GB2615601A

GB2615601A - Autonomous vehicle pressure control

Info

Publication number: GB2615601A
Application number: GB2201991.3A
Authority: GB
Inventors: Matias Dupuy Pablo
Original assignee: Equinor Energy AS
Current assignee: Equinor Energy AS
Priority date: 2022-02-15
Filing date: 2022-02-15
Publication date: 2023-08-16
Also published as: GB202201991D0

Abstract

An autonomous underwater vehicle for transporting a cargo comprises: an outer hull 10 defining an internal volume; one or more internal containers 12 for containing the cargo; a pressure compensation system 14 arranged to control the pressure of the internal volume based on exterior pressure. The pressure compensation system comprises a movable barrier (32, Fig 3) arranged within a housing (31, Fig 3) and wherein the movable barrier separates a bulk fluid provided within the interior volume from fluid outside the vehicle. The movable barrier may comprise a piston member. A method of controlling pressure within an autonomous underwater vehicle using a pressure compensation system is also disclosed.

Description

Autonomous vehicle pressure control

Field of invention

The invention relates to methods and devices for controlling pressure in autonomous underwater vehicles.

Background

Research Disclosure 662093 (published 20 May 2019) describes a subsea shuttle system, using autonomous subsea vehicles for transportation and storage purposes.

Research Disclosure 677082 (published 21 August 2020) provides further detail regarding vehicle shuttle structure and support, applications, e.g., on/offloading of a payload, and the propulsion system of the subsea vehicle.

UK patent number 2585758 (published 22 December 2021) describes an underwater vehicle for transporting a fluid cargo, comprising an outer hull, one or more internal vessels for containing the fluid cargo; a pressure compensation system arranged to control the pressure of the cargo and a barrier for separating a first portion of the vessel from a second portion of the vessel, the barrier permitting pressure communication between the first and second portions.

UK patent number 2585488, published 4 August 2021, describes an assembly for loading or unloading an autonomous underwater vehicle.

Statement of invention

According to a first aspect of the invention, there is provided an autonomous underwater vehicle for transporting a cargo, comprising: an outer hull defining an internal volume; one or more internal containers for containing the cargo; a pressure compensation system arranged to control the pressure of the internal volume based on exterior pressure; the pressure compensation system comprises a movable barrier, wherein the movable barrier is arranged within a housing, and wherein the movable barrier separates a bulk fluid provided within the interior volume from fluid outside the vehicle.

The movable barrier may be arranged to compensate pressure fluctuations within the interior volume, or pressure fluctuations outside the vehicle The compensation takes place by creating an additional volume by movement.

The movable barrier is a piston member received within the housing, and wherein the housing defines a path of travel for the piston member. Optionally, the housing comprises a plurality of piston members arranged in series.

The housing may comprise one or more emergency relief channels, which create a fluid path directly into the internal volume for releasing pressure.

The pressure compensation system may comprise a flexible sheet received within the housing.

The housing may comprise a first inlet fluidly connected to the interior volume of the vehicle and a second inlet fluidly connected to the fluid surrounding the vehicle.

The autonomous underwater vehicle may further comprise a detector arranged to measure displacement of the movable barrier. The autonomous underwater vehicle may further comprise a pressure sensor arranged within the internal volume. The autonomous underwater vehicle may further comprise a gas detector arranged within the internal volume.

The autonomous underwater vehicle may further comprise an emergency relief valve comprises a piston member and a side opening.

The autonomous underwater vehicle may further comprising a controller coupled to one or more of the detectors mentioned above, and an emergency relief valve, wherein the controller is arranged to open the emergency relief valve depending on measurement signals received from the one or more detectors.

The autonomous underwater vehicle may comprise a plurality of said pressure compensation systems, and an active control system, arranged to control the operation of the plurality of pressure compensation systems.

The autonomous underwater vehicle may further comprise one or more valves for opening, closing, or restricting the flow of fluid into the housing from one or both of the internal volume or the outside of the vehicle.

According to a second aspect of the invention, there is provided a method of controlling a pressure within an autonomous underwater vehicle, the vehicle comprising an outer hull defining an internal volume; one or more internal containers for containing the cargo; a pressure compensation system comprises a movable barrier, wherein the movable barrier is arranged within a housing, and wherein the movable barrier separates a bulk fluid provided within the interior volume from fluid outside the vehicle; the method comprising: moving the movable barrier to compensate for a pressure change in the bulk fluid or in the fluid outside the vehicle.

