MULTI-VARIABLE PREDICTIVE CONTROLLER

Abstract

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.

Description

CROSS-REFERENCE TO A RELATED APPLICATION

The present application for patent claims the benefit of United States provisional patent application bearing Ser. No. 61/716,357, filed on Oct. 19, 2012, which is incorporated by reference in its entirety.

TECHNICAL FIELD

The present invention relates to a multi-variable predictive controller, and more specifically, to a multi-variable predictive controller used to reduce condensable material in a gas product stream.

BACKGROUND

Production from offshore platforms, or rigs, generally includes a combination of crude oil and natural gas. Primary separation typically takes place on the offshore platform such that the wet crude oil is transported to shore via a first pipeline and the produced natural gas is dehydrated, compressed and transported to shore via a second pipeline. As used herein, the term offshore platform comprises any platform structure, affixed temporarily or permanently to offshore submerged lands, that houses equipment to extract hydrocarbons from the ocean or lake floor and that processes and/or transfers such hydrocarbons to storage, transport vessels, or onshore. In addition, offshore production can include secondary platform structures, storage tanks associated with the platform structure, and floating production and storage offloading equipment (FPSO).

Once separated from the wet crude oil, the produced natural gas being exported from the offshore platform in the gas product stream typically contains amounts of hydrocarbon components in a liquid state, which are referred to herein as hydrocarbon condensate. The hydrocarbon condensate, which comprises butanes, pentanes, and heavier molecules, increase the British Thermal Unit (BTU) value of the gas entering the offshore facility's compressors, thereby reducing the efficiency of the compressors. Moreover, additional equipment is needed at the onshore gas processing plant for separation and stabilization to extract condensate from the gas product stream. Accordingly, a better method is needed to reduce condensable material in the gas product stream prior to being exported from the offshore platform.

SUMMARY

Embodiments disclosed herein relate to a method for separating a multi-phase fluid on an offshore platform. A separation vessel containing a multi-phase fluid is provided. A pressure and a temperature associated with the separation vessel are measured. A Reid Vapor Pressure for the separation vessel is calculated based on the pressure and the temperature associated with the separation vessel. The pressure and the temperature associated with the separation vessel are accurately controlled such that the calculated Reid Vapor Pressure for the separation vessel is maintained within a predetermined amount of a reference Reid Vapor Pressure.

Embodiments disclosed herein relate to a system for separating a multi-phase fluid on an offshore platform. The system includes a separation vessel, a pressure sensor, a temperature sensor, and a multi-variable predictive controller. The separation vessel receives a multi-phase fluid (e.g., produced fluids). The pressure sensor monitors pressure associated with the separation vessel and the temperature sensor that monitors 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 a calculated Reid Vapor Pressure for the separation vessel is maintained within a predetermined amount of a reference Reid Vapor Pressure.

In embodiments, the pressure and the temperature associated with the separation vessel are adjusted simultaneously based on the calculated Reid Vapor Pressure. The Reid Vapor Pressure can be calculated according to the thermodynamics of butane. For example, the Reid Vapor Pressure can be calculated according to the following equations:

RVP=10^{[A−B/(100+C)]}+Bias

A=Log P₁+B/(T₁+C)

B=(Log P₁−Log P_ref)/[1/(T_ref+C)−1/(T₁+C)]

In embodiments, the calculated Reid Vapor Pressure for the separation vessel is maintained within ten percent of the reference Reid Vapor Pressure. In other embodiments, the calculated Reid Vapor Pressure for the separation vessel is maintained within five percent of the reference Reid Vapor Pressure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of a multi-variable predictive controller, according to an embodiment of the present invention.

FIG. 2 is a schematic of a multi-variable predictive controller, according to an embodiment of the present invention.

FIG. 3 illustrates a graph of Reid Vapor Pressure using a regulatory Single Input Single Output (SISO) control whose set points are given by a human operator and of Reid Vapor Pressure controlled using an automated multi-variable predictive controller.

