CA1048282A - Offshore tower construction and method of erecting same - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A platform is raised from water level up to the tops of prepositioned template legs by means of jacking units comprising jacking mechanisms and jacking legs mounted near the upper ends of the template legs. Bridge beams of open framework construction span the distances between the upper ends of the template legs and these bridge beams provide support for the jacking units as well as reinforcement for the platform when it has been raised.

Description

~ æ~32 This application is a division of my copending Canadian application Serial No. 259,707, filed ~ugust 24, 1976.
BACKGROUND OF THE INVENTION
Field oF the Invention -This invention relates to the erection of-structures and more particularly it concerns novel techniques for the con-struction of offshore towers and platforms such as are used in exploration and recovery of resources such as oil from beneath the sea bed.
Description of the Prior Art Offshore towers and platforms of the type to which the present invention pertains are shown in United States patent No. 3,857,247 to Lindsey J. Phares and in United States patent No. 3,~76,181 to Joseph E. Lucas. In both these patents there is described a two component system comprising a template or tower and a platform or deck, which are separately floated or carried by barge out to a desired offshore location. When the template arrives on location it is affixed in upright or vertical position to the sea bed with its legs extending up well beyond the water surface. The deck or platform is then positioned between the tower legs and is jacked up along the legs so that it is clear of the water surface and of all wave action. The platform is then pinned to the template and drilling and production operations are then carried out from the stably

2~ mounted, elevated platform.
Both the Phares and Lucas patents show arrangements for mounting jacking tubes to be suspended from the upper ends of the , - . - ,. . ~ ~ , , :. : - , . . . .. .

1 1Itemplate legs so they extend down alongside the legs to the 2 platform. J~cking mechanisms are provided on the platform to

3 l grip the jacking legs and pull the platform up via the jacking

4 legs to the top of the template legs. After the elevated platform ,: . !
is secured to the template the jacks and jacking legs may be 6 ~ removed for use in the erection of another offshore tower.

8 ~ SUMMARY OF THE INVENTION
. , g ' The present invention provides improvements to the . . , 10 ~ above described offshore tower erection techniques. More speci-11 fically, the present invention permits a more efficient and lower 12 cost Platform structure that has heretofore been necessary. That ;
13 is, the platform structure may be of lighter weight and less 4 rigid construction than previous platforms. In addition, with the present invention, a platform, once-positioned ad]acent the 1~ template, i5 made ready for a jacking up operation in a much 17 shorter time than has heretofore been necessary. This is im-18 portant because dllring the time the platform is riding on the 19 water adjacent the template legs it is subject to the action of the sea and is vulnerable to damage should the sea conditions 21 become severe.
22 According to one aspect of the present invention there 23 is provided a novel method for erecting an offshore tower platform 24 in which a template, having a plurality of vertical template ~5 legs, is position~d on the sea floor so that the template legs ~6 extend up above the sea level. The template is provided with at 27 least one open framework bridge beam connected to and extending between the upper ends of the ~ertical template legs. A platform ~9 is floated out to the thus positioned template and is jacked up along the template legsl clear of the water, to th bridge beam.
31 The platform is then connected to the bridge beam along its , Il 3 ~ ,.
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1 'llength so that it becomes stiE~ened and reinforced by the bridge 2 beam. The platform thus may be made initially of lighter and "
3 'I less rigid constr~ction than would be required if it had to span t 4 , the dis~ances between the template legs on its own.' , According to another aspect of he inven~ion a platform ' 6 ,may be lifted along rigid jacking legs which ex'tend from the open 7 framework beam to the platform. This arrangement permits the B ;, positioning of the jacking legs at locations where they may most 'efficiently act upon the platform.
~ ' In another o~ its aspects the present invention pro-11 vides rapid interconnection of a jacking unit between a platform 1~ and a template onto which the platform is to be lifted. Accord-13 ing to this aspect a jacking mechanism is mounted on either the 14 template or on the platform and is operative to move an elongated ' ~ ~ .
rigid jacking leg toward the other member until it passes through 16 a slip in that other member. The slip is constructed to allow 17 free movement of the jacking leg in a direction from the slip -1~ away from the first member but is operative to grip the jacking 19 ; leg to prevent xela~ive movement between the jacking leg and the 2~ ,other member in the oppo~ite direction. The jacking mechanism is 21 ~ then operated to move the jacking leg in the opposite direction 22 to elevate the platform clear of the water.
23 1 There has thus been outl,ined rather bro'adly the more ' 24 - important features of the invention in order tha~ the detailed ~5 '~Idescription thereof that follows may be better understood, and in 26 ,,order that the present contribution to the art may he better , 7 appreciated. There are, of course, additional features of the 29 invention that will be described hereinater and which will form ~' 29 ~the subject of the claims appended hereto. Those skilled in the art will appreciate that the conception upon which this dis-31 , closure is based may readily ~e utilized as a basis o~ the _ 1~)482~
1 lldesigni1lg of other structures or rnethods for carrying out the 2 several purposes of the invention. It is important, therefore, 3 1l that the claims be regarded as including such equivalent con-4 structions and me~hods as do not depart from the spirit and scope of the invention.

