CA1127406A - Arctic multi-angle conical structure having a discontinuous outer surface - Google Patents
Arctic multi-angle conical structure having a discontinuous outer surfaceInfo
- Publication number
- CA1127406A CA1127406A CA315,776A CA315776A CA1127406A CA 1127406 A CA1127406 A CA 1127406A CA 315776 A CA315776 A CA 315776A CA 1127406 A CA1127406 A CA 1127406A
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- Canada
- Prior art keywords
- ice
- angle
- lower portion
- horizontal
- walls
- 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
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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
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- Engineering & Computer Science (AREA)
- General Engineering & Computer Science (AREA)
- Mechanical Engineering (AREA)
- Civil Engineering (AREA)
- Structural Engineering (AREA)
- Earth Drilling (AREA)
- Organic Insulating Materials (AREA)
- Laying Of Electric Cables Or Lines Outside (AREA)
- Insulated Conductors (AREA)
Abstract
ABSTRACT OF THE DISCLOSURE
ARCTIC MULTI-ANGLE CONICAL STRUCTURE
HAVING A DISCONTINUOUS OUTER SURFACE
An offshore structure which is able to withstand the ice forces imposed thereon by impinging ice sheets and other larger masses of ice wherein the structure has an upper conical portion coaxially positioned relative to a lower conical portion. The walls forming both the upper and lower portions are inclined at an angle to the horizontal to receive ice masses moving into contact with the structure. The angle of inclination from the horizontal of the upper portion is greater than the angle of inclination of the lower portion, and the cross-sectional diameter of the upper conical portion is less than that at the top of the lower conical portion so that there exists a step-like section between the upper conical portion and the lower conical portion.
ARCTIC MULTI-ANGLE CONICAL STRUCTURE
HAVING A DISCONTINUOUS OUTER SURFACE
An offshore structure which is able to withstand the ice forces imposed thereon by impinging ice sheets and other larger masses of ice wherein the structure has an upper conical portion coaxially positioned relative to a lower conical portion. The walls forming both the upper and lower portions are inclined at an angle to the horizontal to receive ice masses moving into contact with the structure. The angle of inclination from the horizontal of the upper portion is greater than the angle of inclination of the lower portion, and the cross-sectional diameter of the upper conical portion is less than that at the top of the lower conical portion so that there exists a step-like section between the upper conical portion and the lower conical portion.
Description
.z~
al FIELD OF TtiE_I'1VENTION
02 The present invention relates to offshoce structures for 03 use in arctic and other ice-in~ested ~aters, and, more particu-04 larly, to an offshore structure which is able to withstand the ()5 ~orces imposed thereon by impinging ice sheets ~nd other larger 06 ice masses.
07 ~ACKGROUND OF THE INVENTION
_ . . . _ .
0~ In recent yeacs, offshore exploration and production of 09 petroleum products has been extended into arctic and other ice-infested waters in such locations as northern Alaska and Canada.
I~ ~hese ~aters are genecally covered with vast areas o~ sheet ice 9 12 months or ~ore out of the year. Sheet ice may reach a thickness 13 o~ 5 to 10 feet or more, and may have a compressive or crushinq 1~ st~ength in the ranqe of about.200 to 1000 pounds per square inch.
lS Although appearinq stati~nary, ice sheets actually move laterally 16 with wind and water currents and thus can impose very high forces 17 on any stationary structure in their paths.
la A still moce severe problem encountered in arctic ~aters 19 is the pcesence of larger masses of ice such as pressure ridges, rafted ice or Eloebergs. Pressure ridges are ~ormed when two 21 sepacate sheets o~ ice move toward each other and collide, the overthrusting and crushing o the two interacting ice sheets 3 causing the ~ormation of a pressure ridge. Pressure rid~es can be very large, with lengths of hundreds of feet, widths of more than ~5 a hundred feet and a thickness of up to 50 feet~ Consequently, ~6 pressure rid~es can exert a oroportionally greater force on an off-27 shore structure than ordinary sheet ice; thus, the possibility of ~ pressure ridges causing extensive damage to ~n offahore structure 29 or the catastrophic failure of a structure is very great.
A structure built strong enough to resi~st the crush- ~
31 ing force exerted thereon by impin~ing ice, that is, strong eno~gh 32 to ~er~it the ice to be crushed aaainst the structure, enab1ir)g llZ74~a6 01 the ice to ~low around it, would likel~ be very massi1~e and corre-02 spondingly e~?ensive to construct. Therefore, it has been 03 proposed heretotore that structures which are to be used in ice-04 infested waters should be built with a sloping or ramp-like outer 05 surEace ~ather than with a surface which is vec~ically disposed to 06 the impinging ice. As the ice comes into contact with the sloping 07 outer surface, it is forced upwardly above its normal position 08 which causes the ice to fail in flexure by placing a tensile 09 stress in the ice. Since ice has a flexural strength of about 85 c pounds per square inch, a correspondingly smaller force i5 imposed 11 on the structure as the ice impinging thereon fails in flexure 12 rather than compression.
13 Several forms of conical offshore structures havinq 1~ sloping outer surfaces are ill~strated in a paper by J.V. Danys entitled "Effect of Cone-Shaped Structures on Impact Forces of Ice lG Floes", presented to the First International Conference on Port 17 and Ocean E~ngineering under Arctic Conditions, held at the 1~ Technical University of ~orway, Trondheim, Norway, during Auqust 1~ 13-30, 1971. Another papec of interest in this respect is that ~0 presented by Ben C. Gerwick, Jr., and Ronald R. Lloyd, entitled 21 "Design an~ Construction Procedures for Proposed Arctic Offshore 22 Structures", presented at the Offshore Technology Confecence in ~3 Houston, Texas, April 1970.
24 ~s an ice sheet moves relative to an~ in contact with ~5 the sloping outer surface of a conical structure, it will be 2h elevated along the sloping surface. The e1evation of the ice 27 sheet causes initial cracks to be formed in the sheet, which 28 radiate outwardly from the point of contact. Circumferential
al FIELD OF TtiE_I'1VENTION
02 The present invention relates to offshoce structures for 03 use in arctic and other ice-in~ested ~aters, and, more particu-04 larly, to an offshore structure which is able to withstand the ()5 ~orces imposed thereon by impinging ice sheets ~nd other larger 06 ice masses.
07 ~ACKGROUND OF THE INVENTION
_ . . . _ .
0~ In recent yeacs, offshore exploration and production of 09 petroleum products has been extended into arctic and other ice-infested waters in such locations as northern Alaska and Canada.
I~ ~hese ~aters are genecally covered with vast areas o~ sheet ice 9 12 months or ~ore out of the year. Sheet ice may reach a thickness 13 o~ 5 to 10 feet or more, and may have a compressive or crushinq 1~ st~ength in the ranqe of about.200 to 1000 pounds per square inch.
lS Although appearinq stati~nary, ice sheets actually move laterally 16 with wind and water currents and thus can impose very high forces 17 on any stationary structure in their paths.
la A still moce severe problem encountered in arctic ~aters 19 is the pcesence of larger masses of ice such as pressure ridges, rafted ice or Eloebergs. Pressure ridges are ~ormed when two 21 sepacate sheets o~ ice move toward each other and collide, the overthrusting and crushing o the two interacting ice sheets 3 causing the ~ormation of a pressure ridge. Pressure rid~es can be very large, with lengths of hundreds of feet, widths of more than ~5 a hundred feet and a thickness of up to 50 feet~ Consequently, ~6 pressure rid~es can exert a oroportionally greater force on an off-27 shore structure than ordinary sheet ice; thus, the possibility of ~ pressure ridges causing extensive damage to ~n offahore structure 29 or the catastrophic failure of a structure is very great.
A structure built strong enough to resi~st the crush- ~
31 ing force exerted thereon by impin~ing ice, that is, strong eno~gh 32 to ~er~it the ice to be crushed aaainst the structure, enab1ir)g llZ74~a6 01 the ice to ~low around it, would likel~ be very massi1~e and corre-02 spondingly e~?ensive to construct. Therefore, it has been 03 proposed heretotore that structures which are to be used in ice-04 infested waters should be built with a sloping or ramp-like outer 05 surEace ~ather than with a surface which is vec~ically disposed to 06 the impinging ice. As the ice comes into contact with the sloping 07 outer surface, it is forced upwardly above its normal position 08 which causes the ice to fail in flexure by placing a tensile 09 stress in the ice. Since ice has a flexural strength of about 85 c pounds per square inch, a correspondingly smaller force i5 imposed 11 on the structure as the ice impinging thereon fails in flexure 12 rather than compression.
