CA1055790A - Anchored offshore structure with sway control - Google Patents
Anchored offshore structure with sway controlInfo
- Publication number
- CA1055790A CA1055790A CA280,919A CA280919A CA1055790A CA 1055790 A CA1055790 A CA 1055790A CA 280919 A CA280919 A CA 280919A CA 1055790 A CA1055790 A CA 1055790A
- Authority
- CA
- Canada
- Prior art keywords
- elongate member
- guy
- lines
- elongate
- ocean floor
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
Classifications
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B63—SHIPS OR OTHER WATERBORNE VESSELS; RELATED EQUIPMENT
- B63B—SHIPS OR OTHER WATERBORNE VESSELS; EQUIPMENT FOR SHIPPING
- B63B35/00—Vessels or similar floating structures specially adapted for specific purposes and not otherwise provided for
- B63B35/44—Floating buildings, stores, drilling platforms, or workshops, e.g. carrying water-oil separating devices
- B63B35/4406—Articulated towers, i.e. substantially floating structures comprising a slender tower-like hull anchored relative to the marine bed by means of a single articulation, e.g. using an articulated bearing
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B63—SHIPS OR OTHER WATERBORNE VESSELS; RELATED EQUIPMENT
- B63B—SHIPS OR OTHER WATERBORNE VESSELS; EQUIPMENT FOR SHIPPING
- B63B21/00—Tying-up; Shifting, towing, or pushing equipment; Anchoring
- B63B21/50—Anchoring arrangements or methods for special vessels, e.g. for floating drilling platforms or dredgers
Abstract
ANCHORED OFFSHORE STRUCTURE
WITH SWAY CONTROL
ABSTRACT
An offshore structure adapted to carry a drilling or production platform. The structure has a positive buoyancy and is tied-down to the ocean floor by tensioned cables such that the structure is maintained above the ocean floor and is tiltable. A plurality of anchored guy lines connect to the structure and are coupled to sway control apparatus that prevents excessive tilting of the structure by altering the length of the guy lines between the structure and the anchors. The sway control apparatus comprises first and second winches having a separate reel for each guy line, with each guy line being coupled to a reel through a rigging device mounted on the structure. The winches are operable in response to off-vertical movement of the structure as produced by excessive wind, waves and current.
WITH SWAY CONTROL
ABSTRACT
An offshore structure adapted to carry a drilling or production platform. The structure has a positive buoyancy and is tied-down to the ocean floor by tensioned cables such that the structure is maintained above the ocean floor and is tiltable. A plurality of anchored guy lines connect to the structure and are coupled to sway control apparatus that prevents excessive tilting of the structure by altering the length of the guy lines between the structure and the anchors. The sway control apparatus comprises first and second winches having a separate reel for each guy line, with each guy line being coupled to a reel through a rigging device mounted on the structure. The winches are operable in response to off-vertical movement of the structure as produced by excessive wind, waves and current.
Description
`\ ^ ` ~ss~
~NCHORED OTi`FSllORr~' STRUCTUR~
WIT~I SW~Y CONTROI
BACICGROUND OF Tl-IE :[NVENTION
The present invention relates to an offshore structure for disposition in a body of deep water; and more partic-ularly, this invention relates to an offshore structure carrying a platform thereon and held by guy wires secured to anchors on the ocean floor.
offshore structures in current use in connection with the drilling and production of hydrocarbon deposits in offshore locations have generally included a platform held above the surface of the water by support members which rest on the ocean floor. Such structures, referred to as fixed platforms, have been quite successfully utilized in opera tions conducted in shallow water. However, as offshore drilling operations move into areas having deeper waters, such as about 300 feet, the fixed platform design becomes less desirable, primarily because of the great expense to fabricate and install such a structure. As a general rule the cost of fabrication and installation of a fixed platform designed in accordance with proven shallow water methods and technology will increase exponentially with water depth.
~ lthough an alternative to the fixed platform design is the floating platform concept, that approach is found to present additional, and perhaps even more serious disad-vantages. Floating platforms have the disadvantage of being more suseeptible to rough sea conditions and exhibit the undesirab~ie feature of signi~icant heave, pitch and roll motion Recently another type of structure for deep water offshore~locations has been proposed which has a platform supported on buoyant members that are held in position at ",- ~
.- .. . . . .
f~ -~SS7~
the well-site by anchor lines extending to fixed anchors on the ocean floor. Offshore platform structures of -this type are referred to generally as "buoyant restrained platorm"0 The basic principle is to provide a platform with buoyant chambers below the wave area to give a positive buoyancy and to tie the structure down to the ocean floor, allowing the buoyancy of the structure to hold the anchoring cables in tension to prevent heaving. Offshore structures of this type offer significant cost-savings for operation in deep-water environments.
Illustrative of the concept of buoyant restrained plat~form design is that structure disclosed in the article "Tension Leg OfEers Steady Base At Sea" in the November 1973 issue of OFFSHORE magazine, beginning at page 100. The structure disclosed there comprises three buoyant vertical columns having horizontal bracing structure. A working deck is positioned on top of the vertical columns above the surface of the water. Several anchoring cables attach to each vertical column and are secured by dead weight anchors on the ocean floor. Anchor loads to hold the structure are significant, however, due to the resistance in the wave zone o the structure.
` Additional designs based on the buoyallt restrained platform concept are described in the OFFSHO~E ENG-~NEER of May 1975, at page 55. Also, relevant teachings of the buoyant restrained platform concept are found in U.S. Patent 3,256,537 to Clark and U.S. Patent 2,777,669 to Willis.
A yet another approach which has been proposed is th~t of a structure comprising a s]ender column held in a vertical position by guy wires extending from near the top of the column to fixed anchors on the ocean floor. The column .
.,~.
',, ,,. ' ~5~7~
rests on the ocean floor and extends above the surface of the water with a platform supported thereon. Although the vertical column design has less resis-tance in i-ts wave zone than the typical buoyant restrained platform design, some resistance is still pre~ent and forces are developed from periodic wave motion which act on the structure. A problem associatèd with guyed structures of this type is that tilting of the column off-vertical makes it very difficult to con~
duct drilling or production operations. However, increased tension in the guy lines to restrict tilting is not desir-able either.
