DESCRIPTION

The present invention relates to a subsea multiphase fluid separation system, i n particular to a gas-liquid and liquid-liquid gravity separator for subsea applications. The subsea separator according to the invention is particularly useful for separating gas, oil and water phases in multiphase flow streams at oil fields located subsea.

As known, oil, water and gas separation of well fluids is the first core operation before further treatments. Optimal solutions for multiphase oil/water/gas flow separation are vital for further successful oil and gas development and exploitation.

With the rapid development of marginal subsea fields once thought to be unprofitable due to the severe conditions and expense involved for exploiting the available resources, more and more companies are looking towards subsea processing as one of the main methods of reducing costs. Separating fluids subsea will avoid lifting large volumes of water to the surface for processing and disposal. This can reduce lifting costs and allow economies in topside water processing and handling capacities and could extend the economic life of the deep-water projects and reduce development risks.

At present, a number of solutions have been proposed that however are not completely satisfactory due to a number of reasons.

In particular, subsea multiphase fluid separation systems have been disclosed which employ a wide varieties of separation techniques (e.g. gravitational, centrifugal) or combinations of them.

Multiphase fluids separators based on gravity comprise preferably cylindrical vessels horizontally arranged, for separation of fluids such as oil, water and gas suitable in a number of context in process plants. Specific improvements to provide compact and efficient separation vessels which can be used in subsea fluid processing have been disclosed.

For instance, U.S. Pat. Appl. No. 2010/0326922 discloses a cylindrical separator which includes in the separator a multifunctional surface super-hydrophobic with respect to water in order to provide a compact and efficient separation vessel.

In U.S. Pat. No. 4,214,883 and US 7,785,400 spherical separators, particularly suited for use in separating natural gas from high pressure, high velocity production streams, are disclosed. Both the separators comprise an interior surface defining a spherical interior space. In particular, in U.S. Pat. No. 4,214,883 a baffle is provided in the housing for obstructing direct flow from the liquid gas inlet to the gas outlet. In U.S. Pat. No. 7,785,400 a nozzle deflector and an inlet deflector are provided i nside the separation vessel in order to spread and to encourage the flow of a stream introduced into vessel to turn downward and radially outward toward the walls of vessel.

Multiphase fluids separators based on a cyclone separator generally consists of a tube in which a central flow body is arranged. As a result of the centrifugal forces occurring due to the rotation, the relatively heavy fraction of the mixture is flung outward, while the relatively light fraction of the mixture is displaced in a zone along the flow body. Because the light and heavy fractions are displaced in separate zones, a separation of the mixture can be effected by arranging outlet provisions at a suitable location, and the separated light and heavy fractions can be discharged separately.

U.S. Pat. No. 2,588,863 discloses a first attempt of de-sanding spherical centrifugal separation. The shell of the separator has a top fluid discharge nipple and a bottom sediment outlet. It is not explicitly highlighted a possible subsea application. The spherical shell is provided with a co-center, inner vertical axis cylinder purposely angled to be highly favorable in making an additional through-off of suspended particles in the flowing liquid (such as water). Liquid flowing rapidly in the inlet tube onto a ramp will at once begin to rotate in a helical path.

One particularly approach to separating wellbore fluids into gas and liquid phase streams at a subsea location is the vertical annular separation and pumping system (VASPS) that was disclosed in U.S. Pat. No. 4,900,433. A VASPS unit is frequently used as part of a subsea multiphase boosting system and artificial lifting method to increase reservoir production rates. A VASPS is a two-phase (gas-liquid) separation and pumping system which may be installed subsea. A further improvement of the VASPS unit described in U.S. Pat. Appl. No. 20090211763 which discloses a self-contained unit which includes an outer pressure housing, an inner helix separator assembly, a gas discharge annulus, a liquid discharge tube.

Cyclone separators are used in a large number of situations. Inlet cyclones are, for instance, applied in gravity separation vessels. Inlet cyclones ensure that the incoming mixture undergoes a determined pretreatment before a further separation takes place. The inlet cyclone can be connected for this purpose to the inlet of the gravity separation vessel and provided with an outlet for the heavy fraction and an outlet for the light fraction, wherein both outlets debouch in the interior of the gravity separation vessel for further separation of the mixture. Examples of an inlet cyclone are described in the European patent application EP 1 187 667 A2, and in U.S. Patent No. 7,833,298.

Another type of cyclone separator is the so-called in-line separator in which the incoming mixture and at least a part of the outgoing mixture flows through a pipeline, wherein the separator is essentially aligned with the pipeline. In-line cyclone separators can be subdivided into two different types.

In a first type, also known as a " degas ser," the separator separates gas from liquid. The degasser is used, in the case of the gas/liquid mixture, when the continuous phase is liquid. An example of a degasser is known from WO 01/00296. In the degasser, the liquid continuous flow is set into rotation by a number of guide blades causing swirling. Because of the difference in density between the gas and liquid and the initiated centrifugal field, the gas is urged to the middle of the separator, thus producing a stable core of gas. Removal of the gas core is brought about by means of a gas discharge pipe arranged in the middle of the cyclone and provided with outlet openings. Because of the geometry of the separator, removal of the gas via the outlet openings takes place in radial direction.

A second type of in-line cyclone separator is a separator, also referred to as a "deliquidizer", in which a gas continuous feed is set into rotation by a number of guide blades causing swirling. The deliquidizer in this case separates the liquid from the gas. The liquid is urged in the direction of the pipe wall, which results in a stable liquid film (layer) which is displaced in the direction of the gas outlet. In the outlet zone, the gas and the liquid are separated at a fixed position in the flow. The gas outlet is a cylindrical open pipe which is fixed in the flow space of the separator. The gas is discharged in longitudinal direction. An example of a deliquidizer is described in WO 02/056999 Al.

Known subsea separation processes are normally based on the use of conventional equipment that operates in a manner consistent with topside operations. These solutions fail to address all issues associated with separation in the subsea application. Furthermore, it is required an high complexity to match the needed efficiency, reducing the reliability/availability of the system, and/or the size is too large as to make the system economically unfeasible for use at significant depth.

Based on these remarks, there is clearly the need of providing a subsea multiphase fluid separation system which allows the above drawbacks to be eliminated or minimized.

Thus, the main task of the present invention is to provide a subsea multiphase fluid separation system which overcomes or minimizes the drawbacks of known systems.

In particular, within the scope of this task, an object of the present invention is to provide a subsea multiphase fluid separation system that would avoid lifting large volumes of water to the surface for processing and disposal.

Another object of the present invention is to provide a subsea multiphase fluid separation system capable to guarantee successful operation i n a wide range of 3-phase flow regimes also in presence of a solid phase.

Still another object of the present invention is to provide a subsea multiphase fluid separation system, that will not require or will minimize the equipment for post- treatment of the outgoing fluid stream(s).

A further object of the present invention is to provide a subsea multiphase fluid separation system, with increased reliability and limited maintenance,.

Yet another object of the present invention is to provide a subsea multiphase fluid separation system having compact structure and dimensions and being usable even at significant depths.

A further object of the present invention is to provide a subsea multiphase fluid separation system in which the problems associated with the production of solids that may cause clogging of equipment as well as damage to downstream systems are avoided or minimized.

Yet a further object of the present invention is to provide a subsea multiphase fluid separation system which is easy to manufacture at competitive costs.

Thus, the present invention therefore relates to a gas-liquid and liquid-liquid gravity separator which is characterized in that it comprises a main body having a substantially spherical shape and defining an internal volume comprising a first, gas-liquid, separation zone and a second, liquid-liquid, separation zone; said body comprises a feed inlet, a gas outlet, a first liquid outlet and a second liquid outlet, the feed inlet being positioned on the top of said main body and comprising an inlet pipe vertically protruding inside said main body toward its center and extending in said first, gas-liquid, separation zone and in said second, liquid-liquid, separation zone; said inlet pipe comprises a plurality of first openings positioned in said first, gas-liquid, separation zone and a plurality of second openings positioned in said second, liquid-liquid, separation zone; said gas outlet is positioned on the top of said main body, while said first liquid outlet is positioned at the bottom of said main body and said second liquid outlet is positioned on the lateral wails of said main body at an intermediate height with respect to said gas outlet and to said first liquid outlet.

The first liquid is normally a water-based liquid phase, while the second liquid is normally an oil-based liquid phase.

