Abstract:
Disclosed is a floating system. The floating system comprises a floating body, a first propeller and a second propeller. The first propeller is pivotally secured to the floating body. The second propeller is pivotally secured to the floating body and distanced from the first propeller in a first direction and in a second direction perpendicular to the first direction. The first and second directions are generally parallel to a surface of water on which the floating system floats.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to and the benefit of Korean Patent Application No. 10-2008-0024563, filed Mar. 17, 2008, the disclosure of which is incorporated herein by reference in its entirety. 
       BACKGROUND 
       [0002]    1. Field 
         [0003]    The present disclosure relates to a floating system having propulsive devices, and more particularly, to a floating system having propellers for used in moving the ship while sailing and mooring. 
         [0004]    2. Discussion of the Related Technology 
         [0005]    Natural gas which is in a gas state is transported through a gas pipe line installed on the land or in the sea, or natural gas which is in a liquefied natural gas (LNG) state is transported by an LNG carrier (LNG transport vessel) to distant markets while the liquefied natural gas is stored in the LNG carrier. Liquefied natural gas is produced by cooling natural gas at an extremely low temperature of approximately −163° C., and a volume of the liquefied natural gas is approximately 1/600 of a volume of natural gas which is in a gas state, so that marine transportation is suitable for a long-distance transportation of liquefied natural gas. 
         [0006]    In recent, besides an LNG transport vessel employed for loading LNG, sailing on the sea and unloading LNG to a land market, an LNG regasification vessel (RV), which is loaded with LNG, sails on the sea, and re-gasifies the stored LNG to unload the regasified LNG in a natural gas state after arriving a land market, has been employed. Such an LNG RV is equipped with a storage tank for storing LNG and facilities such as an apparatus for re-gasifying LNG and the like. 
         [0007]    In addition, recently, a demand for a floating marine structure such as an LNG floating production storage and offloading (FPSO) or an LNG floating storage and regasification unit (FRSU) also has been increased gradually. Such a floating marine structure is also equipped with the storage tank, which is installed to the LNG transport vessel or the LNG RV, and mounted with an apparatus for liquefying natural gas or re-gasifying LNG, if necessary. 
         [0008]    The LNG FPSO is the floating type maritime structure used for liquefying the produced natural gas directly on the sea, storing it in a storage tank, and delivering the LNG stored in the storage tank to an LNG transport vessel when necessary. In addition, the LNG FSRU is a floating type maritime structure, which stores LNG, which is unloaded from the LNG transport vessel, in a storage tank on the sea far away from the land and then gasifies the LNG, if necessary, and supplies the gasified natural gas to a market on the land. 
         [0009]    The foregoing discussion in the background section is to provide general background information, and does not constitute an admission of prior art. 
       SUMMARY 
       [0010]    One aspect of the invention provides a ship. The ship comprises a hull having a length in a longitudinal direction and a width in a lateral direction perpendicular to the longitudinal direction; a first propeller secured to the hull and configured to propel the ship; and a second propeller secured to the hull and configured to propel the ship, wherein the second propeller is distanced from the first propeller both in the longitudinal and lateral directions. 
         [0011]    In the foregoing ship, at least one of the first and second propellers may be configured to pivotally rotate while secured to the hull. The first and second propellers may be closer to a rear end of the ship than to a front end of the ship. A first distance from the first propeller to a rear end of the ship may be smaller than a second distance from the second propeller to the rear end of the ship. The hull may comprise a center line extending in the longitudinal direction between front and rear ends thereof, wherein the first propeller may be on one side of the center line and the second propeller is on the other side of the center line. The center line may be closer to the second propeller than to the first propeller. The first and second propellers may be arranged along an imaginary straight line which is substantially tilted from the longitudinal direction. The imaginary line may be tilted from the longitudinal direction forming an angle therebetween from about 10° to about 80°. 
         [0012]    Another aspect of the invention provides a floating system. The floating system comprises: a floating body; a first propeller pivotally secured to the floating body; and a second propeller pivotally secured to the floating body and distanced from the first propeller in a first direction and in a second direction perpendicular to the first direction, wherein the first and second directions are generally parallel to a surface of water on which the floating system floats. 
         [0013]    In the foregoing floating system, the floating body may comprise a stem and a stem, wherein the first and second propellers are located closer to the stem than to the stem. The floating body may comprise a stem and a stem, wherein the floating body generally may extend in the first direction from the stem and the stem. The first and second propellers may be arranged along an imaginary line which is slanted with respect to the first direction. The imaginary line and the first direction may define an angle from about 10° to about 80°. The angle may be from about 30° to about 60°. 
