Method and apparatus for installation of an offshore platform

A method and apparatus are disclosed for use in the installation of a platform structure upon the upper elements of a previously-installed substructure. The platform structure is mounted on a barge positionable between the upper elements of the previously-installed substructure. The barge can be flooded and lowered a sufficient distance to allow leg elements of the platform structure to contact and mate with leg support elements of the substructure. Impacts between the structures during the mating operation are absorbed by resilient neoprene pads carried by the leg supports. After the platform structure rests upon the leg supports, and the barge is removed, the platform structure can be lowered and leveled by draining a select volume of sand from the leg supports.

BACKGROUND OF THE INVENTION 
1. Field of the Invention: 
The present invention relates to an apparatus and method for use in the 
installation of an offshore platform structure above a 
previously-installed substructure. 
2. Description of the Prior Art: 
As the exploration for oil and gas reserves has progressed into offshore 
waters, platforms capable of supporting the required drilling and 
production equipment have been installed above promising petroleum 
reservoir locations. 
Each platform is usually installed in two or more separate sections. The 
first section consists of a partially submerged substructure supported by 
the ocean floor. A portion of this substructure extends above the surface 
of the water. A platform structure which forms the upper second section is 
then placed upon the substructure. The platform structure usually has an 
array of four to eight downwardly-depending legs which mate with and are 
welded to a corresponding array of four to eight upwardly-directed leg 
supports which form the upper portion of the substructure. 
The platform structure is placed upon the substructure by a derrick barge. 
This barge carries a crane of sufficient weight-lifting capacity to lift 
the platform structure from an equipment supply vessel, position the 
platform structure over the substructure, and then lower the structure 
downward until the legs of the structure contact the leg supports of the 
substructure. 
As can be imagined, even minor five to six foot ocean swells that 
periodically impact the derrick barge can cause the platform structure, 
when suspended by cables from the crane, to swing back and forth similar 
to a pendulum. Upward and downward movement of the derrick barge will also 
cause similar movements in the suspended platform. The final mating 
alignment between the legs and leg supports is usually very difficult, 
with damaged mating members not uncommon as the platform structure pounds 
up and down on the support legs, until the weight of the platform 
structure is fully transferred to the substructure. 
As can be imagined, failure of the crane as the platform structure is 
suspended in the air could cause catastrophic loss of the platform 
structure with attendant risk to human life on the platform substructure, 
equipment supply vessel, and derrick barge. 
There are economic disadvantages as well in the use of such a derrick 
barge, due to the cost of its operation. In some undeveloped areas of the 
world, the use of such a derrick barge would be economically unfeasible 
due to the high cost associated with the single trip of the barge to a 
country having minimum petroleum reservoir development. 
Accordingly, it is desirable to present a method and apparatus for 
installation of a platform structure without the damage, danger and 
expense associated with use of a derrick barge. 
Additionally, after the platform structure has been placed upon the 
substructure an inordinate amount of time usually must be spent in welding 
the legs of the platform structure to the substructure's leg supports, due 
to misalignments which exist between the legs and leg supports. 
Accordingly, it is also desirable to present a method and apparatus that 
allows final mating adjustments to be made between the legs and leg 
supports prior to welding the legs and leg supports together. 
SUMMARY OF THE INVENTION 
The apparatus of the present invention used to assist in the final 
connection of the platform structure to the substructure comprises a 
plurality of leg-receiving receptacles formed at the upper end of the 
substructure's leg supports. Each receptacle contains a volume of 
particulate material, such as sand, and also a resilient neoprene pad 
carried above the sand that dampens the impacts that occur as the platform 
structure's legs are lowered and subsequently mated with the leg-receiving 
receptacles of the substructure. Once the load of the platform structure 
is transferred to the substructure's leg supports, the sand may be drained 
from the receptacles in order to precisely level the platform structure 
and also to accurately position each leg adjacent each leg support a 
distance sufficient for welding each leg to each leg support. 
