BACKGROUND OF THE INVENTION
This invention relates to an offshore tower of the type adapted to be positioned within a body of water such as, for example, a lake, sea or ocean.
More particularly, the invention relates to an improved offshore tower, method of constructing the offshore tower, method of launching an offshore tower into a body of water and a method of fixing the offshore tower to the bed of a body of water.
Towers have a multiplicity of applications in a marine environment, such as for example supports for radar or sonar stations, light beacons, marine exploration labs and the like. Additionally, offshore towers are frequently utilized in the oil industry in connection with drilling, producing, storing and distributing operations.
Drilling for oil and gas in formations situated beneath the surface of a body of water has in the recent past become an extremely challenging and important segment of activity in the oil industry. In this connection, creative scientists and engineers have made tremendous strides in connection with exploration, drilling, producing, storing and distributing activities in a marine environment often referred to as the last earth frontier. Notwithstanding the successes of the recent past, however, significant challenges remain in this infant segment of the oil industry.
In the initial stages of development, offshore tower operations were conducted in locations of relatively shallow water depths, from a few feet to one or two hundred feet, such as exists along the near shore portions of the Gulf of Mexico. More recently, however, large mineral resources have been detected in water depths ranging from a few hundred to a few thousand or more feet, such as exists along the Pacific Coast continental shelf and the Arctic regions.
In order to exploit mineral resources which exist below such substantial depths of water, tower designs which have been reliable and effectively utilized in the past have undergone considerable redesign for prolonged high stress deep water use. In this connection, offshore towers presently being designed are enormous structures presently truly significant engineering challenges, not only from an initial design aspect, but from a subsequent construction, transportation and erection point of view.
Conventionally, offshore towers are constructed with a plurality of generally upright legs which extend between the bed and the surface of the body of water for supporting a platform above the surface of the body of water. These upright legs are stiffened or reinforced by lateral brace members or crossing struts. The reinforcement members initially require accurate cutting operations to form coped ends to intimately abut against the circular jacket legs and then require an intricate welding operation to fixedly connect the bracing to the jacket legs.
While such a structure and technique of fabrication has been generally satisfactory in the past, significant disadvantages remain. More particularly in dealing with large, heavy structures, it is often difficult to provide an exactly coped end portion which will mate with a similar curved member, particularly in connection with sloping struts. Therefore, excessive welding is often required and in some instances new struts have to be fabricated. Further, cross braces and struts abutting against the tubular jacket legs tend to punch through the jacket legs or at least deform the jacket legs into a generally eliptical or out of round configuration. Merely increasing the wall thickness of the jacket legs is generally unsatisfactory, since the increase in weight of the jacket legs adds significantly to the total weight and cost of the offshore tower. Further, the legs cannot be adequately reinforced or stiffened internally due to space limitations since the legs frequently contain piles, drilling conductors, diver access tubes and the like, which consume the majority of the space in the interior of the tubular leg. Gusset plates and the like have been employed to reinforce the exterior of the legs so that the load transfer takes place across a larger area. However, this indirect transfer causes stress concentrations which can drastically reduce the fatigue life of the joint and structure. Further, such additional plates materially add to the weight of the tower structure, and therefore the overall cost, as previously mentioned.
It would therefore be highly desirable to provide a means for reinforcing or stabilizing jacket legs of an offshore tower which would transfer the load through the connection and not into the tower legs.
Further, in at least some instances, it has previously been the practice to stabilize an offshore tower by forming laterally extending skirt pile casings about the base of the offshore tower and driving piles, having approximately the same diameter as the jacket legs, through the pile casings and into the bed of the body of water.
In areas, however, where loose bottoms exist or seismic or strong hydrodynamic loads are anticipated, it has been found that such conventional towers with skirt pile reinforcing have not been totally satisfactory. In this connection, it would be desirable to provide a jacket piling system which would enhance the capabilities of the offshore tower to withstand large lateral loads such as produced by shifting earth formations, etc.
Another difficulty with previously known offshore towers has been the difficulty in placing piles within the skirt pile casings and then subsequently driving the piles deeply into the bed of the body of water. In this connection a plurality of pile driving guide collars have been established at the end of cantilever arms extending along the lateral surface of the offshore tower at a plurality of elevations with a string of axially aligned guides for each pile casing. Therefore, following the driving of one pile the entire driving string has to be raised in segments and reconstructed in the next string of guides. Such a process is extremely laborious, time consuming and economically undesirable.
It would therefore be desirable to provide a method and apparatus for conveniently guiding the piles into the pile casings and rapidly driving the piles deeply into the bed of the body of water without withdrawing the entire pile driving string following each driven pile.
As previously mentioned, in the early stages of development of the offshore petroleum industry, drilling took place in shallow water depths. Conventional construction equipment and techniques were therefore capable of fabricating the shallow water towers.
It has been found, however, that in trying to meet the current demand for deep water structures, which typically may be five hundred or two thousand or more feet in length, conventional construction techniques are often not suitable to fabricate such enormous towers. In this connection, shipyard facilities to fabricate these enormous structures presently do not exist. Moreover, once fabricated, there remains the substantial problem of transporting the tower to a body of water for subsequent navigation to a desirable marine site.
