Abstract:
An Electrical Insulating Joint (EIJ) for a pipe-in-pipe electrically heated pipeline is provided. A ceramic disk under compressive load and dielectrics in an annulus provide electrical isolation and mechanical strength. An insulative liner extends around the ceramic disk to provide electrical isolation when materials other than hydrocarbons pass through the EIJ. The insulative liner may be extended through a knee joint. Pressure ports may be used to monitor fluid leaks and a built-in transformer may be used to monitor electrical leakage current.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention pertains to subsea pipelines. More particularly, apparatus is provided for electrically insulating and connecting electrical power to a segment of a pipeline that is electrically heated using a pipe-in-pipe configuration. 
     2. Description of Related Art 
     Offshore hydrocarbon recovery operations are increasingly moving into deeper water and more remote locations. Often satellite wells are completed at the sea floor and are tied to remote platforms or other facilities through extended subsea pipelines. Some of these pipelines extend through water that is thousands of feet deep and where temperatures of the water near the sea floor are in the range of 40° F. The hydrocarbon fluids, usually produced along with some water, reach the sea floor at much higher temperatures, characteristic of depths thousands of feet below the sea floor. When the hydrocarbon fluids and any water present begin to cool, phenomena occur that may significantly affect flow of the fluids through the pipelines. At these low temperatures, some crude oils become very viscous or deposit paraffin. Either phenomenon can prevent flow. Hydrocarbon gas under pressure (free gas or solution gas in crude oil) can combine with water at reduced temperatures to form a solid ice-like material, called a “hydrate.” Hydrates can plug pipelines and the plugs are very difficult to remove. In deep water, conventional methods of depressuring the flow line to remove a hydrate plug may not be effective. Higher pressures in the line and uneven sea floor topography may cause excessive time requirements for remediation, which can be costly in terms of lost production. 
     The problem of lower temperatures in subsea pipelines has been addressed by placing thermal insulation on the lines, but the length of some pipelines makes thermal insulation alone ineffective. Increased flow rate through the lines also helps to minimize temperature loss of the fluids, but flow rate varies and is determined by other factors. Problems of heat loss from a pipeline increase late in the life of a hydrocarbon reservoir because production rates often decline at that time. Problems become particularly acute when a pipeline must be shut-in for an extended period of time. This may occur, for example, because of work on the wells or on facilities receiving fluids from the pipeline. The cost of thermal insulation alone to prevent excessive cooling of the lines becomes prohibitive under these conditions. 
     Heating of pipelines by bundling the lines with a separate pipeline that can be heated by circulation of hot fluids has been long practiced in the industry. Also, heating by a variety of electrical methods has been known. Most of the proposals for electrical heating of pipelines have related to pipelines on land, but in recent years industry has investigated a variety of methods for electrical heating of subsea pipelines. (“Direct Impedance Heating of Deepwater Flowlines,” OTC 11037, May, 1999) 
     Two configurations for electrical heating have been considered. In one configuration, a single flowline is electrically insulated and current flows along the flowline. This is called the “SHIP” system (Single Heated Insulated Pipe). In the second configuration for electrical heating, a pipe-in-pipe subsea pipeline is provided by which a flow line for transporting well fluids is the inner pipe and it is surrounded concentrically by and electrically insulated from an electrically conductive outer pipe until the two pipes are electrically connected at one end. Voltage is applied between the inner and outer pipes at the opposite end and electrical current flows along the exterior surface of the inner pipe and along the interior surface of the outer pipe. This pipe-in-pipe method of heating is disclosed, for example, in U.S. Pat. No. 6,142,707, which is commonly assigned and hereby incorporated by reference herein. Other patents related to the pipe-in-pipe method of heating include U.S. Pat. No. 6,292,627 B1 and U.S. Pat. No. 6,371,693 B1, which are hereby incorporated by reference. 
