Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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A gravity base structure for an offshore platform
reslsting to lceberys.
The present lnventlon relates to a gravlty base
structure for an offshore platform and, more partlcularly,
to a structure of thls type deslgned to be lnstalled ln
waters lnfested with icebergs and to withstand collisions
wlth the latter.
French Patent Application No. 2,615,217,, flled by
the appllcant, dlscloses a platform structure of this type
whlch comprises a concrete monolithic caisson suported by
a lower slab resting on the seabed, this caisson having an
array of vertical teeth whlch form defenslve elements
capable of withstanding the impacts of icebergs and of
absorbing the energy transmitted, the caisson also
comprlsing an upper slab directly or lndirectly supporting
the deck of the platform and, between the two slabs,
a peripheral double wall whlch ls formed from two
concentrlc walls lnterconnected by vertical partltion
walls whlch form a framework structure of triangular
prisms, the outer concentric wall carrylng the defensive
elements.
When an iceberg collldes wlth the structure the
defensive elements in the shape of teeth crush the surface
of the iceberg, disslpate the energy of the impact and
transmit the forces resulting from the impact to the
structural members of the perlpheral double wall.
A platform of thls type wlthstands well radial
lmpacts. However, an lceberg may strlke the platform at an
angle and generate forces perpendlcular to certain faces
of the defensive elements whlch extend outwardly from the
outer wall of the caisson. These faces are then subjected
to non-centripetal forces, whlch result ln a conslderable
overall torsion of the platform, as well as conslderable
localized stresses, ln particular in the defensive
elements and thelr supports. When the speclflcation
requires that the platform be able to withstand such ice
forces applled to elements of the outer surface of the
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calsson and disposed ln ths most unfavourable way, this
leads to thicknesses of concrete and relnforclng steel for
the defenslve elements which are excessive, or even
impossible to install.
The obJect of the present inventlon ls therefore to
produce a gravlty base structure for an offshore platform
ln waters lnfested with icebergs, whlch ls deslgned so as
to avoid, or at least to considerably reduce, the
generation of non-centripetal forces applled to the
platform upon lmpact wlth an iceberg movlng in a non-
radial direction relative to this platform.
Another obJect of the present invention is to
produce a platform of this type which ls simple to
construct and economically viable.
These obJects of the invention, as well as others
which will become evident later on in the present
description, are achieved with a gravity base structure
for an offshore platform which is resistant to the impacts
of icebergs, the sald structure comprislng a concrete
monollthlc calsson which lncorporates an upper slab
resting an a seabed, a ring-wall formed by two cyllndrlcal
and concentric perlpheral walls, an outer and an lnner,
respectively, provlded for recelvlng the lmpacts of
icebergs, these two walls being arranged substantlally
vertically between these slabs and rigidly connected wlth
them, and by vertical partition walls being arranged
between the two peripheral walls so as to connect them in
a lattice structure of prisms. According to the present
invention, the outer wall has an external surface, which
is subtantlally smooth and has no unevennesses, allowlng
the lceberg to sllde along the caisson so as to deviate
some of its klnetic energy and to distribute its impact
over a large surface area, and comprises, in its
thickness, reinforcement means capable of ensuring the
resistance of the structure to both radial and oblique
impacts of icebergs.
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Accordlng to a preferred embodiment of the
lnvention, the reinforcement means comprise locally
thickened sections, for example in the form of gussets, to
lncrease the resistance of the system of supportlng the
outer wall on the vertlcal partitlon walls.
The relnforcement means may also comprise means for
prestressing the outer wall.
According to another feature of the present
invention, if a compartmentation of the internal volume of
the caisson ls required, hollow cylindrical volumes are
provided, positioned between the inner wall and
compartments inside the caisson over its entire height,
these hollow volumes having a generally curved horizontal
section so as to render these volumes flexible with regard
to forces generated on the ring-wall by impacts of
icebergs, in order to prevent the transmission of these
forces to the partitions of the inner compartments.
According to yet another feature of the present
invention, the structure comprises strenthened walls of
other internal compartments capable of containing fluids
to be stored inside the caisson. These walls are arranged
so as to allow one or more compartments intended for
storing the fluids to be completely emptied.
Other features and advantages of the present
invention will become evident upon reading the description
below and from the attached drawing, in which:
- Figure 1 ls a perspective view, with a partial
cutaway, of a platform structure according to the
invention,
30- Figure 2 is a cross-sectional view taken across
the caisson of the platform in Figure 1, and
- Figure 3 is an enlargement of the circled part in
Figure 2, which illustrates in detail the construction of
the ring-wall of the caisson of the platform according to
the invention.
Figure 1 shows an oil-drilling platform structure 1
of the type described in the abovementloned French patent
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appllcatlon and modlfied according to the present
invention, as will be seen belo~ in detail.
The structure rest~ on the seabed and comprises a
deck 2 on which the technical facilltles and accommodatlon
modules are arranged. Since the platform ls to be uses ln
cold regions, the Artic for example, the facillties are
covered and air-conditioned.
