US3550549A

US3550549A - Tension anchor system for offshore apparatus

Info

Publication number: US3550549A
Application number: US796788A
Authority: US
Inventors: Edward E Horton
Original assignee: Deep Oil Technology Inc
Current assignee: Fluor Corp
Priority date: 1969-02-05
Filing date: 1969-02-05
Publication date: 1970-12-29
Anticipated expiration: 1987-12-29

Description

United States Patent [111 3,550,549

[72] Inventor Edward E. Horton [56] References Cited Portuguese Bend, Calif- UNITED STATES PATENTS Q1;- 38*?? 3,466,680 9/1969 Schirtzinger 114/230x 45 Patented Dec. 2;, 1970 FOREIGN PATENTS [73] Assignee DeepQ il I ec h no l ogy,lnc. 598,037 2/l948 GreatBritain 114/230 Long Beach, Calif.

Primary Examiner-Trygve M. Blix a corporation of California Attorney-Miketta, Glenny, Poms and Smith ABSTRACT: A tension type anchor system for offshore apparatus for conducting well drilling, production or the like operations including a pair of upper and lower tension means, the lower tension means being anchored and vertically maintained by a subsurface buoy at a selected depth. The upper tension means is adapted to move longitudinally relative to the lower tension means and in extended position the upper ten- [54] TENSION ANCHOR SYSTEM FOR OFFSHORE APPARATUS 9 Claims, 4 Drawing Figs.

[52] US. Cl 114/230, sion means and the lower tension means are provided with a l l4/0.5 connecting means for directly transmitting tension forces in- [5 1] Int. Cl B63b 21/00, dependently of the subsurface buoy. ln nonextended relation, B63b 35/00 the upper tension means is supported by the subsurface buoy {50] Field of Search 1 14/05, in loosely interlinked generally parallel relation with the lower 0.5T, 230; 9/80? tension means.

TENSION ANCHOR SYSTEM FOR OFFSHORE APPARATUS BACKGROUND OF THE. INVENTION Offshore well apparatus includes provision of platform structures supported above a water surface at a selected height as determined by various conditions and supported by several different types of supporting constructions. In a type of construction with which the present invention is concerned, a working platform is supported by a subsurface buoyant means which is held at a selected location by a plurality of anchor means and tension means, such as cable or chain, connecting the anchor means with the buoyant means. The buoyant means is provided with a positive buoyancy sufficient to maintain a selected tension on the tension means and to support the working platform under varying conditions of load and environmental conditions. Thus, the tension means for a tension leg platform" requires that the platform be held at a selected location and height, and further when a permanent anchoring system is desired, the tension means should remain in a vertical position at a sufficient distance below the height of the water so that the anchoring system will not present a navigational hazard.

Prior proposed tension means have included cables extending from the anchor at the ocean floor to a subsurface buoy, the subsurface buoy being then connected by a chain to a surface buoy or platform. In one prior proposed system, the upper chain portion is passed through a hawsepipe on a pon toon for cooperable engagement with a jack means for imparting tension to the tension anchor means. Such prior proposed systems are satisfactory, however, it should be noted that the use of chain in subsea installations presents a difficult problem of preventing rust and corrosion forming on the chain links since they are difficult to completely protect with corrosion resistant coatings, particularly at adjacent loop ends of each chain link. Thus, over a chain of substantial length there are a multitude of places which, although initially protected by a protective or corrosion resistant coating, a may rapidly be subjected to wear and penetration of the coating by relative movement between chain links to thus expose the chain metal to corrosion. In addition, chain of this type has a relatively low strength to weight ratio. Use of such chain also requires a relatively large surface buoy to support the weight of the chain. In the event of failure of the surface buoy and an insufficient design or failure of subsurface buoys which support lower tension means, a progressive failure of the entire tension anchor system may occur since the entire anchor tension array may sink. Moreover, in such tension systems employing heavy anchor chain supported by a surface buoy, when water level would vary because of wave or tide conditions, the upper chain portion would slacken as the water level fell and such slack. would accumulate at the lower end of the chain just above the subsurface buoy and would often cause damage to or fouling of the anchor system. Obviously this condition could be avoided by shortening the upper chain portion and maintaining the chain taut by pulling the surface buoy under water. Such underwater location close to the surface is not desirable because of the navigational hazards created.

