A multi-variable predictive controller is used to separate a multi-phase fluid on an offshore platform, thereby reducing condensable material in the gas product stream. A separation vessel containing a multi-phase fluid is provided, and pressure and temperature associated with the separation vessel are monitored. A Reid Vapor Pressure is calculated for the separation vessel based on the pressure and the temperature associated with the separation vessel. The multi-variable predictive controller actively controls the pressure and the temperature associated with the separation vessel such that the calculated Reid Vapor Pressure for the separation vessel is maintained within a predetermined amount of a reference Reid Vapor Pressure.

DETAILED DESCRIPTION

Aspects of the present invention describe a system and method for reducing condensable material in a gas product stream. As will be described, a multi-variable predictive controller is utilized to achieve better separation of oil and gas material. In particular, the multi-variable predictive controller utilizes Reid Vapor Pressure (RVP) identification of the butane molecules in the Dry Oil Tank of the offshore facility and controls two or more manipulated variables to ensure each controlled variable is tightly controlled to an economically advantageous point.

FIG. 1is a schematic of system100used for reducing condensable material in a gas product stream. The hydrocarbon product stream101is input into dry oil tank103. The temperature and pressure within the dry oil tank are respectively monitored via temperature sensor105and pressure sensor107. Temperature sensor105and pressure sensor107can be positioned within dry oil tank103, or in close proximity to dry oil tank103, such that temperature sensor105and pressure sensor107can accurately measure the temperature and pressure conditions (i.e., measure within specifications set by the International Organization for Standardization (ISO) for temperature and pressure sensors) within dry oil tank103. In some embodiments, temperature and pressure are measured by a single unit (i.e., both temperature and pressure are measured by sensor107).

The measured temperature and pressure associated with dry oil tank103are sent to indicator109, which computes the Reid Vapor Pressure for dry oil tank103. In embodiments, the Reid Vapor Pressure is computed by indicator109with respect to the thermodynamics of butane, which allows operations to know what amount of butane, and heavier molecules, are in the oil and what amount are in the gas. For example, Reid Vapor Pressure can be computed according to the following equations:

Indicator109communicates the calculated Reid Vapor Pressure to multi-variable predictive controller111. While indicator109and multi-variable predictive controller111are illustrated inFIG. 1as two separate components, one skilled in the art will appreciate that Reid Vapor Pressure calculations can alternatively be computed directly by multi-variable predictive controller111. In other embodiments, both indicator109and multi-variable predictive controller111compute Reid Vapor Pressure calculations. The multi-variable predictive controller111operates both the temperature and pressure control associated with the dry oil tank103to maintain a desired Reid Vapor Pressure specification. In embodiments, both the temperature and pressure associated with the dry oil tank103are controlled simultaneously. In embodiments, the RVP within dry oil tank103is maintained within 10% of the target RVP. In embodiments, the RVP within dry oil tank103is maintained within 5% of the target RVP.

In embodiments, a multi-variable predictive controller111, such as AspenTech DMCplus®, is used to collect empirical process information by means of a design of experiment (DOE), also referred to as a “step test” in control engineering, on an offshore oil and gas production facility. This empirical data can then be used to build a dynamic process model of the facility's behavior using the multi-variable predictive controller111. The dynamic process model can then be commissioned to operate inside the facility's process control network. As previously described, the RVP calculation continuously reads the dry oil tank pressure and temperature. A move plan is calculated for the dry oil tank outlet pressure and input temperature and predictions of RVP values are generated based on the facility's current state. Using the RVP predictions and the economic value of making the best changes, the multi-variable predictive controller carries out set point changes to the aforementioned temperature and pressure controllers to keep the RVP under tight, economic control.

