CA2025417A1 - A gravity base structure for an offshore platform resisting to icebergs - Google Patents
A gravity base structure for an offshore platform resisting to icebergsInfo
- Publication number
- CA2025417A1 CA2025417A1 CA 2025417 CA2025417A CA2025417A1 CA 2025417 A1 CA2025417 A1 CA 2025417A1 CA 2025417 CA2025417 CA 2025417 CA 2025417 A CA2025417 A CA 2025417A CA 2025417 A1 CA2025417 A1 CA 2025417A1
- Authority
- CA
- Canada
- Prior art keywords
- wall
- walls
- caisson
- icebergs
- structure according
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Abandoned
Links
Classifications
-
- E—FIXED CONSTRUCTIONS
- E02—HYDRAULIC ENGINEERING; FOUNDATIONS; SOIL SHIFTING
- E02B—HYDRAULIC ENGINEERING
- E02B17/00—Artificial islands mounted on piles or like supports, e.g. platforms on raisable legs or offshore constructions; Construction methods therefor
- E02B17/02—Artificial islands mounted on piles or like supports, e.g. platforms on raisable legs or offshore constructions; Construction methods therefor placed by lowering the supporting construction to the bottom, e.g. with subsequent fixing thereto
- E02B17/025—Reinforced concrete structures
-
- E—FIXED CONSTRUCTIONS
- E02—HYDRAULIC ENGINEERING; FOUNDATIONS; SOIL SHIFTING
- E02B—HYDRAULIC ENGINEERING
- E02B17/00—Artificial islands mounted on piles or like supports, e.g. platforms on raisable legs or offshore constructions; Construction methods therefor
- E02B17/0017—Means for protecting offshore constructions
- E02B17/0021—Means for protecting offshore constructions against ice-loads
-
- E—FIXED CONSTRUCTIONS
- E02—HYDRAULIC ENGINEERING; FOUNDATIONS; SOIL SHIFTING
- E02B—HYDRAULIC ENGINEERING
- E02B17/00—Artificial islands mounted on piles or like supports, e.g. platforms on raisable legs or offshore constructions; Construction methods therefor
- E02B2017/0056—Platforms with supporting legs
- E02B2017/0069—Gravity structures
-
- E—FIXED CONSTRUCTIONS
- E02—HYDRAULIC ENGINEERING; FOUNDATIONS; SOIL SHIFTING
- E02B—HYDRAULIC ENGINEERING
- E02B17/00—Artificial islands mounted on piles or like supports, e.g. platforms on raisable legs or offshore constructions; Construction methods therefor
- E02B2017/0056—Platforms with supporting legs
- E02B2017/0073—Details of sea bottom engaging footing
- E02B2017/0086—Large footings connecting several legs or serving as a reservoir for the storage of oil or gas
Landscapes
- Engineering & Computer Science (AREA)
- General Engineering & Computer Science (AREA)
- Mechanical Engineering (AREA)
- Civil Engineering (AREA)
- Structural Engineering (AREA)
- Devices Affording Protection Of Roads Or Walls For Sound Insulation (AREA)
- Revetment (AREA)
Abstract
ABSTRACT
The platform comprises a caisson (3) which rests on the seabed and supports a deck (2) by means of columns (4). The caisson incorporates a ring-wall for protection against icebergs which is formed from a peripheral double wall (6, 7) braced by partition walls (8) in a lattice structure of prisms. According to the invention, the outer wall (7) is smooth so as to prevent the formation of tangential forces upon impact by icebergs, and reinforcement means are provided in the thickness of this wall. These reinforcement means comprise gussets at the connection with the partition walls (8), and, if required, prestressing means.
The platform comprises a caisson (3) which rests on the seabed and supports a deck (2) by means of columns (4). The caisson incorporates a ring-wall for protection against icebergs which is formed from a peripheral double wall (6, 7) braced by partition walls (8) in a lattice structure of prisms. According to the invention, the outer wall (7) is smooth so as to prevent the formation of tangential forces upon impact by icebergs, and reinforcement means are provided in the thickness of this wall. These reinforcement means comprise gussets at the connection with the partition walls (8), and, if required, prestressing means.
Description
5 '~
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 5 ~ . 7 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 J; ~
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 2 ~ -~J .j ~ 7 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 ~, 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 2 ~ L~
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.
, .
2 ~ ~ 3~ J
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.
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 5 ~ . 7 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.
~ J~
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 J; ~
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 2 ~ -~J .j ~ 7 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 ~, 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 2 ~ L~
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.
, .
2 ~ ~ 3~ J
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.
