Abstract:
An anode for cathodic protection of underwater equipment is provided. The anode comprises: a support body; sacrificial material retained by the support body; and an attachment mechanism for releasably attaching the anode to the underwater equipment.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The field of invention relates generally to anodes for cathodic protection of underwater-located equipment and a method for providing corrosion protection of underwater-located equipment. More specifically, the field of invention relates to cathodic protection of well trees for subsea hydrocarbon fluid extraction. 
         [0003]    2. Description of Related Art 
         [0004]    Metallic equipment that is to be deployed underwater is at risk of corrosion. To protect the equipment, it is common to provide a galvanic anode (also known as a “sacrificial anode”) on the equipment, thus providing cathodic protection (CP) for the equipment. Such anodes comprise sacrificial metals which have a lower electrochemical potential than the metal of which the equipment is made, such that when deployed, the sacrificial material of the anode is corroded more readily than the metal of the equipment. Currently, the most common metal materials used as the sacrificial material for galvanic anodes include alloys of aluminium, magnesium and zinc, with aluminium and zinc being favoured for subsea use. 
         [0005]    Although all sea-deployed equipment is at risk of such corrosion, there are particular problems associated with hydrocarbon fluid extraction facilities deployed on the sea bed, for example well trees. These are relatively large, very expensive structures, which are often required to be in place for over twenty years, and there may be major safety and environmental issues if corrosion occurs. Typically, well trees are provided with U-shaped aluminium galvanic anodes, with the “legs” of the U being welded to the tree to ensure good electrical continuity. A typical such anode is schematically shown in  FIG. 1 . As shown, the anode comprises a block of aluminium  1 , with a metallic leg  2  extending from each end. In use, the free ends of legs  2  are welded onto the well tree. Current practice is to weld the anodes to the tree frame (or other structure, manifold, template, flowbase etc) at the manufacturing stage. This gives good electrical continuity for the CP system to work efficiently. 
         [0006]    The number of anodes/quantity of aluminium used is a function of the intended field life of the tree. For trees with a long life spans, many anodes are required. For example, if calculations show that eight anodes will be required for a field life of twenty years, then all eight anodes will be welded to the tree frame (or other structure) in the workshop. This approach makes the tree densely populated, and fitting so many anodes in may be difficult, and can result in anodes being closer to vulnerable areas than desired. In addition, placement of the anodes is important. Anode positions need to be considered carefully to both minimize the dangers of hydrogen embrittlement and to optimize cathodic protection. 
         [0007]    It is an aim of the present invention to overcome these problems, and enable effective cathodic protection of underwater equipment, such as well trees, for the duration of the equipment&#39;s operational life. 
       BRIEF SUMMARY OF THE INVENTION 
       [0008]    In one embodiment of the present invention, an anode for cathodic protection of underwater equipment is provided. The anode comprises: a support body; sacrificial material retained by the support body; and an attachment mechanism for releasably attaching the anode to the underwater equipment. 
         [0009]    In another embodiment of the present invention, an anode comprising a buoyancy device configured to enable the density of the anode to be selected is provided. 
         [0010]    In a further embodiment of the present invention, a method for providing corrosion protection of underwater equipment is provided. The method comprises providing an anode for cathodic protection of the underwater equipment, wherein the anode comprises: a support body, sacrificial material retained by the support body, and an attachment mechanism for releasably attaching the anode to the underwater equipment; and attaching the anode to the underwater equipment. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0011]    The invention will now be described with reference to the accompanying drawings, in which: 
           [0012]      FIG. 1  schematically shows a known aluminium galvanic anode for use at a subsea well tree; and 
           [0013]      FIG. 2  schematically shows a cross-sectional view of an anode in accordance with an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0014]    In the following discussion and claims, the term “anode” is used to denote an item which includes sacrificial material, and not the sacrificial material itself 
         [0015]    An embodiment of the invention is schematically shown in  FIG. 2 . Here, an anode  3  in accordance with an embodiment of the invention is shown just prior to deployment at a well tree  4 . The anode  3  is designed for manipulation by an ROV, however for clarity the ROV is not shown. Anode  3  comprises an elongate support body  10 , which supports and retains a mass of sacrificial material  5 . As shown, the sacrificial material  5  is circumferentially moulded around the support body  10 . The material  5  may, for example, comprise a material selected from the group consisting of aluminium, zinc and magnesium, although other materials may be used. 
