Mobile offshore structure for arctic exploratory drilling

An offshore exploratory drilling floatable structure ballastable to rest on a sea floor but to extend above water level when so supported and adapted to withstand arctic ice loads, comprising a substantially vertical wall capable of withstanding arctic ice loads; a structural load bearing bottom rigidly connected to a lower portion of the wall; and a floatable vertically displaceable load bearing structural deck inside the wall.

This invention relates to offshore structures used for exploratory drilling 
for oil and gas. More particularly, this invention is concerned with an 
offshore structure, adapted to withstand arctic ice loads, which can be 
floated into position in shallow water and then be ballasted to rest on 
the sea floor. 
BACKGROUND OF THE INVENTION 
To drill in the shallow waters (less than 100 feet depth) of the outer 
continental shelf off Alaska year-round, new types of exploratory drilling 
structures are needed. This is mainly because of the enormous 
environmental loads imposed on drilling structures by sea ice conditions 
during the winter months and during summer break-up. The ice loadings will 
generally exceed normal offshore structure design loadings, such as storm 
wave conditions. As a consequence, structures designed to operate under 
these conditions will tend to be more massive and structurally stronger 
than conventional offshore exploratory structures. 
In addition to remaining structurally intact during operations in sea ice 
conditions, the drilling structure must remain stationary, on site, during 
these same conditions. Thus, the structure must have little or no 
horizontal movement when subjected to ice loadings. To accomplish this, it 
must transfer the substantial horizontal ice loads to some sort of 
foundation, typically the sea floor. For shallow water depths, this can 
usually be accomplished by load transfer between the structure and the 
foundation by two main means. The first involves frictional load transfer 
between the structure and foundation. It relies on the horizontal shear 
resistance to keep the structure sited. The second involves mechanical 
means of load transfer through the use of piles driven into the sea floor. 
The piles absorb the load from the structure and then transfer it into the 
sea floor. A third possible variation involves the use of both friction 
and piles to effect load transfer. 
Several of the above factors combine to complicate oil exploration in 
arctic regions where many of the most promising sites lie under relatively 
shallow water. The same shallow water sites are also invaded by drifting 
ice features that will apply very large lateral loads to a stationary sea 
surface-protruding structure. 
It is also possible that these loads could be applied to the structure at 
any time of the year. It is therefore desirable to have a structure which 
can be installed and brought up to full strength quickly. Piles could be 
used to provide a significant portion of the total strength requirement 
but they take a relatively long time to install, so there is a danger that 
a pile-retained structure may be subjected to an ice load before it is 
completely "up to strength". 
As an alternative, the structure could be ballasted with a high density 
material such as iron ore or gravel, but iron ore is not locally available 
at all and gravel is relatively scarce in many arctic regions. Either one 
of these materials would have to be transported in large quantities over 
distances greater than 100 miles. This becomes prohibitively expensive in 
an arctic environment. They also require substantial amounts of time and 
effort to put in place or remove to refloat the structure. 
In general, an offshore structure for drilling exploratory wells in shallow 
ice-covered waters, such as arctic waters, desirably meets many, if not 
all, of the following criteria: 
1. The structure should have a draft as shallow as possible in the 
lightship condition. This would allow the structure to operate in water 
depths as shallow as possible, thus maximizing its operational 
versatility. 
2. The structure should utilize an abundant, cheap, readily available 
ballast material. This would minimize problems relating to the use and 
acquisition of often scarce, expensive ballast materials. 
3. The structure should have the ability to move onto site, take on ballast 
and be ready for drilling operations to begin, and resist environmental 
ice loadings, as soon as possible. Similarly, the structure should be able 
to de-ballast, pick-up and be able to move offsite in a time period as 
short as possible. This would allow for a move to a different site during 
the short summer season in the target region. 
4. The structure should have the ability to conduct normal drilling 
operations year-round. Due to large periods of complete ice cover in 
target areas, this would necessitate that the structure be able to store 
sufficient quantities of drilling material and supplies to allow 
continuous operation for extended periods of time, as up to 270 days, 
without major resupply of the structure. 