The present invention contemplates a tension anchor system which obviates many of the undesirable characteristics of prior proposed tension anchor means and provided provides a novel anchor tension means. The tension anchor means contemplated by the present invention provides a structurally reliable safe tension anchor system utilizing tension members having a greater strength to weight ratio than provided by chain-type tension members and which does not become a navigational hazard when not is use. The present invention also provides an anchor construction which may be effectively protectively coated with corrosion-resistant materials throughout its entire length and thus becomes lee less susceptible to corrosion. Moreover, the tension anchor system contemplates an arrangement of tension members wherein force transmitting areas are reduced to a minimum.

LII

The primary object of the present invention is to design and provide a tension anchoring system of novel construction for maintaining a platform of tension leg-type at a selected site and for maintaining a subsurface buoyant or pontoon means supporting such a platform at a selected depth in the water.

An object of the present invention is to disclose and provide a tension anchoring system wherein the entire tension means is maintained in vertical relation beneath the ocean surface and without employment of a surface buoy.

Another object of the present invention is to disclose and provide a tension anchoring system readily adapted to be protectively coated and wherein the tension anchor system may be utilized with minimum or no damage to the protective coating.

A further object of the invention is to disclose and provide a tension anchoring system wherein a pair of tension members are employed and are interlinked together so that they may be maintained in a vertical position beneath the ocean surface and without reliance upon a surface buoy.

A still further object of the invention is to disclose and provide a tension anchoring system wherein upper and lower tension members are supported in collapsed or nonextended rela tion by a subsurface buoy which maintains at least one of said tension members under a selected tension force and which is adapted and capable of supporting the dead weight of the upper or there other tension member.

A still further object of the invention is to disclose and provide a tension anchoring system wherein upper and lower tension members interrelated as described above when in nonextended position, are interrelated by a connecting means for directly transmitting longitudinal tensional forces from one member to the other without transmitting such forces through a subsurface buoy which normally supports and maintains tension on the lower tension member.

Various other objects and advantages of the present invention will be readily apparent from the following description of the drawings in which an exam exemplary embodiment of the present invention is h shown.

In the drawings:

FIG. 1 is a side elevational view of a tension means embodying this invention in nonextended relation.

FIG. 2 is a side elevational view showing the tension means of FIG. l in extended relation and connected to a floating submersible pontoon means prior to submerging.

FIG. 3 is an enlarged fragmentary side elevational view of tension member connecting means at the submersible buoy means.

FIG. 4 is a vertical sectional view taken in the plane indicated by line lV-lV of FIG. 3.

In FIG. I there is shown a tension means generally indicated at It) for anchoring offshore well apparatus of tension leg type wherein a submersible buoy support means is anchored in water over a selected location and at a selected depth for supporting a working platform at a selected height above the water surface. Tension means 10 generally comprises a pair of tension members 11 and I2 associated with a submersible or subsurface buoy means 14 which is located below the surface 15 of the water a selected depth which is sufficient to prevent buoy 14 from becoming a navigational hazard. A marker buoy 16 floats on the water surface 15 to indicate location of the anchor site and may be attached to tension member 12 by a light pennant line 17. Tension means may be anchored at selected location by an anchor means 18.

Anchor means 18 bay comprise any suitable anchor body 19, such as a deadweight mass of concrete or other heavy dense anchor material, or it may comprise a type of c anchor means which is driven or drilled into the ocean floor, such as a pile or the like. Anchor body 19 may be provided with an eye 20 which provides a connecting means for the lower end of tension member 11.

The pair of tension members 11 and 12 may be similar in construction each including a pair of

tension cables

21,21 and 22, 22 respectively. The lower ends of cables 21 are con nect'ed and joined by a suitable metal bail 23 which passes through and engages anchor eye 20. The upper ends of cables 21 are connected to an upper bail 24. Bail 24 is provided with an upstanding plate 25 having a port 26 at each upper plate corner for receiving chain link 27 connected to a parted lug 28 secured as by welding at 29 to wall 30 of one of the buoy members 31 forming buoy means 14. Pivotal connections provided by ported lug 28 and link 27, plate 25 and link 27, and pivotal connections provided between ends of cable 2ll and the

bails

23 and 24 distributes the load between tension cables 21. Tension member 11 is thus pivotally supported and hung from buoy l4.

