Slug mitigation by increasing available surge capacity

A system includes a controller and an apparatus that includes a housing having a volume and an inlet. The inlet can receive a fluid that includes a gas and a liquid. The apparatus also includes a baffle that partitions the volume into a first portion and a second portion. The baffle extends from a base of the housing. The first portion can receive the liquid and separate the liquid into a first part and a second part. The second portion can receive the second part of the liquid from the first portion. The controller regulates an amount of the second part of the liquid in the second portion such that a level of the second part of the liquid is higher than a height that the baffle extends from the base of the housing, thus enabling the use of the extra surge capacity in the vessel.

TECHNICAL FIELD

This disclosure relates generally to process control systems and more specifically to slug mitigation by increasing available surge capacity.

BACKGROUND

Processing facilities, such as manufacturing plants, chemical plants and oil refineries, typically are managed using process control systems. Valves, pumps, motors, heating/cooling devices, and other industrial equipment typically perform actions needed to process materials in the processing facilities. Among other functions, the process control systems often control industrial operation in the processing facilities.

A very common problem encountered in many industries, such as the oil production and petrochemical industry, is compensating for variations in the flow rate of fluids (liquid and/or gases) coming into a particular processing unit. Such disturbances are usually common and ordinary events in the routine operation of the process.

As an example, in oil production processes, production from oil wells includes a mixture of oil, water and gas that flows from the wells to surface vessels, such as separators. The separation of phases can cause an undesirable operating condition when production flow becomes discontinuous with periods of large volume of liquid phase followed by periods of predominately gaseous phase. This phenomenon, referred to as “slugging,” causes the liquid flow into the separator to swing significantly. Slugging can lead to unstable operation of the process equipment downstream from the separators, which can shut down the production platform and result in a significant economic loss to the oil producing company.

Microprocessor-based proportional, integral and derivative (PID) controllers are commonly used for level control to reduce variations in the flow supplied to a downstream process. However, PID algorithms run by PID controllers often have two significant limitations. First, PID algorithms are typically unable to address non-linearities. Second, PID algorithms often cannot be used to specify high and low limits for liquid levels explicitly. Moreover, if the inlet flow has a large noise component, such as due to an upstream process that is noisy, control using a PID algorithm becomes increasingly ineffective.

SUMMARY

This disclosure provides a system and method for slug mitigation.

In a first embodiment, an apparatus includes a housing having a volume and an inlet. The inlet is configured to receive a fluid that includes a gas and a liquid. The apparatus also includes a baffle configured to partition the volume into a first portion and a second portion. The baffle extends from a base of the housing. The first portion is configured to receive the liquid and separate the liquid into a first part and a second part. The second portion is configured to receive the second part of the liquid from the first portion. The apparatus further includes a controller configured to regulate an amount of the second part of the liquid in the second portion such that a level of the second part of the liquid is higher than a height that the baffle extends from the base of the housing.

In a second embodiment, a method includes receiving a fluid from an upstream process at a separator. The separator includes a baffle that separates a first portion and a second portion. The method also includes separating the fluid into two or more parts and storing at least a portion of the first part of the fluid in the second portion of the separator. The method further includes maintaining a level of the first part of the fluid stored in the second portion in a range above the baffle.

In a third embodiment, a system includes a central processing unit and a computer readable medium electronically coupled to the central processing unit. The computer readable medium includes a control program that uses a vessel with a baffle that separates the vessel into at least two portions. The control program causes the central processing unit to regulate a level of a first part of a fluid stored in at least one portion of the vessel in a range above the baffle.

DETAILED DESCRIPTION

FIGS. 1 through 5, discussed below, and the various embodiments used to describe the principles of the present invention in this patent document are by way of illustration only and should not be construed in any way to limit the scope of the invention. Those skilled in the art will understand that the principles of the invention may be implemented in any type of suitably arranged device or system. Also, it will be understood that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some elements in the figures may be exaggerated relative to other elements to help improve the understanding of various embodiments described in this patent document.

FIG. 1illustrates an example system100for processing a fluid through a separator according to this disclosure. The embodiment of the system100shown inFIG. 1is for illustration only. Other systems could be used without departing from the scope of this disclosure.