One approach to solving the problem of tilting is that described in U.S. Patent 3,903,705, assigned to Exxon Pro-duction Research Company. There is disclosed in that patent a guyed tower platform using a clump weighted guy line system to control swaying and still relieve the structure of excessive moment forces produced by waves. The wei~hts are designed to lift off the bottom during large s-torm waves, but remain in position during normal sea conditions. The guy line system provides adequate sway restriction, yet, relieves the guy lines of excessive stress and removes the need for heavier anchoring equipment.
Another concept proposed for an offshore structure to provide restricted movement with waves is that of the buoyant tower. The buoyant tower is an elongate tower s-tructure that is hèld near the ocean Eloor by a universal joint that permits t~he tower to tilt. The force required to prevent the tower from tilting excessively is provided by the es~
tablishment of buoyancy for the tower near the surface of the water. One design based on the buoyant tower concept is the structure illustrated and described in T~IE OIL AND G~S
OURNAL of October 28, 1~7~, beginning at page ~0.
:, .
~L~S5~9~
S[JMM~RY OF TEI:I~ INVENTION
srieEly, in accordance with the present invention,there is provided a guyed offshore structure carrying a plat-form thereon from which drilling o:r pxoduction operations may be conducted. Specifically, the instant invention provides an offshore structure suitable for use in bodies of deep water, which structure permits restricted movement with waves, yet prevents excessiVe tilt;ng O.e the structure.
Thus, broadly, the invention contemplates an offshore structure ~or placement in a body of water which comprises an elongate member for disposition ~n a body of water *o extend above and below the surface of the water, with the member being anchored to the ocean floor to permit ti,lting, a plurality of ~uy lines radially arranged around the elongate member and connecting between the memher and anchors on the ocean floor, and sway control apparatus operably connected to each of the guy lines fo~ altering the length of the guy lines between t~e elongate member and the anchors in response ~ to tilt~ng of ~he elongate member. The sway control apparatus includes first and second winch means, with the first winch -~eans ~ncluding a first group of reels mounted on a common ~drive sha~t which connects to a first group of guy lines and ; tilts the elongate structure along one direction by pulling in and lettin~ out the first ~roup of ~uy lines, and with the second winch means including a second group of reels mounted on a common drive shaft which connects to a second group of guy lines and tilts the elongate structure a}on~ another ~directLon at a right angle to the one direction by pulling in and lettin~ out the second group of ~uy li~es~
: -~L~S5~7~
The present invention makes use of the concep-ts of -the buoyant restrained platform and the guyed structure, by providing an offshore structure comprising an elonga-te member for vertical disposition in a body of water with a buoyancy tank giving a po.si`ti~ve buoyancy to the member.
Anchoring lines connected between the lower end of the elongate member and anchors on the ocean floor tie~down the - member, and in addition permit tilting of the structure. To restrict movement of the structure with waves~ a plurality of guy lines connect between the elongate member and sepa-rate clump weight anchors that are disposed on the ocean floor. The clump weight anchors are adapted to raise off the ocean floor upon o~f-vertical movement of the elongate membex. Finally, sway control apparatus operably connected to the guy lines alters line length between the connection point on the elongate member and the respective clump weight anchor. The sway control apparatus operates in response to off-vertical movement of the elongate member and prevents excessive tilting of the structure by reeling in or letting out guy line to keep the structure substanti.ally vertical.
The sway control apparatus also regulates tension in the guy lines.
~ . ' ,.
.
.
. ~... .
.
1~155~
In a more specific embodiment of the presen-t inven-tion, the anchoring lines connecting to the lower end of the elongate member define a universal pivot point. ~lso, the guy lines attach to the elongate member below the surface of the water at a point between the longitudinal mid-point of the elongate member and the top of the elongate member. The sway control apparatus comprises a separate reel for each guy line with means for driving the reels in response to an inclinometer device that detects off-vertical movement of a predetermined degree. The means for driving the reels may comprise an electric motor or similar type of prime mover device.
An offshore structure in accordance with the present invention has the advantage of offering significant cost-savings for operations in deep-water environments, with smaller anchor loads being xequired to hold the struckure than usually found in buoyant restrained platform designs.
Also, tilting of the elongate member, as usually found in most guyed structures, is much less because of the dynamic sway control apparatus provided. This invention, although especially suitable for use in drilling and production operations in deep water, is also useful in shallow water operations.
Other aspects of this invention not outlined in the above will be covered in the detailed description presented below.
BRIEF DESCRIPTION OF THE DR~WXNGS
A more complete appreciation of the invention may be .
had by re~erence to the accompanying drawings illustrating a preferred embodiment of the invention to be described in . ~ . ., . .,,; ~ . . .
.
~ ~ ,.:, . . .
;i7~
detail, in which like reference numberals designate iden-tical or corresponding parts throughout the several use and wherein:
Fig. 1 is an elevation view of an offshore structure in accordance with the present invention having a drilling or production plat~orm thereon;
.
Fig. 2 is a planned view of the embodiment of the off-shore structure of Fig. l, which view illustrates the ar-rangement of the guy lines extending from the offshore structure;
Fig. 3 illustrates a guy line tackle arrangement for the preferred embodiment illustrated; and Fig. 4 is a plan view of the sway control apparatus dlsposed on the lower deck of the platform.
DESCRIPTION OF THE PRE~FERRED EMEiODIMENT
: ~ , Referring now to Fig. 1, there is shown an oEfshore : structure I0 in accordance with the concepts of the present : invention. The offshore structure lO is disposed in a body of water 12 o~ any depth sufficient to accommodate the ~ particular dlmensions of the structure. However, the struc-ture may be most advantageously used in a body of deep : water, wherein the depth will be at least 300 feet, with a prefe.rable depth range of 700-1500 feet.
An elongate tubular member 14, having an internal buoyancy tank 16 extending over a substantial portion of its : ~- _7 16~5~i7~
length, is the primary s-tructural component of the struc-ture. The elongate tubular member 14 which houses the buoyancy tank 16 is designed to withstand the total hydro-static and hydrodynamic pressure of the surrounding sea wikh only atmospheric pressure on the inside. The upper portion of tubular member 14 consists of skeleton structure ]8 which reduces resistance in the wave zone of the structure. The skeleton structure comprises several upwardly extending struts along with several diagonal braces which form the lo X-braciny shown.
