Method of supporting a shallow water drilling barge

A shallow water drilling barge (46) is supported on a silty water bed (50) of low bearing capacity by providing a submersible platform (74) such as the platform of a jackup drydock (40) with jacking legs (42), floating the jackup drydock into position, ballasting it until it rests on the bottom, floating the jackup drydock into position, ballasting it until it rests on the bottom, floating the drilling barge into position over the platform and floating the barge so that it comes to rest on the platform, with the weight of the barge and jackup drydock as so submerged being less than the load bearing capability of the underlying soil and then jacking down the legs of the jackup drydock to increase its load carrying capability.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to arrangements for supporting drilling barges and 
more particularly it concerns a novel method for securely positioning a 
shallow water drilling barge on an underwater bed of low bearing capacity. 
2. Description of the Prior Art 
Many oil deposits in the United States and in other countries have been 
found to be located in areas characterized by shallow water overlying a 
bed of silty soil having very low bearing capacity, e.g. less than 8,900 
pounds per square foot (3,900 kg/m.sup.2). The technique most widely used 
for drilling in these areas has been to float a drilling barge having 
shallow draft into position over the drilling site and then to flood the 
barge hull and allow it to settle onto the underlying soil. Sometimes the 
barge is "posted", that is, its deck containing the drilling materials and 
equipment is elevated above the hull so that when the hull is resting on 
the bottom, the deck and the materials and equipment carried on it will be 
maintained up above the water level. 
Problems have been encountered where the soil under the water has low 
bearing capacity. Although the soil may be able to support the barge 
initially, it often is incapable of maintaining support as drilling 
progresses and the drill string extending down from the barge into the 
earth extends in length to impose a steadily increasing "hook load" on the 
barge. This hook load may eventually exceed the supporting capability of 
the soil, and the barge will no longer be stably supported but instead 
will begin to shift or sink. 
One prior technique which has been used to overcome this problem of low 
soil bearing capacity involved the building up of a pad of suitable dense 
material, e.g. oyster shells, on the water bottom before the barge was 
brought to the site. The pad would provide a large surface area to reduce 
the unit pressure on the underlying soil. In many instances this technique 
is prohibitively expensive. Also, depending on the soil conditions, even 
the pad itself would gradually sink into the underlying soil. 
It also has been proposed to provide a jackup barge to support the drilling 
barge during drilling operations. This technique, as described in Drilling 
Contractor Volume 37, No. 11, November, 1981 on pages 57 and 60, would 
involve the floating of a "shallow water jackup leveling barge" to the 
drilling site, jacking vertical legs down from the barge and into the 
underlying soil until the barge is lifted up out of the water, pinning the 
elevated barge to the legs and ballasting the barge with water to drive 
the legs down into the soil. The barge would then be lowered down along 
the legs until it is submerged. A drilling barge would then be floated 
over the submerged jackup leveling barge and would be flooded so that it 
comes to rest on the jackup leveling barge. 
The above described technique would have the disadvantage of requiring 
substantially all of the weight of both the jackup levelling barge and the 
drilling barge to be supported on the jackup legs. In many instances the 
underlying earth will not provide sufficient bearing resistance to support 
the loads imposed through the legs except at depths which are for beyond 
their practical length. Also, the raising of the jackup barge and the 
ballasting to drive the legs down into the underlying soil could create 
unstable and potentially dangerous conditions. 
SUMMARY OF THE INVENTION 
This invention will serve to provide stable and reliable support for 
shallow water drilling barges in shallow water locations where the bearing 
capacity of the underlying soil is small. Moreover this support would be 
obtained safely and economically without need for the fabrication of a 
special bed on the water bottom and without the need for excessively long 
jackup legs. 
According to the present invention there would be provided a submersible 
jackup platform which is floated to and positioned at the drilling site 
and which would support the drilling barge. However, in the present 
invention the jackup platform would be supported not by the jackup legs 
alone but rather by a combination of buoyancy, underlying soil support and 
jackup leg support. 