The platform structure is initially secured to a variable-draft equipment 
vessel. The equipment vessel with attached platform structure is 
thereafter accurately positioned between the leg supports of the 
substructure which have been previously designed and fabricated to allow 
such a vessel to pass between them. The vessel is then lowered by flooding 
with water or by tidal action or combination of the two. As the vessel 
lowers, the legs of the platform structure mate with the substructure's 
leg supports. 
In this manner, the equipment vessel that transports the platform structure 
to the substructure can be used to mate both structures together, without 
the use of a derrick barge. Since the vessel can be designed or modified 
to fit snugly between the leg supports during the mating operation, very 
little sideward movement will be encountered between the legs and leg 
supports, with a corresponding reduction in the amount of damage sustained 
by both structures. 
It is an object of the present invention to provide apparatus for use in 
the installation of a platform structure upon a substructure. It is a 
further object of the present invention to describe a method of 
installation of a platform structure upon a substructure. 
These and other features, objects, and advantages of the present invention 
will become apparent from the following detailed description, wherein 
reference is made to the figures in the accompanying drawing.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to FIG. 1, an offshore platform structure 10 is shown mounted 
on the deck of a variable-draft equipment vessel 11. The structure 10 may 
have been securely welded to the vessel 11 during transportation of the 
structure 10 to the general vicinity of the substructure 12. The height or 
draft of the vessel 11 may be varied by partially flooding portions of the 
vessel 11 as explained later. The vessel 11 is shown positioned adjacent 
the substructure 12 that has previously been attached to the ocean floor 
32 (FIG. 5). Upwardly-directed legs 13A-F extend above the surface 31 of 
the body of water 30 (FIG. 5). Winches 14, 15, are shown carried by the 
deck of the equipment vessel 11. Cables 16A-H, assist in positioning the 
vessel 11 between auxiliary vessels 17A-D, and substructure 12. Auxiliary 
vessel 17D is shown moored to buoy 18G. Buoy 18G is connected by means of 
another cable 19G to a subsea anchor 20G well known to the art. The other 
auxiliary vessels 17A-C are also moored by means of buoys 18A-C, cables 
19A-C, and anchors 20A-C. 
Referring now to FIG. 2, for the purpose of clarity of offshore structure 
platform 10 is not shown carried upon the deck of vessel 11. Vessel side 
extensions 21, 21A, 22, 22A are shown attached to or respectively form a 
portion of the sides of vessel 11. These side extensions 21, 21A, 22, 22A 
may be formed from wooden members attached to the side of the vessel 11, 
or by members welded to the side of the vessel 11. 
Referring now to FIG. 4, the vessel 11 is shown positioned within the leg 
supports 13A-F of the substructure 12. Vessel side extensions 21, 21A, 22, 
22A are shown contacting leg supports 13A, 13D, 13C and 13F respectively. 
A cable 23 such as a five part nylon rope 10 inches in diameter is shown 
used in combination with a cable 24 such as 11/4 four-part steel cable. 
Both cables 23, 24 can be tensioned by winch 25 and in combination with 
cable sheaves 26, bollard 27, and fishplate 28 form a system of securing a 
portion of the vessel 11 to leg support 13B. In a similar manner cables 
23A, B, C cables 24A, B, C, winches 25A, B, C, cable sheaves 26A, B, D, 
bollards 27A, B, C and fishplates 28 A, B, C can be used to secure other 
portions of the vessel 11 to leg supports 13B or 13E. 
Referring now to FIG. 5, the offshore platform structure 10 is shown 
positioned above vessel 11 and carried by support 29. The vessel 11 is 
shown floating in the body of water 30 having surface 31 and bottom 32. A 
flood valve 33 carried by the vessel 11 when opened may place a portion of 
the body of water 30 in fluid communication with the vessel's 11 ballast 
tank 34. A pump 35 well known to the art may add or remove the water 30 
from the ballast tank 34 when desired. It is recognized that many ballast 
tanks 34 and pumps 35 may be incorporated within a single vessel 11. A 
vent line 36 defined upwardly through the vessel 11 allows air to be 
vented from the ballast tank 34. 
A docking pole 37 such as a cylindrical steel member having a tapered lower 
end and an upper retainer shoulder is shown passing downwardly through 
guides 38 and 39. 
