Abstract:
A liquefied natural gas (LNG) sump is provided for a gravity based structure (GBS). The sump includes a sump containment structure having a predetermined volume and floor surface area. The volume and floor surface area are selected in order to allow spilled LNG contained within the sump containment structure to vaporize at a predetermined, safe rate. A trough is provided for collecting spilled LNG on the GBS, such as at the processing area and on a jetty, and delivering the spilled LNG to the sump containment structure.

Description:
RELATED APPLICATION DATA 
       [0001]    This application claims the benefit pursuant 35 U.S.C. §119 of the filing date of U.S. provisional patent application No. 60/611,388, filed Sep. 21, 2004, the entire contents of which are hereby incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates generally to sumps and, more particularly, to a liquefied natural gas (LNG) sump for containment of LNG spills on an offshore gravity based structure or other offshore installation. 
         [0004]    2. Description of the Related Art 
         [0005]    Gravity based structures (GBS) have been used in the design of offshore oil and gas storage facilities for more than 30 years in many parts of the world, predominantly in areas of severe storms for years to design. Due to their strength and reliability, gravity-based structures have been such as the North Sea offshore Norway and Scotland, and in an area offshore Newfoundland where energy operators must contend with floating icebergs. Recently, GBS&#39;s are being proposed for the storage of LNG. 
         [0006]    LNG is natural gas cooled to cryogenic temperatures (−161 C (−256 F)) through liquefaction, and is transported and stored in its liquid state. Since LNG has never before been stored in a GBS, dealing with LNG spills in a GBS environment is a new issue. 
         [0007]    U.S. Pat. No. 3,657,895 discloses a platform for offshore wells for which a curbing around the periphery of the platform contains oil spills onto the deck platform, and the deck of the platform slopes downwardly from the curbing to an opening in the platform surrounding the well. 
         [0008]    U.S. Pat. No. 3,675,431 discloses an off-shore storage tank for liquefied gas. The tank has an outer shell of which at least the lower part, which is immersed in the water, is made of solid concrete, and does not include a sump. 
         [0009]    U.S. Pat. No. 3,727,418 discloses a sub-aqueous storage of liquefied natural gas. The patent discloses a submerged, jacketed container having an interior which is coupled to a balancing fluid in another container. The balancing fluid is coupled to the water surrounding both containers. 
         [0010]    U.S. Pat. No. 3,984,059 discloses a marine loading system for liquefied natural gas handling. Various plants sites in the system are connected by coaxial ducting. There is no disclosure of a sump. 
         [0011]    U.S. Pat. No. 4,188,157 discloses an offshore structure handling cryogenic fluids such as liquefied natural gas. The structure includes a cell-matrix. The patent does not disclose an LNG sump. 
         [0012]    U.S. Pat. No. 4,202,648 discloses a floating plant for offshore liquefaction, temporary storage, and loading of liquefied natural gas. The plant includes a semi-submersible platform with storage tanks for LNG arranged in the submerged section of the platform. The patent does not disclose an LNG sump. 
         [0013]    U.S. Pat. No. 4,209,267 discloses an improvement in storage systems for liquefied natural gas, to provide increased security in cases of tank rupture. The safety system includes a dike, impounding wall or drainage channel constructed of compacted earth, concrete, metal and/or other suitable substance, surrounding an aboveground steel insulated tank used to store the liquefied gas. A drop shaft is used to communicate the diked area with an underground tunnel for temporary accumulation and subsequent safe disposal of liquid which has escaped from the storage tank. Inert gas is used to neutralize the hazards of a spill, and stored liquid is pumped to a storage tank. 
         [0014]    U.S. Pat. No. 4,217,848 discloses a floating gas liquefaction installation having a liquefaction unit and a sealed, thermally insulated tank space. The patent does not disclose of an LNG sump. 
         [0015]    U.S. Pat. No. 4,302,130 discloses an offshore platform with storage facilities for natural gas. This patent also does not disclose of an LNG sump. 
