GB2178786A - Flexible structural joint - Google Patents

Abstract

The joint (18) is located between first and second sections (14, 16) of an articulated structure and accommodates relative lateral pivoting of the sections through elastic flexing of certain of its members without using moving parts. In a first embodiment (Figure 1), it comprises an elongated axial load member (20) and at least three shear and torsion links (21) which lie substantially in a plane perpendicular to the axis of the structure, each link being connected at one of its ends to the first section and at its other end to the second section by respective support members (23, 25). In a second embodiment (Figure 2), four shear and torsion links (22a-d) are joined to form a shear and torsion frame connected to the respective section (14, 16) by respective support members (26, 30, 24, 28) attached, respectively, to the frame at opposite ends of mutually perpendicular diametral lines (X, Y) lying in the plane of the frame. Figure 8 shows an arrangement embodying a redundant shear and torsion frame. <IMAGE>

Description

SPECIFICATION Flex joint The invention relates to a flexible structural joint or flex joint for use in an articulated structure subjectto transverse loads. More particularly though not exclusively, the invention pertains to a flex joint for use in an articulated offshore structure.

Since its beginnings in the 1ate1940's,theoffshore petroleum industry has been moving into progres sivelydeeperwaters. Until recently, offshore petroleum drilling and producing operations typically have been conducted from rigid, bottom-founded offshore structures such as conventional steel jacket structures or concrete or steel gravity structures.

Such structures are designed to rigidly resist environmental forces such as wind,waves, and ocean currents. However, in wates deeper than about 1,000 feet,the steel tonnage, and hence the cost, required to rigidly resist environmental forces increases so rapidly that an economic limit is soon reached, even given the most favorable economic conditions.

Waterdepths of interest to the offshore petroleum industry have now increased to the pointwhere rigid, bottom-founded offshore structures are, in many cases, no longertechnically oreconomicallyfeasible.

This problem has resulted in the development of new types of offshore structures generally known as "compliant towers". Compliant towers are offshore structures that do not rigidly resist environmental forces. Bather a compliant tower is designed to yield to the environmental forces in a controiled manner.

Basically, the tower is allowed to oscillate a few degrees from vertical about its base in response to the applied environmental forces. This oscillation, which may be characterized as that of an inverted pendulum, creates an inertial restoring force which opposes the applied environmental forces.

One such compliant tower is the "guyed tower".

Basically, a guyed tower is a trussed structure of generally uniform cross-section that extends up wardlyfrom the bottom of the body of water to a deck supported above the water surface. The structure is held upright by an array of guy lines which are spaced around the periphery of the structure and radiate outwardly and downwardlyto anchor points located on the bottom ofthe body of water. The guy lines permit the tower to pivot laterally a few degrees about its base in response to surface wind, wave, or currentforces. See generally, Finn, L,D., "A New"ANew Deep-Water Platform - The Guyed Tower",Journal of Petroleum Technology, April 1978, pp.537-544.

Asecondtypeofcomplianttoweristhe "buoyant tower". Basically, a buoyant tower is a trussed struc turesimilartoaguyedtower; however, no guylines are used. The entire restoring force for the structure is provided by iarg6 buoyancy tanks located on the structure, preferably at or near the surface ofthe body of water. See, for example,the buoyant tower illustrated in U. S. Patent No.3,636,716 issued January 25, 1972 to Castellanos.

Yetanothertypeofcomplianttower,the "hybrid tower", is disclosed in U.S. PatentApplication Serial No.635,942 fiied July 1934 and entitled "Hybrid Offshore Structure". Basically, a hybrid towercomprises a compliant upper section such as a guyed tower or a buoyant tower mounted on a rigid, bottomfounded lower section. As more fully described in the referenced patent application, the compliant upper section is permitted to pivot laterally a few degrees about a pivot point located at or near the upper end of the lower section in response to the applied environmental forces.

The compliant towers described above are generally designed for use as petroleum drilling and producing platforms. Another type ofarticulated offshore structure is the "single anchor leg mooring" or "SALM " which is used to transfer hydrocarbon products from the bottom of a body of water a floating storagefacilityortanker. Basically, a SALM is a riser pipe extending from a base located on the bottom of the body of water to a mooring buoy located on the surface ofthe body of water. Afloating storagefacility ortanker is moored to the mooring buoy and is allowed to "weathervane" bout the buoy in response to environmental forces. Typically, a SALM is articulated orjointed at both the bottom and the top ofthe riser pipe.Hydrocarbon productflowlines extend from the base through the articulated joints, riser pipe, and mooring buoy to the floating storage facility ortanker. See, for example, the SALM illustrated in Figure 1 of U. S. Patent 4,337,970 issued July 6,1982 to Gunderson.

Each of the articulated offshore structures described above requires the use of an articulated joint or pivot which will permit the desired lateral pivoting movement. Typically, the articulated joint or pivot must also be capable oftransmitting largeverticai loads between adjacent sections ofthe structure.

Generally, the articulated joint or pivot is located near the bottom of the structure; however, as noted above, a SALM is typically articulated orjointed at both the bottomandthetopoftheriserpipe.

As an added complication, articulated offshore structures typically require use of a means to transmit torsional loads between adjacent sections of the structure. Offshore structures are seldom, if ever, per fectly symmetrical.Wind, waves, and ocean currents impinging on an asymmetrical structure create uneven forces which tend to twist the structure about its vertical axis. These twisting forces must be transmitted to and resisted by the foundation ofthe structure in orderto prevent damage to or destruction ofthe flow lines, well conductors, and other components of the structure.

