Abstract:
Counter effecting of local corona discharges by inside metallization of ceramic penetrator for high pressures, high temperatures and high electric voltages.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a method for reducing local corona discharges in a penetrator for feeding electricity through a wall in a pressure vessel or a pressurized pipe. 
       BACKGROUND 
       [0002]    Earlier subsea facilities for the production of oil and gas involved locating complex technology close to a permanent or fixed oil or gas field installation, the permanent or fixed installation typically being a platform construction made from steel or concrete. At greater water depths, permanent or fixed platform installations tend to become unreasonably costly. Floating installations are not sensitive to water depth to the same degree, although, as floating installations tend to move along with sea motion, they require more complex riser systems in order first to get the well stream on board and then again to have it exported. A complicating factor being that the unprocessed well stream cannot be transported directly in long pipelines. Furthermore, the pressure drop from the sea bed and up to the surface reduces the productivity of the well with the consequence that considerable quantities of oil and gas remain unextracted in the reservoir. 
         [0003]    The well stream is advantageously processed to separate the well stream into oil and gas at the bottom of the sea, on the subsea field. Thereby the oil and gas fraction are ready to be pumped directly to the receiver. This requires subsea operation of pumps and compressors, which typically include electrically driven rotating machines. Operation of this type of electrically driven rotating machines require supply of considerable electrical power, necessitating high voltage transmission of electrical power over electrical installations that cannot be in direct contact with surrounding sea water. They need to be located in special chambers that also need to withstand substantial pressures. From this comes the need for a device for feeding high voltage electrical power through one or more subsea pressure barriers. Such a device is often called a “penetrator”. 
         [0004]    An important property of a penetrator is that it does not cause corona discharges, frequently referred to as “CD”, from the English trade term “partial discharge”. Penetrators for subsea applications may advantageously be made from a central copper conductor which is metallic connected with a ceramic hollow cylinder, as disclosed in Norwegian patent application (20111094). 
         [0005]    Absence of PD is most important for the parts of an electric power supply that are insulated by polymers such as cross polymerized polyethyls, commonly designated PEX. This material will be broken down by discharges, leading to loss of insulating properties and short circuits. The penetrator itself, to the extent that it consist of only metals and ceramics is insensitive to PD, but it is advantageous that it does not exhibit PD because eventual PD in the penetrator will complicate detection of PD in other parts of the system. 
         [0006]    The relative permittivity, also referred to as the dielectric constant, for ceramics is very large. This leads to large potential leap at those parts of the surface of the penetrator that are located close to the central conductor. In order to reduce PD, it is advantageous that such potential leaps do not lie on the surface of the ceramics. This can be achieved by metallizing the affected surfaces. These surfaces will generally be located on the inner diameter of the central bore of the ceramics. This bore can be called the inward cylindrical cavity. 
       SUMMARY OF THE INVENTION 
       [0007]    The present invention consist of a method for reducing partial corona discharges by metallizing inward surfaces in subsea high voltage penetrators for thereby to improve exploitation of insulating capability and rupture strength in modern high performance ceramics in combination with technologies for metallic bonding in such penetrators. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  illustrates how the main parts in a possible embodiment are located with respect to each other. 
           [0009]      FIG. 2  illustrate the most essential details in the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0010]      FIG. 1  shows a device according to the invention, characterized in that a ceramic insulator  1  is connected to a metallic flange  5 . Through the insulator  1  is passed a conductor  2  which is affixed in the insulator by a fixed sleeve  3  and a unfixed sleeve  6 . 
         [0011]    According to the illustration of  FIG. 1 , the conductor is connected with cables  12  and  13  with sleeves according well known art. The cables  12  and  13  may also be welded or soldered to the conductor  2  according well known art. 
         [0012]      FIG. 2  shows the components in a different embodiment of a penetrator according to the invention. 
         [0013]    A conducting metal rod  2  is passed through a cylindrical cavity  10  in an insulator. In the arrangement of a conducting metal rod  2  is passed through a cylindrical cavity  10  in an insulator, the inward cylindrical cavity  10  is coated with a metal layer. 
         [0014]    According to a further aspect, the invention provides a method for preventing or substantially reducing PD in penetrators for high voltage supply for underwater facilities, characterized in that an inward cylindrical cavity  10  in a ceramic insulation element along the conductor  2  is coated with a metallic conducting layer. 
         [0015]    The present invention provides a high voltage electrical power feed-through, or “penetrator”, apparatus according to any one of the attached patent claims  1  to  10 . 
         [0016]    The penetrator apparatus, illustrated by exemplary embodiments in the accompanying figures, comprises a flange  5 , an elongated tube  1 , first and second attachment sleeves  4 , an elongated electrical conductor  2 , and a third attachment sleeve  3 . 
         [0017]    In an embodiment of the invention, the third attachment sleeve  3  is axially offset from an adjacently located one of said first and second attachment sleeves  4 . 
