Note: Descriptions are shown in the official language in which they were submitted.
The present invention relates to ~n offshore platform structure,
preferably made of concrete, and particularly but not exclusively designed to
operate in waters where drifting ice and/or icebergs may appear. More
particularly, the platform structure is of a gravity type lntended to be
supported by the sea bed in areas where collision between the drifting
icebergs and the platform may take place. The plaiform structure comprises a
substructure intended to rest on the sea bed and a superstructure rlgidly
affixed to the substructure and extending therefrom up above the sea level
preferably to support a deck superstructure above the sea level.
The exploration of and dlscoveries of subaqueous oil and gas in the
arctic waters require drilling and production platform structures which are
able to resist the impact forces caused by colliding drifting icebergs of
enormous siæe.
Principally1 there are two main ways to solve the problems caused by
drifting icebergs - namely, by designing the platform structure to enable
evacuation of the platform structure on short notice whenever a collision is
likely to occur, or by designing the platform structure to be sufficiently
large, rigid and strong to resist the impact loads caused by a colliding
drifting lceberg.
The latter solution is to be preferred since continuous oil and gas
production and protection of the environment from pollution are imperative.
The impact energy or forces imposed by icebergs colliding with the
platform structure may be enormous. By far the governing loading condition
for the platform structure is the one exerted by ice impact which gives a
concentrated loading. Consequently, the structure should provide a wide
distribution of internal forces so that all or most members contribute to
sharing the load. Further, the geometry of the platform structure should be
such as ,o minimize sectional forces which are out of the plane of the members
in the caisson and one should instead aim at a configuration giving only
forces which are in the plane of such members.
The structure should also be designed to withstand iceberg impact
from any direction and, conventionally, the external wall(s) of the caisson
should preferably be provided with vertica] triangular protrusions evenly
distributed along the entire periphery of the caisson. The purpose of said
vertical protrusions is to dissipate significant energy by crushing of the ice
and/or the pro~rusion wall(s). The protrusions are preferably of relatively
small size in order to be equally effective in crushing ice irrespective of
the impact direction of the iceberg. Also, the walls in the protruding parLs
are able to withstand relatively high ice forces acting locally on a single
wall.
A ma~or ob~ect of the present invention is to provide a pla~form
structure where the vital parts of the platform - such as, for example, the
superstructure supporting the deck superstructure - are arranged in spaced
relation to those sections exposed to impact forces caused by drifting
icebergs. It should be appreciated that the sections intended to be exposed
to environmental forces, such as impact forces, are designed to withstand
forces up to a certain level and flnally to collapse locally if exposed to
excessive, continuous overloading. Vital sections such as oil storage
compartments, living quarters, drilling and production equipment, conductors
etc. are centrally arranged, these structures being structurally separated
from those sections intended to be subjected to environmental loads.
A further ob~ect of the present invention ls to design a simple,
rigid structure which preferably may be constructed using slip forming
techniques and which may be built and mechanically outfitted in sheltered
waters, preferably inshore.
Thus, according to the present invention, there is provided an
offshore platform structure for lnstallatlon on a sea bed in waters where
lmpact from drifting icebergs may occur, said platform structure comprlsing:
a substructure having an upper portion and a lower portion, said
lower portion being adapted for support by the sea bed and said substructure
having intermediate walls located between said upper portion and said lower
portion;
a superstructure having an end rigidly fixed to said upper portion of
the substructure, said superstructure extending perpendicular to said upper
portion of said substructure, and said superstructure having smaller lateral
dimensions than said substructure;
a deck supported by said superstructure, said deck being supported
above sea level; and
a fender structure rigidly supported at a lower end thereof by said
substructure, said fender structure extending circumferentially around the
superstructure, and sa~d fender structure further being spaced from said
superstructure and structurally separated therefrom along at least a major
portion oE a length of said fender structure.
Preferably, the substructure is circular or polygonal in
cross-section. According to one embodiment, the outer wall and/or the
diaphragm walls are dimensioned to yield locally and ultimately collapse when
exposed to an excessive, continuing impact load caused by a drifting iceberg.
Further, the weight of the structure and optionally the weight of the added
b~llast should be sufficient to keep the platform in position wlthout tilting
when subjected to overturning moments caused by a drifting iceberg.
The fender structure preferably extends upwardly from the
substructure above the sea level~ It should be appreciated, however, that the
fender structure optionally may be terminated below the sea level.
The platform structure according to the present invention is
preferably made of concrete, the substructure, the superstructure and the
fender structure preferably forming an integral monolithic structure, the
horizontal and vertical diaphragm walls forming an integral unit with the
vertical outer and/or inner walls.