In its more general aspects the present invention would be carried out by 
floating a submersible platform having jacking legs and jacking mechanisms 
to a drilling location, ballasting the platform, for example, by flooding, 
until it rests on the bottom, floating a shallow water drilling barge into 
position over the submerged platform and flooding the barge so that it 
comes to rest on the platform. The total weight of the flooded platform 
and barge would be so related to the bottom surface area of the platform 
that the pressure exerted by the platform on the water bed is less than 
the supporting capability of the water bed. At this point the legs would 
be jacked down into the underlying soil to provide additional bearing 
capacity. Thereafter, as drilling progresses, and the "hook load" 
increases, this additional weight will automatically be borne by the 
jacking legs so that the bearing capacity of the soil will not be 
exceeded. 
In the preferred manner of carrying out the invention the submersible 
platform would be in the form of a drydock having a pair of hollow wing 
walls along opposite sides of a central horizontal platform. The wing 
walls would extend up above the surface of the water even when the 
platform is resting on the bottom. This will allow the buoyancy of the 
submerged platform to be controlled so that the total weight of the 
platform and barge combination will not exceed the supporting capability 
of the underlying soil. 
There has thus been outlined rather broadly the more general features of 
the invention in order that the detailed description thereof that follows 
may be better understood, and in order that the present contribution to 
the art may be better appreciated. There are, of course, additional 
features of the invention that will be described more fully hereinafter. 
Those skilled in the art will appreciate that the conception on which this 
disclosure is based may readily be utilized as the basis for the designing 
of other arrangments for carrying out the purposes of this invention. It 
is important, therefore, that this disclosure be regarded as including 
such equivalent arrangements as do not depart from the spirit and scope of 
the invention.

The shallow water drilling barge shown herein has been built but is not new 
and is not claimed herein per re. The jackup drydock per se and in 
combination with the shallow water drilling barge are described in a 
copending U.S. patent application Ser. No. 349,459 filed Feb. 17, 1982, 
now U.S. Pat. No. 4,456,404. The jackup drydock shown and described 
therein has not yet been built. 
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The jackup platform and drilling barge shown in FIG. 1 comprise a jackup 
drydock 40 resting on a shallow water bed 50 and carrying thereon a 
shallow water drilling barge 46. The jackup drydock 40 is provided with 
jacking legs 42 having pods 48 at their lower ends. The legs 42 extend 
down through the soil under the jackup drydock to a depth where the earth 
provides adequate bearing support for the legs. The jacking legs 42 extend 
up from the pods 48 to the jackup drydock 40 where they pass through wing 
walls 52 on the jackup drydock. The jacking legs continue on up through 
jacking towers 54 mounted on top of the wing walls. Jacking mechanisms 
(shown and described hereinafter) are provided in the jacking towers 54 to 
raise and lower the jacking legs 42 relative to the wing walls 52; and, 
when the jackup drydock is resting on the water bed the jacking mechanisms 
drive the legs 42 down into the underlying earth and thereafter raise them 
again when the jackup drydock is to be moved. The wing walls 52 are of 
hollow construction and they have variable buoyancy. This buoyancy is 
controlled by allowing water to flow in and out of compartments formed by 
bulkheads 55 inside the wing walls. Sea chest valves 56 are provided at 
various locations along the lower edge of the wing walls to admit water 
into the compartments and to drain water out from them. Sea chest valve 
operators 57 extend from the sea chest valves up to the deck to control 
the operation of the valves 56. By so controlling the wing wall buoyancy, 
the jackup drydock can be partially submerged to rest on the water bed 50. 
As shown in FIG. 1, the shallow water drilling barge 46 includes a drilling 
mast 58 extending up from an elevated drilling platform 59 at one end of 
the barge. A casing 60 extends down from the mast and through the drilling 
platform and the jackup drydock to the water bed 50. The casing 
accomodates a drill string during drilling operations and a conductor 
string during production operations. It is the weight of the drill string 
and conductor string which constitutes the "hook-load". Lengths of casing, 
drill pipe and conductor pipe (not shown) are carried on the barge 46 and 
are fed to the drilling mast in a manner well know to those in the field 
of oil well drilling and production. A derrick 62 is also arranged on the 
drilling barge 46 to place the drill pipe and to move other heavy 
equipment as needed for operation of the barge. 
