Note: Descriptions are shown in the official language in which they were submitted.
12987~Z
01 -1 -
RE~IOVAE~L~ l~()'r'rO~1 FOlJNl)EI~ STRUC'rlJ~E
o5 ~ L(I (~F ~:h(? rnv~n~. ion
____ __ _ _ ___ _ _ __ _ __
This invention generally relates to offshore oil
drilling and producing structures. More specifically, to
a structure that may be removably detached from a base
located on the sea floor.
BACKGROUND OF THE trlvENTIoN
. .
As oil exploration continues in remote loca-
tions, the use of offshore drilling techni~ues and struc-
tures will become more common~lace in ice-infested areas.
Platforms are continually erected in isolated areas that
have extremely severe weather conditions. However, the
structures that operate in more temperate climates cannot
usually be employed here because they must~be able to
cope, not only with severe arctic storms and sea ice
incursions, ~ut also with large and small icebergs that
are driven by wind, current and wave action. Because of
these conditions, many difEerent types of platform designs
have arisen in an attempt to cope with the harsh weather
and other natural elements.
Currently, much exploration is conducted in the
arctic and in the ice-inEested waters off Alaska, Canada,
and Greenland. To cope with the iceberg and weather
problem, some structures attempt to resist these large ice
masses by simply being large enough to withstand the
largest conceivable impact forces. Examples of these
designs may be seen in dual cone structures, such as U.S.
Patent No. 4,245,929, large reef-like structures, or many
other gravity based large concrete-steel configurations,
see also U.S. Patent No. 4,504,172. However, these struc-
tures are either very heavy, very expensive, or are per-
manently affixed to the sea bottom. As such, they do notlend themselves to either reuse or quick site evacuation
in the case of an emergency situation. In addition, ulti-
mate removal and abandonment of these structures upon oil
field depletion is extremely diEficult. Due to the wide
variability in iceberg characteristics and lack of data
lZ98~71Z
01 -2-
about them, a more problematic issue with these structuresconcerns the definition of the largest iceberg to design
05 for - the selection of the design iceberg requires a rea-
sonable balance of risks and costs, made di~ficult by the
inherent uncertainties.
~ nother actor to be considered is cost. Gener-
ally, the type of large gravity based structure that may
be used for arctic exploration and production is very
expensive and time consuming to build. With the unproven
nature of some of the oil prospects, the harshness of the
environment, the increased costs and delays due to the
weather down ti1ne, the probability of failure, and even
the political climate, it becomes even more risky for an
oil company to invest a large amount of money or time. In
the event of an accident or other type of misadventure,
losses could be greatly multiplied.
To overcome many of the disadvantages of these
previously discussed arctic structures, it would be advan-
tageous to combine some of the principles of the gravity-
based structures with those of the floating structures.
This is accomplished by constructing a platform that has
subsurface hull chambers that may alternatively provide
buoyancy or ballast and a subbase upon which the platform
may rest. The cornplete structure may then be towed in a
floating mode to an offshore drilling/production site and
slowly filled with ballast until both the platform and the
subbase rest on the sea floor in a gravity-based mode.
I~hen a situation, threatening to the structure, presents
itself, the platform may be deballasted back to a floating
condition and removed from the site to leave the subbase
behind. However, this deballasting procedure is quite
slow (on the order of 6 to 7 hours) and since it is
probably going to be done in rough seas, there is a large
chance that the platform, and/or the subbase on which it
rests, may be damaged when it "bounces around" due to wave
action as it approaches neutral buoyancy on the subbase
and then while it slowly ascends to its final floating
draft.
1298~71Z
01 ~3~
A solution to this probLem is to keep the plat-
form on the subbase with a temporary hold-~own means while
05 it is heing deballaste(l. Once it has fully deballasted,
the hold-down means may then be released to allow the
platform to quickly ascend to its ~loating draft and
escape damage.
This hold-down system maY be mechanical or
hydraulic, however, because a mechanical system: may not
as.sure a simultaneous release of all connection units; is
expensive; requires a sophisticated controi system; and is
diEficult to reuse or to replace damaged or used connec-
tion units, a hydrostatic sealing system is chosen. This
IS hydrostatic system will hold the platform to the subbase
from the beginning of the deballasting procedure to the
tirne "hen deballasting is complete. After deballasting,
the platform rnay be quiclcly detached by releasing the
hold-down system and then floated away from the impending
iceberg danger.
To eliminate most oE the problems of these
previously mentioned arctic structures for use in iceberg-
infested waters, the Removable 80ttom Founded Structure
(RBFS) concept was developed to provide a platform which
may be removably detached on short notice from its subbase
and, if necessary, transported to a safer iocation. Other
advantages of this structure include providing: (1) a
wellhead protection device (i.e., the subbase) against
those icebergs large enough to scour the sea floor, (2) a
capacity for a higher deck load than floating structures
(as the RBFS rests on the sea bottom in the normal oper-
ating mode), (3) the ability to quiclcly evacuate the plat-
form from its fixed location on the sea floor by
deballasting and then releasing the hold-down means,
(4) reduced capital costs from the gravity based struc-
tures due to a more economical design, (5) greater flexi-
bility in structure siting lue to the platEorm's mobility,
(G) direct subsea well access from the fixed deck over-
head, (7) protection of the vertical production risers
from waves and ice due to their placement within the
12~15 71Z
01 _4_
platform columns, and (~) the ability to relocate most oE
the structure to a new site if dictated by changing
05 reservoir information (only a new subbase would be
required for each relocation).
