You are an expert at summarizing long articles. Proceed to summarize the following text:

You are an expert at summarizing long articles. Proceed to summarize the following text: 
FIELD OF THE INVENTION 
     The present invention relates to a seat portion or foundation structure on the sea bottom or on the bed of a river or a stream, for holding in position one or several hydraulic turbine engines, in particular a hydraulic turbine engine for supplying electricity by recovery of the energy of marine or river currents. 
     DISCUSSION OF PRIOR ART 
     Among natural non-polluting power sources, a currently rather unexploited power source corresponds to water currents naturally present on our planet, for example, high sea currents, tidal currents, strait and estuary currents, stream or river currents. Indeed, whilst hydroelectric power plants providing electric energy from the potential energy contained in an impoundment, for example, dams installed on streams or rivers, are widespread, devices supplying electric energy directly from the kinetic energy of marine or river currents are generally still currently at the stage of projects. 
     Even though sites which could be used for the supply of electric power from marine or river currents generally correspond to currents of low velocity, from 0.5 m/s to 6 m/s, the size of the sites and the large number of possible sites make such a power source particularly attractive. Indeed, from rivers to large ocean currents, the exploitable surface areas crossed by a current typically vary from 100 m 2  to 100 km 2 , which corresponds, for a 2-m/s velocity, to respective theoretically recoverable powers ranging from 400 kilowatts to 400 gigawatts. 
     Devices for recovering and converting the kinetic energy of sea or river currents generally comprise a turbine comprising an assembly of blades adapted to rotate a shaft when they are immersed in the current. Among the different types of turbines, one can distinguish axial flow turbines for which the flow direction is parallel to the turbine rotation axis and cross-flow turbines for which the flow direction is inclined, and generally perpendicular with respect to the turbine rotation axis. An example of a cross-flow hydraulic turbine engine is described in patent FR2865777 filed by the applicant. 
     A general feature of hydraulic turbine engines is the presence of a drag force in the incident current direction. The drag force tends to sweep the hydraulic turbine engine away with the current and increases along with the extracted mechanical power. A seat or foundation structure thus has to be provided for the hydraulic turbine engine on the sea or river bed to resist the drag force. Peter Fraenkel&#39;s communication entitled “Tidal &amp; Marine Current Energy” (Franco-British Marine Energies Seminar, Le Havre, Jan. 19-20, 2006) describes examples of hydraulic turbine engine foundation structures. Foundation structures may be distributed in five large groups, each having many variations: 
     (i) Piles: these are prefabricated (steel or concrete) elements which may be driven into the sea or river bed by piling or be installed by drilling. This type of foundation structure is reliable (good resistance to pulling out) and long-lasting. It however has several disadvantages. The drilling or piling operations necessary to install the piles are technically difficult, which limits exploitable depths to 40 m, while many interesting sites have deeper beds. Then, the bottom of the site where the foundation structure must be installed must have good geomechanical features, especially for the drilling. Finally, such foundation structures require the presence of underwater monitoring devices. 
     (ii) Suction anchors: these are hollow anchors having, for example, a cylindrical or trihedral shape. They are driven into the ground by pumping of the water inside of them. Such anchors may have a height ranging from up to 10 to 25 m with a diameter ranging from 3 to 7. The vacuum which forms inside makes the anchor difficult to pull off. The equipment necessary to install a suction anchor is simpler than that to be provided for a pile, since nothing but a pump is required to create vacuum inside of the anchor. This enables to consider the securing of hydraulic turbine engines at great depths. Suction anchors however remain difficult to install since a proper orientation and depressurization of the anchor have to be ensured. Further, suction anchors are heavy and bulky. They thus have a high installation cost (especially due to the transportation to the site) in common with piles. Further, like piles, suction anchors require specific grounds (sands, clays). 
     (iii) Gravity foundations: the hydraulic turbine engine is attached to a heavy body, for example, a strengthened concrete block or plate, which is placed at the bottom of the water. The hydraulic turbine engine attached to the heavy body is stabilized by its weight and by the friction exerted by the heavy body on the ground. This is also the operating principle of VLAs (Vertical Load Anchor), similar to boat anchors, which spontaneously penetrate into the ground due to their weight. A heavy body has a low manufacturing cost. However, transporting the heavy body to the installation site is expensive. Further, to install the heavy body, the sea or river bed of the installation site must be prepared, which may be difficult. Moreover, such heavy bodies do not respond well to horizontal loads and are further sensitive to scouring. Besides, VLAs require soft grounds. 
     (iv) Floating structures: such floating structures may be emerged, like oil drilling barges, or partly submerged. In all cases, they are moored to the bottom the water base plate by cables connected to anchoring systems which may correspond to the above-mentioned foundation structure examples. The holding of the floating structure must take into account the load due to the most dangerous waves. Accordingly, the floating structure and the associated anchoring systems must be oversized with respect to the nominal operating rate of the structure. Such a solution is thus expensive. Further, the use of cables and their mooring to the floating structure are a source of wearing and accidents due, in particular, to the vertical oscillations of the floating structure. Further, floating structures take up the sea surface (hindrance or incompatibility with the fishing or marine transportation activity, visual pollution, etc.). 
