Abstract:
A pontoon-type floating structure comprising an upper deck that is to be maintained above water level and that is to receive and support a load by the load resting thereon; and a horizontal array of chambers disposed underneath the upper deck, with the chambers providing a first set of chambers that provide the structure with buoyancy, and a second set of chambers with water having access thereto so that the second set of chambers, under steady state conditions, do not provide buoyancy.

Description:
FIELD OF INVENTION 
       [0001]    The invention relates to a pontoon-type floating structure. 
       BACKGROUND 
       [0002]    As population and urban development expand in land scarce island countries (or countries with long coastlines), city planners and engineers may resort to land reclamation to ease the pressure on existing heavily-used land and underground spaces. Using fill materials from seabed, hills, deep underground excavations, and even construction debris, engineers are able to create relatively vast and valuable land from the sea. However, land reclamation has its limitations. It is only suitable when the water depth is shallow (less than 20 m). When the water depth is large and/or the seabed is extremely soft, land reclamation may no longer be cost effective or even feasible. Moreover, land reclamation may destroy the marine habitat and may even lead to the disturbance of toxic sediments. 
         [0003]    Very Large Floating Structures (VLFS) are an alternative method to create “land” on the sea. There are two types of VLFS; the semisubmersible-type and the pontoon-type. Semi-submersible type floating structures are raised above the sea level using column tubes or ballast structural elements to minimize the effects of waves while maintaining a constant buoyancy force. Thus they can reduce the wave induced motions and are therefore suitably deployed in high seas with large waves. Floating-platforms used for drilling for and production of oil and gas are typical examples of semi-submersible-type VLFSs. When these semi-submersibles are attached to the seabed using vertical tethers with high pretension as provided by additional buoyancy of the structure, they are referred to as tension-leg platforms. 
         [0004]    In contrast, pontoon-type floating structures lie on the sea level and are typically for use in calm waters, often inside a cove or a lagoon and near the shoreline. The larger category of pontoon-type floating structures or Mega-Floats have at least one length dimensions greater than 60 m. 
         [0005]    When a Mega-Float is heavily loaded, in the central portion for example, the floating structure will deflect with the centre vertically displaced relative to the corners. The resulting differential deflection may cause equipment to malfunction, the superstructure on the floating structure to be subjected to additional stresses or in extreme cases may lead to structural failure under high stress conditions. 
         [0006]    A need therefore exists to address at least one of the above problems. 
       SUMMARY OF THE INVENTION 
       [0007]    In accordance with a first aspect of the present invention there is provided a pontoon-type floating structure comprising an upper deck that is to be maintained above water level and that is to receive and support a load by the load resting thereon; and a horizontal array of chambers disposed underneath the upper deck, with the chambers providing a first set of chambers that provide the structure with buoyancy, and a second set of chambers with water having access thereto so that the second set of chambers, under steady state conditions, do not provide buoyancy. 
         [0008]    A plurality of walls preferably depend from the upper deck and co-operate therewith to provide the chambers separated by the walls. 
         [0009]    Said walls are preferably generally perpendicular to said deck, with the walls including a first set that are generally parallel and transversely spaced and a second set, with the walls of the second set being generally parallel and transversely spaced and generally normal to the first set so that the chambers in horizontal transverse cross-section are generally square or rectangular. 