In this example, the system100includes one or more upstream processes105that provide an outlet flow of at least one fluid. The fluid includes a gaseous portion and a liquid portion. The liquid portion of the fluid may fluctuate significantly and therefore be “noisy.” The outlet flow from the upstream process105is coupled via one or more vessels110to an inlet flow of one or more downstream processes115. The downstream process115can be, for example, a separator vessel having equipment such as valves, pumps and motors. The downstream process115can also be a distillation tower or a furnace.

Each vessel110may be coupled to a non-linear level controller (NLLC)120. The NLLC120can be a non-linear level controller as described in U.S. patent application Ser. No. 11/859,432, which is hereby incorporated by reference in its entirety. In this example, the NLLC120includes a central processing unit (CPU)125, as well as associated memory. The memory can be any computer readable medium, for example, the memory can be any electronic, magnetic, electromagnetic, optical, electro-optical, electro-mechanical, and/or other physical device that can contain, store, communicate, propagate, or transmit a computer program, software, firmware, or data for use by the microprocessor or other computer-related system or method. The NLLC120stores or receives a plurality of inputs, such as a setpoint (SP), a high level (HL), a low level (LL), and tuning constants (T1and T2), as well as current level (inventory) data from the vessel110. The NLLC120also executes a control algorithm and adjusts system flows, including a flow of the fluid to the downstream process115via a flow controller (FC)130, which adjusts the outlet flow from the vessel110together with a control valve135.

FIG. 2illustrates an example vessel110according to this disclosure. The embodiment of the vessel110shown inFIG. 2is for illustration only. Other vessels could be used without departing from the scope of this disclosure.

In this example, the vessel110includes a liquid shown reaching a current level205. The vessel110is associated with several parameters such as a low level limit210, a setpoint215, and a high level limit220. The desired level is shown as the setpoint215. The operating capacity of the vessel110is specified in terms of the low limit210and the high level limit220, outside of which operation is not desirable. Low limit210, setpoint215and high level limit220, together with the current level205, define two differential parameters—a setpoint deviation (SD)225and a capacity deviation (CD)230. To keep the liquid level at the setpoint215would mean that the outlet flow follows fluctuations in the inlet flow. As shown inFIG. 2, the current level205is above the setpoint215. However, the current level205can also be at or below the setpoint215.

The CD230is a measure of discrepancy in the current level205and a prevailing limit, which is generally either the low limit210or the high limit220. AlthoughFIG. 2shows the prevailing limit as being the high limit220, the prevailing limit can also be the low limit210such that the CD230is a measure of the discrepancy between the actual inventory (the current level205) and the low limit210. The prevailing limit can be detected by the NLLC120based on, for instance, the sign of the time derivative of the liquid level in the vessel110. For example, if the level in the vessel110is increasing, the sign of the derivative is positive, and the prevailing limit can be the high limit120. Conversely, if the level is decreasing, the sign of the derivative is negative, and the prevailing limit can be the low limit210. The CD230is thus measured in terms of the volume (available or excess) from the current level205to generally reach the prevailing limit.

The SD225is a measure of discrepancy of the current level205from the setpoint215. The SD225can be measured in terms of the volume (available or excess) from the current level205to generally reach the setpoint215.

A first time (T1) to reduce the CD230by a first specified percentage (such as 100%/exhaust) and a second time (T2) to reduce the SD225by a second specified percentage (such as 100%/exhaust) are generally provided. For example, values for T1and T2can be derived from conventional empirical process tuning tests. In addition, the tuning constants (T1and T2) can be derived from observations of plant operating data, vessel dimensions, the nature and magnitude of disturbances experienced based on actual plant operating data, and control objectives defined by engineers or other plant personnel. T2may generally be greater than T1. It may also be possible to determine T1and T2through modeling. In some embodiments, the specified percentages can both be 100%, T1=15 minutes, and T2=60 minutes.

FIG. 3illustrates an example system configuration for processing a fluid through a separator according to this disclosure. The embodiment of the system100shown inFIG. 3is for illustration only. Other systems configurations could be used without departing from the scope of this disclosure.

