CA1181601A - Offshore platform structure intended to be installed in arctic waters, subjected to drifting icebergs - Google Patents
Offshore platform structure intended to be installed in arctic waters, subjected to drifting icebergsInfo
- Publication number
- CA1181601A CA1181601A CA000397321A CA397321A CA1181601A CA 1181601 A CA1181601 A CA 1181601A CA 000397321 A CA000397321 A CA 000397321A CA 397321 A CA397321 A CA 397321A CA 1181601 A CA1181601 A CA 1181601A
- Authority
- CA
- Canada
- Prior art keywords
- superstructure
- substructure
- platform structure
- fender
- offshore platform
- 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.)
- Expired
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
- E02B17/0017—Means for protecting offshore constructions
- E02B17/0021—Means for protecting offshore constructions against ice-loads
Abstract
Abstract An offshore platform structure, particularily intended to be installed in waters where drifting iceberg frequently appear, the platform structure being intended to be founded in a sea bed and comprises a substructure,a superstructure rigidly affixed to the substructure and extending vertically up above the sea level supporting a deck superstructure at its upper end. The horizontal cross-sectional area of the substructure is substant-ially greater than that of the superstructure. The substructure rigidly supports a fender structure, the fender structure com-prising an outer peripherally arranged wall and an inner cylin-drical wall the inner and outer wall being rigidly interconnected by means of a plurality of vertical and/or horizontal partition walls, dividing the fender structure into a plurality of cells or compartments. The fender structure is arranged in spaced relation with respect to the superstructure.
Description
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 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 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.
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 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 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.
Claims (14)
PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. An offshore platform structure for installation on a sea bed in waters where impact from drifting icebergs may occur, said platform structure comprising:
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 said fender structure further being spaced from said superstructure and structurally separated therefrom along at least a major portion of a length of said fender structure.
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 said fender structure further being spaced from said superstructure and structurally separated therefrom along at least a major portion of a length of said fender structure.
2. An offshore platform structure as claimed in claim 1, wherein the fender structure is of such design and dimensions that local deformations or local collapse of the cell walls may occur without causing a total collapse, if subjected to excessive impact force imposed by a drifting iceberg.
3. An offshore platform structure as claimed in claim 1 wherein the substructure is of such dimensions and weight that the structure may rest stably on the sea bed without tilting even when subjected to excessive impact forces by a drifting iceberg.
4. An offshore platform structure as claimed in claim 1, wherein the fender structure extends up above the sea bed and terminates at a level corresponding to the level of the deck.
5. An offshore platform structure as claimed in any of claims 1 to 3, wherein the superstructure comprises a cylindrical cell having a bottom slab, the cylindrical cell being supported by and being affixed to the upper portion of the substructure.
6. An offshore platform structure as claimed in claim 4, wherein the superstructure comprises two concentrically arranged cylindrical walls, forming a cell the lower end of each wall being rigidly affixed to the upper portion of the substructure, said upper portion of said substructure serving as a bottom slab for the cell formed between said two concentrically arranged cylindrical walls.
7. An offshore platform structure as claimed in claim 6, wherein the platform structure is provided with a central well extending from the sea bed to the deck.
8. An offshore platform structure as claimed in claim 7, wherein the deck comprises a plurality of horizontal beams extending laterally at least to an upper end of the fender structure.
9. An offshore platform structure as claimed in claim 1, wherein the substructure comprises a peripherally arranged, cellular section extending radially out beyond the periphery of the fender structure.
10. An offshore platform structure as claimed in claim 8, wherein the fender structure supports the beams forming the deck.
11. An offshore platform structure as claimed in claim 9, wherein the fender structure terminates below the sea level when installed on the offshore site.
12. An offshore platform structure as claimed in any of claims 1 to 3, wherein the superstructure which supports the deck is centrally located and consists of a cylindrical column and a cylindrical storage cell, supported by the substructure and surrounding the column, the superstructure being arranged in spaced relation with respect to the fender structure.
13. An offshore platform structure as claimed in claim 1, wherein the fender structure comprises an external wall and an internal wall, said fender structure being divided into a plurality of separate cells by means of horizontal and vertical diaphragm walls.
14. An offshore platform structure for installation on a sea bed in waters where impact from drifting icebergs may occur, said platform structure comprising:
a substantially cylindrical substructure resting on a sea bed;
a substantially cylindrical superstructure extending vertically upwardly from an upper portion of said substructure;
a lower end of the superstructure rigidly fixed to the substructure;
a deck supported by the superstructure, said deck being above the sea level, the substructure having a larger radial dimension than that of the superstructure;
a fender structure, said fender structure being supported by said substructure and said fender structure comprising a substantially cylindrical inner wall of substantially greater diameter than that of the superstructure, an outer peripheral wall, and a plurality of vertical and horizontal diaphragm walls rigidly connecting the inner and outer peripheral walls of said fender structure for dividing the fender structure into a plurality of separate cells; and said superstructure comprising a substantially horizontal bottom wall and said substructure having a substantially horizontal top wall portion supporting said bottom wall of the superstructure.
a substantially cylindrical substructure resting on a sea bed;
a substantially cylindrical superstructure extending vertically upwardly from an upper portion of said substructure;
a lower end of the superstructure rigidly fixed to the substructure;
a deck supported by the superstructure, said deck being above the sea level, the substructure having a larger radial dimension than that of the superstructure;
a fender structure, said fender structure being supported by said substructure and said fender structure comprising a substantially cylindrical inner wall of substantially greater diameter than that of the superstructure, an outer peripheral wall, and a plurality of vertical and horizontal diaphragm walls rigidly connecting the inner and outer peripheral walls of said fender structure for dividing the fender structure into a plurality of separate cells; and said superstructure comprising a substantially horizontal bottom wall and said substructure having a substantially horizontal top wall portion supporting said bottom wall of the superstructure.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000397321A CA1181601A (en) | 1982-03-01 | 1982-03-01 | Offshore platform structure intended to be installed in arctic waters, subjected to drifting icebergs |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000397321A CA1181601A (en) | 1982-03-01 | 1982-03-01 | Offshore platform structure intended to be installed in arctic waters, subjected to drifting icebergs |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1181601A true CA1181601A (en) | 1985-01-29 |
Family
ID=4122189
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000397321A Expired CA1181601A (en) | 1982-03-01 | 1982-03-01 | Offshore platform structure intended to be installed in arctic waters, subjected to drifting icebergs |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1181601A (en) |
-
1982
- 1982-03-01 CA CA000397321A patent/CA1181601A/en not_active Expired
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|---|---|---|---|
| MKEC | Expiry (correction) | ||
| MKEX | Expiry |