Abstract:
An offshore floating structure ( 10 ) for the drilling and production of oil and gas includes a generally circular toroidal, hollow pontoon ( 11 ) of substantially the same radial width throughout a perimeter of the pontoon. The offshore floating structure includes a plurality of columns ( 12 ) of substantially a same cross-sectional area, each coupled at a coupling point, on a bottom end thereof to the pontoon at an equidistant point along the perimeter of the pontoon, and adapted to be coupled on a top end to a deck structure. The diameter ( 23 ) from a center of the radial width of the pontoon is greater than a distance ( 21 ) from a center of one column to a center of an adjacent column.

Description:
CLAIM OF PRIORITY OR CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of priority to Provisional Patent Application Ser. No. 61/416,570, filed Nov. 23, 2010, the contents of which are incorporated herein by reference in their entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates generally to an offshore floating platform for the drilling and production of oil and gas. Specifically, the invention relates to a circular cylindrical semi-submersible platform (C-Semi) for offshore drilling and production. 
       BACKGROUND OF THE INVENTION 
       [0003]    Floating structures used for offshore oil and gas drilling and production are known. One such floating structure is conventional semi-submersible hulls. A conventional semi-submersible hull has a square pontoon structure. The square pontoon structure is coupled to four square shaped columns placed at the four corners of the pontoon structure. Therefore, the pontoon section length is the same as the length separating the columns. 
         [0004]    In a conventional semi-submersible hull, the columns do not have strakes. Each column is connected to a deck structure to support topside facilities. A spread mooring or dynamic positioning system is used for station keeping. 
         [0005]    Conventional semi-submersible hulls have several limitations. They are subject to large heave, roll and pitch motions. A conventional semi-submersible hull is unable to support steel catenary risers in extreme weather conditions. These steel catenary risers also have fatigue problems in long term operating conditions. Furthermore, a conventional semi-submersible hull is unable to be used for dry tree production applications while undergoing these motions. 
         [0006]    There are also known variants of this structure that alter the draft and the column distance of the floating platform. In traditional structures, the length of the pontoon structure is considerably larger than the draft. In an attempt to reduce the effects of motions experienced in extreme and operating weather conditions, structures were developed with an increased draft and/or modified column distance. However, these deep draft variants are still operationally limited. 
         [0007]    Another type of known floating structure is an extendable draft platform (EDP). An EDP structure includes a buoyant equipment deck. The buoyant equipment deck is either rectangular or triangular. Column wells are coupled to each corner of the buoyant equipment. On an opposite end, the columns are coupled to a heave plate. In an EDP structure, each of the columns has an upper portion with a diameter that is different from that of a lower portion, which is usually smaller. The columns can move vertically in the column well to adjust the draft. 
         [0008]    EDP structures have several limitations. EDP structures are difficult to manufacture and maintain because they use complex, large moving components. Additionally, strong sub-surface currents can cause vortex-induced vibrations (VIV). A structure that has prolonged exposure to VIV can experience fatigue damage to components and is subject to structural failure. 
         [0009]    A dual column semi-submersible hull is a known floating structure as well. A dual column semi-submersible hull has a deck structure that is supported by vertical columns arranged in pairs. In these structures, one set of the paired columns is displaced a distance outward from the other set of paired columns. The other set of paired columns is in line with a pontoon structure. The lower ends of this set of vertical columns are connected to the pontoon structures. 
         [0010]    The dual column semi-submersible hull, at a much deeper draft, has better performance than a conventional semi-submersible hull at a much shallower draft. However, at the same draft, the dual column semi-submersible hull only marginally improves the motions of a conventional semi-submersible hull. In addition, the dual columns complicate design, fabrication and operation. 
         [0011]    There is also a central pontoon semi-submersible floating platform. The central pontoon structure is disposed inboard of the columns, with each of said vertical support columns having a transverse cross sectional shape with a horizontal major axis oriented radially outward from a center point of said hull. However, the vertical wave force on the central pontoon not substantially cancelled by the forces on the columns. This arrangement has adverse effects, and can result in worse vertical motions than a conventional semi-submersible hull at the same draft. 
