Abstract:
A method of constructing a semi-submersible unit with a side wall. To construct the semi-submersible unit, the method can include placing a hull in a body of water and connecting a deck structure to the hull. In addition, there is a kit for constructing a semi-submersible unit, with a side wall.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims benefit of U.S. Provisional Patent Application No. 61/057,301 filed on May 30, 2008. 
    
    
     BACKGROUND 
     Field 
     The present invention relates to a method of constructing a semi-submersible unit comprising a side wall, wherein the method comprises the steps of: placing a hull in a body of water, the hull comprising an opening for receiving an end piece, the end piece constituting at least a portion of the side wall; lowering the hull into the body of water, and connecting a deck structure to the hull to form a semi-submersible unit. 
     The present invention also relates to a kit for constructing a semi-submersible unit comprising a side wall, wherein the kit comprises hull comprising an opening and a deck structure. Moreover, the present invention relates to a semi-submersible unit constructed according to the method and/or from a kit according to the present invention. 
     Semi-submersible units are inter alia used for drilling for, or production of, natural resources such as gas and oil, when the natural resources for example are located in a source located at the bottom of a sea or lake. Furthermore, semi-submersible units are used for quartering staff working out at sea. 
     Over the years, the sizes of semi-submersible units have generally increased which results in various problems when constructing the units. For instance, there may be few, if any, dry docks available in the world adapted to accommodate a complete semi-submersible unit the displacement and load carrying capacity of which meets the requirements of a modern drilling or production unit. Moreover, semi-submersible units are often provided with a deck structure adapted to accommodate e.g. drilling or production equipment and such a deck structure is often too heavy and/or large to be lifted by conventional lifting arrangements, such as cranes. 
     Furthermore, within the field of semi-submersible units, there has recently been an interest in units comprising a side wall or, even more preferred, being provided with an outer wall delimiting an enclosed area within the unit wherein the outer wall substantially constitutes the hull of the unit. The latter unit may be referred to as a ring wall type of semi-submersible unit. 
     When constructing a semi-submersible unit of a ring wall type with a deck structure, the prior art, see e.g. WO 02/092425, proposes a method of constructing a platform structure comprising a hull structure and a self-floating deck module, which method comprises the steps of lowering the hull structure into a body of water, floating the deck module over the hull structure and connecting the hull structure with the deck module. The steps as defined hereinabove as regards the floating in and connection of the deck module is often referred to as a mating procedure. However, as may be realized by a person skilled in the art, lowering the complete hull structure as proposed by &#39;425 requires a large amount of e.g. ballast water—due to the large water line area of the hull structure—which results in that the hull structure will be subjected to large loads. Moreover, when the &#39;425 hull structure is lowered to a position in which the deck structure may be connected to the hull structure, only a small portion of the hull structure is intersecting a still water level, resulting in a low stability of the hull structure. As may be realized by a person skilled in the art, there is generally a desire to avoid large loads, as well as a low stability, of the hull structure during a mating procedure. 
     Moreover, U.S. Pat. No. 6,125,780 discloses a method of constructing a hull for a ring wall semi-submersible unit which method comprises the steps of providing four substantially identical buoyant modules and attaching them to one another to thereby form a hull of a ring wall type. However, as in the &#39;425 case, the hull obtained using the &#39;780 method will also exhibit a large water plane area, hence requiring a large amount of e.g. ballast water in order to lower the hull to a position in which a deck structure may be connected to the hull. 
     As such, as may be realized from the above, when a constructing semi-submersible unit comprising a side wall and/or which is of a ring wall type, there is a need for improving the methods by which such units are constructed. 
     SUMMARY OF THE INVENTION 
     A first object of the present invention is to provide a method of constructing a semi-submersible unit comprising a side wall, wherein no one of the components constituting the semi-submersible unit is subjected to excessive loading during the construction. 
     A second object of the present invention is to provide a method of constructing a semi-submersible unit comprising a side wall, which method ensures that the floating components of the unit provide an appropriately high stability during the construction. 