5. Based on the above requirement for large amounts of drilling 
consumables, the structure should have a large storage capacity for such 
materials. Since several of these items are quite bulky and heavy, the 
structure must provide sufficient space and structural load-carrying 
capacity to store these materials. 
6. The structure should minimize any necessary site preparation and 
alteration. This will reduce the extent of expensive, slow offshore 
foundation work and cause the least possible adverse impact to the 
existing environment. By reducing foundation preparation requirements, the 
use of expensive, scarce materials such as gravel and sand typically used 
for this purpose is minimized. 
SUMMARY OF THE INVENTION 
According to the invention there is provided an offshore exploratory 
drilling floatable structure ballastable to rest on a sea floor but to 
extend above water level when so supported and adapted to withstand arctic 
ice loads, comprising a substantially vertical wall means capable of 
withstanding arctic ice loads; a structural load bearing bottom rigidly 
connected to a lower portion of the wall means; and a floatable vertically 
displaceable load bearing structural deck inside the wall means. 
The offshore structure will most always include a heater means for heating 
a liquid ballast, in the space defined by the wall means and the bottom, 
used to increase the weight of the structure resting on a sea floor site. 
The offshore structure will generally include a moonpool-forming vertical 
shell which extends entirely through, and is connected to, the structural 
bottom. A hole is provided in the deck so that the moonpool-forming shell 
can project through it loosely. A drilling derrick, supported by the 
structure so as to drill through the moonpool, is usually provided for 
exploratory drilling. 
More specifically, the invention provides an offshore exploratory drilling 
floatable structure ballastable to rest on a sea floor but to extend above 
water level when so supported and adapted to withstand arctic ice loads 
comprising a wall means having an outer vertical cylindrical, desirably 
substantially circular, shell, and an inner vertical cylindrical, 
desirably substantially circular, shell; the inner shell being positioned 
radially inwardly of the outer shell with the inner and outer shells 
having a common vertical axis thereby defining an annular space of uniform 
width between the inner and outer shells; reinforcing means, in the 
annular space, extending between the inner and outer shells in the lower 
portion thereof which may be subjected to arctic ice loads when the 
structure rests on a sea floor; a structural load bearing bottom having a 
pair of substantially equally spaced apart horizontal upper and lower 
discs connected to the wall means and with the lower disc and lower end of 
the wall means being about in the same plane; a structural load bearing 
means between and connecting the upper and lower discs together; and a 
floatable vertically displaceable substantially horizontal circular load 
bearing structural deck inside of and having a diameter slightly less than 
the inner shell. 
The space between the upper and lower discs can be substantially void. 
Means to feed liquid ballast to the void and remove it therefrom to 
deballast the structure are desirably included. 
The annular space between the inner and outer shells of the wall means can 
be partially or substantially void, and means provided to feed liquid 
ballast to the annular space and remove it therefrom to deballast the 
structure. However, if desirable, the annular space between the inner and 
outer shells of the wall means can be partially or fully insulated to 
retard heat loss from any liquid placed in the structure, beneath the 
deck, to the sea around the outside of the wall means. 
Means is also included to fill the offshore structure space defined by the 
upper disc and the inner shell with a liquid to float the deck to near the 
top of the inner shell. Mechanical means could be included to support the 
deck near the top of the inner shell after it is floated into said 
position. 
The bottom will generally have an overall height which is at least 20% of 
the overall height of the wall means to provide the desired strength 
against ice loads. 
A heater means for heating a liquid in the space defined by the lower disc 
and the inner shell, with the deck near the top of the inner shell, is 
usually included to prevent the liquid from solidifying, particularly when 
water is the liquid, when the structure is in a cold environment. 
A moonpool-forming vertical cylindrical shell extending entirely through 
and connected to the structure bottom is generally provided to facilitate 
drilling. To permit the deck to move vertically a hole can be provided in 
the deck through which the moonpool-forming shell can project so that the 
deck can slide or telescope relative to the shell. The moonpool-forming 
shell is preferably circular in horizontal section and has a vertical 
axis. 