         [0012]    The other known semi-submersible is octabuoy. The draft of octabuoy is substantially greater than the distance between the columns&#39; central axes. The columns have quite large diameter relative to the length of pontoon section, and the pontoon section length is around 2 times column diameter. As a result, the column displacement is a few times greater than the pontoon displacement, and the wave forces on the columns will make a greater contribution than the force on the pontoon. The most preferred draft of octabuoy is at least 60 meters, and the most preferred ratio of draft to the distance between central axes of columns is 1.3 to 1.35. The substantially deep draft required makes it cannot integrate topsides at quaysides because of water depth limitations. Additionally, float over operations near the shore are required. The nonlinear shape and variant cross section of columns also increases fabrication complexity. 
         [0013]    Therefore, for the drilling and production of offshore oil and gas, there is a need for a simple floating structure that is subject to minimized environmental forces and platform motions compared with known semi-submersibles. 
       SUMMARY OF THE INVENTION 
       [0014]    According to an embodiment of the present invention, a C-Semi floating platform for offshore production and drilling includes a generally circular toroidal, hollow pontoon, a plurality of columns, a deck structure, and topside facilities. The circular cylindrical pontoon can be comprised of straight and curved sections. The diameter from a center of the radial width of the pontoon is larger than the distance from one column center to an adjacent column center. At the intersection points of columns and pontoon, the cross-sectional area of columns is generally greater than, but can be equal to or less than, the corresponding area of pontoon. The columns have a cross section that is either circular or square with rounded corners. If desired, each column can be provided with overlapping helical strakes, which extend across the entirety of the column perimeter below the waterline. 
         [0015]    In one embodiment of the present invention, the offshore floating structure for the drilling and production of oil and gas includes a generally circular toroidal, hollow pontoon of substantially the same radial width throughout a perimeter of the pontoon. The offshore floating structure includes a plurality of columns of substantially a same cross-sectional area, each coupled at a coupling point, on a bottom end thereof to the pontoon at an equidistant point along the perimeter of the pontoon, and adapted to be coupled on a top end to a deck structure. The diameter from a center of the radial width of the pontoon is greater than a distance from a center of one column to a center of an adjacent column. 
         [0016]    According to another embodiment of the present invention, the offshore floating structure is a hollow, oval toroidal pontoon of substantially a same radial width throughout the perimeter of the pontoon. The offshore floating structure includes four large columns of substantially a same cross-sectional area, each coupled on a bottom end thereof to the pontoon at an equidistant point along the perimeter of the pontoon forming two non-shortest diameters. Each large column is also adapted to be coupled on a top end to a deck structure. The offshore drilling structure also includes two small columns of substantially a same cross-sectional area, each coupled on a bottom end thereof to the pontoon at an equidistant point along the perimeter of the pontoon forming the shortest diameter. Each small column is also adapted to be coupled on a top end to a deck structure. 
         [0017]    According to another embodiment of the present invention, the offshore floating structure is a hollow, rectangular cuboid pontoon of substantially a same radial width throughout a perimeter of the pontoon. The offshore floating structure includes four columns of substantially a same cross-sectional area, each coupled on a bottom end thereof to the pontoon at the center of each side of the pontoon. Each column is also adapted to be coupled on a top end to a deck structure. 
         [0018]    The present invention offers utility for semi-submersible drilling and production units including wet trees with steel catenary risers (SCR) and/or dry trees with top tensioned risers (TTR). Additionally, the C-Semi hull is applicable for Tension Leg Platforms (TLPs). 
         [0019]    The C-Semi offers several advantages, including minimized wave, current and vortex induced motions, and structural forces. These advantages significantly improve hull, mooring and riser system performance. Additionally, the present invention reduces the costs and risks typically in offshore oil and gas field development. 
