Patent Application
20230287763
The present invention relates to the field of subsea systems, and in particular to subsea systems comprising a preconditioning unit upstream of a pressure boosting device.
As new oil and gas production wells are discovered and the old ones lower their production curve, or there is an increase in the production of other fluids (such as water or gas), different from oil, new challenges are faced in order to continue producing with high efficiency. Various technologies are under research and can be applied for the improvement of productivity. One of the applied technologies is the monophasic or multiphasic fluid pressure elevation system.
These systems elevate the pressure of the fluids so that they can be transported to an oil rig, to an onshore location or to offshore production and/or processing units. Another application is to elevate the pressure through the utilization of subsea systems, and subsequently re-injecting them.
With the use of these pressure elevation systems, challenges arise regarding the different types of fluids that can be used in these systems and the high pressure increases that they can be subjected to. One consequence of the increase in fluid pressure is the increase in temperature.
This increase in temperature in the fluid is proportional to the increase in pressure. Thus, the greater the pressure gain of the system, the greater its increase in temperature.
Due to these characteristics, the demand for subsea heat exchangers such as coolers may be required in these pressure elevation systems.
The subsea cooling system for hydrocarbons is an increasingly current demand for various applications in fluid conditioning in order to: meet flexible duct requirements, improve the efficiency of machines and adjust the fluid for the best separation conditions.
Currently, many solutions exist in which heat exchange subsea is used to adjust the process fluid using the thermal exchange with seawater. In the current scenario, heat exchange systems are divided into two different types of systems: active and passive.
In a passive subsea heat exchanger, the process fluid passes through tubes in which the heat exchange occurs with the seawater, simply using the principle of thermal conduction. In this case, there is no active form of controlling the thermal exchange.
For the active heat exchangers, the principle of thermal convection is also used to improve and/or control the thermal exchange. The principle of convection is used by controlling the flow of seawater. This control of marine currents can be carried out by increasing the marine flow in the vicinity of the heat exchanger through systems that increase the marine current. Another strategy used for the system is to reduce the marine current, when necessary, to control the temperature of the heat exchanger.
WO 2013/174584 A1 relates to a subsea cooler system for active control of passive coolers. The subsea cooler system comprising at least a first and a second cooler arranged in a series connection, and a third cooler arranged in parallel with said first and second coolers. At least one of said coolers comprises a recirculation loop.
There are however drawbacks related to the prior art solutions above, because all required parameters of the process fluid exiting the cooler system is not known, resulting in a potential risk of damaging any equipment downstream of the cooler system.
One of the objectives of this invention is thus to provide a subsea system which ensures that the process fluid entering a pump or pressure boosting device has fluid characteristics that will not damage the equipment.
The invention is set forth in the independent claims, while the dependent claims describe other characteristics of the invention.
Using a passive cooler with a secondary line for controlling the temperature of the process fluid, renders it possible to control the outlet temperature of the process fluid exiting the cooler in a way that the thermal load at the inlet of the cooler can be varied. This control adjusts the process fluid for the overall cooler system's outlet condition without the need to add new rotary equipment and consequently increasing the number of failure points.
The process fluid preconditioning system on the suction, i.e. upstream, of the pressure boosting device aims to guarantee the pressure boosting device operation, maintaining the temperature at the discharge, i.e. upstream, of pressure boosting device and any interstage of this device, in accordance with system requirements in terms of at least density and minimum temperature requirements.
The invention relates to a subsea system connected to a subsea well for boosting a process fluid flowing out of the well, comprising:
The sensors used for measuring temperature and pressure may be standard temperature and pressure sensors used subsea. The sensors may be arranged at the outlet of the cooler(s) or they can be arranged elsewhere in the preconditioning arrangement.
Normally, the composition of the process fluid is known either by taking a sample and or from measurements from e.g. a multiphase meter etc. Although the composition of the process fluid, e.g. water-cut etc., varies with time, the changes on a day-to-day or month-to-month basis is normally insignificant. Thus, it is not common practice to perform real tests of the process fluid composition too often. The means for estimating maximum and minimum allowable density of the process fluid may then be a pre-made diagram for the specific process fluid for this subsea well where density can be read based on the measured temperature and pressure. The maximum and minimum allowable density may be decided based on parameters such as, in addition to temperature, pressure and composition of the process fluid, hydrate formation temperature.
The cooler system provides for thermal exchange between the process fluid and the surrounding seawater and can be of the type described in WO 2013/174584, which content is hereby incorporated in its whole. The system can have two or more stages of thermal exchange. Each of these parameters contemplates cooling tubes, where the heat transfer occurs between the process fluid and the seawater. These coolers can be organized in series and/or in parallel allowing different scenarios and modes of operation to be attended.
The cooler system may comprise one or more coolers. Each cooler may be composed of parallel tubes, forming horizontal sections. The number of horizontal sections and the length of each section is determined in accordance with the value of maximum design thermal load at the inlet of the cooler.