8 Certain specific embodiments of the i~vention have been g chosen for purposes of illustration and description, and are shown in the accompanying drawings t forming a part of the 5pP,Ci~
1 fication wher~in:
12 Fig. l is an elevational view of an offshore tower 13 structure in which the present invention is embodied;
Fig. ~ is a ~op plan view of the offshore tower struc-ture of Fig. l;
6 Fig. 3 is a side elevational view showing the placement ~.
17 of a template portion of the offshore tower of Fig. l as a first 18 step in the erection of the tower;
~9 Fig. 4 is ~ view similar to Fig. 3 but showing the positioning of a platform at the templaté portion as a second 21 step in the erection of the offshore tower of Fig. l;
22 Fig. 5 is a top plan view illustrating the platform and 23 template of Fig. 4;
24 Fig. 6 is a view similar to Fig. 4 but show1ng the platform fully positioned at the template and ready to be raised 26 thereon;
27 Fig. 7 is a section view taken along line 7-7 of Fig. 6;
z8 ; Fig. 8 is an enlarged fragmentary diagramatic view, .: ~
29 , taken along line 8-8 of Fig. 7;
, Fig. 9 is a view similar to Fig. 7 but showing the 31 platform in fully raised condition; 5 .'' , ~ ,' ..
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t 1l Fig~ 10 is a view similar to Fig. 4 but showing the ~ positioning of a baxge mounted platform positioned at the 3 ' template;
4 ; Fig. 11 is a view similar to Fig. 7 but showing a barge mounted platform;
6 Fig. 12 is a view simi~ar to Fig. 11 but showing a 7 raised barye mounted platform;
8 Fig. 13 illustxates a modified arrangement whereby a 9 bridge beam is mounted atop a previously installed template; and Fig. 14 illustrates a modified arrangement wherein a 11 tilt-up type template is installed.