A two-level platform 20 is carried atop elongate member 14, and has mounted thereon a derrick 22 for carrying out drilling or production operations. In addition, various pieces of equipment such as, for example, a crane 2~ are provided on the upper deck, as is a control building 26.
The upper deck 28 is built atop the lower deck 30, which lower deck has a smaller floor area. Upper deck 28 is supported above lower deck 30 by means of a support and bracing structure generally denoted by the referenced nu-meral 32. Located on the lower deck 30 is the sway control apparatus 34 to be discussed in more detail in regard to Fig. 4. The elevations of the upper deck 28 and the lower deck 30 will be approximately 100 feet and 80 feet, respec-tively, above khe surface of the water.
The buoyancy chamber 16 comprises a selectively bal-lastable and deballastable chamber. Selective ballasting of the buoyancy chamber 16 permits the exact amount of buoyancy effect necessary to offset the weight of the struc-ture and the equipment mounted on the pla-tform 20. Offshore s~ructure 10 is floated in the water utilizing the positive ... .. . .
~SS~7~0 buoyancy effec-ts provided by the buoyancy -tank 16, with anchoring lines 36, 38, 40 and 42 tying the structure to the ocean floor. The anchor lines connect between the lower end of elongate member 14 and individual dead-weight anchors 44, 46, 48 and 50 (not shown). The portion 17 oE elongate member 14 proximate the connection point of the anchor lines is preferably ballasted with sea water or drilling mud to provide a negative buoyancy in that region.
In order to prevent an excessive -tension from being placed on thè anchoring lines, it is necessary to properly ballast the buoyancy tank 16. To properly se-t and main-tain the correct load on the anchoring lines, the weight of the structure and the equipment which it supports must be determined during construction to provide an accurate account of total weight, as well as the resulting center of gravity for the structure. In addition~ the buoyancy effect through water displacement by elongdte member 14 must be considered.
As well as the buoyant effect of the structure and the weight of the equipment load on the platform, in order to ? determine the amount of ballast necessary to be placed in the buoyancy tank 16, the water depth as well as changes in depth due to tidal variation should be considered.
By virtue of the tie-down arrangement provided by the anchoring cables connected to the lower end of elongate tubular member 14, the offshore structure 10 i5 in effect pi~oted at its lower end, and therefore suscep-tible to ~ilting. In order to counteract swaying of the structure, guy lines, of which only guy lines 50 and 52 are shown in Fig. 1, extend radially outwardly (see ~ig. 2) rom the `~ 30 eIongate tubular member 14. The guy lines attach to the ~.
_ g ._ .
~55~
elongate member 1~ at a poin-t between -the longitudinal mld-point of elongate member 14 and the top of the elongate member, and extend down at approximately a 60-degree angle from vertical to a clump weight on the ocean floor. ~ach clump weight serves as an anchor and consists of a number of parallel segments oE large-diameter chain connected to the guy line. The clump weights are designe~ to lift off the bottom during the existence of large waves. A more detailed discussion of the use of clump weights in connection wi-th a guyed structure may be had by reference to U.S. Patent No.
3,903,705 and to the July 14, 1975 issue of THE OIL ~ND GAS
JOURNAL, at page 86 thereof. Beyond the clump weights 54 and 56, guy lines 50 and 52 extend to an anchor piles 5~ and 60, respectively.
Referriny next to Fig. 2, the symetrical pattern o~ the guy lines 50, 85, ~7, 89, 52, 93, 95 and 97 may be appre-ciated. Also shown in dotted outline is the platform 20 and derrick 22. Although a fewer or greater number of guylines than the eight illustrated may be used, the arrangement illustrated is preferred. In any event, in determining the number of guy lines to be used it must be appreciated that a sufficient number must be used to distribute the load im-posed. An excessive number of guy lines may make the sway control apparatus 34 (see Fig. 4) too unwieldy, and thus impractical.
The upper ends of the guy lines connect to guy line tackle devices 62 and 64 which are secured to the outer surface of the elongate member 14. The guy lines may often comprise rather large diameter cables which would not be adapted for winding on a winch drum. Hence, it is desirable to permit adjustment oE the heavy guy lines using a smaller ~55~9al diameter cable windable on a winch. Turnlng to Fly. 3, one guy line tackle device is shown in de-tail in position on a section of elongate men~er 14. Specifically, yuy line tackle device 62 comprises a fixed sheave 64 that is secured to the outside o~ elongate member 14, with its axis of rotation 66 being substantially perpendicular to the radi-ally extending guy line S0 which passes over it. The end of guy line 50 terminates in an eyelet 68 which further con-nects to a traveling block 70. A pair of traveling block pulleys 74 and 76 receive a cable 78. Cable 78 encircles pulley 76 on traveling block 70 and passes over a fixed sheave 80 that is fixed relative to elongate member 14. The cable 78 then passes over the second pulley on traveling block 70 before its fixed end 7.2 terminates at dead end 82 which is also secured to the outside of elongate member 14.
Thus, when the free end 79 of cable 78 is reeled in or let out via a winch or the like ~not shown) the guy line 50 is either pulled in or let out. Movement in this fashion ~ results in an increase or decrease in tension on guy line 50.
Furthermore, use of the tackle mechanism illus-trated main-tains cable 78 under one-fourth the tension of guy line 50, and permits four times the footage movement o~ the cable 7 as compared to guy line 50 to accomplish more accurate adjustments of tension in the guy lins 50. The tackle arrangement shown is illustrative only, and it will be understood that various other mechanisms might be used to adjust tension of the guy lines. Movement of guy line 50 in this manner also adjusts the length of line between clump weight S~ and the attachment point on elongate member 14 which is defined by sheave 66.
s7~
Referring next to Fig. ~, there is illustrated one configuration for sway control apparatus 34 which is dis-posed on lower deck 30, and which regulates guy line te~sion and alters the len~th of the guy lines between thelr point of attachment to elonga-te member 14 and their respective clump weight. Specifically, there is provided a separate winch line for each guy line, which winch lines are desig-nated with referenced numerals 78, 84, 86, 88, 90, 92, 94 and 96, with winch line 78 corresponding to the cable 78 illustrated in the Fig. 3. Each winch line connects to a separate reel which either reels in or lets out its partic-ular winch line to alter the length of and tension in the guy line to which that particular winch line is connected.