In the design of the preconditioning system, the cooler system/cooling stages can be aligned in series and/or parallel. The design of the cooler system can be such that the different cooling stages have different cooling capacity.
Each or some of the coolers may comprise a bypass line that permits that part of the fluid is diverted from the cooler and allowed to enter the bypass line instead. This deviation is accomplished through the manipulation of the control valve present in this bypass line. The amount of process fluid flowing in the bypass line is determined in order to meet the criteria of specific temperature in the system.
The flow that was deviated from the cooler through the bypass line is preferably mixed with the flow coming from the cooler downstream of the cooler, in which a thermal equilibrium is obtained at the outlet of the cooler. The greater the flow that passes through the bypass line, the higher the equilibrium temperature of the system.
The process fluid is preferably a so-called dense gas, which is a natural gas rich in CO2. This gas has a composition similar to the natural gas produced in Brazilian Pre-Salt well fields, with a high-density value as a differential, similar to fluids in the liquid state. The process fluid comprises at least 30 volume percentage of CO2 at operational subsea conditions, i.e. at the conditions where the pressure boosting device is arranged. Additionally, typical characteristic parameters for the process fluid is in the range of:
The density or specific mass of the process fluid varies dependent on the pressure and temperature. Simulations carried out with different temperatures for the process fluid verified that if reducing the temperature, the density of the process fluid increases. Specific mass values lower than 260 kg/m3 make it impossible to utilize the pressure boosting device, in which fluid preconditioning is necessary, reducing the temperature in a controlled manner, reaching the value of specific mass that permits operation of the pressure boosting device.
In addition to dictating operational parameter in terms of maximum and minimum allowable density, the operational window may have at least maximum and minimum operational parameters of pressure and temperature.
This system may be arranged downstream a separation device. The process fluid flowing through the subsea system may be re-injected into a reservoir. Therefore, another determining factor for the parameters of this system may be the temperature limit in the injection lines used to inject the process fluid discharged from the pressure boosting device. In certain operating modes, the temperature at the discharge of the pressure boosting will change, requiring a fluid preconditioning system at its suction, adjusting the temperature of the discharge. The preconditioning arrangement of the subsea system will enable the operation of the pressure boosting device, in addition to keeping the required temperature allowed by the injection line.
The bypass lines containing their respective control valves, can be added to allow for an active temperature control at the outlet of each cooler stage of the preconditioning system.
When the demand for thermal load required by the process fluid is reduced, it is necessary to manipulate the bypass line control valve. Then a larger portion of the process fluid is guided or deviated through the bypass line resulting in less decrease in temperature (compared to guiding all process fluid through the cooler) and thus less increase in the density of the process fluid (compared to guiding all process fluid through the cooler resulting in an even lower temperature). In this way, if operating on the limit of the minimum acceptable density for the process fluid, the required specific mass value is obtained such that the process fluid can enter the pressure boosting device.
The proposed active temperature control system, besides guaranteeing the specific mass or density required in the system output, also acts in the prevention of hydrate at each cooler. The prevention of hydrate formation in the cooler(s), may be achieved using a temperature controller that manipulate the recirculation line of the pressure boosting device.
The active control described above, can be applied in natural and/or forced convection heat transfer process. This control linked to the diverse possibilities of stage arrangements, guaranties the possibility of the preconditioning system attending a large variety of work temperature at any point of the system.
The system is designed to attend the process fluid's maximum thermal load. In this condition, i.e. at maximum thermal load, 100% of the flow will pass through the coolers by the main line and the control valves of the bypass lines will be closed.
This invention enables the subsea dense gas pressurization system and the subsequent re-injection of the process fluid into a reservoir.
The system can be installed at a depth of up to 3,000 meters.
The main cooler inlet line may be specified in order to attend a uniform distribution between all tubes connected to it. This configuration enables uniform distribution between all the parallel process fluid tubes entering into the cooler, without causing preferential flow.
The main cooler outlet line may be specified in order to attend a uniform distribution between all tubes connected to it. This configuration enables uniform distribution between all the parallel process fluid tubes that exit the cooler, resulting in a uniform mixture of the process fluid entering the pressure boosting device.
An additional control valve or restriction orifice may be positioned in the suction, i.e. upstream, or discharge, i.e. downstream, of the cooler, performing the pressure equalization.
The subsea system may comprise a recirculation loop connected downstream of the pressure boosting device and upstream of the preconditioning arrangement. The recirculation loop may comprise a pump recirculation valve which is connected to a temperature transmitter measuring temperature of the process fluid downstream of the first cooler. The pump recirculation valve may be controlled by the temperature transmitter downstream of the first cooler. If the temperature of the process fluid downstream of the first cooler is low (e.g. due to reduced flow from the well) with the risk of hydrate formation in the cooler(s), the pump recirculation valve opens thereby recirculating process fluid which has been pressurized by the pressure boosting device into the preconditioning arrangement. As such, the risk of hydrate formation resulting from reduced flow, and thereby reduced temperature of the process fluid exiting the first cooler, is reduced. I.e. the recirculation loop may be necessary if the process fluid has not reached satisfying temperature at the outlet of the first cooler.
