Patent Publication Number: US-2012045285-A1

Title: Offshore structure

Description:
The present invention relates to an offshore structure for sealing off, confining and controlling a flow of fluid from an offshore well, especially in an uncontrolled blowout from a location on a seabed, and more particularly to an offshore structure which is adapted to store the fluid from the offshore well for later retrieval or conveyance to the surface of the sea. 
     In the exploration for, and drilling of wells for oil and gas at offshore locations, sometimes crude oil and/or gas escapes uncontrollably from the holding stratum and passes upwards into the water. In many instances the condition might be corrected by immediately capping the well or by using of plugging materials such as cement or the like poured into the aperture through which the fluid is escaping. 
     While the consequences of such crude and/or gas leakage or spillage can often be minimized, they can also turn into economic as well as natural disasters. 
     For example, in the event of a blowout of offshore oil wells, liquid and gaseous hydrocarbons erupt under high pressure. The high pressure values make it very difficult, if not impossible, to directly attempt to shut-in operation of the damaged oil well. 
     In order to stop the oil flow to allow repair of, for instance, a wellhead it is often necessary to drill relief wells to intersect the blowout well in the pressurized formation and then to pump adequate fluid through these relief wells so as to control the escaping pressure. However, the drilling of relief wells is a lengthy operation that may take months to perform. During that time, oil continues to flow out freely into the sea which may cause severe damage to the marine and coastal environment. 
     There are already several methods and apparatuses known if which can be used in connection with uncontrolled blowouts from oil/gas sources on the seabed and which aim at achieving confinement and collection and consequently control over the fluid gushing out from the source. Different types of dome or hood structures are then placed and installed above the blowout location on the seabed. 
     U.S. Pat. No. 3,548,605 describes a submersible structure for emergency offshore gas leakage, where the submersible structure is adapted to be lowered through a body of water to form a provisional passage for crude oil and/or gas escaping from a well or substratum fissure. The structure includes a support frame forming a fluid-guiding and entrapping means, together with a collapsible conduit which communicates the source of escaping fluid with the water&#39;s surface whereby forming a confined pool. 
     Transporting offshore structures to an offshore site has usually required floating them, with all the problems of buoyancy and stability, plus structural integrity that this involves. Upon arriving at the site, the offshore structures must be lowered until they are supported by, or rest upon, the floor or bed of the body of water. As the structure buoyancy is decreased, floating stability varies. Consequently the threat or danger of capsizing is ever present. Thus, experience has shown a definite need for offshore structures which can be floated to a site with stable buoyancy and then submerged to be supported by the floor of the body of water while maintaining control of the offshore structure against unstable dangerous listing or tilting and with full control of the descent. 
     It is therefore an object of the present invention to mitigate or possibly alleviate the above-mentioned drawbacks of the prior art, the offshore structure being designed to possess unusually stable buoyancy while floating and when submerged. 
     It is a further object of the present invention to provide an offshore structure which can be utilized to seal off a well and/or to store crude oil and/or gas generated by an underwater blowout. 
     It is a further object of the present invention to provide an offshore structure that can be retrofitted and connected to an already existing well. 
     These objectives are achieved with an offshore structure for sealing off and/or storing fluid from an offshore well according to the invention as defined in the enclosed independent claim, where embodiments of the invention are given in independent claims. 
     The present invention regards an offshore structure for sealing off and for confining and controlling a flow of fluid from an offshore well, where the offshore structure comprises an outer and inner wall forming a double walled construction in the offshore structure. A space between the outer and inner wall of the offshore structure is divided into a plurality of compartments, where the plurality of compartments extends over at least a part of the offshore structure&#39;s vertical axis and may be in fluid communication with each other. In order to control the immersion of the offshore structure when the offshore structure is to be installed over a location on the seabed, the offshore structure is open to an ambient fluid through an open top and bottom and a plurality of openings in the outer and inner wall, the openings being arranged around the circumference of the offshore structure. 
