Abstract:
The moonpool includes: a first space formed by being penetrated from a bottom surface through an upper deck of the drillship so as to carry out a drilling operation; and a second space formed on a side of the first space in a lengthwise direction of the drillship, a bottom of the second space being open toward a lower side of the drillship. The second space is formed on a side of the first space in the lengthwise direction of the drillship so that the overall length of the moonpool is increased.

Description:
PRIORITY CLAIM 
       [0001]    This application is a continuation and claims the benefit of priority under 35 U.S.C. §§120, 365, and 371 to Patent Cooperation Treaty Patent Application No. PCT/KR2009/005703, filed on Oct. 6, 2009. This application further claims the benefit of priority to Korean Application Nos. 10-2008-0105224 filed Oct. 27, 2008, 10-2008-0125800 filed Dec. 11, 2008, and 10-2009-0091728 filed Sep. 28, 2009. The disclosures of the above applications are incorporated herein by reference in their entireties. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to a moonpool and a drillship having the moonpool, more specifically to a moonpool and a drillship having the moonpool that has a modified moonpool structure to reduce vibrations and resistance caused by the flow of seawater inside the moonpool during the sailing of the drillship. 
       BACKGROUND 
       [0003]    With the rapid industrial and manufacturing development in the global scale, use of fossil fuels, such as petroleum, has been increased, and international oil prices have been steadily soaring. Accordingly, stable production and supply of crude oil has become an increasingly important issue. 
         [0004]    For this reason, petty deep sea oil fields, which have been neglected until recently for their technical difficulties of drilling and lack of economic feasibility, have begun to receive attention, and ships having drilling equipment fitted for oil field development have been developed in step with the development of resource development technologies. 
         [0005]    For the conventional sea drilling equipment, a rig ship or a fixed-type platform have been commonly used, which can be moved by another tugboat and carries out its drilling operation while being fixed at a location in the sea by mooring equipment. 
         [0006]    Developed and used more recently for deep sea drilling is a drillship, which is closer to an ordinary vessel with the state-of-the-art drilling equipment and can sail with its own locomotive power. Such drillship needs to be designed to have optimal sailing capacities as well as drilling capabilities because it needs to frequently move its location for the development of petty oil fields. 
         [0007]    The drillship is installed with a large opening (referred to as “moonpool” hereinafter) for lowering a drilling pipe. Although this structure is indispensably essential for the use of the drillship, this structure is very disadvantageous in terms of the sailing speed. 
         [0008]    In other words, due to periodic oscillation of the water surface inside the moonpool caused by the relative movement between the seawater flowed in and out of the moonpool and the seawater outside the hull, and the resulting movement of the hull, resistance is increased during the sailing of the drillship, resulting in the reduced speed and more fuel consumption. It has been observed that this resistance is increased by as much as 50%. 
         [0009]    To date, designed and utilized for the purpose of reducing such increase of resistance have been affixture affixed inside the moonpool, affixture on the bottom of the ship around the moonpool, movable opening/closing devices inside the moonpool, etc. However, the affixture inside the moonpool has a complicated structure compared to its effect, and the movable opening/closing device is not widely used because its cost for installation and maintenance is very high. 
         [0010]    Contrived to solve the above problems, the present invention provides a moonpool and a drillship having the moonpool that can reduce the resonance and resistance caused by the vertical movement of the seawater inside the moonpool while the drillship is sailing. 
         [0011]    The present invention also provides a moonpool and a drillship having the moonpool that can reduce the amplitude of a sloshing movement and the resistance caused by the sloshing movement of the seawater inside the moonpool while the drillship is sailing. 
       SUMMARY 
       [0012]    An aspect of the present invention features a moonpool formed in a drillship that includes: a first space formed by being penetrated from a bottom surface through an upper deck of the drillship so as to carry out a drilling operation; and a second space formed on a side of the first space in a lengthwise direction of the drillship, a bottom of the second space being open toward a lower side of the drillship. 
         [0013]    A maximum length and a maximum width of a transverse section of the second space can be smaller than a maximum length and a maximum width of a transverse section of the first space. 
         [0014]    The moonpool can include a partition wall, which is formed between the first space and the second space. An upper line of the partition wall can be formed at a predetermined height from a bottom surface of the drillship in such a way that the seawater flowed into the first space can flow into the second space. 
         [0015]    The upper line of the partition wall can be formed between two meters below a water line of the drillship and two meters above the water line of the drillship. 