Skirt means could be connected to the lower surface of the lower disc and 
extend vertically downwardly to further aid in restricting horizontal 
movement of the structure once it is positioned on a sea floor site.

DETAILED DESCRIPTION OF THE DRAWINGS 
To the extent it is reasonable and practical the same or similar elements 
or parts which appear in the various views of the drawings will be 
identified by the same numbers. 
With reference to FIGS. 1 to 3, the offshore structure 10 has a vertical 
cylindrical substantially circular wall 12 connected to bottom 14. Wall 12 
has an inner vertical cylindrical substantially circular shell 16 and an 
outer vertical cylindrical substantially circular shell 18 spaced apart 
from the inner shell. The inner and outer shells 16, 18 have a common 
axis, the result being that they define an annular space 20 between them. 
The top and bottom ends of the annular space 20 are closed by structural 
means. The shells 16, 18 can be metal shells, or any possible combination 
of metal and concrete. 
The lower portion of wall 12 which is calculated to extend from a sea floor 
site to above sea level and to be subjected to ice loadings is reinforced 
or strengthened by providing a plurality of substantially parallel 
vertical spaced apart plates 22, and a plurality of substantially parallel 
horizontal spaced apart plates 24 in annular space 20. The ends of plates 
22 and 24 are desirably connected to the inner surfaces of inner and outer 
shells 16, 18. Each vertical plate 22 contains an access hole 26 (FIG. 4). 
Similarly, each horizontal plate 24 contains an access hole 28 (FIG. 5). 
The holes 26 and 28 provide means for ballast communication throughout the 
annular space. However, it may be desirable for controlled ballasting and 
deballasting to divide the annular space into liquid tight chambers which 
can be filled and emptied of liquid at will for controlled immergence and 
refloating of the structure. 
To retard heat flow through the wall 12, thermal insulation 29 can be 
contained within the annular space (FIGS. 1 to 3). 
The bottom 14 (FIGS. 1 to 4) includes a horizontal lower disc 30 and an 
upper horizontal disc 32. The lower disc 30 has its bottom surface in the 
same plane as the bottom or lower end of wall 12. 
The lower disc 30 is a composite of spaced apart top and bottom horizontal 
plates 34, 36 between which a layer of possibly reinforced concrete 38 is 
positioned (FIG. 4). Similarly, upper disc 32 is a composite of spaced 
apart top and bottom horizontal plates 40, 42 between which a layer of 
concrete 44, possibly reinforced, is positioned. 
A structural truss or framework 50 (FIGS. 1 and 6) is located between and 
joined to discs 30, 32. Framework 50 includes vertical structural members 
52 and angled structural members 54. The ends of structural members 52, 54 
are connected to elongated members 56, 58 connected to heavy metal plates 
60 in each disc. As shown in FIG. 6, plate 60 is connected to vertical 
webs 62 which are connected to top plate 64 located at the top of upper 
disc 32. A similar arrangement is located in the lower disc 30 to connect 
the truss to it. It should be understood that a plurality of truss 
arrangements as described are used to interconnect the discs. 
Extending completely through the bottom 14 is a vertical cylindrical shell 
70 which defines a moonpool 72. The shell 70 desirably is circular in 
horizontal section. Shell 70 is desirably located concentric to wall 12 
(FIGS. 1 to 3). However, it is not essential that shell 70 be 
concentrically located as described and illustrated in the drawings. Thus, 
some users of the structure may prefer to have the shell 70 located closer 
to wall 12. 
A load bearing structural deck 80 is located inside the offshore structure 
(FIGS. 1 to 3). The deck is generally circular with spaced apart top and 
bottom horizontal plates 82, 84 and a vertical peripheral rim 86 connected 
to plates 82, 84. Deck 80 contains a vertical hole 88 defined by vertical 
rim plate 90 which is joined to plates 82, 84. Hole 88 is slightly larger 
than the diameter of moonpool defining shell 70 so as to permit the deck 
to move or slide up and down relative to the shell. Furthermore, the 
overall diameter of deck 80 is slightly less than the inner diameter of 
wall 12 so that the deck can move vertically relative to the wall. 