In this example, the system100includes a separator305coupled to a compressor310and other downstream equipment320. The separator305receives, via an inlet, fluid from one or more upstream processes. The fluid can include a gaseous (G) portion and a liquid (L) portion. The separator305separates the fluid into a gaseous portion and a liquid portion. The gaseous portion can be delivered to the compressor (C)310coupled to a first outlet to the separator305. The liquid portion can be delivered via a second outlet of the separator305to additional process equipment320in the downstream process115.

In some embodiments, the separator305can receive fluid produced as part of an oil production process from a number of oil wells in the upstream process105. The fluid produced from the oil production process includes a mixture of oil, water, and gas. The separator305separates the oil and water in the liquid phase from the gas and provides the gas to the compressor305. The oil and water are drawn out of the separator305and delivered to the downstream equipment320.

The compressor310can be damaged if liquid-phase material is processed through the compressor310. Therefore, the separator305separates the liquid from the gas and provides the gas to the compressor310as the gas is separated from the liquid. However, conventional separators305cannot compensate for slugging where, for example, no gas or no liquid is received by the separator305. When no gas is received by the separator305, the separator305may deliver liquid, such as oil, water or both, to the compressor310. As a particular example, this can occur if all of the gas is drained from the separator305. This can damage the compressor310.

In addition, the separator305may be unable to deliver liquid to the downstream equipment320if only gas is received for a period of time. For example, the separator305may receive only gas for thirty seconds. During that period, the liquids may be fully drained from the separator305. Therefore, the separator305does not deliver either water or oil to the downstream equipment320, which may cause damage to one or more pieces of the downstream equipment320.

FIGS. 4A through 4Dillustrate an example separator305according to this disclosure. The embodiments of the separator305shown inFIGS. 4A through 4Dare for illustration only. Other separators could be used without departing from the scope of this disclosure.

In this example, the separator305includes a vessel volume contained by a housing405. In some embodiments, the separator305includes a cylinder housing that is placed horizontally or vertically. The separator's vessel volume is partitioned into two parts by a baffle410positioned vertically and attached to a lower half of the housing405. An oil and water portion415forms on a first side417of the baffle410, and an oil-only portion420forms on a second side422of the baffle410. The oil and water portion415can be significantly larger than the oil-only portion420. For example, the size of the oil and water portion415can be ten times larger than the oil-only portion420. The larger oil and water portion415includes enough volume to enable the oil to separate from the water.

Production from the wells flows into the oil and water portion415via an inlet425. In the oil and water portion415, separation between gas and oil and water occurs. Gas flows into an overhead portion430. The oil and water settle into the oil and water portion415, where the oil and water separate such that the oil rises to the top and the water settles to the bottom of the oil and gas portion415. Since (i) the oil has a low density compared to water and (ii) the oil includes non-polar organic compounds (while water is a polar compound), the oil does not substantially mix with water and floats on the water's surface. Therefore, an immiscible liquid mixture of oil and water is received from the production of wells.

The gas is drawn out of the separator305via a first outlet435, while water is drawn out of the separator305via a second outlet440. The water is drawn out of the separator305at a rate such that the water level in the oil and water portion415remains below the height of the baffle410. As the water and oil separate in the oil and water portion415, the oil settles on top of the water. Therefore, the immiscible liquid mixture of oil and water separate into a light phase418and a heavy phase419. The heavy phase419settles to the bottom of the vessel volume, and the light phase418settles on top of the heavy phase419. The water is drawn out at a rate that enables at least a portion of the oil on the surface of the water to flow over the top of the baffle410into the oil-only portion420. The oil is drawn out of the separator via a third outlet445.

The separator305includes or is otherwise associated with an NLLC450. The NLLC450can include similar structure and functionality as the NLLC120and, as such, can include a central processing unit (CPU)455and associated memory. The NLLC450stores or receives inputs such as setpoint, high level, low level, tuning constants, and current level (inventory) data for oil in portions415and420of the separator305. The NLLC450executes a control algorithm and adjusts flow out of the outlet445to the downstream equipment via flow controller460cand control valve465c. Other control valves465a-465band respective flow controllers460a-460badjust the outlet flows from the separator305under control of one or more additional controllers (not shown).