SUIlII'IARY OF THE INVENTI()N
The present invention holds a buoyant platform
onto a subbase that rests permanently under its own weight
on the sea floor. The structure is called a Removable
~ottom Founded Structure (RBFS) and it is designed for the
arctic environment. The RBFS resembles a very large sub-
mersible drilling platform which, by virtue of its direct
overhead access to the subsea wells, functions in many
ways like a conventional fixed drilling and production
platform. Normally the plat~orrn would be fully ballasted
on the subbase with water ballast. However, in the event
of an approaching iceberg (larger than one which the RBFS
is designed to resist), the hold-down sealing system is
enga~ed, the platform is deballasted to a positive
buoyancy condition, the risers are disconnected from the
subbase, then the hold-down sealing system is released,
and the platEor~ floats and propels itselE off location to
leave the subbase behind.
The platform must be disconnected from the
subbase to reach its floating draft very quickly so that
there is no collision between the platform and subbase
during platform liftoff due to wave action. To do this,
the hold-down system is engaged to hold the platform down
on the subbase, the platforrn columns and pontoons are
deballasted to achieve a large net buoyant upward force,
and then the hold-down mechanism is quickly released. The
above operations, up to release of the hold-down system,
are always controllable by the platform's operators. If a
threatening iceberg subsequently leaves the area before
the point of actual liftoff, the operations can easily be
reversed.
A load transfer system is describerd in which a
vertical member of the offshore structure has a concave
base plate; and in which a convex raised section generally
~ 1298712
adopted to fit into the concave base plate; and in which
a convex frame is generally adopted to fit between the
convex raised section and the concave base plate, and in
which there are hydraulic capsules mounted on the
convex frame, the capsules adopted to be inflated such
that a load on the capsules can be adjusted. In a
preferred embodiment, the capsules are flatjacks and are
spaced along a perimeter of said frame. There is also
a means for adjusting the load in the capsules with
hydraulic fluid and means for subsequently filling the
capsules with grout. The concave base plate further
comprises a horizontal doughnut shaped bearing plate
and an inclined truncated cone shaped bearing plate.
The convex frame is a tubular steel frame with recesses
in the convex raised section, the recesses adopted to
receive the frame. Resilient bumpers are mounted to
the concave base plate.
An aspect of the invention is as follows:
A load transfer system for an offshore0 structure mounted to a subbase comprising:
an offshore structure;
a vertical member of said offshore structure, said
vertical member having a concave base plate:
an convex raised section on the subbase, the raised5 section generally adapted to fit into said concave base
plate;
a convex frame generally adopted to fit between
said convex raised section and said concave base plate;
and
hydraulic capsules mounted on said convex frame,
said capsules adopted to be inflated whereby a load on
said capsules can be adjusted.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a view of the assembled platform5 resting on the subbase;
FIG. 2 is a cross-sectional view of a portion
of the subbase;
FIG. 3 is a partially cut-away and overhead
plan view of the subbase (the footprint of one of the
six platform columns is shown on the overhead view);
~Z~371Z
-5a-
FIG. 4 is a representation of the forces that
act on the underside of a buoyant column;
FIG. 5 is a cross-sectional view of the seal
system mounted in the underside of a column;
FIG. 5A is an alternate cross-sectional view
of the seal system and the subbase/column interface;
FIG. 6 is an overhead plan view of both
sealing systems;
FIG. 7 is a cross-sectional view of a passive,
elastomeric seal;
FIG. 8 is an overhead view of a segment of the
passive, elastomeric seal;
FIG. 9 is a side view of a segment of the
passive, elastomeric seal;
lZ9871Z
FIG. 10 is an overhead view of the arrangement of
the inflatable seal;
FIG. 11 is a cross-sectional view of the inflatable
seal with its accompanying hardware;
FIG. 12 is an enlarged overhead view of a section
of the inflatable seal mounting plates; and
FIG. 13 is a schematic representation of the
mechanical equipment inside each sealed column involved
in operating the hold-down system.
FIG. 14 is an isometric view of the load transfer
system.
FIG. 15 is an enlarged view of the load transfer
system.
DETAILED DESCRIPTION OF THE INVENTION
The Removable Bottom Founded Structure (RBFS) is an
offshore structure for petroleum drilling and producing
operations and is intended for deployment in waters with
severe weather and iceberg conditions. The RBFS is a
two-part structure. The first part generally comprises
a platform and is made up of multiple columns which are
affixed at substantially 90 to the deck structure.
Cross bracing and other horizontal members are also used
to make the platform more stable. The second component
is a reinforced concrete subbase that rests on the sea
floor and upon which the platform is founded.
The RBFS is designed to withstand severe conditions
of wind, wave and current action, and many of those ice
conditions which could normally be expected during the
lZ98712
structure~s life. For example, the RBFS is designed to
withstand a 150-year return period storm; an iceberg
with a 20-year return period kinetic energy; and to
survive (with some damage) an impact with an iceberg
having a 100-year return period kinetic energy.
However, if an iceberg large enough to cause damage to
the RBFS threatens to come in contact with the
structure, the platform is evacuated from the site to
leave the subbase behind. To ensure that the
inhabitants and operators of the RBFS are apprized of
all iceberg and storm dangers, they maintain visual
lookouts for clear days and shorter distances, whereas
they use a platform-based radar system for longer
distances and less clear weather (they may also rely on
~5 ships and aircraft). Danger zones, having specified radii
1298712
01 --7--
from the platform, may also be established to allow the
platEorm personnel to ~au~e the possibility of actual
05 iceberg incursion and take appropriate action. The towing
of some icebergs by various boats supporting the platform
is also possible, although the variability and number of
potential icebergs prevent a complete reliance on iceberg
towing.
