US3709182A

US3709182A - Anchor means and method of installing the same

Info

Publication number: US3709182A
Application number: US00013536A
Authority: US
Inventors: E Horton
Original assignee: Deep Oil Technology Inc
Current assignee: Fluor Corp
Priority date: 1970-02-24
Filing date: 1970-02-24
Publication date: 1973-01-09
Anticipated expiration: 1990-01-09

Abstract

A drilled-in anchor means and method of installing such an anchor means in a subsea formation, the anchor means including prestressed cable and casing means imbedded in cement which fills an anchor hole in the subsea formation. A subsurface buoy at a selected distance above the surface of the subsea formation provides a connection to a mooring line which may extend from a platform means or other floatable structure. In the method of installing such an anchor means after a drill hole has been made by using a drill casing, the casing is raised a selected distance and then is subjected to stress by a tension cable cooperable therewith, and then the casing and cable means is lowered into the hole. The casing means and hole are then filled with a cement slurry whereby the drill casing and cable are protected against corrosive action of the water.

Description

United States Patent 1 Horton Jan. 9, 1973 ANCHOR MEANS AND METHOD OF INSTALLING THE SAME [75] Inventor: Edward E. Horton, Portuguese Bend, Calif;

[73] Assignee: Deep Oil Technology, 11112.,v Long 7 Beach, Calif. 221 Filedz Feb. 24, 1970 [211' Appl No.: 13,536

[52] U.S. Cl. ..ll4/206 R [51] Int. Cl..; ..B63b 21/24, B63b 21/50 58] Field of Search ....1 1 4 /.5 o, .5 F, .5 T, .5 R, 114/43.:5, 206 R, 230; '9/8 P,"8 R; 61146.5, 46, 53.68, 53.5, 53.6; 175/7, 320; 166/.5, .6; f 52/726 [56] References Cited UNITED STATES PATENTS 3,359,742 12/1967 7 Blatter ..61 /53.68 x 2,988,893 6/1961 Borrmann et al... ..6l/46 3,221,506 12/1965 Strattonet a1. 114/.5 D X 3,015,512 l/1962-- Oakes ..l75/320X 3,330,338 7/1967 Dozier "114/206 X FOREIGN PATENTS OR APPLICATIONS 511,789 6/1952 Belgium ..52/726 Primary Examiner-Milton Buchler Assistant Examiner- -lesus D. Sote'lo Attorney-Miketta, Glenny, Poms & Smith [57] ABSTRACT means after a drill hole has been made by using a drill casing, the casing is raised a selected distance and then is subjected to stress by a tension cable cooperable therewith, and then the casing and cable means is lowered into the hole. The casing means and hole are then filled with a cement slurry whereby the drill casing and cable are protected against corrosive action of the water.

SCIaims, 12 Drawing Figures PATENTEDJAN 9 ma SHEET 1 OF 3' INVENTO/Z. Eon 420 E. H02 ra/v WM, f #Jm'if 4T roeA/Es J PATENTEUJAH 9 I973 SHEET 2 [IF 3 I INVENTO/Q. Eawaea E. Hoe TON I 6K4 firrae/vEs a'.

PATENTEDJAH 9 ms SHEET 3 [IF 3 ANCIIOR MEANS AND METHOD OF INSTALLING THE SAME BACKGROUND OF THE INVENTION Offshore well systems have included various types of platform means which are floatable, semi-submerged, or fully submerged adjacent a well site at the subsea formation. Such platform means have been anchored and positioned by rigid leg structures and by tension line anchoring or mooring systems. In the prior proposed tension line mooring systems the bottom end of the tension line was usually secured to a deadweight anchor means seated on the floor of the subsea formation. Such deadweight anchor systems are disclosed in U.S. Pats. Nos. 2,399,611, 3,154,039 and my co-pending application Ser. No. 735,320 filed June 7, 1968, now U.S. Pat. No. 3,540,396. Prior proposed deadweight anchor means for tension anchor lines required special handling equipment because of their large size and deadweight in the nature of from 200 to 1000 tons. Thus transportation to a well site of a thousand ton deadweight anchor required special high capacity lifting devices for lifting and then lowering the deadweight anchor to the sea floor. It is readily understandable that precise positioning of such a large deadweight mass becomes difficult. Usually such prior proposed anchor systems employed chains, cables or pipes which were connected to the deadweight anchor by specially constructed means in order to properly transfer the load from the deadweight anchor to the tension line. Moreover, in such prior proposed installations the anchor chain, cable or pipe was exposed directly to the corrosive effects of the sea water. Thus anchor systems for offshore platform rigs and other similar floatable structures were very costly, complex, expensive and difficult to precisely and accurately locate and install at a selected subsea site.