Heretofore, a variety of mechanical devices, such as universal joints and ball joints, have been proposed for use as articulated joints or pivots in articulated offshore structures. Use of a universal joint has the benefit of combining the articulated joint or pivot and the torsion means. See, for example, Figure 10 of U. S. Patent 3,626,701 issued December 14,1971 to Laffont. Use of a ball joint as the pivot requires use of a separate torsion means. See, for example, the hinge disclosed in U.S. Patent 3,735,597 issued May 29, 1973 to Guy. Othertypes of mechanical pivots have also been proposed for use in articulated offshore structures. See, for example, U.S. Patent 3,636,716 issued January 25,1 972 to Castellanos and U.S.Patent 4,231,632 issued November 4,1 980 to Tuson.

The mechanical devices previously proposed for use as articulated joints or pivots in articulated offshore structures typically incorporate moving parts which are subject to wear and must be maintained overthe life ofthe structures. In deep waters, maintenance of such devices is, at best, extremely difficult and expensive. Repair or replacement of such devices may be a practical impossibility. For large structures, or structures in deep waters, the loads which must be transmitted by such devices are so great that commonly used mechanical joints or pivots are impractical. Further, use of a mechanical joint or pivot typically requires that loads be transmitted through a single point which has no redundancy.

Accordingly, the need exists for a pivot or joint suitableferusein an articulated offshore structure which has no moving parts, is capable oftransmitting large loads, is capable of providing redundancy, and requires little or no maintenance over the life ofthe structure.

According to the invention from one aspect there is provided a flex joint for use in an articulated structure having a first section and second section, said first and second sections being located along a longitudinal axis of said structure, said flexjoint comprising: - an axial load member attached to said first and second sections; and -at least three elongated shear and torsion links located between said first and second sections of said structure and each oriented at, at most, a small angle to a plane substantially perpendicularto said longitudinal axis of said structure, each of said links having a first end connected to said first section and a second end connected to said second section.

According to the invention from a second aspect there is provided a flexjeintfer use in an articulated structure having a first section and a second section, saidfirstand second sections being located along a longitudinal axis efsaid structure, said flex joint com- prising: - an axial load member attached to said first and second sections; - a shear and torsion frame located between said first and second sections, said frame being oriented so as to lie in a plane substantially perpendicular to said longitudinal axis; - a first pair of support members, each of said first pair being attached to both said frame and said first section, said first pair being attached to said frame, respectively, at opposite ends of a first diametral line lying in the plane of said frame; and - a second pair of support members, each of said second pair being attached to both said frame and said second section, said second pair being attached to said frame, respectively, at opposite ends of a second diametral line lying in the plane of said frame.

According to the invention from a third aspectthere is provided a flex jointfor use in an articulated structure having a first section and a second section, said first and second sections being located along a longitudinal axis of said structure, said flexjoint com- prising - an axial load member attached to said first and second sections; - a primary shear and torsion frame located between said first and second sections, said primary frame being oriented so as to lie in a plane substantially perpendicular to said longitudinal axis;; -a first pair of primaryframe support members, each of said first pair being attached to both said primary frame and said first section, said first pair being attached to said primary frame, respectively, at opposite ends of a first diametral line lying in the plane of said primary frame; - a second pair of primary frame support members, each of said second pair being attached to both said primary frame and said second section, said second pairbeing attached to said primaryframe, respectively, at opposite ends of a second diametral line lying in the plane of said primaryframe; - a secondary shear and torsion frame, said secondaryframe being located in the same plane as and substantially concentricwith said primaryframe;; - a first couple of secondary frame support members, each of said first couple being attached to both said secendaryframe and said first section, said first couple being attached to said redundant frame, respectively, at opposite ends of a third diametral line lying in the plane of said secondary frame; and -a second couple of secondary frame support members, each of said second couple being attached to both said secondary frame and said second section, said second couple being attached to said secondaryframe, respectively, at opposite ends of a fourth diametral line lying in the plane of said secondary frame.

According to the invention from a fourth aspect there is provided an articulated structure having a longitudinal axis, said articulated structure comprising: (a) first and second sections located along said longitudinal axis; (b) an axial load member extending between and attached to said first and second sections; and (c) at least one flex joint located between said first and second sections, each such flex joint comprising: (i) a shear and torsion frame oriented so as to lie in a plane substantially perpendicularto said longitudinal axis; (ii) a first pair of support members, each of said first pair being attached to both said frame and said first section, said first pair being attached to said frame, respectively, at a first pairofdiametrically opposite points on said frame thereby defining a first diametral line lying in the plane of said frame; and (iii) a second pair of support members, each of said second pair being attached to said frame and said second section, said second pair being attached to said frame, respectively, art a second pair of diametrically opposite points on said frame thereby defining a second diametral line lying in the plane of said frame.

Theflexjeints to be described in detail hereinbelow can accommodate the desired lateral pivoting of the first section through elastic flexing or bending of certain of its members. No moving parts are used.

In a first embodiment, the flex joint consists of an axial load member attached to the first and second sections ofthe structure and at leastthree elongated shearandtorsion links located betweenthefirst and second sections and oriented seas to lie substantially in a plane perpendicular to the longitudinal axis ofthe structure. Each of the shear and torsion links is connected at one of its ends to the first section ofthe structure and at its other end to the second section of the structure.

Preferably, the axial load member is an elongated tubularmemberwhich is substantially coincident with the longitudinal axis of the structure. The axial load member is capable oftransmitting all longitudinal loads between the first and second sections ofthe structure. The axial load member bends to accommo- date lateral pivoting of the first section. Thus, the axial load memberfunctions essentially in the manner of a long columnar or barspring. The stiffness ofthe axial load member may be easily varied by changing either or both efitsflexural rigidity and the unsupported length subject two bending. The axial load member may be either a single tubular element or a plurality of concentric tubular elements each of which is attached to both the first and second sections of the structure.