Referring now to the drawings, FIG. l di~closes
that the RsFS comprises two portions, a platform 1, which
is further divided into a hull 7 and a deck 5, and a sub-
base 3. The hull 7 is the frame assembly that extends
from the upper side of the subbase 3 to the underside of
15 the deck 5. It has six columns, four corner columns 15
and two central columns 15a, each are 20 m in diameter
and configured in a two by three arrangement. The corner
columns are spaced at center line dimensions of
83 m x 116 m. The four corner columns can be sealed,
while the two center columns enclose the production
risers. Six upper horizontal braces 10 extend between
the columns 15 and 15a around the hull perimeter at a
center line elevation of 47 m above the bottom of the
hull 7. A seventh horizontal brace 19 connects the cen-
25 tral columns 15a at the same elevation. Each of the sevenupper horizontal braces are 13 m in diamet~er. Six hori-
zontal pontoon braces 13 of 12.5 m in diameter extend
between the columns 15 and 15a around the hull perimeter
at a center line elevation of 6.8 m. The hull 7 has ten
30 10 m diameter diagonal braces 11 in both the transverse
and longitudinal directions. Two diagonal braces 11 are
located on each face of the hull perimeter between the
upper horizontal braces 10 and the pontoon braces 13,
while the remaining two connect the seventh horizontal
35 brace 19 with the central columns 15a.
The outer shell of the members of the hull 7
which are directly exposed to iceberg impacts consists of
a hybrid steel-concrete-steel sandwich. These rnembers
include the six columns 15 and 15a, the six perimeter
1~9871Z
01 -3~
upper horizontal braces 10, and the eight perimeter diago-
nal braces 11. The sandwich desiyn provides strength to
05 withstand the high local ~ressures associated with iceberg
impacts. The steel sections are welded by standard ship-
yard construction techniques and extensive use o~ auto-
matic welding. The in~ill concrete would be high-
strength, lightweight concrete and would be placed between
the steel sections using standard techniques and equipment
adapted to this application.
The subbase 3 is a permanent reinforced concrete
structure, the configuration of which is shown in FIGS. 2
and 3. When the platform 1 is not present, it is designed
to withstand a 100-year iceberg impact with practically no
movement and no structural damage and to survive a 2000-
~ear iceberg (while protecting a subsea well template
located inside it), with limited damage and movement. The
subbase 3 provides a bearing surface for vertical and
lateral load transfer from the platform 1 during normal
operations and it first anchors the platform and later
protects the template from iceberg scour during iceberg
emergency operations. The principle components of the
subbase 3 include outboard 30 and inboard walls 32, top 34
and bottom slabs 36, and interior walls 38 and slabs 40.
The outboard wall 30 extends around the outer perimeter of
the subbase 3, and the inboard wall 32 forms the inner
perimeter. The top 34 and bottom slabs 36 form the roof
and the floor of the subbase 3, respectively.
Interior walls 38 and slabs 40 are used to par-
tition the subbase 3 into compartments 44, a typical cross
section is three compartments high and three wide as shown
in FIG. 2. These interior walls 38 and slabs 40 divide
the subbase 3 into solid ballast compartments 44 (filled
via a surface controlled system during initial RBFS
installation) and greatly increase the shear capacity of
the cross section. Fourteen water ballast compartments in
the subbase 3, required for subbase tow and installation,
are each composed of 27 solid ballast compartments 44
(3 x 3 compartments 44 in plan view over the full height
~Z98712
0 1
oE the subbase). rhose interior walls 3~ betwe~n water
ballast compartments, as well as top 34 and bottom
05 slabs 35 and outboard 30 and inboard walls 32, are all
designed for watertightncss. Furtllerlnore, concrete skirt
sections 42 extend frorn the unclerside of the subbase into
the soil and help transfer lateral forces into the soil
and, with a sand undergrouting systeln, accommodate uneven
seabed conditions. The skirt pattern in plan view is the
same as the pattern oE 1lL w~terti~lht wa1Ls.
There are at least two ways to install the plat-
form l onto the subbase 3. In one method for initial
installation oE the RBFS (which could also be used to re-
site the platform l), the subbase 3 as the foundation isaffixed alone to the sea floor 9 by many of the means used
for the installation of gravity based structures, such
as: tow to the ofshore site; hookup of the floating
subbase 3 to a pre-installed onsite mooring system (a
spring buoy and clum~weight system, which is not illus-
trated); lowering and placement on the sea floor 9 by
controlled flooding of water ballast compartments to a
slightly positive buoyancy condition and thereafter by
mooring system tension adjustments; leveling and penetra-
tion of the skirts 42 into the soil by continued con-
trolled flooding and by solid ballast placement inside the
compartments 44; and sand undergroutincJ between the
skirts 42, the bottom slab 36, and the sea floor 9. All
of the above operations are controlled at the surface from
various ships. The subbase 3 is permanently founded by
virtue of its own structural weight, the weight of the
solid ballast placed inside the colnpartments 44, the
undergrouting and the skirt penetrAtion into the soil. In
the rough seas normally encountered at exposed ofEshore
arctic sites, the subbase installation operations may be
quite difficult, particularly the ~lowerincJ and placement.
Once the subbase 3 is installed, the clumpweight
mooring system is removed and replaced with a standard
chain-and-buoy mooring system. The platforin l is floated
4~ over the subbase 3, hooked ul) to the mooring system and
lZ98'i~1Z
01 -1 O-
positioned. A tendon relocation system is then e~ployed.