     Any method of electrical heating of a segment of a pipeline requires that the segment be electrically insulated from other parts of the pipeline. The pipe-in-pipe method of heating disclosed in the referenced patents requires, when power is applied at one end of the segment to be heated, an Electrical Insulating Joint (herein “EIJ”) at the powered end of the segment. The powered end is normally on or attached to an offshore platform or other structure where electrical power is generated. The voltage drop across the EIJ determines the amount of heating available and the length of a segment that can be heated; for a pipeline a few miles long a voltage drop of thousands of volts is expected. Electrical currents through the pipeline may be in the range of hundreds of amperes. 
     subsea pipeline may contain, along with hydrocarbons, water, grease, pipe dope, well treating chemicals, inhibitors or other contaminants and, from time-to-time, even metallic parts from subsurface equipment such as sand screens or chokes. Water may condense above the EIJ as fluids in the heated segment cool. Therefore, there is need for an electrical insulating joint that can maintain electrical isolation even in the presence of harsh chemical and mechanical environments. The insulating joint should be able to survive repeated exposure to all these materials without failing electrically or reducing the heating capability of the system. The primary protection should be passive, i.e., not dependent on instrumentation, but instrumentation may be used for monitoring. The device should also be capable of transmitting the large static loads of a subsea pipeline riser that is tied to the structure. Under no circumstances should there be a pressure release or exposure of an ignition source. 
     SUMMARY OF THE INVENTION 
     Apparatus is provided for applying electrical power to a pipe-in-pipe heated pipeline. An Electrical Insulating Joint (EIJ) provides mechanical joining, pressure containment and electrical isolation of a heated and an unheated portion. A ceramic ring under compression and dielectrics in the annulus separate inner and outer pipe hubs. A dielectric liner is placed over the ceramic ring and the wall of the flow channel for a selected distance in each direction from the ceramic ring. Additional lined piping (e.g., a knee joint) may be used to extend this distance above the ceramic ring and to place the EIJ at a selected angle with respect to vertical. An additional ceramic ring may be placed between shoulders in the EIJ. O-ring seals may be placed on the ceramic ring and in the annulus dielectrics. Pressure ports may be placed so as to indicate pressure build-up across an o-ring or other seal. A transformer may be placed so as to indicate electrical leakage current along the liner. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a view of apparatus for heating a riser and a segment of a pipe-in-pipe pipeline near an offshore platform. 
     FIG. 2 shows a schematic cross-section of a pipe-in-pipe electrical heating apparatus. 
     FIG. 3 shows a partial cross-section of a prior art insulating joint. 
     FIG. 4 shows a composite cross-section view of the electrical insulating joint disclosed herein. 
     FIG. 5 is a cross-section view of the knee-joint portion of the electrical insulating joint disclosed herein. 
     FIG. 6 is a cross-section view of the body of the electrical insulating joint disclosed herein. 
     FIG. 7 is an end view of the body of the electrical insulating joint disclosed herein. 
     FIG. 8 is the cross-section view of the body of the electrical insulating joint disclosed herein through the angle showing the electrical input connection. 
     FIG. 9 is a schematic cross-section view of the electrical transformer disclosed herein and flow of leakage current. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 1, the environment of use of an Electrical Insulating Joint (EIJ) is illustrated. Here remote satellite well  12  is connected to platform  14  with subsea pipe-in-pipe pipeline  10 . Subsea pipeline  10  may consist of seafloor section  19  and riser  18 . Electrical Insulating Joint  38  is placed in riser  18 , whereby electrical power is supplied to riser  18  and seafloor section  19 . Surface facilities  16  on platform  14  include an electrical power supply. Seafloor section  19  may be up to 20 or more miles long. Pipe-in-pipe flowline  10  may be composed of 40-ft joints of pipe welded together. It is common to form individual 160 ft segments of pipe, called quads (four joints), which are then welded together as they are placed subsea to form pipe-in-pipe flowline  10 . Seafloor section  19 , which may be a half-mile or more below surface  28  of the ocean, may terminate at sled  20 , where the outer pipe and inner pipe of the pipeline are electrically connected by a bulkhead or other apparatus on sled  20 . 
     FIG. 2 illustrates one embodiment of an electrically heated pipe-in-pipe pipeline. In the embodiment shown in FIG. 2, pipeline  10  includes electrically conductive outer pipe  32  and electrically conductive product flowline or inner pipe segment  34  arranged concentrically. Annulus  36  is defined between inner pipe segment  34  and outer pipe  32 . Electrical Insulating Joint (EIJ)  38 , which is normally in proximity to platform  14 , structurally joins and electrically insulates heated segment  34  of inner pipe from outer pipe  32  and from inner pipe in an unheated segment. The structural connection in FIG. 2 is illustrated by a bulkhead in proximity to electrical insulation in inner pipe  34 . Electrical power supply  40  is connected across inner pipe  34  and outer pipe  32  at the end of a segment of the pipeline to be heated. Thus the heated segment of pipe-in-pipe flowline  10  serves as a power transmission line, with the circuit completed by an electrical pathway connecting inner pipe  34  and outer pipe  32  at a second end  44  of the pipeline, which is normally in proximity to sled  20  (FIG.  1 ). By transmitting power, the entire heated segment of pipeline  10  serves as an electrical heater. The connection for joining the inner and outer pipes may be provided by electrically conductive bulkhead  46  (FIG.  2 ). To prevent electrical shorts across annulus  36 , inner pipe  34  must be electrically isolated from outer pipe  32  along the entire length of heated segment  10  except at bulkhead  46 . 
