METHOD AND APPARATUS FOR TRANSPORTATION OF HYDROCARBONS

Abstract: A method of maximising the flow capacity of a conduit for carrying fluids such as hydrocarbons from a source (2) to a processing facility comprising the steps of monitoring the velocity of fluid through the conduit and maintaining said velocity within set parameters to discourage operation within unfavourable, multi-phase flow regimes and therefore encouraging constant flow of fluid through the conduit.

Description: METHOD AND APPARATUS FOR TRANSPORTATION OF HYDROCARBONS This invention relates to a method of controlling flow of fluids in a pipeline, and particularly but not exclusively to a method and apparatus for converting gas export pipelines from dense phase flow to wet hydrocarbon/two-phase flow, typically in pipelines carrying production fluids from oil and gas fields. Gas is conventionally transported through pipelines in one of three phases i.e. single phase (gas), two phase (gas and liquid) and dense phase (above cricondenbar) . These are primarily controlled by the pressure of the fluid in the pipeline during transfer i.e. the operating pressure. For the purposes of this description we will define single phase (gas) as being in the fluid phase envelope below the cricondenbar but above the cricondentherm. Operation of the pipeline in a multiphase regime is problematic; slugs of liquid forming in a predominantly gaseous flow reduces the efficiency of the transfer, and the slugs can cause stability problems with downstream equipment. Capacity of existing dense phase pipeline systems, with specified and unalterable design pressures, can be enhanced by transferring the fluids at a lower pipeline operating pressure, giving a higher available pressure differential for driving fluid flow. The problem associated with lowering the pipeline pressure below the fluid cricondenbar is that retrograde condensation can occur, resulting in a two phase flow regime. Operation within this two phase regime can restrict capacity due to the higher pressure drop, dependent upon the two phase flow regime. In particular the effects of slugging flow can have a significant detrimental effect on the flow capacity and downstream equipment stability. Most pipeline flow management systems currently available concentrate on maintaining the pipeline operating pressure within set parameters, in order to control the flow of fluid through them. The present invention provides a method of maximising the flow capacity of a conduit for carrying fluids such as hydrocarbons from a source to a processing facility comprising the steps of monitoring the velocity of fluid through the conduit and maintaining said velocity within set parameters to discourage operation within unfavourable, multi -phase flow regimes and therefore encouraging constant flow of fluid through the conduit. The invention also provides apparatus for maximising the flow capacity of a conduit for carrying fluids such as hydrocarbons from a source to a processing facility comprising means for monitoring the velocity of fluid through the conduit and a regulator for maintaining the velocity of the fluid within set parameters to discourage operation within unfavourable multi -phase flow regimes and therefore encourage constant flow of fluid through the conduit. Typically the velocity of the fluid is maintained substantially constant. Minor variations of l-5m/s can be tolerated depending on other characteristics of the conduit and/or fluid. Typically the conduit is a gas conduit, and more preferably a gas pipeline for carrying gas and other production fluids from an oil or gas field or well . For a typical pipeline the desired velocity of fluids is around 5-20m/s, typically 10-15m/s and more preferably 15m/s, but the velocity can vary with the characteristics of the pipeline, such as internal diameter and volume and the characteristics of the fluid being transferred because the fluid characteristics such as cricondenbar and/or cricondentherm for the fluid (s) may affect their optimum velocity. Typically the velocity is maintained within set parameters by a compressor at one end of the pipeline, or in a section of the pipeline, that can be used to adjust the pressure of the fluid. A sensor is typically also provided to sense and optionally to indicate the velocity of the fluid. The compressor may be linked to the sensor by electronic or manual control means to adjust the velocity of the fluids through the conduit in response to the readings from the sensor. The control means can be automatic . If the gas velocity can be maintained at a specific value, typically 15m/s at the reception facilities, then the two phase flow regime will generally be in mist flow, a homogenous flow regime, and the capacity of the pipeline can be increased. The desired flow velocity is typically high enough to prevent stagnation of fluids and creation of slugs of liquid in the dense gas phase, and low enough to prevent erosion problems. By controlling the arrival superficial gas velocity, changes in entrant gas mass flowrate will result in changes to the arrival pressure. Increasing the entrant mass flow rate will increase the arrival pressure and decreasing the entrant mass flowrate will decrease the arrival pressure. In a preferred embodiment the compressor maintains a constant suction volume flow by allowing the compressor suction to vary. The compressor is typically operated away from its surge curve. Suitable compressors include those manufactured by NuovoPigone, Dresser and Rand. An embodiment of the invention will now be described by way of example and with reference to the accompanying drawing, which is a schematic representation of a system for controlling the flow of fluid through a gas pipeline. Referring now the drawing, a 30" diameter gas pipeline 1 from an offshore gas source 2, such as a collection of fields carries production fluids from the source 2 to an onshore processing site. Here the production fluids are separated into liquids and gasses in a suction scrubber/separator vessel 5. A suitable vessel includes those made by Burgess Manning; in separator 5, liquids fall under gravity and are drawn off and the gasses are drawn off at variable pressures from the top of the separator 5 by a compressor 10. The compressor 10 is controlled so that it operates at a relatively constant suction volume flow and draws gasses from the separator at a relatively constant rate. The gas flow from the compressor 10 is optionally passed through a heat exchanger or discharge cooler 15 because the pressurised gas flowing from the compressor 10 is often at a very high temperature, and a control valve 20 maintains the pressure ratio across the compressor within a fixed range in order to keep the flow rate of fluid through the compressor substantially constant. A suitable discharge cooler includes those supplied by Motherwell Bridge and Heatric. The compressor is typically a constant suction volumetric flow compressor, such as a centrifugal compressor, and is typically located at the reception end of the pipeline. A centrifugal compressor's operating curve is a map of polytropic head against actual suction volume flow, therefore for a fixed speed compressor, a relatively constant actual volume flow can be maintained by achieving a constant polytropic head across the compressor. Changes in entrant flowrate will change the arrival pressure. A fixed speed compressor must operate at a fixed polytropic head to ensure a constant actual suction volume flow. Polytropic head is a function of pressure ratio therefore the discharge pressure must be controlled, through operation of the discharge control valve to ensure that the polytropic head, and thus the actual suction volumetric flow, remains constant. In the case of a variable speed compressor there is a greater control potential since the velocity can be maintained by adjusting the speed (i.e. adjusting the required polytropic head, which is effectively the pressure ratio across the machine) . Modifications and improvements can be made without departing from the scope of the invention.