Tension member 12 similarly includes tension cables 22, the lower ends of which are pivotally connected at 33 to a bail 34. The upper ends of tension cables 22 are similarly pivotally connected at 35 to a bail 36. It should be noted that the lower bail 34 lies between cables 21 and that the continuous loop formed by the lower tension cables 21 and

bails

23, 24 is linked and interconnected with the loop formed by the upper cables 22 and

bails

34, 36. Tension members ill and l2 are thus arranged along adjacent generally vertical parallel lines, tension member 12 being longitudinally telescoped or collapsed for a substantial part of its length with respect to tension member 11 (FIG. 1) and when extended (FIG. 2) tension member 12 lies along a line which is generally a continuation of the vertical direction or line of lower tension member ill.

The submersible buoy means 14, in this example, comprises a pair of parallel horizontally spaced apart elongated buoy members 3i. of cylindrical cross-sectional configuration. Buoy members 31 are held in spaced apart relation to define an opening 44 by a plurality of longitudinally spaced top transverse bars 40 secured in suitable manner as by welding to the wall of buoy members 31. A plurality of bottom transverse bars 41 may also interconnect buoy members 31 below a horizontal diametrical plane passing through the buoy members 31. Bars 41 may be suitably secured as by welding to buoy member 311.

As best shown in FIG. 1 when the tension members I! and 12 are in nonextended relation and alongside each other bail 36 of upper tension member 12 is supported and carried by one of bars 40 of buoy 14, the spaced tension cables 22 as shown in FIG. 4 passing on opposite sides of a set of vertically aligned

bars

40 and 41. Thus, in nonextended relation buoy M supports and carries tension member 12 and also maintains a selected tension on tension member 11 so that tension member ill will remain substantially vertical.

in fully extended relationship of tension members 11 and 12, the bottom bail 34 of tension member 12 engages at 42 top bail 24 of tension member ill. Opening 44 provided between buoy members 3i permits free passage of cables 22 therethrough so that tension forces or leads imposed upon tension member 12 by a suitable tension applying means 45 carried by a working platform structure generally indicated at 46 may be transmitted directly between tension members Ill and 12 at 42 and fully independently of the buoy 14.

A working platform structure is only partially illustrated in FIG. 2 and generally comprises a submersible buoyant support means 47 carrying platform legs 48 for supporting thereabove a platform (not shown). As described in my copending application Ser. No. 735,320, the submersible buoyant means 47 may be submerged to a selected depth for supporting the platform a selected distance above the water surface and may then be imparted sufficient positive buoyancy so as to maintain a predetermined tension force on tension means 10.

When the tension anchor system described above is employed, it will be apparent from a consideration of FIG. 1 that the entire tension means, including upper and lower tension members 12and ll, is supported in vertical relation by subsurface buoy 114 located a selected distance below the surface of the water so that it will not constitute a navigational hazard and so that it will be a sufficient distance below the surface of the water to permit the positioning of a subsurface support buoyant means such as 4'7 thereabove for anchoring an offshore platform structure. In such nonextended or collapsed position, the tension anchor means may be marked by the pennant floating buoy 16 which is of very light weight and pennant line 17 carries virtually no load. In such collapsed relation, it will be apparent that the only steady or continuous contact between the upper tension members 12. and subsurface buoy M will occur at the contact area between the inner surface of bail 36 and bar 40. Thus, a very minimum of contact area which may produce wear and corrosion is provided.

Also in extended relation of the upper and lower tension members it will be readily apparent that the only steady and continuous contact under tension conditions will be at 42 between top bail 24- of lower tension member 11 and bottom bail 34 of upper tension member 12. As above a minimal contact area is provided.

The interlinking of tension loops of cable with minimal contact of the cable under load and no-load conditions provides a construction which a can be readily uniformly coated with a suitable protective composition and in which the coating is subjected to force loads at only the minimal contact areas described above.