The reels are arranged in groups, in this case of four, and are skid mounted along with the prima mover device that drives them to form winch means 99 and 101. For example, winch means 99 comprises skid 98 having reel 100 for winch line 96, reel 102 for line 94, reel 104 for line 86, and reel 106 for line 88, The reels are all mounted on a single 20~ drive shaft 108 which is coupled to an electric motor 110 through a gear box 112 and a brake 114. Reels 102 and 100 are wound clockwise, while reels 104 and 106 ara wound counterclockwise. This type of winding arrangement permits a single direction of rotation for -the reels to be used to provide the appropriate reeling in and letting out of the various~winch lines.
A similar arrangement exists for winch means 101 com-prising skid 115 having reels 116, 118, 120 and 122 mounted thereon. These enumerated reels connect respectively to winch lines 90, 92, 84 and 78. An electric motor 124 simultaneously drives the reels through a gear bo~ 126 with a brake 128 being provided.
,~
~5575~1 In order to supply the required electric power for the electric motors 110 and 124, a diesel generator set is provided with a skid mounted diesel en~ine 130 driving an electric generator 132 to genera.te the required ~lectric power for the sway control apparatus.
The sway control apparatus is responsive through motor controls to detection means 134 which senses off-vertical movement of the elonga-te member 14. Detection means 134 may be an inertial detector or an i.nclinometer, either of which may be utilized to generate a signal that activates the winch motors to drive the winches in the aforementioned manner to alter the length and tension of the guy lines. The sway control apparatus activates when the structure is displaced from vertical due to wind, waves, or current.
To illustrate the operation of the apparatus of this invention, if the tilting forces on the tower provide a resultant force from a direction between the extensions of lines 94 and 96 in Figure 4, then only reels operating off : winch means 99 likely need be activated to accomplish any adjustment. If, however, the resultant force is from a direction between the extensions of lines 78 and 96, both winch means 99 and 101 will require adjustment~ Similarly, a resultant force from the direction between the extensions of cables 90 and 92 requires only adjustment of wlnch means 101, whereas a resultant force between the extensions of cables 92 and 94 would again require adjustments from both winch means. : :
: The foregoing description of the invention has been : :
directed to a particular preferred embodiment of the present invention for purposes of explanation and illustration. It will be apparent, however, to those skilled in this art that . -13--~S57~
many modifica-tions and changes in the appaxa-tus may be made without depar-ting from the scope and spiri-t of the invention.
It is therefore intended that the following claims cover all equivalent modifications and variations as fall within the scope of the invention as defined by the claims.
, , ~1~-- , '
~NCHORED OTi`FSllORr~' STRUCTUR~
WIT~I SW~Y CONTROI
BACICGROUND OF Tl-IE :[NVENTION
The present invention relates to an offshore structure for disposition in a body of deep water; and more partic-ularly, this invention relates to an offshore structure carrying a platform thereon and held by guy wires secured to anchors on the ocean floor.
offshore structures in current use in connection with the drilling and production of hydrocarbon deposits in offshore locations have generally included a platform held above the surface of the water by support members which rest on the ocean floor. Such structures, referred to as fixed platforms, have been quite successfully utilized in opera tions conducted in shallow water. However, as offshore drilling operations move into areas having deeper waters, such as about 300 feet, the fixed platform design becomes less desirable, primarily because of the great expense to fabricate and install such a structure. As a general rule the cost of fabrication and installation of a fixed platform designed in accordance with proven shallow water methods and technology will increase exponentially with water depth.
~ lthough an alternative to the fixed platform design is the floating platform concept, that approach is found to present additional, and perhaps even more serious disad-vantages. Floating platforms have the disadvantage of being more suseeptible to rough sea conditions and exhibit the undesirab~ie feature of signi~icant heave, pitch and roll motion Recently another type of structure for deep water offshore~locations has been proposed which has a platform supported on buoyant members that are held in position at ",- ~
.- .. . . . .
f~ -~SS7~
the well-site by anchor lines extending to fixed anchors on the ocean floor. Offshore platform structures of -this type are referred to generally as "buoyant restrained platorm"0 The basic principle is to provide a platform with buoyant chambers below the wave area to give a positive buoyancy and to tie the structure down to the ocean floor, allowing the buoyancy of the structure to hold the anchoring cables in tension to prevent heaving. Offshore structures of this type offer significant cost-savings for operation in deep-water environments.
Illustrative of the concept of buoyant restrained plat~form design is that structure disclosed in the article "Tension Leg OfEers Steady Base At Sea" in the November 1973 issue of OFFSHORE magazine, beginning at page 100. The structure disclosed there comprises three buoyant vertical columns having horizontal bracing structure. A working deck is positioned on top of the vertical columns above the surface of the water. Several anchoring cables attach to each vertical column and are secured by dead weight anchors on the ocean floor. Anchor loads to hold the structure are significant, however, due to the resistance in the wave zone o the structure.
` Additional designs based on the buoyallt restrained platform concept are described in the OFFSHO~E ENG-~NEER of May 1975, at page 55. Also, relevant teachings of the buoyant restrained platform concept are found in U.S. Patent 3,256,537 to Clark and U.S. Patent 2,777,669 to Willis.
A yet another approach which has been proposed is th~t of a structure comprising a s]ender column held in a vertical position by guy wires extending from near the top of the column to fixed anchors on the ocean floor. The column .
.,~.
',, ,,. ' ~5~7~
rests on the ocean floor and extends above the surface of the water with a platform supported thereon. Although the vertical column design has less resis-tance in i-ts wave zone than the typical buoyant restrained platform design, some resistance is still pre~ent and forces are developed from periodic wave motion which act on the structure. A problem associatèd with guyed structures of this type is that tilting of the column off-vertical makes it very difficult to con~
duct drilling or production operations. However, increased tension in the guy lines to restrict tilting is not desir-able either.
One approach to solving the problem of tilting is that described in U.S. Patent 3,903,705, assigned to Exxon Pro-duction Research Company. There is disclosed in that patent a guyed tower platform using a clump weighted guy line system to control swaying and still relieve the structure of excessive moment forces produced by waves. The wei~hts are designed to lift off the bottom during large s-torm waves, but remain in position during normal sea conditions. The guy line system provides adequate sway restriction, yet, relieves the guy lines of excessive stress and removes the need for heavier anchoring equipment.