         [0014]    Still in the foregoing system, the floating system may further comprise a mooring structure configured to be fixed to a seabed or land, wherein the floating body is pivotally connected to the mooring structure. The first and second propellers may be configured to, in combination or alone, apply a propulsive force to the floating body so as to move around the mooring structure. The first propeller may be configured to pivot with respect to the floating body between a first sailing position and a first mooring position, wherein the second propeller may be configured to pivot with respect to the floating body between a second sailing position and a second mooring position. 
         [0015]    Further in the foregoing system, the floating body may be configured to sail substantially straight when the first and second propellers are set in the first and second sailing positions. The floating body may be configured to move around the mooring structure when the first and second propellers are set in the first and second mooring positions. The first propeller may be configured to apply a first propulsive force to the floating body at the first sailing position generally in a fourth direction, wherein the second propeller may be configured to apply a second propulsive force to the floating body at the second sailing position generally in a fifth direction, wherein the floating body may further comprise a rudder configured to control sailing of the floating body in the first direction. The fourth and fifth directions may be substantially parallel to the first direction. The first propeller may be configured to apply a first propulsive force to the floating body at the first sailing position generally in a fourth direction, wherein the second propeller may be configured to apply a second propulsive force to the floating body at the second sailing position generally in a fifth direction, wherein the fourth and fifth directions may be substantially nonparallel. The floating body may be selected from the group consisting of an LNG tank ship, an LNG RV, an LNG FPSO and an LNG FSRU. 
         [0016]    Still another aspect of the invention provides a method of operating a floating system. The method comprises: providing the foregoing floating system; engaging the floating body with a mooring structure fixed to a seabed; and running the first and second propellers so as to rotate the floating body around the mooring structure while the floating body s engaged with the mooring structure. 
         [0017]    In the foregoing method, the method may further comprise disengaging the floating body from the mooring structure; and running the first and second propellers so as to sail the floating body away from the mooring structure. The method may further comprise: pivoting the first propeller with respect to the floating body to a first sailing position; and pivoting the second propeller with respect to the floating body to a second sailing position, wherein running the first and second propellers so as to sail the floating body comprises operating the first propeller to apply a first propulsive force to the floating body at the first sailing position generally in a fourth direction; and operating the second propeller to apply a second propulsive force to the floating body at the second sailing position generally in a fifth direction, wherein the fourth and fifth directions are substantially parallel to the first direction. 
         [0018]    Still in the foregoing method, the method may further comprise pivoting the first propeller with respect to the floating body to a first sailing position; and pivoting the second propeller with respect to the floating body to a second sailing position, wherein running the first and second propellers so as to sail the floating body comprises operating the first propeller to apply a first propulsive force to the floating body at the first sailing position generally in a fourth direction; and operating the second propeller to apply a second propulsive force to the floating body at the second sailing position generally in a fifth direction, wherein the fourth and fifth directions are substantially nonparallel. 
         [0019]    Further in the forgoing method, the floating system may comprise a LNG tank configured to contain LNG, wherein the method may further comprise loading or unloading LNG into or from the LNG tank while the floating system moors. The floating system may comprise a LNG tank containing LNG and a LNG vaporizer, wherein the method may further comprise vaporizing the LNG and transmitting the vaporized LNG to a valve port connected to an onshore LNG supplying system. 
         [0020]    An aspect of the present invention is to provide a floating structure having a pair of thrusters, acting as propulsive devices, installed to a stem bottom thereof slantingly with respect to each other to enable the pair of thrusters to control both propulsion and heading angle thereof. 
         [0021]    According to an aspect of the present invention, there is provided a floating structure sailing or moored on the sea, which comprises a plurality of propulsive devices rotatably installed to a stem bottom of the floating structure, wherein the propulsive devices are disposed on the stem bottom slantingly with respect to each other, thereby preventing the propulsive devices from interfering with each other when they operate. 
         [0022]    The plurality of propulsive devices may be arranged in a longitudinal direction of the floating structure to generate propulsive force when the floating structure sails, and the plurality of propulsive devices may be arranged in a right or left side direction of the floating structure to control a heading angle when the floating structure is moored, thereby performing the propulsion and the heading angle control of the floating structure when it sails and is moored, respectively. 
         [0023]    The plurality of propulsive devices are preferably a pair of azimuth type thrusters capable of changing a propelling direction while rotating in its origin place. 
         [0024]    A pair of the propulsive devices are preferably installed to the stem bottom of the floating structure slantingly at an angle θ with respect to each other, the angle θ being in a range of 10 to 80°. 
