United States Patent (19) 11) 4,427,320 Bhula 45) Jan. 24, 1984 IC OFFSHORE PLATFORM 54 ARCTIC OFFSHO O (75) Inventor: Dilipkumar N. Bhula, Houston, Tex. FOREIGN PATENT DOCUMENTS 1081483 7/1980 Canada ................................ 405/211 73) Assignee: Shell Oil Company, Houston, Tex. Primary Examiner-David H. Corbin (21) Appl. No.: 350,458 57) 22 511 52 58) Filed: Feb. 19, 1982 3 Int. Cl. ............................................. E02B 17/00 U.S.C. .......................... '05/211; 05/217 Field of Search .................. 405/61, 195, 211, 217 56) References Cited U.S. PATENT DOCUMENTS 4,215,952 8/1980 Baardsen ............................. 405/211 ABSTRACT An offshore structure for use in drilling and producing wells in arctic regions having a conical shaped lower portion that extends above the surface of the water and a cylindrical upper section. The conical portion is pro vided with a controlled stiffness outer surface for with standing the loads produced by ice striking the struc ture. The stiffness properties of the outer shell and flexi ble members are designed to distribute the load and avoid high local loads on the inner parts of the struc ture. 4,252,471 2/1981 Straub ................................. 405/21 4,335,980 6/1982 DePriester .......................... 405/217 6 Claims, 6 Drawing Figures UPPER level z H - MIDDLE LEVEL g - 11 LOWER LEVEL 12 F- sts "" 2 rim- a , -m. fo - se - .. -/- - s-NN re "" ', - , -l. ris. A v\ SN w ww. w w &v V V w - - - - - v. FA N = s = -s. m w - - s bro zS 4,427,320 Sheet 1 of 4 Jan. 24, 1984 US. Patent 37 73/137 33/1107 11/. 3:7 73/137 3700114 '7 73/137 HEIdc/n - US. Patent Jan. 24, 1984 Sheet 2 of4 4,427,320 US. Patent Ja?. 24, 1984 Sheet 3 of4 4,427,320 FIG. 4 37 US. Patent Jan. 24, 1984 Sheet 4 of4 4,427,320 ?\ca 4,427,320 2 FIG. 3 is a horizontal section taken along line 3-3 of FIG. 2. ARCTIC OFFSHORE PLATFORM FIG. 4 is a portion of FIG. 3 drawn to an enlarged scale. FIG. 5 is a horizontal section of the flexible beam BACKGROUND OF THE INVENTION The present invention relates to offshore structures and particularly structures for conducting drilling and producing operations in the Arctic regions. More par ticularly, the structure is particularly adapted for con ducting operations in the shallow waters of the Beaufort Sea. As is well known, the Beaufort Sea at particular times of the year contains large movements of relatively thick sea ice and offshore drilling structures must with stand this movement. In the past, it has been suggested that any offshore structures based in the shallow waters structure. FIG. 6 is a section taken along line 6-6 of FIG. 5. DESCRIPTION OF PREFERRED 10 15 of the Beaufort Sea have a conical base section to force the moving ice upward, causing it to break due to the tension forces imposed upon the ice. This will cause the large ice features to break into smaller pieces which then can pass safely around the offshore structure. While the use of conical-shaped bottom sections obvi ously solves the problem of breaking the large moving ice sheets into smaller sections, the problem still remains of how to provide an outer skin for the conical section that can withstand the loads imposed by the moving ice sheets. One solution suggested by the prior art is de 20 EMBODIMENTS The structure of the present invention is designed to resist loads due to first year and multiyear ice sheets and ridges, rubble piles and dynamic impacts from storms accompanied by ice invasions. The overall form of the structure comprises a frustum of a cone for the lower portion with a base diameter, for example, of 350 feet. The base portion has a cone angle of typically 45 de grees and is joined to a cylindrical upper section. The upper section contains the drilling and production equipment and facilities. The conical portion of the structure consist of a sys rial is disposed between the wear surface of the conical base section and the support portion thereof. The use of 30 tem of radial and circumferential bulkheads supported by a continuous base plate. The ice and wave loads are borne by a stiff outer shell comprising an orthotropic structural system consisting of radial and meridional webs and top and bottom flanges. The outer shell trans fers the loads to a bulkhead system through an indirect load path created by supporting the outer shell on a system of flexible beams which span the space between the radial bulkheads. The stiffness properties of the upon the structure by the large ice floes. While this is a outer shell and flexible beams are selected to distribute 25 scribed in U.S. Pat No. 4,215,952 where a resilient mate the resilient section is intended to reduce loads imposed possible solution, it requires the use of relatively flexible outer surfaces on the conical base in order that the load can be transmitted to the resilient material positioned between the support structure and the outer surface. 