Patent Publication Number: US-3879952-A

Title: Pressure resistant caisson

Description:
United States Patent m1:  
 Mo Apr. 29, 1975 PRESSURE RESISTANT CAISSON OTHER PUBUCATIONS [76] Inventor: Olav Mo, Groensundveien 94. T l4 1 6 I 360Nesbru Norway he Oll and Gas Journal of Sept. 970. pp. 0 l. [22] Filed: Apr. 23, 1973 Primar Emminer-Jacob Shapiro [2 I] Appl&#39; No: 353,538 Attorney, Agem, or Firm-Larson, Taylor and Hinds 30 F A I r P 0 D [57] ABSTRACT I l I f pp &#34;9&#34; am 7 A monolithic offshore platform includes a number of Ma 197; vertical, cylindrical cells which are monolithically at- DOC. 5. l97 447N727 tached to each other at the Contact points. The cells will therefore circumferentially be subjected to pure [5?] IU.S. Cl til/46.5; 6l/50 compressive forces In one embodiment a&#34; the cells 2 27/38 [.3653 5/00 have the same outer diameter. There is one central I e 0 T cell and around this there are first six cells with the 5 centers on a circle concentric with the central cell, 6] References cued thereafter l2 cells are placed outside. If desired. new  
  N T STATES PATENTS rows of cells can be added on the outside. The struc- 2.661.600 12/1953 Hopkins..... 6l/46.5 ture can also be constructed without the central cell. 3.434.442 3/!969 Manning.... 61146.5 X The cells can be closed with a spherical shell in each 3.535.884 [0/1970 Chaney (Si/46.5 end 3.708.981 H1973 Roulet 61/46 3.793.842 2/1914 Lucroix 6l/46.5 8 a 13 Drawing Figures .QAMVH,  
  T: I l .r -6&#39;9 i /0 awe-.522  
  mam; 45 -,P- I swam w IZZY/M4474 PRESSURE RESISTANT CAISSON The invention relates to a caisson of concrete for location on the sea floor. Such caissons are particularly useful as foundations for platforms and as oil storage.  
  A concrete structure should principally not be subjected to tension. This increases cost because reinforcements are necessary to take tensile forces. Tensile forces also increase the danger of cracks with accompanying leakage, corrosion of the reinforcing material, etc. On the other hand, a concrete construction is very well suited for taking compressive forces because concrete per se is cheaper than steel in connection with compressive forces and because concrete structures become so coarse that breaking and local stresses rarely become problems.  
  The object of the present invention is to find a caisson which generally is subjected to compressive forces while having marine qualities necessary for structures which are to be floated and submerged at sea.  
  FIG. 1 is a schematic side elevational view partially in cross section of a caisson showing the principle features of the invention;  
  FIG. 2 is a view taken along line 1-1 of FIG. 1 showing a cell configuration according to the invention;  
  FIGS. 3 through I2 are schematic diagrams showing successive stages of construction of a caisson according to the invention; and  
  FIG. 13 is a view similar to that shown in FIG. 2 showing an alternative cell configuration.  
  The invention will be described in the following with reference to the example shown on the FIGS. 1-2. The caisson consists of a number of vertical, cylindrical cells which are monolithicly attached to each other at the contact points. The cells will therefor circumferentially be subjected to pure compressive forces. Since all the cells are of the same size, there will be no fixed moments at the contact points, deformations appearing only as changes of scale. Practically speaking, the cells will act independently which is of great importance if a wall should collapse in an accident. If the number of cells is great enough, as for instance is shown on the drawing, the structure is functional even when it is collapsed locally. Looking at the individual operations, the following result is evident:  
  During construction (see below) collapse of one cell will only make the structure take a certain list. The same is true during towing. on the condition that the free-board is sufficient. If a list is present, it will increase during submerging but, if the structure has one or more towers as shown on the figures, the structure will not capsize. Even if the caisson should sink, each cell will resist water pressure so that the structure later can be brought to the surface and repaired.  
  Furthermore, dividing the structure in many small, independent cells has the advantage that the ballast cannot shift appreciably. and the reduction in metacenter height due to internal free water surface becomes insignificant.  
  A division into small cells also gives the advantage in that trimming is easily done. If the sea floor is not level, this can be compensated to a certain degree by more weight on the highest side.  
  The round form is advantage with respect to breakage of the walls.  
  To obtain the effects mentioned above, the number of cells must not be too small. The number should not be below 8.  
 A particularly preferred form is shown on FIG. 2. As  
 5 is seen from this figure, all the cells have the same outer diameter. There is one central cell and around this there are first 6 cells with the centers on a circle concentric with the central cell, thereafter 12 cells are placed outside as shown on the figure. If desirable, new rows of cells can be added on the outside as shown on FIG. 13, where a new row of I8 cells is added.  
  The structure can also be constructed without the central cell.  
