METHOD AND SYSTEM FOR SUPPRESSING AND CONTROLLING SLUG FLOW IN A MULTI-PHASE FLUID STREAM Background of the Invention The invention relates to suppression and the control of slug flow in a multi-phase fluid stream. More particularly, the invention relates to a method and system for suppressing and controlling liquid slugs and gas surges in a stream of multi-phase fluid flowing through a pipeline or a flowline system, which may include a riser section, and a gas/liquid separator or slugcatcher located downstream of the pipeline outlet or the riser section.

In the oil and gas industry it is common practice to transport a multiphase fluid containing crude oil or condensate, water and gas from a well through a single pipeline system to a process facility. For example, in case of offshore oil production crude oil, production water and associated gas are generally simultaneously transported through a single subsea pipeline to gas/liquid separating equipment located onshore or on an offshore platform. Several flow regimes are known to occur in such a stream of multiphase fluid, including stratified flow, core flow, mist flow and slug flow. Of these flow regimes slug flow is generally to be avoided as it consists of alternating batches of liquid (termed slugs) and gas surges. Under certain flow conditions growth of liquid slugs may easily occur, which leads to severe slugging whereby a flow pattern of alternating production starvation (no flow), large liquid slugs and strong gas surges at the exit of the flowline system occur. Large liquid slugs can also be generated by operational changes, e. g. the increase of the fluid

production during the start-up of a pipeline. Supplying such an alternating pattern of liquid slugs and gas surges to a gas/liquid separator strongly reduces the efficiency of the separator, as the gas/liquid separator must be operated with acceptable pressure fluctuations and should deliver an acceptably low liquid content in the gas outlet conduit and an acceptably low gas content in the liquid outlet conduit.

A method for preventing slug growth in a pipeline system during simultaneous transportation of a gas and a liquid through the pipeline system is disclosed in Oil & Gas journal, Nov. 12,1979. In this known method a valve is arranged at the top of a riser, which valve is manually or automatically regulated so as to minimise the pipeline pressure upstream of the riser and to minimise the differential pressure fluctuations in the riser.

Transmitters are used to transmit pressure signals for regulating the valve, which transmitters are installed at a subsea part of the pipeline system. This known method is based on the assumptions that severe slugging only occurs in pipelines having a section of downward inclination when seen in the direction of flow and that slug growth can be prevented by regulating the volumetric fluid flux as a function of fluid pressure variations.

JP-A-63-274408 discloses a separator control apparatus which adjusts a valve in a gas outlet of a gas liquid separator by adding the outputs of a supersonic multiphase flowmeter in the inlet of the separator and of a manometer within the separator to exert a constant pressure in the separator.

EP-B-410522 discloses a method of preventing slug growth in a stream of multiphase fluid flowing from a flowline into a gas/liquid separator, whereby fluid rate control means are provided to manipulate the fluid flow rate.

This known method comprises measuring the liquid flow rate in the liquid outlet of the separator and the gas flow rate in the gas outlet thereof, determining the fluid flux defined as the sum of the liquid flow rate and the gas flow rate, and operating the fluid flow rate control means so as to reduce a variation of the fluid flux. A drawback of this known method and of the method disclosed in JP-A-63-274408 is that measurement of the sum of the liquid and gas flow rates in a multiphase fluid stream is difficult and requires complex measuring equipment.

US patent Nos. 5,256,171; 5,377,714; 5, 494,067; 5,544,672 and 5,711,338 disclose slug suppression methods in which the liquid level in a gas-liquid separation vessel is held as constant as possible. A disadvantage of these methods is that said liquid level is not always the best control parameter and that occasionally, e. g. during start-up still manual control or intervention is required.

The method according to the preamble of claim 1 is known from EP-B-767699.

This known method comprises the steps of: -measuring at least one control variable selected from the group of the liquid level in the separator, the liquid flow rate in the liquid outlet of the separator, the gas flow rate in the gas outlet of the separator, the sum of the liquid flow rate in the liquid outlet and the gas flow rate in the gas outlet, and the fluid pressure at or near the separator; and -dynamically adjusting liquid and gas flow control valves in the liquid and gas outlets so as to reduce a difference between one or more selected control variables and a selected reference value of the selected control variable (s).

Field experience with the method according to EP-B-767699 has learned that this known method significantly suppresses slug flow in a multi-phase fluid stream but that, due to the origin of the liquid slugs, some slugs were suppressed to a more limited extent than other slugs.

The method and system according to the present invention aims to provide a slug suppression and control method which even further controls slug flow than the known method and that can be operated automatically without requiring human intervention.

Summary of the Invention In the method and system according to the invention the control variable which is used to adjust the liquid and/or gas flow control valve is changed automatically from time to time if one or more control variables reaches a pre-set value.

Preferably during normal operation of the slug suppression system the control variable is the sum (QL+QG) of the liquid flow rate in the liquid outlet and the gas flow rate in the gas outlet and the control variable is changed into liquid flow rate (QL) in the liquid outlet of the liquid level LLIQ in the separator or the liquid flow rate (QL) in the liquid outlet reaches a pre-set value and wherein the control variables is changed back again into the sum (QL+QG) of the liquid flow rate in the liquid outlet and the gas flow rate in the gas outlet when said liquid level (LLIQ) in the separator or liquid flow rate (QL) in the liquid outlet is below said pre-set value.