As shown in FIG. 2 the drilling barge 46 itself has a flotatable hull 66 
which is provided with sea chest valves 68 to admit and to drain sea 
water. The hull 66 has a shallow draft which permits the barge to be 
floated to desired drilling locations in shallow water and then to be 
settled down on the bottom for carrying out drilling operations. Such use 
of shallow water drilling barges is well known and the drilling barge by 
itself does not constitute the present invention. The drilling barge 146 
has a main deck 70 which is supported above the hull 66 by "posts" 72. 
This ensures that when the hull 66 is submerged and resting on the bottom 
at a shallow water location the deck 70 will be above the water surface. 
As can be seen in FIG. 2 the mast 58, the derrick 62 and the drilling 
platform 59 are all mounted above the main deck 70. Also, for ease in 
transporting the barge 46, the mast 58 may be pivoted back to extend along 
and above the main deck 70 as shown in FIG. 2. 
As can be seen in FIG. 3, the wing walls 52 of the jackup drydock 40 are 
elongated and are arranged parallel to each other in spaced apart relation 
along opposite sides of a horizontal barge support platform 74. The width 
and length of the platform 74 is sufficient to allow the shallow water 
drilling barge 46 to rest on the platform between the wing walls 52. 
Elongated support rails 75 extend along the upper surface of the platform 
74 spaced apart from each other and parallel to the wing walls 52. These 
rails each rest on a plurality of roller assemblies 76 which allow the 
rails to move freely lengthwise of the jackup drydock even when carrying 
the drilling barge 46 or a production rig. 
There are also provided, along the upper surface of one of the wing walls 
52, an engine room 78 with an operating room 80 mounted thereon. A crew 
quarters 82 is provided along the upper surface of the other wing wall. 
Also, cranes 84 are mounted on each of the wing walls 52 for lifting 
equipment onto the jackup drydock. 
Winches 86 are arranged along the upper surface of the wing wall 52 near 
each end thereof. These winches pull on cables 90 attached to the drilling 
barge 46 to position it over the platform 74. 
A plurality of fenders 90 are arranged along the inner surfaces of the wing 
walls to assist in guiding the drilling barge 46 or production rig into 
place between the wing walls. Also, the wing walls are flared out, as 
shown at 91, at one end of the jackup drydock to facilitate the entry of 
the drilling barge into place. 
At the end of the platform 74 over which the drilling mast 59 of the 
shallow water drilling barge 46 is to be positioned there is formed a 
working slot or opening 92 which accomodates the casing 60 when the jackup 
drydock is drilling position. 
Turning now to FIG. 4 it will be seen that for the drilling operation a 
blowout preventer 94, well known in the art, is mounted on top of the 
casing 60 to seal off the casing in the event that an upward surge of oil 
and gas should occur. During the drilling operation the jackup drydock 
rests on the water bed 50 while the drilling barge 46 rests on the 
platform 74 of the jackup drydock; and the drill string 60 extends down 
from the drilling platform 59 on the drilling barge, and through the 
blowout preventer 94 and, finally, down through the working slot 92 in the 
jackup drydock into the underlying earth below the water bed 50. Also, the 
jackup legs 42 are extended downwardly from the jackup drydock so that the 
pods 48 are submerged in the underlying soil at a depth where the soil has 
sufficient strength to ensure that the jackup drydock and the drilling 
barge along with the drill string will be stably supported. In this 
condition the jackup drydock and the drilling barge are supported by a 
combination of three effects, namely their own partial buoyancy, the 
support of the underlying soil forming the water bed 50 and the support of 
the soil acting on the pods 48. 
