Abstract:
An anode column for protecting a marine structure from corrosion includes: (a) an elongated guide having upper and lower ends, and adapted to be physically supported in an upright position in a body of water which overlies a seabed, independent of the marine structure; (b) an elongated conductive anode carrier surrounding the upright guide; (c) at least one sacrificial anode carried by the anode carrier and; and (b) an electrical conductor extending from the column and adapted to be connected to the marine structure at a location accessible from a surface of the body of water, wherein the at least one anode is electrically connected to the conductor through the anode carrier. A method is provided for installing the anode column from the surface.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of Provisional Application Ser. No. 60/890,855, filed Feb. 21, 2007, and Provisional Application Ser. No. 60/912,957 Filed Apr. 20, 2007. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates generally to marine structures, and more particularly to a cathodic protection system for controlling corrosion of such structures. 
     Known marine structures such as oil and gas structures typically include a platform which is supported above sea level by an arrangement of steel legs anchored on or driven into the sea bed, and coupled together by steel truss members. If unprotected, seawater will rapidly corrode such steel structures. 
     Accordingly, it is well known to apply cathodic protection to steel marine structures by providing sacrificial anodes, for example of aluminum or zinc, which are electrically coupled to the steel structure. The anodes preferentially corrode to produce an electrical current that protects the steel structure from corrosion. 
     Often the sacrificial anodes take the form of many individual masses which are attached directly to the legs and/or truss members of the structure. Installation of such anodes, or replacement at the end of their useful life, requires the efforts of a diver. Offshore structures may be set in waters far beyond the practical diver working depth of about 91 m (300 ft.), for example about 366 m (1200 ft.). Maintenance or replacement of anodes at such depths requires the use of underwater remotely operated vehicles (ROVs), which are very expensive. 
     It is also known that sacrificial anodes can be configured in a vertical column supported by the marine structure, similar to a tubing string. These columns are configured to be attached to the marine structure using special brackets. By attaching the columns, additional weight is added to the marine structure and there is a limit to the number of columns that can be physically installed. Furthermore, this type of column may not be suitable for retrofit situations where the marine structure was not designed to carry the weight of the anodes, and where the specific brackets needed to attach a vertical anode column were not included in the initial construction of the marine structure. 
     BRIEF SUMMARY OF THE INVENTION 
     These and other shortcomings of the prior art are addressed by the present invention, which according to one aspect provides an anode column for protecting a marine structure from corrosion, including: (a) an elongated guide having upper and lower ends, and adapted to be physically supported in an upright position in a body of water which overlies a seabed, independent of the marine structure; (b) an elongated conductive anode carrier surrounding the upright guide; (c) at least one sacrificial anode carried by the anode carrier and; and (b) an electrical conductor extending from the column and adapted to be connected to the marine structure at a location accessible from a surface of the body of water. The at least one anode is electrically connected to the conductor through the anode carrier. 
     According to another aspect of the invention, a cathodically protected apparatus includes: (a) a marine structure disposed in a body of water which overlies a seabed, the marine structure including at least one corrodable metallic member submerged below a surface of the body of water; and (b) at least one anode column, including: (i) an elongated guide having upper and lower ends, the guide being physically supported in an upright position in the body of water, independently from the marine structure; (ii) an elongated conductive anode carrier surrounding the upright guide; (iii) at least one sacrificial anode carried by the anode carrier; and (iv) an electrical conductor extending from the anode column and connected to the marine structure at a location accessible from the surface, such that the at least one anode is electrically connected to the conductor through the anode carrier. 
     According to another aspect of the invention, a method of installing an anode column for protecting a marine structure includes: (a) positioning an elongated guide having upper and lower ends in a body of water which overlies a seabed, such that the guide is supported independently of the marine structure; (b) placing an elongated conductive anode carrier which has at least one sacrificial anode secured thereto over the guide, so it surrounds the guide; and (c) connecting an electrical conductor between the anode column and the marine structure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention may be best understood by reference to the following description taken in conjunction with the accompanying drawing figures in which: 
         FIG. 1  is a partially-sectioned side view of an exemplary anode column constructed according to an aspect of the present invention; 
         FIG. 2  is another side view of the anode column of  FIG. 1  with some of the components removed to reveal a central guide thereof; 
         FIG. 3  is a view taken along lines  3 - 3  of  FIG. 1 ; 
         FIG. 4  is a cross-sectional view of an alternative central guide; 
         FIG. 5  is a side view of an alternative anode carrier; 
         FIG. 6  is a view taken along lines  6 - 6  of  FIG. 5 ; 
         FIG. 7A  is an exploded side of a portion of the anode column shown in  FIG. 1 , showing a connection of a lower portion of an anode carrier to a central guide; 
         FIG. 7B  is a side view of a portion of an anode column showing an alternative configuration of the lower portion of the anode carrier; 
         FIG. 8  is another exploded side of a portion of the anode column shown in  FIG. 1 , showing a connection of an upper portion of an anode carrier to a central guide; 
         FIG. 9  is a side view of an alternative anode column configuration 
         FIG. 10  is a view taken along lines  10 - 10  of  FIG. 9 ; 
         FIG. 11  is a schematic side view of a marine structure installed in a body of water with several anode columns installed nearby; 
         FIG. 12  is a side view of an alternative guide incorporating an auger at a lower end thereof; 
         FIG. 13  is a schematic side view illustrating the process of installing an anode column near a marine structure; 
         FIG. 14  is a schematic side view illustrating another portion of the process of installing an anode column near a marine structure; and 
         FIG. 15  is a schematic side view illustrating a final portion of the process of installing an anode column near a marine structure. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to the drawings wherein identical reference numerals denote the same elements throughout the various views,  FIGS. 1-3  illustrate an exemplary anode column  10  constructed according to an aspect of the present invention. The basic components of the anode column  10  are a guide  12 , an elongated anode string  14  which includes sacrificial anodes  16 , and an electrical conductor  18 . 