The structure comprlses a calsson 3 from the bottom
of which one or more compartments rise in the form of
columns 4 supporting the deck 2, above a slab 16 which
encloses the upper part of the caisson.
The caisson 3 is a monolith, generally cylindrical
in shape. It has a lower slab 5 which rests on the seabed
and from which, in an area close to its periphery, a
double wall rises which is formed from two concentric
walls 6, 7 and is connected to the upper slab 16. The two
concentric walls 6, 7 are interconnected by vertical
partition walls 8 which form a lattice structure of
prisms. This double peripheral wall forms an encloslng
protectlve wall agalnst the impacts of icebergs.
According to the embodlment shown, the concentric
walls 6, 7 have the shape of regular polygons and the
apexes of one of the polygons are angularly offset by half
a side relative to the apexes of the other polygon. The
partition walls 8 connect the apexes of one polygon to the
ad~acent apexes of the other polygon.
These features are clearly shown in Figure 2, which
is a cross-sectlon of Flgure 1. Thus, the wall 6 whlch
forms the inner polygon has lts apexes 9, 10, ... offset
by half a side relative to the apexes 11, 12 of the wall 7
whlch forms the outer polygon. Each of the apexes 9, 10,
... of the inner polygon is connected to the ad;acent
apexes 11, 12 of the outer polygon by partition walls 8
which form a lattice framework of triangular prisms.
Accordlng to an important feature of the platform
structure to the invention, the external surface of the
outer wall 7 is substantlally smooth and, therefore, has
no unevennesses or protuberances. Thls external wall may
be a cyllnder of revolution or, as ln the embodlment
shown, a prim wlth a large number of faces so as to be
analogous to the cylinder of revolution circumscribed by
thls wall. The structure according to the invention
dlffers clearly ln thls respect from that of the
abovementioned patent appllcation wh$ch comprises a
caisson, the outer wall of which is equipped with
protruding defensive elements in the shape of pointed or
rounded teeth (see Figures 2 and 4 of this document). In
the structure according to the invention, the absence of
such defensive elements allows an iceberg striking the
caisson askew, in other words in a non-radial direction,
not to generate the tangential forces on this caisson
resulting, as in the structure of the abovementioned
patent application, from the impact of the iceberg on
parts of the surface of the defensive elements generating
non-centripetal forces. These defensive element parts
receive then the full impact of the iceberg. In contrast,
in the structure according to the invention, the iceberg
advantageoulsy slides along the caisson so as to deviate
part of its kinetic energy. In addition, the residual
kinetic energy to be received will be distributed over a
larger surface area. As seen above, the strengthening of
the defensive elements and of the caisson to make them
resistant to such "skew" iceberg impacts, in the most
unfavourable circumstances envisaged by the
specifications, results in excessive quantities of
materials required, and even in making construction
impossible.
According to an essential feature of the platform
structure according to the invention, the outer wall 7 of
the double wall of the caisson 3 has, in addition to an
external surface which is substantially smooth and has no
unevennesses, reinforcement means provided in its
thickness so as to ensure the mechanical resistance of the
structure to both radial and oblique impacts of icebergs.
As shown in Figure 3, these reinforcement means
comprlse gussets 13 which increase the resistance of the
system for supporting the outer wall 7 on the partition
walls 8.
These relnforcement means may additlonally comprlse
prestressing means which consist of prestressing cables,
such as that designated 14 in Flgure 3. Each cable ls
tensloned along a passage provlded in a horizontal section
of the outer wall. As shown in Figure 3, this passage is
preferably wave-shaped in such a section so as to compress
the outer wall at the places where the bending moments
induced by an iceberg impact could create tensions in the
concrete of the wall, in other words in the vicinity of
the external face of the wall at locations of supports by
the partition walls 8, and in the vinicity of the internal
face of the wall in the middle span.
The gussets, which may be associated with the above-
described prestressing means, also improve resistance to
shear forces and allow optimization of the use of
materials according to the technique known in art.
Gussets of an appropriate size may also reinforce
the connection of the partition walls 8 to the inner wall
6, with or without the use of associated prestressing
means. -
All these reinforcement means may supplement that
which consists, of course, in increasing the thickness of
the partition walls 8 and of the walls 6, 7 appropriately.
However, they allow this increase to be considerably
reduced and to permit so substantial material savings
compared to the solution given in the abovementioned
patent application.
It will be noted that the platform structure
according to the invention retains the arrangement of
these partition walls 8 in a lattice framework of prisms,
as already used in the platform of the abovementioned
patent application, since this framework is particularly
well suited to taking up the considerable forces, which
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are generated upon lcebergs lmpacts, by a contlnuous
network of members operatlng either ln tenslon or ln
compression. The lattice arrangement structure of the
framework of prisms is particularly advantageous if
compared with a structure of radial partitions in the
double wall, a structure in which only the partitions
operate mainly in tension/compression, while the two
walls, outer and inner, operate malnly in bending mode,
which requlres larger quantities of concrete and
reinforcing steel and considerable prestressing means.