         [0010]    The chambers preferably have respective bottom walls, the bottom walls being displaced from the upper deck, with the bottom walls of said second set of chambers having an aperture providing for the flow of water. 
         [0011]    Said second set of chambers are preferably located adjacent a periphery of said structure. 
         [0012]    Said second set of chambers are preferably aligned in rows adjacent said periphery. 
         [0013]    Each row is preferably displaced from the periphery by at least one chamber of the first set. 
         [0014]    Said structure is preferably square or rectangular in configuration when viewed in plan so as to have four sides, with each row extending generally parallel to one of said sides. 
         [0015]    Said structure is preferably formed of one or more of a group consisting of steel, concrete, and reinforced concrete. 
         [0016]    Said structure preferably includes a generally horizontally oriented bottom slab that is to be submerged and that is generally parallel and co-terminus with respect to said top deck but vertically spaced therefrom. 
         [0017]    Said array of chambers is preferably a first array, and said structure includes a second horizontal array of chambers located beneath the first array of chambers, the first and second chambers separated by a generally horizontally oriented middle slab and that is generally parallel and co-terminus with respect to said top deck but vertically spaced therefrom. 
         [0018]    Said top deck preferably has apertures and/or is air pervious to provide for the flow of air with respect to the chambers of the second set. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    Example embodiments of the invention will be better understood and readily apparent to one of ordinary skill in the art from the following written description, by way of example only, and in conjunction with the drawings, in which: 
           [0020]      FIG. 1  is a schematic side elevation view of a floating structure according to an example embodiment. 
           [0021]      FIG. 2  is a cross sectional schematic view of a section of the floating structure of  FIG. 1 . 
           [0022]      FIG. 3   a  is a schematic bottom view of a zero-buoyancy chamber of the floating structure of  FIG. 1 . 
           [0023]      FIG. 3   b  is a schematic bottom view of another zero-buoyancy chamber of the floating structure of  FIG. 1 . 
           [0024]      FIG. 4  shows a plurality of schematic side elevations of different mooring arrangements for the floating structure of  FIG. 1 . 
           [0025]      FIG. 5  is a schematic plan view of a floating structure according to another example embodiment (dimensions in metres). 
           [0026]      FIG. 6   a  is a schematic cross sectional view of a water tight chamber of the floating structure of  FIG. 5 . 
           [0027]      FIG. 6   b  is a schematic cross sectional view of a zero-buoyancy chamber of the floating structure of  FIG. 5 . 
           [0028]      FIG. 7   a  shows a deflection surface of a floating structure without zero-buoyancy chambers and subjected to a 7-tier container loading (deflections in metres). 
           [0029]      FIG. 7   b  shows a deflection surface for the floating structure of  FIG. 5  and subjected to a 7-tier container loading (deflections in metres). 
           [0030]      FIG. 8   a  shows a stress contour of a bottom slab for the major principal stresses in a floating structure without zero-buoyancy chambers and subjected to a 7-tier container loading (stresses in MPa). 
           [0031]      FIG. 8   b  shows a stress contour of the bottom slab for the major principal stresses in the floating structure of  FIG. 5  and subjected to a 7-tier container loading (stresses in MPa). 
           [0032]      FIG. 9   a  shows a stress contour of a top slab for the major principal stresses in a floating structure without zero-buoyancy chambers and subjected to a 7-tier container loading (stresses in MPa). 
           [0033]      FIG. 9   b  shows a stress contour of the top slab for the major principal stresses in the floating structure of  FIG. 5  and subjected to a 7-tier container loading (stresses in MPa). 
       