For the purposes of the present invention, the relative terms, e.g. above/below, top/bottom., as well as the terms vertical horizontal relate to the separator configuration under operation conditions. Also, the term "substantially spherical shape" is meant to include also geometries approaching the sphere, e.g. an ellipsoid or two hemispheres vertically connected by a relatively short cylindrical portion, the maximum extension in the vertical direction being not greater than 1.5x the maximum, diameter of the separator in the horizontal plane.

In practice, in the separator of the present invention the shape and the internal configuration have been configured so as to achieve effective gas-liquid and liquid-liquid separation, maintaining at the same time the advantages of a compact size that makes it particularly suitable for subsea applications.

As better explained in the following description, thanks to the interior configuration and the positioning of the various openings in the inlet pipe, as well as the relative positioning of the liquids and gas outlets, it is possible to maximize the separation effectiveness and minimize the total volume required by the separator, with consequent advantages in terms of transportation and location in crowded or demanding processing areas, like in subsea applications.

In practice, it has been seen that it is advantageous to dedicate more than half of the internal volume of the main body of the separator to the second, liquid-liquid, separation zone, and less than half to the first, gas-liquid, separation zone. This can be achieved by positioning the second liquid outlet at least at half-height, or slightly above, of the internal volume and by having an appropriate liquid hold-up inside the separator.

Advantageously, the subsea separator according to the invention comprises a first deflector positioned in said first, gas-liquid, separation zone in an intermediate position between said first openings and said gas outlet. Preferably the positioning of said first deflector is such that it is closer to the first opening than to said gas outlet.

As an example, the first deflector, that can be, e.g., a substantially horizontal circular plate, is fixed around said inlet pipe and radially extends from it toward the walls of the main body.

Moreover, the subsea separator according to the invention advantageously comprises a second deflector which positioned in the second, liquid-liquid, separation zone. Preferably, said second deflector is fixed at the lower end of said inlet pipe so as to close it, immediately below said second openings.

As for the first deflector, also the second deflector can be a substantially horizontal circular plate which extends from the inlet pipe toward the walls of the main body.

The dimensions, e.g. diameter, of the second deflector, are generally smaller than those of the first deflector.

The length of the inlet pipe may vary according to the needs, provided that the requirements concerning the positioning of the first and second openings are met. Preferably said second openings are positioned at an height lying between said first liquid outlet and said second liquid outlet. In particular, it can be preferable to position said second openings immediately below said second liquid outlet. With the term "immediately below" it is meant that the distance of the second openings from, the second liquid outlet is less than 10% of the distance between said first liquid outlet and said second liquid outlet.

In a particularly preferred embodiment of the subsea separator according to the present invention, said subsea separator comprises a cylindrical weir positioned in said second, liquid- liquid, separation zone around said inlet pipe, the base edge of said weir being positioned at an height lying between said first liquid outlet and said second openings, the upper edge of said weir being positioned at an height lying above said second liquid outlet. In particular, it has been seen that the presence of a weir allows to avoid, or at least minimize, the possible drag of the first liquid, i.e. water, through the second liquid outlet, i.e. the oil outlet.

Preferably, the base edge of the cylindrical weir is fixed on the internal walls of said main body, while the upper edge of said weir is positioned, as in the previous case, at an height lying above said second liquid outlet. In this case, the weir preferably comprises a number of grooves positioned in correspondence of said base edge, said grooves helping avoiding sand deposition and packing and providing an outlet to the water and sand eventually carried by the oil beyond the cylindrical weir.

Preferably said first openings comprises a number of vertically extending slots circumferentialiy positioned on said inlet pipe and, more preferably, said vertically extending slots comprise a series of short slots and a series of long slots circumferentialiy positioned in alternated sequence on said inlet pipe. Indeed, it has been seen that in order to reduce the gas exit velocity and to obtain a more uniform flow field, the inlet flow area should preferably be as large as possible. To this purpose, instead of using circular holes it is preferable to use slots. A particular configuration with short slots alternate with long slots has been seen to give better results in terms of separation efficiency and flow distribution.

