Patent Publication Number: US-4094161-A

Title: Submerged apparatus and method for submersing the same

Description:
This invention relates to a method of immersion of massive parts and to the immersed structure obtained as a result of the practical application of said method. 
     In more exact terms, the present invention is concerned with a method in which a massive part can be submerged so as to rest on the ocean floor while being guided during its immersion. 
     In particular, said massive part can be a tank for the storage of hydrocarbons. 
     It is known that the immersion of a hydrocarbon storage tank gives rise to major difficulties, especially when said tank has a substantial volume. In particular, the tank must be well guided during the immersion operation in order to ensure that the laying of the tank on the ocean floor takes place exactly at the desired point and can be controlled at each moment. Moreover, there are many risks of overturning of the storage tank, especially when this latter has a large volume. 
     In addition, it must be made possible to stop the raising or lowering of the storage tank at any moment in order to deal with any unexpected occurrence. 
     The precise aim of the present invention is to provide a method for the immersion of massive parts and in particular of a tank for the storage of hydrocarbons which overcomes the disadvantages mentioned in the foregoing. 
     The method of immersion of a massive part on an ocean bed essentially comprises the following steps: 
     Said massive part is brought into position above the immersion site after being fitted with ballastable floats, 
     Action is produced on said ballasts in order to sink said part in such a manner that the top portion thereof is in the vicinity of the surface of the water, 
     The lower end of a first rigid column fitted with floats which perform the function of ballasts is fixed on the massive part and action is produced on said ballasts of said massive part and/or of said column in order to cause the sinking of the part and a pivotal displacement of the column and in order to bring said column into a vertical position until the upper extremity of said column is brought level with the surface of the water, 
     A plurality of columns are attached successively to the upper extremity of the first column until said massive part attains the sea bed. 
     Preferably, the first column is attached to the massive part and the columns are attached to each other by means of members which permit of free pivotal motion of one of the elements with respect to the element which is attached thereto. 
     In accordance with another characteristic feature, the rigid columns can have a metallic trelliswork structure. 
     In the event that the massive part is a storage tank, each column is fitted with a section of internal or external duct extending over the full length of each column, each duct section being provided with means for connecting said section to adjacent duct sections of said adjacent column or columns. 
     The present invention is also concerned with the submerged structure which is obtained as a result of the practical application of the method and which makes it possible to provide a continuous articulated connection between the submerged storage tank and a platform which emerges above the level of the water. 
     The submerged structure aforesaid comprises a hydrocarbon storage tank equipped with means for maintaining the pressure existing within the storage tank at a value substantially equal to the pressure which prevails in the vicinity of said storage tank, said tank being intended to rest on the sea bed, and a plurality of rigid columns pivotally coupled to each other, the column located at the greatest depth being connected to said storage tank and the top portion of the uppermost column being intended to emerge above the surface of the water and to comprise at least one platform, each column being provided with ballasts and with one duct section, said duct sections being connected to each other so as to constitute a continuous duct between said storage tank and said platform. 
    
    
     A more complete understanding of the invention will in any case be obtained from the following description of one mode of application of the method which is given by way of example and not in any sense by way of limitation, reference being made to the accompanying drawings, in which: 
     FIG. 1 illustrates one form of construction of an immersion column; 
     FIGS. 2 to 6 illustrate the different phases of positioning of a storage tank and of the columns. 
    