     The offshore structure according to the present invention may be used both to secure and “protect” a new or already existing offshore well, or in connection with an offshore well that has an uncontrolled blowout. When the offshore structure is to be used in connection with new wells or to be retrofitted on already existing wells, the offshore structure is immersed over a well head. In such a case an upper part of the offshore structure may be provided with a closeable opening, through which different equipment, for instance a blow out preventer, may be guided in order to be connected to the well head. Once the well is completed, the opening can be closed by one or more elements. The one or more elements are then formed with a recess to which a pipe or tube can be connected. 
     If the offshore structure according to the present invention is to be used to “close” an uncontrolled blow out in a well, the offshore structure is immersed over the well, and a conduit or pipe is connected to a hollow element. This hollow element is arranged in an opening in the top of the offshore structure. The conduit or pipe will then extend a certain height, for instance 60-70 meters, above the offshore structure, where the conduit or pipe is provided with an arrangement that can close or open a flow through the conduit or pipe. The conduit or pipe is then used to relieve the pressure that is built up inside the offshore construction, as the fluid is allowed to rise up the conduit or pipe. The conduit or pipe is then connected to another conduit, such that the fluid can be transported away. 
     Such arrangement includes closing means where the closing means comprises a valve gate connected with an externally actuable driving means for movement of the valve gate between positions in which the conduit or pipe is open and closed, respectively. 
     The plurality of openings in the outer and inner wall of the double walled construction are arranged in the vicinity of the bottom of the offshore structure, where they will provide an open fluid communication between an outside and inside of the offshore structure. 
     The plurality of compartments inside the double walled construction is provided with partition walls, where the partition walls may be manufactured from concrete, steel or combinations thereof. Furthermore, some of the partition walls may be provided with fluid communication means, where this will create a fluid communication between two adjacent compartments. The fluid communication means may be connected to valves or the like such that the fluid communication means can be opened or closed. This arrangement, during an immersion of the offshore structure, provides a more even distribution of a fluid or seawater between the different compartments in the double walled construction, such that the offshore structure can be stabilized during this process. 
     The plurality of compartments inside the double walled construction are primarily intended to be used to control and regulate the buoyancy of the offshore structure during the transport to and immersion over a location offshore. A fluid (a light fluid) having a density less than that of seawater will then, through suitable means, be filled inside one or more of the compartments. This will give the offshore structure the desired buoyancy. The means may also be able to pressurize the fluid, and in this way regulate the filling of the seawater inside the offshore structure. When the offshore structure is to be immersed on the sea bed, the fluid having a density less than the seawater can be emptied, whereby seawater is allowed to stream in inside the offshore structure through the plurality of openings in the outer and inner wall arranged near the bottom of the offshore structure. In this way the buoyancy and the immersion of the offshore structure can be controlled. 
     However, some of the compartments may also be filled with concrete, sand or the like, such that the weight of the offshore structure is increased. Preferably, this is done when the offshore structure is fully immersed on the sea bed. 
     As the offshore structure is open at top and bottom and have a plurality of openings providing an open fluid communication between the outside and the inside of the offshore structure, the offshore structure is only subjected to a differential pressure, equal to the difference in the unit weights of seawater and the light fluid multiplied by the height over which the difference occurs. 
     The offshore structure according to the present invention may be manufactured only from concrete. However, it should be understood that the offshore structure can also be manufactured from steel or a combination of concrete and steel. 
     The offshore structure according to the present invention may have a generally “box” or “dome-shaped” rigid continuous enclosure, open at top and bottom and through the plurality of openings in the double walled construction, surrounding or defining a large volume. Furthermore, the offshore structure can be of any suitable shape which will define a volume inside and at the same time ensure sealing, confining and controlling a flow of fluid from an offshore well. 
     In horizontal section the offshore structure may be circular, quadratic or polygonal or a combination of straight and arced segments. In vertical section through its axis, the offshore structure may be gradual tapered towards the top, conical, a circular or elliptical segment or a combination thereof. However, the peripheral edge of the offshore structure advisably lies in a horizontal plane and is adapted to rest on a seabed. 