Another concept proposed for an offshore structure to provide restricted movement with waves is that of the buoyant tower. The buoyant tower is an elongate tower s-tructure that is hèld near the ocean Eloor by a universal joint that permits t~he tower to tilt. The force required to prevent the tower from tilting excessively is provided by the es~
tablishment of buoyancy for the tower near the surface of the water. One design based on the buoyant tower concept is the structure illustrated and described in T~IE OIL AND G~S
OURNAL of October 28, 1~7~, beginning at page ~0.
:, .
~L~S5~9~
S[JMM~RY OF TEI:I~ INVENTION
srieEly, in accordance with the present invention,there is provided a guyed offshore structure carrying a plat-form thereon from which drilling o:r pxoduction operations may be conducted. Specifically, the instant invention provides an offshore structure suitable for use in bodies of deep water, which structure permits restricted movement with waves, yet prevents excessiVe tilt;ng O.e the structure.
Thus, broadly, the invention contemplates an offshore structure ~or placement in a body of water which comprises an elongate member for disposition ~n a body of water *o extend above and below the surface of the water, with the member being anchored to the ocean floor to permit ti,lting, a plurality of ~uy lines radially arranged around the elongate member and connecting between the memher and anchors on the ocean floor, and sway control apparatus operably connected to each of the guy lines fo~ altering the length of the guy lines between t~e elongate member and the anchors in response ~ to tilt~ng of ~he elongate member. The sway control apparatus includes first and second winch means, with the first winch -~eans ~ncluding a first group of reels mounted on a common ~drive sha~t which connects to a first group of guy lines and ; tilts the elongate structure along one direction by pulling in and lettin~ out the first ~roup of ~uy lines, and with the second winch means including a second group of reels mounted on a common drive shaft which connects to a second group of guy lines and tilts the elongate structure a}on~ another ~directLon at a right angle to the one direction by pulling in and lettin~ out the second group of ~uy li~es~
: -~L~S5~7~
The present invention makes use of the concep-ts of -the buoyant restrained platform and the guyed structure, by providing an offshore structure comprising an elonga-te member for vertical disposition in a body of water with a buoyancy tank giving a po.si`ti~ve buoyancy to the member.
Anchoring lines connected between the lower end of the elongate member and anchors on the ocean floor tie~down the - member, and in addition permit tilting of the structure. To restrict movement of the structure with waves~ a plurality of guy lines connect between the elongate member and sepa-rate clump weight anchors that are disposed on the ocean floor. The clump weight anchors are adapted to raise off the ocean floor upon o~f-vertical movement of the elongate membex. Finally, sway control apparatus operably connected to the guy lines alters line length between the connection point on the elongate member and the respective clump weight anchor. The sway control apparatus operates in response to off-vertical movement of the elongate member and prevents excessive tilting of the structure by reeling in or letting out guy line to keep the structure substanti.ally vertical.
The sway control apparatus also regulates tension in the guy lines.
~ . ' ,.
.
.
. ~... .
.
1~155~
In a more specific embodiment of the presen-t inven-tion, the anchoring lines connecting to the lower end of the elongate member define a universal pivot point. ~lso, the guy lines attach to the elongate member below the surface of the water at a point between the longitudinal mid-point of the elongate member and the top of the elongate member. The sway control apparatus comprises a separate reel for each guy line with means for driving the reels in response to an inclinometer device that detects off-vertical movement of a predetermined degree. The means for driving the reels may comprise an electric motor or similar type of prime mover device.
An offshore structure in accordance with the present invention has the advantage of offering significant cost-savings for operations in deep-water environments, with smaller anchor loads being xequired to hold the struckure than usually found in buoyant restrained platform designs.
Also, tilting of the elongate member, as usually found in most guyed structures, is much less because of the dynamic sway control apparatus provided. This invention, although especially suitable for use in drilling and production operations in deep water, is also useful in shallow water operations.
Other aspects of this invention not outlined in the above will be covered in the detailed description presented below.
BRIEF DESCRIPTION OF THE DR~WXNGS
A more complete appreciation of the invention may be .
had by re~erence to the accompanying drawings illustrating a preferred embodiment of the invention to be described in . ~ . ., . .,,; ~ . . .
.
~ ~ ,.:, . . .
;i7~
detail, in which like reference numberals designate iden-tical or corresponding parts throughout the several use and wherein:
Fig. 1 is an elevation view of an offshore structure in accordance with the present invention having a drilling or production plat~orm thereon;
.
Fig. 2 is a planned view of the embodiment of the off-shore structure of Fig. l, which view illustrates the ar-rangement of the guy lines extending from the offshore structure;
Fig. 3 illustrates a guy line tackle arrangement for the preferred embodiment illustrated; and Fig. 4 is a plan view of the sway control apparatus dlsposed on the lower deck of the platform.
DESCRIPTION OF THE PRE~FERRED EMEiODIMENT
: ~ , Referring now to Fig. 1, there is shown an oEfshore : structure I0 in accordance with the concepts of the present : invention. The offshore structure lO is disposed in a body of water 12 o~ any depth sufficient to accommodate the ~ particular dlmensions of the structure. However, the struc-ture may be most advantageously used in a body of deep : water, wherein the depth will be at least 300 feet, with a prefe.rable depth range of 700-1500 feet.
An elongate tubular member 14, having an internal buoyancy tank 16 extending over a substantial portion of its : ~- _7 16~5~i7~
length, is the primary s-tructural component of the struc-ture. The elongate tubular member 14 which houses the buoyancy tank 16 is designed to withstand the total hydro-static and hydrodynamic pressure of the surrounding sea wikh only atmospheric pressure on the inside. The upper portion of tubular member 14 consists of skeleton structure ]8 which reduces resistance in the wave zone of the structure. The skeleton structure comprises several upwardly extending struts along with several diagonal braces which form the lo X-braciny shown.
A two-level platform 20 is carried atop elongate member 14, and has mounted thereon a derrick 22 for carrying out drilling or production operations. In addition, various pieces of equipment such as, for example, a crane 2~ are provided on the upper deck, as is a control building 26.