The interlinking of the loops of tension members 11 and 12 by a pivotally mounted or hung top bail 24 on subsurface buoy l4 and free passage of member 22 through an opening in buoy l4 eliminates transmission of tension forces through buoy 14 to upper tension member l2 and longitudinal or axial loads are directly transmitted between the upper and lower tension members. Thus. subsurface buoy 14 serves to provide a sufficient upward tension force on lower tension means 11 to maintain the lower tension means in vertical relation under virtually all conditions and also serves to support the deadweight of the upper tension means when tension forces are not applied to the system and the anchor system is not in use. A permanent anchor system is thus provided; the system being readily available for use while not presenting a hazard to navigation.

It will be readily apparent to those skilled in the art that various modifications and changes may be made in the tension anchor system described above which come within the spirit of the invention and all such changes and modifications coming within the scope of the appended claims are embraced thereby.

Iclaim:

ll. Tension means for c anchoring offshore well apparatus comprising:

a pair of longitudinally relatively movable tension members arranged along adjacent generally vertical parallel lines; one tension member having its lower end portion adapted to be anchored;

submersible buoy means for holding said one tension member under selected tension at a selected depth and for supporting the other tension member alongside said one tension member in depending relation to said buoy means when said tension members are in nonextended relation; and

means at said buoy means for direct connection of said tension members when said tension members are in extended relation to transmit tension forces independently of said buoy means.

2. Tension means for anchoring offshore well apparatus comprising:

a pair of longitudinally relatively movable tension members arranged along adjacent generally vertical parallel lines; each tension member being in the form of a continuous vertical loop;

one tension member having its lower end portion adapted to be anchored;

submersible buoy means for holding said one tension member under selected tension at a selected depth and of for supporting the other tension member when said tension members are in nonextended relation; and

3. A tension means as stated in claim 2 where in said loops of each tension members are loosely interlinked.

4. A tension means as stated in claim 1 wherein said submersible buoy means includes an opening for vertical passage of said other tension member.

5. A tension means as stated in claim 4 wherein said submersible buoy means includes means at said opening for supporting engagement with means at the upper end portion of said other tension member.

6. A tension means as stated in claim 1 wherein said submersible buoy means includes spaced buoyant members defining opening for passage of said other tension member.

7. A tension means as stated in claim 1 wherein each tension member includes a pair of spaced cables, and bail members connecting adjacent ends of said cables.

8. In an anchor system for use on offshore operations, the combination of:

anchored vertically extending tension means including a force connecting means at the upper end thereof; longitudinally relatively movable tension means extending alongside and loosely interengaged with said anchored tension means;

one end of said movable tension means being supported by said upper end of said anchored tension means and adapted to be attached to a pennant line leading to a surface buoyant means under nonforce transmitting conditions;

the other end of said move movable tension means being engageable with said connecting means for directly transmitting tension forces between said anchored and said tension means under force transmitting conditions.

9. In an anchor system as stated in claim 8. wherein said anchored means includes:

a subsurface buoy at the upper end of said anchored vertically extending tension means; and

said subsurface buoy imparting tension to said anchored tension means in sufficient amount to additionally support in vertical relation said movable tension means under nonforce transmitting condition whereby said anchor system is maintained in vertical position at a selected depth below the surface of the water.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No, 3,550,549 Dated December 29, 1970 Edward E. Horton Inventor(s) It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

2 line 27, cancel "there"; line 38 cancel "exam"; line 39 cancel "h"; line 66, cancel "bay" and insert may line cancel "c". Column 3, line 5, "parted" should read ported line 52, "leads" should read loads Column 4, line 45, cancel "c". Column 5, line 5, "1" should read 2 line 1 18, "on" should read in Signed and sealed this 25th day of May 1971.

(SEAL) Attest:

EDWARD M.FLETCHER,J-R. WILLIAM E. SCHUYLER, Attesting Officer Commissioner of Paten USCOMM- DC 603 Column 1 line 39 cancel "a"; line 72, cancel "lee". C01

"1" should read Z line 15, "1" should read 2 1i: