Abstract:
Semi-submersibles are subjected to loading from waves, causing racking, longitudinal shear and parallelogramming, or differential movement of the pontoons. The cyclic wave loading makes the various connections, where stress concentrations occur, susceptible to fatigue damage throughout the hull structure. This is most evident at the connections between the braces and the main hull structure. A revised brace to main hull connection with reduced bending stiffness is employed to reduce the moment being transferred from the brace to the hull, thereby reducing the bending stress and susceptibility to fatigue damage. This improved connection employs an internal member to transfer the loads between the brace and hull structure mainly as tension and compression. As a consequence of the improved fatigue performance, the structural weight of the connection can be greatly reduced, thus increasing the capacity with which the semi-submersible hull can operate.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a Non-Provisional Application claiming priority to U.S. Provisional Patent Application No. 62/302,905, entitled “Bending Stiffness Reducer for Brace to Hull Connection,” filed Mar. 3, 2016, which is herein incorporated by reference. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    This invention was not made under federally sponsored research or development. 
       REFERENCE TO SEQUENCE LISTING, A TABLE, OR A COMPUTER PROGRAM LISTING COMPACT DISC APPENDIX (IF APPLICABLE) 
       [0003]    This is not applicable. 
       BACKGROUND OF THE INVENTION 
       [0004]    This invention relates to mobile offshore units. Mobile offshore units are used in the offshore industry mainly for drilling and production operations, but also for general construction operations, crew accommodation, wind-turbine installation, etc. Semi-submersibles are a type of floating mobile offshore unit designed to provide a stable platform to support the necessary offshore operations in water depths where an on-bottom structure is not feasible. 
         [0005]    The invention provides permanent means of structural connection, between the multiple hulls or multiple legs of the semi-submersible. 
         [0006]    Semi-submersibles typically consist of a deck or deck box supported by a plurality of columns connected by large longitudinal pontoons and a series of transverse braces, at least two per vessel, typically one at the forward column and one at the aft column [see U.S. Pat. No. 4,436,050]. The braces extend from column to column, column to pontoon, or pontoon to pontoon, depending upon the design, but essentially, the braces connect parts of the main hull. 
         [0007]    During operation, a semi-submersible is ballasted to a depth at which its longitudinal hulls are submerged, its columns penetrate the surface of the water and its braces are typically submerged. The hull can be partially de-ballasted to float at a reduced draft, to provide a greater clearance between the hull deck box and the surface waves. 
         [0008]    In transit mode, a semi-submersible is completely de-ballasted resulting in it floating at its minimal draft. In this condition, it floats purely on the pontoons, with the columns completely above the water surface. The braces are typically above the water surface in this condition. 
         [0009]    Weight in a semi-submersible is a critical design parameter. With Variable Deck 
         [0010]    Load around 15 percent of operating displacement, any lightship weight reduction has a multiplicative advantage to carrying capacity. 
         [0011]    Throughout its life, a semi-submersible is subjected to global wave loadings which are resisted by the brace working in concert with the deck or deck box. Due to the wave loads on the semi-submersible, significant loading of the braces can occur, particularly at their connections. 
         [0012]    The brace loading can be separated into two components; 1) an axial load due to squeeze/pry loads, where the hulls are forced together or pulled apart, by wave action, and 2) bending due to direct action of the waves perpendicular to the axis of the brace and due to the racking and parallelogram deflection, resulting in longitudinal and vertical displacement of the brace ends, relative to each other. 
         [0013]    Considering that the wave loading is cyclical, the fatigue life considerations typically drive the design details, and scantlings of the brace members and their connections. 
         [0014]    From this description, it can be appreciated that the braces of a semi-submersible are typically very robust and able to withstand compression, tension and bending loads, with due consideration made to assure adequate fatigue life. The brace is a beam column, with fatigue loading. 
         [0015]    In the past, the approach has been to size the braces for the squeeze and pry forces, considering the minimum slenderness ratio required of the brace to withstand damaged condition loads and reinforce, or increase the cross-section at the end connection [see U.S. Pat. No. 4,771,720] of the brace ends to withstand the bending induced by the global parallelogram and racking deflections of the hull. Naturally, to achieve the required slenderness ratio, the braces are designed with a significant cross-section resulting in essentially a fixed ended brace. In a fixed ended traditional brace design, the bending stress is typically of the same magnitude as the axial stress, requiring heavy reinforcement to withstand the unintended parasitic bending stress. 
         [0016]    Typically, from the brace at vessel centerline to their end connections at the hull, port and starboard, the brace walls are progressively increased in thickness to handle the hull deflection induced bending and its resulting cyclic fatigue stresses. Naturally, as the brace ends are reinforced, they are stiffened, and tend to attract more bending load, caused by the hull deflection. With greater load comes incremental stress, requiring increased reinforcement and weight. 
         [0017]    Reinforcing the brace to hull connection increases the rotational stiffness of the connection, attracting more load, making reinforcement an ineffective way to address the connection fatigue issues. The reinforcement added to the brace is of little value to the vessel, other than to assure the survivability of the brace itself. The brace is intended to resist the axial squeeze/pry loads caused by hydrodynamic wave loading. The bending of the brace is the result of hull deflections over which the brace has no control. In other words, that bending is due to the hull parallelogram and racking deflections which are controlled by the stiffness of the hull box structure, which has orders of magnitude greater torsional stiffness than the braces, and therefore not greatly influenced by the stiffness of the braces. Increasing the stiffness of the braces to bending, only adds weight, without significantly reducing the magnitude of the hull deflection. 
         [0018]    Besides having to keep the final stresses low to achieve adequate fatigue life, which finally requires very thick and heavy sections, the complex geometry at the intersection of the braces with the hull may require measures such as weld toe grinding and weld profiling [see CN 203,612,180] or making the entire hull to brace connection as a cast piece. As a result, the brace to hull connection can be very costly to construct, requiring lots of planning, inspection and lead-time. 
         [0019]    Another brace solution has been to utilize more than  2  braces, per hull, typically two at the forward column and two at the aft column [see U.S. Pat. No. 6,378,450 B1]. As the squeeze and pry loads are shared, this arrangement has the advantage that the braces can be made smaller in cross-section and thereby less stiff. As a result, these braces attract less bending, given the same magnitude of hull deflections. However, this design suffers the same cost and weight deficiencies of the 2 brace design when the one brace damaged condition is considered. 
         [0020]    It has been attempted to eliminate the braces entirely and rely on the columns and deck box connection to withstand the squeeze and pry forces [see U.S. Pat. No. 6,009,820]. This arrangement converts squeeze and pry forces between the pontoons from axial loads on the braces to loads which create bending moments at the column to deck box connection and increase the bending due to racking at the column to deck box connection. In practice, this arrangement resulted in deck box plate cracking, at the column to deck box connection, and braces were retrofitted to take the squeeze and pry loads directly, thereby reducing the deck box deflections to acceptable limits. 
         [0021]    Other designs have added a truss-work of braces to prevent hull relative deflection and brace end relative displacement, but this results in a still heavier structural design. 
       SUMMARY OF THE INVENTION 
       [0022]    The present invention looks to reduce the rotational stiffness of the brace to hull connection, thereby reducing the induced bending moment and reducing the need for local reinforcement requirements of the connection needed to achieve the target fatigue life. By reducing the local reinforcement requirements, a reduced structural weight of the brace connection can be attained, resulting in greater Variable Deck Load capacity. The brace with reduced bending stiffness withstands the squeeze/pry loads, for which it was intended, without attracting significant bending stresses from the hull deflection, which is controlled by the deck box. 
         [0023]    It is therefore, an objective of this invention to provide a means of connecting a brace such to reduce the bending stress at the connection. 
         [0024]    It is therefore, an objective of this invention to provide a brace connection on a semi-submersible with improved fatigue performance by providing a means for reducing the end moment in a structural brace member, and thereby greatly reducing the bending stress at its connection. 
         [0025]    It is therefore, an objective of this invention to provide a brace connection with less weight than a standard connection and thus provide an increased semi-submersible hull payload. 
         [0026]    The objectives of the present invention are achieved by a brace connection that is optimized to transfer the loads on the brace as compression and tension as opposed to compression and tension in combination with high moment. 
         [0027]    This is accomplished by designing the brace to act more like a pin-ended column and less like a fixed end column. 
         [0028]    Rather than connect the brace at its end through a constant or enlarged section, reinforced to withstand induced bending, this invention reduces the stiffness of the of the axial load bearing member of the brace at that connection, resulting in an end element which is more flexible in bending. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0029]    So that the basic need fulfilled by and demands placed upon the brace can be better understood, the drawbacks of the prior art appreciated and improvement on and benefits from this invention revealed, a more particular description and invention embodiments is provided in the following figures, followed by their detailed description. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
           [0030]      FIG. 1  is a typical structural arrangement showing a semi-submersible in section view. 
           [0031]      FIG. 2  is a typical structural arrangement showing a semi-submersible in section view, in a diagrammatic representation. 
           [0032]      FIG. 3  is a typical structural arrangement showing a semi-submersible in profile view. 
           [0033]      FIG. 4  is a typical structural arrangement showing a semi-submersible in profile view, in a diagrammatic representation. 
           [0034]      FIG. 5  is the diagrammatic section view showing a depiction of the behavior of the brace members in pry, with the brace in tension. 
           [0035]      FIG. 6  is the diagrammatic section view showing a depiction of the behavior of the brace members in squeeze, with the brace in compression. 
           [0036]      FIG. 7  is the diagrammatic section view showing a depiction of the semi rolling and the brace behavior with fixed end connections, as the hull parallelograms. 
           [0037]      FIG. 8  is a diagrammatic plan view, showing a depiction of the longitudinal racking displacement of the hull in quartering waves and brace behavior with fixed end connections. 
           [0038]      FIG. 9  is an isometric view of the standard brace to hull connection in isometric view. 
           [0039]      FIG. 10  is a profile section view showing axial plus bending loads at the standard brace to hull connection. 
           [0040]      FIG. 11  is the diagrammatic section view showing a depiction of the semi rolling and the brace behavior with hinged end connections, as the hull parallelograms. 
           [0041]      FIG. 12  is a diagrammatic plan view, showing a depiction of the longitudinal racking displacement of the hull in quartering waves and brace behavior with hinged end connections. 
           [0042]      FIG. 13  is an isometric view of the improved brace to hull connection in isometric view. 
           [0043]      FIG. 14  is a profile section view showing axial plus greatly reduced bending loads at the improved brace to hull connection 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0044]    Referring now to the invention in more detail, a typical structural arrangement is shown in  FIG. 1 , showing a semi-submersible in section view and  FIG. 2 , shows a semi-submersible in section view in a diagrammatic representation. A semi-submersible, or more particularly the main hull structure of a semi-submersible is typically composed of a series of pontoons  1 , columns  2 , and an upper box structure  4 . The hull of the semi-submersible is buoyant, operating at a waterline  3  approximately as indicated. The main deck  5  structure varies in its arrangement depending upon the intended use of the semi-submersible such as drilling, oil production, construction support, accommodations, etc. The brace structure  6  is shown, in this case, the standard way with built-in, or fixed ends  7 . 
         [0045]    For better understanding,  FIG. 3  shows a semi-submersible in profile view, showing the same elements as in the section view, pontoons  1 , columns  2 , operating waterline  3 , deck box  4 , main deck  5  and brace structure  6 . 
         [0046]      FIG. 4  shows the diagrammatic representation of the semi-submersible in profile view, showing pontoons  1 , columns  2 , operating waterline  3 , deck box  4 , and brace structure  6 . 
         [0047]    Throughout its life, a semisubmersible is subjected to global wave loadings which are resisted by the brace  6  working in concert with the deck or deck box  4 . When the semisubmersible is in beam seas, the pontoons  1  are alternatively pried apart and squeezed together and rolled into a parallelogram shape, by the passing waves. To prevent undue bending moment at the column  2  to deck box  4  connection, the braces  6  are intended to take the tension  8  and compression  13  loads generated by the hull-wave interaction as depicted in  FIGS. 5 and 6 , respectively. 
         [0048]      FIG. 5  shows the semi-submersible in diagrammatic section view to illustrate the tension loads  8  the braces  6  are intended to carry, which are tension loads  8  when the wave crest  9  is at the semi-submersible centerline  10 , and the pontoons are pried apart  12 . The brace  6  is deflected with the tension loads  8  in this scenario primarily down  11 , with little bending induced in the brace to column connections  7 . 
         [0049]      FIG. 6  shows the semi-submersible in diagrammatic section view to illustrate the compression loads  13  the braces  6  are intended to carry, when the wave trough  14  is at the semi-submersible&#39;s centerline  10  and the pontoons are squeezed together  15 . The brace  6  is deflected with the compression loads  13  in this scenario primarily up  16 , with little bending induced in the brace to column connections  7 . 
         [0050]    Referring now to  FIG. 7 , as a wave  17  approaches, the semi rolls, the ring formed in section view by the deck box  4 , columns  2  and horizontal brace  6  will parallelogram. The connection between the deck box  4  and columns  2  is rigid  18 , and transmits moment  19  to resist the deflection. In the prior art, the connection between the brace and columns  7  is also rigid  20  and the deflection of the hull distorts the braces  6  into an “S” shape, with a radius of curvature at the connection  7  “Rho,” resulting in bending moment  21  at the brace to column connection  7 . The maximum stress along the brace  6  is then seen at the brace to column connection  7 . The cyclical nature of the bending due to wave loading makes the brace to column connection  7  susceptible to fatigue damage. 
         [0051]    Because the deck box  4  is orders of magnitude stiffer than the braces  6 , the deck box  4  resists this load, but the hull still suffers significant flexure. The braces  6  adapt to the slope of the columns  2  at their ends  7 . This flexure, from the perspective of the horizontal brace  6  looks like vertical displacement of the brace ends  22 , free to translate vertically but not rotate  20 . This distorts the brace into an “S” shape in section view, creating bending moment in the brace  21 . As a result, the brace is not a purely tension  8 -compression  13  member, but a beam column, suffering bending moment  21  due the interaction of its fixed ends  7  and the inevitable hull deflection, in addition to either the tension  8  or compression  13  load. 
         [0052]    Similarly, when the hull is in quartering seas  23 , as shown in  FIG. 8 , the hull racks, which moves the two pontoons  1  longitudinally relative to one another  24 , resulting in the brace  6  adopting an S-shape in plan view due to the rigidity of its end connection  20 . This deflection is analogous to the deflection described in  FIG. 7 , however this form of hull deflection causes brace bending  20  in the horizontal plane. 
         [0053]    These vertical  22  and longitudinal  24  deflections can be quite high and the parallelogramming and racking deflection induced brace bending  21  stresses are typically roughly equivalent to the brace axial stresses caused by prying  12  and squeezing  15 . However, because the deck box  4  is orders of magnitude stiffer than the braces  6 , reinforcing the brace ends  7  does not appreciably reduce the hull flexure, it only reinforces the braces  6  locally, attracting more bending moment  21  and adding weight and cost to the hull. 
         [0054]    For minimum weight, the sole purpose of the brace  6  should be to resist the pry  12  and squeeze forces  15  on the pontoons  1  and columns  2 , while the loads from parallelogramming, as shown in  FIG. 7 , and racking, as shown in  FIG. 8 , deflections of the hull should be resisted by the deck box  4 . 
         [0055]      FIG. 9  shows the standard brace  6  to hull  2  connection, where typically, a brace  6  with a large cross-section is connected  7  to the hull  2  and backed up with hull internal structure  25  to resist both the axial  26  and bending  27  loads, as shown in  FIG. 10 . With such a large cross-section for the brace  6 , connected  7  to the hull structure  2 , it is inevitable that the hull deflection induces large magnitude bending  27 . 
         [0056]    Clearly, what is needed is to decouple the brace  6  from bending due to hull deflection, as depicted in  FIGS. 7 and 8 , by reducing the bending stiffness of the brace  6  to hull  2  connection  7 . In this way, the brace can be sized optimally, for tension  8  and compression  13  loads, without attracting bending moments  21  which do not significantly reduce those hull deflections. 
         [0057]      FIGS. 11 and 12 , in a way analogous to  FIGS. 7 and 8  respectively, show how braces  6  free to rotate at their ends  7  do not induce bending  21  in the brace. As a result, they can be designed for almost pure axial tension  8  and compression  13 , without being reinforced to withstand and attract bending moments  21 . From  FIGS. 11 and 12 , it can be appreciated that the braces  6  do not adopt an “S” shape, but instead remain virtually straight, with their end moments  21  greatly reduced. 
         [0058]    The following embodiment is considered to be the preferred means for achieving this invention. Other arrangements may exist, which reduce the bending stiffness of this connection, so are intended to be hereby covered by the disclosure of this invention. 
         [0059]    The preferred embodiment of this invention is shown in  FIGS. 13 and 14 . In  FIG. 13 , it can be appreciated that the design begins with the full cross-section of the brace  6  but after transitioning through an outer band of increased thickness  29 , for local strength, the cross-section is reduced conically through a conical transition piece  30  which attaches to the flexible element  33 , which is of reduced cross-section  35 . A central flexing element  33  is disclosed, with one end of the flexing element  33  fixed to the hull structure  25  and the other end fixed to the brace  6  as shown in  FIG. 14 . Owing to the minimal cross-section of this element  33 , the structure of the brace connection has a reduced “y” (distance from the neutral axis to the extreme fiber of the element in bending) from that employed on the brace itself for a reduced moment attraction. This cylindrical flexing member  33 , internal to the brace  6 , has a high axial load  26  capacity allowing for the safe transfer of loading as tension  8  or compression  13 , without attracting significant bending stresses  21  due to hull deflections. Pictorially, this is represented by the same axial loads  26 , but greatly reduced bending loads  27 . As a consequence, detailed analysis has proven that the backing structure  25  is also of less weight as it is withstanding primarily axial loads  26 , rather than roughly equal amounts of axial stress  26  and bending stress  27 . 
         [0060]    To withstand transverse loads and to align the flexing element with its axial loads, the brace end is constrained from transverse translation by a “Warping Plate  31 ,” which can withstand angular deflection at the flex member  33 , while behaving rigidly in a direction radial to the brace  6 . The warping plate  31  can flex to accommodate angular deflection of the brace  6  about the center of pivot  34  at the flex element  33 , mid-span, with minimum stress, due to it being relatively thin plate material, on the order of thickness of the rest of the hull in that area. At the same time, the warping plate  31  is very rigid to in-plane-shear, so maintains the brace  6  end, and consequently its central flex element  33 , at the center of axial force  26  action and pivot center  34 . The warping plate  31  also transmits any transverse loads imparted to the brace  6 , into the hull structure  25  through the outer transition piece  32 .