Patent Publication Number: US-2021179229-A1

Title: Super e-column on semi-submersible vessel

Description:
CROSS REFERENCED APPLICATIONS 
     This application is a national phase filing of PCT Application No. PCT/SG2016/050099 filed Mar. 3, 2016 that is hereby incorporated by reference for all reasons as if set forth herewith. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to a semi-submersible vessel. Particularly, this invention relates to a semi-submersible vessel having a particular support structure between the deck and the pontoons of the semi-submersible vessel. 
     PRIOR ART 
       FIG. 1  illustrates a typical semi-submersible vessel  100  comprises a deck  110  for working space, which is supported by four or more columns  120  which are again supported by two or more pontoons  130 . The pontoons  130  are submerged inside the water, providing majority of buoyance. Braces  140  are provided connecting a pair of columns  120  forming a loop between the deck  110 , columns  120  and pontoon  130 . 
     According to vessel&#39;s functions and displacement size, traditional two pontoon semi structural format as shown in  FIG. 1  are either using many braces to form truss structure to resist wave load on pontoons and columns, or using big column with haunch to fix with deck box to resist wave load. While using many braces to form truss structure can achieve thin deck box, such brace connections are heavily stressed to become critical fatigue points. Further, using big column requires a big deck box and haunch to resist the wave load acting on pontoon and column. Hence, there are limitations to such traditional two pontoon semi structural format. Thus, those skilled in the art are striving to provide an improved structural format of supporting the deck and to improve vessel hydrodynamic performance and functional space. 
     SUMMARY OF THE INVENTION 
     The above and other problems are solved and an advance in the art is made by a semi-submersible vessel in accordance with some embodiments of this invention. A first advantage of a semi-submersible vessel in accordance with some embodiments of this invention is that the semi-submersible vessel includes double skin columns that improves current drag load during operation. This also improves dynamic positioning capability. A second advantage of a semi-submersible vessel in accordance with some embodiments of this invention is that the double skin columns require lesser materials to manufacture. This further leads to lower cost of manufacturing. Still further, weight of the columns is reduced. A third advantage of a semi-submersible vessel in accordance with this invention is that the inner space is functional and creates storage space. 
     In accordance with some embodiments of this invention, a semi-submersible vessel is configured in the following manner. The semi-submersible vessel includes a deck, at least four columns supporting the deck, a first pontoon, a second pontoon, a first brace, and a second brace. Each column comprises an outer skin, an inner skin, and a number of reinforcing plates connecting the inner skin to the outer skin forming a double skin arrangement. The first pontoon is connectable to a first pair of columns and the second pontoon is connectable to a second pair of the columns. The first brace is connectable to the first of each of the two pairs of columns and the second brace connects the second of each of the two pairs of columns. 
     In accordance with many embodiments of this invention, a perimeter of the outer skin at an upper part of the column is larger than a perimeter of the outer skin at a lower part of the column. Further, the perimeter of the outer skin at the lower part of the column conforms to a super ellipse function with the following expressions, 
       x=a cos 2/r t 
       y=b sin 2/r t 
     where a is not equal to b and r is equal to 4. 
     In accordance with some particular embodiments of this invention, a perimeter of the inner skin is the same throughout an upper part and a lower part of the column. Particularly, the perimeter of the inner skin is octagonal in shape. 
     In accordance with a number of embodiments of this invention, each of the columns further comprises a solid block at a bottom of the lower part of the column. The solid block has an indent at a top surface of the solid block for supporting respective first and second braces. Further, the indent conforms to the surface of the first and second braces. 
     In accordance with many embodiments of this invention, the reinforcing plates are evenly spaced apart from each other. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other features and advantages in accordance with various embodiments of this invention are described in the following detailed description and are shown in the following drawings: 
         FIG. 1  illustrating a perspective view of a known semi-submersible vessel; 
         FIG. 2  illustrating a perspective view of a semi-submersible vessel in accordance with an embodiment of this invention; 
         FIG. 3  illustrating a side view from a column of the semi-submersible vessel in accordance with an embodiment of this invention 
         FIG. 4  illustrating a drag load analysis of the known semi-submersible vessel and the semi-submersible vessel in accordance with an embodiment of this invention; 
         FIG. 5  illustrating another drag load analysis of the known semi-submersible vessel and the semi-submersible vessel in accordance with an embodiment of this invention; 
         FIG. 6  illustrating a cross sectional view of the lower and upper parts of the column in accordance with an embodiment of this invention; 
         FIG. 7  illustrating a perspective view of a portion of the column in accordance with an embodiment of this invention; 
         FIG. 8  illustrating a plan view of an arrangement between the columns and the deck of the known semi-submersible vessel; and 
         FIG. 9  illustrating a side view of an arrangement between the columns and the deck with haunch structures of the known semi-submersible vessel. 
     