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. 1 is a schematic of system 100 used for reducing condensable material in a gas product stream. The hydrocarbon product stream 101 is input into dry oil tank 103. The temperature and pressure within the dry oil tank are respectively monitored via temperature sensor 105 and pressure sensor 107. Temperature sensor 105 and pressure sensor 107 can be positioned within dry oil tank 103, or in close proximity to dry oil tank 103, such that temperature sensor 105 and pressure sensor 107 can 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 tank 103. In some embodiments, temperature and pressure are measured by a single unit (i.e., both temperature and pressure are measured by sensor 107).

The measured temperature and pressure associated with dry oil tank 103 are sent to indicator 109, which computes the Reid Vapor Pressure for dry oil tank 103. In embodiments, the Reid Vapor Pressure is computed by indicator 109 with 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:

RVP=10^{[A−B/(100+C)]}+Bias

A=Log P₁+B/(T₁+C)

B=(Log P₁−Log P_ref)/[1/(T_ref+C)−1/(T₁+C)]

Indicator 109 communicates the calculated Reid Vapor Pressure to multi-variable predictive controller 111. While indicator 109 and multi-variable predictive controller 111 are illustrated in FIG. 1 as two separate components, one skilled in the art will appreciate that Reid Vapor Pressure calculations can alternatively be computed directly by multi-variable predictive controller 111. In other embodiments, both indicator 109 and multi-variable predictive controller 111 compute Reid Vapor Pressure calculations. The multi-variable predictive controller 111 operates both the temperature and pressure control associated with the dry oil tank 103 to maintain a desired Reid Vapor Pressure specification. In embodiments, both the temperature and pressure associated with the dry oil tank 103 are controlled simultaneously. In embodiments, the RVP within dry oil tank 103 is maintained within 10% of the target RVP. In embodiments, the RVP within dry oil tank 103 is maintained within 5% of the target RVP.

In embodiments, a multi-variable predictive controller 111, 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 controller 111. 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 valve 113, which controls the flow of gas effluent out of dry oil tank 103. In particular, multi-variable predictive controller 111 communicates a control signal to pressure controller 115 to adjust the position (e.g., open, close, or position somewhere in between) of valve 113 to increase or decrease the flow of gas effluent out of dry oil tank 103 based on the calculated RVP by indicator 109. The position of valve 113 is adjusted in real-time to maintain the pressure in the dry oil tank 103 within a desired range of the target RVP. While valve 113 and pressure controller 115 are illustrated in FIG. 1 as two separate components, one skilled in the art will appreciate that valve 113 and temperature controller 115 can be combined into a single assembly.

The dry oil tank temperature is controlled by adjusting valve 117, which controls the flow of temperature of hydrocarbon product stream 101 entering the dry oil tank 103. In particular, multi-variable predictive controller 111 communicates a control signal to temperature controller 119 to adjust the position (e.g., open, close, or position somewhere in between) of valve 117 to increase or decrease the flow of fluid flowing through heat exchanger 121, thereby exchanging heat with hydrocarbon product stream 101. In embodiments, heat exchanger 121 can act as a cooler or chiller to hydrocarbon product stream 101 (i.e., reduce the temperature). For example, if the temperature of hydrocarbon product stream 101 is above the desired temperature (based on the calculated RVP by indicator 109) and heat exchanger 121 is a cooler to hydrocarbon product stream 101, multi-variable predictive controller 111 can communicate a control signal to temperature controller 119 to further open valve 117, thereby increasing the flow of fluid through heat exchanger 121 to reduce the temperature of hydrocarbon product stream 101 entering dry oil tank 103. Likewise, if the temperature of hydrocarbon product stream 101 is below the desired temperature (based on the calculated RVP by indicator 109), multi-variable predictive controller 111 can communicate a control signal to temperature controller 119 to further close valve 117, thereby reducing the flow of coolant through cooler 121 to increase the temperature of hydrocarbon product stream 101 entering the dry oil tank 103. In other embodiments, heat exchanger 121 can act as a heater to hydrocarbon product stream 101 (i.e., increase the temperature). In this case, valve 117 is opened and closed in the opposite manner to if heat exchanger 121 acts as a cooler or chiller to hydrocarbon product stream 101. Therefore, the position of valve 117 is adjusted in real-time to maintain the temperature in the dry oil tank 103 within a desired range of the target RVP. While valve 117 and temperature controller 119 are illustrated in FIG. 1 as two separate components, one skilled in the art will appreciate that valve 117 and temperature controller 119 can be combined into a single assembly.