SUMMARY OF THE INVENTION.

The present invention contemplates anovel anchor system and method of installation thereof which avoids the problems of the prior proposed anchor means for tension line mooring systems and' which does not require complex special handling equipment. The present invention contemplates a prestressed anchor means including tension members in which the stress imposed upon said members may be essentially equal so that the tension members will cooperate together to withstand tension forces imposed thereon by a tensioned mooring line connected to the anchor means and to a floatable platform structure.

The primary object of the present invention'therefore is to disclose a novel anchor means and method of installation thereof which is readily installed without special equipment.

An object of the present invention is to disclose and provide an anchor means whereinv tension members are employed, the. tension members being virtually entirely" protected from the corrosive-effects of sea water while an outer member is protected therefrom over a substantial portion of its length.

Another object of the present invention is to disclose and provide a novel anchor means wherein the connection to the tension mooring line is made at a selected distance above the subsea floor or below the water surface instead of at the seafloor.

Another object of the present invention is to provide a novel anchoring system or means which is permanently affixed to a subsea formation and is readily made capable of withstanding maximum tension forces applied to its upper end through a tension mooring line.

A still further object of the present invention is to disclose and provide a novel method of installing such an anchor means which briefly includes the steps of drilling an anchor hole in a subsea formation byusual drilling equipment including a drill casing, raising the casing a selected distance after a selected depth of hole has been drilled, imparting tension to a tension member and compression to a compression member forming part of the anchor means, lowering the prestressed tension and compression means into the hole, and then introducing a cement slurry into the hole through the compression means and also within the compression means until the cement slurry return reaches the surface of the subsea formation, whereby a permanent fixed anchor means is provided thereby.

A still furtherobject of the invention is to disclose and provide an anchor means wherein a special tapered or cross section of varying thickness is provided at the sea floor to afford limited bending of the upper portion of the anchor means without exceeding stress limits therein.

Generally speaking in the practice of the invention a drilling hole is provided adjacent a well site by usual drilling practices employing a drill casing and a bit at the lower end thereof. The drill casing which extends to the rotary table of the drilling platform is then raised a selected distance to shorten the length of the drill casing so that its top end will be located at a selected depth below the water surface. After cutting the drill casing to selected length, a tension cable member is lowered through the casing and secured by slip means at the bottom end of the drill casing. The upper end of the tension cable is connected to a yoke having an adjustable nut for tensioning the cable against the drill casing. After the casing and cable have been stressed as desired, the drill casingis lowered with a landing easing into the hole until it reaches the bottom. A cement slurry is then pumped through a landing casing connected to the top end of the drill casing until the cement slurry appears on' the sea floor between the casing and the wallsof the holein the formation. There is thus provided a prestressed anchor permanently secured to the subsea formation and protected against the corrosive effects of the sea water.

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

IN THEDRAWINGS FIG. 1 is a schematic vertical sectional view showing an initial step in the practice of this invention, that is drilling a hole in a subsea formation, the formation being shown in section and the drilling rig being schematically shown;

FIG. 2 is a view similar to FIG. 1 illustrating the raising of the drilling string a selected amount within the drilled hole;

FIGL'S is a view similar to FIGS. 1 and 2 and showing lowering of the drilling string to the bottom of the hole after a subsurface buoy has been attached to the end thereof and a landing string is attached thereto for lowering the casing to the bottom of the hole and to conduct a cement slurry therethrough;