     In one embodiment the offshore structure may have a circular base and a gradual tapering towards the top of the offshore structure. In other embodiments the offshore structure may be formed as a rectangular or quadratic container. 
     As the offshore structure according to the present invention is to be placed on a sea bed in order to seal off, confine and control a flow of fluid from an offshore well, the offshore structure is provided with an arrangement that can provide a closed or “leak proof” connection between the offshore structure and the sea bed, as the sea bed may be uneven or rough. The arrangement may for instance be a steel skirt that is arranged over a part or whole of the offshore structure&#39;s circumference. The steel skirt will then extend below the offshore structure. Another arrangement may be that a plurality of separate plates is arranged around a part of or whole of the circumference, where the plates can separately be adjusted in a lengthwise direction of the offshore structure. The adjustment of the separate plates may be achieved by hydraulic or pneumatic means. A person skilled in the art would know how this can be done, and this will therefore not be discussed further here. 
     The offshore structure can be built with a size and capacity considered most suitable for a particular offshore location. Strength against underwater turbulence and pressure is also a feature of the offshore structure provided by this invention. 
    
    
     
       The foregoing and other objects, features and advantages of the present invention will be apparent from the following more particular description of preferred non-limiting embodiments of the invention, as illustrated in the accompanying drawings: 
         FIG. 1  illustrates a principal configuration of an offshore structure according to the present invention; 
         FIG. 2  illustrates in greater detail the double wall construction, where a part of the outer wall is removed, 
         FIG. 3  illustrates how the offshore structure according to the present invention may be moored to the seabed, 
         FIG. 4  illustrates another embodiment of the offshore structure according to the present invention, and 
         FIG. 5  illustrates a finished offshore structure that is ready to be installed. 
     
    
    
     An offshore structure  1  according to the present invention is primarily intended to be installed on new or already existing offshore wells. Once the offshore structure is installed over the well, it will secure and “seal off” the well, thereby reducing the risk of oil leakage, blowouts and damage of well equipment (i.e. a wellhead, blow out preventer etc.) caused by falling objects etc. However, the offshore structure  1  according to the present invention may also be used to stop already existing uncontrolled blow outs or to seal off a well that is to be closed. 
       FIG. 1  shows the principal configuration of the offshore structure  1  according to the present invention. The offshore structure  1  is designed with a double wall, where an outer wall is denoted  2 , while an inner wall is denoted  3 . Lower ends of the outer and inner walls  2 ,  3  are terminated by an annular ring  4 , where the annular ring  4  will have a width that is larger than the width of the double wall, such that the annular ring  4  will extend a certain distance outwardly from the outer wall  2  of the offshore structure  1 . This annular ring  4  (see also  FIG. 2 ) will lie against the seabed floor S when the offshore structure  1  is submerged at the desired place, for instance over a Blow out Preventer (BOP)  6  arranged on the seabed S. 
     At a top T of the offshore structure  1 , upper ends of the outer and inner walls  2 ,  3  are terminated by a hollow element  5 , where the hollow element  5  will extend a certain distance above the offshore structure  1 . The hollow element  5  is provided with connections means (not shown) for a conduit or tube  100 . 
     The conduit or tube  100  is connected to a pipeline (not shown) or the like, such that fluid stored in the offshore structure  1  can be conveyed away from the offshore structure  1 . Furthermore, the conduit or tube  100  is provided with closing means  101 . The closing means  101  comprises a valve gate connected with an externally actuable driving means for movement of the valve gate between positions in which the conduit or tube  100  is open and closed, respectively. 
     Furthermore, the offshore structure  1  is provided with a plurality of openings  102  in the outer and inner walls  2 ,  3 , thus providing an open communication between the outside, the space between the outer and inner walls  2 ,  3  (the compartments  7 ) and the inside of the offshore structure  1 . The openings  102  are arranged in a suitable manner around the circumference of the offshore structure  1 . 