    
    
     DETAILED DESCRIPTION 
     Various embodiments of this invention relate to a semi-submersible vessel. Particularly, some embodiments of this invention relate to a semi-submersible vessel having a particular support structure between the deck and the pontoons of the semi-submersible vessel. 
       FIG. 2  shows a perspective view of a semi-submersible vessel  200 . Similar to a known semi-submersible vessel  100 , the semi-submersible vessel  200  also comprises a deck  210 , columns  220 , pontoons  230  and braces  240 . Water level  260  illustrates the water level in which the semi-submersible vessel  200  is in transit, i.e. moving to a new location. Water level  270  illustrates the water level in which the semi-submersible is in operation, i.e. anchored at a location. In use, the pontoons  230  are partially submerged under the water with the braces  240  above the water level  260  during transit. This ensures that the braces  240  are not subjected to drag forces during transit. This is also advantageous for visual inspection. 
     The deck  210  provides a top surface where offshore activities can be carried out. The deck  210  is supported by the columns  220 . Particularly, columns  220  are connected to the bottom surface of the deck  210 . Although  FIG. 2  shows deck  210  being quadrilateral, one skilled in the art will recognise that other shapes can be implemented without departing form the invention.  FIG. 8  illustrates a plan view of a common arrangement of the columns and the deck where a portion of each of the columns extends outside the area of the deck  210 . In such arrangement, full support of the deck  210  will not be rendered by the columns  220  since only a portion of each of the columns is supporting the deck  210 . Hence, haunch structures  910  are required to adjoin the columns to the bottom surface of the deck which is shown in  FIG. 9  being the side view of the arrangement of the columns and the deck. Differently, as shown in  FIG. 2 , the four columns  220  of the semi-submersible vessel  200  are provided within the area of the deck  210  and proximate or at corners of the deck  210 . This ensures that the deck  210  is fully supported by the columns  220 . Hence, additional haunch structures are not required. Although  FIG. 2  illustrates that the columns  220  are arranged at corners of the deck  210 , one skilled in the art will recognise that the columns  220  need not be exactly at the corners of the deck  210  without departing from the invention. 
     Although only four columns  220 , with one obscured, are illustrated in the semi-submersible vessel  200  as shown in  FIG. 2 , one skilled in the art will recognize that other number of columns  220  may be provided depending on the dimension of the deck  210  and the exact number of columns  220  is left to those skilled in the art. 
     The pontoons  230  houses a number of ballast tanks to control the buoyancy of the pontoons  230 . By controlling the ballast tank, the semi-submersible vessel  200  can be maintained at a certain level with respect to the water level. Essentially, the pontoons  230  are maintained at a certain level of buoyancy by controlling the ballast tank such that the deck  210  is above the water. Particularly, the buoyancy of the pontoons  230  are kept about the water level  270  when the semi-submersible vessel  200  is in operation and water level  260  when the semi-submersible vessel  200  is in transit. One skilled in the art will recognise that the water level  260  and water level  270  are meant for illustrative purposes. In other words, the water level  260  and water level  270  are meant to be rough guide of the water levels when in operation and transit. 
       FIG. 3  illustrates a side view  301  of one of the columns  220 , a first cross sectional view  302  between X-X of the side view  301 , and a second cross sectional view  303  between Y-Y. Each of the columns  220  comprises an inner skin  320 , an outer skin  310  and a number of reinforcing plates  330  between the inner and outer skins. The columns  220  further comprise an upper part  221  and a lower part  222 . The upper part  221  and lower part  222  may be integrally manufactured to obtain a better structural stability. Alternatively, the upper part  221  and lower part  222  may be manufactured separately and joined together by welding. 
     The reinforcing plates  330  are provided between the inner skin  320  and outer skin  310  to bind the inner skin  320  to the outer skin  310 . Further, the reinforcing plates  330  are arranged vertically, extending between the top and bottom of the columns  220 . This double skin arrangement of reinforcing plates  330  between the inner skin  320  and outer skin  310  creates voids within the inner skin  320  and outer skin  310  and hence reduces the amount of materials required. Inevitably, the mass of the columns are greatly reduced and costs of material to manufacture the columns are lowered as well. Further, the void created increases the buoyancy of the semi-submersible vessel  200 . Still further, the void can also be used for storage, creating functional spaces within the inner skin. One skilled in the art will recognise that the reinforcing plate  330  may be replaced with reinforcing bars without departing from the invention. Further, additional reinforcing plates or reinforcing bars may be provided to interlock the vertically arranged reinforcing plates  330  to strengthen the columns  220 . 
     The shape of the outer skin  310  is based on super ellipse function which can be described parametrically by the following expressions, 
     