System 100 can 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 line 123 out of dry oil tank 103) or oil export stream (through oil effluent line 125 out of dry oil tank 103).

FIG. 2 is a schematic of system 200 used for reducing condensable material in a gas product stream. The hydrocarbon product stream 201 is input into dry oil tank 203. The pressure within the dry oil tank is monitored via pressure sensor 207. The temperature within the dry oil tank is monitored indirectly by monitoring the temperature of a fluid in communication with hydrocarbon product stream 201. The measured temperature and pressure associated with dry oil tank 103 are sent to indicator 209, which computes the Reid Vapor Pressure (similar to system 100) for dry oil tank 203. Indicator 209 communicates the calculated Reid Vapor Pressure to multi-variable predictive controller 211. Again, computation of Reid Vapor Pressure could alternatively be computed directly by multi-variable predictive controller 211. The multi-variable predictive controller 211 operates both the temperature and pressure control associated with the dry oil tank 203 to maintain a desired Reid Vapor Pressure specification. In embodiments, both the temperature and pressure associated with the dry oil tank 203 are controlled simultaneously.

The dry oil tank pressure is controlled by adjusting valve 213, which controls the flow of gas effluent out of dry oil tank 203. In particular, multi-variable predictive controller 211 communicates a control signal to pressure controller 215 to adjust the position (e.g., open, close, or position somewhere in between) of valve 213 to increase or decrease the flow of gas effluent out of dry oil tank 203 based on the calculated RVP by indicator 209. The position of valve 213 is adjusted in real-time to maintain the pressure in the dry oil tank 203 within a desired range of the target RVP. In the embodiment shown in FIG. 2, gas stream flows through gas effluent line 229 out of dry oil tank 203 to vapor recovery unit 216 where the gas stream is further separated. Gas is fed into a compressor 217 where it is compressed and delivered to a sales gas pipeline. Any condensate from vapor recovery unit 216 can be delivered to a sales oil pipeline (e.g., oil export stream through oil effluent line 231), back into hydrocarbon product stream 201, or into other processing equipment. Liquid level sensor 219 can monitor the level of liquid within vapor recovery unit 216 and open valve 221 as necessary to purge vapor recovery unit 216 of condensate.

The dry oil tank temperature is controlled by adjusting valve 223, which controls the flow of temperature of hydrocarbon product stream 201 entering the dry oil tank 203. In particular, multi-variable predictive controller 211 communicates a control signal to temperature controller 225 to adjust the position (e.g., open, close, or position somewhere in between) of valve 223 to increase or decrease the flow of fluid flowing through heat exchanger 227, thereby exchanging heat with hydrocarbon product stream 201. In embodiments, heat exchanger 227 can act as a cooler or chiller to hydrocarbon product stream 201 (i.e., reduce the temperature) or act as a heater to hydrocarbon product stream 201 (i.e., increase the temperature). In this embodiment, the temperature of the fluid communicating with the hydrocarbon product stream 201 in heat exchanger 227 is monitored downstream of heat exchanger 227. Based on any temperature change to this fluid, multi-variable predictive controller 211 adjusts valve 223 to increase or decrease heat exchange from hydrocarbon product stream 201, thereby adjusting the temperature of the hydrocarbon product stream 201 entering the dry oil tank 203.

System 200 can 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 line 229 out of dry oil tank 203) or oil export stream (through oil effluent line 231 out of dry oil tank 203).