The method may further comprise detecting a displacement of the movable barrier, and controlling the opening of a pressure relief valve depending on the displacement.

The method may further comprise measuring a pressure of the bulk fluid, and/or measuring the presence of a gas within the internal volume, and operating the movable barrier or a pressure relief valve in response to said measuring.

The bulk fluid may be selected depending on the density of the bulk fluid.

Figures Some embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings, in which: Figure 1 is a vertical cross section of a schematic drawing of an autonomous underwater vehicle; Figure 2 is a vertical cross section of a schematic drawing of a pressure compensating tank; Figure 3 is a schematic drawing of a pressure compensator including a piston; Figure 4 is an illustration of an active pressure control system; Figure 5 is a vertical cross section of a schematic drawing of a gas release valve; Figure 6 is a schematic drawing of an autonomous underwater vehicle at different depths; and Figure 7 is a flow diagram of a method.

Detailed description

The subsea transportation of a fluid cargo (e.g. liquid or gas), or other cargo, is enabled by an autonomous underwater vehicle. A pressure compensation system is provided to maintain or control the internal pressure of a bulk fluid within the vehicle based on the external hydrostatic pressure. Pressure within the hull of the vehicle but outside a pressure vessel holding the cargo fluid may be maintained and controlled at or near external hydrostatic pressure, based on a pressure compensation system comprising a movable barrier, keeping external and internal fluids separate while allowing pressure communication.

Although the word 'sea' and 'seawater' are used throughout, these may equally be understood as 'lake' and 'freshwater', respectively, and the invention is envisaged to be used in any large body of water. Similarly, when the words 'seabed' or 'sea surface' are used, this is not intended to be limited to a sea in a strict sense but should also be understood to cover 'ocean bed' or 'ocean surface', or similar terms for any large body of water.

Figure 1 illustrates a schematic underwater vehicle according to an embodiment of the invention. The underwater vehicle (or 'shuttle') may be an autonomous underwater vehicle (AUV), or a remotely operated underwater vehicle (WV). The vehicle comprises an outer hull 10, having a hydrodynamic shape to reduce drag. An elliptical outer hull 10 is shown in Figure 1, but other hydrodynamic shapes known in the art are suitable. Within the outer hull 10, an internal pressure vessel is provided 12. The pressure vessel 12 may be fixed within the outer hull 10 using a frame or other rigid supports (not shown). In this way, a space between the outer hull 10 and pressure vessel 12 is formed. A pressure communication device 14 is provided, as described in more detail below. When the vehicle is submerged, the space between the outer hull 10 and inner pressure 12 vessel is at least partially filled with a bulk fluid, depending on the net buoyancy requirements. In some embodiments, a part of the outer hull 10 volume is occupied by one or more compartments 18 for containing gas (e.g. ballast tanks). In this way, the gas remains separate to the bulk fluid, meaning that sloshing is avoided.

Although pressure vessel 12 in Fig. 1 is illustrated as a longitudinal vessel in a horizontal orientation, in some configurations, the vehicle may also carry one or more longitudinal vessels in a vertical orientation. More specifically, a plurality of vertical vessels may be carried by the vehicle, perhaps as many as a few hundred, while the overall shape of the outer hull still has a longitudinal hull in a horizontal orientation during normal use.

At the stern (back end) of the vehicle, a propeller 16 is provided. The propeller is coupled to a power source and control unit (not shown) to enable autonomous and/or remote operation of the vehicle. An electric power source is preferably used.

The vehicle structure shown in Figure 1 has some similarities to a double hull structure used in some conventional submarines. However, a difference in the present technology is that instead of having a pressure hull maintained at or near to atmospheric pressure to accommodate personnel, the inner structure is maintained at a pressure similar to the external hydrostatic pressure. Advantageously, this means that the vehicle can be designed with a much lower collapse pressure capacity than a standard pressure hull, which results in significant weight savings (e.g. by reducing the need for stiffeners). By "similar' to the external hydrostatic pressure, it is meant that the internal pressure is kept suitably close to the external pressure so that the pressure differential dP (i.e. overpressure or under-pressure) is not too large. If the overpressure is too large, the vehicle may undergo a burst failure. Conversely, if the under-pressure is too large, the vehicle is at risk of a collapse failure. The collapse pressure threshold is sensitive to geometric imperfections in the vehicle shape, meaning that a smooth, curved shape is preferable.