FIG. 4 is a view similar to FIGS. 1, 2 and 3 and shows the drilled-in anchor means at its completion and with a pennant line attached to a surfacemarking buoy and to the upper end of a mooring cableat the subsurface buoy; FIGS. 4 and 4b are enlarged fragmentary sectional views showing the anchor construction above and below the sea floor;

FIG. 5 is an enlarged fragmentary vertical sectional view showing the top and bottom connections of the cable tension member with the drill casing compression member;

FIG. 6 is an enlarged transverse sectional view taken in the plane indicated by line VI-Vl of FIG. 5;

FIG. 7 is a fragmentary enlarged view, partly in section, showing construction of said anchor means at the surface of the sea floor;

FIGS. 8a, 8b and 8c illustrate a method ofmooring a tension leg platform offshore structure to an anchor means of this invention.

With first reference to FIG. 4, an anchor means 10 embodying the present invention and installed in accordance therewith generally comprises a drilled hole 11 in a subsea formation 12 having a subsea surface 14 which may be located at depths up to 2000 feet or more from surface 15 of a body of watenExtending within hole 11 and below and above the subsea surface 14 is a drill casing compression means 16 which carries therewithin a tension cable means 17 extending from the bottom end portion 19 of the drill casing means 16 to a yoke means 20 carried by a subsurface buoy 21. Yoke means 20 provides a connection to a pennant line 22 for a surface buoy 23 which serves to mark the location of anchor'means l0. Drill casing 16 with tension cable 17 therein is filled cement means 24 and the hole' 11 is likewise filled with the cement means 24 for the depth of hole 11. Yoke means 20 provides a connection to mooring cable lines 26 shown in non-operative or lowered position in FIG. 4.

The invention contemplates utilizing well-known drilling equipment and techniques to drill subsea hole 11 in formation 12 to a selected depth at a selected location for an anchor to be used for mooring and securing a floatable structure, such as various types of offshore platform means or vessels which are desired to be positioned near or adjacent a well drilling or production operation. Drilling operations have been conducted in depths of water up to 2000 feet or more and it is contemplated that anchor means 10 in an anchor hole 11 may be provided virtually wherever an offshore subsea drilling operation can be performed.

Anchor means 10 may be best described in detail by reference to the method of installing the anchor means. In FIG. 1 a drilling rig 30 of an suitable well-known type used in offshore well drilling is schematically illustrated and includes a derrick 31 which may be equipped with usual drilling equipment including crown and traveling sheaves 32 for raising and lowering a drill casing through a rotary table and Kelly arrangement (not shown) and the coupling drill casings together. In the drilling operation, drill casing 16 may be connected to and drivesa well known suitable drill bit 34. The drilling string may include drill collars 35, a bumpersub 36 and any other type of well tool normally employed in drilling a subsea hole. Hole 11 may be drilled to aselected depth depending upon the type of floatable structure to be anchored and may be drilled to a depth of, for example from 300 to 1000 feet.

After hole 11 has been drilled to itsselected depth, drilling string 16 is raised a selected height, in this ex ample illustrated as 125', which determines the distance below the surface 15 of the water at which the subsurface buoy 21 will be located. It is desirable that the anchor means 10 and subsurface .buoy 21 at the upper end of anchor means 10 be located at sufficient depth below the surface of the water so that it is virtually unaffected by weather conditions and by vessels moving in the location of anchor means 10. The lower end portions of drilling string 16 is not removed from hole 11 during raising thereof. When drill casing 16 has been lifted a selected distance and the upper raised portion of the casing removed,a subsurface, buoy 21 is secured to the top end of drill. casing 16 in suitable manner. Subsurface buoy 21 is-provided with sufficient buoyancy to exert an upwardly directed buoyant force on the anchor means 10 at least equalto the portion of the drill casing which extends above subsea surface 14 when casing 16 carries cable 17 and cement means 26 therein. Thus, buoy 21 supports the deadweight of this upper portion of the drill casing l6-whenthe anchor means is completed.