         [0016]    U.S. Pat. No. 4,365,576 discloses improvements in an offshore platform and submarine storage facility for highly chilled liquefied gas. This patent also does not disclose of an LNG sump. 
         [0017]    U.S. Pat. No. 4,404,988 discloses an apparatus for draining a liquid which has been inadvertently freed from a primary storage container and captured in a containment space. A pump delivers spilled liquid to a remote location. 
         [0018]    Thus, there is a need for systems and methods for handling LNG spills when LNG is stored in a GBS. 
       SUMMARY OF THE INVENTION 
       [0019]    The present invention is directed to an LNG sump that provides containment for an LNG spill on an offshore GBS or other offshore installation. 
         [0020]    According to an embodiment of the present invention, a sump includes a preferably long, relatively narrow cylindrical structure that provides sufficient LNG spill containment while minimizing the rate of evolution of gaseous methane at the surface due to the relatively small surface area. The cylinder can be insulated and is preferably arranged to fit neatly into a GBS compartment. Spills from a loading platform can enter the sump area through a trough through the GBS wall. 
         [0021]    The LNG sump may require insulation and/or heat tracing to protect the GBS walls from low temperature during a spill event. The LNG sump may be supported by GBS floor sitting on insulation blocks. A cryogenic sump pump maybe installed in the sump to remove rainwater or the like. Spilled LNG will be allowed to boil-off within the sump. 
         [0022]    According to an embodiment of the present invention, an LNG sump is provided for a GBS. The LNG sump includes a sump containment structure having a predetermined volume and floor surface area. The volume and floor surface area are selected in order to allow spilled LNG contained within the sump containment structure to vaporize at a predetermined, safe rate. A trough is provided for collecting spilled LNG on the GBS, such as at the processing area and on a jetty, and delivering the spilled LNG to the sump containment structure. 
         [0023]    According to an embodiment of the present invention, an LNG sump for a gravity based structure (GBS) includes containment means for collecting spilled LNG and for allowing the spilled LNG contained to vaporize at a predetermined rate. Collecting means is also provided for collecting spilled LNG on the GBS and delivering the spilled LNG to said containment means. 
         [0024]    According to an embodiment of the present invention, a method for handling LNG spills on a GBS includes a step of collecting spilled LNG and delivering the spilled LNG to a containment means, and a step of allowing the spilled LNG contained to vaporize at a predetermined rate. 
         [0025]    Further applications and advantages of various embodiments of the present invention are discussed below with reference to the drawing figures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]      FIG. 1  is a diagram of an LNG sump according to an embodiment of the present invention. 
           [0027]      FIG. 2  is a perspective view showing installation of a LNG sump in an offshore GBS in accordance with an embodiment of the invention. 
           [0028]      FIG. 3  is second perspective view showing installation of the LNG sump according to an embodiment of the invention. 
           [0029]      FIG. 4  is a top plan view showing installation of the LNG sump according to an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0030]    As shown in  FIG. 1 , an LNG sump  102  has a long relatively narrow cylindrical shape, which has the effect of providing sufficient LNG spill containment while minimizing the rate of evolution of gaseous methane at the surface due to the relatively small surface area. The cylinder may be insulated ( 106 , not shown as completely covering sump) and should fit neatly into a GBS compartment  104 . Electrical heat tracing, such provided by a heat blanket  108 , can be provided on the outside of the LNG sump  102  and used to protect the surrounding concrete from cryogenic temperatures while allowing some control over the rate of vapor evolution. The electrical tracing or heat blanket fitted outside of the insulation. 
         [0031]    A trough  110   a  collects spilled LNG from the process area and delivers it to the LNG sump  102 . A second trough  110   b  can be used to collect spilled LNG from a jetty or the like, and delivers the spilled LNG to the LNG sump  102  via separate inlet  112  through a slot  120  in the GBS wall. Consideration may be given to the expansion and contraction of the pipes due to chilling and warming. 