The autonomous vehicle comprises a pressure compensation system 14. The pressure compensation system includes a barrier between the seawater surrounding the vehicle, and the internal fluid. The internal fluid may be seawater, freshwater, or any other suitable fluid. What constitutes a suitable fluid is determined by some of the properties of the vehicle. The internal volume is large (example), and the choice of water is therefore good because it is readily available in large quantities. The difference in density between the surrounding seawater and the bulk fluid is preferably small, so salt water or fresh water are therefore also preferred options. The bulk fluid is preferably also inert, to protect the internal equipment of the vehicle. The bulk fluid may also include additives, such as chlorine, to prevent growth of algae or other organisms. Although water and seawater are preferred choices, the bulk fluid is not restricted to these choices and hydrocarbons or an oil-in-water mixture may also be used. An advantage of using other bulk fluids than seawater is that it is not necessary to satisfy the same corrosion resistance requirements for tanks and other internal components as for seawater. The bulk fluid can be fresh water or another liquid with a slightly lighter density than seawater. By removing oxygen, growth of organic matter or oxidation of equipment is inhibited. The choice of fluid can also be used as a design parameter for controlling buoyancy, by choosing the density in accordance with requirements.

An example of a pressure communication system is an open fluid channel between the surrounding seawater and the internal volume of the vehicle. However, a disadvantage would be the entering of smaller or larger organisms or pollution through the open fluid channel into the internal volume of the vehicle. Preferably, a filter such as a mesh filter is therefore provided to block any undesired pollutants entering the internal volume.

Instead of a mesh, a membrane may also be used, whereby the membrane lets fresh water through, while blocking salt. An overpressure is required for such a membrane, and is therefore only used if the hull structure can withstand a pressure difference between the internal volume and the surrounding seawater, or alternatively the pressure difference is only applied over a specific vessel before letting the fresh water into the internal volume.

The bulk fluid is preferably loaded and subsequently isolated from the surrounding seawater, such that no fluid communication is possible between the seawater and the internal volume, while allowing pressure communication. A barrier is provided between the seawater and the bulk fluid. Optionally, an intermediate fluid is provided and the pressure communication is multi-stage, rather than direct over a single barrier. The multi-stage barrier may comprise a plurality of barriers arranged in series, or in parallel.

One example of a barrier is a flexibly deformable material layer. The layer may be a sheet of plastics, rubber, reinforced rubber, or synthetic rubbers such as nitrile butadiene rubber, NBR, or hydrogenated nitrile butadiene rubber, HNBR. The barrier can be provided within a channel, or within a dedicated tank. The tank encompasses the barrier and thereby protects the barrier. The walls of the tank also define a limit to the deformation of the barrier, because when the barrier is extended against the walls it cannot deform further. The tank can therefore also accommodate a maximum pressure variation corresponding to the maximum internal movement of the barrier. If more than the maximum pressure variation is required, a larger tank may be used, or a plurality of tanks may be used to collectively achieve the desired pressure variation range.

An example of a pressure compensating tank is illustrated schematically in Fig. 2. The tank comprises an outer wall 21, an internal bladder 22, a first inlet 23 and a second inlet 24. The internal bladder separates the internal volume defined by the outer wall into a first portion connected to the first inlet, and a second portion connected to the second inlet. An example of numerical values for the tank illustrated in Fig. 2 are: a diameter of 2.38 m and an internal volume of 60m3. The tank could be installed horizontally. The inlets may further comprise a valve to control the flow through the inlets.

Another example of a barrier is a piston arrangement. A piston member is received within a tubular body. The piston member is able to travel within the tubular body to accommodate a pressure change. On one side of the piston member, the external seawater is received, while on the other side of the piston member the bulk fluid is received. A multi-stage arrangement may be used, comprising two or more piston members arranged in series within the tubular body.

In one particular example, illustrated schematically in Fig. 3, a tubular body 31 has an inner wall with a generally cylindrical shape, and a piston member 32 has an outer wall with a matching cylindrical shape. The rotationally symmetric shape of the tubular parts means that the rotational orientation of the piston member is not relevant and can change during operation. Inlets 33 and 34 are provided, connecting the variable volumes on either side of the piston member 32 to the surrounding seawater and the internal bulk fluid, respectively.