     A third object of the present invention is to provide a method of constructing a semi-submersible unit comprising a side wall, which reduces, preferably eliminates, a risk of obtaining large residual stresses in any part of the semi-submersible unit. 
     A fourth object of the present invention is to provide a method of constructing a semi-submersible unit comprising a side wall, which requires a minimum of construction time in a dry dock and/or at a quay. 
     At least one of the objects above is achieved by a method for constructing a semi-submersible unit comprising a side wall according to claim  1 . 
     As such, the present invention relates to a method of constructing a semi-submersible unit comprising a side wall, wherein the method comprises the steps of: placing a hull in a body of water, the hull comprising an opening for receiving an end piece, the end piece constituting at least a portion of the side wall; lowering the hull into the body of water, and connecting a deck structure to the hull to form a semi-submersible unit. 
     According to the present invention, the method further comprises the steps of: floating the end piece into the opening, and attaching the end piece to the semi-submersible unit. 
     Preferably, the two last steps of the method as presented hereinabove are executed after the deck structure has been connected to the hull. 
     Thus, by utilizing the aforementioned method of the invention, the mating of the deck structure with the hull may be performed when the hull provides appropriate hydrostatic characteristics, such as water plane area and stability. Then, the end piece is inserted in the semi-submersible unit thus formed, wherein the introduction of the end piece will further contribute to the hydrostatic characteristics of the hull without impairing the above mentioned mating procedure. 
     Moreover, since the hull and deck structure are connected to one another, thus preferably forming a continuous unit prior to the insertion of the end piece, initial deflections of the hull—e.g. due to differences in the inner and outer pressure of the hull—may be reduced, or even avoided since the deck structure generally is a member with a high stiffness. As may be realized by a person skilled in the art, if the end piece was to be inserted and connected to the hull when the hull was subjected to initial deflections—this may result in residual stresses in at least a portion of the complete semi-submersible unit. These residual stresses may be reduced, or even avoided, by the construction method of the present invention. 
     It should be noted that the step of attaching the end piece to the semi-submersible unit need not necessarily be an additional method step, the end piece and semi-submersible unit may namely be arranged to become attached to one another on insertion of the end piece and semi-submersible unit with mating mechanical parts arranged to fasten and/or lock on insertion of the end piece into the semi-submersible unit. 
     It should also be noted that an end piece need not necessarily fill the entire opening but that several end pieces may be arranged to be inserted into the same opening; said one or more end pieces not necessarily filling the opening. Additionally, and end piece may itself comprise one or more openings. 
     According to a preferred embodiment of the method of the present invention, the method further comprises a step of elevating the semi-submersible unit after the deck structure has been connected to the hull in order to facilitate the floating in of the end piece. 
     According to another embodiment of the method of the present invention, the method further comprises a step of elevating the semi-submersible unit after the end piece has been inserted into the opening such that the step of attaching the end piece to the semi-submersible unit may be performed above a still water level of the body of water. 
     This ensures that the end piece may be attached to the semi-submersible unit by means of conventional attachment techniques, such as dry welding, which is considerably cheaper and more efficient than for instance wet welding, i.e. welding below the still water surface. 
     According to a further embodiment of the method of the present invention, the end piece is buoyant such that the end piece may be self-floating into the opening, preferably the end piece is towed into the opening. Thus, if the end piece is self-floating, the need for additional floating members, such as barges, during the construction of the semi-submersible unit, may be reduced, or even omitted. 
     According to another embodiment of the method of the present invention, the hull comprises a truss assembly at least partially delimiting the opening and the step of attaching the end piece to the semi-submersible unit in turn comprises the steps of: attaching the end piece to the truss assembly, and sealing the truss assembly from ambient water by attaching a sealing arrangement, which sealing arrangement preferably comprises a plate such as a steel plate, to the truss assembly. 
     The truss assembly as mentioned hereinabove, may provide additional structural stiffness to the hull—hence preferably reducing the risk for initial deflections—without substantially increasing the water plane area of the hull. 
