US3834432A

US3834432A - Transfer system for suboceanic oil production

Info

Publication number: US3834432A
Authority: US
Inventors: H Lilly; G Morgan
Original assignee: SUBSEA EQUIPMENT ASS Ltd
Current assignee: SUBSEA EQUIPMENT ASS Ltd; SUBSEA EQUIPMENT ASS Ltd GB
Priority date: 1969-09-11
Filing date: 1969-09-11
Publication date: 1974-09-10
Anticipated expiration: 1991-09-10

Abstract

A transfer system for offshore petroleum production has a vertically movable riser extending from a collection tank on the ocean floor to a storage tanker on the ocean surface. The riser is releasably attachable by a pivotal connection to the tanker during flowing operation and disconnectable during storms or otherwise violent sea states. In the disconnected mode, the riser remains submerged under the ocean surface to avoid excessive structural loading. The riser is articulated and moored by a system of weights and floats to maintain tension within acceptable stress limits throughout a wide range of changes in vertical and some horizontal movement of the tanker.

Description

' [111 2 3,834,432 Sept. 10, 1974 TRANSFER SYSTEM FOR SUBOCEANIC OIL PRODUCTION [75] Inventors: Herbert J. Lilly, Jr., Mission Viejo; George W. Morgan, Anaheim, both of Calif.

[73] Assignee: Subsea Equipment Associates Limited, Hamilton, Bermuda [22] Filed: Sept. 11, 1969 [21] Appl. No.: 857,108

3,409,055 11/1968 Bily 137/236 X 3,479,673 ll/1969 Manning 141/387 3,517,110 6/1970 Morgan 61/46 X Primary Examiner-Harold W. Weakley Attorney, Agent, or Firm-Gausewitz, Carr & Rothenberg [5 7] ABSTRACT A transfer system for offshore petroleum production has a vertically movable riser extending from a collection tank on the ocean floor to a storage tanker on the ocean surface. The riser is releasably attachable by a pivotal connection to the tanker during flowing operation and disconnectable during storms or otherwise violent sea states. In the disconnected mode, the riser remains submerged under the ocean surface to avoid excessive structural loading. The riser is articulated and moored by a system of weights and floats to maintain tension within acceptable stress limits throughout a wide range of changes in vertical and some horizontal movement of the tanker.

5 Claims, 6 Drawing Figures TRANSFER SYSTEM FOR SUBOCEANIC OIL PRODUCTION BACKGROUND OF THE INVENTION In offshore petroleum production, offloading of crude oil collected from various points on the ocean floor to a storage tanker or other floating facility is necessary. One method in current wide-spread use involves attachment of a riser to a floating swivel buoy which in turn forms a connection point for storage tankers. During storms or severe sea states, the tanker can be disconnected from the buoy, but the buoy can thereafter tear loose from the riser resulting in damage and possible complete loss of the riser.

Another severe problem in the use of risers is associated with their particular sensitivity to structural overstress, especially in waters of great depth or in offshore locations characterized by wide variation of tide levels. Thus, when a tanker heaves upward and downward clue to surface wave conditions, the riser connected to such tanker cannot usually respond to such movement at a sufficient rate to accommodate the resulting loads, although risers are generally stronger under tensile loads than any other type loading. During the downward displacement of the tanker, compression forces are applied to the riser which tend to move the same downwardly. The huge length, inertia and relative inflexibility of the riser as well as tremendous hydrostatic pressures severely retard its downward movement through the water, whereby compression loads identified with the mentioned operating conditions involve greater risk of damage than any other type of load or stress imposed thereon. Distortion of the riser such as identified with column bending in beams or the like or any distortion resulting in curvature of the riser cannot be accommodated in view of its mass and axial rigidity.

SUMMARY OF THE INVENTION The invention contemplates a riser consisting essentially of two separate runs or

links

12 and 14 for communicating between a collection or storage tank 16 on the ocean floor and a tanker 18 on the ocean surface.

Links

12 and 14 are both substantially straight and are joined to each other by a pivotal connection 20. Diverging mooring lines connect joint 20 with two

floats

26, 27 and two

anchors

28, 29, respectively, as shown in FIG. 2, to maintain link 12 substantially under tension throughout a relatively wide range of vertical and horizontal displacements of joint 20 as suggested in FIG. 1. Weighted mass 30 is secured to joint 20 to apply downward force to vertical link 14, while upward force is applied to the same riser link by buoyant mass or float 32, thus maintaining this portion of the riser under continuous tension during the mentioned range of movement. The upper end of riser 14 is releasably connected to tanker 18 in the offloading operation, and when disconnected therefrom remains submerged under the ocean surface and therefore less exposed to violent forces associated with stormy seas or collision with surface vessels.