As shown in FIG. 5, the barge support platform 74 of the jackup drydock 40 
interconnects the wing walls 52 along their lower edges. Both the wing 
walls 52 and the barge support platform 74 are of hollow constructon and 
they are provided with internal stiffeners 96 to maintain their strength 
and rigidity. By virtue of this construction the barge support platform 74 
may also be of variable buoyancy. Internal bulkheads 98 and valves 100 are 
also provided inside the wing walls and the platform to control the flow 
of water between them. This permits selective ballasting of different 
portions of the jackup drydock. 
It will be noted from FIG. 5 that the wing walls 52 have substantial 
freeboard, i.e. vertical height above the sea surface 44, in the normal 
floating position of the jackup drydock. The barge support platform 74, 
however, has little or no freeboard. Thus the wing walls are not 
completely submerged when the jackup drydock rests on the water bed 50. 
This enables the ballast and buoyancy of the jackup drydock to be 
controlled while it rests on the water bed. 
FIGS. 6-9 show the construction and arrangement of the jacking legs 42 and 
the manner in which they are mounted in the wing walls 52. 
As can be seen in FIGS. 6, 8 and 9, the jacking legs 42 are of cylindrical 
configuration and they comprise an outer cylindrical wall 102, an inner 
axial wall 104 and three equispaced radial ribs 106. Along the outside of 
the cylindrical wall 102, in line with the radial ribs 106, there are 
provided gear racks 108. 
Turning now to FIG. 7 it will be seen that there are provided jackup leg 
guides 110, 112 and 114 mounted respectively on the floor and the upper 
surface of the wing walls 52 and at the top of the jack housing 54. As 
shown in FIGS. 8 and 9, the guides closely accomodate the outer surface of 
the legs 42 and they are provided with recess formations 116 to accomodate 
the gear racks 108. 
Inside the jack housings 54 there are provided pairs of elongated gear 
support plates 118 which extend between the guides 112 and 114. Each pair 
of support plates straddles one of the leg gear racks 108. The support 
plates 118 also serve to mount pinion gears 120 (FIG. 8) which are meshed 
with the gear racks 108. Hydraulic drive motors 122 are also mounted on 
the support plates 118 and are connected to drive the pinion gears. The 
general construction of the gear rack, pinion and hydraulic drive motors 
arrangement is well known and is available, for example, from Superior 
Lift-Boat and Rig Mfg. Inc., Route 3, Box 555, AB Lafayette, La. 70505. 
Accordingly the details of the construction of these items will not be 
described herein, suffice it to say that the motors 122 for each jacking 
leg 42 are driven in unison to turn the pinions in one direction or 
another to move the jacking leg up or down relative to the jackup drydock 
40 or, conversely, to move the jackup drydock down or up relative to the 
leg. The motors 122 can be independently controlled from leg to leg so 
that substantially all of the weight of the jackup drydock will be imposed 
on each leg individually to drive it deeply into the earth below the water 
bed 50. Other jacking arrangements, using leg gripping devices and 
hydraulic or pneumatic piston and cylinder arrangements, which are well 
known in the art, may also be used. 