         [0016]    Perforations can be formed in the partition wall. 
         [0017]    The second space can be formed at least at one of a stern side and a bow side of the first space in a lengthwise direction of the drillship. 
         [0018]    An upper surface of the second space can be open toward an upper side of the drillship. 
         [0019]    A transverse section of the first space and the second space respectively can have the shape of a quadrangle that is extended in a lengthwise direction of the drillship. 
         [0020]    A transverse width of the second space that is perpendicular to the lengthwise direction of the drillship can be formed to be smaller than a transverse width of the first space. 
         [0021]    A length of the second space that is extended in the lengthwise direction of the drillship can be formed to be smaller than a length of the first space. 
         [0022]    The length of the second space can be 10% to 50% of the length of the first space. 
         [0023]    An opening opened toward a lower side of the first space can maintain a constant transverse width and then become narrower in a stern-side direction of the drillship. 
         [0024]    A baseplate that is placed on a same plane as a bottom surface of the drillship can be installed on both corners of an end of the opening opened toward the lower side of the first space, wherein the both corners of the end are located in a stern-side direction of the drillship and the baseplate has the shape of a triangle. 
         [0025]    A transverse section of the second space can have the shape of a semi-circle or a polygon. 
         [0026]    Another aspect of the present invention features a drillship having the moonpool described above. 
         [0027]    According to the present invention, a second space is formed on a side of a first space in the lengthwise direction of a drillship so that the overall length of a moonpool is increased. Therefore, vertical movements of the water surface inside the moonpool can be changed, and the amplitude of the water surface movement inside the moonpool and the resistance of the drillship can be reduced. 
         [0028]    Moreover, by installing a partition wall having a particular upper line height between the first space and the second space, the amplitude of sloshing movements of the water surface inside the moonpool can be reduced, and the resistance caused by the sloshing movements can be reduced. 
         [0029]    Furthermore, by forming an opening opened toward a lower side of the first space to maintain a constant transverse width and become narrower in a stern-side direction of the drillship, the amount of seawater flowed into the first space can be relatively reduced, and the resistance applied to the drillship can be reduced. 
     
    
     
       BRIEF DESCRIPTIONS OF THE DRAWINGS 
         [0030]      FIG. 1  is a plan view of a moonpool in accordance with a first embodiment of the present invention. 
           [0031]      FIG. 2  is a cross-sectional view of  FIG. 1  seen along the II-II line. 
           [0032]      FIG. 3  shows a modification example of a second space included in the moonpool in accordance with the first embodiment of the present invention. 
           [0033]      FIG. 4  shows another modification example of a second space included in the moonpool in accordance with the first embodiment of the present invention 
           [0034]      FIG. 5  shows yet another modification example of a second space included in the moonpool in accordance with the first embodiment of the present invention 
           [0035]      FIG. 6  is a plan view of a moonpool in accordance with a second embodiment of the present invention. 
           [0036]      FIG. 7  is a cross-sectional view of  FIG. 6  seen along the VII-VII line. 
           [0037]      FIG. 8  is a cross-sectional view of  FIG. 6  seen along the VIII-VIII line. 
           [0038]      FIG. 9  is a plan view of a moonpool in accordance with a third embodiment of the present invention. 
           [0039]      FIG. 10  is a cross-sectional view of  FIG. 9  seen along the X-X line. 
           [0040]      FIG. 11  shows the result of a towing experiment of model ships in which moonpools in accordance with the embodiments of the present invention are formed. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0041]    Hereinafter, certain embodiments of the present invention will be described with reference to the accompanying drawings. 
         [0042]      FIG. 1  is a plan view of a moonpool in accordance with a first embodiment of the present invention, and  FIG. 2  is a cross-sectional view of  FIG. 1  seen along the II-II line. 
         [0043]    Referring to  FIGS. 1 and 2 , a moonpool  5  in accordance with a first embodiment of the present invention is formed between a bow and a stern of a drillship  1 , and includes a first space  10  for carrying out a drilling operation and a second space  20  formed adjacent to the first space  10 . 
         [0044]    The first space  10  is formed by penetrating an upper deck  2  from a bottom surface  3  of the drillship  1 . In this case, the first space  10  can be vertically formed from the bottom surface  3  of the drillship  1 , and the first space  10  can be limited by inner walls  51 ,  52 ,  53 ,  54 ,  55  that are extended vertically in a hull  50  of the drillship  1 . 