     The guide  12  is a vertically-elongated, tube-like member. The guide  12  may be constructed from a plurality of steel pipe guide sections  20  which are joined to each other at threaded connections  22  of a known type. In the illustrated example, the pipe inner diameter is about 7.62 cm (3 in.). The size is not critical and may be varied to suit a particular application. The interior of the guide  12  may be filled with cement  24 , expanding foam, or a similar material to stiffen and stabilize the guide  12 . The primary functions of the guide  12  are to provide structural support and a means for guiding installation and removal of the anode string  14 , as described in more detail below. Accordingly, while the guide  12  is depicted as having a circular cross-section, the specific cross-sectional shape is not critical, and other shapes such as a polygon, or solid or lobed cross-sectional shapes could be substituted. Furthermore, any type of joint, for example threads, mechanical fasteners or welding, may be used between the guide sections  20  so long as the joint retains them together securely. 
     While  FIG. 3  depicts the guide  12  as a single-walled structure, it is possible that it could comprise multiple walls. For example,  FIG. 4  illustrates an alternative guide  12 ′ having an inner wall  26  and an outer wall  28  which cooperatively define two spaces, either or both of which that may be filled with cement  24 , expanding foam, or a similar material to form a strong composite structure. 
     The anode string  14  comprises an anode carrier  30  and sacrificial anodes  16 . Like the guide  12 , the anode carrier  30  is a vertically-elongated, tube-like member. In the illustrated example, the anode carrier  30  may be constructed from a plurality of steel pipe carrier sections  32  which are joined to each other at threaded connections  34  of a known type. As shown, the pipe inner diameter is about 10.2 cm (4 in.). The size is not critical and may be varied to suit a particular application. The anode carrier  30  need only be sized and shaped to fit over and surround the guide  12 . Accordingly, while the anode carrier  30  is depicted as having a circular cross-section, the cross-sectional shape is not critical, and other shapes such as a polygon or a lobed cross-sectional shape could be substituted. Furthermore, any type of joint, such as threads, mechanical fasteners, or welding, may be used between the carrier sections  32 . 
     The sacrificial anodes  16  comprise a material which is anodic to steel, such as aluminum, magnesium, or zinc. In the example shown in  FIGS. 1-3 , the anodes  16  are cast or otherwise fabricated into generally cylindrical shapes, and are secured to the outer surface of the anode carrier  30 , for example by being shrunk thereon or by mechanical fasteners.  FIGS. 5 and 6  illustrate an alternative anode carrier  30 ′. Bars  36  of sacrificial material are shrunk onto steel tubes  38  which are welded, bolted, or otherwise secured to the outer surface of the anode carrier  30 ′. It will be understood that neither the specific physical configuration of the sacrificial material nor its method of attachment to the anode carrier  30  is critical, so long as the sacrificial material is mechanically supported and an electrically conductive path is provided to the guide  12 . For purposes of descriptive simplicity only, the installation and use of the anode columns  10  will be further described with the configuration of sacrificial material shown in  FIG. 1 . 
     One or both of the upper and lower ends of the anode string  14  may be secured to the guide  12  so that the guide  12  can provide structural support and an electrical conduction path to a protected structure. 
       FIG. 7A  illustrates how the lower end of the anode string  14  may be attached to the guide  12 , which is in turn secured to an anchorage (shown schematically at  39 ). A fitting  40  is attached to the guide  12 . This takes the form of an annular component having upper and lower sections  42  and  44  of different diameters, such that a step  46  is defined. The upper section  42  is sized to fit inside of the anode carrier  30 , while the lower section  44  is sized so that the anode carrier  30  sits on top of it. This prevents the anode carrier  30  from dropping below a predetermined height above the seabed when installed. The upper section is  42  externally threaded and the bottom-most section of the anode carrier  30  would be screwed thereto. Standard hardware such as “go/no-go” fittings or threaded collets may be used for this purpose as well. 