For purposes of clarity a single platform structure leg 40 is shown with 
its lower end positioned within leg-receiving receptacle 41. It is 
recognized that a plurality of legs 40 fit within receptacles 41 located 
at the upper end of leg supports 13A-F. The lower end of structure leg 40 
contacts upper bearing plate 42, which rests upon resilient material means 
43 such as a neoprene pad. The resilient material means 43 rests upon 
lower bearing plate 44 which is carried by the upper surface of a volume 
of particulate material 45 such as sand well known to the art. Drain means 
46, 46A such as conduits placed in fluid communication with the 
particulate material 45 may be used to drain a portion of the particulate 
material 45 from the receptacle 41. The drain means 46, 46A pass upwardly 
through closure means 47 such as a steel plate well known to the art which 
supports the lower portion of the particulate material 45. Valve means 48 
can be used to control the draining of the particulate material 45 from 
the receptacle 41. A leg cone 51 forming the lower end of the platform 
structure leg 40 has a downwardly-directed tapered configuration in order 
to aid in the alignment and subsequent mating of structure leg 40 to 
receptacle 41. Substructure leg support 13C is shown forming a portion of 
the substructure 12 which enters the bottom of the body of water 32. 
A draft-variance dimension 52 is shown to indicate the depth to which the 
vessel 11 may be submerged without decreasing the clearance dimension 53 
sufficiently to cause the vessel 11 to contact the substructure 12. 
The apparatus shown in FIGS. 1-5 is used to position, lower, and connect 
the lower end of the downwardly-depending legs 40 of the offshore platform 
structure 10 to the upper ends of upwardly-directed leg supports 13A-F of 
the substructure 12. One of several leg-receiving receptacles 41 is shown 
having an open top and a chamber defined downwardly through the upper end 
of at least a plurality of the substructure leg supports 13A-F. Although 
receptacles 41 may be defined at the top of each substructure leg 13A-13F, 
it is well recognized that receptacles 41 need not be defined at the top 
of every leg support 13A-F. 
The closure means 47 formed at the lower end of the receptacle 41 may also 
be formed from a cementaceous substance such as concrete poured into each 
of said respective leg supports 13A-F. Closure means 47 need be positioned 
at a selected distance from the top of the receptacle 41 in order to allow 
a selected mass of the particulate material 45 to be positioned in the 
lower portion of each receptacle 41. The location of the closure means 47 
should also be positioned a sufficient distance down from the top of the 
receptacle 41 so the drain means 46, 46A, which are connected in fluid 
communication with the lower portion of the receptacle 41, may be allowed 
to drain enough particulate material 45 from the receptacle 41 in order to 
provide sufficient space above the material 45 to accommodate leg cone 51 
of the leg 40 as it enters the receptacle 41. The closure means 47 may 
comprise steel plate means positioned perpendicular to a longitudinal axis 
54 which is defined along the length of leg supports 13A-13F. 
In a preferred embodiment the drain means 46, 46A may comprise three 
particulate material 45 conduits 55 of a size and location to allow 
approximately one-third of the particulate material 45 to be drained from 
the receptacle 41 through each of said conduits 55. It is necessary to 
drain the particulate material 45 evenly from the receptacle 41 in order 
to allow the lower bearing plate 44 to settle evenly downward through the 
receptacle 41 so the structure leg 40 will settle within the receptacle 41 
without binding. It is recognized that one or more conduits 55 may be used 
to drain material 45 from the receptacle chamber 41, depending upon the 
overall size of the receptacle 41. 
The particulate material 45 conduits 55 may pass upwardly from a position 
outside the leg supports 13 A-F and thereafter upwardly through the 
closure means 47. In this manner personnel located outside of leg supports 
13A-F may easily monitor the rate of removal of the particulate material 
45 from the receptacle 41. Conduits 55 may have a removable cap 56 placed 
in threaded engagement with the conduit 55 in order to prevent flow of 
particulate material 45 from the receptacle 41 when cap 56 is placed in 
threaded engagement with the conduit 55. 