     A prior art EIJ, disclosed in U.S. Pat. No. 6,142,707, is illustrated in FIG.  3 . EIJ  38 A includes annular rings  62  to isolate the inner and outer pipes. Annular rings  62  may be formed from epoxy or zirconia. Other annular spaces  63  within EIJ  38  are filled with similar high-strength electrically insulating materials. According to this patent, liner  54  is bonded over each side of insulator interface  64  to prevent electrical breakdown due to brine in the well fluids. Electrical terminal  46  is connected to the inner pipe by penetrator  46 C, which passes through port  46 D. Liner  54  terminates in swage ring liner termination  66 . 
     Referring to FIG. 4, EIJ  38 B, disclosed herein, is illustrated. Inner pipe connector  102  is connected to the inner pipe of a segment of pipe-in-pipe electrically heated pipeline. Outer pipe hub  104  is connected to the outer pipe of the segment. Inner pipe hub  106  forms the end of the inner pipe and forms an end surface for sealing and applying a compressive joining force. Knee joint  110  may be joined to body  112  of the EIJ by bolts  114 A and nuts  114 B. Insulative liners  108 A and  108 B extend through at least a portion of knee joint  110  and at least partially through body  112  of the EIJ. Preferably, the liners are installed such that the inside surface is flush with the remaining flow channel through the EIJ, as shown. Liner  108 A preferably includes an increased outside radius where the liner contacts dielectric ring  126 . The purpose of the increased outside radius is twofold: (1) to increase thermal isolation between the inside surface of the liner, where high-temperature arcing may occur, and dielectric ring  126 ; and (2) to provide additional sealing capability to protect against contamination behind liner  108 A. The additional sealing is realized by plastic deformation of the liner material  108 A between the steel components  106  and  122 . 
     Liner  108 , consisting of liner  108 A and  108 B, is electrically insulative, should maintain dimensional stability in the presence of fluids passing through the EIJ, should have high damage resistance after repeated arcing and aging, should have high dielectric strength after repeated arcing and aging, should be hydrophobic to minimize continuous water tracks along the liner, should have a temperature rating of at least 200° F., and preferably should be flexible enough to allow flaring of the ends of the liner to enable sealing at a flange, as shown in FIG. 4 where liner pieces  108 A and  108 B join. The liner material should also have a high tracking-path resistance after water arcing and contaminant degradation. These properties will prevent thermal degradation of the liner or excessive power loss. Preferably, liner  108  is formed from PVDF (polyvinylidine fluoride), which is sold by ATOFINA Chemicals of Philadelphia, Pa. Nylon 11 or other insulative polymers may also be used. 
     FIG. 5 illustrates knee joint  110  in more detail. Weldments  117  are used to attach flanges  118 A and  118 B to the ends of the joint. Flanges  118  may be ordinary API flanges. Liner  108 B is preferably about 0.25 inch thick and is selected to have a length inside joint  110  that decreases voltage gradient from the end of inner pipe hub  106  to the end of the liner to a value that will limit arc energy per unit length below a value that can cause failure of the polymer. Preferably, liner  108 , consisting of parts  108 A and  108 B, will have a length greater than 12 inches, and more preferably, will have a length in the range from 24 to 48 inches, but may have a greater length. 
     Additional electrical insulation between fluids passing through knee joint  110  and the metal wall of the joint may be provided by drip ring  119 A and coating  119 B. Coating  119 B may be an epoxy selected for high thermal and electrical properties. Coating  119 B may extend under liner  108 . Drip ring  119 A has a contour selected to break-up a stream of water that is flowing along coating  119 B, so as to prevent a continuous water phase that could short from the end of inner pipe hub  106 . The material of liner  108  should also be selected to be hydrophobic, so as to aid in preventing continuous water flow along the inside surface of the liner. 
     O-rings  116  may be placed near the ends of liner  108 B so as to assist in sealing the annulus between liner  108 B and the inside wall of knee joint  110 . Preferably, a hydrophobic, electrically insulating grease will be applied to the inside wall before liner  108 B and  108 A are installed. A suitable grease is a polyurea grease that was developed for high-voltage electric motors, such as Shell—Dolium or Texaco—Polystar. 