13 Several forms of conical offshore structures havinq 1~ sloping outer surfaces are ill~strated in a paper by J.V. Danys entitled "Effect of Cone-Shaped Structures on Impact Forces of Ice lG Floes", presented to the First International Conference on Port 17 and Ocean E~ngineering under Arctic Conditions, held at the 1~ Technical University of ~orway, Trondheim, Norway, during Auqust 1~ 13-30, 1971. Another papec of interest in this respect is that ~0 presented by Ben C. Gerwick, Jr., and Ronald R. Lloyd, entitled 21 "Design an~ Construction Procedures for Proposed Arctic Offshore 22 Structures", presented at the Offshore Technology Confecence in ~3 Houston, Texas, April 1970.
24 ~s an ice sheet moves relative to an~ in contact with ~5 the sloping outer surface of a conical structure, it will be 2h elevated along the sloping surface. The e1evation of the ice 27 sheet causes initial cracks to be formed in the sheet, which 28 radiate outwardly from the point of contact. Circumferential
2~ cracks then form and cause the ice sheet to bceak up into we~ge-shaoed pieces. The approximate total force exerted on a conical 31 stcucture then consists primarily of the force required to ~ail 32 the i~pinging ice sheet in flexuce, that is, the force required to ~7~
01 form the initial radial or subsequent circumferelltial cracks, and 02 the Eorce caused by the broken ice pieces riding up on the outer Q3 surface of the stcucture and interacting there-~ith.
04 The force associated with the for~ation of initial and 05 circumferential cracks in the ice sheet is primarily a function of 06 the particular mechanical and geometrical oroperties of the ice 07 impinging on the structure. The ride-up force is due to the 08 broken ice pieces interacting with the structure and thus is depen-09 dent upon the surface area of the structure above the water line.
Therefore, to reduce the total ice forces imposed on a conical 11 structure, it is always desirable to keep the waterline dia~eter 12 of the structure as small as possible.
13 Larger ice masses such as pressure ridges impacting a 14 conically shaped structure will be.lieted along the sloping outer lS surface of the structure to cause the ridges to fail in flexure.
16 As with ice sheets, a radial crack will form in the ridge at the 17 point of impact; the fo~mation of ~ radial crack is followed by 18 the formation of hinge cracks that occur at a relatively ~reater 19 distance rom the structure. As the ridge continues to move into ~0 the structure, it will break into large blocks oE ice which fall 21 away Erom the structure.
22 ~ As indicated above, the force imposed on a struct~re by 23 an impin~3in~ pressure ridge is much greater than that of an 24 impinging ice sheet. ~he approximate total force exerted on a conical structure by a pressure ridge is a combina~ion of the 26 Eorce required to f~il the impinging ridge in flexure and the ~7 force caused by the broken ice pieces, ormed by the failure of 28 the Ice sheet advancing ahead oE the presure ridge, riding up on 29 the outer surface of the structure and interacting therewith. The large blocks of ice formed when a ~ressure ridge fails in flexure 31 tend not to ride up the outer surface of the structure, there-32 fore, the ride-up force is essentially a result of pieces of sheet 33 ice riding up the structure's outer surface.
~74~6 01 Since struct~res located in ~aters in ~Ihich larger ice 02 masses are ~esent are ex~sed to ~elatively ~reater ice ~orces, 03 they must be built strong enough to withstand tnese greater ice ~ orces. Utilizin~ present bottorn-supported conical structure (~`s~ designs requires supportiny the structure by means of a~ditional 06 f~undation support, such as piling; however, this would increase 07 the cost and time of installation of the structure. ~ithout 08 additional foundation support, the structure would have to be made 09 larger and stronger to resist the greater ice forces, which would ln necessitate increasing its waterline diameter. This, however, 11 would increase that component of the total ice force associated 12 with the ride up of ice pieces on the structure, since the ride~up 1`3 orce is proportional to the surface area o the structure a~ove 1~ the waterline. For a very lar.ge cone waterline dia~eter, this component of the force would be substantially greater than the 16 force required to fail the impinging ice in flexure. Addi-17 tionally, as these structures are designed for use in deeper 13 waters, their over-all size would likely increase.
I9 Rccordingly, present conical structures built ~or use in deeper waters and built strong enough to withstand the orces asso-~1 ciated with larger ice masses would be correspondingly more 22 expensive to construct and install. In fact, such structures 23 could be so massive as to ~e impractical and economically prohibi-tive to build. The present invention is directed to an offshore ~5 structure which is able to withstand the forces associa~ed with 26 large impinging ice masses, and at the sa;ne ti~e is feasible Erom 27 an economic and size standpoint.
28 ~ UF THE _NVENTION
29 ~roadly speaking, the present invention comprises an off-shore structure which is designcd ~or operation in ice-infested 31 waters and which is particularl~ suited for use in deeper ~aters,
01 form the initial radial or subsequent circumferelltial cracks, and 02 the Eorce caused by the broken ice pieces riding up on the outer Q3 surface of the stcucture and interacting there-~ith.
04 The force associated with the for~ation of initial and 05 circumferential cracks in the ice sheet is primarily a function of 06 the particular mechanical and geometrical oroperties of the ice 07 impinging on the structure. The ride-up force is due to the 08 broken ice pieces interacting with the structure and thus is depen-09 dent upon the surface area of the structure above the water line.
Therefore, to reduce the total ice forces imposed on a conical 11 structure, it is always desirable to keep the waterline dia~eter 12 of the structure as small as possible.
13 Larger ice masses such as pressure ridges impacting a 14 conically shaped structure will be.lieted along the sloping outer lS surface of the structure to cause the ridges to fail in flexure.
16 As with ice sheets, a radial crack will form in the ridge at the 17 point of impact; the fo~mation of ~ radial crack is followed by 18 the formation of hinge cracks that occur at a relatively ~reater 19 distance rom the structure. As the ridge continues to move into ~0 the structure, it will break into large blocks oE ice which fall 21 away Erom the structure.
22 ~ As indicated above, the force imposed on a struct~re by 23 an impin~3in~ pressure ridge is much greater than that of an 24 impinging ice sheet. ~he approximate total force exerted on a conical structure by a pressure ridge is a combina~ion of the 26 Eorce required to f~il the impinging ridge in flexure and the ~7 force caused by the broken ice pieces, ormed by the failure of 28 the Ice sheet advancing ahead oE the presure ridge, riding up on 29 the outer surface of the structure and interacting therewith. The large blocks of ice formed when a ~ressure ridge fails in flexure 31 tend not to ride up the outer surface of the structure, there-32 fore, the ride-up force is essentially a result of pieces of sheet 33 ice riding up the structure's outer surface.
~74~6 01 Since struct~res located in ~aters in ~Ihich larger ice 02 masses are ~esent are ex~sed to ~elatively ~reater ice ~orces, 03 they must be built strong enough to withstand tnese greater ice ~ orces. Utilizin~ present bottorn-supported conical structure (~`s~ designs requires supportiny the structure by means of a~ditional 06 f~undation support, such as piling; however, this would increase 07 the cost and time of installation of the structure. ~ithout 08 additional foundation support, the structure would have to be made 09 larger and stronger to resist the greater ice forces, which would ln necessitate increasing its waterline diameter. This, however, 11 would increase that component of the total ice force associated 12 with the ride up of ice pieces on the structure, since the ride~up 1`3 orce is proportional to the surface area o the structure a~ove 1~ the waterline. For a very lar.ge cone waterline dia~eter, this component of the force would be substantially greater than the 16 force required to fail the impinging ice in flexure. Addi-17 tionally, as these structures are designed for use in deeper 13 waters, their over-all size would likely increase.
I9 Rccordingly, present conical structures built ~or use in deeper waters and built strong enough to withstand the orces asso-~1 ciated with larger ice masses would be correspondingly more 22 expensive to construct and install. In fact, such structures 23 could be so massive as to ~e impractical and economically prohibi-tive to build. The present invention is directed to an offshore ~5 structure which is able to withstand the forces associa~ed with 26 large impinging ice masses, and at the sa;ne ti~e is feasible Erom 27 an economic and size standpoint.