Four tendons 6, similar to those on tension leg platforms
05 (TLPs), are lowered from the platform 1, stabbed into
receptacles on the subbase 3, and tensioned with a jacking
system on the deck 5. ~hile maintaining a constant ten-
sion and controlling lateral movements by mooring system
adjustments, the platfonn 1 is lowered to just above the
subbase 3 by selective admission o~ seawater ballast into
the interior of tlle hull members. Employing a pin-and-
cone docking system for Einal lateral alignment, continued
water ballasting allows the platform 1 to land on the
subbase 3. Further water ballast is added to weight the
platform 1 down on the subbase 3 in a gravity-based mode.
The only requirement is that the platform 1 be
properly weighted down on the subbase 3 with a water
ballast quantity large enough to provide sufficient resis-
tance to all possible platforrn movements (e.g., rocking or
sliding) due to wave action. At this point, the plat-
form 1 is stable and connection of the production
risers may begin as well as drilling operations. The
above platform installation operation will be used to
relocate the platform 1 after its evacuation for an ice-
berg emergency, as well as for the first method here ofinitial RBFS installation.
However, the second preferred method for initial
RBFS installation is to join the platform 1 and subbase 3
in a floating mating operation prior to their transporta-
tion to the final offshore site. Tile floating subbase 3is towed to a sheltered deepwater location near land,
hooked up to a spring buoy and clumpweight mooring system
already installed there, and lowered to a depth just above
the sea floor 9. Lowering is accornplished by controlled
compartment flooding with water ballast to a slightly
buoyant condition and then by pulling the subbase 3 down,
and keeping it stationary, with the mooring system. This
` operation is similar to that performed offshore for the
first method above, although the protected waters make it
less difficult and risky.
.
lZ98~71Z
Next the platEorrn 1 is hooked up to a separate,
conventional chain-and-buoy mooriny system (not illus-
trated) at the site, and positioned over the submerged
subbase 3 . The four tendons 6 di scussed above are lowered
from the platform 1 and stabbed into the subbase recep-
tacles. The tendons 6 are slightly tensioned. By
deballasting the subbase 3, pulling it up with the ten-
dons 6, making ad justments with the two mooring systems
and using docking devices, the subbase 3 is raised and
then mated with the platEorm 1. The tendons 6 are fully
tensioned to serve as a sea fastening, securing the
subbase 3 to the platEorrn 1 for the sea tow. Within naval
architectural limits, they are also secured together by
increasing the buoyancy of the subbase 3 and reducing the
buoyancy of the platform 1, squeezing the two components
together. The hydrostatic hold-down system then undergoes
preliminary tests at the deepwater construction site. The
platform 1 and subbase 3 are disconnected from their
respective mooring systems, and the mooring systems are
la te r re t r i eved .
The RBFS is then towed to location, hooked up to
a pre-installed onsite mooring system (conventional chain-
and-buoy type), ballasted down (by controlled flooding of
water ballast compartments in the hull members and in the
subbase 3 ) and positioned by mooring system ad justments,
until the subbase 3 rests in its f inal desired position on
the sea floor 9. RBFS leveling and skirt penetration,
solid ballast placement in the subbase 3, and sand under-
grouting proceed in much the same manner as discussed
above for the first method, although these operations are
considerably simplified by the presence of the platform 1.
The platform 1 is weighted down with water ballast as
above. These operations result in a completed bottom
founded structure. The tendons 6 are retrieved, the
mooring system is disconnected, and the hydrostatic hold-
down system is thorougllly tested. Drilling and production
operations may then cornmence.
~29~3712
01 -l2-
There may be times when the platform l willhave to be moved from its location due to a threatening
05 iceberg. Before the platform l can abandon site, it
must be deballasted to reach a desired ~loating draft.
I~owever, if it is deballasted and permitted to rise
slowly off the subbase 3 in rough waters, there is the
risk that the platfonn 1 may come in contact with the
subbase 3. This could cause a considerable amount of
damage to both the plat~orm l and the subbase 3 and may
even go so far as to cause the platform 1 to flood and
sink. As a result, a hydrostatic hold-down system 50
keeps the platform l down onto the subbase 3 while it is
being deballasted. The hold-down system 50 is disengaged
once sufficient liquid ballast has been removed from the
platform l so that it may rise to its floating draft, in a
rapid fashion, without incurring any damage.
As previously stated, the platform l must rise
quickly to its floating draft to prevent potential colli-
sion between the platform l and the subbase 3 during an
iceberg avoidance operation. Furthermore, to shorten the
time required for the overall iceberg avoidance procedure,
the operators of the platform will shut in wells, and
purge and disconnect the drilling and production risers,
while concurrently deballasting the platform l. The
hydrostatic pressure that acts on the platforrn l is tempo-
rarily reduced by the hold-down system to hold the plat-
form l onto the subbase 3 while it is being deballasted
(and thus becomes more buoyant). To accomplish this, a
system of hold-down seals SO enclose the perimeter of the
base of each corner column l5. After the spaces, enclosed
by this system of seals 50, are separated from the outside
seawater, the hold-down system is activate~d. This is done
by reducing the hydrostatic pressure that acts on the
bottom of the column, effectively holding the platform l
on the subbase 3 by virtue of the platform's own weight.
The hydrostatic hold-down system 50 reduces the
hydrostatic head on the area underneath the column 15.
This is shown in FIG. 4 which represents the buoyancy
12g.8~12
01 -13-
forces acting on a column 15 before and after the sealing
system is engaged. In normal operations, the buoyant
05 force that acts on a column 15 may be shown by Pl = ~ .
hl . A where Pl is the total buoyant force, ~ is the
density of water, hl is the height of water in a standpipe
(the depth below water surface in normal operations), and
A is the area underneath the column 15. However, when the
hold-down system is activated, the water level in the
standpipe can be reduced to h2 by pumping water out of the
sealing spaces. This decreases the buoyant force to a new
value which can be expressed as P2 = ~ . h2 . A and which
can equal zero as h2 is lowered to zero. The difference
in hydrostatic pressure between the outside environment
and the space underneath the column 15 is maintained by
the seals around the perimeter of the column. While the
seals are enga~ed, the pressure difference keeps the plat-
form 1 on location.