Alternatively, the axial load member may be a closely-spaced cluster of tubular elements grouped around the longitudinal axis of the structure.

The shear and torsion links elastically flex or bend to accommodate lateral pivoting of the first section.

So long as the maximum stress in a given linkdoes not exceed the elastic limit of the structural material, the linkwill return to its undeflected position when the applied load is released. The links are also capable of transmitting all shear and torsion loads between the first and second sections.

In a second embodiment ofthe invention,four shearandtorsien links joined to form a substantially planar shear and torsion frame located between the first and second sections ofthe structure. The,, frame lies in a plane substantially perpendicularto the longitudinal axis of the structure. Preferably, the frame is square in shape; however, other planar shapes may also be used. The geometric center of the frame is preferably located at or nearthe longitudinal axis of the structure.

Afirst pair of the support members are used to connect the shear and torsion frame to the first section of the structure. Preferably, the first pair of support members are attached, respectively, to diametrically opposite points on the frame. In otherwords, the first pair are attached to the frame at opposite ends of a first diametral line lying in the plane ofthe frame and passing substantially through the geometric center ofthe frame. A second pair of support members are used to connect the shear and torsion frame to the second section ofthe structure. The second pair of support members are attached to the frame at opposite ends of a second diametral line lying in the plane ofthe frame. Preferably, thefirst and second diametral lines are substantially mutually perpendicular.In the case of a square shear and torsion frame,thefoursupport members are preferably connected atthefeurcerners of the frame, and the firstandsecnddiametral lines are diagonal linescen- necting opposite corners of the frame.

The shearandtorsion frame elasticallyflexes or bends to accommodate lateral pivoting of the first section ofthe structure. Torsional and shear loads on the first section are transmitted by the first pair of support members to the shear and torsion frame and thence bythesecend pair of support members to the second section.

Multipleflexjoints may be used to accommodate large angle deflections of the first section. It may be desirable to provide means for limiting the maximum deflection of each individual jointto prevent harmful everstressing thereof.

In an alternative embodiment, a second shear and torsion frame is used to add desirable redundancyto the flex joint. The second frame is smallerthatthefirst frame and lies in the same plane asthefirstframe.

Preferably, the geometric centers ofthe first and second frames are substantially coincident. The secendframe is connected to thefirstand second sections ofthe structure in the same manner as described above forthe first frame and the connection pointsforthe second frame preferably lie on the same diametral lines as the connection pointsforthefirst frame; however, the direction of support at a given connection point the second frame is opposite that ofthe corresponding connection point on the firstframe. For example, if thefirstframe is connected to the first section at a given connection point, the second frame would be connected to the second section at the corresponding connection point.

The invention will be better understood by referring, bywayofexample,tothefollowing detailed description and the attached drawings in which: Figure lisa perspective view illustrating the principal components of a first embodiment ofthe inven- tion.

Figure 2 is a perspective view i 11 ustrati ng the prin cipal components of a second embodiment of the invention; Figures3A through 3D are single-line elevational sketches illustrating, respectively, the deflection of each efthefeurside members efthe shearandter- sienframefera given direction oftilt; Figure 4 is an elevational view, in partial section, taken along a diagonal line of the structure and illustrating onearrangementfertheaxial load member of the present invention; Figures5and 6are elevational views illustrating an embodiment of the present invention which uses a closely-spaced cluster of elongated tubular elements as the axial lead member;; Figure 7 is a perspective view of an embodiment of the present invention in which multiple shear and torsion frames are used to increase the maximum allowable angle oftilt; Figure 8 is a perspective view illustrating an embo diment of the present invention which uses a redun dant shear and torsion frame; and Figure 9 is a perspective view illustrating one corner ofthe shear and torsion frame and the associated support member.

The flexible structural joints or "flexjoints" to be described are suitable for use in an articulatedstruc- ture, have no moving parts, can easilytransmit large loads, are capable of providing redundancy, and require little or no maintenance throughoutthe life of the structure. These and other advantages will be apparent from the following detailed description. It should be noted that although the invention will be described in connection with an articulated offshore structure, it is also suitable for use in other types of articulated structures.

Turning nowto Figure ian offshore structure 10 located in a body of water 12 and having an upper section 14and a lower section 16 is illustrated. Upper section 14 and lower section 16areshown in block form for simplicity and ease of understanding.

However, in actual practice it is likely that upper section 14 and lower section 16 would consist of trussed frameworks, as is well known in the art. Alternatively, upper section 14 and lower section 16 may be any othertype of structure, such as a riser pipe in a SALM.

Upper section 14 extends upwardlytoward the surface (not shown) of body of water 12. Lower section 16 extends downwardlytoward the bottom (not shown) of body of water 12. Lower section 16 may be the base of structure 10 and have a height of only a fewfeet. Alternatively, lower section 16 may be the lower section of a hybrid tower or the riser pipe of a SALM and, accordingly, have a height of several hun- dred feet. Upper section 14 is subject to transverse loads resulting from wind, waves, and currents. It is desired to permit upper section 14to pivot laterally with respect to lower section 16 in response to such transverse loads.