28 ~ UF THE _NVENTION
29 ~roadly speaking, the present invention comprises an off-shore structure which is designcd ~or operation in ice-infested 31 waters and which is particularl~ suited for use in deeper ~aters,
3~ but not restricteà to such use, in which sheet ice and other 33 _ 5 _ 7~6 larger masses of ice, sucn as pressure ridges, are present.
The offshore structure of this invention includes a lower portion in the shape of a -truncated cone coaxially position-able on top of a base portion. An uppex portion o~ the structure is in the shape of a second truncated cone and is coaxially positionable on top of said lower portion. The walls forming the upper and lower portions of the structure are inclined at an angle to the horizontal to receive ice masses moving relative to and in contact with the structure in order to cause the ice masses to fail in flexure. The angle of inclination from the horizontal of the walls of the upper portion is greater tha~ that of the lower portion, and the cross-sectional diameter of the upper portion is less than that of the top of the lower portion so that there exists a step~like section or discontinuity between the walls of the upper and lower conical portions.
~ The angle of inclination of the walls of the upper portion is between approximately 26 and 70 from the horizon-tal, with the preferred range being between approximately 54 and 58 from the horizontal. The angle of inclination of the walls of the lower portion is between approximately 15 and 25 from the horizontal, with the preferred range being between approximately 19 and 23 from the horizontal.
The above offshore structure configuration permits the structure to be utilized in relatively deeper waters which contain ice sheets and relatively larger ice masses without unnecessarily increasing the mass ~and the cost of a structure.
In accordance with one aspect of this invention there is provided an offshore structure for use in a body of water that contain~ ice masses, comprising a lower portion `Jl`~ ~ substantially in the shape of a first truncated cone sloping ~7~6 inwardly and upwardly so that the walls of said lower portion are inclined at an angle between 15 and 25 to the horizontal to provide a ramp-like surface to receive ice masses moving relative to and in contact with said structure; means for affixin~ said lower portion to the bottom of a body of water;
and an upper portion coaxially positionable above said lower portion, said upper portion substantially in the shape of a second truncated cone sloping inwardly and upwardly so that the walls of said upper portion are inclined at an angle between 26 and 70 to the horizontal to provide a ramp-like surface to receive ice masses moving relative to and in contact with said structure, the cross-sectional diameter of the base of the second truncated cone forming said u,pper portion being less than the cross-sectional diameter at the IS top of the first truncated cone forming said lower portion so that there exists a step~like section between the walls of s~id upper portion and the walls.of said lower portion.
In accordance with another aspect of this invention there is provided a marine structure for use in a body of water that contains ice masses, comprising a base portion; means for affixing said base portion to the bottom o a body`of water; a lower portion coaxially positionable on top of said base portion for joining thereto, said lower portion substantially in the shape of a first truncated cone sloping inwardly and upwardly so that the walls of said lo~er portion are inclined at an angle between 15 and 25 to the horizontal to provide a ramp-like surface to receive ice masses movin~ relat.ive to and in contact with said structure; and an upper portion coaxially positionable on top of said lower portion for joininy thereto, said upper portion substantially i~ the shape of a second truncated - 6a~-.
cone sloping inwardly and upwardly so that the walls of saidupper portion are inclined at an angle between 26 and 70 to the horizon-tal to provide a ramp-like surface to receive ice masses moving relative to and in contact with said struc-ture, the cross-sectional diameter of the base of the second truncated cone forming said upper portion being less than the cross-sectional diameter at the top of the first truncated cone forming said lower portion so that there exists a step-like section between the walls of said upper portion and the walls of said lower portion.
In accordance with another aspect of this inven-tion there is provided an offshore structure for use in a body of water which becomes frozen through natural conditionsj comprising: a supporting base portion positioned in a body of water; means securing said base portion to the underwater bottom; a lower portion directly joined to and rigidly support-ed on said base portion, said lower portion forming a first circum~erential wall which converges upwardly and inwardly of said base portion at an angle between 15 and 25 to the horizontal to receive and support an edge portion of a sheet of ice or other ice mass which moves in contact with said lower portion so as to elevate said ice above its natural level an amount to cause said ice to fracture continuously adjacent said offshore structure; an upper portion directly joined to and rigidly supported on said lower portionJ said upper portion forming a second circumferential wall which ` ' converges upwardly and inwardly of said lower portion at an angle between 26 and 70 to the horizontal to receive and support an edge portion of a sheet of ice or other ice mass which moves into contact with said lower portion so as to elevate said ice above its natural level an amount to cause ~ ~ .
~ 6~
7~6 said ice to fracture continuously adjacent said offshore structure, the base diamRter of said second circumferential wall being less than ~he top diameter of said first cireum-ferential wall so that there exists a discontinuity between 5 said first and said second circumferential wall.
_BJECTS OF THE INVENTION
An object of an aspect of the present invention is to provide an offshore structure which is able to with-stand the orces imposed thereon by impinging ice sheets 10 and larger ice masses and whieh incorporates less struetural material and whieh is eorrespondingly less massive and eost-ly.
~ - .
.
~J~
- 6e -01 ~-~dditional objects and advantages o the invention .Yill02 become apparent from a detailed reading of the specification and 03 drasYinqs which are incorpora~ herein and made a part of this 04 specification.
05 BRIEF DESCRIPTION OF THE DRA~INGS
06 FIG. 1 is a schematic side elevation view, partly in sec-07 tion, illustrating the preferred embodiment of the invention;
08 FIG. 2 is a schematic ilustcation in section taken along 09 line 2-2 of FIG. l;
FIG. 3 is a partial perspective view showing the upper 11 and lower conical portions and the thcoat ,oortion as being fabri-12 cated from steel plate.
13 DESCRIPTION O~ THE PRE~ERRED EMBODIMENT
,.
14 Referring now to the.dratYinqs, FIG. 1 represents a macine structure 15 located in a body of water 30 and particularly 16 designed ~or installation in arctic waters upon which thick sheets 17 of ice 20 and larger masses of ice such as pressure rid~es 22 may lR be forme~. The structure is held in place on the underwater 19 bottom 12 by its own weight plus the wei~ht of any ballast, as will be discussed in more detail below, added to the structure.
2l ~ work platform 10 of structure 15 is illustrated in 2~ FIG. 1 with a drilling rig 45 located on its deck 42; othec conven-~3 tional drilling equipment, which is not illustrated, may also be located on work platform 10. The invention, however, is not restricted to offshore structures used to support drilling rigs.
26 It is suitable for any type of offshore operation conducted in 27 arctic waters in which there is a need for protection a~ainst ice 23 masses formed on such waters.
29 The work platform 10 may actually contain several addi-tional levels of decks 40 and 41 which serve as living q~arters 31 and working areas for the personnel on the structure. The decks 32 may be enclosed and heated to provicle a reasonably comfortable ~'7~
01 wor~ing environment which offers ?rotection for men and e(~uipment 02 durin~ winter weathec, during wnich temperatures may drop to the 03 r~nge o~ -60F. The in~erior of the structure may also contain 0~ storage and equipment comparments which are illustrated generally 05 by reference numeral 60.
06 Ofeshore structure 15 i5 constructed to be reaclily estab--07 lished with full operating capacity at a selected drilling site 08 and with the ability to be moved from one drilling site and estah-09 lished at another in operating condition without delay. To this 1~ purpose, ballast tanks 62 are integrally built into the interior 11 of the structure to provide appropriate stability when the struc-12 ture is heing towed and to enable the structure to be lowered 13 through the water and into contact with the sea bottom. The 1~ ballast tanks may, of course, be t~immed as necessary to compen-sate foc any uneven distrihution of weight within the structure.
16 The ballast tanks are each provided with appropriate means, such 17 as sea cocks and a blowdown pipe, neither of which is illustrated, 1~ for remotely controlling the amount of wate~ in the tanks so that 19 the buoyancy of the structure is adjustable.