       
         
           
             x 
             = 
             
               a 
                
               
                   
               
                
               
                 cos 
                 
                   2 
                   r 
                 
               
                
               t 
             
           
         
       
       
         
           
             y 
             = 
             
               b 
                
               
                   
               
                
               
                 sin 
                 
                   2 
                   r 
                 
               
                
               t 
             
           
         
       
     
     Where a is not equal to b and r is equal to 4. 
     An analysis on the current drag load of a conventional semi-submersible vessel and a semi-submersible vessel  200  in accordance with this invention is shown in  FIGS. 4 and 5 .  FIG. 4  illustrates current moving towards the front of the semi-submersible vessel. The left of  FIG. 4  shows the current drag load of a conventional semi-submersible vessel while the right of  FIG. 4  shows the current drag load of the semi-submersible vessel  200 . Under this condition, it is noted that the semi-submersible vessel  200  has a current drag load reduction of about 18% compared to the conventional semi-submersible vessel.  FIG. 5  illustrates current moving towards the side of the semi-submersible vessel. The left of  FIG. 5  shows the current drag load of a conventional semi-submersible vessel while the right of  FIG. 5  shows the current drag load of the semi-submersible vessel  200 . Under this condition, it is noted that the semi-submersible vessel  200  has a current drag load reduction of about 5% compared to the conventional semi-submersible vessel. This reduction in current drag load can be attributed to the shape of the outer skin  310  at the lower part  222  of the columns  220  which provide a better hydrodynamic performance than the conventional semi-submersible vessel. 
     Cross sectional view  302  illustrates the cross sectional view of the lower part  222  along X-X of the column  220  while cross sectional view  303  illustrates the cross sectional view of the upper part  221  along Y-Y of the column  220 . As shown, the perimeter of the inner skin  320  in the upper and lower parts of the column is the same while the perimeter of the outer skin  310  at the upper part  221  of the column is larger than the perimeter of the outer skin  310  at the lower part  222  of the column. In particular, the perimeter of the outer skin  310  at top of the upper part  221  of the column is quadrilateral with rounded vertices. A larger perimeter at the upper part  221  of the outer skin  310  allows for better connection with the deck  210 . Although  FIG. 3  shows that the shape of both inner skin  320  and outer skin  310  at the lower part  222  is in accordance with the super ellipse function mentioned above, one skilled in the art will recognise that other shapes may be implemented without departing from the invention as long as the outer skin  310  in the lower part  222  of the columns  220  conforms to the super ellipse function mentioned above.  FIG. 6  illustrates another configuration of the columns  220  where reference number  601  shows the cross sectional view of the lower part  222  of the column  220  and reference number  602  shows the cross sectional view of the upper part  221  of the column  220 . Particularly, the cross section of the inner skin  320  is an octagon. The void within the inner skin can be used as storage space. Hence, the shape of the inner skin is a design choice left to the skilled in the art. 
       FIG. 7  illustrates a perspective view of a portion of the column  220  with one end of the brace  240  connected to proximate bottom of the lower part of the column  220 . As the double skin arrangement of the columns  220  is hollow in nature, a block structure  710  is provided at the bottom of the lower part  222  of the column  220 . An indent  720  extends a length of the top of the solid block  710  and is shaped according to the surface of the braces  240  so that the brace  240  rests on the indent  720 . This provides better structural support to withstand deformation induced force acting on the braces  240 . 
     The above is a description of exemplary embodiments of a semi-submersible vessel in accordance with this invention. It is foreseeable that those skilled in the art can and will design alternative structure or assembly based on this disclosure that infringe upon this invention as set forth in the following claims.