         [0018]    In the accompanying drawing, a straight broken or dotted line has been drawn through a central part of the illustrated penetrator embodiments in different views thereof to indicate what will be referred to as a longitudinal direction or longitudinal axis of the high voltage electrical power feed-through or “penetrator” apparatus of the invention. 
         [0019]    In  FIG. 1  is also indicated by broken lines optional features of a penetrator when suitably applied or installed in a facility for supply electric power to power consuming devices, and indicated by reference signs relating to these optional features only in  FIGS. 1 and 3  are insulated cables  12 , connector means  11  for connecting the insulated s cables  12  and  13  to the electrical conductor  2  of the penetrator device, and a shroud or cap arrangement on each side of the flange  5 , covering the insulating tube  1 , the attachment  3  and  4  sleeves on respective sides of the penetrator device, and the exposed ends of cables  12  and  13  and their respective connector means  11 . 
         [0020]    In  FIG. 2 , an embodiment of a penetrator apparatus according to the invention is shown in a partial cross section in a perspective view, illustrating the penetrator apparatus comprising a circular disc shaped flange  5 , with a plurality through holes  9  provided for bolting the flange to a wall of a pressure vessel (not shown). Flat surfaces on oppositely located sides  7  and  8  of the flange disc are advantageously provided with respective circular groove type features located next to the arrow heads related to reference signs  7  and  8  for providing a suitable high pressure metal-to-metal seal, however, such sealing features on any of the surfaces of the flange disc are optional, since sealing may be provided by other suitable means, such as e.g. a conventional gasket or an o-ring arrangement, depending e.g. on the pressure to be sealed, on the media on respective sides of the flange, etc. Accordingly, the shape of the flange, here shown to be circular, could be any suitable shape for adapting it to the vessel or other device to which it is to be attached or bolted, such as e.g. any of a square, rectangular hexagonal, or elliptic shape. 
         [0021]    The flange  5  is typically of a first thickness, and has an opening providing a passage between first  8  and second  7  oppositely located sides of said flange. The elongated tube  1  is advantageously of a solid, electrically insulating material, is of a first length greater than said first thickness, and is advantageously positioned in the passage of the flange, with first and second parts of said tube protruding from respective ones of said first and second sides of said flange. The flange is provided with a suitable number of through holes  9  for attaching it by bolts or similar fastener to a pressure vessel or other device. The first and second attachment sleeves  4  for attaching the insulating tube to the flange  5  are positioned on said tube and are advantageously sealingly attached to respective ones of said first and second parts of the tube, at a distance from respective first and second ends of said tube. The first and second attachment sleeves  4  are advantageously sealingly attached to respective ones of said first and second oppositely located sides  7  and  8  of said flange  5 . Thereby, a space may formed between a central part of the tube and a wall or walls of the passage in the flange, the space being advantageously sealed off from the surroundings of the feed-through apparatus on at least one side of the flange. The elongated electrical conductor  2  is advantageously of a second length that is greater than said first length of the tube, and is advantageously positioned in a lumen  10  of said tube with first and second parts of said conductor protruding from respective ones of said first and second parts of said tube. The third attachment sleeve  3  is positioned on said conductor and is advantageously sealingly attached to one of said first and second parts of said conductor, at a distance from a respective one of a first and second end of said conductor, and is advantageously sealingly attached to a respective one of a first and second end of said tube. 
         [0022]    Although it has been stated that the conductor  2  in the feed-through apparatus of the present invention is of a length greater than the length of the tube  1 , and extends beyond ends of the tube  1 , it will be understood that the conductor  2  as a unitary element could be of a shorter length than the length of the tube for not extending beyond the tube  1  at one or both ends, as its length may be extended by a suitable conducting extension element to form a conductor assembly having a length that is greater than the length of the tube  1 , or extending beyond one or both ends of the tube  1 , 
         [0023]    In an advantageous embodiment of the invention, as illustrated in  FIGS. 1 to 6 , an end cap  6  is advantageously provided at one end of the tube. The end cap  6  is contemplated to serve as a means for sealing off a space between the tube and the conductor, and/or to serve as a means for positioning the conductor within the lumen of the tube, e.g. in order to maintain it centered with respect to the lumen, or to maintain a desired space or gap between an outer surface of the conductor and an inward surface of the lumen of the tube, e.g. in order to obtain a desired electrical field configuration in the space or gap. Advantageously, the end cap  6  is not firmly attached to the conductor or to the tube to allow for relative movement of these items in axial or radial direction, such as e.g. in case of different thermal expansion or extension. Advantageously, the end cap  6  could made from a resilient and/or flexible material to maintain a seal between and/or maintaining relative positions of the conductor and the tube in case of relative movements of these items. 
         [0024]    In an advantageous embodiment of the invention, a space  20  between the tube and the conductor is filled with an electrically insulating fluid being resistant to high voltage breakdown, such as e.g. a high voltage transformer oil or other fluid or liquid having a corresponding high voltage insulation property. 