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l3 DETAILED DESCRIPTION OF T~IE PREFERRED EMBODIMENTS
14 The offshore tower structure of Fig. 1 comprises a ;s platform 20 which is held, by means of a template 22, in s~ation-1~ ary position up above a sea surace 24 so as to be free of waves 17 ~ and sea currents.
Ia The template 22 comprises forward and rearward pairs of 19 vertical template legs 26 and 28 which extend up from a base 30.
The base 30, in turn, comprises a base plate 32 which is pinned/
21 as by anchor piles 34, to a sea bed 35, and a cluster of tubular ~22 elements 36 which extend upwardly from the plate 32 to surround 23 the lower portion of the vertical legs 26 and 28.
2~ Forward and rearward bridge beams 38 and 40, of truss-., , like open framework construction, extend aoross and span the 26 distance between the upper ends of the forward and rearward legs 27 respectively. These bridge beams are secured to pedestals 42 and 28 44 on top of the legs 26 and 28.
~29 As shown in Figs. 1 and 2, the platform 20 lS of ~ -expansive, generally flat configuration; and in plan view it is 31 shaped as a T, with a base portion 46 and a cross portion 48.
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I I~The ~ase portion 46 o~ the T shaped platform extends lengthwise 2 between and beyond the forward and rearward pairs of legs 26 and 3 ! 28 and in its widthwise direction it extends the full distance ~ , between the corresponding legs of each pair. The cross portion S ,48 of the T shaped platform extends out forwardly and laterally ., I
6 ;beyond the forward legs 26.
7 i' The platform 20 is secured to the bridge beams 38 and 8 l 40 along their length; and consequently it is stlffened and 9 strengthened by them~ Because of this, the pla~orm, although o expanslve in size, may be made of lighter construction than would 11 have been necessary if the platform had to bridge the entire distance between the template legs without external bracing.
13 The offshore tower structure shown in Figs. 1 and 14 2 is arranged for oil well drilling. As can be seen in Fig. 2, ~5 the forward bridge beam 38 is formed to define a grid like array 16 O~ drilling locations 50 and to support a drilling tower 52 ~bove 7 dif~erent ones of those locations for exploratory drilling. The la forward bridge beam 38 also supports a tower crane 54 which is ~9 used to position additional lengths of drill pipe in the tower 52.
Although the basic operative elements, i.e. the drilling Zl `tower 52 and the tower crane 54, are supported on the bridge beam 22 38, the platform 20 is required to support personnel, auxiliary ~3 ;equipment and supplies~ Thus, as can be seen in Figs. 1 and 2, 24 there is provided a crew quarters 56 at the outer end of the 23 platform base portion 46, and a helicoptor landing platform 58 26 atop the rearward bridge beam 40. Power generating and control 27 equipment 60 is arranged on the platform cross portion 48 and a , drill p~pe storage ledge 62 is construc~ed thereabove. A supply 29 hoist 64 is also mounted along the forward edge of the platform cross portion 48 for bringing supplies and equipment up from ship 3 or barges moored to the template. A tower and crane positioning ., .