~s indicated above, a drill rig 45 is located on decks 42 21 alony with other conventional drilling equipment, not shown, ~or ~2 use in drilling a well bore 90 within the subsurfaces. A moon-~3 pool or drillway 50 thus extends from deck 42 down through the ~ structure to watec bottom 12 so that drill string 92 may be extended into wellbore ~0. Since it is both expensive and diffi-26 cult to construct and install a structure in arctic waters, it is 27 desirable that the structure be provided with the capability to 28 drill a number of wells at any particular site. For example, a 29 structure may be designed to drill two or more wells to a depth of approximately 20,000 feet. Accordingly, the structure must be 31 made lar~e enough to accornmodate the equipment necessary ~or this 32 purpose.
74~6 01 .~n o~shore structure lar~e enough to carry out the 02 above-described drilling activities will weiqh several thousand 03 tons beore it receives any of the equipment necessary for the 0~ drilling operation. Moreover, the weight of e~istinq designs for ds bot~om-supported structures increases proportionately as the struc-06 ture is desiqned for use in deeper waters and to withstand greater 07 natural ice forces such as those associated with lar~er ice masses 08 such as pressure ridges. Since the weiyht oE the stcucture is 09 dicectly relate~ to its costs, the cost will proportionate1y increase as the weight increases. The present invention is 11 directed toward an oEfshore structure configuration which is par-12 ticularly adaptable foc use in deeper waters, but which is not 13` restricted to use in deeper waters, and which minimizes the Eorces 1~ imeosed on the structure by impinging ice sheets and larger masses of ice, and, at the same time, permits less structural material to 16 be incorporated in the structure and correspondingly reduces its 17 mass and cost.
18 As discussed hereinabove, an ice sheet that moves into 19 contact with the sloping surface of a conically shaped offshore ~0 structuce will fail in flexure resulting in the ice sheet being 21 broken into wedge-shaped segments. As the ice sheet continues to 22 move against the structure, the wedge-shaped pieces of ice will 23 ride up the outer surfaces of the structure and ideally fall away 24 r^rom and be swept around the structure. As the ice masses impinging on the structure become larger, the forces imposed 26 thereon are likewise increased. To prevent failure of the present 27 designs for bottom-supported conical structures, when a larger 28 mass of ice such as a pressure ridge moves into contact with the 29 structure, several things may possibly be done. First, the base diameter of the structure and thus its size l~ay be incr~ased to 31 resist the larger ice forces. Second, the structure may be ~-32 pcovided with a rather gently sloping surface, which also 33 _ 9 _ ~2~
01 increases its size, to receive the impinging pressure ridge; this 02 has the effect o~ reducing the total ice force imposed on the 03 structure by the impinging ridge, since that component of total 04 force due to Elexural failure of a ridge decreases as the angle of 05 inclination from the horizontal of the slopinq surface decreases.
06 Third, the structure may be supported by piling; however, this is 07 undesirable because the cost and time of installation for the 08 structure at a selected drilling site would be increased.
09 To resist the greater forces associated with larger impinging ice masses, the si2e of present designs for bottom-11 supported conical structuces would then have to be increased, 12 which necessitates incorporating more structural material in the li structure which increases its mass and thus its cost, mak~nq it 14 prQhibitively expensive to build. ~oreQVer~ the size of the structure also tends to increase as the structure is designed Eor 16 use in deeper waters. As these structures are built larger, the 17 total ice force imposed on the structure increases. As pointed 13 out previously, the total ice force exerted on a conical offshore lg structure essentially consists of the force required to ~ail the impin~ing ice mass in Elexure and the Eorce caused by broken 21 ~ieces o~ sheet ice riding up the outer surEace of the structure 22 an~ interacting therewith. This ride-up force depends upon the 23 weight of the ice pieces as well as the force of friction existing 24 between the ice and the outer surfaces of the structure. Thus, it can be seen that ride-up ice force is proportional to the surEace area oE the conical structure above the waterline. Therefore, as 27 the size of the structure is increased, the ride-up ~orce imposed 28 on the structure is ]ikewise increased, and for conical structures 2~ having relatively large waterline diameters, the ride-u2 force may well exceed the force require~ to fail the impinging ice mass in 31 Elexure.
~:~L27~ 6 01 .~ccordingly, there is provided in accordance with th~
02 present i~vention an of~shore structure adaptable for use in 03 deeoer waters which is able to withstand the forces imposed 04 thereon by an impinging ice sheet 20 or some other large~ mass of ~5 ice such as 3 pressure ridge 22 wherein the mass and cos~ oE the Q6 structure is not unnecessarily incceased. This structure basi-07 cally has, as illustrated in FIGS. 1-3, a lower conically shaped oa ~ortion 4 and upper conically shaped portion ~ coaxially posi-09 tioned with respect to one another to form a continuous external ld shell which has a discontinuity 200 ~herein and which is adapted 11 to receive ice masses moving relative to and in contact with the 12 structure. It is contemplated that the e~ternal shell o~ the li structure is to be constructed from steel plate, as illustrated in 14 ~IG. 3r but other materials, s~ch as prestressed concrete, may be used.
16 The upper portion 6, as can be seen, is in the shape of 17 a truncated cone wherein the walls form a ramp-like surface 16 18 which is inclined at an angle to the horizontal so that surface 16 19 converges upwardly and inwardly of lower portion 4. The lower ~0 portion 4 oE the structure likewise is in the shape of a truncated 21 cone, but is of larger cross-sectional diameter than upper portion 22 6; that is, the base diameter of the cone forming upper portion 6 23 is less than the top diameter of the cone-forming lower portion 4 24 so that there exists a step-like section 200 between the walls of the upper portion 6 and the walls of the lower portion 4. The walls of lower portion 4 converge upwardly and inwardly o base 27 portion 2 to form a ramp-like surface 14 which is inclined at an 23 angle to the horizontal, but at an angle of inclination from the 29 horizontal which is greater than that o~ lower portion 4.
Thus, the waterline dia~eter of upper section 6 is kept 31 as small as practicable to reduce the ride-up forces acting on the 3~ structure. ~n the other hand, to enable the structure to wit'n-3~ - 11 -~Z~4~6 01 stand the forces associated with larger itnpinging ice ~asses, a 02 relatively large lower section 4 with a reduced angle o~ inclin-03 ation is provided~ The reduced angle of inclination of lower 0~ section 4 offers the advantage of reducing the Eorces imposed on ~0-~ the structure by the flexural failure oE a pressure ridge.
06 ~dditionally, the relatively large lower section 4 decreases the 07 likelihood of foundation failure of the structure, as well as 0~ improving i~s flotation stability. ~oreover, the discontinuity or 09 step-like section 200 existing between section 4 and section 6 reduces the over-all mass of the structure and thus its cost, 11 making it feasible for use in deeper waters.
12 The ba5e portion 2 of the structure may also have a 13 conical shape so that its walls converge upwardly and inwardly of 14 the underwater bottom 12, ~ith the top diameter of the b~se por-tion being approximately equal to the bottom diameter of lower 16 portion 4. This particular shape is useful from the point of view 17 that it imparts additional stability to the structure when it is 1~ bein~ moved through thé water. In addition, the ra~p-like surface 19 of ~ase portion 2 may assist in failing an impinging pressure ridge. Of course, base portion 2 may have other appropriate ~1 shapes, such as that of a cylinder, so that walls of the base 2 portion are vertically disposed to the underwater bottom.
23 In deeper waters, larger ice masses, such as pressure 24 ridge 22, extend a considerable distance below the surface of the water; therefore, when they move relative to and in contact with ~6 structure 15, the edge portion of the ridge 22 will be received by ~7 wall of the lower portion 4 and lifted along surface 14, causing th~ ridge to fail in flexure. As the pressure ridge is elevated ~9 along surEace 14, it breaks into blocks of ice which tend to slide beneath the ice sheet advancing behind the ridge; the blocks oE
3l ice are then swept laterally around the structure. Surface 16 oE -3 upper portion 6 will receive the ice sheets impinging on the struc-33 ture, and, as d2scribed, cause them to fail in flexure.
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7~6 01 if the structure were located in relatively snallow 02 ~aters, the lower conical portion 4 woulc~ receive anà fail in ~3 flexure ice sheets and smaller pressure ridges irl?ingin~ on the n4 structure. The only force im~osed on the upper portion ~ ~ould be ~S~ that associated with the ride up oE oieces of sneet ice on surEace Oh 16.