    
     There is shown in FIG. 1 a preferred form of construction of a rigid column for the practical application of the method. The column which is generally designated by the reference 2 is constituted by a cylindrical shell 4 of metallic trelliswork construction. The cylindrical shell 4 has an upper end-fitting provided with a coupling member 6 of known design, for example of the knuckle-joint type. Similarly, provision is made at the lower end of said shell for a second end-fitting provided with a coupling member 8 which is capable of cooperating with the coupling member 6. The column is also provided with two floats 10 and 12 each having the shape of a torus. Said floats are fixed on the shell and perform the function of ballasts. Provision can be made within the floats for radial internal partitions in order to prevent rotational displacements of the column when this latter is moved into position over the immersion site. A pipe 14 fitted with a valve serves to establish a connection between the two floats. The upper float is fitted with a pipe 16 for the injection of gas under pressure in order to regulate the quantity of water within the floats. Closable orifices such as the orifice 13 are also formed in the floats in order to adjust the quantity of water contained in these latter. Opening of said orifices is remotely controlled by means of electric cables 18. Said cables can also serve to provide a connection with instruments for checking apparent weight. 
     Finally, the column 2 is preferably provided along its axis with a duct 20 for the flow of hydrocarbons. The duct 20 is provided at each end with a connector 22 which serves to connect the ducts 20 of the different columns to each other. 
     FIG. 2a illustrates the storage tank 32 in the position in which this latter is towed by a ship. 
     By way of example, said tank has the shape of a cylinder with a vertical axis. In actual practice, the diameter of said cylinder can be considerably greater with respect to its height than is shown in the figure. The storage tank is of course constituted by a bottom wall 34, a side wall 36 and a cover 38. The central portion of the cover 38 is provided with a coupling member 40 and one or a number of floating columns 41 which make it possible to improve the stability of the storage tank at the moment of immersion. The tank has a portion 42 which forms a ballast and is filled with water during transportation whilst the remainder of the tank is empty. 
     FIG. 2b illustrates the first stage of the method of immersion which consists for example in filling the storage tank with a liquid which is lighter than water and is designated by the reference 44. This introduction of liquid makes it possible to reduce the apparent weight of the storage tank to zero or practically to zero. 
     During the different immersion stages which will be described hereinafter, this nullification of the apparent weight can take place continuously or in successive steps. There is therefore a continuous connection (not shown in the figures) between an enclosure for the storage of said liquid above the level 46 of the sea and the storage tank. 
     The floating column or columns serve to support the storage tank during the stage of connection of the first column 2a. 
     During the different immersion stages, there is maintained within the tank a pressure equal to the pressure existing in the water which surrounds the storage tank. This result can be obtained by means of the closable orifices 47 formed in the wall of the storage tank. 
     In the following stage, a first column 2a is brought into position in proximity to the storage tank. 
     In the next stage shown in FIG. 3a, the column 2a is displaced in pivotal motion from its horizontal position in FIG. 2c to a vertical position shown in FIG. 3b. 
     This pivotal displacement is obtained by producing action conjointly on the ballasts of the column 2a and of the storage tank 32 and in particular by opening the valve of the pipe 14, thus permitting the water contained in the float 12 to flow towards the float 10. 
     On completion of the pivotal motion which naturally accompanies a downward displacement of the storage tank, the situation is again the same as in FIG. 3b; in other words the upper end of the column 2a emerges above the surface of the water in the vicinity of the level of the top ballast 12. 
     In the following stage, action is again produced on the ballasts in order to bring the top portion of the column into the position shown in dashed lines and designated by the reference 56. 
     A second column 2b which is of the same type as the column 2a is brought into position in proximity to the upper end of said column 2a. 
     There is shown in FIG. 4 a further stage of immersion of the storage tank which consists in securing the second column 2b to the upper end of the first column 2a. 
     The second column just mentioned is also provided with pivotal coupling devices. The operation carried out with the column 2b takes place in the same manner as the operation described in connection with the first column 2a. 
     The pivotal coupling members preferably comprise a pivotal articulation which is fixed on the column or on the storage tank; the free end of the pivotal articulation is provided with a coupling member and this latter is capable of cooperating with a corresponding coupling member which is rigidly fixed to the column or to the storage tank. These pivotal coupling members can be of the type described in the French certificate of Addition No. 96,425 filed in the name of the present Applicant on Nov. 20, 1968. 
     The platform can also comprise means for treatment of the hydrocarbons or the waste waters contained in the storage tank. In particular, this treatment may be intended to remove the traces of water contained in the hydrocarbon. 
     The column 2c mentioned earlier is placed in position in the same manner as the column 2b and, after pivotal displacement, is located in the position shown in FIG. 6. 
     The total height of the three columns is adjusted so as to ensure that, after pivotal displacement of the last column 2c, the bottom wall of the storage tank 2 rests on the sea bed 54. 
     As can readily be understood, the number of columns required in this example of construction is not given in any limiting sense and it is apparent that this number of columns must simply be sufficient to equal the intended depth of immersion below the surface of the sea. This number is therefore dependent on the one hand on the immersion depth and on the other hand on the height to be given to each column. 
     During all the immersion operations, the apparent weight is checked and the admission of water into the ballasts or the discharge of water from these latter is controlled from a servicing ship. 
     It is also readily apparent that each column can advantageously have sections of piping over the entire length of the column, said sections being provided with pipe connections at the level of each coupling member so as to obtain a continuous pipe from the storage tank 32 to the platform 52. 
     Said columns can also be fitted with all desirable cables for devices which serve to detect or check the level of hydrocarbons contained in the storage tank or for any other function.