  The cells can be closed with a spherical shell in each end as shown on the figures. Also the ends will thereby be pressure structures. If the edge angle of the spherical shell is too small, tensile stresses will arise at the ends of the cylinder. These must be taken by prestressed reinforcements if cracks in the structure are to be avoided. It has been found, however, that if the edge angle of the spherical shell is larger than approximately 55, no tensile stresses in the cylinder will arise, and the structure will only have compression when it is subjected to pure water pressure.  
  One or more of the cells may be extended up over the water&#39;surface and form the foundation for a working platform. The extention, or the tower, ought to be conical because this will substantially reduse the wave forces without redusing the critical cross section at the bottom.  
  It can often be advantageous to fit the caisson with a skirt to prevent the sea floor from being washed out from under the structure etc. This is easily obtained according to the invention by extending the cylinder walls downwards. The skirt will thereby have the same good resistance to breakage as the cell walls while, at the same time, transmission of forces between the skirt and the cell becomes very simple.  
  By use of ballast, the center of gravity of the caisson can be brought low enough for the caisson to be stable per se during towing and submerging. Several towers with sufficient distance between them will also add to the stability.  
  Small deviations from the even thickness and circular form of the cells are contemplated, for instance for obtaining a form which can better sustain shear forces.  
  If desirable, a caisson according to the invention will be very easy to prestress. Due to the form, however, the water pressure will act as prestressing and prestressing will therefor normally not be necessary.  
  Prestressing necessary due to tension is conceivable at the following locations:  
 a. the towers in vertical direction due to wave, wind and current forces;  
 b. the caisson as a whole if the internal pressure hecomes greater than the external pressure which is conceivable if the caisson is used for instance for oil storage and the oil surface is not kept below the water surface;  
 c. the bottom section when the caisson is filled with water due to the weight of the ballast;  
 d. upper and lower part of cell wall if the spherical shells are too flat.  
 By keeping the internal pressure always lower than the external pressure, all the above mentioned cases of tension can be avoided.  
  A method for construction a caisson in accordance with the invention is shown schematicly on FIGS. 3-12. As will be seen. the bottom section is first made in a dry dock, this section is floated to deep water and the caisson is finished there. On the figures it is shown that the cylinder walls are made with a sliding form and that the same is true for the tower. This method makes it very easy to construct the cells monolithicly connected which is of great importance for the strength of the caisson.  
  Although but two embodiments of the invention and one method of construction have been described in the preceding and showed on the figures. it will be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the spirit of the invention or the scope of the appended claims.  
 I claim:  
  1. A monolithic offshore platform comprising: a heavy sub-structure which is heavier than the rest of the platform. the sub-structure being formed by a plurality of rigidly interconnected. vertical elongated cells; and comprising six cylindrical cells connected monolithically at their contact points with centers on a circle with radius equal to the outer diameter of the cells and twelve cylindrical cells of equal size placed outside the six cells and also connected monolithically at their contact points; a superstructure having a crosssectional area which is exposed to wave action which area is considerably less than the cross-sectional area of the sub-structure; the superstructure being formed by at least three vertical elongated cells. each of which having an outer and inner wall at least a portion of which are conical with a larger lower diameter and a smaller upper diameter; and a deck structure supported by the superstructure; none of said parts of the platform being moveable in relation to each other.  
 2. An offshore platform as claimed in claim 1 including a central cell of equal size.  
  3. An offshort platform as claimed in claim 1, wherein the superstructure is formed by at least three vertically elongated cells superposed above three of the cells in the substructure, the lower diameter of each of the cells in the superstructure being equal to the diameter of the corresponding cell in the sub-structure.  
  4. A monolithic off shore platform comprising in combination a heavy substructure which is heavier than the rest of the platform, the substructure being formed by a plurality of rigidly interconnected vertical elongated cells; a superstructure having a cross-sectional area which is exposed to wave action, which area is considerably less than the cross-sectional area of the substructure; the superstructure being formed by at least one vertical elongated cell. each such cell being formed as a static and continual elongation of one or more cells in the substructure and each being conical at least along a portion of its length with a larger outer and inner lower diameter and a small outer and inner upper diameter; and a deck structure supported by the superstructure; none of said parts of the platform being moveable in relation to each other.  
  5. A monolithic off shore platform as claimed in claim 4, wherein each cell in the substructure which is lengthened to form the superstructure, has a circular cross-sectional area.  
  6. A monolithic off shore platform as claimed in claim 5. wherein the lower diameter of each of the cells in the superstructure is substantially equal to the diameter of the corresponding cell in the substructure.  
  7. A monolithic off shore platform as claimed in claim 4, wherein the portion of the cells in superstructure which is exposed to water level, has a cylindrical shape.  
 8. A monolithic off shore platfomt as claimed in claim 1 wherein said cells are cylindrical.  
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