As shown in Figure 2, the platform according to the
invention has, inside the protective ring-wall formed by
the double wall 6, 7, in addition to the compartments
forming the columns 4 for supporting the deck 2,
cylindrical compartments 17 to 25 which extend vertically
between the two slabs of the caisson. These compartments
are intended to receive, for example, crude oil, other
fluids, or the ballast for the platform, such as:
seawater, sand, heavy metal ore, etc... According to an
advantageous feature of the structure according to the
invention, the walls of these compartments are ln practice
dissociated from the transmission of the forces resulting
from the impacts of icebergs since they are connected to
the rlng-wall only through the caisson slabs or by the
casings of the volumes 26 to 29 of the platform according
to the invention (see Figure 2) each of which is
positioned, for example, between the wall 6 and a column
4.
According to another feature of the invention, each
of these volumes 26 to 29 is hollow, cylindrical about a
vertical generatrix and has a horizontal cross-section in
the shape of a curved shell. The sections of these
volumes, shown in Figure 2 merely by way of example, have
the shape of a flexible circular tube partially squashed
along a diameter. These volumes are substantially flexible
with regard to transverse forces. Thus, when the double
wall 6, 7 receives an iceberg impact, only a small part of
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the energy of the lmpact is transmitted to the volume
which absorbs it elastlcally and prevents lt to be
transmitted to the ad~acent column 4 and to the outer
compartments. The latter are thus effectively protected
from any deterioration by the dis~unction of the double
wall and the column, established by the flexible volume
positioned between them.
The presence of these flexible volumes 26 to 29,
which form concrete vertical diaphragms of an appriopriate
curved shape connecting the protective double wall 6, 7
and the inner compartments of the platform, thus ensures
structural continuity whilst at the same time providing
mechanical flexibility which allows almost complete
dis~unction (except at the level of the slabs of the
caisson) and the taking up of the iceberg forces by the
ring-wall alone.
The section of the volumes 26 to 29 could, of
course, take other appropriate shapes, for example a
circle.
Another advantageous feature of the platform
structure according to the invention will also be noticed
in Figure 2, namely the presence of plane walls 30 to 33
which form both parts of partition walls for the
compartments l9 to 21 and rigidifying elements for the
compartments as a whole. Since the compartments 17, 18,
19, 20 are arranged inside (in cross-section) a square
circumscribed by the four columns 4, at a tangent to these
columns and to a central compartment 21, the walls 30 to
33 define this rectangular central compartment 21, whereas
the extensions of these walls, beyond the compartment 21,
meet the columns 4 and the compartments 17, 18, 19 and 20
tangentially. Each of the walls 32, 33 extends from a
point of contact of a compartment/column pair to the point
of contact of a pair symmetrical to the former relative to
a plane 34 of symmetry which is the axis of the caisson of
the platform. The walls 30, 31, beyond their contact
points with the columns 4, are connected by semi-circular
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shells 35, 36. This arrangement of appropriately combined
plane walls and curved shells provldes the rlgldlflcatlon
and the mechanlcal reslstance of the compartments 17 to 21
ln order to allow thelr dralnage or fllllng up
lndependently of each other or of the compartments 22 to
25 wlthout runnlng the rlsk of dlfferentlal hydrostatlc
pressure applled to a wall separatlng two ad~acent
compartments which may cause this wall to break.
The platform structure according to the invention
allows numerous advantages, the essential one consistlng
of a substantlal reduction, or even a cancellation of the
tangential forces generated on the platform structures
having defensive elements protruding from the external
wall of the protective ring-wall ln the event of collision
with an iceberg moving in a non-radial direction.
A saving in materials results therefrom, of the
order of 10% relative to a solution of the type described
in the abovementioned patent application, as well as a
considerable simplification of the method of construction,
by slipforming of the outer wall of the caisson to the
extent that the reinforcing and prestressing are
considerably simplified as a result of an outer surface
having no protrusions which are difflcult to relnforce.
The ~unction nodes of the outer wall 7 with the partition
walls 8 have no more than four branches, as compared to
slx ln the abovementloned solution.
To the dlrect saving ln the quantities of materials,
there may be added an indirect saving due to the fact that
the structure floats during the construction phases and
that the direct saving ln materlals allows, with the same
buoyancy, the marine stability to be improved by replacing
the weight of the materlals saved ln this way by solid
ballast placed in the bottom of the compartments. This
added marlne stability permits either an increase in the
payload carried on the deck level or a decrease in the
overall diameter of the structure, and hence a further
reduction in the quantities of materials.
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The present lnvention also allows the passage of
risers, J-tubes, and other condults through the peripheral
ring-wall to be lmproved and s:Lmplified since the number
of walls to be crossed is considerably smaller.
The invention is not, of cour~e, limited to the
embodiment described and illustrated which has only been
given by way of example. Thus, although a hollow volume
(26 to 29) is positioned between a column 4 and the wall 6
in this embodiment, this volume could also be installed
between this wall and one of the compartments 17 to 20
without going beyond the scope of the present invention.