The inner wall may be made of metal, while the piston member may be made of Teflon, such that the piston member can travel by sliding within the tubular body. However, other suitable materials can be used, such as two metal members. The seal between the piston member and the tubular body does not need to be perfect to relax the design parameters, especially when the bulk fluid is fresh water, possibly with some additives, and the outside fluid is seawater. Some leakage of seawater into the internal freshwater fluid would not pose a significant problem in some operating conditions, while larger polluting objects are still kept out of the internal volume of the shuttle by the barrier.

The example of a cylindrical shaped piston member has been provided, but alternatively a different geometrical shape can be used which is not rotationally symmetrical and thereby prevents rotation of the piston member. For example, the piston member can be rectangular. In addition, or alternatively, the piston member may have a protrusion in radial direction which travels through a rail provided within the wall of the piston tubular body. The protrusion locks the position of the piston member in rotational direction.

The interface between the piston member and the tubular body has been described as a low-friction sliding contact between two surfaces, such as Teflon to metal. Additional features may be provided. One example of an additional feature to aid the movement is a set of rollers or bearings on the piston member. The rollers or bearings improve the movement when compared to a pure frictional connection, but, on the other hand, provide for a more complicated arrangement with more parts that could become stuck during operation while being in a location relatively difficult to access for repairs during normal operation.

Another possible additional feature is one or more seals, such as 0-rings, between the piston member and the tubular body. The one or more seals reduce the chance of a leak, but, like for the rollers or bearings, also provide more parts with possible risks for failure. The skilled person will be able to weight out the advantages and disadvantages depending on the particular operational conditions. For example, the presence of hazardous materials within the shuttle may necessitate the use of seals to achieve a required safety standard; while operating conditions that cause frequent and rapid adjustments of the piston member may necessitate the use of rollers or bearings.

The piston arrangement may further comprise a failsafe mechanism for accommodating large pressure fluctuations outside the expected range of fluctuations. Expected fluctuations are those due to a change of depth of the vessel within an expected range of depths, for example. An example of an unexpected fluctuation is the bursting of a high pressure container within the vessel. The failsafe mechanism comprises one or more channels in the wall of the tubular body. In Fig. 3, a channel 35 is illustrated, and if inlet 33 is connected to the interior, and inlet 34 to the exterior, then channel 35 is connected to the exterior of the vehicle, such that there is a direct fluid communication channel through inlet 33 and channel 35. When a large pressure fluctuation occurs, the piston member moves outside its normal operating range and past the one or more channels. As a consequence, there is direct fluid communication through the one of more channels between the internal and external of the vehicle. The fluid communication allows inflow or outflow of fluids to relieve the extreme pressure change. Channels may be arranged on either side of the normal working range: one to accommodate an extreme overpressure, and another one to accommodate an extreme under-pressure.

During activation of a failsafe mechanism, some fluid may be expelled from the interior volume through a pressure relief channel during excessive overpressure, or drawn in during underpressure. Subsequently, the expelled fluid or drawn in fluid will be replaced when operating conditions return to normal. For example, is a gas leak from a cargo tank causes a surge in pressure, fluid may be expelled, and when the gas has been removed a replacement volume of fluid such as seawater is taken on board to compensate for the fluid that has been expelled.

The example of the failsafe mechanism based on one or more pressure relief channels described above can be considered a passive system. Alternatively, or in addition, an active pressure relief mechanism may be provided. Figure 4 illustrates a schematic vertical cross section of a vehicle with an active failsafe mechanism. The piston member of the pressure compensation system 41 is shown as moved to an extreme position. The active pressure relief mechanism comprises a pressure monitor, arranged to measure the pressure within the vessel and/or the pressure differential over the outer wall of the vessel. The mechanism further comprises a control unit 42, arranged to analyse the output of the pressure monitor, and send a control signal when a pressure outside a normal operating range is detected. The control signal is arranged to be sent to an actively controlled pressure relief valve 43, which opens or closes depending on a received control signal. The illustration of Fig. 4 includes an illustration of gas bubbles 44, which are released from the vehicle following detection of a pressure increase outside normal operational limits.