FIG. 5 shows the entire hold-down system 50.
Concentric seals 56 and 58 at the perimeter of each corner
column 15 enclose hold-down chambers 51 and 52 between the
column 15 and the subbase 3. During normal platform oper-
ation, when the R~3FS behaves as a gravity structure and a
hold-down force is not needed, the chambers 51 and 52 are
open to the ambient hydrostatic pressure. In an iceberg
emergency requiring platform evacuation, the platform
operators would first activate the seals 58, then create a
hold-down force by reducin~ e hydrostatic pressure in
chambers 51 and 52 (by dewatering), and finally deballast
the platform 1 to make it more buoyant. The hold-down
force equals the product of the plan area of the
chambers 51 and 52 and the differential pressure in the
chambers 51 and 52 which is ~P = ~(hl-h2) (the differen-
tial pressure is the ambient hydrostatic pressure at thetop of the subbase 3 less the pressure in the chambers 51
and 52 which corresponds to the hydrostatic head in the
chambers 51 and 52). The sum of the hold-down forces at
each corner column 15 is suEficietlt to prevent platform 1
lift-off under the combined effects of the buoyancy of the
129871Z
Ol -14-
deballasted platform 1 and the design storm loads. Theoperators of the platform 1 eliminate the hold-down force
OS when they open the chambers 51 and 52 to the ambient
hydrostatic pressure and simultaneously deactivate the
seals 58 (to protect the seals during liftoff).
There is a space between the underside of each
corner column 15 and the subbase 3, (except at a column
IO weight bearing area 54 which carries the axial load of the
platform 1). Two concentric seals 56 and 58 define this
space. Elastomeric compression seals 56 are mounted on
brackets 60 on the outside of the column's'outer ~all
plates 62 (and are the outer seals), and inflatable
seals 58 are mounted on the underside oE the column base
plates 64 concentrically spaced within the elastomeric
compression seals (and are the inner seals). The inflat-
able seals may be set in a recessed area 66 and inflated
via line 65. The space between the column 15, the
subbase 3, and bounded by the inner seal 58, is referred
to as the inner chamber 51, and the annular space between
the two sets of seals, the column 15, and'the subbase 3,
as the outer chamber 52. As shown in FIG. 6, if one could
look down through the column at these spaces, the outer
space 52 would appear to look like a donut and the inner
space 51 the hole. The bearing area 54 is not fluid-tight
and therefore has no effect on the chambers 51 and 52.
An alternate arrangement of the column 15/subbase
3 interface is shown in FIG. 5A. To receive both vertical
and lateral loads from the platform 1, the subbase 3 may
be designed to have a raised portion 3a that will fit into
an indented portion 15b in the column 15. 'The sides of
the raised portion 3b and the sides of the indented por-
tion 15c may be sloped and expandable grout bags may be
placed between and against these sides 15c'and 3b. This
will compensate for construction tolerances Eor the plat-
form 1 and subbase 3, and will provide a bearing surface
' for proper vertical/lateral load transfer between the
platform 1 and subbase 3. This alternate arrangement
98712
01 -15-
should have little effect on the design or operation of
the hold-down system 50 shown in FIG. 5.
05 The outer seal 56 mounts on brackets 60 on the
outside of the four corner columns 15, as shown in FIG. 5.
The outer seals 56 project below the underside of the
columns 15 to ensure sufficient compression and allow for
the construction tolerances of the platform hull 7 and
subbase 3. Virtually all vertical loads will be borne by
area 54. As the platform 1 settles onto the su~base 3,
the seals 56 compress by the weight of the platform 1 to
create an essentially fluid-tight barrier.
As shown in FIGS. 7, 8, and 9, the compression
seal 56 consists of 30 segments 70 (see FIG. 8) which may
be made of a castable polyurethane elastomer. Bolts 72
are embedded into each segment 70 to properly mount the
segment onto the support bracket 60. The segment ends 74
are mitered at 45 to produce lapped joints between seg-
ments 70.
The compression seal 56 creates a fluid-tight
barrier around the outside of the column 15 with the
Eollowing advantages:
- No mechanical systems are required to deploy the
seal, which eliminates the chance of equipment
failure;
- The compression seal 56 is continuously deployed
when the platform 1 is resting on the subbase 3
which keeps sediment and debris from entering
the hold-down chambers 51 and 52;
- Although some leakage may occur after several
lift-offs and reinstallations (due to a possible
permanent set of the elastomer material), the
total loss of the seal 56 is unlikely and
leakage should be small and manageable;
- The compression seal 56 requires little mainte-
nance; and
- The segments 70 may be easily replaced if
damaged or excessively deformed.
lZg871~
Ol -16-
The outer seal, 56 would ~e tested on a regularbasis. An operator simply reduces the pressure in the
05 inner 51 and outer chamber 52 to subject the compression
seal 56 to a differential pressure. Water in the cham-
bers 51 and 52 can then ~e Inonitored by the operator for
leakage.
When the outer seal 56 is activated by a
differential pressure across the seal, it is compressed in
all three directions: vertically, by the weight of the
platform 1 on the subbase 3; tangentially, by hoop com-
pression induced by the greater outside pressure; and
radially, by pressing the seal against the outer wall
plate 62. The bolts 72, support brackets 60, and outer
wall plates 62 rigidly Eix the top of the outer seal 56.
