of said elongate first riser link.

3. The system set forth in claim 2 above, wherein; said vertically movable joint is adapted for pivotal movement between said second and third riser links about a horizontal axis normal to the longitudinal center axis of said elongate second riser link.

4. The system set forth in claim 3 above, wherein: said third riser link is secured to said second storage facility at the uppermost end of said third riser link, and said system further includes at least one weight secured to said third riser link proximate the lowermost end thereof to maintain said third riser link under substantially continuous tensile stress.

5. The system set forth in claim 4 above, further including: a plurality of mooring lines secured at one end thereof to said second storage facility and at the other end thereof to a corresponding plurality of anchors for limiting excursions of said second storage facility.

6. The system set forth in claim 5 above, further including: A weight suspended from each of said mooring lines proximate said one end thereof, a tether line extending from each of said weights to each of said mooring lines, a lateral load-reaction line connected between each of said weights and said third riser link so that lateral movement of said third riser link away from any one of said mooring lines causes the lateral load-reaction line connected to the particular weight suspended from said any one mooring line to apply lifting force to said particular weight.

7. A spider mooring system for use with a generally vertical riser connected to a floating storage facility, said system comprising: at least three generally downwardly depending mooring lines secured to said storage facility and defining a corresponding number of substantially equal angles in plan view with said riser located at the apices of said angles, anchor means connected to the lower end of each of said mooring lines, said anchor means being spaced apart substantially equidistantly, weight means suspended from each of said three mooring lines, and lateral connection means between said weight.

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: a first elongate and straight riser link extending from said first storage facility to an anchored joint in fixed relationship with said first storage facility, a second elongate and straight riser link extending from said anchored joint to a vertically movable joint and communicating with said first riser link, and a third elongate and straight riser link extending substantially vertically from said vertically movable joint to said second storage facility and communicating with said second riser link, said first and second riser links being substantially horizontal plane, and said second and third riser links defining a substantially vertical plane, said anchored joint being adapted for pivotal movement between said first and second riser links about a substantially horizontal axis defined by the longitudinal center axis of said elongate first riser link, said vertically movable joint being adapted for pivotal movement between said second and third riser links about a horizontal axis normal to the longitudinal center axis of said elongate second riser link, said third riser link being secured to said second storage facility at the uppermost end of said third riser link, and at least one weight secured to said third riser link proximate the lowermost end thereof to maintain said third riser link under substantially continuous tensile stress, a plurality of mooring lines secured at one end thereof to said second storage facility and at the other end thereof to a corresponding plurality of anchors for limiting excursions of said second storage facility, a weight suspended from each of said mooring lines proximate said one end thereof, a tether line extending from each of said weights to each of said mooring lines, a lateral load-reaction line connected between each of said weights and said third riser link so that lateral movement of said third riser link away from any one of said mooring lines causes the lateral load-reaction line connected to the particular weight suspended from said any one mooring line to apply lifting force to said particular weight.