Moreover, the subsea separator according to the invention preferably comprises a cylindrical shield positioned in said first, gas-liquid, separation zone in order to avoid the deposition of water droplets directly in the oil recovery zone. When a weir is present, the diameter of said cylindrical shield is preferably lower, e.g. from 100 to 200 mm less, than the diameter of said cyl indrical weir, and the base edge of the cylindrical shield extends below, e.g. from. 50 to 100 mm. the upper edge of the cylindrical weir in order to enhance the separation of oil droplets entrained by the oil flow toward the edge of the cylindrical weir. Further features and advantages of the present invention will be more clear from the description of preferred but not exclusive embodiments of a gas-liquid and liquid-liquid gravity separator according to the invention, particularly for subsea applications, shown by way of examples in the accompanying drawings, wherein:

Figure 1 is a section view of a first embodiment of a subsea separator according to the invention;

Figure 2 is a perspective view of the subsea separator of Figure 1 ;

Figure 3 shows some details of the subsea separator of Figure 1 ;

Figure 4 is a perspective view of the subsea separator of Figure 3;

Figure 5 shows some details of an alternative embodiment of the subsea separator of

Figure 1 ;

Figure 6 is a section view of a second embodiment of a subsea separator according to the invention;

Figure 7 is a perspective view of the subsea separator of Figure 6;

Figure 8 is a section view of a third embodiment of a subsea separator according to the invention;

Figure 9 is a perspective view of the subsea separator of Figure 8;

Figure 10 is a perspective view of a fourth embodiment of a subsea separator according to the invention.

With reference to the attached figures, a gas-liquid and liquid-liquid gravity subsea separator, designated with the reference numeral 1, in its more general definition, comprises a mai n body 2 which has substantially spherical shape. In the embodiments represented in the attached figures the main body 2 comprises a first 11 and second 12 hemisphere directly joined together. As previously explained, other possible geometries of the main body 2, e.g. two hemispheres with an interposed cylindrical section of relatively low height, are also possible.

One of the distinguishing features of the subsea separator of the present invention is given by the main body 2 that defines an internal volume which comprises a first, gas-liquid, separation zone 21 and a second, liquid-liquid, separation zone 22. In practice, the first gas- liquid, separation zone 21 is positioned in the upper half of the main body 2, while the second, liquid-liquid, separation zone 22 in the lower half thereof.

In the gas-liquid separation zone 21 the gas is separated from the liquid phases, while in the liquid- liquid separation zone 22 a first liquid phase 221 (i.e. water phase) is separated from a second liquid phase 220 (i.e. oil phase) by gravity, as better explained hereinafter. The main body 2 further comprises a feed inlet 3 wh ich comprises an inlet pipe 31 which vertically protrudes inside said main body 2 toward its center. More in particular, as shown in the figures, a further distinguishing feature of the subsea separator of the present invention is given by the feed inlet 3 which is positioned on the top of said main body 2 and by inlet pipe 31 which vertically extends in said first, gas-liquid, separation zone 21 and in said second, liquid-liquid, separation zone 22.

Furthermore, the inlet pipe 31 comprises a plurality of first openings 311 which are positioned in said first, gas-liquid, separation zone 21 and a plurality of second openings 312 which are positioned in said second, liquid-liquid, separation zone 22.

The main body 2 further comprises a gas outlet 4 which is positioned on the top of said main body 1, as well as a first liquid outlet 5 and a second liquid outlet 6.

According to preferred embodiments, the gas outlet 4 is concentrically positioned around said inlet pipe 31, as shown in the attached figures.

A still further distinguishing feature of the subsea separator of the present invention is given by the relative positioning of the various gas and liquids outlets. In particular, the first liquid outlet 5 is positioned at the bottom of said main body 2 and said second liquid outlet 6 is positioned on the lateral walls of said main body 1 at an intermediate height with respect to the gas outlet 4 and to said first liquid outlet 5.

Thanks to the particular configuration of the subsea separator 1 it is possible to optimize its internal volume exploitation and maximize the separation efficiency. As already explained, it has been seen that it is advantageous to dedicate more than half of the internal volume of the main body 2 of the separator 1 to the second, liquid-liquid, separation zone 22, and less than half to the first, gas-liquid, separation zone 21.

In particular, with reference to the figures, this can be achieved by positioning the second liquid outlet 6 at least at half -height, or slightly above, of the internal volume and by maintaining an appropriate liquid hold-up inside the separator.

Thus, with reference to figure 1, substantially all gas present in a multiphase flow stream (i.e. gas, water and oil) entering the separator via the inlet pipe 31 is "vented off, together with some liquid, through the openings 311 into the first, gas-liquid, separation zone 21. In other words, from the openings 311 the gas-containing stream 101 is radially projected into the first, gas-liquid, separation zone 21.