The dry oil tank pressure is controlled by adjusting valve113, which controls the flow of gas effluent out of dry oil tank103. In particular, multi-variable predictive controller111communicates a control signal to pressure controller115to adjust the position (e.g., open, close, or position somewhere in between) of valve113to increase or decrease the flow of gas effluent out of dry oil tank103based on the calculated RVP by indicator109. The position of valve113is adjusted in real-time to maintain the pressure in the dry oil tank103within a desired range of the target RVP. While valve113and pressure controller115are illustrated inFIG. 1as two separate components, one skilled in the art will appreciate that valve113and temperature controller115can be combined into a single assembly.

The dry oil tank temperature is controlled by adjusting valve117, which controls the flow of temperature of hydrocarbon product stream101entering the dry oil tank103. In particular, multi-variable predictive controller111communicates a control signal to temperature controller119to adjust the position (e.g., open, close, or position somewhere in between) of valve117to increase or decrease the flow of fluid flowing through heat exchanger121, thereby exchanging heat with hydrocarbon product stream101. In embodiments, heat exchanger121can act as a cooler or chiller to hydrocarbon product stream101(i.e., reduce the temperature). For example, if the temperature of hydrocarbon product stream101is above the desired temperature (based on the calculated RVP by indicator109) and heat exchanger121is a cooler to hydrocarbon product stream101, multi-variable predictive controller111can communicate a control signal to temperature controller119to further open valve117, thereby increasing the flow of fluid through heat exchanger121to reduce the temperature of hydrocarbon product stream101entering dry oil tank103. Likewise, if the temperature of hydrocarbon product stream101is below the desired temperature (based on the calculated RVP by indicator109), multi-variable predictive controller111can communicate a control signal to temperature controller119to further close valve117, thereby reducing the flow of coolant through cooler121to increase the temperature of hydrocarbon product stream101entering the dry oil tank103. In other embodiments, heat exchanger121can act as a heater to hydrocarbon product stream101(i.e., increase the temperature). In this case, valve117is opened and closed in the opposite manner to if heat exchanger121acts as a cooler or chiller to hydrocarbon product stream101. Therefore, the position of valve117is adjusted in real-time to maintain the temperature in the dry oil tank103within a desired range of the target RVP. While valve117and temperature controller119are illustrated inFIG. 1as two separate components, one skilled in the art will appreciate that valve117and temperature controller119can be combined into a single assembly.

System100can be implemented on any upstream facility where both temperature and pressure of a separation vessel can be controlled simultaneously. A predictable, well controlled, RVP parameter will provide a very detailed separation at the molecular level of hydrocarbons for correct placement in either the gas export stream (through gas effluent line123out of dry oil tank103) or oil export stream (through oil effluent line125out of dry oil tank103).

FIG. 2is a schematic of system200used for reducing condensable material in a gas product stream. The hydrocarbon product stream201is input into dry oil tank203. The pressure within the dry oil tank is monitored via pressure sensor207. The temperature within the dry oil tank is monitored indirectly by monitoring the temperature of a fluid in communication with hydrocarbon product stream201. The measured temperature and pressure associated with dry oil tank103are sent to indicator209, which computes the Reid Vapor Pressure (similar to system100) for dry oil tank203. Indicator209communicates the calculated Reid Vapor Pressure to multi-variable predictive controller211. Again, computation of Reid Vapor Pressure could alternatively be computed directly by multi-variable predictive controller211. The multi-variable predictive controller211operates both the temperature and pressure control associated with the dry oil tank203to maintain a desired Reid Vapor Pressure specification. In embodiments, both the temperature and pressure associated with the dry oil tank203are controlled simultaneously.

The dry oil tank pressure is controlled by adjusting valve213, which controls the flow of gas effluent out of dry oil tank203. In particular, multi-variable predictive controller211communicates a control signal to pressure controller215to adjust the position (e.g., open, close, or position somewhere in between) of valve213to increase or decrease the flow of gas effluent out of dry oil tank203based on the calculated RVP by indicator209. The position of valve213is adjusted in real-time to maintain the pressure in the dry oil tank203within a desired range of the target RVP. In the embodiment shown inFIG. 2, gas stream flows through gas effluent line229out of dry oil tank203to vapor recovery unit216where the gas stream is further separated. Gas is fed into a compressor217where it is compressed and delivered to a sales gas pipeline. Any condensate from vapor recovery unit216can be delivered to a sales oil pipeline (e.g., oil export stream through oil effluent line231), back into hydrocarbon product stream201, or into other processing equipment. Liquid level sensor219can monitor the level of liquid within vapor recovery unit216and open valve221as necessary to purge vapor recovery unit216of condensate.

