Platform crane

A mobile crane for use on an offshore oil well platform, comprising parallel base beams adapted to span the capping beams of the platform, and a pedestal and a rotating upper works movably mounted on the base beams. A method is described for erecting a workover rig on the offshore platform by means of the crane, and for expeditiously erecting workover rigs on a plurality of platforms in succession.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to mobile cranes for use on offshore oil well 
platforms, and to the utilization of such cranes in the erection of 
workover rigs on such platforms. 
2. Description of the Prior Art 
Offshore oil and gas wells are often completed at the dock of a platform 
which is supported above the water by means of a plurality of legs. 
Usually each platform will have a plurality of wells completed thereon, 
ranging from a few wells up to 40 or more. From time to time after the 
completion of a well, it is necessary to bring a workover rig to the 
platform to perform certain operations on the well. One form of workover 
rig suitable for this purpose is disclosed in applicant's U.S. Pat. No. 
3,802,137, issued Apr. 9, 1974. 
Such a workover rig is designed to rest on the capping beams of the 
platform which may be from 25 feet to 55 feet apart and which extend from 
one side of the platform to the other. Such beams are extremely heavy and 
strong, having sufficient strength to support the loads imposed by the 
workover Fig. The portion of the deck between the capping beams and 
extending outwardly therefrom usually has insufficient strength to support 
such loads. In addition, those outwardly extending portions of the deck 
usually have mounted thereon various tanks, pumps and other production 
equipment which may be necessary in connection with the production of oil 
or gas from the well and for the transporting of the oil or gas from the 
well to the shore. 
Such platforms also normally are equipped with a small affixed crane which 
is positioned for on loading and off loading of equipment. However, such 
platform cranes have been inadequate for placing a workover rig on the 
platform, since the platform crane is in a fixed position and has 
insufficient capacity at a long enough distance to allow it to place the 
workover rig in position for operating on all of the wells of the 
platform. Thus, it has heretofore been necessary to use a derrick barge to 
erect the workover rig on the platform and to remove it from the platform 
after the work on each platform is completed. Since derrick barges are 
quite expensive, it often cost from $60,000 to $200,000 just to move a rig 
from one platform to another. Then when the rig's work on that platform 
was finished, it was necessary to bring the derrick barge back to remove 
the rig from that platform and move it to and erect it on another 
platform. 
SUMMARY OF THE INVENTION 
According to the present invention a workover rig is erected on and removed 
from a platform by means of a mobile platform crane having a pedestal 
which is supported on a pair of base beams which span the existing capping 
beams of the platform. The permanent fixed deck crane may be used to lift 
the elements of the mobile crane onto the platform. and the mobile crane 
may then be used to erect and to dismantle the workover rig, thereby 
eliminating the necessity of a derrick barge. Since all the load can be 
supported directly on the existing capping beams, no modification of the 
structure of the platform is required. 
In one embodiment of the invention substantial time is saved in doing 
workover operations on a plurality of platforms by utilizing a pair of 
mobile cranes and a single workover rig. In this operation a crane is 
erected on a second platform while workover operations are being performed 
on a first platform, and then the workover rig is transferred to the 
second platform where it performs operations while the crane from the 
first platform is moved to a third platform. This results in a saving of a 
day or more of rig time on each platform.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIGS. 2 and 3 illustrate a typical offshore platform 10 supported on a 
plurality of platform legs 12 which are surmounted by horizontal beams 14. 
A truss 16 supports a pair of capping beams 18. 
A production deck 20 extends between the capping beams and production 
equipment decks 22 and 24 extend outwardly of the capping beams for the 
support of various equipment (not shown) which is required for the 
handling of oil or gas produced from the wells identified at 26, nine such 
wells being shown in the drawing. 
A fixed platform crane is schematically identified at 28, mounted on the 
equipment deck 22, and a mobile crane according to one embodiment of this 
invention is schematically identified at 30. The mobile crane 30 is 
mounted on a pair of base beams 32 which span the production deck 20 and 
rest upon the capping beams 18. 
This structure is illustrated more clearly in FIG. 1 wherein the capping 
beam 18 is shown to be constructed, in this embodiment, from a pair of 
I-beams, and the decks 20 and 24 are shown to be supported on smaller wide 
flange beams such as the beam 34. The capping beams are very heavy, strong 
beams designed to support tremendous weights, as for example the weight of 
a drilling rig or a workover rig. The decks, however, will support only 
relatively small loads, and will not support a workover rig. 