It would therefore be highly desirable to provide a method for fabricating large tower structures of indefinite length and a corresponding means of launching the towers thus constructed into a body of water for transport to a desired offshore site.
OBJECTS AND SUMMARY OF THE INVENTION
It is therefore a general object of the invention to provide a method and apparatus which will obviate or minimize problems of the the type previously described.
It is a particular object of the invention to provide an offshore tower with a novel cross bracing system which will minimize the stress concentrations which typically exist between offshore tower jacket legs and stiffening horizontal braces and struts while simultaneously minimizing the total tower weight.
It is a further object of the invention to provide an offshore tower which will transfer lateral loads through a cross bracing junction and not into the tower jacket legs.
It is a further object of the invention to provide a method for fabricating an offshore tower which will not be limited by the size of the tower required to be constructed.
It is a still further object of the invention to provide a stabilizing structure connected to the base of the offshore tower capable of withstanding large lateral loads such as produced by seismic disturbances.
It is another object of the invention to provide a method and apparatus for guiding and driving piles into jacket pile casings in a convenient and rapid manner.
It is a still further object of the invention to provide a method for launching a deep water offshore tower into a body of water for transportation to a preselected marine site.
It is yet a further object of the invention to provide a method of fabricating an offshore tower segmentally whereby any length may be fabricated if desired.
It is yet another object of the invention to provide a method and apparatus for increasing the capacity of a tower to support drilling operations in a plurality of locations.
It is still another object of the invention to provide a convenient manner of fabricating an offshore tower which will maintain the geometrical integrity of the offshore tower during the construction and the subsequent launching operations.
Other objects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings wherein:
FIG. 1 is an isometric view of an offshore tower, with a lower corner removed for illustration purposes, positioned within a body of water and resting upon the bed of the body of water with an upper portion thereof extending above the surface for stably supporting a platform with drilling equipment thereupon;
FIG. 2 is a fragmentary corner view of an offshore tower which in full embodiment would be illustrated identically as in FIG. 1 with the exception that surrounding the bottom portion of the tower, twice as many jacket legs are provided which extend alternately from the base of the tower upwardly and coextend with twice as many cross bracing connections to a position intermediate the length of the offshore tower;
FIG. 3 is an isometric view of an offshore tower according to the invention and is provided with a longitudinally extending pile guiding and driving truss circumferentially riding on the outer periphery of the tower and is further provided with a circumferentially extending belt of jacket pile casings and piles about the base for pinning the offshore tower to the bed of the body of water;
FIG. 4 is an isometric view of a girder ring forming a portion of the invention;
FIG. 5 is a segmental detailed view of a portion of the girder ring disclosed in FIG. 4;
FIG. 6 is a detailed view of a bracket for connecting the girder ring spokes to the hub of the girder ring;
FIG. 7 is a cross-sectional view of FIG. 6 taken along section line 7--7 therein;
FIG. 8 is a detailed segmental view of a portion of a girder ring disclosing the brackets for connecting the hub spokes with the inner rim of the girder ring;
FIG. 9 is a sectional view of FIG. 8 taken along section line 9--9 therein;
FIG. 10 is a sectional view of FIG. 8 taken along section line 10--10 therein;
FIG. 11 is a segmental view of a section of the outer periphery of an offshore tower disclosing the relationship of the tower legs, the cross bracing shell and the girder ring;
FIG. 12 is an isometric view of a segment of an offshore tower disclosing an alternate girder ring configuration;
FIG. 13 is an isometric view of a segment of the outer portion of an offshore tower disclosing an alternate connection arrangement of the girder ring disclosed in FIG. 12, with the tower legs and cross bracing shell;
FIG. 14 is an isometric view of a segment of the outer portion of an offshore tower disclosing a still further alternate girder ring arrangement;
FIG. 15 is an isometric view of a portion of an offshore tower disclosing an alternate connection configuration of the girder ring arrangement disclosed in FIG. 14;
FIG. 16 is a plan view of a unitary cross forming a part of the invention;
FIG. 17 is a top view of the unitary cross disclosed in FIG. 16;
FIG. 18 is a plan view of a segment of a skirt piling casing ring which may be deployed around the base of an offshore tower as illustrated in FIG. 3;
FIG. 19 is a sectional view taken along section line 19-19 of FIG. 18;
FIG. 20 is a detailed view of a connection bridge between a tower jacket leg and a horizontally disposed brace, spanning adjacent skirt pile casings;
FIG. 21 is a plan view of the bridge shown in FIG. 20;
FIG. 22 is a sectional view of FIG. 21 taken along section line 22--22 therein;
FIG. 23 is a detailed elevational view of an alternate connection bridge between a tower jacket leg and a horizontally disposed brace spanning adjacent skirt pile casings;
FIG. 24 is a plan view of the bridge shown in FIG. 23;
FIG. 25 is a side elevational view of a pile driving truss as isometrically illustrated in FIG. 3;
FIG. 26 is a side elevational view of the pile driving truss shown in FIG. 25;
FIG. 27 is a cross-sectional view of FIG. 25 taken along section line 27--27 therein;
FIG. 28 is a detailed view of the truss driving mechanism;
FIG. 29 is a segmental elevational view of a portion of the vertical columns utilized to hold the girder rings during the tower construction operation;
FIG. 30 is a detailed view of an upper portion of one of the support columns shown in FIG. 29 specifically illustrating the adjustable upper bracket and pillow blocks;
FIG. 31 is a side elevational view of FIG. 30;
FIG. 32 is a cross-sectional view of FIG. 31 taken along section line 32-32 therein;
FIG. 33 is a schematic elevational view of an offshore tower in a partially completed state of construction;
FIG. 34 is a plan view of an offshore tower positioned within a construction bay adjacent a body of water;
FIG. 35 is an end elevational view of the base of the tower as illustrated in FIG. 34, resting upon supporting blocks;
FIG. 36 is an end elevational view of the top of the offshore tower, as illustrated in FIG. 34, resting upon a rail bearing guide;
FIG. 37 is a detailed elevational view of the rail bearing guide;
FIG. 38 is a sectional view of the rail bearing guide of FIG. 37 and taken along section line 38-38 therein; and
FIGS. 39-41 disclose in a sequential schematic array a method of launching the previously constructed offshore tower along the monorail of the construction bay and into the body of water for transport to a desired marine site.