         [0032]    A sump pump  114  may be provided in the bottom of the sump  102  for removing water and other liquid. Guides  116  can be provided for supporting the sump  102  inside compartment  104 . Guides  116  can be insulated with insulation blocks  117  where it contacts sump  102 . An insulation block  118  can be provided at the bottom of the LNG sump  102 . 
         [0033]    In the event of a spill, LNG is collected by gravity in the LNG sump  102  via the troughs  110   a, b  provided about the GBS where spills may occur. Since the surface area in the bottom of the LNG sump  102  is small, LNG can be allowed to vaporize at a calculated rate that is safe and acceptable to operators. As shown, the exemplary LNG sump  102  is a cylinder more than 30 meters in height with only a 3 meter diameter. 
         [0034]    Further, the LNG vaporization rate can be adjusted by adjustment means—it may be reduced by use of foam insulation or increased by use of the electrical/heat tracing on the LNG sump  102  outer walls. Insulation and heat tracing may be used together. The concrete GBS compartment  104  is prevented from reaching cryogenic temperatures by the use of the insulation and/or heat tracing. 
         [0035]    Since LNG has never before been stored in a GBS, dealing with spills in this environment has never before been an issue. The inventive geometry of the present invention avoids the large footprint required by deck mounted sumps. Further, the fact that it is mounted within the GBS avoids wave action on the LNG sump  102 . The present invention may include other LNG spill containers such as deck-mounted sumps, outboard mounted sumps, etc. 
         [0036]    The LNG sump preferable includes, but is not limited to, a long thin cylindrical containment structure extending into the GBS compartment. A single LNG spill sump is preferred for the GBS. One skilled in the art will understand that volume (due to spill size) and location (due to elevation) can be controlled by the loading platform (jetty) area spill. Preferably, volume of the sump  102  should be in range of 200 to 220 cubic meters, and the area of the open top should be minimized. 
         [0037]    One skilled in the art will understand that the sump may not need to be insulated for process reasons. For example, during a spill event the exterior of the sump  102  can ice up and be self insulating to an extent. 
         [0038]    The LNG sump  102  may be constructed of suitable materials for handling cryogenic temperatures, such as, but not limited to, concrete and aluminum. Aluminum may be preferred due to its low weight and compatibility with the cryogenic temperatures. 
         [0039]    The LNG sump  102  may be located exterior to the GBS. An exterior location requires a portion of the sump to be below sea level. Support, wave loading, and fatigue will need to be considered. The sump is preferably located as close as possible to the GBS to minimize support requirements. In the unlikely event that the LNG sump  102  ruptures, rapid phase transition (RPT) loads may occur. One skilled in the art should consider a rupture when locating the sump. 
         [0040]    The location within the GBS (in an exterior compartment) is preferred. This location makes the LNG sump  102  easier to support. Spills from the loading platform will enter the LNG sump  102  area via a trough  110   a  through the GBS wall. The LNG sump  102  may require insulation and heat tracing to protect the GBS walls  104  from low temperatures during a spill event. The LNG sump  102  may be supported by GBS floor sitting on insulation blocks  118 . 
         [0041]    The diameter (surface area) of the LNG sump is based on the volume requirement, the fixed elevation of the GBS floor and fixed elevation of the loading platform. A cryogenic sump pump  114  may be installed in the LNG sump  102  to remove rainwater or the like. As described above, spilled LNG will be allowed to boil off. 
         [0042]      FIG. 2  is a perspective view showing installation of a LNG sump  102  in an offshore GBS  100 , in accordance with an embodiment of the invention. As shown, a trough  110   a  feeds the sump  102 . 
         [0043]      FIG. 3  is second perspective view showing installation of the sump according to an embodiment of the invention. From this view, both troughs  110   a, b  can be seen. 
         [0044]      FIG. 4  is a top plan view showing installation of the sump according to an embodiment of the invention. 
         [0045]    Thus, a number of preferred embodiments have been fully described above with reference to the drawing figures. Although the invention has been described based upon these preferred embodiments, it would be apparent to those of skilled in the art that certain modifications, variations, and alternative constructions would be apparent, while remaining within the spirit and scope of the invention.