Connection of riser 14 with tanker 18 occurs through a mooring swivel 76 on the storage tanker, at the top of which suitable locking means and conduit flow connections are situated.

BRIEF DESCRIPTION OF THE DRAWINGS 0 section, of a detail from the structure shown in FIG. 3,

FIG. 5 shows a transverse view, partly in crosssection, through a storage tanker connected to the riser shown in FIGS. 1 and 2, and

FIG. 6 is a top plan view of the connection system shown in FIG. 5.

DESCRIPTION OF THE PREFERRED EMBODIMENT From FIG. 1, it may be seen that the invention in this case contemplates and includes a collection or storage container 16 for oil or gas accumulated from one or more suboceanic well drillings (not shown) supplied to tank 16 by one or more conduits as suggested by conduit 10. A transfer riser consisting of two sections or

links

12 and 14 communicates the internal area of tank 16 with a storage tanker or other floating facility 18 on the ocean surface. Riser link 12 is operatively joined to tank 16 through pivotal joint 36 which may take any convenient form known to the prior art, including joint 17 shown in US. Pat. No. 3,236,266 issued Feb. 22, 1966 and permitting rotational movement of link 12 in a substantially vertical plane perpendicular to horizontal axis 38 through joint 36 as shown in FIG. 2. Link 12 of the riser is connected to link 14 through a second pivotal joint 20 which is movable in an are defined by a center of rotation coinciding with axis 38. The arcuate path of movement involves both vertical and lateral displacement of joint 20 as suggested by the dash lines in FIG. 1. Restraining means are provided to limit the transition rate at which movement of joint 20 may occur, the stated means including relatively heavy mass or weight 30. Link 12 is rendered neutrally buoyant by suitable hydrostatic balance means such as by securing any required number of floats spaced along the length of link 12 as may be necessary to compensate for the weight and mass distribution of riser link 12, which will naturally include due consideration of the ocean depth and displacement volume of link 12 in any particular installation.

Due to the particular sensitivity of

riser links

12 and 14 to damage by compressive loads applied axially thereto, means are provided in this case to maintain each of the separate links under substantially axial tension. The stated means includes one or more mooring line connections between joint 20 and one or more anchors or dead weights. Preferably, a minimum of two separate mooring line connections are used to apply tensile loading of link 12. Thus, referring to FIG. 2,

lines

22 and 23 are attached to joint 20 and connect the same with buoyant masses or

floats

26 and 27, respectively.

Mooring lines

24 and 25 each connect

floats

26 and 27 with separate anchor or deadweight means 28 and 29, respectively. Anchor means 28 and 29 are positioned on the ocean floor equidistantly from the longitudinal axis of riser link 12 so that lateral movements of link 12 either in a clockwise or counterclockwise direction about joint 36 as shown in FIG. 2 will be resisted by substantially equal reaction loads in

lines

22 and 23. In achieving the foregoing action affects, the included angle 40 between

lines

22 and 23 should preferably be within a range from to 90, the higher range limit being preferred. Tensioning of riser 12 is achieved by both of the separate branches in the same manner and may be illustrated by the one shown in FIG. 1 comprising

elements

22, 24, 26, and 28. Due to buoyancy of float 26, it will be understood that tension is maintained in

lines

22 and 24, for example, connected to joint 20 and anchor 28, respectively, and that a force component due to the stated tension in line 22 pulls joint 20 generally toward the right in the view shown by FIG. 1. Since joint 20 is structurally connected to link 12, the pulling force thus applied to the joint results in tensile loading of link '12 as the link is unable to move axially due to the restraint offered by its connection with joint 36 and stationary tank 16. The vertical position of joint 20, to which downward pulling force is continuously maintained by attachment of weight thereto, is dependent upon the location of the upper end of riser link 14, which in turn depends upon the buoyancy characteristics of riser buoy 32 in the disconnected mode or the position of tanker 18 when theriser is connected thereto.

In the connected mode suggested by FIG. 1, it will be understood that ocean surface 42 will cause variations in the vertical position of tanker 18, either through wave action or tides, and also through loading of the tanker as its hold is filled. Changes in the vertical position of tanker 18 with respect to the ocean floor 44 during oil transfer operation of the riser will result in displacement of joint 20 along the arcuate path within which the joint .is movable. If, for example, tanker 18 heaves upwardly from the initial position shown in FIG. 1, upward force transmitted through riser 14 will pull joint 20 upwardly, and such upward movement will necessarily result in lateral movement of riser 14 and joint 20 due to the restraint offered by riser portion 12 through its connection with the joint. The stated displacement due to upward heaving of tanker 18 is suggested by dashed line 14 denoting the displaced position of riser 14 toward the left in FIG. 1. In the stated condition of displacement, float 26 will be forced in a generally downward direction as

lines

22 and 24 attached thereto are pulled into closer linear alignment by movement of joint 20 with respect to stationary anchor means 28. The displaced position of float 26 due to movement of riser 14 and joint 20 toward the left in FIG. 1 is suggested by reference numeral 26. In the displaced condition thus represented by numerals 14' and 26 in FIG. 1, it will be understood that tensile force is continuously maintained axially through both

riser links

12 and 14 due to the horizontal force component of the tension in

lines

22 and 24 and through the continuous downward pull applied to joint 20 by mass 30.