Claims (7)
1. A gravity base structure for an offshore platform which is resistant to the impacts of icebergs, the said structure comprising a concrete monolithic caisson (3) which incorporates an upper slab (16) and a lower slab (5) resting on a seabed, a ring-wall formed by two cylindrical and concentric peripheral walls (6, 7), an inner and an outer, respectively, provided for receiving the impacts of icebergs, these two walls (6, 7) being arranged substantially vertically between the slabs (5, 16) and rigidly connected with them, and by vertical partition-walls (8) being arranged between the two peripheral walls so as to connect them in a lattice structure of prisms, characterized in that the outer wall (7) has an external surface which is substantially smooth and has no unevennesses, allowing the iceberg to slide along the caisson so as to deviate some of its kinetic energy and to distribute its impact over a large surface area, and comprises, in its thickness, reinforcement means (13, 14) capable of ensuring the resistance of the structure to both radial and oblique impacts of icebergs.
2. Structure according to Claim 1, characterized in that the reinforcement means comprise locally thickened sections (13) to increase the resistance of the system of supporting the outer wall (7) on the vertical partition-walls (8).
3. Structure according to either of Claims 1 and 2, characterized in that the reinforcement means comprise means for prestressing (14) the outer wall (7).
4. Structure according to Claim 3, characterized in that the said prestressing means comprise cables (14) tensioned along substantially horizontal passages provided in the thickness of the outer wall (7), each passage running close to the external face of the wall at locations of supports by the partition walls (8) and to the internal face of this wall in the middle span.
5. Structure according to any one of Claims 1 to 4, characterized in that it comprises hollow cylindrical volumes (26 to 29) positioned between the inner wall (6) and compartments (4; 17 to 20) inside the caisson over its entire height, these volumes having a generally curved horizontal section so as to render these volumes flexible with regard to forces generated on the peripheral ring-walls by impacts of icebergs, in order to prevent the transmission of forces to the partitions of the inner compartments.
6. Structure according to Claim 5, characterized in that it comprises rigidifying walls (30 to 33) for the compartments inside the caisson.
7. Structure according to any of the preceding claims, characterized in that the prisms of the lattice structure are triangular prisms.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| FR8912273 | 1989-09-19 | ||
| FR8912273A FR2652104B1 (en) | 1989-09-19 | 1989-09-19 | GRAVITY STRUCTURE OF ICEBERG RESISTANT MARINE PLATFORM. |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2025417A1 true CA2025417A1 (en) | 1991-03-20 |
Family
ID=9385636
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 2025417 Abandoned CA2025417A1 (en) | 1989-09-19 | 1990-09-14 | A gravity base structure for an offshore platform resisting to icebergs |
Country Status (2)
| Country | Link |
|---|---|
| CA (1) | CA2025417A1 (en) |
| FR (1) | FR2652104B1 (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US6371695B1 (en) | 1998-11-06 | 2002-04-16 | Exxonmobil Upstream Research Company | Offshore caisson having upper and lower sections separated by a structural diaphragm and method of installing the same |
Family Cites Families (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4422804A (en) * | 1981-12-10 | 1983-12-27 | Mobil Oil Corporation | Gravity base of offshore production platform with ice-pentrating peripheral nose sections |
| US4470725A (en) * | 1982-03-01 | 1984-09-11 | Ingenior Thor Furuholmen A/S | Offshore platform structure intended to be installed in arctic waters, subjected to drifting icebergs |
| US4478537A (en) * | 1982-07-02 | 1984-10-23 | Brian Watt Associates, Inc. | Arctic caisson system |
| GB2136033A (en) * | 1983-02-28 | 1984-09-12 | Taylor Woodrow Const Ltd | Off-shore structures |
| FR2615217B1 (en) * | 1987-05-13 | 1990-12-21 | Doris Engineering | GRAVITY STRUCTURE OF A MARINE PLATFORM FOR ARCTIC AREA |
-
1989
- 1989-09-19 FR FR8912273A patent/FR2652104B1/en not_active Expired - Fee Related
-
1990
- 1990-09-14 CA CA 2025417 patent/CA2025417A1/en not_active Abandoned
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US6371695B1 (en) | 1998-11-06 | 2002-04-16 | Exxonmobil Upstream Research Company | Offshore caisson having upper and lower sections separated by a structural diaphragm and method of installing the same |
Also Published As
| Publication number | Publication date |
|---|---|
| FR2652104A1 (en) | 1991-03-22 |
| FR2652104B1 (en) | 1993-01-08 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| EEER | Examination request | ||
| FZDE | Dead |