Referring now to the drawings, and more particularly to FIG. 1 thereof, there will be seen an offshore tower 50 situated upon the bed 52 of a body of water 54 and extending above the surface 56 of the body of water for stably supporting a platform 58 thereabove. The body of water 54 typically may range from 200 to 2,000 or more feet in depth. The offshore tower, as previously mentioned, may be used for a multiplicity of applications such as, for example, a support for radar stations, light beacons, marine exploration labs and the like. More predominantly, however, offshore towers of the type illustrated are utilized in the oil industry for drilling, producing, storing and distributing operations.
In this connection, the platform 58 frequently is composed of at least two decks, a main deck 60 and a cellar deck (not shown) positioned therebelow. The main deck may serve to support a plurality of drilling rigs 62 which progressively and simultaneously drill in a plurality of locations around the periphery of the offshore tower. Further, the main deck may be provided with a plurality of cranes and various mud tanks and other equipment suitable for sustaining a continuous drilling operation. The cellar deck typically may contain housing units, generators, compressors, control centers, test facilities and the like.
The offshore tower 50 is composed of a plurality of jacket legs 64 disposed symmetrically about a central vertical axis 66 and forming an outer tower surface generally in the geometrical configuration of a truncated cone. The peripherally disposed upright jacket legs 64 are supportingly interconnected by a diamond patterned shell of cross bracings 68 which serve to take lateral loads imposed upon the offshore tower 50. It will be appreciated by those skilled in the art that the diamond patterned shell of cross bracings shown in FIG. 1 in a preferred outer encompasing posture may in some instances be placed within the interior of the inner tower peripher formed by the jacket legs 64. Surrounding the upright jacket legs 64 and the surrounding shell of cross bracings 68 are a plurality of girder rings 70 positioned around the outer periphery of the tower.
The girder rings 70 lie in a plurality of mutually parallel planes all lying normally to the central tower axis 66. As will be readily realized by viewing FIG. 1, the girder rings 70 incrementally diminish in diameter from the base 72 of the offshore tower to the top 74 thereof. Each of the girder rings 70 is supported against out of round deformation by a bicycle bracing network 76 (note FIG. 4) which will be more fully described hereinafter.
The upright jacket legs 64 are columnar structures and are sufficiently dimensioned to receive concentrically within the interior thereof a conductor 78 which is driven into the bed of the body of water 52. The conductor may be grouted to the interior of the jacket leg and serves to guide a drilling string (not shown) for drilling through the jacket legs into formations positioned beneath the offshore tower 50. Moreover the conductors serve the at least two additional significant purposes of strengthening and supporting the tower.
As illustrated in FIG. 1, it may be desirable in some instances to drill in locations between adjacent jacket legs. In this connection, conductor strings 80 shown by dotted lines may be guided through a plurality of axially aligned conventional funnel shaped collars 82 (not FIG. 8) fixedly connected to the girder rings 70.
In those instances where additional lateral stability is desired, the base 72 of the offshore tower may be provided with a tighter shell of cross bracing struts 86 identical in general configuration with the cross bracing shell 68. The spacial area, however, within an individual diamond is diminished by a factor of four while there are twice as many crossing junctions at a given planar level. The lateral structural strength of the offshore tower is thereby significantly increased.
Referring now to FIG. 2, there will be seen a lower corner segmental view of an offshore tower 87, the remainder of which being substantially identical with the tower as disclosed in FIG. 1. Tower 87 includes a plurality of jacket legs 88 extending about the periphery of the tower and containing therein conductors 90, which extend coaxially within the jacket legs and deeply into the bed of the body of water. Drilling strings are then lowered through the conductors 90 for drilling earth formations situated beneath the offshore tower.
The conductor legs 80 are surrounded by a cross bracing shell 92 and girder rings 94, identically as described in connection with the tower illustrated in FIG. 1.