Differently from conventional cylindrical separator in which the front section of the advancing gas is constant, in the separator of the present invention the section increases while the gas is advancing in the gas-liquid separation zone 21 due to the spherical geometry, with consequent stronger decrease of velocity. As a consequence, the efficiency of separation of entrained liquid droplets from the gas stream is considerably increased.

Then, liquid-liquid separation takes place in the second separation zone 22 in which the stream 100 enters through the openings 312. In such zone, the water phase 221 (first liquid phase) separates by gravity from the oil phase 220, as schematically represented by the arrows 103 (water falling down and leaving the separator 1 through the outlet 5) and 102 (oil lifting up and leaving the separator 1 through the outlet 6).

The interface between the two phase 220 and 221 defines the water level 223, while the oil level 222 defines the interface between the liquid phases and the gas phase, as well as the distribution of the total internal volume of the main body 2 between the gas-liquid separation zone 21 and the liquid- liquid separation zone 22. The distribution of the total volume between the two separation zones 21 and 22 can be adapted by appropriately positioning the oil outlet 6 and regulating the hold-up level 222, e.g. by regulating the pressures.

As shown in the figures 1 and 3-5, the positioning of the second openings 312 can be varied according to the needs but it is preferable that such positioning is above the water level 223 inside the second separation zone 22. In particular, it can be preferable to position said second openings 312 immediately below the second liquid outlet 6. As already said, with the term "immediately below" it is meant that the vertical distance of the second openings 312 from the second liquid outlet 6 is less than 10% of the vertical distance between said first liquid outlet 5 and said second liquid outlet 6.

According to a preferred embodiment of the subsea separator 1 of the present invention, said separator 1 advantageously comprises a first deflector 51 which is positioned in said first, gas-liquid, separation zone 21 in an intermediate position between the first openings 311 and the gas outlet 4. The first deflector 51 divides the first, gas-liquid, separation zone 21 into two sub-zones communicating through a restricted flow area 510 and allows exploiting at maximum the available volume for gas-liquid separation. It is worth noting that the volume of the sub-zone below the first deflector 51 is normally much higher than the volume of the sub-zone above the first deflector 51

In practice, as shown in the attached figures, the first deflector 51 can be a substantially horizontal circular plate which is fixed around the inlet pipe and radially extends from it toward the wails of the main body 2, leaving an annular gap 510 for the passage of the gas flow stream 101. The positioning of the first deflector 51 on the inlet pipe 31 may vary according to the needs. It has been seen that particularly good results can be obtained when the deflector 51 is closer to the first opening 311 than to the gas outlet 4, as shown in figure 7, 9 and 10. This can be achieved also by positioning the first openings 311 at a certain distance from the liquid level 222, as shown in figure 7, 9 and 10, instead of being close to it, as shown in figure 1.

According to a further preferred embodiment of the subsea separator 1 of the present invention, said separator 1 advantageously comprises also a second deflector 52 which is positioned in said second, liquid-liquid, separation zone 22.

As shown in the attached figure, the second deflector 52 is preferably fixed at the lower end of said inlet pipe 3 1 so as to close it, the second openings 3 1 2 being positioned immediately above it. In this way the flow stream 100 leaving the inlet pi e 3 1 flows onto the second deflector 2 and is radial ly distributed in the second liquid-liquid separation zone 22.

In other words, the presence of the second deflector 52 allows achieving much higher separation efficiencies, since the second deflector 52 greatly contributes to better distribute the liquid flow 100 in the second, liquid- liquid, separation zone 22.

As for the first deflector 5 1 . also the second deflector 52 can be a substantially horizontal circular plate which extends from the inlet pipe 3 1 toward the wal l s of the mai n body 2. As shown in the attached figure, the diameter of the second deflector 52 is generally smaller than the diameter of the first deflector 51.

In a preferred embodiment of the subsea separator 1 according to the present invention, represented in figures 6- 10, the separator 1 further comprises a cylindrical weir 7, 8 which is positioned i n said second, l iquid-l iquid, separation zone 22 around the inlet pipe 3 1 .

In general, the base edge 7 1 . 81 of said weir 7, 8 is positioned at an height lying between the first l iquid outlet 5 and the second openings 3 1 2. while the upper edge 72. 82 of the weir 7, 8 is positioned at an height lyi ng above the second l iquid outlet 6.