The desired lateral pivoting of upper section 14 is accommodated by the flex joint, generally indicated at 18. As will be hereinafter described in greaterde tail,flexjoint 18 accommodates lateral pivoting of uppersection 14through elasticflexing or bending of certain of its members. Flex joint 18 is also capable of transmitting all shear and torsional loads from upper section 14to lower section 16. As illustrated in Figure 1, flex joint 18 consists of an axial load member 20 and atleastthree elongated shearand torsion links 21.Each ofthe links 21 is connected at one of its ends to uppersection 14 by afirstsupport member 23 and at its other end to lower section 16 by a second sup port member 25. Thesupport members 23, 25 may form extensions of the upper and lower sections 14, 16, respectively, and may be part of the trusswork of these sections. Preferably, the links 21 and the sup port members 23 and 25 are constructed from steel tubulars ofthe type generally used forthetrusswork of an offshore structure. Other types of structural elements may also be used, if desired.

Figure 1 illustrates a plane "P" located between upper section 14and lowersection 16. Plane "P" is substantially perpendicularto the vertical axis (longitudinal centerline) of structure 10. Preferably, each shear and torsion link 21 is oriented so that its longitu dinal centerline lies substantially in plane "P"; however, if desired, one or more of the links 21 may be inclined to plane "P" at an angle of upto about 15 .

The three shear and torsion links 21 may be of equal our unequal lengths, as desired. Further, no particular orientation ofthe links 21 within plane "P" is neces sary; however, preferably, no morethan two ofthe three links 21 should be collinear. The links 21 may be located either inside or outside ofthe perimeter of structure 10, and the support members 23 and 25fora given link21 may be interchanged, as desired. In a preferred embodiment ofthe invention, the three shearandtorsion links 21 would be of substantially equal lengths and would be oriented along the sides of an equilateral triangle located substantially in plane "P".

Preferably, axial load member 20 is an elongated tubular memberthat is substantially coincident with the vertical axis (longitudinal centerline) of structure 10. However, if desired, axial load member 20 may be laterally offset from the vertical axis. As will be hereinafter described in greater detail, axial load member 20 is attached to both upper section 14 and lower section 16 and is capable oftransmitting vertical (longitudinal) loadstherebetween.

As illustrated in Figure 1, support members 23 and 25 comprise trusses constructed from steel tubulars, each ofwhich has a substantially vertical centercolumn andtwo angled braces. Other suitable support members will be apparentto those skilled in the art.

Figure 2 illustrates a second embodiment of the invention in which fourshearandtorsion links 22a, 22b, 22c, and 22d are joined to form a substantially planarshearandtorsion frame 22. The remainderof the description will be directed toward the embodiment illustrated in Figure 2 and certain variations thereof.

Shear and torsion frame 22 is located between uppersection 14 and lower section 16and, preferably, lies in a plane that is substantially perpendicularto the vertical axis of structure 10. Preferably, the geometric center of frame 22 is located at or nearthe vertical axis (longitudinal centerline) of structure 10.

As illustrated in Figure 2, frame 22 comprises a squareframe; however, otherplanarshapes may also be used. Preferably, frame 22 is constructed from steel tubulars of the type generally used for the trusswork of an offshore structure; however, othertypes of structural elements may also be used.

Shear and torsion frame 22 is attached to upper section 14 by support members 26 and 30 and to lowersection 16 by support members 24and 28.

Support members 26 and 30 are attached, respective ly, to frame22 at opposite ends of diametral line "X".

As used herein and in the claims, "diametral line" means a line segment lying in the plane of the frame which passes substantially through the geometric centeroftheframe and connects diametricallyopposite points on the frame. Support members 24 and 28 are attached, respectively, to frame 22 at opposite ends of diametral line "Y". Preferably, diametral lines "X" and "Y" are substantially mutually perpendicu lar. In the case of a square shear and torsion frame, as illustrated in Figure 2, the preferred support points are the four corners of the frame and diametral lines "X" and "Y" are diagonal lines connecting opposite corners.

As illustrated in Figure 2,frame22 is supported at four points spaced around its periphery with the direction of support alternating from pointto point.

Thus, as with the embodiment illustrated in Figure 1, each ofthe individual side members 22a, 22b, 22c, and 22d is connected atone of its ends to upper section 14 and at its otherendto lower section 16.

Referring now to Figures 3Athrough 3D, the man ner in which shear and torsion frame 22 flexes or bends to accommodate lateral pivoting of upper section 14will be described. Figures 3Athrough 3D are elevational one-line sketches, respectively, ofthe deflection of the four side members 22a, 22b, 22c, and 22d of frame 22 for a given direction of tilt. The direction of tilt is indicated at the top of each sketch.The deflection of upper section 14 has been exaggerated for purposes of illustration. Typically, the maximum deflection of upper section 14 will not exceed a few degrees.The undeflected positions of upper section 14, axial load member 20, the relevantsupport mem ber26 or30, and the relevant side member22a, 22b, 22c, or 22d offrame 22 are indicated by dashed lines and the deflected positions by solid lines. As illus traded in Figure 3A, for the indicated direction of tilt, side members 22a is elastically flexed or bent up- wardly in a single smooth curve. As illustrarted in Figures 3B through 3D, forthe same direction oftilt, side member 22b is elastically flexed downwardly in a double curve, side member 22e is elastically flexed downwardly in a single curve, and side member 22d is elastically flexed upwardly in a double curve.

For simplicity, the direction of tilt illustrated in Figures 3Athrough 3D was assumed to be perpendicularto side members 22b and 22d offrame 22. In actual practice the direction oftilt is likely to be obliqueto each ofthe side members offrame 22 (orto the individual shear and torsion links 21 in the embodiment illustrated in Figure 1). However, the deflection of side members 22a, 22b, 22c, and 22d (or links 21 ) for any direction of tilt may be determined by resolving the applied load into its orthogonal components and adding the deflections resulting from each component. So long as the maximum bending stress in each ofthe side members 22a, 22b, 22c, and 22d (or the links 21) does not exceed the elastic limitofthe structural material, the side members (or links) will return to their undeflected positions when the applied load is released.