In conventional separator systems, oil is drawn at a rate such that the oil level is maintained at a fixed level, such as level L2, below the baffle height. The conventional separator systems are unable to maintain the oil level above the baffle's height because a mathematical discontinuity occurs when the oil level exceeds the baffle height.

In accordance with this disclosure, the NLLC450executes an optimal surge control process to control the oil level in the separator305such that the oil level can be controlled in a range above the baffle410. The NLLC450also addresses any non-linear behavior in the operation of horizontal and vertical separators. In particular, the NLLC450can maintain the oil at level L1(shown inFIG. 4D) instead of L2(shown inFIG. 4C). The NLLC450can maintain the oil at level L1by using a lower limit that is established above or below the baffle height. In addition, the NLLC450can use an established upper limit and setpoint. In some embodiments, the lower limit is at or above level L1.

Maintaining the oil level above the baffle410can increase or maximize the use of the available surge capacity in the separator305and thus provide an opportunity to stabilize the oil flow from the separator305. The available surge capacity can equal the volume of the oil-only portion420between the baffle's height and level L2plus the volume of the entire separator vessel between the level L1and the baffle's height. For example, when the separator305receives only gas for a period of time, such as for thirty seconds, the oil can continue to be drawn out of the separator at the same rate to “ride-out” the gas-only flow because of the increased surge capacity of the oil created by maintaining the oil at level L1instead of L2.

In some embodiments, the NLLC450calculates an unforced time (referred to as T*) to reduce the CD by the first specified percentage. For example, if the first specified percentage is 100%, T* is thus defined as the time to completely exhaust the CD corresponding to a volume of the fluid comprising liquid if no changes to the flows are made by the controller (e.g., the status quo). Thus, the conditions for calculating the unforced time can be based on the most recent measurement of the current level205that is stored in the memory associated with the NLLC450and/or the change in the current level205since the last execution of the NLLC process.

FIG. 5illustrates an example process500for controlling surge capacity of a separator vessel according to this disclosure. In this particular example, the process500is configured to exhaust the majority of CD in T1minutes and/or exhaust the majority of SD in T2minutes. Non-linear control is based on the differential parameters CD and SD and the relative speed of the flow disturbance, which controls surge capacity for the separator305.

In block505, SD and CD as described above are calculated. In block510, T* can be calculated or otherwise provided, such as stored from an earlier computation. In block515, T* is then compared to a time constant T1.

If T*≦T1, the variation/disturbance in inlet flow is relatively fast. The NLLC450computes a first minimum change in the outlet flow (for outlet445from the separator305) to reduce CD by the first specified percentage in a time T1. The minimum change is calculated and implemented as a flow change in block520.

If T*>T1, the variation/disturbance in inlet flow is relatively slow. The NLLC450computes a second minimum change in the outlet flow (for outlet445from the separator305) to reduce or exhaust SD by the second specified percentage in a time T2. The minimum change is calculated and implemented as a flow change in block525.

Accordingly, the NLLC450is configured to perform an optimal surge control process to maintain the oil at a level sufficient to provide a surge capacity. The NLLC450can determine the current “inventory” level of the oil and access a previous level stored in memory. If the current level is outside a limit and worsening, the NLLC450computes a change to the flow rate through the outlet445to keep the oil level from getting worse. If the unforced time to exhaust CD (T*) is less than a specified time (T1), the NLLC450computes a capacity change. If the unforced time to exhaust CD (T*) is greater than the specified time (T1), the NLLC450computes a setpoint change. The flow change can be implemented using a flow control parameter. The NLLC450can control the output flow setpoint of the flow controller460cthat is coupled to the control valve465cto implement the first minimum flow change (when T*≦T1) or the second minimum flow change (when T*>T1).

Although the figures above illustrate specific systems, structures, and processes, various changes may be made to the figures. For example, various components in the systems and structures can be combined, omitted, further subdivided, or moved according to particular needs. Also, while shown as a series of steps, various steps inFIG. 5could overlap, occur in parallel, or occur multiple times.