    
    
     DETAILED DESCRIPTION 
       [0034]      FIG. 1  shows a floating structure  100  according to an example embodiment. The floating structure  100  may be moored to a mooring facility  102  and may include an access connection  104  to land  108 , another structure or a vessel. A breakwater  106  may be optionally provided to reduce large wave forces impacting the floating structure  100 . 
         [0035]      FIG. 2  shows a schematic cross sectional drawing of a section of the floating structure  100 . The structure  100  includes a top deck  200  provided by a top slab in the example embodiment. Depending from the deck  200  are a plurality of walls e.g.  202 ,  204 . The walls  202 ,  204  extend generally perpendicular to the deck  200  so as to provide a plurality of chambers e.g.  206 ,  208 . The chambers  206 ,  208  are arranged in a horizontal array underneath the deck  200 . A horizontal bottom wall or slab  210  is provided. The walls  202 ,  204 , as well as the slab  210  are made from a water impervious material, with each of the walls  202 ,  204  sealingly connected to the horizontal bottom slab  210 . In this respect it will be appreciated that the majority of the chambers e.g.  206  are sealingly enclosed so that water may not enter them. At the same time, apertures  212 ,  214  are provided in the bottom slab  210  in the area of selected chambers  208 , allowing water to enter those chambers e.g.  208 . To facilitate the venting of air from the chambers  208  as the water enter, the deck  200  may be provided with apertures (not shown) or may be otherwise air pervious, at least in areas of the chambers  208 . Under steady state conditions, the chambers  208  are thus filled with water up to a level, indicated at  216 , equivalent to the sea level, indicated at numeral  218 . 
         [0036]    As the water is free to flow in and out of the chambers  208 , those chambers, which may be referred to as gill cells, provide zero-buoyancy to the floating structure  100 . At the same time, the remaining chambers  206  provide buoyancy to the structure  100 . Thus, buoyancy forces are acting on the bottom slab  210 , apart from areas underneath the chambers  208 . 
         [0037]    In the example embodiment, the chambers  208  are provided along an edge  216  of the structure  100 , and as a result of the zero-buoyancy of the chambers  208 , a restraint to vertical movement of the edge  216  is provided. This was found to decrease the differential deflection of the edge  216  when loads are applied at or near the centre of the floating structure  100 . By adjusting the number and geometry of the chambers  208 , the floating structure  100  can be designed to maintain the differential deflection within acceptable limits under varying loads. 
         [0038]    In the example embodiment, the apertures  212 ,  214  are designed such that the structural integrity of the bottom slabs  210  is maintained. The aperture size is chosen to be sufficiently large to allow water to freely enter so that the water level in the chamber is equal to the sea water level. 
         [0039]      FIGS. 3   a  and  3   b  show example apertures  300 ,  302  for individual zero-buoyancy chambers  304 ,  306 . In choosing aperture designs, sharp points in the apertures may be avoided as they can cause starting points for cracks. The size of the apertures may be balanced between avoiding weakening of the chambers&#39; structure, and blockage of particularly small apertures. 
         [0040]    In the example embodiment the walls and slabs are constructed from steel, concrete, reinforced concrete such as stell reinforced concrete, or any other suitable watertight material with the requisite stiffness and strength. Since watertightness of concrete avoids or limits corrosion of the reinforcement, either watertight concrete or offshore concrete may be used. For example high-performance concrete containing fly ash and silica fume would be suitable. It will be appreciated that other combinations of structural materials may be used in different embodiments. 
         [0041]    Corrosion protection techniques may be applied to the reinforcing and other steel work using for example coatings, cathodic protection, corrosion allowance and corrosion monitoring. In areas where marine organisms are active, antifouling coatings may be used to reduce marine growth. In areas of potential severe low corrosion, such as directly beneath the mean low water level, cathodic protection may be applied, while coating methods may be applied for remaining parts shallower than the depth of 1 m below the mean low water level. Coating methods may include painting, titanium-clad lining, stainless steel lining, thermal spraying with zinc, aluminium and aluminium alloy. 
         [0042]    Returning now to  FIG. 1 , the mooring facility  102  ensures that the floating structure  100  is kept in position so that the facilities installed on the floating structure can be reliably operated. Preventing the structure  100  from drifting away under critical sea conditions and storms is an example design consideration for a mooring facility  102 . A free or drifting floating structure  100  may lead to damage to the surrounding facilities and may also lead to the loss of human life in a collision with vessels.  FIG. 4  shows a number of types of mooring systems such as the dolphin-guide frame system  400 , mooring by cable and chain  402 , tension leg method  404  and pier/quay wall method  406 . Choice of the type of mooring system depends on the local conditions and the performance requirements. 
         [0043]    Once the type of mooring system is chosen, the shock absorbing material, the quantity and layout of devices to meet the environmental conditions and the operating conditions and requirements can be determined. Layout of mooring dolphins for example may be such that the horizontal displacement of the floating structure is adequately controlled and the mooring forces are appropriately distributed. The layout and quantity of the mooring dolphins may be adjusted so that the displacement of the floating structure and the mooring forces do not exceed the allowable values. 
         [0044]    In order to reduce the wave forces impacting the floating structure, optionally one or more breakwaters  106 , may be constructed nearby. A breakwater may be useful if the significant wave height is greater than 4 m. 