35 The key design problem is to avoid excessive concen tration of load on the supporting bulkheads. BRIEF DESCRIPTION OF THE INVENTION The present invention solves the above problems by 40 spreading the load over a larger area before it is trans ferred to the bulkheads. This is achieved by using a stiff conical outer shell which is supported by a system of beams spanning between radial bulkheads. When the ice 45 load is applied to the stiff outer shell it, in turn, transfers the load to the supporting beams. Since these beams are more compliant than the outer shell they will deflect, permitting the shell to move inward and transfer the load to adjacent sets of beams. The upper end of the conical outer shell is attached to a cylindrical upper shell which houses three decks which contains the drilling and production equipment. The entire structure may be constructed in a less hostile environment, towed to location under its own buoyancy, and installed on location by water ballasting. It will resist ice and wave loads by a gravity foundation using a system of steel skirts to transfer the loads into the foundation soil. BRIEF DESCRIPTION OF THE DRAWINGS The present invention is more easily understood from the following description when taken in conjunction with the attached drawings in which: 50 55 the load over a number of bulkheads instead of a single bulkhead. This avoids the imposition of high local loads on individual bulkheads as is the case with previous designs. Referring now to FIG. 1, there is shown an elevation view of the conical base section 10 coupled to a cylin drical upper section 11 that may be either a drilling or production facility. The upper section is provided with three levels; 12, 13 and 14. which can contain drilling supplies, the drilling equipment, production equipment and living quarters for the drilling or production per sonnel. The base section is provided with sufficient height so that the upper portion of the truncated conical section extends above the normal water line 15. The extension should be sufficient so that moving ice will ride up the conical section and break due to tension stress before it impacts on the upper section i. Referring to FIG. 2, there is shown a partial vertical section of the conical base member shown in FIG. I. The conical base member includes a base plate 20 which is attached to a skirt 21 which extends down into the ocean bottom to assist in anchoring the conical base in position. Normally, the conical base will be ballasted or flooded so that the weight of the conical base, plus the sea water, will cause it to sink and rest on the ocean floor with the skirt 21 penetrating the ocean floor. The base 20 may be constructed of a stiffened plate system. The outer skin 22 of the conical base is actually con posed of two spaced-apart plate members having a se ries of circumferential stiffening webs 24 and meridional webs 32. The actual construction of the outer surface will be described in detail with relation to FIGS. 3 and 4. The space between the outer and inner plates at the ous plate members 25 and 26. The composite outer shell is spaced from the outer ends of the radial bulkheads 27 FIG. 1 is a schematic elevation view of the base sec 65 lower end of the outer surface is closed by two continu tion of the invention attached to a circular production platform. FIG. 2 is a vertical section of the conical base section. so that it is free to move within this restricted distance 4,427,320 3 as the supporting beams flex. The flexible beam systems 23 are positioned between the radial bulkheads 27 as particularly shown in FIGS. 5 and 6. As shown, the 4. 9-degree centers while the flexible beam system com posed of I-beans having a flange width of approxi mately 1.5 feet and a thickness of 2.75 inches with a 12-inch high web. The total weight of steel in the struc ture is approximately 40,000 tons which would provide the reserve buoyancy of approximately 50,000 tons while towing the base structure in an upright position. The above described conical base structure provides flexible beam system adjacent the water line is provided with a double set of beams since this is the area which is subject to the greatest load by the moving ice. Referring now to FIGS. 3 and 4, there is shown the detailed construction of the outer shell and the flexible supporting beams of the conical member. In particular, the outer surface 22 of the base member consists of an O a flexure of approximately two-inches when the outer surface of the conical base is subjected to its maximum outer skin 31 and an inner skin 34 which are spaced apart. The inner and outer skins are held apart by the load of 26,000 kips. In order to withstand greater loads, it may be necessary to increase the diameter of the conical base and increase the thickness or strength of some of the members. While increasing the diameter It should be noted that the outer shell is spaced a supported at the mid-span of the flexible beam system 23 by parallelogram shaped load transfer boxes 33. As system. horizontal or circumferential webs 24 shown in FIG. 2 and a series of meridional webs 32. The combination of the meridional and the circular webs form a cellular or 15 and the thickness of the members, it should be borne in egg crate type structure for the outer shell. mind that one should still maintain the