     Alternatively, the lower end of the anode string  14  may be attached directly to the anchorage  39 , as shown in  FIG. 7B . In this configuration, a base fitting  41  is provided which is like one of the carrier sections  32  of the anode carrier  30 . The remainder of the anode carrier  30  may then be joined to the base fitting  41  in the same manner that the carrier sections  32  are attached to each other, e.g. by a threaded joint. 
     In order to permit easy disassembly for inspection, maintenance, or replacement, means are provided for selective disconnection of the lower end of the anode carrier  30  from the guide  12  or the base fitting  41 . This could be accomplished by using left-hand threads on the connection between the guide  12  or base fitting  41  and the anode carrier  30  (where the joints between the carrier sections  32  have right-hand threads), by using a low-torque threaded joint so that the connection of the lower end of the anode carrier  30  can be unscrewed from the guide  12  or base fitting  41  without separating the carrier sections  32 , or the like. 
       FIG. 8  illustrates one method by which the upper end of the anode string  14  may be attached to the guide  12 . An annular hanger  48  has a threaded inner bore  50  which is connected to a threaded portion  51  of the uppermost section of the guide  12 , and a threaded outer wall  52  that is connected to the uppermost section of the anode carrier  30 . Other types of hardware such as threaded collets may be used for this purpose as well. The uppermost section of the anode carrier  30  may be provided with a coupling structure such as external threads  55  (e.g. left-hand threads) in order to facilitate removal of the anode carrier  30  without disturbing 
     As shown in  FIG. 8 , the conductor  18  is mechanically and electrically connected to the guide  12 , for example by a braze joint  53 , a swage, mechanical fasteners, or the like. In this example, a conduction path is provided from the anodes  16  through the conductive anode carrier  30 , the conductive hanger  48 , the conductive guide  12 , and finally to the conductor  18 . However, other configurations may be used so long as a conduction path is provided from the anodes  16  to the conductor  18 . 
       FIGS. 9 and 10  illustrate an alternative anode column  110 . Like the anode column  10 , it includes a central guide  112  anchored into the seabed B or otherwise supported in an upright position, and a plurality of sacrificial anodes  116 . Instead of a single tube, the carrier  130  comprises a tower  117  having an open truss construction to reduce water drag forces. A pipe  119  of relatively small diameter is disposed in the center of the tower  117  and serves to locate the tower  117  on the guide  112 . A plurality of arms  121  extend out laterally from the tower. As illustrated, the arms  121  take the form of flat plates, but other shapes such as I-beams may be used. The tower  117  may include several vertically spaced-apart levels of arms  121 , as shown. One or more upright columns  131 , similar in construction to the carriers  30  described above, extend between the arms  121 . The anodes  116  are attached to the columns  131 . A conductor  118  connects the anode column  110  to a protected marine structure (not shown) This configuration allows increased density of anode placement using a single guide. 
       FIG. 11  illustrates how a marine structure may be protected by one or more of the anode columns described above. In this example, the structure is a drilling rig  54  erected in a body of water W, such as the ocean. A platform  56  is supported by a plurality of metallic legs  58  that are driven into the seabed B below the body of water W and interconnected by metallic truss members  60 . One or more drill strings  62  extend downward from the platform  56  to the seabed B. Substantial portions of the drilling rig  54  are constructed from ferrous alloys and are thus subject to rapid corrosion in seawater. 
     While a drilling rig  54  is illustrated, any marine structure may be provided with cathodic protection using the principles of the present invention. The protected structure could be permanently mounted in the seabed, as in the case of the drilling rig  54 , or it could be free-floating, or it could be floated on anchored spars in a known manner. 
     One or more anode columns  10 , constructed as described above, are placed in convenient proximity to the drilling rig  54 . Each anode column  10  is structurally supported independently from the drilling rig  54  and electrically connected to the drilling rig  54  via an electrical conductor  18 , such as the illustrated cables. Known methods may be used to compute the total mass of sacrificial material required to protect a specific structure, and this sacrificial material may be distributed among as many anode columns as desired.  FIG. 11  is merely intended as an example of the different kinds of possible installation configurations, and greater or fewer anode columns  10  may be used in a particular application. As illustrated, a first anode column  10 A is placed on a piling  64  driven into the seabed B within the perimeter defined by the legs  58 . A second anode column  10 B is placed on a piling  66  driven into the seabed B outside the drilling rig  54 . A third anode column  10 C is mounted on a truss structure  68  which is placed on or driven into the seabed B. This configuration may be used to elevate the anode column  10 C a substantial distance above the seabed B when desired. For example, this may be necessary if the seabed B is at a depth that might cause crushing of the anode column  10 C. 