In this way the most turbulent portion of the second separation zone 22 is kept separated from the oil stream outlet 6, thereby avoiding water entrain ment in the oil stream leaving the separator 1.

In practice, with reference to figures 6 and 7. the weir 7 can be a cylinder fitted inside the main body 2 of the separator 1, around the inlet pipe 31, in particular around a portion of the inlet pipe 3 1 compri sing the second opening 3 1 2. In the embodiment of figures 6 and 7. the weir 7 is fixed to the main body 2 of the separator 1 via a supporting annular structure 7 1 1 .

In an alternative, much preferred solution, represented i n figure 8, 9 and 10, the subsea separator 1 comprises a cylindrical weir 8 which is fitted i nside the mai n body 2 of the separator 1 . around a portion of the inlet pipe 31 comprising the second opening 312. In this case, the base edge 81 of the weir 8 is fixed directly on the internal walls of the main body 2 of the separator 1.

In practice, differently from the previous case, in this embodiment the weir 8 extends down in the second liquid- liquid separation zone, below the first liquid (water) level 223. As in the previous case, the upper edge 82 of the weir 8 is positioned at an height lying above the second liquid outlet 6. Moreover, in order to avoid or reduce at minimum sand deposition and packing, and allow the outflow of water entrained by the oil flow over the weir, the weir 8 can preferably comprise a number of grooves 810 positioned in correspondence of said base edge 81.

In general, the shape and dimensions of the first 311 and second 312 openings may vary according to the needs. For instance, the first openings 311 may be substantially circular openings as shown in figures 1, 3, 4, 5, 6, and 8.

However, according to a preferred embodiment, the first openings 311 may comprise a number of vertically extending slots 321, 322 which are circumferentially positioned on said inlet pipe 31, as shown in figures 7, 9 and 10. In practice it has been seen that by using slots instead of circular holes a higher separation efficiency is achieved, as gas velocity is reduced and a more uniform flow field is obtained. Particularly good results have been obtained by using a series of relatively short slots 321 and a series of relatively long slots 322 which positioned in an alternated sequence (i.e., long, short, long, ...) on the circumference of the inlet pipe 31.

In a further preferred embodiment, illustrated in figure 10, the subsea separator 1 according to the present invention advantageously comprises a cylindrical shield 9 which is positioned in said first, gas-liquid, separation zone 21, and which has a diameter lower than the diameter of said cylindrical weir 8.

In practice, under operating conditions, water condensation phenomena on the internal walls of the main body 2 in the first, gas-liquid separation zone 21 may occur. As a consequence, water droplets, slipping or dropping from the walls of the gas-liquid separation zone 21, will fall into the oil phase 220 contaminating it. Thus, according to this embodiment, possible water condensation will take place on the surface of the cylindrical shield 9. Consequently, by appropriate dimensioning the cylindrical shield 9, it will be possible to convey condensed water in the separation zone 22 inside the weir 8 and limit its entrainment in the oil stream leaving the separator. In presence of solids (sand), it is expected that the solids, which are heavier than water, follow the path of the water phase toward the water exit, at the bottom of the separator.

As is clear from the above description, the technical solutions adopted for the subsea separator according to the present invention allow the proposed aims and the objects to be fully achieved.

The subsea separator according to the invention is extremely compact and easy to install and maintain due to its simplicity. Indeed, the compact structure is a great advantage considering the transportation and installation problems connected to the positioning in operation subsea.

At the same time, its particular design, specially of its interior, allows to exploit at maximum its internal volume for separation, thereby achieving a very high separation efficiency in spite of its relatively small dimensions. Such high efficiency allows avoiding lifting large volumes of water to the surface for processing and disposal.

Contrary to prior art equipment, the subsea separator of the present invention is capable of carrying out effective gas-liquid and liquid-liquid separation without the need of further or ancillary equipment, such as a cyclone separator for the pre-treatment of the incoming fluid stream.

Also, since there are substantially no mechanical moving parts and devices for its functioning, grant high reliability and limited maintenance, which is a considerable advantage due to its operating location subsea.

Several variations can be made to the subsea separator thus conceived, all falling within the scope of the present invention. In practice, the materials used and the contingent dimensions and shapes can be any, according to requirements and to the state of the art.