     During the immersion of the offshore structure  1 , the offshore structure  1  will be open both at the top and bottom. Additionally there will be, through the plurality of openings  102 , an open fluid communication between the outside and inside of the offshore structure  1 . This will provide a structure where the differential pressure over the construction is low. 
     Furthermore, one or more of the openings  102  may be provided for connection to supply means (not shown) for injection of a filler material, such as concrete, sand or the like, in the compartments  7 . 
     In order to ease the removal of fluid stored inside the offshore structure  1 , both side walls  2 ,  3  and “roof” of the offshore structure  1  have a sloping design. However, it should be understood that the offshore structure  1  according to the present invention may be designed with any suitable form, as long as the purpose of the invention can be obtained. 
     The internal space of the double wall is divided into a plurality of compartments  7  by partition walls  8 , see  FIG. 2 . The compartments  7  will extend over the entire length of the offshore structure&#39;s vertical axis, where these compartments  7  are used to control the buoyancy of the offshore construction during the lowering and positioning of the offshore structure on the desired place on the seabed S. The compartments  7  are then ballasted or de-ballasted with sea water and/or a fluid having a density less than that of sea water. 
     Fluid communication means  104  (see  FIG. 2 ) is arranged in the partition walls  8 , where the fluid communication means  104  are used to provide a fluid communication between two adjacent compartments  7 . 
     A ballasting/de-ballasting system (not shown) will then be used to fill the compartments  7  with sea water and/or a fluid having a density less than that of sea water, where the system can control and distribute the sea water and fluid in each compartment  7  separately. 
     If the seabed S around the offshore well, on which the offshore structure is to be placed, is uneven or rough, the offshore structure  1  may be provided with a steel skirt  105  or the like. The steel skirt  105  will then be dig into the seabed S and provide a closed or “leak proof” connection between the offshore structure and the seabed. The steel skirt  105  is then arranged over a part or whole of the offshore structure&#39;s  1  circumference. 
       FIG. 3  shows an offshore structure  1  according to the present invention arranged on the seabed S, where the offshore structure  1 , as an extra precaution, is moored to the seabed S. Three cables or wires  9  are then connected at one end to a surface structure (not shown), for instance a platform or vessel, and an opposite end of the cables or wires  9  are then connected to the offshore structure  1 . Furthermore, the cables or wires  9  are, before being connected to the offshore structure  1 , run or guided over an anchor  10 . The anchors  10  are in appropriate ways fixed to the seabed S. This arrangement allows the offshore structure  1  to be forced and held on the seabed S, as the cables or wires  9  can be tightened for instance by a winch arrangement arranged on the surface structure. This will prevent the offshore structure, when exposed to currents, tides, buoyancy etc., from being removed from the seabed S. 
     Another embodiment of the offshore structure  1  according to the present invention is shown in  FIG. 4 , where the top T of the offshore structure  1  is provided with a larger closable opening  12 . This is advantageous when the offshore structure  1  according to the present invention is installed on new wells, as the closable opening  12  can be used to lower down larger equipment, for instance a blow out preventer  6 , through the closable opening  12 . When the equipment is installed on the seabed, one or more elements  13  are used to close the closable opening  12 . 
       FIG. 5  shows a finished offshore structure  1  according to the present invention, where the offshore structure  1  can be used to seal off an oil well, thus preventing oil during an unwanted event, for instance a blowout, from contaminating the environment, as the oil is stored in the offshore structure  1 . 
     The offshore structure  1  according to the present invention may also be used to store oil and/or gas when a blowout of an offshore oil well has already occurred, as the offshore structure  1  is then lowered over the well. 
     The invention has now been explained by means of several non-limiting embodiments. A person skilled in the art will appreciate that it will be possible to carry out a number of variations and modifications of the offshore structure for sealing off and/or storing oil from an underwater oil and/or blowout as described within the scope of invention, as defined in the attached claims.