         [0045]    Referring to  FIG. 1 , the transverse section of the first space  10  is shaped to be a quadrangle such as, for example, a rectangle that is extended in a lengthwise direction of the drillship. In this case, the transverse section of the first space  10  is symmetric about a center line from the bow to the stern of the drillship  1 . The first space  10  formed as described above can be used as a pathway to lower a drilling device (not shown), a drilling pipe (not shown), etc. to the seabed. 
         [0046]    An opening  13  opened to a lower side of the first space  10  in accordance with the first embodiment of the present invention is shaped to be a quadrangle such as, for example, a rectangle that is extended in a lengthwise direction of the drill ship. 
         [0047]    However, it shall be appreciated that the shape of the first space  10  in accordance with the first embodiment of the present invention is only an example, and a variety of shapes can be used as long as the first space  10  can be used as a pathway for carrying out a drilling operation. 
         [0048]    A second space  20  is formed on a side of the first space  10  in the lengthwise direction of the drillship. In this case, referring to  FIG. 2 , a bottom of the second space  20  is formed to be open toward a lower side of the drillship  1 , and the second space  20  can be limited by inner walls  56 ,  57 ,  58  extended vertically in the hull  50  of the drillship  1 . 
         [0049]    Through the opening of the second space  20  formed as described above, the seawater can be flowed in and out of the second space  20 . In this case, an upper surface of the second space  20  can be opened toward an upper side of the drillship. 
         [0050]    Referring to  FIG. 1 , the transverse section of the second space  20  is shaped to be a quadrangle that is extended in a lengthwise direction of the drillship. In this case, the transverse section of the second space  20  is symmetric about a center line from the bow to the stern of the drillship  1 . 
         [0051]    Moreover, the second space  20  is formed to be in contact with a rear side of the first space  10  (i.e., the stern side of the first space  10  in the drillship  1 ). Here, it is possible that the second space  20  is formed in contact with a front side of the first space  10  (i.e., the bow side of the first space  10  in the drillship  1 ), and it is also possible that the second space  20  is formed on both sides of the first space  10  in the lengthwise direction of the drillship, that is, the bow side and the stern side of the first space  10 . 
         [0052]    Accordingly, compared to a moonpool having only the first space  10  formed therein (referred to as “conventional moonpool” hereinafter), the length of the moonpool  5  (the length in the bow-stern direction of the drillship  1  in  FIG. 1 ) in accordance with the first embodiment of the present invention is increased. 
         [0053]    This increase of length changes a movement pattern of the seawater occurring in the conventional moonpool. More specifically, in the moonpool  5  in accordance with the first embodiment of the present invention that is relatively longer, vertical movements of the seawater that predominantly occurred in the conventional moonpool are reduced, and instead sloshing movements predominantly occur. 
         [0054]    Here, since the vertical movements of the water surface inside the moonpool cause greater resistance to the drillship than the sloshing movements do, the moonpool  5  in accordance with the first embodiment of the present invention can give less resistance to the drillship  1  than the conventional moonpool. 
         [0055]    According to the first embodiment, a length L 2  of the second space  20  is formed to be smaller than a length L 1  of the first space  10 . In this case, it is preferable that the length L 2  of the second space  20  is between 10% and 50% of the length L 1  of the first space  10 . 
         [0056]    If the length L 2  of the second space  20  becomes excessively great, the area of the opening of the moonpool  5  becomes excessively great, adversely increasing the resistance occurring while the drillship  1  is sailing. Therefore, it is preferable that the length of the second space  20  is small compared to the length of the first space  10 . 
         [0057]    It is preferable that a width W 2  of the second space  20  is smaller than a width W 1  of the first space  10 . If the width W 2  of the second space  20  is greater than or equal to the width W 1  of the first space  10 , the area of the opening is excessively increased, adversely increasing the occurred resistance due to the seawater flowed into the moonpool of the drillship. 
         [0058]    The second space  20  in accordance with the first embodiment of the present invention has a quadrangular sectional shape with a smaller length and width than the first space  10 . However, this is only an example, and the second space  20  can have a variety of shapes as long as the maximum sectional length and maximum sectional width of the second space  20  are smaller than the sectional length and sectional width of the first space  10 , respectively. 
         [0059]    In this regard,  FIG. 3  to  FIG. 5  show modification examples of the second space included in the moonpool in accordance with the first embodiment of the present invention. 