The exact dimensions of the jackup drydock are not critical to the present 
invention. Nevertheless, the basic dimensions of an illustrative 
arrangement are given below: 
______________________________________ 
Overall length 230 ft. (70.1 m) 
Overall width 98 ft. (29.9 m) 
Height of wing walls 18 ft. (5.5 m) 
Width of wing walls 14 ft. (4.3 m) 
Width of platform 70 ft. (21.3 m) 
Height of platform 4 ft. (1.2 m) 
Width of working opening 
50 ft. (15.2 m) 
Length of working opening 
40 ft. (12.2 m) 
Distance between centers 
104 ft. (31.7 m) 
of jacking legs 
Distance of center of jacking 
56 ft. (17.1 m) 
legs closest to working 
slot end to end 
Diameter of jacking legs 
6 ft. (1.8 m) 
Length of jacking legs 
125 ft. (38.1 m) 
______________________________________ 
FIGS. 10-14 show the proposed use of the jackup drydock 40 to support the 
drilling barge 46 in a shallow water location. As shown in FIG. 10 the 
jackup drydock is floated, with its legs 42 in their fully raised 
position, to a drilling site. This site is characterized by a shallow 
water depth, e.g. 8-16 feet (2.4-4.8 meters). The depth must be sufficient 
to permit the jackup drydock to be floated with the pods 48 on the bottoms 
of the legs 42 clear of the water bed 50 and to allow the drilling barge 
46 to be floated onto the platform 74 when the jackup drydock rests on the 
bottom. Also, the depth of the water should not be so great that its 
surface is above the wing walls 52 of the jackup drydock 40 when it is 
resting on the water bed 50. In an illustrative example the water depth 
will be about twelve feet (3.6 meters). 
When the jackup drydock 40 is floated to the proper location its sea chest 
valves 56 (FIGS. 1 and 5) will be opened to flood and supply ballast to 
the platform 74 and the wing walls 52. The platform will then become 
partially submerged and brought to rest on the water bed 50 as shown in 
FIG. 11. At this time the drilling barge 46 will be floated into position 
between the wing walls 52 and over the platform 74 as shown. 
When the drilling barge 46 has been brought to proper position between the 
wing walls 52, its sea chest valves 68 will be opened to flood the hull of 
the barge and allow it to sink down until it comes to rest on the platform 
74 of the jackup drydock as shown in FIG. 12. 
At this point the combined weight of the submerged jackup drydock 40 and 
the drilling barge 46 will be supported by the buoyant effects of the 
surrounding water as well as by the underlying soil of the water bed 50. 
In an illustrative example the combined basic weight of the jackup drydock 
40 and the drilling barge 46 would be 7,400 k (k=1,000 pounds) or 3,357 
mkg. (mkg=1,000 kilograms) but with fuel and supplies this might be as 
much as 11,650 k (5,284 mkg). In addition, in flooding, the jackup drydock 
40 and barge 46 would take on an additional 7,000 k (3,175 mkg) of ballast 
for a total downward force of 18,650 k (8,459 mkg). The combined buoyancy 
of the jackup drydock 40 and barge 46 in twelve feet (3.6 m) of water is 
14,686 k (6,662 mkg). The net downward force on the underlying soil would 
be 3,964 k (1,797 mkg) or about 4,000 k (1,814 mkg). 
Now in the proposed example the jackup drydock would have a bottom bearing 
area of about 20,540 ft.sup.2 (1,908 m.sup.2); and, allowing for about 20% 
erosion of the underlying soil, the bottom of the jackup drydock would 
have about 16,600 ft.sup.2 (1,542 m.sup.2) of effective bearing area to 
support the 4,000 k (1,542 mkg) load. This amounts to a pressure on the 
underlying soil of about 0.25 k per ft.sup.2 (1.220 mkg per m.sup.2). 
If the underlying soil has a strength of 0.8 k per ft.sup.2 (3.904 mkg per 
m.sup.2) the pressure of the jackup drydock and barge on the soil will be 
well below that which would allow a factor of safety of two; and the 
underlying soil will therefore support the jackup drydock and barge. 
Now, during drilling operations the drill string will extend downward from 
the working slot 92 at the stern end of the jackup drydock; and as this 
drill string lengthens its weight, which is known as "hook load", will add 
significantly to the total downward force of the jackup drydock and 
drilling barge. In the example chosen this hook load may be expected to be 
about 1,500 k (680 mkg). 