The upper deck 28 is built atop the lower deck 30, which lower deck has a smaller floor area. Upper deck 28 is supported above lower deck 30 by means of a support and bracing structure generally denoted by the referenced nu-meral 32. Located on the lower deck 30 is the sway control apparatus 34 to be discussed in more detail in regard to Fig. 4. The elevations of the upper deck 28 and the lower deck 30 will be approximately 100 feet and 80 feet, respec-tively, above khe surface of the water.
The buoyancy chamber 16 comprises a selectively bal-lastable and deballastable chamber. Selective ballasting of the buoyancy chamber 16 permits the exact amount of buoyancy effect necessary to offset the weight of the struc-ture and the equipment mounted on the pla-tform 20. Offshore s~ructure 10 is floated in the water utilizing the positive ... .. . .
~SS~7~0 buoyancy effec-ts provided by the buoyancy -tank 16, with anchoring lines 36, 38, 40 and 42 tying the structure to the ocean floor. The anchor lines connect between the lower end of elongate member 14 and individual dead-weight anchors 44, 46, 48 and 50 (not shown). The portion 17 oE elongate member 14 proximate the connection point of the anchor lines is preferably ballasted with sea water or drilling mud to provide a negative buoyancy in that region.
In order to prevent an excessive -tension from being placed on thè anchoring lines, it is necessary to properly ballast the buoyancy tank 16. To properly se-t and main-tain the correct load on the anchoring lines, the weight of the structure and the equipment which it supports must be determined during construction to provide an accurate account of total weight, as well as the resulting center of gravity for the structure. In addition~ the buoyancy effect through water displacement by elongdte member 14 must be considered.
As well as the buoyant effect of the structure and the weight of the equipment load on the platform, in order to ? determine the amount of ballast necessary to be placed in the buoyancy tank 16, the water depth as well as changes in depth due to tidal variation should be considered.
By virtue of the tie-down arrangement provided by the anchoring cables connected to the lower end of elongate tubular member 14, the offshore structure 10 i5 in effect pi~oted at its lower end, and therefore suscep-tible to ~ilting. In order to counteract swaying of the structure, guy lines, of which only guy lines 50 and 52 are shown in Fig. 1, extend radially outwardly (see ~ig. 2) rom the `~ 30 eIongate tubular member 14. The guy lines attach to the ~.
_ g ._ .
~55~
elongate member 1~ at a poin-t between -the longitudinal mld-point of elongate member 14 and the top of the elongate member, and extend down at approximately a 60-degree angle from vertical to a clump weight on the ocean floor. ~ach clump weight serves as an anchor and consists of a number of parallel segments oE large-diameter chain connected to the guy line. The clump weights are designe~ to lift off the bottom during the existence of large waves. A more detailed discussion of the use of clump weights in connection wi-th a guyed structure may be had by reference to U.S. Patent No.
3,903,705 and to the July 14, 1975 issue of THE OIL ~ND GAS
JOURNAL, at page 86 thereof. Beyond the clump weights 54 and 56, guy lines 50 and 52 extend to an anchor piles 5~ and 60, respectively.
Referriny next to Fig. 2, the symetrical pattern o~ the guy lines 50, 85, ~7, 89, 52, 93, 95 and 97 may be appre-ciated. Also shown in dotted outline is the platform 20 and derrick 22. Although a fewer or greater number of guylines than the eight illustrated may be used, the arrangement illustrated is preferred. In any event, in determining the number of guy lines to be used it must be appreciated that a sufficient number must be used to distribute the load im-posed. An excessive number of guy lines may make the sway control apparatus 34 (see Fig. 4) too unwieldy, and thus impractical.
The upper ends of the guy lines connect to guy line tackle devices 62 and 64 which are secured to the outer surface of the elongate member 14. The guy lines may often comprise rather large diameter cables which would not be adapted for winding on a winch drum. Hence, it is desirable to permit adjustment oE the heavy guy lines using a smaller ~55~9al diameter cable windable on a winch. Turnlng to Fly. 3, one guy line tackle device is shown in de-tail in position on a section of elongate men~er 14. Specifically, yuy line tackle device 62 comprises a fixed sheave 64 that is secured to the outside o~ elongate member 14, with its axis of rotation 66 being substantially perpendicular to the radi-ally extending guy line S0 which passes over it. The end of guy line 50 terminates in an eyelet 68 which further con-nects to a traveling block 70. A pair of traveling block pulleys 74 and 76 receive a cable 78. Cable 78 encircles pulley 76 on traveling block 70 and passes over a fixed sheave 80 that is fixed relative to elongate member 14. The cable 78 then passes over the second pulley on traveling block 70 before its fixed end 7.2 terminates at dead end 82 which is also secured to the outside of elongate member 14.
Thus, when the free end 79 of cable 78 is reeled in or let out via a winch or the like ~not shown) the guy line 50 is either pulled in or let out. Movement in this fashion ~ results in an increase or decrease in tension on guy line 50.
Furthermore, use of the tackle mechanism illus-trated main-tains cable 78 under one-fourth the tension of guy line 50, and permits four times the footage movement o~ the cable 7 as compared to guy line 50 to accomplish more accurate adjustments of tension in the guy lins 50. The tackle arrangement shown is illustrative only, and it will be understood that various other mechanisms might be used to adjust tension of the guy lines. Movement of guy line 50 in this manner also adjusts the length of line between clump weight S~ and the attachment point on elongate member 14 which is defined by sheave 66.
s7~
Referring next to Fig. ~, there is illustrated one configuration for sway control apparatus 34 which is dis-posed on lower deck 30, and which regulates guy line te~sion and alters the len~th of the guy lines between thelr point of attachment to elonga-te member 14 and their respective clump weight. Specifically, there is provided a separate winch line for each guy line, which winch lines are desig-nated with referenced numerals 78, 84, 86, 88, 90, 92, 94 and 96, with winch line 78 corresponding to the cable 78 illustrated in the Fig. 3. Each winch line connects to a separate reel which either reels in or lets out its partic-ular winch line to alter the length of and tension in the guy line to which that particular winch line is connected.