The method and system according to the invention are able to suppress and control liquid slugs and gas surges in multiphase fluid streams automatically without human intervention. Typically during start up the liquid valve

is closed and the gas valve is operated such that the fluid pressure at or near the entrance of the separation remains fairly constant. When the liquid level in the separation has reached a pre-set level the liquid valve is opened automatically and dynamically adjusted to maintain the liquid level at said pre-set value whereas the gas valve is dynamically adjusted such that the mixture flow rate remains fairly constant. This hybrid constant liquid level/constant mixture flow rate control mode remains the default control mode until the liquid level in the separator and/or the liquid flow in the liquid outlet reaches a pre-set threshold value whereupon the system is automatically adjusted such that the gas valve is temporarily closed substantially and the liquid valve is dynamically adjusted such that the liquid flow rate in the liquid outlet remains fairly constant.

As soon as the liquid level in the separator and/or liquid flow rate in the liquid outlet is again below said pre-set threshold value the system is automatically switched back into the default hybrid constant liquid level/constant mixture flow rate control mode.

The gas/liquid separator described hereinbefore can have sufficient capacity for processing the fluid stream, in which case said separator can be the only separator in the system. Alternatively, the separator can form a mini- separator which is located upstream a slug catching separator. The mini-separator then forms a primary separator of which the gas outlet and the liquid outlet debouche into the slug catching separator which forms a secondary separator.

Description of a preferred embodiment The invention will now be described in more detail and by way of example with reference to the accompanying drawings in which:

Fig. 1 shows schematically a flowline system for carrying out the method according to the invention; Fig. 2 shows schematically how the system operates in its default mode; and Fig. 3 shows schematically how the system occasionally operates in its constant liquid flow and no gas flow mode.

The flowline system of Fig. 1 comprises an oil and/or gas production pipeline 1 extending on the seafloor 3 from a wellhead (not shown) of an oil and/or gas production well to an offshore platform 5, an upwardly inclined pipeline section in the form of a riser 7 connected to the platform 5 and a gas/liquid separator 9 having a liquid outlet conduit 13 and a gas outlet conduit 14. The liquid outlet conduit 13 is provided with a liquid flow control valve 15, and the gas outlet conduit is provided with a gas flow control valve 16. The valves 15,16 can be of any suitable type such as vortex amplifier disclosed in the Oil man, August 1987, pp. 82-85. A gas flow meter 17 is provided in the gas outlet conduit 14 and a liquid flow meter 19 is provided in the liquid outlet conduit 13. The separator 9 is further provided with a liquid level gauge 25 and a pressure gauge 27. A control system 30 is provided which receives signals from the gas flow meter 17, the liquid flow meter 19, the liquid level gauges 25, and the pressure gauge 27, which control system 30 controls the valves of 15,16 in a manner depending on the signals received by the control system 30. The liquid outlet conduit 13 and the gas outlet conduit are in fluid communication with the interior of a slug-catching separator (not shown) located downstream the separator 9.

The slug-catching separator is of a size considerably larger than the separator 9.

During start up of the system the liquid valve 15 is closed until the liquid level LLIQ in the separator 9 reaches a selected level, whereas the gas valve (16) is adjusted dynamically such that the pressure measured by the pressure gauge 27 is maintained at a selected level.

As soon as the liquid level LLIQ in the separator 9 has reached a selected level the system is automatically switched into the default total volumetric flow control mode shown in Fig. 2.

In the total volumetric flow control mode shown in Fig. 2 the liquid valve 15 is controlled to maintain a liquid level LLIQ set point. In addition, the gas valve 16 is controlled to maintain a total volumetric flow QLIQ + QGAS set-point. The actual flow rate QLIQ and QGAS are measured by the gas and liquid flow meters 17,19 in the liquid and gas outlets 14 and 13. The sum of the output of the flow meters 17 and 19, is the variable to be controlled. The set-point of the total volumetric flow controller 30B is given by a pressure controller 30A in combination with algorithms which depend on the pipeline system 1,7.

For slugs, the total volumetric flow control scheme shown in Fig. 1 will not work optionally due to the fact that the level controller 30C is not bounded by the total volumetric flow. It will open the liquid valve 15 completely to keep the liquid level on its set-point.

Therefore, the liquid flow control mode of operation shown in Fig. 3 is switched on when: (i) the liquid level LLIQ in the separator 9 reaches a threshold value or (ii) the liquid flow rate QL in the liquid outlet 13 reaches a threshold value. The threshold value of the liquid flow rate QL may depend on external factors such as the liquid level or liquid drainage capacity of the first stage separator. This threshold value can also be used as the

set-point of the liquid flow controller 30C. The set- point of the total volumetric flow remains constant during liquid flow control.

Liquid flow control prevents acceleration of the slug. When the liquid level in the mini-separator 9 is decreased and the liquid flow rate is below the threshold value, the total volumetric control mode shown in Fig. 2 is switched on again. In the total volumetric control mode, the compressed gas phase (behind the slug) can be produced without any gas surge.