In FIG. 5 only a portion of subsurface buoy 21 is illustrated and includes an outer shell 40 of suitable wellknown construction defining buoyant chambers and provided with a central through opening 41 for drill casing 16 and with two parallel through openings 42 for slidable reception therethrough of mooring cable lines 26.

Above subsea buoy 21, yoke means 20 may be connected to the top end of drill casing 16 as by a threaded connection at 44. Yoke means 20 may comprise a body member 46 of suitable shape provided with diametrically opposite through openings 47 for reception therethrough of mooring cable lines 26 and for providing a seat at 48 for a lower enlarged socket end 49 of mooring cable line 26 for transmitting tension forces through the yoke means 20 to anchor means 10. The body member 46 also includes an central throughbore 50 extending from adjacent the threaded connection at 44 and having circumferentially spaced longitudinal through slots 51, receiving adjustable tensioning and securing means for the top end of tension cable l7..The upper end of body member 46 may be provided a threaded connection 52 for'a landing string 53 which is attached thereto for lowering the' shortened drill casing until the drill bit 34 reaches the bottom of hole 11 and to thereby position the subsea buoy 21 at its selected distance below the water surface 15.

Before lowering drill casing 16 to the bottom of hole 11 and while drill casing 16 has'its upper end accessible at the rotary table for connecting yoke means 20 thereto and before the landing string 53 is attached thereto, tension cable 17 of selected strength, such as bridge strand type is inserted and lowered into drill casing 16for connection thereto. Adjacent'its lower end drill casing 16 is provided with an enlarged portion 56 providing an'enlarged internal chamber 57 which forms a downwardly directed shoulder 58 on the drill casing.

The lower end of tension cable 17 may be provided with a socket member 59 having an outer generally frustoconical or tapered surface 60. A plurality of circumferentially arranged slips 61, which maybe carried in radial collapsed relation against the tension cable 17 above socket member 59 so that they may pass with cable 17 downwardly through the drill casing 16, are movable radially outwardly when socket 59 passes below shoulder 58 at least the length of a slip 61 so that the slips 61 will be positioned between the internal surfaces of the enlarged chamber 57 and the outer tapered surface 60 of the socket member'59. When tension cable 17 is then moved upwardly, the top edges of slips 61 seat against the internal annular shoulder 58 and as the member 59 moves upwardly in wedging relationship with the slips 61, the lower end of tension cable 17 is securely frictionally held against further upward movement.

The upper end of tension cable 17 is provided with a socket member 63 having a top elongated threaded portion 64 and a nut 65 threaded thereon. Asnut 65 is tightened on the threaded shaft 64 it bears against a thrust washer 71 having a conical seat on the upper end of body member 46 as at 72. Tension cable 17 is placed under tension while at the sametime drill casing 16 is placed under compression through the connections of body member 46 therewith at 44 and of socket 59 with shoulder 58.

The tension force applied to cable 17 may be suitably selected depending upon the projected ultimate loads to be applied to the anchor means when connected to a floatable structure. Since the purpose of applying the tension force to the cable is to develop maximum strength of the cable and casing as a unit, the amount of prestressing becomes a function of the elastic properties or modulus of elasticity of the material of the cable and casing, their ultimate strength, the geometry and size and dimensions of the system including the dead weight of the length of casing and cement therein when installed. Thus, the amount of tension force applied to the tension cable is such that when tension is applied to the anchor means by the floatable structure, the prestressed cable and casing will be capable of withstanding maximum forces by developing maximum restraint in both members. Thus before lowering the drill easing into hole 11, the casing means 16 and cable means 17'are prestressed to develop such maximum force.