         [0025]    The floating structure may further comprise an automatic control system for correcting an advance direction of the floating structure by utilizing a rudder, by adjusting directions of the pair of propulsive devices, or by adjusting a propulsive force ratio of the pair of propulsive devices to correct an error caused by a difference in moment when the floating structure sails by the pair of propulsive devices slantingly installed. 
         [0026]    The floating structure may be any one selected from an LNG transport vessel, an LNG RV, an LNG FPSO and an LNG FSRU. 
         [0027]    According to another aspect of the present invention, there is provided a method of controlling a heading angle of a moored floating structure using a plurality of propulsive devices rotatably installed to a stem bottom thereof, wherein a heading angle of the moored floating structure is controlled using the plurality of propulsive devices disposed on a stem bottom of the floating structure slantingly with respect to each other to prevent the propulsive devices from interfering with each other when the propulsive devices operate. 
         [0028]    The method of controlling a heading angle of a moored floating structure may comprise the step of generating propulsive force in a state where the propulsive devices are rotated to be directed to a right or left side of the floating structure. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0029]      FIG. 1  is a plan view schematically showing a floating structure according to one embodiment of the present invention; and 
           [0030]      FIG. 2  is a side view schematically showing the floating structure according to one embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0031]    Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. 
         [0032]    The floating structures, such as an LNG regasification vessel, an LNG FPSO, an LNG FRSU and the like, for transporting and storing liquid cargo such as LNG on the sea can be used in a state where the floating structure is moored by a single point mooring system such as a turret or yoke mooring system. Such a single point mooring system is mainly applied to a floating structure for operating on a sea area in which a direction of environmental force such as wind, tide and wave is frequently changed. 
         [0033]    According to the single point mooring system, a mooring rope used for mooring a floating structure is fixed to a device such as a turret, so that the mooring rope serves to maintain a location of the floating structure. At this time, the floating structure can freely turn about the turret, so that a direction of the floating structure can be converged to minimize external force exerted on the floating structure. 
         [0034]    Various external forces are exerted on the floating structure according to external environmental conditions. Wind, tide and wave as external force may be applied to the floating structure. A heading angle of the floating structure moored in the single point mooring system is naturally stabilized to minimize magnitudes of the various external forces caused by an environment. 
         [0035]    However, in a case where a direction of wind and tide applied to the floating structure differs from a direction of wave applied thereto, wave is frequently applied to not a front of the hull but a side thereof in a state where a heading angle is stable. At this time, if the wave is high, a motion of the hull, in particular a roll motion of a stem wherein the hull rolls from side to side, becomes large. 
         [0036]    If the roll motion of the stem becomes excessively large, it is difficult to operate various kinds of facilities provided in the floating structure or a severe sloshing phenomenon can be caused in the storage tank, so that there is need to control intentionally a heading angle of the floating structure according to the circumstances. The sloshing phenomenon means that a liquid state material, i.e., LNG, stored in the storage tank flows when a floating body such as a vessel moves under various marine conditions, and causes a wall surface of the storage tank to be severely damaged. 
         [0037]    In a floating structure, accordingly, in addition to a propulsive device provided at the stern and used for sailing (or moving, coming back to harbor), other propulsive devices are additionally mounted to left and right surfaces of the stern for intentionally controlling a heading angle when the floating structure is moored in a single point, as described above. 
         [0038]    In the floating structure as described above, a propulsive device for sailing the floating structure and propulsive devices for controlling a heading angle are separately provided. Thus, the cost required for installing the propulsive devices is increased and a control system used for controlling a plurality of propulsive devices becomes complicated. 
         [0039]      FIG. 1  is a plan view schematically showing a floating structure according to one embodiment of the present invention; and  FIG. 2  is a side view schematically showing the floating structure according to one embodiment of the present invention. 
         [0040]    In certain embodiments, a floating structure mentioned herein is a concept including a structure or a vessel, which floats on the sea on which flows occur. In one embodiment, the structure or vessel has a storage tank for storing liquid cargo such as LNG loaded at an extremely low temperature, LNG re-gasification facilities or LNG liquefaction facilities. For example, the floating structure includes a marine structure, such as an LNG FPSO or an LNG FSRU, as well as a vessel, such as an LNG transport vessel or an LNG RV. The floating structure can be utilized while being moored by a single point mooring system such a turret or yoke mooring system. As an example,  FIGS. 1 and 2  show that a floating structure  1  is single-point-moored by a turret  2  provided at a stem portion of the floating structure  1 . However, the present invention is not limited thereto. The floating structure of one embodiment of the present invention can be moored by any known system in addition to the turret. 