The resilient material means 43 mentioned earlier may comprise alternating 
horizontal layers of steel plate and neoprene with the upper and lower 
ends of the resilient means 43 formed with layers of steel plate. The 
resilient material means 43 may be sized and ordered from Oil States 
Industries, Inc., Bearing Pad Division, P. O. Box BPD, Athens, Tex. 75751. 
Depending upon the impact loads expected from the platform structure 10 
during the mating operation with the leg supports 13A-F the number of 
layers of neoprene incorporated within each resilient material means 43 
may be designed to yield different overall deflection of the resilient 
means 43. In this manner the contact forces between the structures 10, 12 
may be cushioned within an acceptable range of values, depending upon the 
anticipated sea state, the load of the structure, and the relative 
movement between the vessel 11 and the leg supports 13A-F. 
Referring to FIG. 1, in operation the vessel 11 is initially secured by 
cables 16F and 16G and cables 16E and 16D connected to winches 14, 15 to 
the respective legs 13C, 13F, 13D, and 13A of the substructure 12, and 
also secured by cables 16H, 16A, B, C to auxiliary vessels 17A, B, C, D. 
The length of the cables 16A-H is adjusted in a manner to draw the vessel 
11 within the substructure legs 13A-F. 
As shown in FIG. 2, in a preferred embodiment cables 16E and 16D may now be 
connected to leg supports 13A and 13D respectively in order to continue 
movement of the vessel 11 within the leg supports 13A-F. The vessel side 
extensions 21, 21A have not yet passed by leg supports 13C and 13F. 
As shown in FIG. 3 the vessel 11 is now near its final position within the 
leg supports 13A-13F of the substructure 12. Vessel side extensions 21, 
21A and 22, 22A have not yet contacted their respective leg supports 13A, 
D, C, F respectively. It is preferred that there be a minimum of five feet 
mean clearance between the leg cone 51 (shown in FIG. 5) and the upper end 
of the receptacle 41 (shown in FIG. 5) prior to entry of the vessel 11 
into the slot defined between the leg supports 13A-F. 
Referring now to FIG. 4, the vessel 11 is shown finally positioned between 
leg supports 13A-F. Side extensions 21, 21A and 22, 22A are now adjacent 
leg supports 13A, D, C and F respectively. In a preferred embodiment the 
vessel's side extensions 21, 21A, 22, 22A do not fully contact the leg 
supports 13A, D, C and F until the vessel 11 is lowered by flooding a 
portion of the draft variance distance 52 (FIG. 5). As the vessel 11 
reaches this final position the cables 23, 23A and cables 24, 24A are 
first rigged around legs 13B and 13E and back to bollards 27, 27A by means 
well known to the art. Once the vessel 11 is centered in the desired 
position the cable 23B, 23C and cables 24B and 24C are rigged from winches 
25B, 25C around leg supports 13B and 13E and back to bollards 27B and 27C, 
by means well known to the art in order to stabilize the vessel 11 within 
the slot defined between the leg supports 13A-F. 
Referring now to FIG. 5 at this point in time the docking pole 37 is 
allowed to drop down through guides 38 and 39 of leg support 13C. In a 
preferred embodiment another docking pole (not shown) is also dropped at 
leg support 13D (FIG. 4). Similar docking poles are then also dropped 
downwardly at legs 13A and 13F in order to continue the stabilization of 
the vessel 11 within the leg supports 13A-F. The legs 40 of the platform 
structure 10 have now been positioned substantially centrally above the 
substructure leg supports 13A-F. It should be noted that cables 23, 23A 
and cables 23B and 23C (FIG. 4) may be alternatively tensioned by their 
respective winches 25, 25A, B, C in order to apply a sufficient lateral 
restraining force to the vessel 11 in order to reduce to acceptable limits 
any movement of the vessel 11 due to waves which impact the vessel 11. 