Support members 24, 26, 28, and 30 are rigidly attached (e.g., by welding or bolting)to shear and torsion frame 22 and to upper section 14 or lower section 16, whichever is applicable. Preferably, the connection between each individual support member and frame 22 should be designed to minimize stress concentrations (and, therefore, fatigue). FigureS illustrates one possible design forthe connection between support member30 and the corner offrame 22 formed by side members 22a and 22d. As illustrated, support member 30 consists of a substantiallyvertical center column 30a and two angled braces 30b and 30c. Typically, the center column 30a would have a larger diameterthan either the side members 22a and 22d or the braces 30b and 30c. The joints between the various structural elements are welded.Other possible connection designs will be apparent to those skilled intheart.

As described above, shear and torsion frame 22 and support members 24, 26, 28, and 30 are capable of transmitting to lower section 1 6 all loads resulting from lateral pivoting of upper section 14. These loads are primarily transmitted through bending orflexing ofthe individual side members 22a, 22b, 22c, and 22d offrame 22, as illustrated in Figures 3Athrough 3D.

As will be apparentto those skilled in the art, frame 22 and support members 24,26,28, and 30 are also capable of transmitting to lower section 16 all torsion- al loads resulting from twisting of upper section 14 about its vertical (longitudinal) axis. Thesetorsional loads are transmitted bysupport members 26 and 30 toframe22 and thence bysupport members 24and 28to lower section 16. However, duetothe unique method of supporting frame 22, described above, frame 22 is not capable of transmitting largevertical (longitudinal) loads between upper section 14and lowersection 16.Large vertical loads applied toframe 22 through support members 26 and 30 would cause excessive bending in side members 22a, 22b, 22c, and 22d, thereby damaging or destroying frame 22.

Accordingly, means must be provided fortransmit tingvertical loads between uppersection 14andlow- ersection 16 and forisolating frame 22fromthe harmful effects thereof. Further, such means for transmitting vertical loads should not excessively in terferewiththeflexing of joint 18.

Axial load member 20 satisfies the foregoing criteria. Member 20 is attached to both upper section 14 and lower section 16 and is capable of transmitting all vertical (longitudinal) loads therebetween. As noted above, member 20 is preferably an elongated tubular memberthat is substantially coincident with the ver- tical axis of structure 10. For purposes of the following discussion, the vertical axis of structure 10 will be assumed to be coincident with the neutral bending axis of structure 10 for any given direction oftilt.

Therefore, as illustrated in Figures 3A and 3C, mem ber20will be placed in simple bending by lateral pivoting of upper section 14. Thus, member 20 functions essentially as a long columnar or bar spring and will not excessively interfere with the flexing ofjoint 18. The stiffness of member 20 will be primarilyde- pendent on its flexs ray rigidity and the unsupported length being subjected to bending.

Axial load member 20 may be a single tubular element ofthetype commonly used in offshore structures. Alternatively, to add desirable redundancy, member 20 may consist of a plurality of concentric tubular members, as illustrated in Figure 4. Figure 4 is a cross-sectional view of structure 1 taken long a diagonal line such as diametral line "X" (see Figure2).

In Figure 4, member 20 consists ofthree concentric tubular members 20a, 20b, and 20cwhich areterminated at different levels above and below joint 18.

Tubular member 20a is the longest ofthethreecon- centric tubular members. Tubular member20b is shorter than tubular member 20a and fits loosely thereover so that tubular member 20a extends beyond the upper and lower ends of tubular member 20b. Tubular member 20c is the shortest of the three concentric tubular members and fits loosely over tubular member 20b so that both tubular members 20a and 20b extend beyondtheupperand lower ends of tubular member 20c. The upper and lower ends of the three concentric tubular members are welded or otherwise attached, respectively, to the trusswork (e.g., horizontal beam members 36 and angled brace members38) of uppersection 14and lowersection 16.

Guides 40 are rigidly attached to the lower end of upper section 14 and the upper end of lower section 16.Axial load member20extendsthroughandis guided by guides 40. If desired, additional guides (not shown) may be attached to uppersection 14and lower section 16 and spaced along the length of member 20. The guides 40 permit axial load member 20 to spread its bending over a long distance and,therefore, reduce the bending stress in member 20.

As noted above, the stiffness of axial load member 20 is primarily dependent on its flexural rigidity and the unsupported length being subjected to bending.

Thus, the stiffness may be easily varied by varying the locations at which member 20 is attached to upper section l4andlowersection 16.Forexample,mem- ber 20 may be attached to upper section 14 only at or near its upper end. In this case, the unsupported length of member20 would be quite large and the stiffness nest of m member 20 would be quite low. Alterna- tively, member 20 may be attached to upper section 14 near its lower end in which case the stiffness of member 20would be substantially higher.

Turning now to Figures 5 and 6, an alternative embodimentoftheaxial load memberwill bede- scribed. As illustrated in FigureS 5 and 6, the axial load membercomprisesa pluralityofmain piles42 (two shown). Main piles42 may be singletubular elements orconcentrictubulars, as described above in connection with Figure 4. Preferably, each main pile 42 is attached to upper section 14 onlyatornear the upper end of upper section 14. Main piles42 extend downwardlythrough a series of main pile guides 44 spaced along the length of upper section 14. Main pile guides44are rigidlyattached to horizontal beam members 36 which from partofthetrusswork of upper section 14.