         [0045]    In the following, results of calculations illustrating the performance of an example embodiment of the present invention will be described.  FIG. 5  shows a schematic top view of a floating container terminal  500  according to the example embodiment, and used for the calculation discussed below. In  FIG. 5 , a central container area  502  is provided, as well as a rail area  504  at one edge of the structure  500 . Dimensions indicated in  FIG. 5  are in meters. The location of the zero-buoyancy chambers are schematically indicated at numerals  506 ,  508 , and  510 . 
         [0046]    A finite element method (FEM) calculation was used to compare the structure  500  against the same structure without zero-buoyancy chambers. An example concern is the differential deflection between the corners and the middle portion of the floating structure  500 . For example a quay crane may not be able to operate if the between-rail  504  gradient goes above certain gradient specification, for example 0.4%. 
         [0047]    For the calculations, the structure  500  is assumed to be of a double layer structure, which will now be briefly described.  FIGS. 6   a  and  b  show schematic cross-sectional views of a water tight chamber  600 , and a zero-buoyancy chamber  602  of the structure  500  ( FIG. 5 ) respectively. In  FIG. 6   a , the water tight chamber  600  is partitioned by a middle slab  604  disposed between the top and bottom slabs  606 ,  608  respectively. Similarly, as shown in  FIG. 6   b , the zero-buoyancy chamber  602  is partitioned by the middle slab  604  disposed between the top and bottom slabs  606 ,  608  respectively. Apertures  610 ,  612  are provided in the bottom slab  608  in areas of the zero-buoyancy chamber  602 , with corresponding apertures  614 ,  616  provided in the middle slab  604 . Beam stiffeners  618 ,  620  are provided underneath the top slab  606  and on top of the bottom slab  608  respectively, and extend in two orthogonal sets of horizontally spaced rows across the top and bottom slabs  606 ,  608 . 
         [0048]    Table 1 summarises the data adopted for the calculation including the dimensions and construction material properties of the example floating structure, the selfweight and weight of quay cranes. 
         [0000]                                                TABLE 1                   Data Adopted for Calculation                Data   Units                        Dimensions of Floating Structure               Total length   470   m       Total width   520   m       Total height   10   m       Thickness of top and bottom slabs   0.4   m       Thickness of intermediate level slab   0.2   m       Thickness of vertical walls   0.3   m       Width of beam stiffeners   0.5   m       Depth of beam stiffeners   1.0   m       Material Properties and Allowable Stresses       Density of high performance concrete   1900   kg/m 3         Modulus of high performance concrete   22.9   GPa       Poisson&#39;s ratio of high performance concrete   0.2       Compressive stress   70   MPa       Flexural tensile stress   7.2   MPa       Splitting tensile stress   4.3   MPa       Allowable compressive stress   42   MPa       Allowable flexural tensile stress   4.32   MPa       Allowable splitting tensile stress   2.58   MPa       Dead Loads       Total selfweight of container terminal   737250   ton       Weight of one quay crane   1360   ton       Number of quay cranes   8                    
ABAQUS software was used for the calculation. The model for the calculation consists of
       4-node thin-plate elements for the top, middle and bottom slabs and the vertical walls. Each element for the slab has dimensions 5 m×5 m with different thicknesses and each element for the vertical wall has dimensions 5 m×4.8 m   2-node beam elements for modelling the beam stiffeners. Each beam stiffener has a length of 5 m.   Lateral springs are attached to the nodes of the bottom plate elements to model the buoyancy forces. The spring coefficient is taken as 250 kN/m (=1.03×9.81×5×5), which is equivalent to the buoyancy force.         
         [0052]      FIGS. 7   a  and  b  show the calculated deflection surfaces  700 ,  702  for the floating structure without zero-buoyancy chambers, and with zero-buoyancy chambers according to the example embodiment, respectively. The deflection surfaces  700 ,  702  were calculated under 7-tier container loading, and the quay crane load and the terminal selfweight as listed in Table 1. As can be seen from a comparison of  FIGS. 7   a  and  b , the floating structure in accordance with the example embodiment ( FIG. 7   b ) experiences significantly reduced differential deflection of the floating structure, as illustrated by the substantially “flat” deflection surface  702 . 
         [0053]      FIGS. 8   a  and  b  show the calculated stress contours  800 ,  802  of the bottom slab for the major principal stresses for the floating structure without zero-buoyancy chambers, and with zero-buoyancy chambers according to the example embodiment, respectively. The stress contours  800 ,  802  were calculated under 7-tier container loading, and the crane load and selfweight as listed in Table 1. As can be seen from a comparison of  FIGS. 8   a  and  b , the floating structure in accordance with the example embodiment ( FIG. 8   b ) experiences significantly reduced stresses. 
         [0054]      FIGS. 9   a  and  b  show the calculated stress contours  900 ,  902  of the top slab for the major principal stresses for the floating structure without zero-buoyancy chambers, and with zero-buoyancy chambers according to the example embodiment, respectively. The stress contours  900 ,  902  were calculated under 7-tier container loading, and the crane load and selfweight as listed in Table 1. As can be seen from a comparison of  FIGS. 9   a  and  b , the floating structure in accordance with the example embodiment ( FIG. 9   b ) experiences significantly reduced stresses. 
         [0055]    Tables 2 and 3 summarise the deflections calculated for the floating structure without zero-buoyancy chambers, and with zero buoyancy chambers according to the example embodiment, respectively. 
         [0000]    
       