The reels are arranged in groups, in this case of four, and are skid mounted along with the prima mover device that drives them to form winch means 99 and 101. For example, winch means 99 comprises skid 98 having reel 100 for winch line 96, reel 102 for line 94, reel 104 for line 86, and reel 106 for line 88, The reels are all mounted on a single 20~ drive shaft 108 which is coupled to an electric motor 110 through a gear box 112 and a brake 114. Reels 102 and 100 are wound clockwise, while reels 104 and 106 ara wound counterclockwise. This type of winding arrangement permits a single direction of rotation for -the reels to be used to provide the appropriate reeling in and letting out of the various~winch lines.
A similar arrangement exists for winch means 101 com-prising skid 115 having reels 116, 118, 120 and 122 mounted thereon. These enumerated reels connect respectively to winch lines 90, 92, 84 and 78. An electric motor 124 simultaneously drives the reels through a gear bo~ 126 with a brake 128 being provided.
,~
~5575~1 In order to supply the required electric power for the electric motors 110 and 124, a diesel generator set is provided with a skid mounted diesel en~ine 130 driving an electric generator 132 to genera.te the required ~lectric power for the sway control apparatus.
The sway control apparatus is responsive through motor controls to detection means 134 which senses off-vertical movement of the elonga-te member 14. Detection means 134 may be an inertial detector or an i.nclinometer, either of which may be utilized to generate a signal that activates the winch motors to drive the winches in the aforementioned manner to alter the length and tension of the guy lines. The sway control apparatus activates when the structure is displaced from vertical due to wind, waves, or current.
To illustrate the operation of the apparatus of this invention, if the tilting forces on the tower provide a resultant force from a direction between the extensions of lines 94 and 96 in Figure 4, then only reels operating off : winch means 99 likely need be activated to accomplish any adjustment. If, however, the resultant force is from a direction between the extensions of lines 78 and 96, both winch means 99 and 101 will require adjustment~ Similarly, a resultant force from the direction between the extensions of cables 90 and 92 requires only adjustment of wlnch means 101, whereas a resultant force between the extensions of cables 92 and 94 would again require adjustments from both winch means. : :
: The foregoing description of the invention has been : :
directed to a particular preferred embodiment of the present invention for purposes of explanation and illustration. It will be apparent, however, to those skilled in this art that . -13--~S57~
many modifica-tions and changes in the appaxa-tus may be made without depar-ting from the scope and spiri-t of the invention.
It is therefore intended that the following claims cover all equivalent modifications and variations as fall within the scope of the invention as defined by the claims.
, , ~1~-- , '
Claims (7)
1. An offshore structure for placement in a body of water comprising:
an elongate member extending above and below the surface of the water, said member being anchored to the ocean floor to permit tilting; a plurality of guy lines radially arranged around said elongate member in connection between said member and anchors on the ocean floor;
sway control apparatus operably connected to each of said guy lines for altering the length of said guy lines between said elongate member and said anchors in response to tilting of said elongate member;
said sway control apparatus includes first and second winch means;
said first winch means includes a first group of reels mounted on a common drive shaft which connects to a first group of guy lines and tilts said elongate structure along one direction by pulling in and letting out said first group of guy lines; and said second winch means includes a second group of reels mounted on a common drive shaft which connects to a second group of guy lines and tilts said elongate structure along another direction at a right angle to said one direction by pulling in and letting out said second group of guy lines.
an elongate member extending above and below the surface of the water, said member being anchored to the ocean floor to permit tilting; a plurality of guy lines radially arranged around said elongate member in connection between said member and anchors on the ocean floor;
sway control apparatus operably connected to each of said guy lines for altering the length of said guy lines between said elongate member and said anchors in response to tilting of said elongate member;
said sway control apparatus includes first and second winch means;
said first winch means includes a first group of reels mounted on a common drive shaft which connects to a first group of guy lines and tilts said elongate structure along one direction by pulling in and letting out said first group of guy lines; and said second winch means includes a second group of reels mounted on a common drive shaft which connects to a second group of guy lines and tilts said elongate structure along another direction at a right angle to said one direction by pulling in and letting out said second group of guy lines.
2. An offshore structure according to Claim 1 further comprising:
a plurality of anchoring lines connected between the lower end of said elongate member and anchors on the ocean floor; and a buoyancy tank within said elongate member providing said elongate member with a positive buoyancy and placing said anchoring lines in tension with the lower end of said elongate member being disposed above the ocean floor.
a plurality of anchoring lines connected between the lower end of said elongate member and anchors on the ocean floor; and a buoyancy tank within said elongate member providing said elongate member with a positive buoyancy and placing said anchoring lines in tension with the lower end of said elongate member being disposed above the ocean floor.
3. An offshore structure according to Claim 2 wherein the connection of said anchoring lines to said elongate member allows universal pivoting of said elongate member.
4. An offshore structure according to Claim 2 wherein said buoyancy tank has means for selectively ballasting and deballasting it.
5. An offshore structure according to Claim 1 wherein said guy lines are attached to said elongate member below the water line at a point between the longitudinal midpoint of the member and the top of the member.
6. An offshore structure according to Claim 1 wherein said sway control apparatus further comprises:
detection means for sensing off-vertical movement of said elongate member.
7. An offshore structure for placement in a body of water comprising:
the elongate member for disposition in a body of water to extend above and below the surface of the water;
a plurality of anchoring lines connected between the lower end of said elongate member and anchors on the ocean floor with the connection of said anchoring lines to said elongate member defining a universal pivot point that permits said elongate member to tilt;
a buoyancy tank within said elongate member providing said member with a positive buoyancy and placing said anchoring lines in tension with the lower end of said elongate
detection means for sensing off-vertical movement of said elongate member.
