Wells – Processes – Cleaning or unloading well
Reexamination Certificate
1999-09-17
2001-09-11
Schoeppel, Roger (Department: 3672)
Wells
Processes
Cleaning or unloading well
C166S345000, C166S370000, C166S372000, C166S375000
Reexamination Certificate
active
06286602
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a method for controlling a device for transporting hydrocarbons, in the form of a mixture of liquid and gas, between off-shore hydrocarbons production means and a unit for processing the said hydrocarbons.
STATE OF THE PRIOR ART
A conventional technique used for exploiting subsea deposits of hydrocarbons consists in pooling the hydrocarbons produced in liquid and gaseous form by several neighbouring wells and transporting them to a processing plant mounted on a floating support or on a support of the platform type, above sea level.
For this purpose, the underwater wellheads with which the production wells are equipped are connected to a single transport pipe which runs along the seabed between the wellheads and the support leg of the treatment plant and then rises, via a riser system, to above sea level, where it is connected, usually through an outlet choke, to the treatment plant.
Under certain operating conditions, the mixture of liquid and gas in the transport pipe is in the form of an alternating series of plugs of liquid and pockets of gas which result in substantial fluctuations in the pressures and flowrates of the two fluids, which fluctuations are incompatible with correct operation of the treatment plant and also disrupt the flow of hydrocarbons in the wells.
One method which aims to avoid the development of plugs of liquid in such a hydrocarbons transport device is described in document EP 0,410,552 A 2 of 30.01.91.
As the outlet choke is used as a means of controlling the flows of gas and of liquid in a downstream section of the transport pipe, this method consists in:
determining a flow of fluid, which is defined as the sum of the flows of gas and of liquid in the said section, and
adjusting the flow-control means so as to minimize the variations in flow of fluids in the said section.
According to this method, the flows of fluid are defined as being the instantaneous volumetric flowrates of the fluids.
This method is not suited to the control of a hydrocarbons transport device which comprises, in addition to the transport pipe, means of injecting gas upstream of the rising section of the said pipe, which allow gas to be injected in order to lighten the mixture of liquid and gas produced so as to make it easier to raise to the surface, because, in this case, the measurement of the flow of gas produced is rendered inaccurate by the flowrate of injected gas.
Furthermore, this method does not allow the transport device to be started up gradually with a minimum of pressure and flowrate surges, nor does it allow the device to be controlled during disrupted particular operations such as the passage of a scraping device, nor does it, in all phases of operation, prevent the formation of plugs of gas in the transport pipe accompanied by plugs of liquid hydrocarbons.
Nor can it be used for producing hydrocarbons at low cost at an established rate, that is to say by injecting a minimum amount of gas for a given amount of hydrocarbons produced.
DESCRIPTION OF INVENTION
The present invention is intended precisely to overcome these drawbacks by proposing a method for controlling a device for transporting hydrocarbons between production means and a treatment plant which makes it possible to prevent the formation of plugs and ensure the stability of the flowrate of hydrocarbons in disrupted situations, thus creating conditions that are favourable to the optimum control of the treatment plant.
Furthermore, by virtue of the invention, a maximum amount of produced hydrocarbons can be transported at the best cost.
To this end, the invention proposes a method for controlling a device for transporting liquid and gaseous hydrocarbons between production means and a treatment plant, which device comprises a pipe, for transporting the hydrocarbons, which has a lower section connected to the hydrocarbon production means and an upper end connected to the treatment plant through an adjustable-aperture outlet choke, the said method being characterized in that the said device additionally comprises a gas-injection pipe which has an upstream end connected to a source of pressurized gas through a control valve and a downstream end connected to the lower section of the hydrocarbons transport pipe and in that, when the outlet choke and the control valve are closed, it includes a start-up phase which consists in performing the following sequence of steps:
a step of initiating the transport of hydrocarbons which consists:
in comparing the pressure in the lower section of the hydrocarbons transport pipe with a predetermined threshold Pf1 and:
a) if this pressure is above the threshold Pf1, in gradually opening the outlet choke to a predetermined value to ensure that the hydrocarbons transported flow at a predetermined minimum flowrate, or
b) if this pressure is below the threshold Pf1, in injecting gas at a predetermined flowrate to encourage the hydrocarbons to flow through the transport pipe, and, when the difference between the pressures upstream and downstream of the outlet choke exceeds a predetermined threshold, in gradually opening the said choke (
9
) to a predetermined value to ensure that the hydrocarbons transported flow at a predetermined minimum flowrate,
in waiting for a predetermined length of time to allow the minimum hydrocarbon flowrate to become established;
a step of ramping up to transport speed, which consists in periodically performing the following operations:
determining a pressure instability factor F in the lower section of the pipe, and comparing the instability factor F with two predetermined thresholds F1 and F2, F2 being higher than F1, and:
a) if the instability factor F is below F1 and if a target transported-hydrocarbons flowrate has not been achieved, opening the outlet choke wider by a predetermined amount,
b) if the instability factor F is below F1 and if a target transported-hydrocarbons flowrate has been achieved, decreasing the flowrate of injected gas by a predetermined amount,
c) if the instability factor F is between F1 and F2 and if the injected-gas flowrate is zero, injecting a predetermined flow of gas to fill the gas-injection pipe as far as its downstream end,
d) if the instability factor F is above F2, increasing the gas flowrate by a predetermined amount to ensure that there is a continuous flow of gas in the lower section of the pipe and so as to increase the pressure difference available across the outlet choke,
repeating the above operations if at least one of the previous actions have been performed within a predetermined space of time.
According to another feature of the invention, the start-up phase is followed by a production phase which consists in ensuring production stability by performing the following monitoring operations:
determining at least one factor G which characterizes the start of an interruption in the circulation of the gaseous hydrocarbons in the lower section of the pipe,
comparing the said factor G with a predetermined threshold and:
if it exceeds the threshold, increasing the gas flowrate to a predetermined value and reducing the aperture of the outlet choke to a predetermined value,
otherwise, comparing the flowrate of the hydrocarbons produced with the target flowrate, and:
a) if it is below the target flowrate, increasing the flowrate of injected gas,
b) if it is above the target flowrate, decreasing the flowrate of injected gas,
if an action has been taken during the preceding monitoring operations, the production phase then consists in periodically performing the following stability-control operations:
determining a pressure instability factor S in the lower section of the pipe, and
comparing the instability factor S with two predetermined thresholds S1 and S2, S2 being higher than S1, and:
a) if the instability factor S is below S1 and if a target transported-hydrocarbons flowrate has not been achieved, opening the outlet choke wider by a predetermined amount,
b) if the instability factor S is below S1 and if a target transported-hydrocarbons flowrate has been achieved,
Lemetayer Pierre
Poublan Jacques
Blank Rome Comisky & McCauley LLP
Elf Exploration Production
Schoeppel Roger
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