Abstract:
A power supply system for electrically heated pipelines is provided. A variable frequency drive supplies power to the pipeline from an external source of electrical power. An isolation transformer and a contactor or isolating switch may be used between the variable frequency drive and the pipeline. A controller may be used in the system to control special sequences of power level and other needs for heating a pipeline.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to electrical heating of subsea pipelines. More particularly the invention relates to a power supply system for supplying electrical power to the pipeline. 
     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. Some crude oils become very viscous or deposit paraffin when the temperature of the oil drops, making the oil practically not flowable. Hydrocarbon gas under pressure combines with water at reduced temperatures to form a solid 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 require excessive time and may create more operational problems and be costly in terms of lost production. 
     The problem of lower temperatures in subsea pipelines has been addressed by a variety of heating methods, including electrical heating. Heating by a variety of electrical methods is well known in industry. 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). One electrical heating method is the pipe-in-pipe method. In one configuration of this method, 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 bulkhead at the distal or remote end of a heated segment. Voltage is applied between the inner and outer pipes at the proximate or electrical input 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. Anther configuration for electrical heating of subsea pipelines is the “single heated insulated pipe” method, in which only one pipe is insulated from seawater by an electrically insulating coating. This configuration is disclosed in commonly owned patent application Ser. No. 09/629,963, entitled “Apparatus and Method for Heating Single Insulated Flowlines.” Other heating configurations include the earthed-current system developed in Norway and described in “Introduction to Direct Heating of Subsea Pipelines,” overview by Statoil, Saga et al, February 1998. 
     In any form of electrical heating of pipelines a source of electrical power supplying at least a few hundred kilowatts is normally needed. The power often needs to be applied in a programmed sequence to achieve selected operating conditions. In some methods of electrical heating, such as the pipe-in-pipe method, it is desirable that the output of the power supply have very low harmonic content and be delivered with little or no direct current. Selection of a particular frequency may be an advantage. Conversion of power from three-phase to single-phase will often be necessary, especially in subsea pipeline applications, where power may be taken from an existing three-phase power grid on an offshore platform. What is needed is an efficient, versatile power supply system that can supply these needs. 
     SUMMARY OF THE INVENTION 
     A system including a variable frequency drive that may be electrically connected to a power bus and a Distributed Control System (DCS) is provided. A medium voltage drive will normally be employed. The output of the variable frequency drive is preferably coupled to a pipeline through an isolation transformed and a contactor or isolating switch. Control of the variable frequency drive may be provided by a DCS that is connected with the variable frequency drive through an Ethernet connection or is directly connected. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows one environment of use of a power supply to heat a subsea pipeline. 
     FIG. 2 shows details of connection of the power supply system of this invention to a subsea pipeline. 
    
    
     DETAILED DESCRIPTION 
     U.S. Pat. No. 6,142,707 and patent application filed concurrently herewith and titled “Annulus for Electrically Heated Pipe-in-Pipe Subsea Pipeline” (Ser. No. 09/910,696) both commonly owned, are incorporated herein by reference. 
     FIG. 1 illustrates the environment of the present invention. 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 section  18 . Seafloor section  19  may be up to 20 or more miles long. Pipe-in-pipe flowline  10  may be comprised of 40-ft joints of pipe welded together. It is common to form individual 150 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, terminates at sled  21 . 
     Surface facility  16  on platform  14  includes a power supply and associated control equipment for applying and controlling electrical power to the pipeline. Power may be applied at one end of the pipeline or at any intermediate point along the pipeline. 
     The power requirements for heating a pipeline may be moderate in comparison with the power available on offshore platforms, and excess power may already be available on a platform. If the equipment available on a platform is not capable of supplying the power needed, the platform must have provisions for adding electrical power. 
     FIG. 2 illustrates a power supply and associated control equipment suitable for heating a pipeline. Drive  20  is preferably a variable frequency drive. A drive in the “Perfect Harmony Series” of drives supplied by Robicon Corporation of New Kingsington, Pa. may be used. This drive has an 18-pulse input AC to DC converter. It produces sine wave output with total harmonic distortion in current of 0.8% and in voltage of 1.2%. Total variable frequency drive (VFD) system efficiency higher than 96%. The drive is powered though three phase bus  22  and provides a single phase output. The output may be directly connected to a pipeline for heating The output of drive  20  may also be fed to the primary of isolation transformer  24 , such that the small direct current in the drive output (which can cause corrosion) is eliminated before the approximately 60 Hz voltage is applied to a pipeline. Transformer  24  may be integral with drive  20 . Contactor or isolating switch  25  may also be placed between the drive and a pipeline. This can make possible electrical diagnostic testing of the pipeline without interference from drive  20  or transformer  24 . Also, open circuit tests can then be performed on drive  20 . 
     Important features of the variable frequency drive are: it presents a balanced load through the three phase bus or generator from the single phase pipeline load, it allows continuous and rapid variation of applied voltage from zero to the rated voltage, it eliminates the need for a costly and space consuming phase balancing matching network, it has very low harmonic content and it is controllable locally or remotely from a Distributed Control System (DCS). Low harmonic content is important because impedance of the line is variable with frequency. DCS  26  may be connected with drive  20  through Ethernet connection  27  or any other communication channel. DCS  26  may be programmed to shut down the drive when high or low conditions exist and for other purposes. DCS  26  includes the capability of controlling the temperature of the pipeline with a closed loop Proportional Integral Differential (PID) temperature controller and includes the capability for high- and low-temperature alarms, high- and low-impedance alarms, graphic displays of power, voltage, current, and impedance, maintaining a history of operating parameters, programming various combinations of voltage or current versus time, and displaying learned temperature response to applied voltage and current versus time. A suitable DCS is supplied by Foxboro Company. 
     Drive  20  may be operated at constant voltage output, constant power output, or variable power in response to temperature or other sensors used to detect conditions in a pipeline. A temperature sensor on pipeline  10  (FIG. 1) may be used to control power output of drive  20  to maintain temperature within a range of a selected set point, for example. As another example, calculated power input from drive  20  and time of operation may be programmed to melt a hydrate plug in a pipeline. A separate drive is normally used for each pipeline segment to be heated. 
     In one application, a drive having a rated capacity of 3250 volts and 400 amperes was selected. For purposes of stable internal phase balancing, the drive frequency was selected to be 63 Hz. 
     Data recorder  29  can be attached to DCS  26  to keep a history of control signals sent to drive  20 , or may be supplied integral with DCS  26 . Alternatively, local controls  28  may be used to control drive  20 . As an example of control signals that may be sent to drive  20 , during start-up of electrical heating a pipeline, it may be desirable to step up voltage in increments, such as 400 volt increments, to help avoid unrecoverable faults in the annulus caused by water-induced arcing. Such control signals can be programmed into DCS  26  and the signals can be applied to drive  20 . As another example of the use of control signals sent to drive  20 , during commissioning activities of a pipeline, it may be desirable to operate the power system over the entire range of current and voltage expected. The system can then be operated briefly at increments of voltage or current over the entire range up to operating voltage and current and power factor can be recorded at each voltage setting. Current may be increased in 50 ampere increments, for example. These values can then be compared with values predicted based on prior measurements of impedance of segments of the pipeline before they are joined together. Other programmed sequences of voltage or current output may be used. 
     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 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 the scope of the invention as defined by the appended claims.