The offshore tower 87 illustrated in FIG. 2, however, in addition to the structure of FIG. 1, is provided with a plurality of jacket legs 96 which extend between the bed of the body of water, upwardly only a partial distance along the offshore tower lateral surface. Above the jacket legs 96, conventional conductor guide collars 82 (note FIG. 4) are positioned within the girder rings 94 in a manner previously discussed. The lower jacket leg segments 96 serve to isolate the lower portion of the conductor strings from excessive compressive forces and current stresses which may be produced when the base of the tower is positioned in relatively deep water. Further, it will be seen that additional cross bracings 95 extend from the leg of the body of water upwardly and coextensively with the jacket legs 96 to increase the lateral structural strength of the offshore tower and distribute loads to the conductor piles.
Referring now to FIG. 3, there will be seen an isometric view of an offshore tower 97 again as substantially described in connection with FIG. 1, including an upper platform 98, a plurality of jacket legs 100 surrounded by a shell of cross bracing struts 102 and a plurality of girder rings 104.
The offshore tower 97 is provided at its base 106 with a plurality of skirt pile guides 108 extending peripherally about the circumference thereof. The pile guides 108 are fixedly interconnected with each other by horizontally disposed braces 110 and sloping struts 112. Piles 114 are guided within the skirt pile guides 108 and are driven into the bed 52 of the body of water 54 by utilization of a rotating truss 116 connected along the periphery of the offshore tower 97 which will be more fully described hereinafter.
Girder Ring Segment
The truncated cone offshore towers 50, 87 and 97, as isometrically illustrated in FIGS. 1-3, all utilize a plurality of girder rings.
As specifically illustrated in FIG. 4, a girder ring 118 is constructed with a circular outer beam 120 and a coaxially disposed circular inner beam 122. The beams are interconnected by sloping braces 124 in a manner which will be more specifically disclosed hereinafter.
The girder ring is supported against out of round deformation by a bicycle bracing system 76 comprising an axially disposed tubular hub 126 having an upper flange plate 128 and a lower flange plate 130 disposed circumferentially about the outer periphery of tubular hub 126. Emanating from the upper flange plate 128 and the lower flange plate 130 are a plurality of spokes 132. These spokes may be composed, for example, of a set of steel wires wrapped with a surrounding cloth with a resin covering to prevent excessive corrosion thereof.
The spokes 132 emanate from both the upper and lower surface of both the upper flange plate 128 and the lower flange plate 130. At each junction location of the spokes 132 with the inner beam 122 of the girder ring 118, one spoke originates from the upper flange plate 128 and a second spoke emanates from the lower flange plate 130.
A segment of spoke junction locations have been labelled in the top portion of FIG. 4 as junction points A through E. The spoke lines have been hatched in coding to more fully illustrate the connection system. In this connection, reference may be had to the legend at the lower corner of FIG. 4, wherein the full dotted line indicates a spoke emanating from the lower flange lower side (L.F.L.S.). The dashed and dotted line indicates a spoke from the upper flange lower side (U.F.L.S.). Spokes radiating from these flange locations will be seen as connecting with the beam 122 at position A. In the next clockwise connection B, there will be seen a solid line emanating from the upper flange which, as noted in the legend, represents the upper flange upper side (U.F.U.S.), while the long dash line represents the lower flange upper side (L.F.U.S.). Position C on the rim 120 then is provided with spokes (note line coding) from the lower flange lower side and the upper flange lower side. At position D, spokes emanate from the upper flange upper side and lower flange upper side. Thus, in alternate locations around the periphery of the girder ring 118 the spokes emanate from the upper flange upper side and lower flange upper side, while in alternate locations the spokes emanate from the lower flange lower side and upper flange lower side.
Referring now to FIG. 5, there will be seen a detailed view of the hub anchoring structure including the tubular hub 126, upper flange plate 128, and the line coded spoke system 76. As specifically illustrated, each individual spoke 132 is formed from a pair of spaced wires 134. The wires are fixedly connected to the flanges 128 and 130 by a suitable bracket arrangement 136.
The bracket attachments 136 may be formed from one of a number of conventional designs currently utilized in conjunction with prestressing operations. One specific embodiment, however, that is satisfactory, is specifically illustrated in FIGS. 6 and 7. The bracket arrangement 136, as specifically there illustrated, is composed of a base plate 138 and a plurality of normally extending legs 140. A head section 142 interconnects the vertical legs and the base plate. The head section 142 is provided with a pair of channels 144 which open upwardly and serve to receive in sliding fashion a flexible wire or braided bundle of strands 134. Juxtaposed against the head section 142 of the bracket 136 are one or more key plates 146 having a pair of downwardly facing channels 148 fashioned therein and being dimensioned to spacially conform with the upward channels 144 in the head plate 142 as specifically illustrated in FIG. 7. The wires 134 are formed with integral head beads 150 and are spaced from the key plate 146 by one or more circular bushing rings 152. The key plate 146 in conjunction with the juxtaposed head plate 142 serves to confine the ends of the prestressed bundle of wires 134.
Referring now to FIG. 8, there will be seen a segmental plan view of the girder ring 118 including an outer beam 120 and an inner beam 122. The beams are spaced, as previously mentioned, by a plurality of sloping supports 124 positioned therebetween about their periphery. Further, there will be seen a junction location 154 of the wires 134 forming one of the spokes 132 of the girder ring arrangement.
As specifically illustrated in FIGS. 9 and 10, it will be seen that the inner rim 122 is composed of a T-beam and is provided with a pair of inwardly extending parallel channels 156 for the reception of wires 134. A key plate 158 is provided with a pair of inwardly extending compatibly dimensioned channels 160 which serve to retain wire head tabs 162 within the channels 156, in a manner previously discussed in connection with FIGS. 6 and 7.