         [0020]    Further features and advantages of the present invention shall be understood in view of the following description with reference to the drawing figures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    While the present invention may be embodied in many different forms, a number of illustrative embodiments are described herein with reference to the accompanying figures. The skilled person should understand that the present disclosure is to be considered as providing examples of the principles of the invention, and such examples are not intended to limit the invention to the preferred embodiment described herein and/or illustrated herein. 
           [0022]      FIG. 1  is a plan view of a C-Semi, according to an embodiment of the present invention. 
           [0023]      FIG. 2  is an elevation view of a C-Semi, according to an embodiment of the present invention. 
           [0024]      FIG. 3  is a perspective view of a C-semi, according to an embodiment of the present invention. 
           [0025]      FIG. 4  is a detailed view of an individual column with strakes, according to another embodiment of the present invention. 
           [0026]      FIG. 5  is a plan view of a C-Semi with the pontoon offset to the outside, according to another embodiment of the present invention. 
           [0027]      FIG. 6  is a plan view of a C-Semi with the pontoon offset to the inside, according to another embodiment of the present invention. 
           [0028]      FIG. 7  is a plan view of a C-Semi with square columns, according to another embodiment of the present invention. 
           [0029]      FIG. 8  is a plan view of a C-Semi with six columns, according to another embodiment of the present invention. 
           [0030]      FIG. 9  is a plan view of a C-Semi with straight pontoon middle sections and circular columns, according to another embodiment of the present invention. 
           [0031]      FIG. 10  is a plan view of a C-Semi with straight pontoon middle sections and square columns, according to another embodiment of the present invention. 
           [0032]      FIG. 11  is a perspective view of a C-Semi with straight pontoon middle sections and square columns, according to another embodiment of the present invention. 
           [0033]      FIG. 12  is a plan view of a C-Semi with a square pontoon and circular columns, according to another embodiment of the present invention. 
           [0034]      FIG. 13  is a graph displaying the heave response amplitude operators (RAO) for a C-Semi (as embodied in  FIG. 3 ) and conventional semi-submersible hull both at the same draft. 
           [0035]      FIG. 14  is a graph displaying the heave response amplitude operators (RAO) around wave peak period for a C-Semi (as embodied in  FIG. 3 ) and conventional semi-submersible hull both at the same draft. 
           [0036]      FIG. 15  is a graph displaying the wave exciting forces on the pontoon and columns in the vertical direction for a C-Semi (as embodied in  FIG. 3 ) and a conventional semi-submersible hull both at the same draft. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0037]      FIG. 1  is a plan view of a C-semi  10  with a circular cylindrical pontoon  11  according to an embodiment of the present invention. As shown, the four circular cylindrical columns  12  are coupled to the pontoon  11  at points along the perimeter of the pontoon  11  equidistant from each other. While the pontoon  11  may be a single structure or several separate structures, for ease in description, the pontoon  11  will be referred to as having four sections or quadrants  19   a ,  19   b ,  19   c  and  19   d ; each section is coupled to and positioned between two adjacent columns  12 . The pontoon  11  is a circular, hollow toroid with an interior edge  11   a  and an exterior edge  11   b . The pontoon can be filled with buoyant material such as air, or ballast such as water. 
         [0038]    In this embodiment, parts of each column  12  can extend radially beyond the interior and exterior edges of the pontoon  11 . The maximum width (in this case the diameter)  13  of the columns  12  is larger than the radial width  16  of the pontoon  11 . Therefore, at the point where the column  12  intersects the pontoon  11 , the cross-sectional area of each column  12  is greater than the corresponding area of the pontoon  11 . The diameter from the center of the radial width of one pontoon section  17  to the center of the radial width of an opposite pontoon section  17  is larger than the distance from the center of one column  18  to the center of an adjacent column  18 . A spread mooring (not shown) can be used for station keeping of the C-Semi. 