A pedestal 36, comprising four legs 38 mounted on a pair of beams 40 and 
supporting a rotary base 42, is mounted on top of the base beams 32. The 
beams 40 may be bolted, clamped or otherwise removably attached to the 
base beams 32, and the base beams may similarly be removably attached to 
the capping beams 18. In FIGS. 1 and 3 C-clamps 43 are used for attaching 
the beams 40 to the base beams 32, and C-clamps 45 are used for attaching 
the base beams 32 to the capping beams 18. 
The rotating upper works 44 of the crane are illustrated only 
schematically, since any suitable design may be used, and the design of 
such upper works does not form a part of this invention. FIG. 3 shows in 
broken lines a possible position for a power unit 46 for the crane. 
The crane is thus readily broken down into a number of relatively small 
portions which can be easily lifted to and from the platform by means of 
the fixed crane 28. Accordingly, this crane can be used to make separate 
lifts of each of beams 32, the pedestal 30, the power unit 46, and the 
rotating upper works 44. The rotating upper works may, if necessary, be 
broken down into smaller units. Thus, the entire mobile crane can be 
erected on the platform by means of the fixed crane 28. The mobile crane, 
once erected, may be used to unload the workover rig from a vessel, such 
as a ship or barge, and erect the rig on the platform. Since the crane 
rests on the capping beams of the platform, it is able to support heavy 
loads, and it can be placed close to the wells so that the workover rig 
can be placed where desired. Thus, it is no longer necessary to have a 
derrick barge available for the erection of the workover rig. Instead of 
the usual $60,000 to $200,000 cost of moving a rig from one platform to 
another, by the use of this invention the cost may be reduced to as little 
as $15,000. 
It is apparent that because of the mobility of the crane 30, it can readily 
be placed in any desired position along the length of the base beams 32, 
and the base beams 32 may be placed at any desired location along the 
length of the capping beams 18. Thus, this crane may be positioned so that 
it can make lifts from a vessel floating alongside the platform and place 
equipment at any desired location between the capping beams. A workover 
rig can therefore be placed directly over any one of the wells 26. 
Furthermore, the crane can be skidded along the base beams 32 and the base 
beams 32 can be skidded along the capping beams 18 as necessary to 
position the crane at any desired location over the production deck. Such 
skidding may be accomplished by means of a snatch block or with hydraulic 
jacks or any other convenient equipment. Usually the crane will not have 
to be moved more than once on each platform. 
The apparatus of this invention is particularly useful and results in 
outstanding savings when workovers are to be done on a number of 
platforms. Under this circumstance, preferably two of the mobile cranes 30 
and one workover rig are carried on the vessel which transports equipment 
to the platforms. A first mobile crane is erected on the first platform 
and this crane is used to erect the workover rig on that platform. While 
workover operations are being performed on the first platform, the vessel 
is moved to the second platform where a second crane is erected. The 
vessel then returns to the first platform and recovers the workover rig 
and mobile crane from this platform. The workover rig is then transported 
to the second platform and erected thereon by means of the mobile crane 30 
which has previously been erected on this platform. The mobile crane which 
has been removed from the first platform is then carried to a third 
platform and erected there while workover operations are being carried out 
on the second platform. This "leap frog" process is repeated until the 
workovers are complete on all of the platforms. 
The advantage of this type of operation is that the amount of time required 
for the erection of a workover rig is substantially reduced, since the 
crane equipment for such erection is already in place when the rig arrives 
at the platform. This results in substantial savings in the cost of 
workover operations since a day or more of rig time is saved on each 
platform. By merely adding a relatively low cost crane to the equipment, 
and using this "leap frog system", the total time required for workover 
operations is substantially reduced, and the cost of workover operations 
is reduced by $4,000.00 or more per platform. 
Many modifications and variations of the apparatus, methods and systems 
disclosed will be apparent to those skilled in the art upon consideration 
of the foregoing. Accordingly, the invention is not to be considered to be 
limited to the specific embodiments disclosed, but to all forms which may 
come within the scope of the appended claims.