     According to a further embodiment of the method of the present invention, the hull comprises guide means for guiding the end piece into the opening and the step of floating the end piece into the opening in turn comprises a step of guiding the end piece into the opening using the guide means. 
     According to another embodiment of the method of the present invention, the step of attaching the end piece to the semi-submersible structure in turn comprises the steps of: attaching the end piece to the hull, and attaching the end piece to the deck structure. 
     A second aspect of the present invention relates to a kit for constructing a semi-submersible unit comprising a side wall, wherein the kit comprises: hull comprising an opening, and a deck structure. 
     According to the second aspect of the present invention, the kit further comprises an end piece adapted to constitute at least a portion of the side wall, the end piece further being adapted to be floated into the opening and attached to the hull. 
     According to a preferred embodiment of the second aspect of the present invention, the hull comprises means for guiding the end piece into the opening. 
     According to another embodiment of the second aspect of the present invention, the hull comprises a truss assembly at least partially delimiting the opening. 
     According to a further embodiment of the second aspect of the present invention, the hull comprises a first and a second longitudinally extending float, wherein each one of the first and second floats is adapted to extend through a still water level of a body of water, wherein the first and second floats are connected to one another by means of a substantially transversally extending bridging arrangement wherein: at least a portion of the first float, at least a portion of the second float and at least a portion of the bridging arrangement at least partially delimits the opening. 
     According to another embodiment of the second aspect of the present invention, the end piece comprises structural tanks, such as ballast water tanks. 
     According to a further embodiment of the second aspect of the present invention, the hull comprises tanks adapted to store liquefied natural gas. 
     According to another embodiment of the second aspect of the present invention, the kit comprises a plurality of end pieces such that the side wall delimits an enclosed area. 
     A third aspect of the present invention relates to a semi-submersible unit constructed according to the method according to the first aspect of the present invention and/or from a kit according to the second aspect of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be described in greater detail by means of non-limiting examples only and with reference to the attached drawings wherein: 
         FIG. 1  illustrates a schematic perspective view of a hull comprising an opening for receiving an end piece; 
         FIG. 2  illustrates a schematic side view of the hull illustrated in  FIG. 1 ; 
         FIG. 3  illustrates a schematic perspective view of the hull of  FIG. 1  as well as a deck structure in a step of the method of the present invention wherein the deck is connected to the hull to form a semi-submersible unit; 
         FIG. 4  illustrates a schematic side view of the semi-submersible unit as illustrated in  FIG. 3 ; 
         FIG. 5  illustrates a schematic perspective view of the of the semi-submersible unit of  FIG. 3 , wherein an end piece is floated into the opening of the hull; 
         FIG. 6  illustrates a schematic perspective view of the semi-submersible unit in  FIG. 3  and  FIG. 5 , wherein the end piece is being attached to the hull; 
         FIG. 7  illustrates a side view of an alternative implementation of a hull suitable for the construction method of the present invention; 
         FIG. 8  illustrates a top view of the hull illustrated in  FIG. 7 ; and 
         FIG. 9  illustrates a schematic perspective view of yet another hull which may be used in the construction method of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The invention will be described using examples of embodiments. It should however be realized that the embodiments are included in order to explain principles of the invention and not to limit the scope of the invention, defined by the appended claims. 
       FIG. 1  illustrates a hull  10  which is suitable to be used in the construction method of the present invention. The hull  10  comprises a first and a second longitudinally extending float  12 ,  14 . Each one of the first  12  and second  14  floats is adapted to extend through a still water level (SWL) of a body of water. The first and second floats  12 ,  14  are connected to one another by means of a substantially transversally extending bridging arrangement, which bridging arrangement in the implementation of the hull  10  illustrated in  FIG. 1  is constituted by two transversally extending bracings, namely a first  16  and a second bracing  18 , wherein each one of the bracings  16 ,  18  connects a lower portion of the first float  12  to a lower portion of the second float  14 . The floats  12 ,  14  and the bracings  16 ,  18  are preferably made out of steel. Purely by way of example, the hull  10  may have a displacement within the range of 50 000-500 000 metric tonnes when in an operational draught. 