         [0039]      FIG. 2  is an elevation view of the C-Semi  10  with a circular cylindrical pontoon  11  according to this embodiment of the present invention. A deck structure  13  can be connected to the top end of columns  12 . Strakes  14  can be provided on the exterior of the columns  12  below the mean waterline  15  to mitigate vortex induced motions. The columns  12  can be attached to the pontoon  11  on the end opposite the deck structure  13 . 
         [0040]    The pontoon sections are positioned radially outward relative to the columns. The diameter  23  from the center of the radial width of one pontoon section  17  to the center of the radial width of an opposite pontoon section  17  is preferably between 1.2 to 1.5 times the distance  21  from the center of one column  18  to the center of an adjacent column  18 . The pontoon sections are substantially longer relative to the column width  13 , and the distance  21  between central axes of adjacent columns is preferably 3.5 to 4 times the column width  13 . The preferred draft  20  is generally between 20 to 50 meters. The draft is between 0.3 and 1 times the distance  21  from the center of one column  18  to the center of an adjacent column  18 . The draft is also typically much less than the distance  21  between central axes of adjacent columns. The pontoon width  16  varies from 0.6 to 1 times the column width  13 . The preferred pontoon height  22  is in the range of 0.4 to 0.8 times the pontoon width  16 . The column displacement is between 0.8 to 2 times the pontoon displacement. The wave forces on the columns contribute less than the force on the pontoon for most wave periods. 
         [0041]      FIG. 3  is a perspective view of the C-semi  10  with a circular cylindrical pontoon  11  according to this embodiment of the present invention. As shown, the four circular cylindrical columns  12  can be coupled to the pontoon  11  at equidistant points along the pontoon  11 . 
         [0042]      FIG. 4  is a plan view of an individual column  12  according to another embodiment of the present invention. The exterior of each column can be provided with three overlapping helical strakes  14   a ,  14   b  and  14   c , which fully cover the column  12  perimeter below the waterline. 
         [0043]      FIG. 5  is a plan view of a C-Semi  50  with a circular cylindrical pontoon  51  according to another embodiment of the present invention. As shown, the four circular cylindrical columns  52  can be coupled to the pontoon  51  at points along the perimeter of the pontoon  51  equidistant from each other. The pontoon  51  is a is a circular, hollow toroid with an interior edge  51   a  and an exterior edge  51   b.    
         [0044]    While the pontoon  51  may be a single structure or several separate structures, for ease in description, the pontoon  51  will be referred to as having four sections or quadrants  59   a ,  59   b ,  59   c  and  59   d ; each section is coupled to two adjacent columns  58 . As shown, the maximum width (in this case the diameter)  53  of the columns  52  is larger than the radial width  54  of the pontoon  51 . Therefore, at the point where the column  52  intersects the pontoon  51 , the cross-sectional area of each column  52  is greater than the corresponding area of the pontoon  51 . In this embodiment, as shown, parts of each column  52  extend radially beyond only the interior edge  51   a  of the pontoon  51 . The edge of a column  52  can be in line with the outer circumferential edge of the pontoon  51 . The diameter from the center of the radial width of one pontoon section  57  to the center of the radial width of an opposite pontoon section  57  is larger than the distance from the center of one column  58  to the center of an adjacent column  58 . 
         [0045]      FIG. 6  is a plan view of a C-Semi  60  with a circular cylindrical pontoon  61  according to another embodiment of the present invention. As shown, the four circular cylindrical columns  62  can be coupled to the pontoon  61  at points along the perimeter of the pontoon  61  equidistant from each other. The pontoon  61  is a circular, hollow toroid with an interior edge  61   a  and an exterior edge  61   b.    
         [0046]    While the pontoon  61  may be a single structure or several separate structures, for ease in description, the pontoon  61  will be referred to as having four sections or quadrants  69   a ,  69   b ,  69   c  and  69   d ; each section is coupled to two adjacent columns  68 . The maximum width (in this case the diameter)  63  of the columns  62  is larger than the radial width  64  of the pontoon  61 . Therefore, at the point where the column  62  intersects the pontoon  61 , the cross-sectional area of each column  62  is greater than the corresponding area of the pontoon  61 . In this embodiment, as shown, parts of each column  62  extend radially beyond only the exterior edge  61   b  of the pontoon  61 . The diameter from the center of the radial width of one pontoon section  67  to the center of the radial width of an opposite pontoon section  67  is larger than the distance from the center of one column  68  to the center of an adjacent column  68 . 