Ii - 7 - ' 1'(14t~Z82 I Illedge 66 is constructed on the platform 20 just b~hind the2 forward brid~e beam 38. This ledge accomodates the drilling 3 1I tower 52 and the tower crane S4 when the platform 20 is being 4 moved into position on the template or when it is being dis-mantled from the template.
6 Figs. 3-9 illustra~e -the manner of erecting the above 7 ,Idescribed offshore tower at a desired location in the sea. As 8 'can be seen in the phantom outline portion oE Fig. 3 the template 9 l 22 is towed, while 10ating, to a desired location; and it is lo then sunk to the sea bed 35 when it has arrived at this location~
11 The flotation control of the template 22 can be carried out by 12 external means ~not shown) attached to it, or the template legs l3 26 and 28, and the tubular elements 36 may be hollow and selec-l~ tively floodable. After the template has been sunk to the sea bottom, it is secured there by installation of the anchor piles 34.
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16 It will be noted from Fig 3 that the pedestals 42 and 17 44 and the bridge beams 38 and 40 are pre-assembled to the upper 1~ ends of the template legs 26 and 28 before the template is towed - 19 out to its desired location. Since the bridge beams 38 and 40 are of open framework construction they axe rela~ively light in - 21 weight and are of minimum bulk. Therefore they do not offer 22 ,appreciable resistance to towing nor do they substantially affect 23 balance or buoyancy of the template during the towing operation.
` 24 In fact, the bridge beams 38 and 40 actually brace ~he upper ends 2S of the template legs so that it can better withstand the rigors 26 of sea and wind action during the towing operation. On the other !
27 hand, if the template 22 were towed with the platform 20 attached, 28 " the platform would be subject to wind and/ox sea action, which 29 woul~ overstress the structure. Moreover, the structure would ~not be stable in floatiny condition and would be likely to 31 capsi~e.
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1 ~ It will also be noted in Fig. 3 that there are provided 2 within each of the bridge beams 38 and 40, a plurality of jacking 3 mechanisms 68 and associated rigid tubular jacking legs 70. The ~ bridge beams 38 and 40 thus serve as mounting members on the template 22 for the jacking mechanisms 68. This axrangement of 6 jacking mechanisms and jacking tubes permits rapid and convenient ~interconnection between the template and platform for lifting of 8 the platform to its proper height, as will be explained more 9 ~fully hereinafter.
~l After the templat~ 22 has been secured to the sea bed 11 35, the plai~orm 20 is towed in a floating condition, as shown inFig. 4, to the template. The platform 20, may be fully con-13 structed at a shore location and there outfitted with the crew 14 quarters 56, the power generating and control equipment 60, the drill pipe storage ledge 62 and the tower and crane positioning 16 ledge 66 with the drilling tower and tower crane 52 and 54 17 mounted thereon. As shown in Fig. 5~ the platform is directed so n that its base portion 46 first enters the region between the , 19 forward legs 26 and then continues on between and beyond the rearward legs 28 until the platform cross portion 48 nearly abuts 21 the forward legs 26. The platform 20 is provided with jacking 22 leg slips 72 which become aligned with the jacking legs 70 on the 23 bridge beams 38 and 40 when the platform is floated into position.
2~ These slips are designed to allow the jacking legs to move freely 2s through then longitudinally in one direction but to grip the legs 26 and prevent relative movement in the opposite direction. As 27 shown in Fig. 6, the jacking mechanism 68 are then operated to 28 lower the jacking l~gs 70 until they enter their corresponding ~9 ~ jacking leg slips 72 on the platform 2Z and thereby provide an interconnection between the platform and the bxidge beams 38 and 31 40 extending across the tops of the template legs 26 and 28.
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As can be seen in Fig. 8,the jacking mechanisms 68 each comprise upper and lower holder assemblies 74 and 76 with the lower holder assembly 76 mounted on a portion of one of the bridge beams 38 and 40. Hydraulic piston and cylinder assemblies 78 are driven and controlled by external means (not shown) to move the holder assemblies 7~ and 76 toward and away from each other. Slips 80 on the holder assemblies 74 and 76 are controlled to grip and release the jacking leg 70 in alternate sequence as the holder assemblies are moved toward and away from each other respectively. Thus, in order to lower the jacking leg 70 the slip 80 on the lower holder assemblies 76 are released while those on the upper holder assemblies 74 are engaged when the assemblies are moved toward each other and vice versa when they are moved - away from each other. In order to raise the jacking leg, the slip engagement and release sequence i-s reversed. This is shown diagrammatically in Fig. 8 by leaf type springs 81 which are held by pins 81a and 87b to bias either the upper or lower portion of the slips 80 against the jacking leg 70.
When the jacking legs 70 engage the slips 72 on the , 20 platform 20 they ~ecome secured to the platform in a manner which allows upward but not downward movement of the platform with respect to the jacking legs. That is, the slips 72 allow relative ;~
movement of the jacking legs 70 with respect to the slips which coincides with movement of the platform and bridge beam toward each other, but they prevent relative movement in the reverse direction. Thus, should the platForm begin to rise as a result of wave or tide action, the slips 72 will allow the platform to move up on the jacking legs. However, the slips 72 act to prevent reverse movement. Thus, wave action is utilized in raising o~
the platform in a manner similar to that described in U.S. patent No. 3,876,181 to Joseph E. Lucas. ~
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1 ,¦ After the jacking legs 70 have engaged th~ir respective 2 jacking tubc slips in th~ platform 20 the jacking mechanisms 68 3 'iare operated to lift the jacking legs and the platform 20 along ~ ! with them to raise the platform up to the bridge beams 38 and 40 as shown in Fig. 9. When the platform is fully raised, it is 6 secured to the bridge beams along their length, as by welding.
7 The jacking mechanisms and jacking legs may then be removed for a use in the erection of other offshore tower structures. Because . i 9 of the truss-like open framework configuration of the bridge beams 38 and 40 they provide a skeletal type support which l1 strengthens and rigifies the platform when it has been raised and 1~ secured to the bridye beams. Also, since the bridge beams span l3 the distance between templa~e legs the platform may be of much 14 lighter and less rigid construction ~han would have been necessary if the platform had to span the template legs on its own.
16 Figs. 10-12 show a modified arrangement wherein the 1~ platform 20 is carried out on a barge 82 to the template 22.
18 This arrangement permits the platform to be designed without 19 regard ~o buoyancy or ability ~o withstand the stresses of sea action. As shown in Fig. 11 the jacking legs 70 are engaged in 2I the jacking tub~ slips 72 on the platform while it is supported 22 by the barge B2; and as shown in Fig. 12 the jacking mechanisms 23 68 operate to lift the platfor~ up off the barge to the bridge 24 beams 38 and 40 for subsequent attachment thereto. The jacking ~5 mechanisms and jacking legs 68 and 70 may then be disconnected 26 and lowered back down to the barge for use in the erection of 27 another offshore tower structure.
2S ~ig. 13 shows another moaification wherein the template 29 22 is floated to location and ins~alled there prior to installa-tion of the bridge beams 38 and 4Q. These beams may then be 3~ installed hy means of a derric~ barge 84. As shown, the jacking 1~