Downward heaving of tanker 18, to which riser link 14 is connected at its upper end, would result in corresponding downward movement of joint 20 due to the continuous force applied to joint 20 by mass 30. The stated downward movement of joint 20 would be accompanied by lateral movement of riser 14 toward the position designated by dashed line 14" to the right as shown in FIG. 1 due to the continuous rightward force applied to joint 20 by float 26 and transmitted through 22. Downward movement of joint 20 in the foregoing manner would cause corresponding downward movement of float 26 to the position suggested by reference numeral 26" by force transmitted through line 22 extending between the joint and the float. In the displaced condition thus denoted by reference numerals 14" and 26" in FIG. 1, it will be understood that

riser links

12 and 14 are both continuously maintained under tension due to the forces respectively applied thereto by float 26 and mass 30, respectively. It may incidentally be noted that the position of the structure shown in FIG. 1 and identifiable with reference numerals l4 and 26" is the one normally assumed by the riser and mooring system when riser 14 is in the disconnected mode as discussed more fully below. In the foregoing disconnected position, mass 30 may descend so far as to touch ocean floor 44, which in many suboceanic locations may comprise a semi-solid surface of mud rather than a smooth hard surface of rock or the like. To prevent embedment of mass 30 by sinkage thereof into a highly tenacious mud mass, a mud mat 46 may be provided in the location suggested by FIG. 1 for supporting mass 30 at its lowermost limit of movement.

Referring to FIG. 3, the upper end of riser 14 is shown in the disconnected mode characterizing its condition when not in use for transferring products from tank 16 into a storage facility or the like on ocean surface 42. Hydrostatic balance means in the form of buoyant body 32 comprises a hollow sphere through which riser 14 penetrates is secured or otherwise formed on the riser. The upper terminal end of the riser is provided with a blind hub or cap 48 which seals and closes off the upper terminal end of riser 14. Cap 48, as shown more particularly in FIG. 4, may include a sealing surface 50 adapted to make contact with a sealing member such as flexible ring 52 nested within the upper terminal edge portion of riser 14, and held firmly in contact therewith by suitable clamping means such as an over-center toggle system as shown in the mentioned figure. The stated system may take any suitable form including those known to the prior art, and illustratively may comprise a projecting boss or lug S4 integrally formed on a cap 48 and supporting a pivotal link 56 having a pivotal lever 58 mounted on the lower end thereof. A projecting flange or curved lug 60 integrally formed or otherwise secure to riser 14 is adapted to receive a rounded portion of lever 58 in securely nesting and load transmitting relationship as shown in FIG. 4. Rotation of lever 58 into or out of engagement with flange 60 provides secure but easily disconnected holding means between the cap and the riser. As further shown in FIGS. 3 and 4, cable connection means are provided on cap 48 for securing visual marker buoy 62 thereto. The stated means illustratively include a hollow cleat 64 having cable 66 joined thereto in the manner shown by FIG. 4. Riser buoy 32 has a predetermined amount of buoyancy which, in combination with the lifting force of buoyant float 32 and the downward forces applied by riser 14 and components attached thereto, result in buoy 32 hanging at a predetermined depth below ocean surface 42 a sufficient amount to avoid the violent effects associated with stormy seas and the like.

As may be further seen from FIG. 4, the terminal end portion of riser 14 is provided with an annular groove 70 defined by two vertically spaced-apart

annular flanges

72 and 74. Referring particularly to FIG. 5, at-

tachment means for connecting riser 14 to tanker 18 may conveniently include pivotally mounted mooring swivel 76 supported within a vertical passage 78 through the center of tanker l8. Suitable bearing support for mooring swivel 76 is provided as schematically shown by

bearings

80 and 82, for example.