         [0047]      FIG. 7  is a plan view of a C-Semi  70  with a circular cylindrical pontoon  71  according to another embodiment of the present invention. As shown, the four square cylindrical columns  72  with round corners can be coupled to the pontoon  71  at points along the perimeter of the pontoon  71  equidistant from each other. The pontoon  71  is a circular, hollow toroid with an interior edge  71   a  and an exterior edge  71   b.    
         [0048]    While the pontoon  71  may be a single structure or several separate structures, for ease in description, the pontoon  71  will be referred to as having four sections or quadrants  79   a ,  79   b ,  79   c  and  79   d ; each section is coupled to two adjacent columns  78 . The maximum width  73  of the columns  72  is larger than the radial width  74  of the pontoon  71 . Therefore, at the point where the column  72  intersects the pontoon  71 , the cross-sectional area of each column  72  is greater than the corresponding area of the pontoon  71 . In this embodiment, as shown, parts of each column  72  extend radially beyond only the exterior edge  71   b  of the pontoon  71 . The diameter from the center of the radial width of one pontoon section  77  to the center of the radial width of an opposite pontoon section  77  is larger than the distance from the center of one column  78  to the center of an adjacent column  78 . 
         [0049]    The pontoon sections are positioned radially outward relative to the columns. The diameter  76  from the center of the radial width of one pontoon section  77  to the center of the radial width of an opposite pontoon section  77  is preferably between 1.2 to 1.5 times the distance  75  from the center of one column  78  to the center of an adjacent column  78 . The pontoon sections are substantially longer relative to the column width  73 , and the distance  75  between central axes of adjacent columns is preferably 3.5 to 4 times the column width  73 . The preferred draft is generally between 20 to 50 meters. The draft is between 0.3 and 1 times the distance  75  from the center of one column  18  to the center of an adjacent column  78 . The draft is also typically much less than the distance  75  between central axes of adjacent columns. The pontoon width  74  varies from 0.6 to 1 times the column width  73 . The preferred pontoon height is in the range of 0.4 to 0.8 times the pontoon width  74 . The column displacement is between 0.8 to 2 times the pontoon displacement. The wave forces on the columns contribute less than the force on the pontoon for most wave periods. 
         [0050]      FIG. 8  is a plan view of a C-Semi  80  according to another embodiment of the present invention. The pontoon is an oval, hollow toroid with an interior edge  81   a  and an exterior edge  81   b . As shown, two small cylindrical columns  83  can be coupled to the pontoon  81  such that the distance between the coupling points of the columns and the interior of the pontoon forms the shortest diameter  85  of the oval pontoon  81 . The two small cylindrical columns  83  can have a maximum width  87  that is equal to the radial distance  86  from the interior edge  81   a  to the exterior edge  81   b  of the pontoon  81 . The other four large columns  82  can be coupled to the oval pontoon  81  at opposite ends of two diameters that do not comprise the shortest diameter of the oval. 
         [0051]    The four large columns  82  can have a maximum width  84  that is larger than the radial width  86  of the pontoon  81 . Therefore, at the point where these four columns  82  intersect the pontoon  81 , the cross-sectional area of each column  82  is greater than the corresponding area of the pontoon  81 . In this embodiment, as shown, parts of each column  82  extend radially beyond both the interior edge  81   a  and exterior edge  81   b  of the pontoon  81 . 
         [0052]      FIG. 9  is a plan view of a C-semi  90  according to another embodiment of the present invention. As shown, the four circular cylindrical columns  97  can be coupled to the pontoon  91  at points along the perimeter of the pontoon  91  equidistant from each other. 