Friction between the seal 56 and subbase 3 prevents the
seal 56 from bending inward and upward about its fixed
to~.
An inflatable reinforced elastomer seal was
selected for the inner seal 58. See U.S. Patent
No. 3,397,490. These inflatable seals 58 mount in cham-
bers 66 in the underside of the four corner columns 15
just inside of the weight bearing area 54. The mounting
chambers 66 are recessed so that the seals 58 do not pro-
ject below the area 54 in their normal (deflated) state.
The inflatable seal 58 is a single donut-shaped
piece (as viewed from above in FIG. 10). It consists of a
flattened tube 80, an integrally molded base 82 and a
neck 84, as shown in FIG. 11. The base 82 fits into a
retainer plate 86 which attaches to the underside of a
seal mounting plate 87 by bolts 88 (see FIG~. 11 and 12).
There are inlets 89 for pressurizing the seal 58 and out-
lets 91 for depressurizing the seal 58 (see FIG. 10).
The tube 80 and base 82 of the seal 58 may be
molded ethylene, propylene, diene snonomer (EPDsV) elastomer
reinforced with Kevlar (tradernark of E.I. Du Pont de
Nemours) fabric. This elastorner, selected for the inflat-
able seal 58, is a 60 durometer EPDM formulation having
e~cellent oxidative aging resistance, and good wear and
~Z!~1~37~LZ
01 -17-
abrasion resistance. It may be reinEorced by a wovenfabric with two plies laminated biaxially around the
05 tube 80, and an additional two plies incorporated along
the neck area ~4 where the base 82 joins the tube 80
structure.
The inflatable seal 58 was selected for the
following advanta~es:
- Inflatable seals are a proven concept and are
used for a wide range of applications, such as
nuclear reactor refueling cavity pool seals;
- Inflatable seals conform well to uneven seating
surfaces, and self-adjust within the range of
vertical gaps anticipated between the undersides
of the columns 15 and th3 top of the subbase 3;
- The seals are stressed only during iceberg
avoidance operations or in-service tests which
prolongs their life; and
- Dissimilar designs and materials for the
outer 56 and inner seals 58 reduce the possi-
bility of simultaneous failure of both seals,
enhancing redundancy.
In one ernbodiment, the subbase 3 is a cellular
structure resembling a rectangular doughnut in plan view
with out-to-out dimensions of approximately 370.6 x 480.9
ft. It is 50 ft high with 7 ft deep skirts 42 extending
from the underside of the subbase into the soil. The
typical subbase cross-section is 85 ft wide by 49 ft high,
with four slabs and four walls in the longitudinal direc-
tion (perpendicular to the cross-section) and with trans-
verse walls spaced along its length. Solid and liquid
ballast are placed in the compartments provided by the
cellular design.
Instrunental to all phases of the platform's
life are the platform/subbase interface components, i.e.,
the "load transfer and hold-down systems".~ These inte-
grated systems prevent problems associated with mating
major components, therefore, providing reliable load
transfer between components, and allowing safe departure
129871Z
0 1
of the platform from the subbase duriny iceberg ernergen-
cies.
05 The functions of the inte(~rated interface compo-
nents are:
a. To provide a means oE compensating for dimen-
sional differences betwe~n the platform and subbase.
Given tilat the platform and subbase are extremely large
structures, there is a chance that the structures would
mate less than perEectly, especially considering that the
two structures could be built in different locations. A
vertical differential tolerance up to 2 inches is
expected.
b. To provide alignment guides for platform/subbase
matiny during initial joining of the platform and subbase
at the deepwater construction site, and any platforrn relo-
cation operations following iceberg emergency lift-off
events.
c. To provide a continuous load path for gravity and
environsnental loads during all phases of operation.
The load transfer systesn is a passive resistance
mechanism with no mechanical eguipment or moving parts.
The mechanism relies on bearing between horizontal and
inclined surfaces to transmit vertical and lateral loads
across the platform/subbase interface.
Variations in the surface contours of the column
and subbase would result in at least three contact points
between the two structures and very large point loads.
Local failures at the contact points would be possible
because of these high-point loads. By uroviding adjust-
ment capabilities at the interface, these failures can be
avoided; moreover, calibration can be performed to ensure
that the dead load distribution between tlle six columns is
per design,
When the platform and subbase are joined
together, it is important that the two components be self-
aligning. This diminishes the ~otential for impact to
occur between portions of the structures not intended for
contact.
lZ98~712
o 1 - 1 9 -
A continuous load path allows the reactionforces, which are created at the platEorm/subbase inter-
O5 face, to be applied to the subbase, which in turn areresisted by bearing and shear reaction forces at the sub-
base/soil interface.
The load transfer system 210 is illustrated in
FIGS. 14 and 15. It comprises a concave column base plate
200 fixed at the bottom of each column 15, corresponding
convex concrete structural elements 201 on the subbase 3,
and jacking devices 202a and 202b that are mounted on a
gasket 203 and located between the column base plate and
the subbase. Alignment guides may be attached to the
column base plate assembly.
The column base plate 200 is comprised of a
horizontal doughnut shaped bearing plate 204, an inclined
truncated cone shaped bearing plate 205, and one or more
horizontal circular diaphragms 206. The doughnut shaped
base plate is 1-1/2 inches thick, 5.9 ft wide, and has an
outside diameter of 65.5 ft. The truncated cone shaped
bearing plate is inclined at 30 from the vertical, 1-1/2
inches thick, 3.8 ft wide, and is 53.8 ft in diameter at
its base. The circular diaphragms are 1-1~2 inches thick
and 65 ft in diameter. Radial and circumferential
stiffeners 207 provide additional structural integrity for
the assembly.