mechanisms and jacki!lg legs 68 and 70 may be previously mounted on the bridge beams so that the entire bridge beam and jacking assembly may be positioned on top of the -template legs 26 and 28.
Because the bridge beams are of open framework construction they may be handled by a derrick barge rather easily, as compared to the platform itself.
Fig. 14 shows the installation of a tilt-up type template 86 with preassembled bridge beams 88. The basic idea `~ of horizontally floatable template which is tilted to upright ` 10 position by selective flooding is well knownand is shown for example, in U.S. patent No. 2,8579744 to W.F. Swiger, et al. In that patent a temporary truss 22 is shown attached to the upper - ends of the template legs; however, it is believed that the ~ present invention represents the first time that bridge beams ;~ 15 have been used in the lifting and reinforcing of a platform ~, structure.
, Having thus described the invention with particular . ~ :
reference to the preferred forms thereof, it will be obvious to those skilled in the art to which the invention pertains, after understanding the invention, that various changes and modifi-cations may be made therein without departing from the spirit and scope of the invention as defined by the claims appended hereto.
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Claims (7)

The embodiments of the invention in which an exclu-sive property or privilege is claimed are defined as follows:

1. A method of erecting an offshore tower, said method comprising the steps of positioning on a sea floor, a template having at least one vertical template leg with a mounting member thereon, so that said template leg with said mounting member extends up above the sea level, floating a platform member out to the thus positioned template, at least one jacking mechanism being secured to one of said members and operative to move an elongated rigid jacking leg in a vertical direction, operating said jacking mechanism to move said jacking leg longitudinally in one direction through a slip mechanism in the other member, said slip mechanism being constructed to allow free movement of said jacking leg in a direction which coincides with movement of said members toward each other and to prevent relative movement in the reverse direction, and thereafter operating said jacking mechanism to move said jacking leg in said opposite direction to elevate said platform member clear of the water.

2. A method of erecting an offshore tower according to claim 1, wherein said jacking mechanism is secured to said mounting member and wherein said slip mechanism is provided in the platform member.

3. A method of erecting an offshore tower according to claim l, wherein said platform member is secured to said template after it has been raised and wherein said jacking mechanism and jacking leg are thereafter disconnected and removed from said members.

4. An offshore tower construction comprising a template constructed to be anchored to a sea floor, said template including at least one vertical template leg which extends out above the surface of the sea when said template is secured to the sea floor, a mounting member connected to the upper end of at least said one template leg, a platform member constructed to be positioned adjacent said template leg and to be lifted upwardly therealong clear of the water, at least one jacking unit comprising an elongated rigid jacking leg and a jacking mechanism operative to produce relative longitudinal movement with respect to said jacking leg, said jacking unit being mounted to extend vertically between said platform member and said mounting member with said jacking mechanism mounted on one of said members and slip means on the other member for receiving said jacking leg, said slip being constructed to allow relative movement of said jacking leg in a direction which coincides with movement of said members toward each other and to prevent relative movement in the reverse direction.

5. An offshore tower construction according to claim 4, wherein said jacking mechanism is on said mounting member and wherein said slip means is on said platform member.

6. An offshore tower construction according to claim 4, wherein said template is formed with a plurality of spaced apart vertical template legs and open framework bridge beams spanning the distances between different pairs of said template legs near the upper ends thereof.

7. An offshore tower construction according to claim 4, wherein a plurality of jacking mechanisms are mounted on said bridge beams near said template legs, each of said jacking mechanisms having an associated jacking leg extending there-through and said platform being provided with plural slip means which become aligned with said jacking legs.