         [0053]    While the pontoon  91  may be a single structure or several separate structures, for ease in description, the pontoon  91  will be referred to as having four sections or quadrants  99   a ,  99   b ,  99   c  and  99   d ; each section is coupled to two adjacent columns. The pontoon  91  is generally in the shape of a circular, hollow toroid with an interior edge  91   a  and an exterior edge  91   b . However, each pontoon section or quadrant  99   a ,  99   b ,  99   c  and  99   d  can have linear portions  93  and non-linear portions  94 . The linear portions  93  can comprise the center of each pontoon section  99   a ,  99   b ,  99   c  and  99   d , while the non-linear portions  94  can be nearest to the coupling points of the columns  97  and pontoon  91 . The maximum width (in this case the diameter)  98  of the columns  97  is larger than the radial width  95  of the pontoon  91 . Therefore, at the point where the column  92  intersects the pontoon  91 , the cross-sectional area of each column  92  is greater than the corresponding area of the pontoon  91 . In this embodiment, as shown, parts of each column  92  extend radially beyond the interior edge  91   a  and exterior edge  91   b.    
         [0054]    The pontoon sections are positioned radially outward relative to the columns. The diameter  96  from the center of the radial width of one pontoon section  93  to the center of the radial width of an opposite pontoon section  93  is preferably between 1.2 to 1.5 times the distance  95  from the center of one column  97  to the center of an adjacent column  97 . The pontoon sections are substantially longer relative to the column width  98 , and the distance  95  between central axes of adjacent columns is preferably 3.5 to 4 times the column width  98 . The preferred draft is generally between 20 to 50 meters. The draft is between 0.3 and 1 times the distance  95  from the center of one column  98  to the center of an adjacent column  98 . The draft is also typically much less than the distance  95  between central axes of adjacent columns. The pontoon width  92  varies from 0.6 to 1 times the column width  98 . The preferred pontoon height is in the range of 0.4 to 0.8 times the pontoon width  92 . The column displacement is between 0.8 to 2 times the pontoon displacement. The wave forces on the columns contribute less than the force on the pontoon for most wave periods. 
         [0055]      FIG. 10  is a plan view of a C-semi  100  according to another embodiment of the present invention. As shown, the four square cylindrical columns  102  with round corners can be coupled to the pontoon  101  at points along the perimeter of the pontoon  101  equidistant from each other. 
         [0056]    In this embodiment, each of the four columns can be positioned to face the center of the interior of the pontoon structure. While the pontoon  101  may be a single structure or several separate structures, for ease in description, the pontoon  101  will be referred to as having four sections or quadrants  109   a ,  109   b ,  109   c  and  109   d ; each section is coupled to two adjacent columns  102 . The pontoon  101  is generally in the shape of a circular, hollow toroid with an interior edge  101   a  and an exterior edge  101   b . However, each pontoon section  109   a ,  109   b ,  109   c  and  109   d  can have linear portions  103  at the center and non-linear portions  104  nearest to the coupling points of the columns  102  and pontoon  101 . The maximum width  106  of the columns  102  is larger than the radial width  105  of the pontoon  101 . Therefore, at the point where the column  102  intersects the pontoon  101 , the cross-sectional area of each column  102  is greater than the corresponding area of the pontoon  101 . In this embodiment, as shown, parts of each column  102  extend radially to flush the interior edge  101   a  and exterior edge  101   b.    
         [0057]      FIG. 11  is an elevation perspective view of a C-semi  100  according to this embodiment of the present invention. As shown, the four square cylindrical columns  102  with round corners can be coupled to the pontoon  101  at points along the perimeter of the pontoon  101  equidistant from each other. 
         [0058]      FIG. 12  is a plan view of a C-Semi  120  according to another embodiment of the present invention. As shown, the four circular columns  122  can be coupled to the pontoon  121  at the center of each side of the pontoon  121 . The pontoon  121  is a hollow rectangular cuboid with an interior edge  121   a  and an exterior edge  121   b.    