Lower section 16 may be a frustum shaped trussed structure as illustrated in Figure 5 5 constant-width trussed structure as illustrated in Figure 6. In either case, lower section 16 is rigidly fixed to the bottom 46 of body of water 12 by piles 48 (FigureS) or 50 (Figure 6). Alternatively, lower section 16may be agravity base, as swell known intheart. Main piles 42 maybe attached to lower section 16 in the manner previously described in connection with Figure 4. Alternatively, as illustrated in Figure 5, a plurality of main pile sleeves 52 may be rigidly attached to the trussworkof lower section 16. Main pile sleeves 52 are located so asto beverticallyaligned, respectively, with each of the main piles 42.The lower ends of main piles 42 extend into main pile sleeves 52 and are grouted or otherwise fixed therein. In another embodiment, illustrated in Figure 6, main piles 42 are not attached to lower section 16. Rather, main piles 42 pass through one or more additional main pile guides 54 attached to the trusswork of lower section 16 and extend into the bottom 46 of body of water 12. In this embodiment, vertical loads are transmitted by main piles 42 directly from upper section 14 to the bottom 46 of body of water 12.

In the embodiment ofthe invention illustrated in Figures 5 and 6, itis likelythata cluster of main piles 42 would be used to provide desirable redundancy.

Such a cluster might include as many as eight or more main piles 42. In this embodiment, the stress in any individual main pile will be dependent on the section modulus ofthe entire cluster. Accordingly, the cluster of main piles should be grouped as closely as possible around thevertical axis of structure 10 so that no individual axial load memberwill be excessively loaded by flexing of joint 18. As upper section 14 pivots laterally, some ofthe main piles 42 will be placed in tension while others will be placed in compression. However, since main piles 42 are quite long, the resu Iting tensile and com pressive forces shou Id not be excessive.

The embodiment illustrated in Figure 6 includes additional pile guides 56 attached to the outside of upper section 14. During launch and upending of structure 10, piles 50 are retracted to the position indicated by dashed lines and are secured to both uppersection 14 and lower section 16. This will prevent harmful overflexing of joint 18 during launch and upending of structure 10. After structure 10 has been positioned on the bottom 46 of body of water 12, piles 50 are driven through legs 58 into bottom 46 thereby releasing joint 18 and securing structure lotto bottom 46. Other means fortemporarily securing joint 18 during launch and upending of structure 10 will be apparent to those skilled in the art.

Figure 7 illustrates an embodiment of the invention in which multiple shear and torsion frames (two shown) are used to increase the maximum allowable angle of tilt of upper section 14with respectto lower section 16. As illustrated in Figure 7, structure 10 consists of upper section 14, lower section 16, and a middle section 60. Twoflexjoints 18 are located, respectively, between upper section 14 and middle section 60 and between middle section 60 and lower section 16. Each of the flex joints 18 consists of a shearandtorsion frame 22 and foursupports 24, 26, 28, and 30 as described above in connection with Figure 2.Axial load member 20 extends through upper section 14, middle section 60, and lower section 16 and is attached to upper section 14 and lower section 16 as previously described. Preferably, middle section 60 includes one or more guides (not shown) ofthetype illustrated in Figure 4to guide member 20.

The embodiment illustrated in Figure 7 will permit larger angles oftiltthan is possible using a singleflex joint 18. Several joints may be used if desired. As illustrated in Figure 7, each ofthe joints has the same angular orientation; however, this is not necessary. In other words, each of the joints 18 may be rotated about the vertical axis of structure 1 to any position such that the corresponding support members (e.g., support members 24) in each joint are not vertically aligned.

In order to prevent harmful overstressing offlex joint 18 (in any of the embodiments described herein), it may be desirable to provide means for limiting the maximum allowable angle of tilt of each individual joint. Onesuitable means isthe mechanical stop 62 located below support member 30 ofthe lowerflexjoint 18 in Figure 7 (illustrated in dashed lines). As illustrated, mechanical stop 62 consists of a three-legged truss having a plate 64 attached to its upperend. Similar mechanical stops (notshown) would be located below support member 26 and above support members 24 and 28. The plates 64 are located a distance below or above frame 22, as appropriate.The maximum allowable deflection of frame 22 (and, therefore, the maximum allowable bending stress in the individual side members of frame 22) will be governed by the distance between theframe 22 and the plates 64. Other suitable means for limiting the angle oftiltwill be apparenttothose skilled in the art.

Figure 7 may also be used to illustrate the use ofthe flex joint in a SALM. In a SALM, middle section 60 would correspond to the riser pipe and, accordingly, would be quite long. Lower section 16wouldcorres- pond to the base of the SALM and upper section 1 4to the buoy. The direction oftiltforthe upperflexjoint would typically be the reverse ofthe direction oftilt forthe lowerflexjoint, as is well known in the art.

Figure 8 illustrates an embodiment ofthe invention in which theflexjoint 18 includes a secondary or "redundant" shear and torsion frame 66 and its four support members 68,70,72, and 74. The secondary frame 66 is " redu ndant" in the sense that it is not normally needed for the purpose of accommodating the tilting or pivoting of the upper section 14 of the articulated structure relative to the lower section 16, but it will serve as a back-up in the eventofmechanic- alfailureefthe primaryframe22. Forclarity,support member30 and one corner of frame 22 have been deleted and replaced by dashed lines. As illustrated, redundant frame 66 is square in shape.The length of each ofthe side members of redundant frame 66 is shorterthan the length ofthe side members offrame 22. Redundantframe 66 is located inside frame 22 and lies substantially in the same plane as frame 22. Pre ferably,the geometric centers efredundantframe 66 and frame 22 are substantialy coincident. Redundant frame 66 is connected to uppersection 14 and lower section 16 in the same manner as frame 22. Support members 68,70,72, and 74 are preferably located on the same diametral lines as support members 24,26,28, and 30; however, the direction of support at each connection point is opposite to the direction of support atthe corresponding connection pointon frame 22.Thus, support members 68 and 72 are located on diametral line "Y" and are connected to upper section 14, and support members 70 and 74 are located on diametral line "X" and are connected to lowersection 16. Additional redundantshearandtor- sionframes may be used, ifdesired,withthethird frame having support directions corresponding to the firstframe, the fourth frame having supportdirections corresponding to the second frame, and soon.