         [0041]    In one embodiment, propulsive devices  4  and  6  are installed to a stem bottom of the floating structure  1 . Although azimuth type thrusters capable of changing the propelling direction while rotating in its origin place are used as these propulsive devices  4  and  6  in one embodiment, a variety of propulsive devices may be employed if they are rotatable or pivotable with respect to the hull. 
         [0042]    As shown in  FIG. 1 , the propulsive devices  4  and  6  are installed to the stem bottom of the floating structure  1  along a line L 2  extending in a direction slanted with respect to a line L 2  at a certain angle θ. 
         [0043]    Assuming that an imaginary line L 1  passes the first propulsive device  4  installed to a stern side of the floating structure  1  and is perpendicular to a line L 3  extending along the longitudinal direction of the floating structure  1 , and that an imaginary line L 2  passes the first propulsive device  4  and the second propulsive device  6  installed to be closer to a stem side than the first propulsive device  4 , the lines L 1  and L 2  define a certain angle θ. The lines L 2  and L 3  define an angle α. 
         [0044]    In one embodiment, the angle α is about 10° to about 80°. In certain embodiments, the angle α is one of about 20°, about 30°, about 35°, about 37°, about 40°, about 42°, about 44°, about 45°, about 46°, about 48°, about 50°, about 52°, about 54°, about 56°, about 58°, about 60°, about 62°, about 65°, about 68°, about 70° and about 75°. In some embodiments, the angle α may be within a range defined by two of the foregoing angles. In an embodiment, the angle α is determined such that the first and second propulsive devices  4  and  6  avoid undesirably interfering with each other when the floating structure  1  sails (or moves, comes to the harbor). In addition, the angle α is determined such that the first and second propulsive devices  4  and  6  avoid undesirably interfering with each other when the floating structure  1  turns in a moored state. 
         [0045]    In a case where the floating structure  1  sails using the first and second propulsive devices  4  and  6  disposed slantingly with respect to each other as described above, they generate propulsive force in a state where the first and second propulsive devices  4  and  6  are directed to the rear, i.e., in a state where the first and second propulsive devices are arranged in the longitudinal direction of the floating structure  1  to be directed to the rear. 
         [0046]    At this time, since the first propulsive device  4  is positioned to be closer to the stern than the second propulsive device  6 , it is concerned that an advance direction of the floating structure  1  can be changed due to a difference in moment generated by the first and second propulsive devices. 
         [0047]    Thus, in order to accurately control the advance direction of the floating structure  1 , it is preferable that the advance direction should be corrected by utilizing a rudder  10 , by adjusting the directions of the first and second propulsive devices  4  and  6 , by adjusting a propulsive force ratio of the first and second propulsive devices  4  and  6 . 
         [0048]    The correction of the advance direction of the floating structure  1  as described above can be controlled precisely by an automatic control system (not shown) mounted in the floating structure  1 . 
         [0049]    In addition, when the moored floating structure  1  turns using the first and second propulsive devices  4  and  6  arranged as described above, the first and second propulsive devices  4  and  6  generate the propulsive force in a state where both the first and second propulsive devices  4  and  6  rotate to the right by about 90° or to the left by 90°, i.e., arranged to be directed to the right or left side of the floating structure  1 . 
         [0050]    At this time, since the first and second propulsive devices  4  and  6  arranged along the line L 2 , it is possible to turn the moored floating structure to control the heading angle thereof without interference between the two propulsive devices. 
         [0051]    Referring to  FIG. 1 , in one embodiment, the first propulsive device  4 , which is positioned to be closer to the stern than the second propulsive device  6 , is provided on the portside of the floating structure  1 , and the second propulsive device  6 , which is positioned to be closer to the stem than the first propulsive device  4 , is provided on the starboard thereof. According to one embodiment of the present invention, however, the first propulsive device  4  is positioned to be closer to the stem than the second propulsive device  6 . 
         [0052]    As described above, according to one embodiment of the present invention, there is provided a floating structure having a pair of thrusters, acting as propulsive devices, installed to a stem bottom thereof to enable the pair of thrusters to both propel the structure to sail or navigate and control heading angle thereof. 
         [0053]    Accordingly, it is advantageous that the number of propulsive devices to be installed can be decreased to save the cost and the number of the propulsive devices to be controlled can also be decreased to simplify the control system. 
         [0054]    Although embodiments of the present invention have been described, it will be apparent to those skilled in the art that various modifications, changes and variations can be made thereto within the scope of the present invention and the appended claims. Therefore, the aforementioned descriptions and the accompanying drawings should be construed as not limiting the spirit or scope of the present invention but illustrating the present invention.