     A fourth anode column  10 D is configured as a “spar” structure. The inner guide and/or the anode carrier thereof are sealed and partially evacuated to provide buoyancy. The anode column  10 D is connected to an anchor  70  by a tether  72  (e.g. a heavy cable or chain). 
     A fifth anode column  10 E is directly mounted to the seabed B. This may be accomplished by using a guide  12 ′ (see  FIG. 12 ) with an auger  73  or other type of cutting tip suitable for cutting into the seabed B during installation. If a configuration such as that shown in  FIG. 7B  is used to anchor the anode column  10 E, the auger or cutting tip could be attached to the anode carrier  30 . 
     The anode column  10  is configured so that it may be easily installed or removed from a surface location with minimal or no use of divers or ROVs. The basic installation process is as follows, with reference to  FIGS. 13-15 : 
     First, the guide  12  is set in place. This may be done by connecting the guide sections  20  in a bottom-to-top sequence and lowering the guide  12  towards the seabed B as it is built up. This step is similar to the known manner in which conventional well drill strings are built up. Additional temporary pipe sections may be added to the top end of the guide  12  as needed to provide sufficient height to reach the seabed B and allow driving force to be applied thereto. The installed guide  12  is shown in  FIG. 13 . The guide  12  is supported in such a way as to remain upright during use, for example using one of the structures shown in  FIG. 9 . Preferably, the guide  12  is supported or anchored in such a way that is can be set completely from the surface S, for example, the guide  12  may be driven into the seabed B in the manner of a conventional piling, or screwed in if an auger  73  or similar type of cutting tip is used. 
     Next, the anode string  14  is installed. This may be done by connecting the carrier sections  32  in a in a bottom-to-top sequence and lowering the guide towards the seabed B, as it is built up. This step is similar to the known manner in which conventional well drill strings are built up. Additional temporary pipe sections may be added to the top end of the anode carrier  30  as needed to provide sufficient height to reach the seabed B. Once in place, the anode carrier  30  is connected at one or both of its upper and lower ends to the guide  12 , so that the guide  12  can provide structural support and an electrical pathway. As shown in  FIG. 7B , the lower end of the anode carrier  30  could be anchored directly to the seabed B rather than being secured to the guide  12 . The installed anode carrier  30  is shown in  FIG. 14 . 
     Once the guide  12  and anode carrier  30  are installed, any extra pipe sections are removed, and an electrical conductor  18 , such as the cable shown in  FIG. 15 , is connected between the guide  12  and the marine structure  54 . For practical purposes, the conductor  18  may be connected to the uppermost guide section  20  before it is lowered into the water. A junction box (not shown) or other appropriate hardware may be provided on the marine structure for this purpose. Once connected, a conduction path is present from the anodes  16  to the marine structure  54 . Using known electrical equipment such as an ammeter or voltmeter, the electrical performance of the anode column  10  can be checked and verified. 
     In some cases, there may be subsurface currents which place substantial forces on the anode column  10 . In such cases, an external guide  74 , shown in dashed lines in  FIGS. 13 and 14  may be erected to protect the anode column  10  during assembly. The external guide  12  may simply be a large-diameter pipe in one or more sections, and is removed after installation is complete. If the external guide  74  is used, a portion of it may be left in place to serve as an anchoring structure for the anode column  10 . For example, a portion of the external guide  74  may be used to serve as the base fitting  41  described above. 
     The exact sequence of installation is not critical, and may variations are possible. For example, the guide  12  and the anode string  14  may be made up and installed simultaneously rather than installing the guide  12  first. 
     The configuration of the anode column  10  allows easy surface access if repair or maintenance is required after installation. For example, when the anodes  16  reach the end of their useful life, they may be replaced by extending pipe sections down to the anode string  14 , connecting them to the anode carrier  30 , disconnecting the anode carrier  30  from the guide  12 , and hauling the anode carrier  30  to the surface. The anodes  16  may then be replaced and the anode carrier  30  reinstalled, or new anode carrier sections  32  may be provided. All of these steps are performed while the guide  12  and conductor  18  remain in place, providing a means to pilot the movement of the anode carrier  30 , again minimizing the amount of diver or ROV intervention required. 
     The foregoing has described a method and apparatus for cathodic protection of marine structures. While specific embodiments of the present invention have been described, it will be apparent to those skilled in the art that various modifications thereto can be made without departing from the spirit and scope of the invention. Accordingly, the foregoing description of the preferred embodiments of the invention and the best mode for practicing the invention are provided for the purpose of illustration only.