         [0060]    Referring to  FIG. 3 , a transverse section of a second space  20   a  can have the shape of a semi-circle. Here, the shape of a semi-circle can include the shape of a semi-ellipse. In this case, a maximum length L 2   a  and a maximum width W 2   a  of the transverse section of the second space  20   a  are smaller than a maximum length L 1  and a maximum width W 1  of the transverse section of the first space  10 , respectively. 
         [0061]    The transverse section of the second space can have the shape of a polygon. For example, as it can be seen in  FIG. 4 , a transverse section of a second space  20   b  can have the shape of a triangle. In this case, a maximum length L 2   b  and a maximum width W 2   b  of the transverse section of the second space  20   b  are smaller than the maximum length L 1  and the maximum width W 1  of the transverse section of the first space  10 , respectively. 
         [0062]    Alternatively, as it can be seen in  FIG. 5 , a transverse section of a second space  20   c  can have the shape of a trapezoid. In this case, a maximum length L 2   c  and a maximum width W 2   c  of the transverse section of the second space  20   c  are smaller than the maximum length L 1  and the maximum width W 1  of the transverse section of the first space  10 , respectively. 
         [0063]      FIG. 6  is a plan view of a moonpool in accordance with a second embodiment of the present invention.  FIG. 7  is a cross-sectional view of  FIG. 6  seen along the VII-VII line, and  FIG. 8  is a cross-sectional view of  FIG. 6  seen along the VIII-VIII line. Referring to  FIG. 6  to  FIG. 8 , a moonpool  65  in accordance with a second embodiment of the present invention is formed between a bow and a stern of a drillship  61 , and includes a first space  10  for carrying out a drilling operation, a second space  20  formed adjacent to the first space  10  and a partition wall  30  formed between the first space  10  and the second space  20 . 
         [0064]    Here, any elements that are identical to those described with reference to the first embodiment will not be described, and unless described specifically, these elements will be considered to be identical to those of the first embodiment, and the description thereof will be substituted by the description provided with reference to the first embodiment. Hereinafter, the elements peculiar to the second embodiment of the present invention will be mainly described. 
         [0065]    According to the second embodiment of the present invention, the partition wall  30  is installed between the second space  20  and the first space  10 . The partition wall  30  is installed in order to partition the entire length of the moonpool  65  into certain lengths. 
         [0066]    Accordingly, sloshing movements with a big amplitude that occurs in a relatively long (i.e., L 1 +L 2 ) space can be changed to sloshing movements with a small amplitude in a relatively short (i.e., L 1  and L 2 , respectively) space due to the presence of the partition wall. 
         [0067]    In this case, the partition wall  30  is formed in such a way that the seawater inside the first space  10  can flow to the second space  20 . To that end, according to the second embodiment, the partition wall  30  is formed in such a way that its upper line is placed at a predetermined_height from the bottom surface  3  of the drillship  61 . 
         [0068]    With respect to the installation height of the partition wall with reference to  FIGS. 7 and 8 , the partition wall  30  is formed in such a way that the upper line of the partition wall  30  is extended to a water line of the drillship  61 . In this case, the upper line of the partition wall  30  can be placed between two meters below the water line and two meters above the water line. 
         [0069]    It shall be appreciated, however, that the shape of the partition wall  30  in accordance with the second embodiment of the present invention is an example only and that the partition wall can be modified in various ways as long as it can reduce the resistance occurred in the drillship pursuant to the seawater in the first space  10  flowing over the upper line of the partition wall  30  to the second space  20 . 
         [0070]    Moreover, the partition wall  30  can be formed with perforations, through which the seawater in the first space  10  and the second space  20  can respectively flow in and out of the second space  20  and the first space  10 . 
         [0071]      FIG. 9  is a plan view of a moonpool in accordance with a third embodiment of the present invention, and  FIG. 10  is a cross-sectional view of  FIG. 9  seen along the X-X line. Hereinafter, the elements peculiar to the third embodiment will be mainly described. Here, any elements that are identical to those described with reference to the first and second embodiments will not be described, and unless described specifically, these elements will be considered to be identical to those of the first and second embodiments, and the description thereof will be substituted by the description provided with reference to the first and second embodiments. 
         [0072]    Referring to  FIGS. 9 and 10 , the transverse section of a first space  10  in accordance with a third embodiment of the present invention is shaped to be a quadrangle, for example, a rectangle, that is extended in a lengthwise direction of a drillship  71 . 