07 To assist the rnovement of ice relative to and over the 08 outer surfaces of the upper portion 6 and lower portion 4 of the n9 structure and to prevent ride-up ice pieces from freezing to these surEaces, appropriate adfreeze prevention apparatus should be 11 us~ec~ Adfreeze prevention procedures include heating the outer 12 surEaces 14 and 16 of the structure, as disclosed in Chevron 13 Rese3rch Com~any's ~.S. Patent 3,831,385, or coatinq the surfaces l~t with a material that reduces ir~e adhesion, as disclosed ~n Chevron lS Research COmQany's U.S. Patent 3,972,199.
16 The angle of inclination of the walls of the lower l7 portion 4 and the upper portion 6 of the structure are indicated l8 by ~1 and 2~ respectively. These two angles are acute angles 19 which should be steep enough to cause failure of an ice rnass in flexure. The value of a1 needs to be small enough so that the 21 ~orce associated with the flexural failing of a large ice mass is 2~ minimized. llowever, the value ~ ~1 should not be too small, as 23 the base of the structure would then be too large, maXing the cost 24 of the structure economically prohibitive. The value of ~2 is large enough such that the surface area of the structure above the 6 waterline is minimized, but not so lar~e as to cause an impingin~
27 ice sheet to Eail in compression rather than flexure. In most multi-angle conical structures, ~1 and a2 may range betYeen ~9 approxilnately 15 and 25 and 2G to 70 from horizontal, respec-tively. The preferred ran~e of ~1 is between approximately 19 to 31 23 from the horizontal, and the oreferred range of ~ 2 is ~etween 32 approximately 54 and 58. The ~referred an~le for 1~ and ~2 - ~27~6 01 respectively, is essentially dependent upon three factors, namelyr 02 the range of water deptl)s in which the structure is to oe located, 03 the ex~oected si~e of ice sheets and oressure ridges in these 04 waters, and the soil characteristics of the sea Eloor on which the `0~ structure is to be supported. Therefore, if a structure havin~ a 05 step-like section is to be operated in the relatively decper 07 waters off northern Alaska, the preferred angle for 1 is aDproxi-03 ~nately 21 from the horizontal and the preferred angle ~or a2 is Og approximately 56 from the horizontal.
~s illustrated in FIG. 1, the throat portion 8 of the 11 structure, which has a cylindrical shape, is coaxially positioned 12 on top of and vertically abuts upper portion 6 and extends wor~
13 platform lO above the surface of the body of water 30 to a height 1~ sufEicient to avoid contact with pieces of sheet ice ricling u~ the structure.
l6 While it is contemplated that structure 15 will be towed 17 to the drlllin~ site in a completely assembled condition with no 1~ additional construction-at the site being necessary, it would cer-1~ tainly be possible and perhaps desirable to tow individual sec-tions o~ the structure from their place of ~abrication to the ~1 drilling site or assernbly. For examDle, base portion 2 could be 22 brou~ht to the drilling site and placed on the under~ater bottom ~3 12. Lower portion 4 could then be brouqht to the drilling site 24 an~ nositioned in abutting relationship on top of and joined by ap~oropriate means to base portion 2. Likewise, upper portion 26 would be brought to the drilling site and E~ositioned on to~ of `2/ lower portion 4 and joined to lower portion 4. In a like manner, 28 the other components of the structure could be assembled at the 29 drilling site.
The advanta~es of this disclosure can also be realized 31 by minor variations in the confiquration of the structure whereill -32 the ramp-like outer surface of the structure has a Inulti-cone .. ..... .. .... . ... .
llZ74~6 01 geo~etry o~ more than two conical sections or a geometry involving 02 two, stepped, continuallv curved surfaces such as portions of 03 hyperboloids of revolution.
04 Although certain specific embodilnents of the invention have ~5` been described herein in detail, the invention is not to be Ofi lirnited to only such embodiments, bu~ rather only by the appended 07 ~laims, ~;' .
:
' 08 ~ - 15 -.~
The offshore structure of this invention includes a lower portion in the shape of a -truncated cone coaxially position-able on top of a base portion. An uppex portion o~ the structure is in the shape of a second truncated cone and is coaxially positionable on top of said lower portion. The walls forming the upper and lower portions of the structure are inclined at an angle to the horizontal to receive ice masses moving relative to and in contact with the structure in order to cause the ice masses to fail in flexure. The angle of inclination from the horizontal of the walls of the upper portion is greater tha~ that of the lower portion, and the cross-sectional diameter of the upper portion is less than that of the top of the lower portion so that there exists a step~like section or discontinuity between the walls of the upper and lower conical portions.
~ The angle of inclination of the walls of the upper portion is between approximately 26 and 70 from the horizon-tal, with the preferred range being between approximately 54 and 58 from the horizontal. The angle of inclination of the walls of the lower portion is between approximately 15 and 25 from the horizontal, with the preferred range being between approximately 19 and 23 from the horizontal.
The above offshore structure configuration permits the structure to be utilized in relatively deeper waters which contain ice sheets and relatively larger ice masses without unnecessarily increasing the mass ~and the cost of a structure.
In accordance with one aspect of this invention there is provided an offshore structure for use in a body of water that contain~ ice masses, comprising a lower portion `Jl`~ ~ substantially in the shape of a first truncated cone sloping ~7~6 inwardly and upwardly so that the walls of said lower portion are inclined at an angle between 15 and 25 to the horizontal to provide a ramp-like surface to receive ice masses moving relative to and in contact with said structure; means for affixin~ said lower portion to the bottom of a body of water;
and an upper portion coaxially positionable above said lower portion, said upper portion substantially in the shape of a second truncated cone sloping inwardly and upwardly so that the walls of said upper portion are inclined at an angle between 26 and 70 to the horizontal to provide a ramp-like surface to receive ice masses moving relative to and in contact with said structure, the cross-sectional diameter of the base of the second truncated cone forming said u,pper portion being less than the cross-sectional diameter at the IS top of the first truncated cone forming said lower portion so that there exists a step~like section between the walls of s~id upper portion and the walls.of said lower portion.
In accordance with another aspect of this invention there is provided a marine structure for use in a body of water that contains ice masses, comprising a base portion; means for affixing said base portion to the bottom o a body`of water; a lower portion coaxially positionable on top of said base portion for joining thereto, said lower portion substantially in the shape of a first truncated cone sloping inwardly and upwardly so that the walls of said lo~er portion are inclined at an angle between 15 and 25 to the horizontal to provide a ramp-like surface to receive ice masses movin~ relat.ive to and in contact with said structure; and an upper portion coaxially positionable on top of said lower portion for joininy thereto, said upper portion substantially i~ the shape of a second truncated - 6a~-.
cone sloping inwardly and upwardly so that the walls of saidupper portion are inclined at an angle between 26 and 70 to the horizon-tal to provide a ramp-like surface to receive ice masses moving relative to and in contact with said struc-ture, the cross-sectional diameter of the base of the second truncated cone forming said upper portion being less than the cross-sectional diameter at the top of the first truncated cone forming said lower portion so that there exists a step-like section between the walls of said upper portion and the walls of said lower portion.
In accordance with another aspect of this inven-tion there is provided an offshore structure for use in a body of water which becomes frozen through natural conditionsj comprising: a supporting base portion positioned in a body of water; means securing said base portion to the underwater bottom; a lower portion directly joined to and rigidly support-ed on said base portion, said lower portion forming a first circum~erential wall which converges upwardly and inwardly of said base portion at an angle between 15 and 25 to the horizontal to receive and support an edge portion of a sheet of ice or other ice mass which moves in contact with said lower portion so as to elevate said ice above its natural level an amount to cause said ice to fracture continuously adjacent said offshore structure; an upper portion directly joined to and rigidly supported on said lower portionJ said upper portion forming a second circumferential wall which ` ' converges upwardly and inwardly of said lower portion at an angle between 26 and 70 to the horizontal to receive and support an edge portion of a sheet of ice or other ice mass which moves into contact with said lower portion so as to elevate said ice above its natural level an amount to cause ~ ~ .
~ 6~
7~6 said ice to fracture continuously adjacent said offshore structure, the base diamRter of said second circumferential wall being less than ~he top diameter of said first cireum-ferential wall so that there exists a discontinuity between 5 said first and said second circumferential wall.