Following the connection operation, the individual wires or bundles of braided wire strands 134 are prestressed by conventional hydraulic stressing devices (not shown), and a suitable number of key plates 158 or 146 may be inserted to retain the wire in the desired tension. It will be readily recognized that such a tensioned spoke system will maintain the girder ring in an approximately circular posture.
Peripheral Tower Elements
Referring now to FIG. 11, there will be seen an isometric segmental view of a preferred embodiment of ring girder 118 and its manner of connection to a plurality of jacket legs 164 and diamond patterned cross bracings 166.
The ring girder 118 is composed of an inner T-beam rim 122 and an outer T-beam rim 120, having the long legs 168 thereof mutually facing each other and being rigidly interconnected by a plurality of sloping braces 124. The braces 124 may be either straps or tubular stock, as preferred or as load requirements dictate. The jacket legs 164 and cross braces 166 are positioned between the inner and outer girder rims and are fixedly connected to the inwardly projecting legs 168 through one or more coped notches 170, fashioned therein as required and one or more horizontal and vertical tying brackets 172 (note also FIG. 9).
Although mutually facing inner and outer T-beam rims 120 and 122 are preferred, alternate ring girder structures may be utilized, as specifically illustrated in FIGS. 12-13 and 14-15.
In the embodiment illustrated in FIGS. 12 and 13, there will be seen a first alternate embodiment having an outer rim 174 formed from a pair of T-beam members 176 interconnected by slanting braces 178 and an inner rim 180 formed from a pair of angles 182. The T-beams 176 and angles 182, respectively, are interconnected by a plurality of bracing straps 184. Both the inner and outer rims 180 and 174 are positioned outside of the jacket legs 164 and diagonal cross bracings 166. Interconnection between the ring girder 118 and the jacket legs 164 and the diagonal bracings 166 may be provided by the provision of bridge members 186, extending from the angle members 182 having coped ends 188 to unite, as by welding, integrally with the jacket legs 164 and across bracing members 166.
A second alternate embodiment of the ring girder 118 is illustrated in FIGS. 14 and 15. There will be seen an outer rim 189 formed from a pair of spaced T-beam members 190. An inner rim 192 is formed from a pair of spaced angle members 194. The angles 194 of the inner rim 192 and the T-beams 190 of the outer rim 188 are fixedly interconnected by a plurality of sloping struts 196. The outer rim 189 is interconnected with the inner rim 192 by a plurality of brace straps 198. Jacket legs 164 and cross braces 166 of an offshore tower are extended between the inner and outer rim members and intimately abut coped projections 200, which extend from the rim segments for uniting contact therewith, such as by welding.
Cross Bracing Reinforcing Shell
As previously mentioned in connection with FIGS. 1-3, the outer tower structure assumes the general geometric configuration of a truncated cone having jacket legs extending about the outer periphery thereof in a symmetric posture about a central axis of the offshore tower. The jacket legs are fixedly interconnected by a cross bracing network 66 (note particularly FIG. 1).
Referring again now to FIG. 11, there will be seen a detailed view of a segment of the outer tower periphery including jacket leg segments 164 and cross bracings 166, fixedly connected thereto. The cross bracing struts 166 are integrally joined, as by welding, at their junction locations by generally hollow unitary crosses 202. The crosses 202 are fixedly connected at the mid points by bridging structures 172 having coped ends, as previously mentioned, to the outer periphery of the jacket legs 164.
Referring now specifically to FIGS. 16 and 17, there will be seen detailed views of the unitary cross 202.
The cross is composed of four arms 204, 206, 208 and 210 of uniform lengths and as best illustrated in FIG. 17 are composed of generally hollow tubular members. The outer ends 212 are formed with normal surfaces relative to the axis of each arm and serve to abuttingly mate with a similar surface of the strut braces 166. This normal abutting surface contact provides a convenient welding junction and serves to uniformly transmit loads through the junction equally around the periphery of the junction. The entire crossing structure preferably is fabricated as a unitary piece such as by forging or casting or in the alternative the inner ends of the legs may be integrally united as by welding.
An angle is formed between adjacent legs with opposing angles alpha, A, and beta, B, being equal. The magnitude of these angles will be determined by the desired slope of the cross bracing arms 166.
Referring now to FIG. 17, the arms 204 and 210 have axes lying in substantially the same plane 214 and arms 206 and 208 have axes lying in substantially the same plane 216. Both of these planes are slightly angled by an amount rho, P, with respect to a plane 218, which lies tangent to the outer periphery of the offshore tower. This slight angle permits the cross 202 to conform to the outer periphery of the curvilinear tower surface.
The unitary cross 202 is equally dimensioned throughout the entire jacket structure including angles alpha, beta and rho. It will be readily realized that this unitary cross uniformly positioned throughout the outer surface of the tower will transmit axial loads through the junction locations, as opposed to the previously known technique of transmitting the loads into the tower jacket legs 164. Further, the uniform nature of the cross permits mass assembly techniques which materially reduces the time and labor involved in constructing the cross bracing system.