         [0025]    In an advantageous embodiment of the invention, at least a part of an inward surface of said lumen  10  is supporting a layer of an electrically conducting material. In an further embodiment of the invention, the layer of conduction material is extended onto one or both end surfaces  23 A,  23 B of the tube, illustrated in  FIGS. 6 and 7 . It should be noted that the illustration in figure is a partial “exploded view” version of the partial assembly drawing of  FIG. 4 , in which the third attachment sleeve  3  and cap  6  are drawn as separated from the partial conductor and tube assembly. In  FIGS. 6 and 7  are also illustrated further features of advantageous embodiments of the invention, particularly a section  22  of the lumen  10  within the tube having a larger inner diameter, for receiving a portion of the third attachment sleeve  3  having an outer diameter that is larger than an inner diameter of an adjacently located part of the lumen  10 , and a first transition or step  12  between sections of the tube having different outer diameters. Advantageously, in connection with the section  22  of the lumen  10 , there is a defined second transition or is step at the point where the inner diameter of the lumen  10  changes. Advantageously, the first transition or step  12  of the outer diameter of the tube  1  is located proximal to an end surface  23 A of the tube at which the section  22  of the lumen of the larger diameter is located. The first and second transition or steps between inner diameters of the lumen  10  and between outer diameters of the tube  2 , provide respective means for ensuring proper location of the respective first or second attachments sleeves  4  and the third attachment sleeve, respectively, and provide also respective strength taking portions of the tube for, in the case of a pressure differential between the side a  8  and  7  of the penetrator, coupling an axial force; exerted by pressure, from the conductor and attachment sleeve  3  assembly to the tube  1 , and from the tube  1  and first attachment sleev 4   4  on side  8  of the penetrator to the flange  5 . The outer diameter transition or step  12  of the tube is more clearly shown in the side view illustration of the tube  1  in  FIG. 7 . The reference  22  in  FIG. 7  has been included only to indicate the location of an opening in the end part  23 A formed by the section of the lumen  10  having the larger inner diameter. 
         [0026]    As illustrated in the expanded detail view of  FIG. 5 , it is contemplated that in an embodiment, the apparatus of the invention comprises an electrically conducting means  21 , being arranged in a space  20  between said conductor and said layer of a first electrically conducting material carried on a surface of the lumen  10 , for establishing an electrically conducting path between said conductor and said layer of a first electrically conducting material. The electrically conducting means  20  could be provided alone, or in combination with an electrically conducting third attachment sleeve for establishing an electrically conducting path between the conductor and the layer of a first electrically conducting material. 
         [0027]    In an advantageous embodiment of the invention, the third attachment sleeve comprises a first electrically conducting material. In an advantageous embodiment of the invention, the first electrically conducting material comprised in said third attachment sleeve  3  is adapted to provide an electrically conducting path between said conductor and said layer of an electrically conducting material, thereby bringing an electrical potential of the layer of an electrically conducting material towards an electrical potential of the conductor. 
         [0028]    In an advantageous embodiment of the electrical feed-through apparatus of the invention, a diameter of a part of the conductor being positioned within said lumen is smaller than a diameter of said lumen so as to provide a gap  20  between an outer surface of the conductor and an inward surface of said lumen, as illustrated in the enlarged cross section detail of  FIG. 5 . 
         [0029]    In an advantageous embodiment of the electrical feed-through apparatus of the invention, said layer said layer of a first electrically conducting material is a layer of metal or a layer of a semiconducting material, applied to the entire inward surface of the lumen. 
         [0030]    According to a further aspect of the invention, said layer said layer of a first electrically conducting material is applied to a longitudinal portion of the inward surface of the lumen, the longitudinal portion extending axially between about the points of attachment to the tube  1  of the first and second attachment sleeves  4 , respectively, or the longitudinal portion extending axially beyond the points of attachment to the tube  1  of the first and second attachment sleeves  4  but not entirely to ends  23 A,  23 B of the tube  1 . 
         [0031]    In an advantageous embodiment of the electrical feed-through apparatus of the invention, said electrically conducting means is constituted by said third attachment sleeve comprising a second electrically conducting material. 
         [0032]    In an advantageous embodiment of the electrical feed-through apparatus of the invention, said electrically conducting means is constituted by a resilient electrical conductor  20  positioned in a gap between an outer surface of the conductor and an inward surface of said lumen. The resilient electrical conductor could e.g. be placed or wrapped around the conductor to form a ring connecting the electrical conductor to the conducting layer on their circumferences, or be located so as to form a longitudinal connection between surfaces of the electrical conductor and the conducting layer. 
         [0033]    The conducting layer is advantageously a layer of metal which has been applied to the surface by a method of vapor deposition, chemical deposition, electrolytic deposition, sputtering, or spraying.