Also, since the hook load is concentrated at the stern end of the jackup 
drydock its effect will be imposed on the underlying soil via the 
rearwardmost 3,000 ft.sup.2 (279 m.sup.2) of the jackup drydock bottom 
surface area; and, when a 20% erosion factor is considered, this 
additional hook load must be considered to be applied to the soil 
supporting about 2,400 ft.sup.2 of (221 m.sup.2) the jackup drydock. The 
hook load therefore would amount to an increase in downward force of about 
0.63 k per ft.sup.2 (3.074 mkg per m.sup.2) which, when added to the force 
produced by the jackup drydock and barge without the hook load (i.e. 0.25 
k per ft.sup.2), (1.220 mkg per m.sup.2) would amount to 0.88 k per 
ft.sup.2 (4.294 mkg per m.sup.2) which is far in excess of the total 
permissible load of 0.4 k per ft.sup.2 (1.952 mkg per m.sup.2). 
This hook load would be accomodated according to the present invention by 
jacking down the legs 42 as shown in FIGS. 13 and 14 to bring the pods 48 
down into a region of the underlying soil which is more consolidated and 
therefore is more capable of supporting a load than the soil at the 
surface of the water bed. 
The amount of additional load which can be carried by the submerged legs 42 
increases with the depth to which they are jacked. This is because at 
greater depths the soil is more firmly consolidated and is therefore more 
capable of supporting downwardly applied loads. In the example given 
above, the excess of hook load i.e. 0.88 k per ft.sup.2 would be well in 
excess of the total permissible load of 0.4 k per ft.sup.2 (1.952 mkg per 
m.sup.2). This excess load, which is spread over 2,400 ft.sup.2 (221 
m.sup.2) amounts to 1,152 k (0.522 mkg). Half of this load, i.e. 576 k 
(261 mkg), is to be taken up by each of the stern end legs 42. In the 
example given, the pod 48 at the bottom of each leg is thirty feet (9.14 
m) in diameter and has a surface area of 700 ft.sup.2 (65 m.sup.2). At a 
depth of forty feet (12.19 m) the underlying soil will apply to each pod 
an additional support capability of 1,089 k (0.494 mkg). This provides an 
acceptable factor of safety of 1.9. 
The stern end legs 42 would be jacked down as shown in FIGS. 13 and 14 
prior to imposition of the hook load. The action of this jacking would 
impose an uplift reaction on the jackup drydock and this uplift reaction 
would be as great as the additional support capability acquired by the 
driver leg. Thus the upward reactions force produced by jacking down each 
leg would be about 1,089 k (0.494 mkg). This, however, would be far less 
than the 4,000 k net downward force of the jackup drydock and barge 
combination. Thus it will be seen that the legs can be driven down into 
the underlying soil without lifting the jackup drydock up off the soil. 
Accordingly, the assembly will be firmly and securely supported throughout 
the entire jacking operation. 
It will be appreciated that the forward end legs 42 could be jacked down in 
the same manner to provide support at that end of the jackup drydock. Also 
the legs may be jacked down in anticipation of further erosion of the 
underlying soil so that as the soil is washed away, e.g. by river 
currents, the weight of the jackup drydock and barge will be automatically 
transferred to the submerged legs 42. 
Once the legs 42 are driven down into the underlying soil they would also 
provide a certain amount of resistance to uplift loads. Thus, as each leg 
is driven down it would automatically provide additional uplift resistance 
which would enable the other legs to be driven down with even greater 
force to deeper locations to provide increased support capability. 
Further, this uplift resistance would serve to stabilize the jacking 
drydock and barge against the effects of winds and waves. 
It will also be appreciated that as drilling continues and the hook load 
increases, the legs 42 may be jacked down even further into the underlying 
soil to provide even greater support to the jackup drydock. The added 
weight of the drill string would provide uplift resistance which allows 
this further jacking down of the legs without lifting the jackup drydock 
up off the underlying soil. After drilling operation have been completed 
the drilling barge 46 may be disconnected from the well it has drilled and 
then it may be deballasted and floated away from the jackup drydock 46. 
The jackup drydock 46 may then be deballasted to assist in jacking up its 
legs 42. Also to assist in raising the legs they may be provided with a 
water jetting arrangement which is well known in the art. Once the legs 42 
have been raised the jackup drydock is then full deballasted and floated 
away to a new location.