With reference to FIGS. 5 and 7, the wall 12 while substantially circular 
is actually shown as a sixty sided polygon having the inner 16 and outer 
18 shells parallel to each other. The inner and outer shells can be 
stiffened by T-sections 90, 92 to better withstand and transfer the loads 
to which they will be subjected (FIG. 7). 
FIG. 2 illustrates the described offshore structure 10 floating with some 
draft in an essentially deballasted condition. A drilling derrick 100 has 
been positioned on deck 80 preparatory to towing the offshore structure to 
a site. At this time, deck 80 rests on the structure bottom 14. 
Once the offshore structure arrives at the site where it is to be located, 
ballasting of the structure begins. The bottom 14 is ballasted by filling 
the void space therein with sea water. Similarly, the reinforced lower 
portion of wall 12 could be ballasted with sea water. As such ballasting 
proceeds the offshore structure will be lowered into the water as shown in 
FIG. 3. Then further ballasting is effected by feeding sea water into the 
space above bottom 14 and below deck 80. The ballasting will cause bottom 
14, having vertical skirt elements 112 extending downwardly from the outer 
surface thereof, to descend until it rests on the prepared foundation on 
the sea floor. At this stage, the water level in the structure will be the 
same as sea level and the deck will be floating at that level. To increase 
the downward bearing force applied by the offshore structure to the 
foundation, additional sea water is fed to the structure to raise the 
water level beneath deck 80 until the internal water level is near the top 
of wall 12. At that point, deck 80 floats near the top of wall 12 as shown 
in FIG. 1. 
To effect the aforementioned ballasting operation, sea water is drawn from 
outside the structure through inlet 144 into pump 141 (FIGS. 1, 4 and 8) 
located in pump chamber 140. During initial immergence, ballast water is 
introduced simultaneously into the wall annular space 20 and bottom void 
14 by means of pipe 142 and outlet 143. After the structure has been 
lowered to the sea floor and the annular space has been flooded to sea 
level, no further ballast is added to the annular space 20. However, 
ballast addition continues into the bottom void so that water passes 
through port 150 in upper disc 32 into the space beneath deck 80 causing 
the deck to float and rise to the level shown in FIG. 1. 
Heating unit 120 provides heated fluid to closed loop coils 122. In this 
way the water is prevented from freezing during cold weather. Furthermore, 
if desired, thermal insulation can be placed in and/or over the space 
between wall 12 and the rim of deck 80, and in the deck, to retard heat 
loss. 
While the deck 80 can usually be permitted to float freely in the structure 
as shown in FIG. 1, it may be desirable at times to detachably support the 
deck on load bearing supports 130 joined to wall 12 and moonpool shell 70. 
After the offshore structure is in position on site as described, derrick 
100 is moved into position over the moonpool 72 to commence drilling. 
EXAMPLE 
A specific offshore structure such as illustrated by FIG. 1 can have an 
outside diameter of 400 feet, an inner diameter of 380 feet, a wall height 
of 130 feet, a bottom vertical height of 45 feet, a deck height of 15 
feet, a moonpool inner diameter of 40 feet and a stiffened or reinforced 
wall of 70 feet measured from the bottom. Such a structure can be used in 
arctic waters which are about 20 to 50 feet deep. 
The offshore structure is towed into position at one offshore site with the 
deck resting on the structure bottom. Then the structure bottom between 
the lower and upper discs is ballasted with sea water. Similarly, the 
annular space between the inner and outer shells of the wall is ballasted 
with sea water. This is generally adequate to lower the structure to the 
sea floor. Then sea water is fed to the space defined by the structure 
bottom and wall to float and raise the deck to near the top of the 
structure wall. Not only does the sea water float the deck but the water 
ballast is raised to a level above the sea surface so as to develop the 
necessary weight to resist horizontal ice loads. Specifically, the water 
level in the structure can be brought to about 10 feet from the top of the 
wall. Since sea water is readily available, it provides an inexpensive 
ballast material and one which can be added and removed rapidly and 
easily. It also provides an efficient and inexpensive means of supporting 
a large deck or drilling platform which can also store the needed drilling 
equipment and supplies. 