As described above, the flex joints described herein overcome deficiencies inherent in the mechanical pivots previously proposed for use in articulated offshore structures. Other advantages will be readily apparent to those skilled in the art. For example, the flex joints provide ample spaceforthe passage of flowlines, well conductors, and othervertical mem bersthrough the joint. One ofthe primary problems in constructing a SALM is passage of multipleflowlines through the articulated joints. The disclosed flex joints solve this problem.

Claims (19)

1. Aflexjointfor use in an articulated structure having a first section and second section, said first and second sections being located along a longitudinal axis of said structure, said flex joint comprising: - an axial load member attached to said first and second sections; and - at leastthree elongated shear and torsion links located between said first and second sections of said structure and each oriented at, at most, a small angle to a plane substantially perpendicularto said longitudinal axis ofsaid structure, each ofsaid links having a first end connected to said first section and a second end connected to said second section.

2. Aflexjointas claimed in claim 1, wherein said first end of each ofsaid links is connected to said first section by a first support member forming an extension of said first section and said second end of each of said links is connected to said section section by a second support memberforming an extension of said second section.

3. Aflexjointasclaimed in claim 1 or2,wherein said shear and torsion links are oriented so asto lie substantially in a plane perpendicularto said longitudinal axis of said structure.

4. Aflex jointas claimed in claim 1 or 2, wherein said shear and torsion links are oriented so as to be inclined at an angle not exceeding 15".

5. A flex joint as claimed in any preceding claim, wherein said axial load membercomprisesanelon- gated member located substantially coincident with said longitudinal axis.

6. Aflexjointasclaimed in any preceding claim, wherein said elongated member comprises a single tubular element.

7. Aflex joint as claimed in any one of claims 1 to 5, wherein said elongated member comprises a plu rality of concentric tubular elements, each ofwhich is attached to both said first and second section.

8. Aflexjointasclaimed in any preceding claim, further comprising at least one guide member attached to each of said first and second sections, said elongated member(s) extending through and being guided by said guide member(s).

9. Aflexjointas claimed in any one of claims 1 to 5, wherein said axial load member comprises a plurality of elongated members grouped in a closely spaced clusteraround and substantially parallel to said longitudinal axis, each of said elongated members being attached to both said first and second sections.

10. Aflex joint as claimed in any preceding claim, wherein said flexjoint has four elongated shear and torsion links, said four links being joined so as to form a substantially planar shear and torsion frame.

11. Aflexjointforuse in an articulated structure having a first section and a second section, said first and second sections being located along a longitudinal axis of said structure, said flexjointcomprising: -an axial load memberattached to said firstand second sections; - a shear and torsion frame located between said first and second sections, said frame being oriented so astro lie in a planesubstantiallyperpendicularto said longitudinal axis; - a first pair of support members, each of said first pair being attached to both said frame and said first section, said first pair being attached to said frame, respectively, at opposite ends of a first diametral line lying in the plane of said frame; and - a second pair of support members, each of said second pair being attached to both said frame and said second section, said second pair being attached to said frame, respectively, at opposite ends of a second diametral line lying in the plane of said frame.

12. A flex joint as claimed in claim 11, wherein the geometric center of said frame is located at or near said longitudinal axis and wherein said first and second diametral lines are substantially mutually per- pendicular.

13. Aflexjointasclaimed inclaim 12,wherein said shear and torsion frame is square in shape and wherein said first and second diametral lines are diagonals of said frame.

14. Aflexjointas claimed in claim 11,12 or13, wherein said axial load member comprises an elongated member located substantially coincident with said longitudinal axis.

15. Aflexjointas claimed in claim 11,11 or13, wherein said elongated member comprises a single tubular element.

16. Aflexjointas claimed in claim 11,12 or 13, wherein said elongated member comprises a plurality of concentrictubular elements, each of which is attached to both said first and second sections.

17. Aflex joint as claimed in any one of claims 11 to 16, said flex jointfurther comprising at least one guide member attached to each of said first and second sections, said elongated member(s) extending through and being guided by said guide members.

18. Aflexjointas claimed in any one of claims11ofclaimsi 1 to 14, wherein said axial load member comprises a plurality of elongated members grouped in a closelyspaced cluster around and substantially parallel to said longitudinal axis, each of said elongated members being attached to both said first and second sections.