Skirt Pile Guides
In instances where the offshore tower will be situated upon a soft bottom, or the tower must withstand large hydrodynamic or seismic loads, it will be desirable to form a belt of skirt pile guides 108 around the outer periphery of the base of the tower 106. Piles 114 are driven within the guides 108 for pinning the offshore structure to the bed of the body of water (note FIG. 3).
Referring now to FIG. 18, there will be seen a top sectional view of a segment of a skirt pile guide structure 220. A plurality of skirt pile guides 108 are horizontally connected by upper and lower tubular brace segments 110.
As best illustrated in FIGS. 18 and 19, the skirt pile casings 108 are integrally attached to adjacent jacket legs 100 by longitudinally extending diamond patterned bracings 222, which slopingly connect between the jacket legs 100 and the skirt pile casings 108.
Cross bracing struts 166 abut against and are weldingly connected to bridge members 224, as shown in FIG. 18 but more specifically illustrated in FIGS. 20-22.
The bridge 224 is composed of a pair of horizontal 226 and vertical 228 plates fixedly interconnected with a crossing plate 230. The horizontal plates 226 have coped surfaces 232 and the vertical plates 228 have coped ends 234 to intimately abut with the adjacent jacket leg 100 and skirt piling brace 110 for fixed interconnection therewith, as by welding.
In those instances where the cross bracing arms 166 do not join at the connection point between the upper and lower horizontal brace arms 110 and the jacket legs 100, an I-beam 236, as generally illustrated in FIG. 18 and more specifically illustrated in FIGS. 23 and 24, is provided which connects between the upper and lower horizontal braces 110 and the jacket legs 100. The I-beams 236 are provided with coped upper and lower surfaces 238 and coped web surfaces 240 for intimately contacting the adjacent jacket leg 100 and cross brace 110 for being fixedly welded thereto to unite the pile brace and jacket legs.
It will be noted by referring to FIG. 18 that the cross sectional dimensions of the skirt pile legs are significantly larger than the cross-sectional dimensions of the jacket legs 100. More specifically, the diameter of the jacket leg 100 is approximately one-half to one-fifth that of the diameter of the pile casing 108. The variation in cross-sectional dimensions is provided to increase the capability of the pile casings 108 and and piles 114 receivable therein to withstand shear and flexture stresses. More specifically, for a tubular member such as the skirt piles, the load carrying capacity is increased in proportion to the second power of the cross-sectional dimension. Thus the large diameter piling may withstand significantly larger loads than corresponding conductor piles while at the same time, the jacket legs which must be larger than the conductor piles may be maintained relatively small through the use of lateral bracing. The resulting structure is a combination with maximum strength and simultaneously the long jacket legs with adequate bracing are maintained with relatively small dimensions to minimize total structural weight and costs.
Pile Driving Truss
In order to position the pile 114 within the pile casings 108, a pile positioning and driving truss 116 (note particularly FIG. 3), is suspended along the lateral surface of an offshore tower.
Referring particularly now to FIGS. 25-27, there will be seen detailed views of the truss 116.
The truss is formed from three generally mutually parallel legs 242 interconnected by a plurality of struts 244 and horizontal braces 246. A plurality of ring girder rolling supports 248 are connected along the legs 242 of the truss 116 and are specifically designed to rest upon and roll about the offshore tower ring girders.
Referring now to FIG. 28, there will be seen a detailed view of a rolling support 248.
The rolling support 248 comprises a channel faced roller 250 adapted to rest upon the upper edge of a T-beam 252 forming at least a portion of the outer rim of the girder ring. The roller 250 may be driven through a gear linkage system which includes a spur gear 254 axially connected with the roller 250 and mating with spur gear 256, which may be driven by a conventional electric or hydraulic motor 258. The roller support assembly 248 is fixedly connected to one of the legs 242 of the truss 116 by a suitable bridge 260.
In operation, the truss is suspended from an offshore tower, such as specifically illustrated in FIG. 3, and axially aligned with a skirt pile guide 108. A pile may then be axially guided into engagement with the interior of the skirt pile guide 108. The truss 116 may then be advanced about the periphery of the tower until the axis of the truss is in alignment with the axis of the next skirt pile guide 108. A pile may then be guidingly lowered into the guide and the procedure repeated circumferentially around the offshore tower.
A driving stinger may then be lowered within the interior of the truss 116 for driving the piles 114 into the bed of the body of water. Following the driving operation of one pile, the truss 116 is advanced into axial alignment with the next succeeding pile and the driving stinger drives the pile into the waterbed. This procedure is duplicated around the outer periphery of the offshore tower until all of the piles 114 are driven deeply within the bed of the body of water. The truss 116 may then be removed for subsequent utilization with other offshore towers.
It will be appreciated that the rotating truss 116 provides a means for initially placing and later driving a plurality of piles about the base of an offshore tower which may be subsequently removed so as not to interfere with subsequent operations and wherein the piles may be placed and driven without retracting and placing a lengthy pile column with each shift pile driving location.
Following the driving operation, the piles 114 may be fixedly connected to the guides 108 as by grouting the interior thereof in a manner such as specifically described in a U.S. Hauber et al. Pat. No. 3,315,473, assigned to the assignee of this application. The disclosure of this patent is hereby incorporated by reference as though set forth at length.