     Furthermore, as may be gleaned from  FIG. 1 , the first and the second floats  12 ,  14  in combination with each one of the bracings  16 ,  18  form an opening in the hull. As such, the first and second floats  12 ,  14  and the first bracing  16  form a first opening  20  whereas the first and second floats  12 ,  14  and the second bracing  18  form a second opening  22 . 
     In the presentation of the construction method of the present invention hereinbelow, reference will be made to the first opening  20  only. However, a person skilled in the art will directly realize that each step in the construction method is equally applicable for the second opening  22 . 
     At least one, but preferably both, of the floats  12 ,  14  are preferably furnished with tanks for storing hydrocarbons, such as gas or oil. More preferred, the floats are furnished with tanks  13  for storing liquefied natural gas (LNG). Moreover, the floats  12 ,  14 —and possibly also the bracings  16 ,  18 —are provided with ballast .water tanks  15  and a ballast water system including inter alia water pumps for controlling the water level in the aforementioned ballast water tanks such that the load and hence the floating condition of the hull  10  may be controlled. 
       FIG. 2  presents a side view of the  FIG. 1  hull  10 , wherein deflections of the hull  10  have been exaggerated. As may be realized from  FIG. 2 , when the hull  10  is floating in water, there is a risk that the hull may be subjected to initial deflections due to e.g. difference in the internal pressure and the external pressure—i.e. the water pressure—of the hull  10 . Moreover,  FIG. 2  illustrates that it is not unlikely that the bracings  16 ,  18  may be subjected to a bending deflection resulting in that the first and second floats  12 ,  14  will be inclined away from one another. As such, if an end piece (not shown in  FIG. 2 ), adapted to be fitted into the first opening  20  was to be attached to the hull  10  in the floating condition illustrated in  FIG. 1  and  FIG. 2 , there is a risk that the hull  10  thus obtained—i.e. comprising the original hull  10  and the end piece—would contain residual stresses which may impair the strength of the hull  10 . This problem is at least reduced by the construction method of the present invention, as will be explained hereinbelow. 
       FIG. 3  illustrates a further step of the construction method of the present invention, wherein the hull  10  has been lowered into the water such that the freeboard, i.e. the distance from the still water level to the uppermost portion of the first and second floats  12 ,  14 , is so small that a deck structure  24  may be floated over the hull  10 . Preferably, the hull  10  is lowered by introducing ballast water into the hull  10 , e.g. by using the aforementioned ballast water tanks (not shown). Since the water plane area of the hull  10  is only constituted by the water plane areas of the first  12  and second  14  floats, respectively, the hull  10  may be lowered to a desired draught without subjecting the hull  10  to excessive loads. Furthermore, the water plane areas of the first  12  and second  14  floats, respectively, preferably are adapted to provide an appropriate stability to the hull  10  in the floating condition illustrated in  FIG. 3 . 
     When the hull  10  is in the position illustrated in  FIG. 3 , the deck structure  24  is floated over the hull  10 . To this end, the deck structure  24  may be loaded onto one or several barges (not shown) wherein at least one of the barges may be guided through the second opening  22  of the hull  10 . Optionally, the deck structure  24  may be buoyant—for instance a centre portion of the deck structure  24  may be adapted to be immersed in water and thus carry the rest of the deck structure  24  when it is introduced through the second opening  22 . 
     The deck structure  24  may have various designs, depending inter alia on the purpose of the semi-submersible unit to be constructed. Purely by way of example, the deck structure  24  may be a buoyant member or at least have a water tight bottom. Optionally, the deck structure  24  may comprise, or even be constituted by, a truss structure (not shown). Moreover, although the deck structure  24  in  FIG. 3  is illustrated as being constituted by one single unit, the deck structure  24  may instead be split up into several sub-structures (not shown) wherein each one of the sub-structures are floated into position and connected to the hull  10 . Additionally, the deck structure  24  may comprise additional equipment, e.g. for drilling, producing or storing hydrocarbons (not shown). Purely by way of example, a deck structure  24  without the additional equipment as presented hereinabove may weigh approximately 10 000 to 20 000 metric tonnes. In a case wherein a deck structure  24  is provided with arrangements for e.g. producing LNG, the deck structure  24 , including such arrangements, may weigh 60 000 to 80 000 metric tonnes. 