19. Aflexjointforuse in an articulated offshore structure having a first, upper, section and a second, lower, section, said first and second sections being located along a substantially vertical axis of said structure, said flex joint comprising: -an axial load member attached to said first and second sections; - a primaryshearand torsion frame located between said first and second sections, said primary frame being oriented so as to lie in a plane substantially perpendicularto said substantially vertical axis;; - a first pair of primary frame support members, each of said first pair being attached to both said primary frame and said first section, said first pair being attached to said primary frame, respectively, at opposite ends of a first diametral line lying in the plane of said primaryframe; - a second pair of primary frame support members, each of said second pair being attached to both said primary frame and said second section, said second pair being attached to said primary frame, respectively, at opposite ends of a second diametral line lying in the plane of said primaryframe; - a secondary shear and torsion frame, said secondaryframe being located in the same plane as and substantially concentric with said primary frame;; - a first couple of secondary frame support members, each of said first couple being attached to both said secondary frame and said first section, said first couple being attached to said secondary frame, respectively, at opposite ends of a third diametral line lying in the plane of said secondary frame; and 29.An articulated offshore structure having a substantially vertical axis, said articulated structure comprising: (a) a first, upper, section and a second, lower, section, the first and second sections being located along said substantially vertical axis; (b) an axial load member extending between and attached to saidfirstand second sections; and (c) at least one flex joint located between said first and second sections, each such flexjointcomprising: (i) a shear and torsion frame oriented so asto lie in a plane substantially perpendicularto said substantiallyvertical axis; (ii) afirstpairofsupport members, each ofsaid first pair being attached to both said frame and said first section, said first pair being attached to said frame, respectively, at a first pair of diametrically opposite points on said frame thereby defining a first diametral line lying in the plane of said frame; and (iii) a second pair of support members, each of said second pair being attached to said frame and said second section, said second pair being attached to said frame, respectively, at a second pair of diametrically opposite points on said frame thereby defining a second diametral line lying in the plane of said frame.

19. Aflexjointforuse in an articulated structure having a first sectiona and a second section, said first and second sections being located along a longitudinal axis of said structure, said flex joint comprising: -an axial loadmemberattachedtesaidfirstand second sections; - a primary shear and torsion frame located between said first and second sections, said primary frame being oriented so asto lie in a plane substantially perpendicularto said longitudinal axis; - a first pair of primary frame support members, each of said first pair being attached to both said primary frame and said first section, said first pair being attached to said primary frame, respectively, at opposite ends of a first diametral line lying in the plane of said primary frame;; - a second pair of primary frame support members, each of said second pair being attached to both said primary frame and said second section, said second pair being attached to said primary frame, respective ly, at opposite ends of a second diametral line lying in the plane of said primaryframe; - a secondary shear and torsion frame, said secondaryframe being located in the same plane as and substantially concentric with said primary frame; - a first couple of secondary frame support members, each of said first couple being attached to both said secondary frame and said first section, said first couple being attached to said secondary frame, respectively, at opposite ends of a third diametral line lying in the plane of said secondary frame; and - a second couple of secondary frame support members, each of said second couple being attached to both said secondary frame and said second section, said second couple being attached to said secondaryframe, respectively, at opposite ends of a fourth diametral line lying in the plane of said secondary frame.

20. Aflexjoint as claimed in claim 19, wherein the geometric centers of said primary and secondary frame are located at or near said longitudinal axis.

21. Aflexjointas claimed in claim 19 or20,where- in said first and second diametral lines are substantially mutually perpendicular and wherein said third and fourth diametral lines are substantially mutually perpendicular.

22. Aflexjoint as claimed in claim 21,wherein said first and fourth diametral lines are substantially coincident and said second and third diametral lines are substantially coincident.

23. A flex joint as claimed in claim 21,wherein said primary and secondaryframes are square in shape and wherein said first and second diametral lines are diagonals of said primaryframe and said third and fourth diametral lines are diagonals of said secondary frame.

24. Aflexjoint as claimed in any one of claims 19 to 23, wherein said axial load member comprises an elongated member located substantially coincident with said longitudinal axis.

25. Aflexjoint as claimed in any one of claims 19 to 23, wherein said elongated member comprises a single tubular element.

26. Aflexjoint as claimed in any one of claims 19 to 23, wherein said elongated member comprises a plurality of concentric tubular elements, each of which is attached to both said first and second sections.

27. Aflexjointas claimed in any one of claims 19 to 26, further comprising at least one guide member attached to each of said first and second sections, said elongated member(s) extending through and being guided by said guide members.

28. Aflexjointas claimed in any one of claims 19 to 23, wherein said axial load member comprises a plurality of elongated members grouped in a closelyspaced cluster around and substantially parallel to said longitudinal axis, each of said elongated members being attached to both said first and second sections.

29. An articulated structure having a longitudinal axis, said articulated structure comprising: (a) first and second sections located along said longitudinal axis; (b) an axial load member extending between and attached to said first and second sections; and (c) at least one flex joint located between said first and second sections, each such flex joint comprising: (i) a shear and torsion frame oriented so asto lie in a plane substantially perpendicularto said longitudinal axis; (ii) a first pair of support members, each of said first pair being attached to both said frame and said first section, said first pair being attached to said frame, respectively, at a first pair of diametrically opposite points on said frame thereby defining a first diametral line lying in the plane of said frame; and (iii) a second pair of support members, each of said second pair being attached to said frame and said second section, said second pair being attached to said frame, respectively, at a second pair of diametrically opposite points on said frame thereby defining a second diametral line lying in the plane of said frame.

30. An articulated structure as claimed in claim 29, wherein said first a nd second diametral linesare substantially mutually perpendicular.

31. An articulated structure as claimed in any preceding claim, wherein said structure is an articulated offshore structure.

32. Aflexjoint substantially as hereinbeforede- scribed with reference to any one ofthe several embodiments depicted by the accompanying drawings.

Amendments to the claims have been filed, and have the following effect: *(a) Claims 11, 19 and 29 above have been deleted or textually amended.

*(b) New ortextually amended claims have been filed asfollows:

11. Aflexjointforuse in an articulated structure having a first section and a second section, said first and second sections being located along a longitudinal axis of said structure, said flex joint comprising: - an axial load member attached to said first and second sections; and - a shear and torsion frame located between said first and second sections, said frame being oriented so asto lie in a plane substantially perpendicularto said longitudinal axis, said frame being connected to said first section at opposite ends respectively'of a first diametral line lying in the plane of said frame, and said frame being connected to said second section at opposite ends respectively of a second diametral line lying in the plane of said frame.