         
               
               
             
               
               
               
             
               
               
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
             
             
               
                   
                   
               
               
                   
                 Differential Deflection (m) 
               
             
          
           
               
                   
                 Corner with 
                 Edge with 
               
             
          
           
               
                   
                 Deflection (m) 
                 respect to 
                 respect to 
               
             
          
           
               
                 Tiers 
                 Corner 
                 Edge 
                 Centre 
                 centre 
                 centre 
               
               
                   
               
             
          
           
               
                 0 
                 −3.53 
                 −3.06 
                 −2.89 
                 −0.64 
                 −0.17 
               
               
                 1 
                 −3.43 
                 −3.62 
                 −3.58 
                 0.15 
                 −0.04 
               
               
                 2 
                 −3.53 
                 −3.85 
                 −4.26 
                 0.73 
                 0.41 
               
               
                 3 
                 −3.53 
                 −4.27 
                 −4.95 
                 1.42 
                 0.68 
               
               
                 4 
                 −3.53 
                 −4.67 
                 −5.64 
                 2.11 
                 0.97 
               
               
                 7 
                 −3.52 
                 −5.90 
                 −7.70 
                 4.18 
                 1.8 
               
               
                 Allowable 
                 −7.5 
                 −7.5 
               
               
                 Deflection 
               
             
          
           
               
                 Draft 
                 OK since deflection 
                   
                   
                   
               
               
                 Check 
                 is less than 
               
               
                   
                 allowable deflection 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
               
             
               
               
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 3 
               
             
             
               
                   
                   
               
               
                   
                 Differential Deflection 
               
             
          
           
               
                   
                 Deflection (m) 
                 Corner w.r.t. 
                 Edge w.r.t. 
               
             
          
           
               
                 Tiers 
                 Corner 
                 Edge 
                 Centre 
                 centre (m) 
                 centre (m) 
               
               
                   
               
             
          
           
               
                 5 
                 −6.15 
                 −6.74 
                 −6.27 
                 0.12 
                 −0.47 
               
               
                 6 
                 −6.48 
                 −7.02 
                 −6.93 
                 0.45 
                 −0.09 
               
               
                 7 
                 −6.69 
                 −7.15 
                 −7.61 
                 0.92 
                 0.46 
               
               
                 Allowable 
                 −7.5 
                 −7.5 
               
               
                 Deflection 
               
             
          
           
               
                 Draft 
                 OK since deflection 
                   
                   
                   
               
               
                 Check 
                 is less than 
               
               
                   
                 allowable deflection 
               
               
                   
               
             
          
         
       
     
       ADVANTAGES 
       [0056]    The zero-buoyancy chambers in example embodiments are passive since the water flows in and out naturally from the chambers. There may be no need for pumps and expensive operating costs as in an active ballast system. The zero-buoyancy chambers may allow the floating structure to have the same draft even when loaded unevenly, provided the acceptable draft is not exceeded. This may lead to cost savings because of uniformity of modules across the whole floating structure. The lower buoyancy chambers may lead to a lighter and cheaper floating structure since the thickness of structural sections may be reduced (due to the reduced stresses and differential deflection) without compromising on the serviceability and strength capacities. The lower buoyancy chambers, being partially filled with water, may also provide hydrodynamic damping, thereby making the floating structure more resistant to movement caused by wave forces and water currents. 
       INDUSTRIAL APPLICABILITY 
       [0057]    Embodiments may be used in
       a floating container terminal, a floating cruise centre, a floating hotel, a floating restaurant, a floating pier/berth or a floating airport,   mooring buoys,   spars,   semi-submersibles,   rafts or mat foundations on soft soils, and   other floating structures such as multi-body floating structures, and comb-type floating structures.       
 
         [0064]    It will be appreciated by a person skilled in the art that numerous variations and/or modifications may be made to the present invention as shown in the example embodiments without departing from the spirit or scope of the invention as broadly described. The example embodiments are, therefore, to be considered in all respects to be illustrative and not restrictive.