     When the deck structure  24  has been placed in position over the hull  10 , the two components may be connected to one another to form a semi-submersible unit  26 . To this end, the deck structure  24  may be fixedly attached to the hull  10 , e.g. by means of welding. Optionally, the deck structure  24  may simply be resting on the hull  10  wherein the deck structure  24  will be held in place in relation to the hull  10  by means of friction forces developed between the two structures. 
       FIG. 4  illustrates a side view of the  FIG. 3  semi-submersible unit  26 . As may be realized when comparing  FIG. 4  and  FIG. 2 , since the  FIG. 4  semi-submersible unit  26  comprises the deck structure  24  which is connected to the hull  10  and since the deck structure  24  often is relatively stiff, the deflections of the hull  10  in the  FIG. 4  configuration are generally substantially less pronounced than in the case illustrated in  FIG. 2 . Thus, the configuration as proposed in  FIG. 4  provides for that an end piece may be inserted in the first opening  20  in the hull  10  and may also be attached to the hull  10  when the hull  10  subjected to small initial deflections, resulting in a semi-submersible unit with low residual stresses. 
       FIG. 5  illustrates a perspective view of the semi-submersible unit  26  obtained from the construction step illustrated in  FIG. 3 .  FIG. 5  further illustrates that an end piece  28  may be floated into the opening  20  and connected to the semi-submersible unit  26 . In this respect, the draught—i.e. the distance from the bottom of the hull  10  to the still water level—of the semi-submersible unit  26  in the condition illustrated in  FIG. 5  is preferably substantially lower than the draught of the hull  10  during the deck structure  24  mating step as illustrated in  FIG. 3 . This reduction of the draught is carried out order to facilitate the floating in of the end piece  28 . However, in some implementations of the construction method of the present invention, it may not be necessary to reduce the draught between the steps of the construction method of the present invention as illustrated in  FIG. 3  and  FIG. 5 . As further may be realized from  FIG. 5 , the end piece  28  preferably constitutes a substantial part of the side wall. Purely by way of example, the end piece  28  may have a height which is in the interval of 0.2-0.8 of the height of the hull  10  and the width of the end piece  28  may also range from 0.2 to 0.8 of the width or length, whichever is the largest, of the hull  10 . 
     In a preferred implementation of the end piece  28  floating in step of the method of the present invention, the end piece  28  is buoyant such that it may be self-floating into the first opening  20 . To this end, the end piece  28  may preferably be connected to an arrangement (not shown) adapted to propel the end piece  28 . Purely by way of example, such an arrangement may comprise one or more tugs (not shown). Optionally, if the end piece  28  is not buoyant or if it by some reasons is so desired, the end piece may be loaded on a floating arrangement (not shown) such as a barge (not shown) which floating arrangement in turn may be floated into the first opening  20 . 
     The end piece  28  is preferably made out of steel and preferably comprises structural tanks, such as ballast water tanks (not shown). Such ballast water tanks may be used to control the floating condition of the end piece  28 , in the case of a buoyant end piece  28 . Moreover, once the end piece  28  has been connected to the semi-submersible unit  26 , the ballast water tanks of the end piece  28  may preferably be connected to a ballast water system (not shown) of the semi-submersible unit  26  such that the ballast water tanks of the end piece  28  may be used for controlling the floating condition of the semi-submersible unit  26 . 
     In order to facilitate the floating in of the end piece  28  into the first opening  20 ; the hull  10  and/or the end piece  28  is preferably provided with guide means for guiding the end piece  28  into position in the first opening  20 . Purely by way of example, such a guide means may comprise a rod (not shown) of the hull  10  which rod extends substantially vertically in the first opening  20  as well as two frames (not shown) located on the bottom of the end piece  28 , which frames form a tapering gap. 