In operation, a line (not shown) may be passed through center passage 78 of tanker 18 and secured to marker buoy 62 or cable 66 in order to pull the sealed and submerged end of riser 14 up through mooring swivel 76 for operative attachment as required during the transfer operation. With the riser and buoy 32 thus positioned as shown in FIG. 5, locking collet or annular flange means mounted on mooring swivel 76 are secured to riser 14 within groove 70 and locked in place by suitable means including those known to the prior art. Blind cap 48 is then removed, exposing the open end of riser l4 and positioning the same substantially in the center of passage 78. A pivotally mounted section of pipe 84, with a suitable pipe swivel 99, appropriately supported on tanker 18 such as by brackets 86 and having mating connection means on the distal end thereof as suggested by flanged end 88 in FIG. 5 is rotated into mating connection with seal 52 at the terminal end of riser 14. Pivotal links 90 secured at one end to swivelling mooring structure by such means as brackets 92 may be engaged to similar brackets 94 proximate the end 88 of pipe 84 and joined thereto by a pin and clevis connection or the like to hold the pipe 84 in tightly sealed fluid transferring relationship with riser 14. Alternatively, a toggle system such as shown in FIG. 4 for cap 48 may be used on flange 88 of pipe section 84 to secure the same together. When thus connected, the contents of tank 16 on ocean floor 44 may be transferred into storage tanks and the like aboard tanker 18 through suitable conduits as suggested by pipe 96 in FIG. 6. Disconnection of riser 14 from tanker 18 is accomplished by simply reversing the con nection operation, and includes replacement of cap 48 in the position shown by FIG. 4 and release of the riser together with marker buoy 62 through passage 78 into the submerged state.

We claim:

1. A transfer riser system for use in offshore oil production comprising:

stationary storage means on the ocean floor;

a riser having an upper end disposed near the ocean surface and a lower end connected to said storage means for transferring the contents thereof to the ocean surface;

weight means secured to said riser at a point between the ocean surface and floor;

flotation means operatively associated with said upper end for providing an upward force sufficient to support the weight of the riser plus said weight means and maintaining said upper end near the ocean surface;

said riser having sufficient flexibility to allow said upper end to rise and fall with the wave action and said weight means being sufficiently large to cause said upper end to follow instantaneously the rising and falling waves to allow said flotation means to always maintain a tension force on the riser, said riser further including an upwardly directed portion and a generally laterally extending portion pivotally connected at one end to the lower end of said upwardly directed portion and pivotally connected at the other end to said storage means; and

a mooring system means having means connected to the pivotal connection for maintaining the laterally extending portion under continuous substantially axial tension.v

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

said mooring system means for maintaining said laterally extending portion under tension includes at least one mooring line having one end thereof secured to the pivotal connection between said laterally extending portion and said upwardly directed portion, and having the other end thereof secured to anchor means on the ocean floor.

3. The structure set forth in claim 2 above, wherein:

a buoyant mass is secured to said mooring line intermediate the said ends thereof.

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

said mooring system means comprises at least two diverging mooring lines, each of said lines extending from the pivotal connection to a separate anchor;

each of said anchors being spaced apart a substantially equal distance on either side of the longitudinal axis of said one riser portion.

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

separate float means secured to each of said two mooring lines intermediate the connection thereof with said second pivotal connection and said anchors, said float means being displaced downwardly upon said vertical movement of said other riser portion in an upward direction.

Claims

1. A transfer riser system for use in offshore oil production comprising: stationary storage means on the ocean floor; a riser having an upper end disposed near the ocean surface and a lower end connected to said storage means for transferring the contents thereof to the ocean surface; weight means secured to said riser at a point between the ocean surface and floor; flotation means operatively associated with said upper end for providing an upward force sufficient to support the weight of the riser plus said weight means and maintaining said upper end near the ocean surface; said riser having sufficient flexibility to allow said upper end to rise and fall with the wave action and said weight means being sufficiently large to cause said upper end to follow instantaneously the rising and falling waves to allow said flotation means to always maintain a tension force on the riser, said riser further including an upwardly directed portion and a generally laterally extending portion pivotally connected at one end to the lower end of said upwardly directed portion and pivotally connected at the other end to said storage means; and a mooring system means having means connected to the pivotal connection for maintaining the laterally extending portion under continuous substantially axial tension.

2. The structure set forth in claim 1 above, wherein: said mooring system means for maintaining said laterally extending portion under tension includes at least one mooring line having one end thereof secured to the pivotal connection between said laterally extending portion and said upwardly directed portion, and having the other end thereof secured to anchor means on the ocean floor.

3. The structure set forth in claim 2 above, wherein: a buoyant mass is secured to said mooring line intermediate the said ends thereof.

4. The system set forth in claim 2 above, wherein: said mooring system means comprises at least two diverging mooring lines, each of said lines extending from the pivotal connection to a separate anchor; each of said anchors being spaced apart a substantially equal distance on either side of the longitudinal axis of said one riser portion.

5. The system set forth in claim 4 above, further including: separate float means secured to each of said two mooring lines intermediate the connection thereof with said second pivotal connection and said anchors, said float means being displaced downwardly upon said vertical movement of said other riser portion in an upward direction.