The dry oil tank temperature is controlled by adjusting valve223, which controls the flow of temperature of hydrocarbon product stream201entering the dry oil tank203. In particular, multi-variable predictive controller211communicates a control signal to temperature controller225to adjust the position (e.g., open, close, or position somewhere in between) of valve223to increase or decrease the flow of fluid flowing through heat exchanger227, thereby exchanging heat with hydrocarbon product stream201. In embodiments, heat exchanger227can act as a cooler or chiller to hydrocarbon product stream201(i.e., reduce the temperature) or act as a heater to hydrocarbon product stream201(i.e., increase the temperature). In this embodiment, the temperature of the fluid communicating with the hydrocarbon product stream201in heat exchanger227is monitored downstream of heat exchanger227. Based on any temperature change to this fluid, multi-variable predictive controller211adjusts valve223to increase or decrease heat exchange from hydrocarbon product stream201, thereby adjusting the temperature of the hydrocarbon product stream201entering the dry oil tank203.

System200can be implemented on any upstream facility where both temperature and pressure of a separation vessel can be controlled simultaneously. A predictable, well controlled, RVP parameter will provide a very detailed separation at the molecular level of hydrocarbons for correct placement in either the gas export stream (through gas effluent line229out of dry oil tank203) or oil export stream (through oil effluent line231out of dry oil tank203).

The system illustrated inFIG. 2was tested on a deepwater oil production platform in the Gulf of Mexico.FIG. 3illustrates a graph of the resultant RVP improvement. Using univariate regulatory control305, which only controls pressure and temperature independently of each other by the human operator, resulted in a mean value of 5.48 PSIG with a standard deviation of 0.13 PSIG. Using the multi-variable predictive controller310resulted in a significantly higher mean value of 9.64 PSIG with a standard deviation of 0.02 PSIG. Accordingly, the multi-variable predictive controller optimized the control process, thereby keeping the RVP under tight, economic control.

As used in this specification and the following claims, the terms “comprise” (as well as forms, derivatives, or variations thereof, such as “comprising” and “comprises”) and “include” (as well as forms, derivatives, or variations thereof, such as “including” and “includes”) are inclusive (i.e., open-ended) and do not exclude additional elements or steps. Accordingly, these terms are intended to not only cover the recited element(s) or step(s), but may also include other elements or steps not expressly recited. Furthermore, as used herein, the use of the terms “a” or “an” when used in conjunction with an element may mean “one,” but it is also consistent with the meaning of “one or more,” “at least one,” and “one or more than one.” Therefore, an element preceded by “a” or “an” does not, without more constraints, preclude the existence of additional identical elements.

The use of the term “about” applies to all numeric values, whether or not explicitly indicated. This term generally refers to a range of numbers that one of ordinary skill in the art would consider as a reasonable amount of deviation to the recited numeric values (i.e., having the equivalent function or result). For example, this term can be construed as including a deviation of ±10 percent of the given numeric value provided such a deviation does not alter the end function or result of the value. Therefore, a value of about 1% can be construed to be a range from 0.9% to 1.1%.

While in the foregoing specification this invention has been described in relation to certain preferred embodiments thereof, and many details have been set forth for the purpose of illustration, it will be apparent to those skilled in the art that the invention is susceptible to alteration and that certain other details described herein can vary considerably without departing from the basic principles of the invention. For example, the invention can be implemented in numerous ways, including for example as a method (including a computer-implemented method), a system (including a computer processing system), an apparatus, a computer readable medium, a computer program product, a graphical user interface, a web portal, or a data structure tangibly fixed in a computer readable memory.