Upon prestressing the drill casing 16 and tension cable 17 and upon connecting mooring cable lines 26 to yoke means 20 and connecting the upper ends of mooring lines 26 to a suitable bail means 74 to which the pennant line 22 is attached, landing string 53 may be connected to the yoke means and the prestressed tension cable 17 and drill casing 16 lowered into the hole 1 1 until the bit 34 rests upon the bottom thereof. It

ble type and composition is introduced under pressure by pumping through the upper end of landing string 53. The cement slurry passes through the passageways 51 in yoke member 46, into the drill casing 16 and then through the passageways 57 between the slips 61, through the passageways in the drill bit 34 which is on or adjacent to the bottom of the hole 11 and then passes upwardly in hole 11 on the outside of drill casing 16. The cement slurry is continuously pumped through the drill casing and hole 11 until it reaches the surface 14 of the subsea formation as shown in FIG. 7. The

' landing string 53 may thenbe disconnected from the -14 to the yoke means 20. Thus throughout its entire length, tension cable 17 is provided with a protective coating and throughout that portion of its length within hole 11 drill casing 16 is provided with a protective coating within the hole 11. That portion of drill casing 16 above surface 14 which is filled with cement to the yoke means 20 will stand upright in relatively rigid fashion and is further supported in said vertical upright position by the buoyancy of subsea buoy 21.

In FIG. 7 a modification of the drill casing 16 is illustrated in which drill casing 16 at the surface 14 may be provided with a plurality of casing portions-16' and 16" of progressively reduced diameter so as to provide a variable decreasing cross-sectional area adjacent surface- 14 to allow bending of the upper portion of the drill casing at the point where it emerges from the concrete 24 without exceeding allowable stresses in the casing 16.

When it is desired to anchor a vessel or a floatable structure to anchor means 10, it will be understood that I such a vessel may approach the surface buoy 23 to engage pennant line 22. Such a vessel is illustrated at 76, FIGS. 8a, 8b and 8c. After pennant line 22 is grasped by a line from vessel 76, the ball 74 and mooring cable lines 26 maybe pulled upwardly by suitable winch means generally indicated at 81, the lines 26 being passed over a guide roller 82 at one side of the hull 83 of the vessel. In this example, vessel 76 may be semisubmersible structure so that by application of suitable power means through the winch means 81 and connection to similar anchor means 10 at opposite ends of the vessel 76, the vessel may be submerged by pulling the vessel downwardly into the water by the anchor means 10. Submergenceof vessel 76 may also be assisted by applying negative buoyancy to the buoyant members of the hull 83 so that whenthe vessel reaches a desired depth, the anchor means 10 and mooring cable lines 26 may be placed under selected tension.

It will thus be understood by those skilled in the art that drilled-in and cemented anchor means 10 provides an immovable composite anchor member readily installed and for use with an offshore floatable structure of the tension leg type. The anchor means may be readily installed by suitable well-known drilling rigs. The tension cable is protected throughout its length against corrosion from the sea water and making a connection to an anchor weight at the surface of the subsea formation is avoided. The combined weight of the drill casing and cement therein together with the bonding and engagement of the cement with the casing and walls of the hole provides an anchor means of exceptional holding strength and which may be readily installed according to varying anchor strength requirements.

lt will be understood that various changes and modifications may be made in the anchor means described above, that other protective coatings other than cement or a cement slurry may be employed and that the drill casing may embody other configurations which may facilitate the anchoring of the drill casing and tension member within the subsea hole. All changes and modifications coming within the scope of the appended claims are embraced thereby.

lclaim:

1. In an anchor means for an offshore flotable structure and including a hole drilled in a subsea formation;-

the provision of:

a drilling casing extending into said hole and above said hole a selected distance;

a cable connected at its ends to corresponding ends of said casing;-

means for prestressing said casing and said cable for combining their strength in tension and for action as a unit under maximum tension forces whereby said casing and said cable will exceed their elastic limits under approximately maximum tension forces; and

cement slurry means in said formation hole and in the portions of said casing in said hole for anchoring said casing and cable in said hole;

said cement means also filling portions of said casing above said hole to the upper end of said casing,

said cement means bonding said cable and said casing together for its entire length whereby said unitary action under maximum tension forces is enhanced and a structurally stiffened anchor portion is provided above said hole.

2. In a method for anchoring tension mooring lines to offshore platforms and the like, comprising the steps of:

drilling a hole in a subsea formation to a selected depth including use of a drill casing;

raising the drill casing to a selected height;

lowering a cable through said casing;

securing the lower cable end at the casing bottom end portion against upward movement of the cable;

securing a rotatable means to the upper cable end in seating engagement with an upper casing yoke means cemented to walls of the formation hole. 3.-ln a method as stated in claim 2 ll'lCllldlllg the steps connecting a subsurface buoy to said casing yoke means; and

connecting a mooring line and surface buoy to said subsurface buoy.