     A slight bend in knee joint  110  is usually preferable, the bend angle being selected depending on the angle from vertical of the riser at the location where the EIJ is to be installed. For example, the bend angle may be 9 degrees. 
     Referring to FIG. 6, body  112  of the EIJ is shown. Retainer flange  122  is joined to outer pipe hub  104  by bolts in boltholes  124 . Typically, 12 bolts of 1 ⅜ inch diameter with cap heads are used. The bolts are used to pre-load insulating rings  126  and  128  to a compressive load, preferably a load of about 1 million pounds. Insulating ring  126  is loaded between the end surface of inner pipe hub  106  and the interior end surface of retainer flange  122 . Insulating rings  126  and  128  are preferably formed from zirconia. Ring  126  may have a thickness of about 1 inch. The annulus between the inner and outer pipe is filled with dielectric material. DELRIN ring  134  may be placed in the annulus before assembly. The DELRIN ring may include o-ring grooves as shown. If the O-ring seals on the upper ceramic rings should fail due to overheating or other cause, or cracks develop in the upper ceramic ring, gas pressure or even liquids can be communicated from the flowline to the EIJ annulus across the ceramic face. To prevent communication of this pressure to the pipe annulus, DELRIN ring  134  and associated o-rings are provided in the lower annulus of the EIJ. The DELRIN ring preferably contains o-ring grooves on both the inside and outside surfaces of the ring. Other dielectric materials may be used in place of DELRIN. 
     Silicone rubber  130  and  136  is preferably injected into the annulus, using ports such as port  132  and other ports opposite the point of injection to allow evacuation of the annulus before rubber injection. Pressure ports  125  may be used for monitoring pressure outside o-rings  123 A and  123 B. A port outside o-ring  123 A can be used to indicate failure of that seal, independently of the state of the seal provided by o-ring  123 B. Retainer flange  138  is used to confine dielectric  136  to the annulus. 
     Referring to FIG. 7, an end view of body  112  is shown with bolts  114 A and bolt holes  124 . Cap  152  for an electrical power connection is shown along with connector  120  for signal transmission of current transformer measurements. Cross-section  8 — 8 , is indicated. FIG. 8 shows cross-section  8 — 8  of body  112 , which is in the plane of electrical input connection  150 . Cap  152  is removed after the EIJ is installed and electrical power is to be connected. For example, 300 amperes at 2000 volts may be applied at connector  150 . Pressure may be monitored behind seals to the annulus at ports  122 . In fact, all ports can be manifolded using separate piping and monitored. 
     FIG. 9 illustrates method and apparatus for monitoring electrical leakage current that may occur along the surface of liner  108  in EIJ  38 B. A transformer, consisting of core  160  and winding  162 , may be inserted in retainer flange  122  above the point where heating current flows and below the top of liner  108 . Any leakage current flowing on the inside surface of liner  108  will then complete a circuit as illustrated by the dotted lines in FIG.  9 . Due to the skin effect and proximity effect, current flow (AC) will occur along the outside surface of inside concentric conductors and along the inside surface of outside concentric conductors and, in general, along the surface of all carbon steel conductors in the current leakage path. The leakage current through the steel then flows outside transformer core  160  and winding  162 , allowing leakage current along the liner, which is inside the transformer core, to be detected. 
     Transformer core  160  may be formed from SUPERALLOY, such as 81% nickel and 14% silicon steel, with 12 layers, each having a thickness of about 0.014 inch, available from Magnetic Metals of Anaheim, Calif. The core may be tack welded and heat treated according to manufacturer&#39;s specifications. Conductor wire  162  is preferably toroidally wound on the core. The wire may be wrapped around core  160  to form a cross-section about 0.125 by 0.18 inch, consisting of two windings of about 1000 turns each. One of the windings is redundant and can be energized to test the other. The diameter of the transformer ring may be about 12 inches. The transformer may be held in place by ring  164 , which is a thin, continuous ring that serves primarily as a shield from electrical and magnetic fields arising from the heating current, but also serves as a retainer for the transformer. 
     While particular embodiments of the present invention have been described, it is not intended that these details should be regarded as limitations on the present invention, except as to the extent that they are included in the appended claims. It should be understood that various changes, substitutions and alterations can be made hereto without departing from the spirit and scope of the invention as defined by the appended claims.