         [0059]    While the pontoon  121  may be a single structure or several separate structures, for ease in description, the pontoon  121  will be referred to as having four sections or quadrants  129   a ,  129   b ,  129   c  and  129   d ; each section is coupled to two adjacent columns  122 . The maximum width (in this case the diameter) of the columns  122  is larger than the width  125  of the pontoon  121 . Therefore, at the point where the column  122  intersects the pontoon  121 , the cross-sectional area of each column  122  is greater than the corresponding area of the pontoon  121 . In this embodiment, as shown, parts of each column  122  extend radially beyond the interior edge  121   a  and exterior edge  121   b.    
         [0060]    The pontoon sections are positioned radially outward relative to the columns. The diameter  126  from the center of the radial width of one pontoon section  129   a  to the center of the radial width of an opposite pontoon section  129   c  is preferably between 1.2 to 1.5 times the distance  123  from the center of one column  127  to the center of an adjacent column  127 . The pontoon sections are substantially longer relative to the column width  124 , and the distance  123  between central axes of adjacent columns is preferably 3.5 to 4 times the column width  124 . The preferred draft is generally between 20 to 50 meters. The draft is between 0.3 and 1 times the distance  123  from the center of one column  122  to the center of an adjacent column  122 . The draft is also typically much less than the distance  123  between central axes of adjacent columns. The pontoon width  125  varies from 0.6 to 1 times the column width  124 . The preferred pontoon height is in the range of 0.4 to 0.8 times the pontoon width  125 . The column displacement is between 0.8 to 2 times the pontoon displacement. The wave forces on the columns contribute less than the force on the pontoon for most wave periods. 
         [0061]    A C-Semi with a circular cylindrical ring pontoon and straked columns is beneficial because the structure minimizes hydrodynamic and structural forces.  FIG. 13  is a graph of heave response amplitude operators for a C-Semi according to the embodiment shown in  FIG. 3  and a conventional semi-submersible hull at the same draft.  FIG. 14  is a graph showing a detailed view around the wave peak period (Tp) in  FIG. 13 . The graphs show that the C-Semi minimizes hydrodynamic loading around both the wave peak period and the natural period through cancellation and redistribution of wave excitation forces on pontoon and columns. Specifically, the C-Semi reduces heave motions by 20% to 30% in extreme hurricane conditions when compared to a conventional semi-submersible hull at the same draft. The C-Semi reduces heave motions by 40% to 50% in fatigue sea states. 
         [0062]      FIG. 15  is a graph of wave exciting forces on the pontoon and columns in the vertical direction corresponding to  FIG. 14 . The C-Semi and conventional semi-submersible hulls have the same draft, column width, and distance between central axes of adjacent columns, and thus the same wave exciting force on columns, A. According to preferred embodiments of the present invention, the wave exciting force on the C-Semi pontoon, C, is noticeably less than the wave exciting force of the conventional semi-submersible pontoon, B, for a dominant wave peak period. Since the wave forces on the pontoon and columns act in the opposite direction, the total force on the C-Semi, C-A, is more significantly reduced than the conventional semi-submersible, B-A. 
         [0063]    A C-Semi also minimizes vortex induced motion (VIM) by mitigating current flows through strakes. In comparison to a conventional semi-submersible hull, the C-Semi reduces VIM amplitude by 50% or more and riser fatigue damage by 80% in current sea states. The C-Semi structure also reduces VIM induced mooring and riser tension and fatigue damage. The C-Semi structure may offer additional benefits by minimizing current forces. 
         [0064]    Furthermore, the C-Semi minimizes structural forces. In comparison to a conventional semi-submersible hull, the C-Semi reduces structural forces and stress concentrations by eliminating the sharp corners between the pontoon sections. 
         [0065]    Thus, the preferred embodiments have been fully described above. Although the invention has been described based upon these preferred embodiments, it would be apparent to those of skill in the art that certain combinations, modifications, variations, and alternative constructions could be made to the described embodiments within the spirit and scope of the invention.