The subbase contains a concrete ~pset 201
(raised section) and (optionally) a horizontal doughnut
shaped bearing plate 213, 1-1/2 inches thick, 3.9 ft wide,
having an outside diameter of 67.8 ft. Thb upset is
conical in shape with a slope of 30 from the vertical,
53 ft in diameter at its base, and 3.3 ft high. When the
platform and subbase are joined, the concrete upset fits
into the indented tconcave) space in the column base plate
200.
The jacking devices 202a and b (Freyssinet
Flatjacks in the preferred embodiment) are hydraulic
capsules in the form of a flat double saucer, made of two
soft grade stainless steel plates that are welded and
129871Z
Ol -20-
heat-treated to ~roduce the required internal pressure and
flexibility. Inlet and outlet lines (not shown) are pro-
05 vided for inflation and adjustment purposes. The ~acksare 3 ft wide, 8 ft long, have a 2-inch stroke (working
range), and a maximum operating pressure of 2,000 psi.
Epoxy resin thrust plates (not shown), l inch thick, are
attached to both sides of the flatjack to provide the
necessary bearing surfaces. Sixteen sets of flatjacks
(each set consisting of horizontal flatjacks 202a and
inclined flatjacks 202b) are spaced at 22.5 along the
perimeter of the circular interface.
Support for the flatjacks is furnished by a
radially configured, tubular steel frame 208 that fits in
recesses 209 cast into~the top of each subbase upset.
Welded to the frame are the horizontal and inclined gas-
kets 203 that serve as mounting surfaces for the flat-
jacks. The frame also functions as a spreader bar and
lifting frame for the entire assembly, as well as a pipe
duct for the inlet and outlet lines required by the flat-
jacks.
The flatjack framés are installed on the subbase
with handling slings 214 during construction and remain
with the subbase after platform lift-off. Replacement of
a flatjack gasket, if damaged during lift-off, would be
completed prior to platform reconnection. A marine vessel
with sufficient lifting capacity would be used during
replacement operations.
Inflatable rubber bumpers 215 and the hard
rubber bumpers (not shown~, both of which $erve as align-
ment guides, are mounted on the underside of the column
base plate 200. The inflatable rubber bumpers are
attached to the horizontal base plate between the inner
and outer hold-down system seals, and the hard rubber
bumpers are evenly spaced at 22.5 along the inclined
bearing plates 205 so not to interfere with the flatjack
arrangement.
The alignment guides are installed onshore
~O during platform construction. Replacement of the bumpers,
~Z9Ei 71Z
01 -21-
if required, would take place in the platform floating
mode with diver and workboat assistance.
OS The principal functions of the load transfer
system are alignment and support adjustment between the
platform and subbase, and to provide a load path for
gravity and environmental loads across the-platform/sub-
base interface.
During initial platform/subbase mating opera-
tions and subsequent relocation operations, the installa-
tion sequence for the load transfer system is identical.
As the two structures are brought together, initial impact
occurs between the inflatable bumpers and the subbase.
lS Once uniform contact is secured, the hard rubber bumpers
align the platform into its final position. Ballasting
operations continue until predetermined seating forces are
established in the inflatable bumpers. The hold-down
system is activated, thereby dewatering the hold-down
chamber 51 and allowing hookup of ~latjack grout and vent
lines via watertight hatches in the column base plate
assemblies. Ballasting is completed and the flat jacks
are then inflated with hydraulic fluid and adjusted to
provide dead load distribution per design. Upon comple-
tion of final adjustments, the hydraulic fluid is purgedfrom the flatjacks and replaced with high strength cement
grout, The hold-down system is deactivated and the load
transfer system installation is complete.
During in-place operations, the load transfer
system requires no maintenance or adjustment. Gravity and
environmental loads are transmitted in bearing from the
platform to the subbase via the grout-filled flatjacks.
Grout or cement 211 may also be introduced into the spaces
formed by sti~fners 207.
During normal operations, the inflatable seal 58
internal pressure equals the hold-down chamber pressures
(i.e., external seawater pressure). To fully deploy the
inner seal 58 for iceberg emergency operations and
in-service tests, the internal pressure of the seal 58 is
~;~98712
01 -22-
first increased using seawater to an overpressure substan-
tially greater than the external seawater pressure. An
05 operator would then reduce the pressure in the inner
chamber 51 to create the hold-down force. The inner
seal 58 functions as a backup to the outer seal 56. If
the outer chamber 52 is at ambient hydrostatic pressure
(due to a leak in the outer seal 56 or to test tAe inner
seal 58), the low pressure in the inner chamber and its
enclosed area still provides a slightly smaller, but auto-
matic, hold-down force.
During iceberg avoidance operations, the inflat-
able seal 58 would be subjected to a large differential
pressure if the outer seals 56 could not maintain a
pressure difference. The seal 58 is rigidly fixed at its
base 82. The differential pressure load, which tends to
compress the seal 58 radially and to bend the tube 80
inward and upward about the neck 84, is su~stantially
resisted by high seal-to-subbase friction. The required
friction is generated by a sufficient internal seal pres-
sure to create large normal forces.
To activate the inflatable sealing system 58
(see FIG. 13), seawater is pumped from a sea chest 90 to
the seal 58 via a first pump 94. Valves 104, 106, and 108
are closed, while valves 92, 102, and 96 are open. Once
the seal 58 is inflated to the proper pressure, the sea-
water is allowed to flow into, and pressurize, an inflat-
able seal head tank 98. When the head tank operating
pressure has been attained, the valves 92 and 96 are
closed. The seal head tank 98 is an accumulator tank
using compressed service air, which provides a means to
adjust the internal pressure on the inflatable seal 58.