The inYention will now be described further by way of example only
and with reference to the accompanying drawings, wherein:
Figure 1 shows schematically a vertical section through a platform in
accordance with the present invention, the section being along line I-I in
Figure 2.
Figure 2 shows a horizontal section through the platform shown in
Figure 1, the section being along the line II-II in Figure l;
Figures 3, 4 and 5 show schematically a vertical section through
three different embodiments of the platform according to the present
invention; and
Figure 6 shows an artist's view, partly in section, of a fully
equipped platform installed on an offshore site.
Figures 1 and 2 shows a platform structure comprising a substructure
1, a superstructure extendi~g vertically upwards from the substructure 1 and a
deck superstructure ~, supported by the superstructure 2 above the sea level.
The substructure 1 has a substantially larger cross-sectional area than that
of the superstructure 2, the substructure 1 extending radially beyond the
-- 3 --
superstructure. The substructure 1 supports a cylindrical ring-shaped fender
structure ~ spaced from and surrounding the superstructure 2. ~he object of
the fender structure 4 i5 to protect the superstruc~ure 2 from iceberg impac~s.
The substructure is buoyant and is divided into a plurality of
segmentally-formed cells or compartments 5 by means of vertical, radially
extending partition walls 6 and one or more concentrically arranged annular
walls 7. At its lower end, the substructure 1 is provided with a downwardly
protruding skirt structure 8~ locating the platform structure upon the sea
bed. Centra~ly through the platform and the skirt structure 8, a vertical
well 9 ls arranged, extending down to the sea bed. As previously pointed out
the substructure 1 rigidly supports and is coaxial with the superstructure 2.
The lower end of the superstructure 2 is integral with the substructure 1,
whereby the substructure 1 and the superstructure form a monolithic unit. The
top 10 of the substructure 1 serves as bottom slab 11 for the superstructure.
The superstructure 2 further comprises a circumferential, cylindrical outer
wall 12 and a cylindrical inner wall 13, the latter also servlr~ as an
enclosure for a well 14, which is coextenslve wlth the corresponding central
well 9 in the substructure. Secured to the upper end 15 of the
superstructure 2~ is a deck superstr~lcture 16 supporting a drilling rig 17,
etcO The deck superstructure 16 is provlded with a central opening 18
colnciding wlth the well 14 of the superstructure 2 and the well 9 of the
superstructure, thereby for~ing a vertical well extending from the sea bed to
~he deck superstructure 16. Said vertlcally extendin~ well contains
conductors, drilling strings, etc. dependent on the functions which the
platform structure is to serve.
According to the embodiment shown in Figures 1 and 2 the platform
structure serves as a platform for production of oil and gas. Consequently
the or each peripherally arranged cell of the superstructure 2 is used for
storing hydrocarbons and is provided with a separate bottom slab 11 whlch may
form an integral unit with the top plate 10 of the substructure. The cells 5
in the substructure 1 may, when the platform structure is installed on the sea
bed, be filled with the appropriate weight of ballast.
The outer cylindrical wall 19 of the cylindrical ring-shaped fender
structure 4 is continuous with the corresponding outer, peripheral wall of the
substructure 1. Further, the inner cylindrical wall 20 of the fender
stntcture 4 is continuous with a corresponding annular wall 7 in the
substructure 1. The inner cylindrical wall 20 is arranged in spaced relation
with respect to the outer wall 12 of the superstructure 2. The radial
distance between the inner wall 20 of the fender structure 4 and the outer
wall 12 of the superstructure 2 is such that forces or impact energy absorbed
by the fender structure 4 may not be transferred to the superstructure 2. The
walls 19 and 20 of the fender structure are rigidly interconnected by means of
horizontal and/or vertical diaphragm walls 21 and 22, respectively, the
vertical walls 22 being radially extending. Consequently, the fender
structure is divided into a plurality of separate compartments or cells 23.
The upper end of the fender structure 4 may preferably function as a
platform deck and/or support sections of the deck superstructure. Further,
equipment such as cranes 24, mooring winches, etc. may be arranged on top of
the fender structure 4. The top end 15 of the superstructure 2 may be
provided with a radially extendlng, horizontal top slab (not shown), extending
laterally to the top 25 of the fender structure 4. The intermediate space 26
between the supporting superstructure 2 and the fender structure 4 is
preferably filled with sea water.