     Moreover, the hull  10  may be provided with supports (not shown) located below the opening on which supports the end piece  28  may rest when it has been inserted into the first opening  20 . 
     When the end piece  28  has been inserted into the first opening  20 , the end piece  28  and the semi-submersible unit  26  are attached to one another. Preferably, the end piece  28  is fixedly attached to the semi-submersible unit  26 , e.g. by means of welding. Moreover, the end piece  28  is preferably fixedly attached to the hull  10  and, if possible, also to the deck structure  24 . 
     The end piece  28  may be attached to the semi-submersible unit  26  when the unit  26  is in the floating condition as illustrated in  FIG. 5 , e.g. immediately after the end piece  28  has been inserted into the first opening  20 . However, in a preferred embodiment of the present invention, the semi-submersible unit  26  is raised—i.e. the draught of the unit  26  is reduced—before the end piece  28  is attached to the unit  26 . This implementation of the preferred embodiment is illustrated in  FIG. 6 . If the end piece  28  is attached to the semi-submersible unit  26  when the unit is in a floating condition as illustrated in  FIG. 6 , attachment procedures, such as welding, may be performed in a dry environment which generally provides for a more rapid and efficient attachment procedure. 
       FIG. 7  illustrates a side view of another implementation of a hull  10  which may be used in the construction method of the present invention. As may be gleaned from  FIG. 7 , the hull  10  comprises a truss assembly  30  at least partially delimiting the first opening  20 . The truss assembly  30  adds additional structural stiffness without adding any substantial hydrostatic stiffness to the hull  10 . As may be realized by a person skilled in the art, the aforementioned changes in the characteristics of the hull  10  obtained by the introduction of the truss assembly  30  may be useful when e.g. performing the construction method of the present invention, for instance as regards deflections of the hull  10  and/or the capability of raising and lowering the hull. With a semi-submersible unit  26  comprising a hull  10  as illustrated in  FIG. 7 , an end piece (not shown in  FIG. 7 ) is preferably attached to the inner portion of the truss assembly  30 . 
     In order to obtain a buoyant—or at least weather proof—side wall extending from the first float  12  to the second float  14  of the hull  10  as illustrated in  FIG. 7 , the truss assembly  30  is preferably sealed from ambient water after the end piece  28  has been inserted in the first opening  20  and attached to the truss assembly  30 . Preferably, such a sealing is obtained by attaching a sealing arrangement to the truss assembly  30 . An example of such a sealing arrangement is illustrated in  FIG. 8 , wherein the sealing arrangement comprises four steel plates  32 ′,  32 ″,  34 ′ and  34 ″. Each one of the steel plates  32 ′,  32 ″,  34 ′ and  34 ″ is placed in an appropriate position on or over the truss assembly and attached to the hull  10  and the end piece  28 . Preferably, the sealing arrangement illustrated in  FIG. 8  further comprises a two more plates (not shown in  FIG. 8 ) adapted to be located at each side of the end piece  28  and adapted to cover the portions of the truss assembly  30  located below the end piece  28 . 
     Finally,  FIG. 9  illustrates another hull  10  for use in the construction method of the present invention may be performed. As may be gleaned from  FIG. 9 , the hull  10  illustrated therein comprises a ring pontoon  36  and four substantially vertically extending columns  38 ,  40 ,  42  and  44  wherein each one of the columns is connected to the ring pontoon  36 . As may be realized from  FIG. 9 , the hull  10  has four openings  46 ,  48 ,  50  and  52  and each one of the openings is adapted to receive and end piece (not shown in  FIG. 9 ). Preferably, when all the four openings of a semi-submersible unit, based on the hull  10  illustrated in  FIG. 9 , have been provided with an end piece, a semi-submersible unit is obtained which has a continuous outer wall, i.e. the unit is of a so called ring wall type. 
     It should be realized that the present invention is not limited to the embodiments described hereinabove and illustrated in the drawings. Rather, a person skilled in the art will realize that many changes and modifications may be performed within the scope of the appended claims.