Pipe control method for transporting liquid and gaseous hydrocarbons between offshore production plant and treatment plant involves controlling an outlet choke and control valve of gas injection pipe in start up and then production phase

摘要

The method uses a hydrocarbons transport pipe (6) that is fitted with an adjustable-aperture outlet choke (9) to which a gas injection pipe (16) fitted with a control valve (15) is connected. A start up phase involves initiating the transport of hydrocarbons and ramping up the transport speed, after which a production phase is performed where the outlet choke and control valve are operated so as to maintain the stability of the pressure in the pipe. The method uses a pipe which has a lower section connected to the hydrocarbon production system (1) and an upper end connected to the treatment plant (10) through an adjustable-aperture outlet choke, with 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. When the outlet choke and the control valve are both closed, the method involves a start up step which involves comparing the lower pipe section pressure with a predetermined threshold and if the pressure is above the threshold then the outlet choke is gradually opened to a predetermined value to ensure that the hydrocarbons transported flow at a minimum flow rate. If the pressure is below the predetermined threshold then gas is injected at a predetermined flow rate to encourage the hydrocarbons to flow through the pipe and, when the difference between the pressures upstream and downstream of the outlet choke exceeds a predetermined threshold the choke is gradually opened to a predetermined value to ensure that the hydrocarbons are transported at a predetermined minimum flow rate. A predetermined time period is then left to allow a minimum hydrocarbon flow rate to become established after which the transport speed is ramped up through a process which involves determining the pressure instability factor in the lower section of pipe and comparing the instability factor with two predetermined thresholds S1 and S2, with S2 being higher than S1. If the instability factor is below S1 and if a target transported-hydrocarbons flow rate has not been achieved, then the outlet choke is opened wider by a predetermined amount, while if the instability factor is below S1 and the target flow rate has been achieved then the flow rate of the injected gas is reduced by a predetermined amount. If the instability factor is between S1 and S2 and if the injected gas flow rate is zero then a predetermined flow of gas is injected to fill the gas injection pipe as far as its downstream end while if the instability factor is above S2 then the gas flow rate is increased by a predetermined amount to ensure that there is a continuous flow of gas in the lower section of the pipe to increase the pressure difference available across the outlet choke. This operation is repeated if at least one of the previous actions has been performed within a predetermined space of time. Preferred Features: The start up phase is followed by a production phase which consists of ensuring production stability by determining at least one factor which characterizes the start of an interruption in the circulation of the gaseous hydrocarbon in the lower pipe section and comparing the factor with a predetermined threshold. If the factor exceeds the threshold then the flow rate is increased to a predetermined level and the aperture of the outlet choke is reduced to a predetermined value while otherwise the flow rate of the hydrocarbons is compared with the target flow rate and the flow rate of injected gas is increased or decreased if the hydrocarbon flow rate is below or above the target flow rate, respectively. If an action has been taken during the preceding monitoring operations then the production phase consists of periodically performing a stability control operation that involves determining a pressure instability factor in the lower section of the pipe which is compared with predetermined thresholds S1 and S2, with S2 being greater than S1, and if the factor is below S1 and the target flow rate has not been achieved then the outlet choke is opened by a predetermined amount. If the instability factor is below S1 and the target flow rate has been achieved then the flow rate of injected gas is decreased by a predetermined amount and if the instability factor is above S2 then the injected gas flow rate is increased 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. This operation is repeated if at least one of the previous actions have been performed within a predetermined space of time and the previous monitoring operations are resumed. The instability factor is calculated from the pressure in the lower pipe section or from the pressure difference between the lower section of the pipe and the pressure upstream of the outlet choke or the outlet from the hydrocarbon production system to which the pipe is connected. The factor which characterizes the start of an interruption in the flow of gaseous hydrocarbons is calculated from the pressure in the lower section of the pipe or the difference between the lower pipe section pressure and the pressure at the outlet of the hydrocarbon production system. The pressure in the lower section is measured using a sensor and the control method includes a preliminary step that consists of opening the valve which controls the flow rate of injected gas so as to obtain an injected gas flow rate Q1 for a predetermined length of time, permanently maintaining the injected gas flow rate at a value at least equal to Q1 and calculating the pressure in the lower section of the hydrocarbons transport pipe from the pressure downstream of the control valve and from the injected gas flow rate.