_BJECTS OF THE INVENTION
An object of an aspect of the present invention is to provide an offshore structure which is able to with-stand the orces imposed thereon by impinging ice sheets 10 and larger ice masses and whieh incorporates less struetural material and whieh is eorrespondingly less massive and eost-ly.
~ - .
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~J~
- 6e -01 ~-~dditional objects and advantages o the invention .Yill02 become apparent from a detailed reading of the specification and 03 drasYinqs which are incorpora~ herein and made a part of this 04 specification.
05 BRIEF DESCRIPTION OF THE DRA~INGS
06 FIG. 1 is a schematic side elevation view, partly in sec-07 tion, illustrating the preferred embodiment of the invention;
08 FIG. 2 is a schematic ilustcation in section taken along 09 line 2-2 of FIG. l;
FIG. 3 is a partial perspective view showing the upper 11 and lower conical portions and the thcoat ,oortion as being fabri-12 cated from steel plate.
13 DESCRIPTION O~ THE PRE~ERRED EMBODIMENT
,.
14 Referring now to the.dratYinqs, FIG. 1 represents a macine structure 15 located in a body of water 30 and particularly 16 designed ~or installation in arctic waters upon which thick sheets 17 of ice 20 and larger masses of ice such as pressure rid~es 22 may lR be forme~. The structure is held in place on the underwater 19 bottom 12 by its own weight plus the wei~ht of any ballast, as will be discussed in more detail below, added to the structure.
2l ~ work platform 10 of structure 15 is illustrated in 2~ FIG. 1 with a drilling rig 45 located on its deck 42; othec conven-~3 tional drilling equipment, which is not illustrated, may also be located on work platform 10. The invention, however, is not restricted to offshore structures used to support drilling rigs.
26 It is suitable for any type of offshore operation conducted in 27 arctic waters in which there is a need for protection a~ainst ice 23 masses formed on such waters.
29 The work platform 10 may actually contain several addi-tional levels of decks 40 and 41 which serve as living q~arters 31 and working areas for the personnel on the structure. The decks 32 may be enclosed and heated to provicle a reasonably comfortable ~'7~
01 wor~ing environment which offers ?rotection for men and e(~uipment 02 durin~ winter weathec, during wnich temperatures may drop to the 03 r~nge o~ -60F. The in~erior of the structure may also contain 0~ storage and equipment comparments which are illustrated generally 05 by reference numeral 60.
06 Ofeshore structure 15 i5 constructed to be reaclily estab--07 lished with full operating capacity at a selected drilling site 08 and with the ability to be moved from one drilling site and estah-09 lished at another in operating condition without delay. To this 1~ purpose, ballast tanks 62 are integrally built into the interior 11 of the structure to provide appropriate stability when the struc-12 ture is heing towed and to enable the structure to be lowered 13 through the water and into contact with the sea bottom. The 1~ ballast tanks may, of course, be t~immed as necessary to compen-sate foc any uneven distrihution of weight within the structure.
16 The ballast tanks are each provided with appropriate means, such 17 as sea cocks and a blowdown pipe, neither of which is illustrated, 1~ for remotely controlling the amount of wate~ in the tanks so that 19 the buoyancy of the structure is adjustable.
~s indicated above, a drill rig 45 is located on decks 42 21 alony with other conventional drilling equipment, not shown, ~or ~2 use in drilling a well bore 90 within the subsurfaces. A moon-~3 pool or drillway 50 thus extends from deck 42 down through the ~ structure to watec bottom 12 so that drill string 92 may be extended into wellbore ~0. Since it is both expensive and diffi-26 cult to construct and install a structure in arctic waters, it is 27 desirable that the structure be provided with the capability to 28 drill a number of wells at any particular site. For example, a 29 structure may be designed to drill two or more wells to a depth of approximately 20,000 feet. Accordingly, the structure must be 31 made lar~e enough to accornmodate the equipment necessary ~or this 32 purpose.
74~6 01 .~n o~shore structure lar~e enough to carry out the 02 above-described drilling activities will weiqh several thousand 03 tons beore it receives any of the equipment necessary for the 0~ drilling operation. Moreover, the weight of e~istinq designs for ds bot~om-supported structures increases proportionately as the struc-06 ture is desiqned for use in deeper waters and to withstand greater 07 natural ice forces such as those associated with lar~er ice masses 08 such as pressure ridges. Since the weiyht oE the stcucture is 09 dicectly relate~ to its costs, the cost will proportionate1y increase as the weight increases. The present invention is 11 directed toward an oEfshore structure configuration which is par-12 ticularly adaptable foc use in deeper waters, but which is not 13` restricted to use in deeper waters, and which minimizes the Eorces 1~ imeosed on the structure by impinging ice sheets and larger masses of ice, and, at the same time, permits less structural material to 16 be incorporated in the structure and correspondingly reduces its 17 mass and cost.
18 As discussed hereinabove, an ice sheet that moves into 19 contact with the sloping surface of a conically shaped offshore ~0 structuce will fail in flexure resulting in the ice sheet being 21 broken into wedge-shaped segments. As the ice sheet continues to 22 move against the structure, the wedge-shaped pieces of ice will 23 ride up the outer surfaces of the structure and ideally fall away 24 r^rom and be swept around the structure. As the ice masses impinging on the structure become larger, the forces imposed 26 thereon are likewise increased. To prevent failure of the present 27 designs for bottom-supported conical structures, when a larger 28 mass of ice such as a pressure ridge moves into contact with the 29 structure, several things may possibly be done. First, the base diameter of the structure and thus its size l~ay be incr~ased to 31 resist the larger ice forces. Second, the structure may be ~-32 pcovided with a rather gently sloping surface, which also 33 _ 9 _ ~2~
01 increases its size, to receive the impinging pressure ridge; this 02 has the effect o~ reducing the total ice force imposed on the 03 structure by the impinging ridge, since that component of total 04 force due to Elexural failure of a ridge decreases as the angle of 05 inclination from the horizontal of the slopinq surface decreases.
06 Third, the structure may be supported by piling; however, this is 07 undesirable because the cost and time of installation for the 08 structure at a selected drilling site would be increased.
09 To resist the greater forces associated with larger impinging ice masses, the si2e of present designs for bottom-11 supported conical structuces would then have to be increased, 12 which necessitates incorporating more structural material in the li structure which increases its mass and thus its cost, mak~nq it 14 prQhibitively expensive to build. ~oreQVer~ the size of the structure also tends to increase as the structure is designed Eor 16 use in deeper waters. As these structures are built larger, the 17 total ice force imposed on the structure increases. As pointed 13 out previously, the total ice force exerted on a conical offshore lg structure essentially consists of the force required to ~ail the impin~ing ice mass in Elexure and the Eorce caused by broken 21 ~ieces o~ sheet ice riding up the outer surEace of the structure 22 an~ interacting therewith. This ride-up force depends upon the 23 weight of the ice pieces as well as the force of friction existing 24 between the ice and the outer surfaces of the structure. Thus, it can be seen that ride-up ice force is proportional to the surEace area oE the conical structure above the waterline. Therefore, as 27 the size of the structure is increased, the ride-up ~orce imposed 28 on the structure is ]ikewise increased, and for conical structures 2~ having relatively large waterline diameters, the ride-u2 force may well exceed the force require~ to fail the impinging ice mass in 31 Elexure.
~:~L27~ 6 01 .~ccordingly, there is provided in accordance with th~
02 present i~vention an of~shore structure adaptable for use in 03 deeoer waters which is able to withstand the forces imposed 04 thereon by an impinging ice sheet 20 or some other large~ mass of ~5 ice such as 3 pressure ridge 22 wherein the mass and cos~ oE the Q6 structure is not unnecessarily incceased. This structure basi-07 cally has, as illustrated in FIGS. 1-3, a lower conically shaped oa ~ortion 4 and upper conically shaped portion ~ coaxially posi-09 tioned with respect to one another to form a continuous external ld shell which has a discontinuity 200 ~herein and which is adapted 11 to receive ice masses moving relative to and in contact with the 12 structure. It is contemplated that the e~ternal shell o~ the li structure is to be constructed from steel plate, as illustrated in 14 ~IG. 3r but other materials, s~ch as prestressed concrete, may be used.