Major lateral ice loadings on the exterior wall of the structure will be 
withstood by the stiff, strengthened outer wall extended 70 feet above the 
wall bottom. This wall serves two purposes: (1) to withstand the ice 
loadings and facilitate transfer of them to the foundation, and (2) act as 
a water-retaining membrane to hold the additional ballast water above sea 
level. This 70 feet tall section of wall is extended high enough to resist 
major horizontal ice loading applied to the structure. The extension above 
this wall section is a continuation of the two discrete inner and outer 
cylindrical shells. The upper inner shell acts as a water retention 
membrane, while the upper outer shell provides additional environmental 
protection as well as a space in which an insulating material may be 
located to reduce heat transfer from inside the structure to the ice and 
sea around the structure. 
To assure that the seawater ballast remains liquid in arctic conditions, a 
ballast water heating system is incorporated into the structure. In 
conjunction with this heating system, the structure has the exterior 
sections, which are exposed to outside temperatures, sufficiently 
insulated to minimize heating requirements and to prevent ice formation 
within the ballast water. 
To transfer horizontal ice loadings into the foundation, the structure has 
a structural load-carrying bottom which includes two flat discs tied 
together by a structural framework such that the two discs efficiently act 
together as flanges of a deep beam. Thus, they become load-carrying 
members, and act to transfer load carried by them to other sections of the 
structure. This distribution of load across the structure is important for 
fairly uniform distribution of foundation shear resistance to the imposed 
ice loadings. 
The actual load transfer of these ice loads into the sea floor is 
complicated, but basically, acts in two ways. Because of the rigidity of 
the wall outer shell and the rounding effect that the two bottom discs 
have on the vertical cylinder, some horizontal load transfer is 
accomplished through the distribution of circumferential shear forces 
around the circumference of the outer wall, then into the bottom disc. The 
other portion of the ice load will go directly into the top disc, to be 
carried and distributed through the top disc and into the bottom disc 
through the structural framework connecting the two discs. 
The floating deck during drilling operations will be quite stable. Because 
of the enclosed environment of the ballast water that supports the deck, 
the deck will essentially act like a standard drilling barge used in 
shallow water areas. The deck will always float at one height for safety 
and operational considerations. To maintain level trim of the deck during 
operation, a variable ballast trim system can be incorporated into the 
deck. This will be used to offset substantial lateral weight shifts to 
ensure a level drill platform. The size and load-carrying capacity of the 
deck is such that it can store sufficient drilling consumables for three, 
14,000 feet deep wells and 270 continuous days of operation without major 
resupply. 
Drilling from the floating deck can be accomplished through the integral 
moonpool that penetrates both the deck and the bottom structure. The 
moonpool is defined by a watertight shell or membrane. It can house the 
required number of drilling slots and possibly act to transfer loads from 
the drill derrick to the bottom structure. The deck can translate 
vertically along the moonpool shell exterior surface during ballasting and 
deballasting operations. 
The lightship draft of the structure is shallow enough to allow access to 
sites having water depths as shallow as 20 feet. Upon reaching the desired 
drilling site, the structure is flooded until settled on the sea floor, 
then additional ballast water is added until the deck reaches operational 
height. The structure can be designed to accomplish this within 48 hours 
after site arrival. Thus, it may be raised, transported, and sited on a 
new location within the very short ice-free summer season in arctic 
regions. To aid deballasting operations, a jetting system to aid in 
breaking the suction bond between structure and foundation can be 
incorporated. This system can utilize high-pressure water jets to 
eliminate the adhesive bond between the structure bottom and the sea 
floor. 
The foregoing detailed description has been given for clearness of 
understanding only, and no unnecessary limitations should be understood 
therefrom, as modifications will be obvious to those skilled in the art.