Method of Fabrication
As previously mentioned, the subject offshore tower may be constructed in water depths ranging from 200 to 2,000 or more feet. The jacket legs are typically two or more feet in diameter. The overall diameter of the base of the tower structure, typically may be three hundred or more feet in diameter, while at the water line, the diameter may be 160 feet or more. When dealing with such large structures, essentially composed of tubular steel members, the construction or fabrication techniques typically satisfactorily utilized in shipyards for smaller tower structures are often either economically unsatisfactory or physically incapable of performing the construction operation.
A preferred method of constructing the above described offshore tower comprises fabricating a plurality of girder ring segments 118, as previously discussed in connection with FIG. 4, in a shipyard.
A plurality of vertically extending columns 270, as illustrated in FIG. 29, are then constructed upon a plurality of pilings 272, which extend deeply into the earth 274 to fixedly support the columns 270. The columns 270 are aligned and correspond in number and spacing to the number of tower girder rings required and to normal spacing therebetween. The height of the columns 270 are designed to be approximately equal. Positioned at the upper ends thereof are vertically adjustable supports 276.
The support 276 comprises four upwardly facing outwardly sloping arms 278 which connect at their lower ends to a quadraped base 280 comprising four upwardly facing members 282 (note FIG. 32) which are interconnected by horizontal braces 284. The members 282 are adjustably connected to the upper end of a column 270 by connection with four axially adjustable hydraulic cylinder and ram assemblies 286. At the upper end of the arms 278 upwardly extending members 288 are longitudinally connected by braces 290 and serve to support a pair of spaced I-beams 292 which in turn support a pair of pillow blocks 294.
As best illustrated in FIG. 29 the girder rings 118 previously assembled are upended and the hubs 126 thereof positioned upon a pair of adjacent pillow blocks 294 of successive columns 270. The pillow blocks 294 are then vertically manipulated until the axes of the hubs 126 are in alignment whereupon the hubs are interconnected by spacer tubes 296 into a column that extends coaxially about the central tower axis.
The girder rings 118 thus suspended in planes being mutually parallel and normal with the central axis of the tower are interconnected by a plurality of generally normally extending jacket leg segments 164, as specifically illustrated in FIG. 33.
Following the connection of the jacket leg segments 164, the outer shell of truss bracings 166 and the unitary crosses 202 are formed surrounding and reinforcing the jacket legs.
The process of connecting the leg segments, which are axially aligned to produce generally hollow legs throughout the tower structure, cross bracings and unitary crosses is duplicated throughout the tower structure until the tower is completed.
Method of Launching
As previously mentioned in connection with the fabrication of the offshore tower, current tower designs are often enormous structures. Therefore, not only is it a truly significant problem to initially fabricate the tower, but the manner of launching the tower into a body of water for transport to the desired site presents a significant challenge.
Referring now to FIG. 34 there will be seen an offshore tower positioned within a launching bay 302. The tower 300 may have been fabricated by the previously discussed technique horizontally upon upright columns 270.
In order to launch the tower, a construction bay 302 is first fabricated along a shore line where water deep enough to support a large tower is present near the shore. The bay 302 is constructed generally normally toward and into the body of water 306 such that the bay 302 is partially on the shore 304 and partially beyond the shore line of the body of water 306. The bay 302 is maintained in a dry condition throughout by the establishment of sheet pile barrier side walls 308 and an end wall 309 which permits the lower portion of the tower to be constructed below the adjacent water level 306. The columns 270 are positioned along a concrete rail 310 and the tower 300 is fabricated in a manner previously discussed.
The launching rail 310 is supported upon a plurality of piles 311 positioned along the length thereof and driven deeply within the bed 312 of the construction bay 302.
Further, in the lower portion of the construction bay, a pair of pads 314 are mounted atop a plurality of piles 316 and serve to support by pillow blocks 318 a pair of floatation vessels 320. The floatation vessels are connected to the outer surface of the offshore tower 300 by a bracing system 322 and an internal framing system 323. At the upper end 324 of the tower 300, a toroidal floatation collar 326 is constructed which at its lower portion is provided with a rail bearing guide assembly 330, which bears upon and is guided along the rail 310 in a manner which will be more fully described hereinafter. The columns 270 are then removed and the tower 300 rests upon a three-point bearing within the construction bay.
Referring now to FIGS. 37 and 38, there will be seen detailed views of the rail guide and bearing assembly 330. The toroidal floatation collar 226 is provided at its lower portion with a support I beam 332 which has a downwardly projecting leg ending in a cylindrical bearing rod 334. The bearing rod 334 rests in a freely pivotal manner upon a rail bearing guide 330.
The rail bearing guide 330 is generally triangular in cross-section as best illustrated in FIG. 38 having a rectangular bottom surface and a pillow block apex 336 for pivotally receiving the cylindrical bearing rod 334. Along the lateral edges of the bearing guide 330 are guide arms 338 (note FIG. 37) which serve to assist in maintaining the bearing guide 330 upon the rail 310.
The bearing guide is divided by walls 340 which add structural rigidity to the bearing assembly 330. An essentially incompressible fluid supplied through a piping system (not shown) is directed downwardly towards the rail in a manner which will be more fully discussed hereinafter.