This tank 98 would also provide solne reserve energy should
the inflatable seal 58 lose differential pressure, and
would allow corrective action prior to any substantial
loss of sealing ability (float valves may be used to
` detect leaks in either of the hold-down chambers 51 and
52, and to trigger the water removal apparatus described
below). When the system is activated, the seal head tank
lZ9871~
Ol -23-
98 pressurizes the inflatable seal 58 and valve 96 acts as
a relief valve. However, in the event of an emergency,
05 valve 110 and drain 112 are also provided to relieve the
water pressure from the seal head tank 98. AEter the
pressure is relieved, proper instrumentation may then be
used to determine when the appropriate internal seal pres-
sure has again been reached and the valve 96 may again be
closed. After the inflatable seal 58 is pressurized, the
inner space 51 and the outer space 52 may then be
dewatered by a second pump 100. The second pump 100 may
be operated to dewater the inner space 51 through valve
106, now open while valve 108 remains closed. (A sump in
the top of the subbase may or may not be necessary for
dewatering and if one is used, then a float valve could
be placed in it to detect leaks and to dewater the space).
The pump system expels the water outside the structure.
At this point, continuous pumping and an atmospheric vent
116 lower and then maintain the pressure inside the inner
space 51 at approximately atmospheric pressure.
Valves 106 and 102 are then closed, and valve 108 opened.
The second pump 100 dewaters the outer space 52 through
valves 104 and 108. Again, water is expelled outside the
structure. Continuous pumping and an atmospheric vent 118
lower and then maintain the pressure inside the outer
space 52 at approximately atmospheric pressure. There is
now a reduced hydrostatic head in the area underneath the
columns 15 and since redundant seals 56 and 58 seal off
this area from the surrounding seawater creating a hold-
down force, the platform 1 remains affixed to the sub-
base 3 even during deballasting, when platform buoyancy is
increased. As leaks in the chambers 51 and 52 are
detected by float valves, the second pump 100 will dewater
the spaces once the correct valves are opened and closed.
Operation of the hydrostatic hold-down
system 50 is not necessary for the RBFS during normal
operating conditions, however, the seals 56 and 58 would
be frequently leak tested. Prior to platform evacuation
for an iceberg emergency, the seals 56 and 58 are engaged,
129871Z
01 -24-
and the platform 1 is deballasted by pumping out the
ballast chambers in the columns 15 and 15a, the upper
05 horizontal braces 10 and 19, and the diagonals 11. The
ballast pumps are sized to deballast the platform 1 in
- approximately five hours. Redundant control of ballast
tanks from several independent pumps is d~signed into the
system, and ballast control is fully automated with manual
10 backup.
Since the RBFS can evacuate the site on
impending impact of a large iceberg, all oil/gas/water
piping and control lines between the platform 1 and
subbase 3 must be readily disconnectible. (None of which
are illustrated.) The production and injection wells and
oil sales lines are first shut in subsea and all pipelines
and individual fluid lines in the integrated riser bundles
are purged with seawater. Before site evacuation can take
place, it is necessary to hydraulically disengage the
production riser mechanical latching systems and lift each
of four integrated riser bundles up into the columns 15a
by means of hydraulic hoists on the deck 5. Two electri-
cal control bundles inside the columns 15a are also dis-
connected from the subbase 3 and retrieved. Drilling
operations are halted, the wells are secured, and the
drilling risers recovered onto the deck 5. These are the
final preparatory steps before platform liftoff and occur
concurrently with platform deballasting.
The platform 1 may lift-off once the hydrostatic
pressure that acts on the bottom of the columns 15 is
restored to the ambient seawater pressure. This may be
done by flooding the inner 51 and outer chambers 52. The
proper procedure would be to simultaneously shut down the
first 94 and second pumps 100 and open valves 92, 102,
104, 108, and 106 that connect the sea chest 90 to spaces
51 and 52 and inner seal 58. This would allow seawater to
flow into the chambers 51 and 52 and reestablish hydrosta-
tic equilibrium. The seal 58 is simultaneously deflated
to prevent it from being damaged during liftoff and to
~0
lZ9~37~2
01 -25-
speed the Elooding process. Valves 110 and 96 are opened
for this purpose.
05 Immediately after the platform 1 lifts off the
subbase 3, the platform 1 moves away under positive navi-
gational control achieved with a thruster system built
into the platform 1. Eight thrusters 17 are positioned at
locations above the horizontal pontoon braces 13 of the
hull 7 (see FIG. 1). The thruster system can steer the
platform 1 in a controlled drift manner, but cannot
stationkeep in severe storm conditions. Tugs in the
vicinity (for iceberg towing, surveillance and other pur-
poses) provide further steering control once sea condi-
tions permit attachment of towing lines.
When sea and ice conditions again permit, theplatform 1 is re-sited on the subbase and the platform 1
is reballasted. Re-iting is performed with the permanent
onsite mooring system, the platform's tendon relocation
system and docking devices, as discussed earlier for the
first method of initial RBFS installation.~ After final
water ballasting is complete, then hold-down system 50 is
fully tested. The integrated riser bundles are then
stabbed into their receptacles in the subbase by hydraulic
hoists on the deck 5 which can stab a riser connector down
onto a connector mandrel in the subbase reçeptacle. Elec-
tric control bundles are reconnected. Drilling risers can
also be reattached to the wellheads in the well template
through a centrally located moon-pool in the deck 5 and
normal drilling operations can resume.
Since many modifications and variations of the
present invention are possible within the spirit of this
disclosure, it is intended that the embodiments disclosed
are only illustrative and not restrictive. For that rea-
son, reference is made to the following claims rather thanto the specific description to indicate thè scope of this
invention.