16 The upper portion 6, as can be seen, is in the shape of 17 a truncated cone wherein the walls form a ramp-like surface 16 18 which is inclined at an angle to the horizontal so that surface 16 19 converges upwardly and inwardly of lower portion 4. The lower ~0 portion 4 oE the structure likewise is in the shape of a truncated 21 cone, but is of larger cross-sectional diameter than upper portion 22 6; that is, the base diameter of the cone forming upper portion 6 23 is less than the top diameter of the cone-forming lower portion 4 24 so that there exists a step-like section 200 between the walls of the upper portion 6 and the walls of the lower portion 4. The walls of lower portion 4 converge upwardly and inwardly o base 27 portion 2 to form a ramp-like surface 14 which is inclined at an 23 angle to the horizontal, but at an angle of inclination from the 29 horizontal which is greater than that o~ lower portion 4.
Thus, the waterline dia~eter of upper section 6 is kept 31 as small as practicable to reduce the ride-up forces acting on the 3~ structure. ~n the other hand, to enable the structure to wit'n-3~ - 11 -~Z~4~6 01 stand the forces associated with larger itnpinging ice ~asses, a 02 relatively large lower section 4 with a reduced angle o~ inclin-03 ation is provided~ The reduced angle of inclination of lower 0~ section 4 offers the advantage of reducing the Eorces imposed on ~0-~ the structure by the flexural failure oE a pressure ridge.
06 ~dditionally, the relatively large lower section 4 decreases the 07 likelihood of foundation failure of the structure, as well as 0~ improving i~s flotation stability. ~oreover, the discontinuity or 09 step-like section 200 existing between section 4 and section 6 reduces the over-all mass of the structure and thus its cost, 11 making it feasible for use in deeper waters.
12 The ba5e portion 2 of the structure may also have a 13 conical shape so that its walls converge upwardly and inwardly of 14 the underwater bottom 12, ~ith the top diameter of the b~se por-tion being approximately equal to the bottom diameter of lower 16 portion 4. This particular shape is useful from the point of view 17 that it imparts additional stability to the structure when it is 1~ bein~ moved through thé water. In addition, the ra~p-like surface 19 of ~ase portion 2 may assist in failing an impinging pressure ridge. Of course, base portion 2 may have other appropriate ~1 shapes, such as that of a cylinder, so that walls of the base 2 portion are vertically disposed to the underwater bottom.
23 In deeper waters, larger ice masses, such as pressure 24 ridge 22, extend a considerable distance below the surface of the water; therefore, when they move relative to and in contact with ~6 structure 15, the edge portion of the ridge 22 will be received by ~7 wall of the lower portion 4 and lifted along surface 14, causing th~ ridge to fail in flexure. As the pressure ridge is elevated ~9 along surEace 14, it breaks into blocks of ice which tend to slide beneath the ice sheet advancing behind the ridge; the blocks oE
3l ice are then swept laterally around the structure. Surface 16 oE -3 upper portion 6 will receive the ice sheets impinging on the struc-33 ture, and, as d2scribed, cause them to fail in flexure.
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7~6 01 if the structure were located in relatively snallow 02 ~aters, the lower conical portion 4 woulc~ receive anà fail in ~3 flexure ice sheets and smaller pressure ridges irl?ingin~ on the n4 structure. The only force im~osed on the upper portion ~ ~ould be ~S~ that associated with the ride up oE oieces of sneet ice on surEace Oh 16.
07 To assist the rnovement of ice relative to and over the 08 outer surfaces of the upper portion 6 and lower portion 4 of the n9 structure and to prevent ride-up ice pieces from freezing to these surEaces, appropriate adfreeze prevention apparatus should be 11 us~ec~ Adfreeze prevention procedures include heating the outer 12 surEaces 14 and 16 of the structure, as disclosed in Chevron 13 Rese3rch Com~any's ~.S. Patent 3,831,385, or coatinq the surfaces l~t with a material that reduces ir~e adhesion, as disclosed ~n Chevron lS Research COmQany's U.S. Patent 3,972,199.
16 The angle of inclination of the walls of the lower l7 portion 4 and the upper portion 6 of the structure are indicated l8 by ~1 and 2~ respectively. These two angles are acute angles 19 which should be steep enough to cause failure of an ice rnass in flexure. The value of a1 needs to be small enough so that the 21 ~orce associated with the flexural failing of a large ice mass is 2~ minimized. llowever, the value ~ ~1 should not be too small, as 23 the base of the structure would then be too large, maXing the cost 24 of the structure economically prohibitive. The value of ~2 is large enough such that the surface area of the structure above the 6 waterline is minimized, but not so lar~e as to cause an impingin~
27 ice sheet to Eail in compression rather than flexure. In most multi-angle conical structures, ~1 and a2 may range betYeen ~9 approxilnately 15 and 25 and 2G to 70 from horizontal, respec-tively. The preferred ran~e of ~1 is between approximately 19 to 31 23 from the horizontal, and the oreferred range of ~ 2 is ~etween 32 approximately 54 and 58. The ~referred an~le for 1~ and ~2 - ~27~6 01 respectively, is essentially dependent upon three factors, namelyr 02 the range of water deptl)s in which the structure is to oe located, 03 the ex~oected si~e of ice sheets and oressure ridges in these 04 waters, and the soil characteristics of the sea Eloor on which the `0~ structure is to be supported. Therefore, if a structure havin~ a 05 step-like section is to be operated in the relatively decper 07 waters off northern Alaska, the preferred angle for 1 is aDproxi-03 ~nately 21 from the horizontal and the preferred angle ~or a2 is Og approximately 56 from the horizontal.
~s illustrated in FIG. 1, the throat portion 8 of the 11 structure, which has a cylindrical shape, is coaxially positioned 12 on top of and vertically abuts upper portion 6 and extends wor~
13 platform lO above the surface of the body of water 30 to a height 1~ sufEicient to avoid contact with pieces of sheet ice ricling u~ the structure.
l6 While it is contemplated that structure 15 will be towed 17 to the drlllin~ site in a completely assembled condition with no 1~ additional construction-at the site being necessary, it would cer-1~ tainly be possible and perhaps desirable to tow individual sec-tions o~ the structure from their place of ~abrication to the ~1 drilling site or assernbly. For examDle, base portion 2 could be 22 brou~ht to the drilling site and placed on the under~ater bottom ~3 12. Lower portion 4 could then be brouqht to the drilling site 24 an~ nositioned in abutting relationship on top of and joined by ap~oropriate means to base portion 2. Likewise, upper portion 26 would be brought to the drilling site and E~ositioned on to~ of `2/ lower portion 4 and joined to lower portion 4. In a like manner, 28 the other components of the structure could be assembled at the 29 drilling site.
The advanta~es of this disclosure can also be realized 31 by minor variations in the confiquration of the structure whereill -32 the ramp-like outer surface of the structure has a Inulti-cone .. ..... .. .... . ... .
llZ74~6 01 geo~etry o~ more than two conical sections or a geometry involving 02 two, stepped, continuallv curved surfaces such as portions of 03 hyperboloids of revolution.
04 Although certain specific embodilnents of the invention have ~5` been described herein in detail, the invention is not to be Ofi lirnited to only such embodiments, bu~ rather only by the appended 07 ~laims, ~;' .
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' 08 ~ - 15 -.~
Claims (13)
1. An offshore structure for use in a body of water that contains ice masses, comprising a lower portion substan-tially in the shape of a first truncated cone sloping inward-ly and upwardly so that the walls of said lower portion are inclined at an angle between 15° and 25° to the horizontal to provide a ramp-like surface to receive ice masses moving relative to and in contact with said structure; means for affixing said lower portion to the bottom of a body of water;
and an upper portion coaxially positionable above said lower portion, said upper portion substantially in the shape of a second truncated cone sloping inwardly and upwardly so that the walls of said upper portion are inclined at an angle between 26° and 70° to the horizontal to provide a ramp-like surface to receive ice masses moving relative to and in contact with said structure, the cross-sectional diameter of the base of the second truncated cone forming said upper portion being less than the cross-sectional diameter at the top of the first truncated cone forming said lower portion so that there exists a step-like section between the walls of said upper portion and the walls of said lower portion.
and an upper portion coaxially positionable above said lower portion, said upper portion substantially in the shape of a second truncated cone sloping inwardly and upwardly so that the walls of said upper portion are inclined at an angle between 26° and 70° to the horizontal to provide a ramp-like surface to receive ice masses moving relative to and in contact with said structure, the cross-sectional diameter of the base of the second truncated cone forming said upper portion being less than the cross-sectional diameter at the top of the first truncated cone forming said lower portion so that there exists a step-like section between the walls of said upper portion and the walls of said lower portion.