Figure 3 shows a second embodiment of the present invention which
2Q basically resembles the embodiment shown in Figures 1 and 2. A major
difference is, however, that the bottom slab of the superstructure 2 and the
top of the substructure 1 are formed as a single uniform plate with a
thickness corresponding to the thickness of the remaining sections of the top
slab of the substructure. Further, the radius of the fender structure is
increased. As in Figure 1, the fender structure extends above the sea level.
According to the embodiment shown in Figure 4, the substructure 1
extends laterally beyond the fender structure 4, forming an annular base
structure 28 which improves the stability of the structure both during
construction and towing and in its installed condition on the sea bed.
The fender structure 4, according to the embodiment shown in
Figure 5, is termlnated below the sea level, allowing small icebergs to float
over the fender structure, impacting the superstructure 2. According to such
an embodiment it becomes easier to handle, transport and lift equipment
suppled by surface supply vessels, the supply vessel being moved to the
platform structure durlng the loading and/or unloading stage. As shown in
_ 5 _
Figure S, the superstructure 2 is formed of a single column while a separate
storage tank 29 is concentrically arranged around the column 20 The storage
tank 29 forms an integral unit both with the column 2 and the substructure 1.
The outer diameter o~ the tank 29 is substantially less than that of the inner
surface of the fender structure 4. Further, the storage tank 29 e~tends to
well below the upper end of the fender structure. A further major advantage
of such embodiment is that the wave resistance is considerably reduced due to
the reduced cross-sectional area at the water line.
Figure 6 discloaes an embodiment which substantially corresponds to
the embodiment shown in Figures 1 and ~. Figure 6 shows an artist's
impression of such a platform structure installed on the sea bed and partly
cut away. As shown in Figure ~, ~he deck superstructure 3 comprises a
plurality of girders or concrete beams 30, extending radially out from the top
of the superstructure 2 to the top section 25 of the fender structure 4. The
horizontal diaphragm walls 21 are shown while the vertical, radially extending
walls 22 which are flush with the corresponding vertical walls of the
substructure are not shown.
Apart from Figure 5, the Figures show a platform structure where the
superstructure 2 is substantially formed as a plurality of concentrically
arranged cylindrical cells. Figure 5 shows an embodiment wherein the deck
supe structure 3 is supported by a single column. It should be appreciated,
however, that the superstructure may be formed of a plurality of separate
columns being arranged in spaced relation. At least one of said columns may
function as a well extendlng from the sea bed to the deck superstructure S,
housing conductors, risers, etc.
If the superstructure 2 is provided with a separa~e bottom slab 11,
the superstructure 2 should be equipped with means for ballasting, preferably
arranged in the vicinity of the slab 11.
The platform structure may be constructed in conventional manner,
i.e. the construction of the bottom raft is executed ln a dry dock, whereupon
the raft is towed out to a deep water site where the remaining construction
work is performed, preferably by means of slip forming, the structure being
successively ballasted to maintain a constant free board during the
constructional stage. Finally, the deck superstructure is constructed, either
by building the girders or beams in situ or by floatîng the deck
-- 6
superseructure 5 over the platform and then deballasting the structure to lift
the deck superstructure off the barges on which the deck superstructure is
transported. Subse~uent to ~he installation of the deck superstructure, the
platform ls towed out to the offshore site and installed by adding ballast to
the various compartments, thereby lowering the structure down on to the sea
bed to partly penetrate it. Sea water :ls preferably used as ballast.
Optionally, sand may be applied.
Alternativel~, the various sections of the platform may be
constructed separately, and assembled in a floating state, either in sheltered
waters inshore or off~hore at the operational site. Openings, w~ch may
preferably be closed, may be arranged in the cell wall(s) between the various
cells or groups of contiguous cells. Further9 a pipe system incorporating
pumps, valves etc. may be incorporated, thereby enabling the cells to be
ballasted or deballasted using sea water.
The platform structure and in particular the fender structure is
designed to be able to wlthstand the maximum impact forces that may occur.
However, if the impact forces imposed on the fender structure exceed the
maximum expected impact forces, the outer section(s) of the fender structure
is or are allowed to be locally deformed absorbing said impact forces.
Optionally, even the inner wall of the fender structure may be locally
deformed. Due to the lateral dlstance between the fender structure and the
superstructure incorporated vital and fragile parts, the latter structure is
still protected against the impact forces. It should further be appreciated
that the design and dimensions are based on the so-called "weak-link
principle - i.e. the platform structure is to be forced laterally along the
sea bed if the impact forces become excessively high. Further, the platform
structure is given such weight and dimensions that the platform is prevented
from tilting lf the impact forces become exceedingly high.