flexible beam distance 30 from the ends of the radial bulkheads 27 and shown in FIG. 4, the load transfer box 33 terminates in a flat flange section 35 which contacts the individual a series of secondary stiffening webs 36 are positioned beam members. In addition to the meriodional webs 32, between each of the meridional webs. What is claimed is: 1. An offshore structure for use in arctic water con 20 taining moving ice masses comprising: a frustum base section and a circular upper section, said base section having sufficient height to extend above the surface of the water; the skin of said frustum section having a controlled stiffness cellular structure formed by an outer plate member and an inner plate member, said outer and inner plate members being separated by a series of radial and meridional webs fastened to said plates Referring now to FIGS. 5 and 6 there is shown the 25 details of the flexible beam system used for supporting the outer shell of the conical base. In particular, each flexible beam member of the system adjacent the water to form a cellular structure; and line comprises two I-beams 40 and 41 which are spaced a flexible beam structure, said beam structure being apart a short distance 42 by spacing members 43 and 44 30 formed by a series of circumferential beams sup positioned at the center and the ends of the beams re ported by a series of radial bulkheads projecting spectively. The spacing members allow the beams to upward from the bottom of said frustum, said skin transmit the load from the outer shell to the radial bulk being attached to said flexible beam structure inter heads while maintaining their ability to flex with respect mediate said radial bulkheads. to each other without shear transfer. To increase their 35 2. A frusto-conical base member for use in an offshore flexibility the ends of the I-beams have a slight clear structure for conducting operations in arctic waters ance 45 at each end adjacent the radial bulkheads. The moving ice, said base member comprising: end of the innermost beam is supported by a T-section having a solid circular bottom; having an end 48 attached to the beams and a web 47 an outer skin for said base, said outer skin being attached to the radial bulkheads. The flanges of the formed by a solid outer plate and a solid inner I-beams are reinforced by web members 46 adjacent the plate, said inner and outer plates having a general center and the ends respectively, to ensure that the conical shape and radially spaced, a plurality of load-bearing portions of the beam and the flanges do not meridional web members positioned between said buckle or collapse. As best seen in FIG. 5, the flat sec inner and outer plates and fasten thereto to main tion 35 of the parallelogram shaped box of the outer 45 tain the spacing between said plates; shell bears against the outermost flange of the I-beam a plurality of radial bulkheads attached to said bottom 40. Thus, the load from the outer shell is transmitted and terminating short of said outer skin; and over a narrow area of the beam which permits the beam a plurality of horizontal flexible beams, said beams to slightly flex to absorb the load imposed on the outer being secured between said radial bulkheads to shell. Since the beams are not connected at their ends to form a series of vertically spaced substantially cir the radial bulkheads but only supported by the T-sec 50 cular beams, said beams supporting said outer skin tions, the beams can readily flex, absorbing the load at points intermediate said bulkheads. from the outer shell. The flexibility of the conical base base member of claim 2 wherein in addition to and limited contact points between the outer shell and said3. The meridional members a series of circumferential supporting structure also prevents excessive loads as a 55 result of temperature fluctuation. Also, an insulating webs are positioned between said inner and outer plates layer can be placed on the inside surface of the outer and fastened thereto. 4. The base member of claim 3 and in addition sec shell of the conical base and upper cylindrical section to ondary stiffening webs, said webs being attached to the provide thermal insulation. In a typical base structure designed for a load of outer shell and positioned between said meridional 25,000 kips and a maximum contact pressure of 1600 psi, 60 members. 5. The base of claim 4 wherein said inner plate is the base structure would have a diameter of approxi mately 350 feet with a height of roughly 70 feet for provided with a series of load transfer boxes that operating in water depths of 30 to 60 feet. The upper project inwardly from said inner plate, said load trans cylindrical drilling platform is 210 feet in diameter with fer boxes having a flat surface that contacts said beams a height of 90 feet. The outer shell of the conical section 65 at approximately the center thereof. 6. The base member of claim 5 wherein said box has includes two-inch outer and two-inch inner plate walls with two-inch meridional webs with one-inch plate a parallelogram cross section. is as radial bulkheads. The radial bulkheads were placed on