2. A marine structure for use in a body of water that contains ice masses, comprising a base portion; means for affixing said base portion to the bottom of a body of water; a lower portion coaxially positionable on top of said base portion for joining thereto, said lower portion sub-stantially in the shape of a first truncated cone sloping inwardly and upwardly so that the walls of said lower portion are inclined at an angle between 15° and 25° to the horizontal to provide a ramp-like surface to receive ice masses moving relative to and in contact with said struc-ture; and an upper portion coaxially positionable on top of said lower portion for joining thereto, said upper portion substantially in the shape of a second truncated cone sloping inwardly and upwardly so that the walls of said upper portion are inclined at an angle between 26° and 70° to the horizon-tal to provide a ramp-like surface to receive ice masses moving relative to and in contact with said structure, the cross-sectional diameter of the base of the second truncated cone forming said upper portion being less than the cross-sectional diameter at the top of the first truncated cone forming said lower portion so that there exists a step-like section between the walls of said upper portion and the walls of said lower portion.
3. The marine structure of claim l or claim 2 wherein the angle of inclination of the walls of said lower portion is between about 19° and 23° from the horizontal and wherein the angle of inclination of the walls of said upper portion is between about 54° and 58° from the horizontal.
4. The marine structure of claim 1 or claim 2 wherein the angle of inclination of the walls of said lower portion is 21° from the horizontal and wherein the angle of inclination of the walls of said upper portion is 56° from the horizontal.
5. The marine structure of claim 2 further including:
a cylindrical throat portion coaxially positionable on top of said upper portion for joining thereto and for extending a work platform above the surface of said body of water.
a cylindrical throat portion coaxially positionable on top of said upper portion for joining thereto and for extending a work platform above the surface of said body of water.
6. The marine structure of claim 5 wherein the angle of inclination of the walls of said lower portion is between about 15° and 25° from the horizontal and the angle of incli-nation of the walls of said upper portion is between about 26°
and 70° from the horizontal.
and 70° from the horizontal.
7. The marine structure of claim 5 wherein the angle of inclination of the walls of said lower portion is between about 19° and 23° from the horizontal and the angle of incli-nation of the walls of said upper portion is between about 54°
and 58° from the horizontal.
and 58° from the horizontal.
8. The marine structure of claim 5 wherein the angle of inclination of the walls of said lower portion is 21° from the horizontal and the angle of inclination of the walls of said upper portion is 56° from the horizontal.
9. An offshore structure for use in a body of water which becomes frozen through natural conditions, comprising:
a supporting base portion positioned in a body of water;
means securing said base portion to the underwater bottom; a lower portion directly joined to and rigidly supported on said base portion, said lower portion forming a first circum-ferential wall which converges upwardly and inwardly of said base portion at an angle between 15° and 25° to the horizontal to receive and support an edge portion of a sheet of ice or other ice mass which moves in contact with said lower portion so as to elevate said ice above its natural level an amount to cause said ice to fracture continuously adjacent said off-shore structure; an upper portion directly joined to and rigidly supported on said lower portion, said upper portion forming a second circumferential wall which converges upward-ly and inwardly of said lower portion at an angle between 26°
and 70° to the horizontal to receive and support an edge portion of a sheet of ice or other ice mass which moves into contact with said lower portion so as to elevate said ice above its natural level an amount to cause said ice to fracture continuously adjacent said offshore structure, the base diameter of said second circumferential wall being less than the top diameter of said first circumferential wall so that there exists a discontinuity between said first and said second circumferential wall.
a supporting base portion positioned in a body of water;
means securing said base portion to the underwater bottom; a lower portion directly joined to and rigidly supported on said base portion, said lower portion forming a first circum-ferential wall which converges upwardly and inwardly of said base portion at an angle between 15° and 25° to the horizontal to receive and support an edge portion of a sheet of ice or other ice mass which moves in contact with said lower portion so as to elevate said ice above its natural level an amount to cause said ice to fracture continuously adjacent said off-shore structure; an upper portion directly joined to and rigidly supported on said lower portion, said upper portion forming a second circumferential wall which converges upward-ly and inwardly of said lower portion at an angle between 26°
and 70° to the horizontal to receive and support an edge portion of a sheet of ice or other ice mass which moves into contact with said lower portion so as to elevate said ice above its natural level an amount to cause said ice to fracture continuously adjacent said offshore structure, the base diameter of said second circumferential wall being less than the top diameter of said first circumferential wall so that there exists a discontinuity between said first and said second circumferential wall.
10. The offshore structure of claim 9 wherein said base portion forms a third circumferential wall which converges upwardly and inwardly of the underwater bottom and wherein the top diameter of said third circumferential wall forming said base portion is approximately equal to the base diameter of said first circumferential wall forming said lower portion.
11. The offshore structure of claim 10 further including a cylindrical throat portion rigidly supported on said upper portion for supporting a work platform above the surface of said body of water.
12. The offshore structure of claim 10 wherein said first circumferential wall converges upwardly and inwardly of said base portion at an angle of between approximately 19°
and 23° from the horizontal and wherein said second circum-ferential wall converges upwardly and inwardly of said lower portion at an angle of between approximately 54° and 58° from the horizontal.
and 23° from the horizontal and wherein said second circum-ferential wall converges upwardly and inwardly of said lower portion at an angle of between approximately 54° and 58° from the horizontal.
13. The offshore structure of claim 10 wherein said first circumferential wall converges upwardly and inwardly of said base portion at an angle of approximately 21° from the horizontal and wherein said second circumferential wall converges upwardly and inwardly of said lower portion at an angle of approximately 56° from the horizontal.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US89142278A | 1978-03-29 | 1978-03-29 | |
| US891,422 | 1978-03-29 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1127406A true CA1127406A (en) | 1982-07-13 |
Family
ID=25398152
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA315,776A Expired CA1127406A (en) | 1978-03-29 | 1978-11-03 | Arctic multi-angle conical structure having a discontinuous outer surface |
Country Status (5)
| Country | Link |
|---|---|
| JP (1) | JPS5913612B2 (en) |
| CA (1) | CA1127406A (en) |
| GB (1) | GB2017793B (en) |
| NO (1) | NO153505C (en) |
| SE (1) | SE438692B (en) |
Families Citing this family (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4325655A (en) | 1980-01-16 | 1982-04-20 | Exxon Production Research Company | Multiple slope structure |
| JPS62128104U (en) * | 1986-02-04 | 1987-08-13 | ||
| GB9113194D0 (en) * | 1991-06-19 | 1991-08-07 | Earl & Wright Ltd | Offshore structure |
Family Cites Families (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JPS5364901A (en) * | 1976-11-24 | 1978-06-09 | Mitsui Shipbuilding Eng | Ice crusher for structure used in icy waters |
-
1978
- 1978-11-03 CA CA315,776A patent/CA1127406A/en not_active Expired
-
1979
- 1979-01-08 JP JP54000326A patent/JPS5913612B2/en not_active Expired
- 1979-01-09 SE SE7900193A patent/SE438692B/en not_active IP Right Cessation
- 1979-01-24 GB GB7902464A patent/GB2017793B/en not_active Expired
- 1979-03-28 NO NO791019A patent/NO153505C/en unknown
Also Published As
| Publication number | Publication date |
|---|---|
| GB2017793B (en) | 1982-05-19 |
| NO153505B (en) | 1985-12-23 |
| GB2017793A (en) | 1979-10-10 |
| NO791019L (en) | 1980-01-22 |
| SE7900193L (en) | 1979-09-30 |
| JPS5913612B2 (en) | 1984-03-30 |
| JPS54133701A (en) | 1979-10-17 |
| SE438692B (en) | 1985-04-29 |
| NO153505C (en) | 1986-04-02 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| MKEX | Expiry |