Claims: CLAIMS 1. A method of maximising the flow capacity of a conduit for carrying fluids such as hydrocarbons from a source to a processing facility comprising the steps of monitoring the velocity of fluid through the conduit and maintaining said velocity within set parameters to discourage operation within unfavourable, multi -phase flow regimes and therefore encouraging constant flow of fluid through the conduit. 2. A method according to claim 1, wherein the flow rate of the fluid is maintained substantially constant. 3. A method according to claim 1 or 2 , wherein the velocity of fluids flowing in the conduit is between 5- 20m/s. 4. A method of according to claim 3 , wherein the velocity of fluids flowing in the conduit is between 10- 15m/s . 5. A method according to claim 4, wherein said velocity is 15m/s. 6. A method according to any one of the preceding claims wherein said velocity is maintained by a compressor. 7. A method according to claim 6 wherein said compressor maintains a constant suction volume flow, within the conduit. 8. A method of maximising the flow capacity of a conduit for carrying fluids such as hydrocarbons from a source to a processing facility substantially as hereinbefore described. 9. Apparatus for maximising the flow capacity of a conduit for carrying fluids such as hydrocarbons from a source to a processing facility comprising means for monitoring the velocity of fluid through the conduit and a regulator for maintaining the velocity of the fluid within set parameters to discourage operation within unfavourable multi -phase flow regimes and therefore encourage constant flow of fluid through the conduit . 10. An apparatus according to claim 9 wherein the means for monitoring the velocity of fluid through the conduit is a sensor. 11. An apparatus according to claim 9 or 10 wherein the regulator is a compressor provided at one end of, or within a section of the conduit. 12. An apparatus according to claim 10 or 11 wherein the regulator is linked to the sensor by control means to adjust the velocity of fluids through the conduit. 13. An apparatus according to claim 12 wherein said control means are electronic control means. 14. An apparatus according to claim 12 wherein said control means are manual control means. 15. An apparatus for maximising the flow of capacity of a conduit for carrying fluids such as hydrocarbons from a source to a processing facility substantially as hereinbefore described in the accompanying Figure .

Inventors: GAYTON PAUL WILLIAM, GAYTON, PAUL, WILLIAM
Applicants: GAYTON PAUL WILLIAM, INGEN PROCESS LTD, GAYTON, PAUL, WILLIAM, INGEN PROCESS LIMITED
Application year: 2001
Main classification: F17D1
IPC codes: F17D1