The system illustrated in FIG. 2 was tested on a deepwater oil production platform in the Gulf of Mexico. FIG. 3 illustrates a graph of the resultant RVP improvement. Using univariate regulatory control 305, 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 controller 310 resulted 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.

Claims

1. A method for separating a multi-phase fluid on an offshore platform, the method comprising:

(a) providing a separation vessel containing a multi-phase fluid;

(b) measuring a pressure and a temperature associated with the separation vessel;

(c) calculating a Reid Vapor Pressure for the separation vessel based on the pressure and the temperature associated with the separation vessel; and

(d) actively controlling 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.

2. The method of claim 1, wherein the pressure and the temperature associated with the separation vessel are adjusted simultaneously based on the calculated Reid Vapor Pressure.

3. The method of claim 1, wherein the calculated Reid Vapor Pressure for the separation vessel is maintained within ten percent of the reference Reid Vapor Pressure.

4. The method of claim 1, wherein the calculated Reid Vapor Pressure for the separation vessel is maintained within five percent of the reference Reid Vapor Pressure.

5. The method of claim 1, wherein the temperature associated with the separation vessel is actively controlled by a heat exchanger upstream of the separation vessel.

6. The method of claim 1, wherein the Reid Vapor Pressure is calculated according to the following equations:

RVP=10[A−B/(100+C)]+Bias

A=Log P1+B/(T1+C)

B=(Log P1−Log Pref)/[1/(Tref+C)−1/(T1+C)].

7. The method of claim 1, wherein the Reid Vapor Pressure is calculated according to the thermodynamics of butane.

8. The method of claim 1, further comprising generating an empirical dynamic process model of the offshore platform and utilizing the empirical dynamic process model to determine adjustments made to the pressure and the temperature associated with the separation vessel.

9. A system for separating a multi-phase fluid on an offshore platform, the system comprising:

(a) a separation vessel that receives a multi-phase fluid;

(b) a pressure sensor that monitors pressure associated with the separation vessel;

(c) a temperature sensor that monitors temperature associated with the separation vessel; and

(d) a multi-variable predictive controller that actively controls the pressure and the temperature associated with the separation vessel such that a calculated Reid Vapor Pressure for the separation vessel is maintained within a predetermined amount of a reference Reid Vapor Pressure.

10. The system of claim 9, further comprising an indicator to calculate the Reid Vapor Pressure for the separation vessel based on the pressure monitored by the pressure sensor and the temperature monitored by the temperature sensor.

11. The system of claim 9, wherein the multi-variable predictive controller maintains the calculated Reid Vapor Pressure for the separation vessel within ten percent of the reference Reid Vapor Pressure.

12. The system of claim 9, wherein the multi-variable predictive controller maintains the calculated Reid Vapor Pressure for the separation vessel within five percent of the reference Reid Vapor Pressure.

13. The system of claim 9, wherein the multi-variable predictive controller further calculates the Reid Vapor Pressure for the separation vessel based on the pressure monitored by the pressure sensor and the temperature monitored by the temperature sensor.

14. The system of claim 9, wherein the Reid Vapor Pressure is calculated according to the following equations:

RVP=10[A−B/(100+C)]+Bias

A=Log P1+B/(T1+C)

B=(Log P1−Log Pref)/[1/(Tref+C)−1/(T1+C)]

15. The system of claim 9, wherein the Reid Vapor Pressure is calculated according to the thermodynamics of butane.

16. The system of claim 9, wherein the pressure and the temperature associated with the separation vessel are adjusted simultaneously based on the calculated Reid Vapor Pressure.

17. The system of claim 9, further comprising a heat exchanger upstream of the separation vessel that is used to adjust the temperature associated with the separation vessel.

18. The system of claim 9, wherein the pressure and the temperature associated with a separation vessel are adjusted simultaneously based on the calculated Reid Vapor Pressure.

19. The system of claim 9, wherein the multi-variable predictive controller is further used to generate an empirical dynamic process model of the offshore platform to determine adjustments made to the pressure and the temperature associated with the separation vessel.