The pressure relief valve is illustrated in more detail in Fig. 5. The valve is shown as closed in part A, and as opened in part B of the figure. A piston member 51 is provided, which moves past a side opening 52 due to excess pressure, thereby releasing a gas, or other matter, 53. A further opening 54 is provided for allowing a free movement of piston member 51 past side opening 52, without fluids becoming 'trapped' within the top part of the housing 55.

The examples illustrated in Figs 3 and 5 can be used both in passive and in active systems. For an active system, the Fig. 5 valve has an additional control which can lock or unlock the piston member (not illustrated).

The examples illustrated in Figs. 3 and 5 optionally comprise a control for moving the piston member away from the maximum position within the housing, as the piston member may get stuck at the maximum position. An example of a control is a spring, or other resiliently deformable element.

The failsafe mechanism may be combined with a serial piston member arrangement. In one example, two piston members are arranged within the tubular body. The two piston members may be attached to each other via a rigid connector such that the two piston members move in tandem, or the two members may move independently. The space between the two piston members may be filled with a signal fluid. An example of a signal fluid is a fluid with a dye or other material such as Styrofoam objects, which has a lower density than water such that it rises to the surface when it is released in the seawater. When the channel for pressure release opens up to the signal fluid, the fluid escapes and a human operator will be able to notice the failure.

The piston pressure release system may be provided at any suitable location of the underwater vehicle. A plurality of release systems may be provided at various locations of the vehicle. If a plurality of systems is used, the combined system may be unstable, in that some pistons move more easily than others and that the pressure is controlled un-evenly. To avoid an unstable system, active monitoring and control of the systems may be used. An active control mechanism can be provided by controlling the piston member, for example through the before mentioned protrusion travelling within a rail system. The controlling may also take place by selectively opening or closing valves provided at inlets or outlets to the pressure control systems. For example, valves can be provided to an inlet or outlet to the volume on either side of the movable barrier. An advantage of using a plurality of systems is that the combined volume change scales linearly with the number of systems. A larger number of systems can therefore accommodate a larger pressure variation. Further, the amount an individual piston member needs to travel to accommodate a pressure variation is smaller when the volume change is distributed over a larger number of systems.

A piston arrangement with an emergency pressure release channel can be provided at the top of the underwater vehicle when used for releasing gas because gas will move to the top of the interior volume of the vehicle, similar to the Fig. 5 arrangement of the relief valve. The vehicle may not be perfectly horizontal, and the gas will in that orientation move to the highest point, for example the bow, if the stern is lower than the bow. The gas may also rise to the highest point in a compartment. The pressure release channels are therefore provided in those places where the gas is most likely to accumulate. The gas release mechanism may be connected to a controller, which is arranged to activate the release mechanism when gas is detected by a gas detector, of when an excess build-up of pressure is detected by a pressure sensor.

The pressure release mechanism described above may further be used for preventing an excessive overpressure or underpressure due to the inertia of the bulk fluid during acceleration or deceleration of the vehicle. Pressure release mechanisms for that purpose are provided at the front or rear of the internal volume containing large amounts of bulk fluid. When the vehicle decelerates, for example, the inertia of the bulk fluid creates an overpressure towards the front, which is the part facing the direction of travel, and an underpressure at the rear. Although typically the vehicle will be designed to be strong enough to withstand pressure fluctuations due to accelerating or decelerating, the pressure release mechanism can be used to prevent excessive pressure fluctuations. A failsafe mechanism may also be used, in which case some bulk fluid will be expelled during a failure occurring during sudden changes of movement of the vehicle, and the expelled fluid is replaced by letting in replacement fluid, such as seawater.

Fig. 6 further illustrates the functioning of the pressure compensation system. If the vehicle rises by an amount H to a shallower depth, the hydrostatic pressure will decrease. The compensation system adapts by moving the piston by a distance D to increase the effective internal volume of the vehicle, and thereby equalises the pressure.

The size of the volume to be displaced by movement of the moveable barrier to compensate for pressure variations will vary depending on the parameters of the particular situation. For example, for a vehicle with nominal payload volume of 15000 m3, and a volume to be filled inside hull of 10000 m3, a temperature variation of 25 degrees will give rise to a change in density of about 1.5%. This will require a displacement of water at the movable barrier similar to 150 m3. This could be achieved by one large system or three parallel systems of 50 m3, e.g. three pressure compensating devices with a volume of at least 50 m3 each, located at the top front, top middle, and top back. However, this is only a specific example for illustration, and each implementation will provide their own set of numerical values.