4. In a method for anchoring tension mooring lines to offshore platforms and the like, comprising the steps of:

drilling a hole in a subsea formation to a selected depth including the use of a drill casing;

raising said drill casing until the bottom end thereof is a selected height above the bottom of the hole;

securing the lower end of the cable at the casing bottom end portion against the upper movement of the cable;

connecting the upper end of the cable to the upper end of the casing and to a tensioning means;

placing said casing and cable, under selected stress by said tension means whereby said casing and cable act as a unit under conditions of maximum forces applied thereto;

lowering the prestressed drill casing and cable to the bottom of the hole in said formation by a landing string means;

introducing a cement slurry in plastic flowable state through said landing string means between said cable and drill casing and between said casing and walls of the formation hole until the hole is filled and the casing is filled; and whereby a prestressed cable and casing embedded in concrete slurry provides a prestressed stiff solid anchor member cemented'to walls of the formation hole.

5. In the method as stated in claim 20 including the step of: I

buoyantly supporting the weight of that portion-of the anchor member which extends above the surface of the subsea formation.

Claims

1. In an anchor means for an offshore flotable structure and including a hole drilled in a subsea formation; the provision of: a drilling casing extending into said hole and above said hole a selected distance; a cable connected at its ends to corresponding ends of said casing; means for prestressing said casing and said cable for combining their strength in tension and for action as a unit under maximum tension forces whereby said casing and said cable will exceed their elastic limits under approximately maximum tension forces; and cement slurry means in said formation hole and in the portions of said casing in said hole for anchoring said casing and cable in said hole; said cement means also filling portions of said casing above said hole to the upper end of said casing, said cement means bonding said cable and said casing together for its entire length whereby said unitary action under maximum tension forces is enhanced and a structurally stiffened anchor portion is provided above said hole.

2. In a method for anchoring tension mooring lines to offshore platforms and the like, comprising the steps of: drilling a hole in a subsea formation to a selected depth including use of a drill casing; raising the drill casing to a selected height; lowering a cable through said casing; securing the lower cable end at the casing bottom end portion against upward movement of the cable; securing a rotatable means to the upper cable end in seating engagement with an upper casing yoke means connected to said casing; placing said casing and cable under stress by adjustably turning said rotatable means; lowering the prestressed drill casing and cable to the bottom of the hole in said formation by a landing string means; introducing a cement slurry in plastic flowable state through said landing string means between said cable and drill casing and between said casing and walls of the formation hole until the hole is filled; and whereby a prestressed cable and casing embedded in concrete slurry provides a prestressed anchor means cemented to walls of the formation hole.

3. In a method as stated in claim 2 including the steps of: connecting a subsurface buoy to said casing yoke means; and connecting a mooring line and surface buoy to said subsurface buoy.

4. In a method for anchoring tension mooring lines to offshore platforms and the like, comprising the steps of: drilling a hole in a subsea formation to a selected depth including the use of a drill casing; raising said drill casing until the bottom end thereof is a selected height above the bottom of the hole; securing the lower end of the cable at the casing bottom end portion against the upper movement of the cable; connecting the upper end of the cable to the upper end of the casing and to a tensioning means; placing said casing and cable under selected stress by said tension means whereby said casing and cable act as a unit under conditions of maximum forces applied thereto; lowering the prestressed drill casing and cable to the bottom of the hole in said formation by a landing string means; introducing a cement slurry in plastic flowable state through said landing string means between said cable and drill casing and between said casing and walls of the formation hole until the hole is filled and the casing is filled; and whereby a prestressed cable and casing embedded in concrete slurry provides a prestressed stiff solid anchor member cemented to walls of the formation hole.

5. In the method as stated in claim 20 including the step of: buoyantly supporting the weight of that portion of the anchor member which extends above the surface of the subsea formation.