The platform structure legs 40 are then lowered towards the substructure 
leg supports 13A-F, by flooding vessel 11 with a portion of the body of 
water 30 which flows through flood valve 33 into ballast tank 34. As can 
be seen as the vessel 11 lowers in the body of water 30, the leg cone 51 
of the structure leg 40 approaches and eventually passes downward through 
the upper portion of receptacle 41. Before the vessel 11 is lowered, the 
sides of the vessel 11 should be outwardly extended adjacent the 
substructure leg supports 13A, D, C, and F a distance sufficient to cause 
the vessel 11 sides to operatively contact at least one pair of leg 
supports 13A, D or 13C, F located on each side of the vessel 11, 
preferably before the leg cone 51 enters the receptacle 41, in order to 
more fully restrain the vessel 11 within the leg supports 13A-F. In a 
preferred embodiment, the vessel side extensions 21, 21A, 22, 22A may be 
formed from sections of hard wood such as mahogany. Use of mahogany will 
allow the side extensions 21, 21A, 22, 22A to abraid at a slow rate when 
they contact the leg supports 13A, D, C and F, and should not cause 
noticeable wear to the leg supports 13A, D, C, and F. 
As the vessel 11 is lowered a portion of the draft variance 52 dimension 
into the body of water 30, the leg cone 51 of at least one of the platform 
structure legs 40 will operatively contact the particulate material 45 
positioned in the lower portion of each receptacle chamber 41. This 
operative contact may be done in a resilient manner by positioning 
resilient material means 43, such as the neoprene pad assembly mentioned 
earlier, between the leg cone 51 and the particulate material 45, prior to 
contact of the leg 40 with the particulate material 45. In a preferred 
embodiment the resilient material means 43 are carried by the upper 
surface of the particulate material 45, although it is recognized that 
resilient material means 43 may also be incorporated into the leg cone 51, 
or in any other mechanically equivalent location to allow the same 
resilient contact between the structure legs 40 and their respective leg 
supports 13A-F. 
In a preferred embodiment the vessel 11 will continue to be flooded until 
the full load the structure 10 has been transferred to the leg supports 
13A-F. The vessel 11 will then be flooded an additional amount to allow 
the supports 29 to be removed from beneath the structure 10. The resilient 
material means 43 will be compressed in each receptacle 41 after the 
supports 29 are removed from beneath the structure 10. Once the supports 
29 are removed, the vessel 11 may be removed from beneath the structure 
10. It is understood that a plurality of supports 29 may be used to 
support the structure 10 on the vessel 11. 
The entire platform structure 10 may now be leveled to a desired horizontal 
elevation by selective draining of particulate material 45 from each 
respective leg support 13A-F. Drainage of particulate material 45 from at 
least one of the conduits 55 of each receptacle 41 may be done by 
actuation of the appropriate valve 48 and measurement of the amount of 
particulate material 45 removed from each conduit 55 in order to evenly 
settle each leg 40 within each receptacle 41. 
The leveling and/or lowering process may be continued until the platform 
structure legs 40 are settled to a desired position adjacent the 
substructure leg supports 13A-F. At this time the legs 40 may be connected 
to the leg supports 13A-F by means such as welding. The final makeup 
dimension required for the welding of the leg supports 13A-F to the 
platform structure legs 40 may be precisely controlled by the continued 
draining of particulate material 45 from each respective leg support 
13A-F. 
At this point in time, the platform structure 10 has been precisely 
positioned, lowered, and connected to the substructure 12 without the use 
of a derrick barge. Damage normally encountered in connection operations 
of this nature has been minimized by use of cables 16A-H which initially 
position the vessel 11, cables 23, 23A-C which secure the vessel 11 to the 
leg supports 13A-F, vessel side extensions 21, 21A, 22, 22A, resilient 
material means 43 which soften the initial impacts of legs 40 to leg 
supports 13A-F, and particulate material 45 which may be removed to lower 
the structure 10 in a controlled manner to the final welding position, 
preferably after the vessel has been removed from beneath the platform 
structure 10. 
Many other variations and modifications may be made in the apparatus and 
techniques hereinbefore described, both by those having experience in this 
technology, without departing from the concept of the present invention. 
Accordingly, it should be clearly understood that the apparatus and 
methods depicted in the accompanying drawings and referred to in the 
foregoing description are illustrative only and are not intended as 
limitations on the scope of the invention.