Figure 7 is a method diagram, illustrating the main method steps described previously.

Si: controlling a pressure within an autonomous underwater vehicle, the vehicle as described above, S2: moving the movable barrier to compensate for a pressure change in the bulk fluid or in the fluid outside the vehicle.

Although the invention has been described in terms of preferred embodiments as set forth above, it should be understood that these embodiments are illustrative only and that the claims are not limited to those embodiments. Those skilled in the art will be able to make modifications and alternatives in view of the disclosure which are contemplated as falling within the scope of the appended claims. Each feature disclosed or illustrated in the present specification may be incorporated in the invention, whether alone or in any appropriate combination with any other feature disclosed or illustrated herein.

Claims

CLAIMS: 1. An autonomous underwater vehicle for transporting a cargo, comprising: an outer hull defining an internal volume; one or more internal containers for containing the cargo; a pressure compensation system arranged to control the pressure of the internal volume based on exterior pressure; the pressure compensation system comprises a movable barrier, wherein the movable barrier is arranged within a housing, and wherein the movable barrier separates a bulk fluid provided within the interior volume from fluid outside the vehicle.
2. The autonomous underwater vehicle of claim 1, wherein the movable barrier is arranged to compensate pressure fluctuations within the interior volume, or pressure fluctuations outside the vehicle.
3. The autonomous underwater vehicle of claim 1 or 2, wherein the movable barrier is a piston member received within the housing, and wherein the housing defines a path of travel for the piston member.
4. The autonomous underwater vehicle of claim 3, wherein the housing comprises a plurality of piston members arranged in series.
5. The autonomous underwater vehicle of claim 3, wherein the housing comprises one or more emergency relief channels.
6. The autonomous underwater vehicle of claim 1 or 2, wherein the pressure compensation system comprises a flexible sheet received within the housing.
7. The autonomous underwater vehicle of any one of the preceding claims, wherein the housing comprises a first inlet fluidly connected to the interior volume of the vehicle and a second inlet fluidly connected to the fluid surrounding the vehicle.
8. The autonomous underwater vehicle of any one of the preceding claims, further comprising a detector arranged to measure displacement of the movable barrier.
9. The autonomous underwater vehicle of any one of the preceding claims, further comprising a pressure sensor arranged within the internal volume.
10. The autonomous underwater vehicle of any one of the preceding claims, further comprising a gas detector arranged within the internal volume.
11. The autonomous underwater vehicle of any one of the preceding claims, further comprising an emergency relief valve comprises a piston member and a side opening.
12. The autonomous underwater vehicle of any one of claims 8 to 10, further comprising a controller coupled to one or more of the detectors of claims 8 to 10, and an emergency relief valve, wherein the controller is arranged to open the emergency relief valve depending on measurement signals received from the one or more detectors.
13. The autonomous underwater vehicle of any one of the preceding claims, further comprising a plurality of said pressure compensation systems, and an active control system, arranged to control the operation of the plurality of pressure compensation systems.
14. The autonomous underwater vehicle of any one of the preceding claims, further comprising one or more valves for opening, closing, or restricting the flow of fluid into the housing from one or both of the internal volume or the outside of the vehicle.
15. A method of controlling a pressure within an autonomous underwater vehicle, the vehicle comprising an outer hull defining an internal volume; one or more internal containers for containing the cargo; a pressure compensation system comprises a movable barrier, wherein the movable barrier is arranged within a housing, and wherein the movable barrier separates a bulk fluid provided within the interior volume from fluid outside the vehicle; the method comprising: moving the movable barrier to compensate for a pressure change in the bulk fluid or in the fluid outside the vehicle.
16. The method of claim 15, further comprising: detecting a displacement of the movable barrier, and controlling the opening of a pressure relief valve depending on the displacement.
17. The method of claim 15 or 16, further comprising measuring a pressure of the bulk fluid, and/or measuring the presence of a gas within the internal volume, and operating the movable barrier or a pressure relief valve in response to said measuring.
18. The method of claim 15, 16 or 17, wherein the bulk fluid is selected depending on the density of the bulk fluid.