7. An offshore structure for placement in a body of water comprising:
the elongate member for disposition in a body of water to extend above and below the surface of the water;
a plurality of anchoring lines connected between the lower end of said elongate member and anchors on the ocean floor with the connection of said anchoring lines to said elongate member defining a universal pivot point that permits said elongate member to tilt;
a buoyancy tank within said elongate member providing said member with a positive buoyancy and placing said anchoring lines in tension with the lower end of said elongate
Claim 7 - cont'd ....
member being disposed above the ocean floor, said buoyancy tank being selectively ballastable and deballastable;
a plurality of guy lines radially arranged around said elongate member and connecting between said member and anchors on the ocean floor;
guy line tackles secured to the outer surface of said elongate member for connecting the upper ends of said guy lines to said member, each of said guy line tackles comprising a fixed sheave mounted to the outside of said elongate member with its axis of rotation being substantially perpendicular to a guy line passing thereover, a traveling block to which the upper end of a guy line attaches, said traveling block having first and second pulleys, and a cable terminating on one end at a dead end fixed relative to said elongate member and encircling the first and second pulleys on said traveling block;
detection means for sensing off-vertical movement of said elongate member; and sway control apparatus disposed on said elongate member to regulate guy line tension and alter the length of the guy lines between their point of attachment to said elongate member and said anchors in response to tilting of said elongate member, said sway control apparatus comprising first winch means having a separate reel for each of the first grouping of said guy lines and having the cable of each respective guy line tackle wound thereon, and second winch means having a separate reel for each of a second grouping of said guy lines and having the cable of each respective guy line tackle wound thereon, said first and second winch means being operable in response to said detection means to either reel in or let out particular ones of said guy lines.
member being disposed above the ocean floor, said buoyancy tank being selectively ballastable and deballastable;
a plurality of guy lines radially arranged around said elongate member and connecting between said member and anchors on the ocean floor;
guy line tackles secured to the outer surface of said elongate member for connecting the upper ends of said guy lines to said member, each of said guy line tackles comprising a fixed sheave mounted to the outside of said elongate member with its axis of rotation being substantially perpendicular to a guy line passing thereover, a traveling block to which the upper end of a guy line attaches, said traveling block having first and second pulleys, and a cable terminating on one end at a dead end fixed relative to said elongate member and encircling the first and second pulleys on said traveling block;
detection means for sensing off-vertical movement of said elongate member; and sway control apparatus disposed on said elongate member to regulate guy line tension and alter the length of the guy lines between their point of attachment to said elongate member and said anchors in response to tilting of said elongate member, said sway control apparatus comprising first winch means having a separate reel for each of the first grouping of said guy lines and having the cable of each respective guy line tackle wound thereon, and second winch means having a separate reel for each of a second grouping of said guy lines and having the cable of each respective guy line tackle wound thereon, said first and second winch means being operable in response to said detection means to either reel in or let out particular ones of said guy lines.
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US69826076A | 1976-06-21 | 1976-06-21 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1055790A true CA1055790A (en) | 1979-06-05 |
Family
ID=24804534
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA280,919A Expired CA1055790A (en) | 1976-06-21 | 1977-06-20 | Anchored offshore structure with sway control |
Country Status (3)
| Country | Link |
|---|---|
| CA (1) | CA1055790A (en) |
| GB (1) | GB1540035A (en) |
| NO (1) | NO146898C (en) |
Families Citing this family (7)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| USRE32119E (en) * | 1980-04-30 | 1986-04-22 | Brown & Root, Inc. | Mooring and supporting apparatus and methods for a guyed marine structure |
| GB2075096B (en) | 1980-04-30 | 1984-08-08 | Brown & Root | Mooring and supporting apparatus and methods for a guyed marine structure |
| GB2177364A (en) * | 1982-07-09 | 1987-01-21 | William A Hunsucker | Anchoring vessel |
| US4681059A (en) * | 1983-09-29 | 1987-07-21 | Hunsucker William A | Roll restraint of anchored vessel |
| NO171773C (en) * | 1988-02-24 | 1993-05-05 | Norwegian Contractors | TENSION PLATFORM AND PROCEDURE FOR AA INSTALLING SUCH |
| US6655312B1 (en) | 1999-08-09 | 2003-12-02 | Single Buoy Moorings Inc. | Active semi-weathervaning anchoring system |
| GB201001302D0 (en) * | 2010-01-26 | 2010-03-10 | Butterrow Offshore Development | Floating caissons |
-
1977
- 1977-06-20 NO NO772160A patent/NO146898C/en unknown
- 1977-06-20 GB GB25670/77A patent/GB1540035A/en not_active Expired
- 1977-06-20 CA CA280,919A patent/CA1055790A/en not_active Expired
Also Published As
| Publication number | Publication date |
|---|---|
| NO146898C (en) | 1983-01-05 |
| NO772160L (en) | 1977-12-22 |
| NO146898B (en) | 1982-09-20 |
| GB1540035A (en) | 1979-02-07 |
Similar Documents
| Publication | Publication Date | Title |
|---|---|---|
| US4170186A (en) | Anchored offshore structure with sway control apparatus | |
| CA2202151C (en) | Deep water offshore apparatus | |
| US7686543B2 (en) | System for mounting equipment and structures offshore | |
| CN103786837B (en) | Asymmetric anchoring system for supporting offshore wind turbine | |
| US9592889B2 (en) | Submersible active support structure for turbine towers and substations or similar elements, in offshore facilities | |
| CA2197942C (en) | Floating caisson for offshore production and drilling | |
| US5118221A (en) | Deep water platform with buoyant flexible piles | |
| CN107021190A (en) | Can be from the floating tension leg type offshore floating wind turbine foundation installed and its installation method | |
| CA1102570A (en) | Sea-floor template | |
| WO2002048547A1 (en) | Method and apparatus for placing at least one wind turbine on open water | |
| US4417831A (en) | Mooring and supporting apparatus and methods for a guyed marine structure | |
| CA1055790A (en) | Anchored offshore structure with sway control | |
| CN115812066A (en) | Crane vessel for lifting an offshore wind turbine or a component thereof | |
| EP0179776B1 (en) | Offshore multi-stay platform structure | |
| GB2147549A (en) | Minimum heave offshore structure | |
| US3369511A (en) | Marine floating structure | |
| USRE32119E (en) | Mooring and supporting apparatus and methods for a guyed marine structure | |
| EP0225324A1 (en) | Submarine element driving means. | |
| JPS6389715A (en) | Method for arranging prefabricated structure to sea bottom or river bottom and said structure | |
| IE43330B1 (en) | Platform for marine work | |
| EP2440708B1 (en) | Foundation for offshore tower | |
| EP4353577A2 (en) | Mooring system and method for installing a floating platform using said mooring system | |
| JPS5929476B2 (en) | Tension leg offshore marine equipment | |
| JP2708072B2 (en) | Floating bridge pier | |
| JP2953818B2 (en) | Construction method for deep water structures |