         [0016]    One end of the support body  10  comprises a member formed as a tapered projection  6 . This is shaped to enable frictional engagement with the well tree  4 , this frictional engagement causing the anode  3  in use to be retained at the tree. More particularly, the projection  6  is tapered so as to be inserted (“stabbed”) and retained within a substantially correspondingly shaped receptacle  7  provided at the well tree  4 . The taper helps to ensure that electrical continuity is maintained between the anode  6  and tree  4 . 
         [0017]    Receptacle  7  is specifically adapted for retaining anodes  3 , which must be located at the tree prior to anode deployment, e.g. during manufacture. Receptacles  7  are placed at suitable locations on the tree frame that allow relatively simple deployment by ROV. Generally, receptacles would be positioned at the exterior of the tree  4 , but in other embodiments receptacles could be placed in the body of the tree  4 , such that, for example, anode  3  may be inserted into the receptacle in the body of the tree from above (e.g. through a hole in the roof) or below (e.g. using horizontal tracks). 
         [0018]    Friction is needed to force the surfaces of the projection  6  and receptacle  7  to merge and ensure good electrical contact. Once frictionally-engaged, a locking mechanism (not shown) such as a holding latch acts to retain the anode  3  within the receptacle. This locking mechanism may also be actuable by ROV. The latch may have various designs, as would be appreciable to those skilled in the art, such as a screw thread arrangement, or a lock bar/pin, latch, which is relatively simple and relatively immune to crustacean damage. 
         [0019]    As mentioned above, the anode  3  is designed for manipulation by an ROV. To this end, the anode comprises an ROV-friendly grab handle  8 . As shown, the handle  8  is mounted on the support body  10 , at the distal end to the projection  6 . 
         [0020]    Anode  3  also includes buoyancy means for enabling the density of the anode to be selected, in this case comprising an air-filled cavity  9  located within the support body  10 . In effect therefore, the support body  10  could be considered to comprise a sealed, hollow pipe, of sufficient strength to withstand the ambient pressure at the installation location. The dimensions and/or filling material of the cavity may be selected to make the anode  3  substantially buoyancy neutral. This provides various advantages, particularly that the operation to install/replace the anode  3  would be much simpler and more cost effective. Below about one kilometre depth (i.e. the depth of the pycnocline), the density of water does not vary greatly with increasing depth, however selection of the appropriate anode buoyancy maybe dependent on the depth of installation. As an alternative, the anode may be made slightly more dense than the sea water at the installation, such that in the event of accidental release, the anode would sink to the sea floor to facilitate recovery. 
         [0021]    A suitable installation procedure may be as follows. As many anodes  3  as required are loaded onto an ROV launch frame, such launch frames being known in the art. The launch frame is picked up by an ROV and taken to the required installation location. Individual anodes  3  are placed in respective well tree receptacles  7  by the ROV, to create a friction fit between projection  6  and receptacle  7 . The positive locking mechanism is engaged to more securely retain the anode  3  in the receptacle  7 . Following initial installation, old, spent, anodes would be removed by ROV before a new anode may be inserted. This would require disengagement of the positive locking mechanism, by the ROV. The anodes may be included in a regular inspection process using ROV (or diver) and camera. Thus, visual inspection would determine when replacement would be necessary. 
         [0022]    The above-described embodiment is exemplary only, and other possibilities and alternatives within the scope of the invention will be apparent to those skilled in the art. For example for some installations, the buoyancy cavity  9  may be filled with materials other than air. For example, the cavity  9  may be filled with a solid material, so that the support body is more capable of withstanding high ambient pressure without undue deformation. Alternatively, the buoyancy means may comprise buoyancy tanks attached to the support body. The anodes may optionally be fitted with simple current/voltage monitoring means to detect when CP protection lowers to unacceptable levels, indicating that replacement of the anode is required. In this case the current/voltage monitoring means could be connected to a condition monitoring system of the well. 
         [0023]    Embodiments of the present invention provide various advantages over the prior art, including, but not limited to the following. Each anode may take up less space on a tree than a conventional anode. Instead of requiring many anodes on a tree, relatively few need be used, these being replaced as required. Consequently, there is more surface space available on the tree for other purposes. Since anodes are replaced as required, there is the potential for trees to have very long lives. No welding is required for embodiments of the present invention. Since anodes are modular, the overall weight of the tree is reduced. There is no need to run each anode from the surface to the tree. Since use of replaceable anodes means that a stock of anodes may be run to the sea bed by ROV deployment, the ROV may then pick up each anode in turn to place it in position as required. This simplifies the activity by having only one deployment trip that could cover all equipment, e.g. trees and manifolds for example, in the local area; and all operations may be carried out by ROV with no surface operations, i.e. replacement can be carried out in bad weather. Replacement may be performed for example during a tree inspection visit.