         [0073]    In this case, an opening  73  that is opened toward a lower side of the first space  10  is formed to keep a fixed transverse width and become narrower toward a stern of the drillship  71 . To that end, in the third embodiment of the present invention, a baseplate  40  that is placed on the same plane as a bottom surface  3  of the drillship  71  is installed on both corners of a rear-side end (i.e., an end part located in the stern-side direction of the drillship  71 ) of the opening  73 , which is opened toward the lower side of the first space  10 . Here, the baseplate  40  can have the shape of a triangle. 
         [0074]    As such, by forming the opening  73  that is opened toward the lower side of the first space  10  to become gradually narrower along the moving direction of the seawater that moves from the bow to the stern of the drillship  71  when the drillship sails forward, the amount of the seawater that flows into the first space  10  becomes relatively reduced, thereby reducing the resistance applied to the drillship  71 . 
         [0075]    The size and shape of the opening  73  opened toward the lower side of the first space  10  shall be determined in such a way that a drilling pipe, etc. that are descended toward the seabed are not interfered. The size of the baseplate  40  shall be also determined in the same respect. 
         [0076]      FIG. 11  shows the result of a towing tank experiment of model ships in which moonpools in accordance with the embodiments of the present invention are formed. Illustrated in  FIG. 11  are results on the relations between speed and effective horsepower by conducting an experiment in a towing tank with a model ship in which the conventional moonpool (having the first space only) is formed (referred to as the “first model ship” hereinafter), a model ship in which the moonpool in accordance with the first embodiment of the present invention (having the first space and the second space only) is formed (referred to as the “second model ship” hereinafter),_a model ship in which the moonpool in accordance with the second embodiment of the present invention (having the partition wall between the first space and the second space, the opening toward the lower side of the first space having a quadrangular shape) is formed (referred to as the “third model ship” hereinafter), and a model ship in which the moonpool in accordance with the third embodiment of the present invention (having the partition wall between the first space and the second space, the opening toward the lower side of the first space maintaining a fixed width and becoming narrower in the stern-side direction) is formed (referred to as the “fourth model ship” hereinafter). Here, the values indicated in the effective horsepower axis refer to relative values with an assumption that the effective horsepower required to tow the first model ship with 13 kts is  100 . 
         [0077]    Describing the experiment results by referring to  FIG. 11 , it can be seen that when the first model ship (with the conventional moonpool) and the second model ship (with the first embodiment) are compared, the second model ship has approximately 4% less resistance than the first model ship at the speed of 13 kts. This means that the second model ship can sail with less engine horsepower than the first model ship at the same sailing speed. 
         [0078]    This trend is more prominent when the speed of the model ships is greater. For example, at the speed of 15 kts, the resistance is decreased by about 16%. This means that the second model ship can sail with a significantly less engine horse power than the first model ship at the sailing speed of 15 kts. 
         [0079]    As such, the second model ship can sail with less engine horse power at a particular speed because the resistance occurred during the sailing is less than the first model ship. 
         [0080]    Comparing the first model ship (with the conventional moonpool) with the third model ship (with the second embodiment) referring to  FIG. 11 , it can be seen that the third model ship has approximately 10% less resistance than the first model ship at the speed of 13 kts. This means that the third model ship can sail with less engine horse power than the first model ship at the same sailing speed. 
         [0081]    This trend is more prominent when the speed of the model ships is greater. For example, at the speed of 15 kts, the resistance is decreased by about 30%. This means that the third model ship can sail with a significantly less engine horse power than the first model ship at the sailing speed of 15 kts. 
         [0082]    As such, the third model ship can sail with less engine horse power at a particular speed because the resistance occurred during the sailing is less than the first model ship. 
         [0083]    Comparing the third model ship (with the second embodiment) with the fourth model ship (with the third embodiment) referring to  FIG. 11 , it can be seen that the fourth model ship has approximately 3% less resistance than the third model ship in the entire range of sailing speeds. This means that the fourth model ship can sail with relatively less fuel than the third model ship at any particular sailing speed. 
         [0084]    While some embodiments of the present invention have been described above, the technical ideas of the present invention are not restricted to the embodiments presented above, and it shall be appreciated that anyone skilled in the art to which the present invention pertains can present a variety of other embodiments by supplementing, modifying, deleting and adding the elements within the scope of the same technical ideas, but such varieties shall be considered to be included in the scope of technical ideas of the present invention.