Referring now to FIGS. 39-41, there will be seen in a generally schematic array a sequential depiction of launching the offshore tower into a body of water.
Referring now specifically to FIG. 39, the end wall 309 of the fabrication bay 302 is removed and the body of water 306 enters the construction bay seeking its own level. The water lifts the base 342 of the offshore tower 300 from supporting contact with the pillow blocks 318 due to the buoyancy of the pair of floatation vessels 320. It will be noted that the spaced relationship of the floatation vessels 320 provides, in cooperation with the bearing 330, a three point bearing arrangement which stably supports the tower. As the base 342 of the offshore tower rises, the upper portion of the tower 324 will pivot about the bearing rail guide 330.
A source of incompressible fluid 344 is connected into the interior of the bearing guide 330 by a line 346. Incompressible fluid, such as for example water, oil, soap suds or the like, is then pumped into the chambers formed by the partitions 340. The bearing rail guide 330 is then lifted from frictional contact with the rail 310. At this juncture, the tower will begin to slide down the sloping rail 310 toward the body of water 306. If downward movement does not immediately occur upon unloading of the rail bearing guide, a tug (not shown) may be connected to the base of the tower to initiate the downward movement of the structure may be jacked down the rail by conventional jacking devices. The line 346 is of sufficient length to maintain contact with the source 340 and the bearing guide 330, for a sufficient period of time for the tower to gain momentum as it descends down the launching rail. When sufficient momentum is achieved, however, the lubricating fluid is no longer necessary and the line 346 may be severed. Alternately, the incompressible fluid source may be stored within the tower.
Referring now to FIG. 40, it will be seen that as the tower slides down the rail 310, the toroidal floatation collar 326 will come into contact with the surface of the body of water 306 and by its buoyancy lift the upper portion of the tower 324 off of bearing contact with the guide bearing 330.
Referring to FIG. 41, it will thus be seen that the offshore tower 300 may be completely supported on a floatation system which includes an upper toroidal floatation collar 326 and a pair of spaced floatation vessels 326 connected to the outer periphery of the base of the tower 342. The tower 300 may then be towed to a desired offshore site and sunk to the bed of a body of water in a manner more fully disclosed and specifically claimed in applicant's copending U.S. application, Ser. No. 29,831 now U.S. Pat. No. 3,693,361.
While the specific floatation system utilized in conjunction with the launching operation has been described as comprising a toroidal collar and a lower floatation vessel or vessels, it will be realized that other floatation systems may be utilized in conjunction with the previously described launching technique. One such floatation system is specifically described and claimed in copending U.S. application, Ser. No. 29,994 now U.S. Pat. No. 3,633,369 by Joseph Benton Lawrence, said application being of common assignment with the present application.
In those instances where it is desired to utilize this latter floatation system to transport the offshore tower to a desired marine site, it will be recognized by those skilled in the art that the rail bearing guide 330 may be connected directly to an upper brace affixed to the offshore tower 300 and the articulated string of floatation vessels may be attached to the lateral surface of the offshore tower and supported upon pillow blocks substantially similar to pillow blocks 318.
SUMMARY OF THE MAJOR ADVANTAGES
It will be appreciated that the above disclosed offshore tower is of universal design which may readily be constructed for any desired water depth.
Another significant aspect of the tower invention is the shell of cross bracings which join through unitary crosses being uniformly dimensioned throughout the tower structure which serve to transmit loads through the crossing junctions rather than into the tower legs. The junctions may be fabricated in large quantities by mass production techniques since the dimensions of the junctions are uniform throughout the tower design. Therefore, fabrication time and labor are minimized.
Another significant aspect of the invention is the provision of large diameter pilings juxtaposed peripherally around the base of the offshore tower to withstand large shear and bending loads while permitting the very long jacket legs to remain economically slender.
A further significant aspect of the invention is the provision of the rotating pile guiding and driving truss which serves to accurately align and drive a plurality of piles with a minimum amount of time and labor spent in pulling and setting pile strings.
A further significant advantage of the offshore tower is the ready adaptability of the design to vary the number of drilling locations about the periphery by the addition of guide collars and conductors which may be pinned generally parallel with existing jacket legs through a plurality of girder rings. In the alternative and where large base loads are anticipated, jacket legs may be fabricated partially from the bed of the body of water upwardly along the surface of the tower coextensive with an increased number of cross bracings to add strength to the base of the tower and to receive conductors within the interior thereof in a protective manner.
Further, girder rings are positioned throughout the tower design having a radiating bicycle bracing system which supports the jacket legs and the cross bracings from out of round deformation.
A significant method aspect of the invention is the provision of a novel method of fabricating an offshore tower of indefinite size and length suitable to meet the increasing demands for larger offshore towers.
A further significant aspect of the invention is the manner of launching the thus constructed offshore tower within a body of water for transportation to an offshore site. In this connection, lifting the base of the tower and unloading the rail bearing guide permits a smooth and efficient launching operation.
An additional significant advantage is the utilization of the conductors extending within the jacket legs in the dual function to guide a drilling string and as pilings.
While the invention has been described with reference to preferred embodiments, it will be appreciated by those skilled in the art that additions, deletions, modifications and substitutions, or other changes not specifically described, may be made which will fall within the purview of the appended claims.