In an aspect, the cooler system comprises a second cooler arranged in series or parallel connection with the first cooler. The second cooler may have equal, higher or lower cooling capacity than the first cooler.
In an aspect, the cooler system comprises a third cooler which is arranged in parallel connection with the first and second cooler. If the first and second coolers are arranged in series, and the third cooler in parallel, there is a total of two cooling branches, whereas if the first, second and third coolers are in parallel connection, there is a total of three cooling branches. The different cooling branches preferably have different cooling capacity such that different cooling requirements or cooling demands may be met without modifying the system.
The cooler system may comprise at least one flow control device, e.g. a valve, for directing flow through at least one of the cooling branches dependent on the cooling requirement.
In an aspect, some or all the coolers may comprise a recirculation loop for recirculating process fluid back into an inlet of the cooler.
In an aspect, some or all the coolers may comprise a chemical injection line. The preconditioning system presents the possibility of inserting a chemical injection point at the inlet of each cooling stage. The point of injection allows for the complete distribution of chemicals added to all the cooler's tubes. The chemical injection fluid can be Mono Ethylene Glycol (MEG) and this chemical injection fluid can be injected into the cooler if there is a risk that hydrates may form in the cooler, e.g. in the tubes forming the cooler. Each of the coolers may have a chemical injection line to prevent the formation of hydrates in operation and for preservation with no flow.
The subsea system may include an active control system of the temperature, complementary to the arrangement of the cooler stages. This control system makes it possible to obtain the specific mass required at the outlet of the preconditioning arrangement in addition to potentially prevent hydrate formation. This control system utilizes subsea temperature transmitters for monitoring temperatures in real time.
It is further described a of operating a subsea system, the subsea system comprises:
The operational parameters which is measured and estimated in the method, may be density pressure and/or temperature and is dictated by the operational window of the pressure boosting device.
The system uses an active control of the temperature, complementary to the arrangement of the cooling stages. This control makes it possible to obtain the specific mass required at the outlet of the system, and the prevention of hydrate formation.
This control system utilizes subsea temperature transmitters for monitoring temperatures in real time.
Summarized, the subsea system and method may have at least one of the following advantages:
These and other embodiments of the present invention will be apparent from the attached drawings, where:
In the following, embodiments of the invention will be discussed in more detail with reference to the appended drawings. It should be understood, however, that the drawings are not intended to limit the invention to the subject-matter depicted in the drawings.
In operation, process fluid from e.g. a well (not shown in
A recirculation line 65 is connected to the outlet line 64 downstream of the pressure boosting device 3 and the main line 45. An operable pump recirculation valve 66 is arranged in the recirculation line 65 to control minimum flow of the boosting device 3 and minimum temperature in the preconditioning arrangement 2. The pump recirculation valve 66 is connected to temperature transmitter 23 measuring temperature of the process fluid downstream of the first cooler 20 via control lines 69. The pump recirculation valve 66 is controlled by the temperature transmitter 23. If the temperature of the process fluid downstream of the first cooler 20 is low (e.g. due to reduced flow from the well) with the risk of hydrate formation in the cooler(s) 20, 21, the pump recirculation valve 66 opens thereby recirculating process fluid which has been pressurized by the pressure boosting device 3 into the preconditioning arrangement 2. As such, the risk of hydrate formation resulting from reduced flow, and thereby reduced temperature of the process fluid exiting the first cooler, is reduced. I.e. the recirculation loop 65 may be necessary if the process fluid has not reached satisfying temperature at the outlet of the first cooler 20.
The operational conditions of the disclosed cooler system in terms of cooling capacity is as follows:
The amount of process fluid is thus dependent on the active control of the operated valve 22 and how much of the process fluid which flows through the bypass line 58.
A chemical injection line 68 is connected to the process fluid line 45 upstream of the first cooler 20. Alternatively, the chemical injection line 68 could be connected downstream of the first cooler 20 but upstream of the second cooler 21.
Fluid exiting the second cooler 21 is typically directed to or towards the pressure boosting device 3 (as shown in
The cooler system provides large flexibility with regards to the cooling requirement. Being able to provide a cooler system having different cooling capacities dependent on the cooling need, is advantageous bearing in mind that the hydrate formation temperature and/or flow rates may vary during the lifetime of a field.
The invention is now explained with reference to non-limiting embodiments. However, a skilled person will understand that there may be made alterations and modifications to the embodiment that are within the scope of the invention as defined in the attached claims.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/IB2020/058152 | 9/2/2020 | WO |