     (v) Anchored base plates: they are comprised of a plate comprising an upper surface and a lower surface. The floor or base plate is attached to an anchoring or foundation system on its lower surface side. The hydraulic turbine engine is attached to the upper surface of the base plate. The hydraulic turbine engine is thus indirectly connected to the anchoring or foundation system. The presence of the base plate has several advantages. First, it eases the design of the connection of the hydraulic turbine engine to the base plate (clamping, pin joint, etc.). Similarly, there is more liberty as to the anchoring or foundation system. Instead of a single anchoring system, for example, such as the foundation structures previously described at points (i), (ii), or (iii), the types of anchoring systems may be multiplied at the base plate periphery, and of smaller size. As an example, an embodiment of French patent FR2865777 describes hydraulic turbine engines which are attached to a common base plate, called a raised floor in this patent, itself connected to the bottom by cables attached to anchor studs. In patent GB2434413, a gravity solution is provided wherein a ballast system enables to set the horizontality of the base plate. However, although the use of a base plate enables to use smaller anchoring systems, the specific disadvantages of the installation of each of these anchoring systems remain. 
     SUMMARY 
     The present invention aims at a foundation structure for a hydraulic turbine engine comprising a base plate and which may adapt to a sea or river bed having any geomechanical features, for example, sands or clays, including beds having poor geomechanical features, for example, rocky beds or beds formed of cobbles or stones, or a bed having poor geometric features, for example, a non-planar bed, inclined with respect to Earth&#39;s gravity, having an uneven surface, etc.). 
     According to another object, the installation cost of the foundation structure is lower than or comparable to the cost of the actual turbine engine. 
     According to another object, the installation of the foundation structure is technically simple, fast, with no risk, and does not require heavy-duty technology or the presence of divers. 
     Thus, to achieve all or part of these and other objects, an embodiment of the present invention provides a foundation structure for at least one hydraulic turbine engine on a ground, comprising:
         a base plate comprising first and second opposite surfaces, said at least one turbine engine being intended to be arranged on the side of the first surface;   a first bearing element connected to the second surface in central position and intended to be in contact with the ground;   at least three arms, each arm comprising first and second opposite ends and being connected at its first end to the base plate by a pin joint, the arms being capable of pivoting with respect to the base plate between a first position in which the second ends are close to one another and a second position in which the arms extend radially from the base plate;   for each arm, a second bearing element connected to the second end and intended to be in contact with the ground;   for at least one arm, a positioning device capable of modifying the distance between the second end and the second associated bearing element; and   for each arm, a device for locking the arm in the second position.       

     According to an embodiment of the invention, the positioning device comprises a double-acting jack connecting the second end of the arm to the second associated bearing element. 
     According to an embodiment of the invention, the double-acting jack is oriented perpendicularly to the axis of the arm. 
     According to an embodiment of the invention, the foundation structure comprises a platform having third and fourth opposite surfaces, said at least one turbine engine being intended to be attached to the third surface, the fourth surface being opposite to the first surface of the base plate, the platform being capable of pivoting with respect to the base plate around an axis perpendicular to the first surface. 
     According to an embodiment of the invention, at least one bearing element from among the first bearing element and the second bearing elements corresponds to a mooring having a weight greater than 500 kilograms or to a suction anchor. 
     According to an embodiment of the invention, at least one bearing element from among the first bearing element and the second bearing elements comprises an elongated and/or pointed portion intended to be in contact with the ground. 
     According to an embodiment of the invention, the locking device comprises a deformable portion, a lock, and a stop element resting on said deformable portion, the associated arm bearing against the stop element and compressing said deformable portion in the second position, the stop element being capable of locking the lock when the arm is not in the second position and being capable of releasing the lock when the arm is in the second position, the arm being sandwiched between the lock and the stop element in the second position. 
     According to an embodiment of the invention, the first bearing element is connected to the second surface by a ball joint. 
     According to an embodiment of the invention, the foundation structure comprises, for each arm, a device for damping the pivoting of the arm from the first position to the second position. 
     An embodiment of the present invention also provides a method for installing the foundation structure such as defined hereabove. The method comprises the steps of:
         bringing the foundation structure to the ground level, the arms being in the first position;   pivoting the arms from the first to the second position;   bringing the second bearing elements into contact with the ground, the first bearing element already being in contact with the ground; and   setting the horizontality of the base plate via positioning devices and a system for measuring the horizontality of the base plate.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other objects, features, and advantages of the present invention will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings, among which: 
         FIG. 1  is a perspective view of an embodiment of a foundation structure according to the invention once installed on a sea or river bed; 
         FIG. 2  is a perspective view of the foundation structure of  FIG. 1  before an installation operation; 
         FIGS. 3A to 3C  are three simplified side views of the foundation structure of  FIG. 1  at successive steps of an installation operation; 
         FIGS. 4 and 5  are partial simplified views illustrating two embodiments of the connection between the base plate and the central mooring of the foundation structure of  FIG. 1 ; 
         FIGS. 6A to 6D  are partial simplified top views of embodiments of foundation structures provided with an increasing number of arms; 
         FIGS. 7 and 8  respectively are a perspective view and a cross-section view of the device for locking an arm of the foundation structure of  FIG. 1 ; 
         FIGS. 9 and 10  respectively are a side view and a cross-section view of the connection between one of the arms and the peripheral mooring associated with the foundation structure of  FIG. 1 ; 
         FIGS. 11A and 11B  are side views illustrating two embodiments of the peripheral mooring of the foundation structure of  FIG. 1 ; 
         FIG. 12  is a side view of a detail of the damping device of the foundation structure of  FIG. 2 ; 
         FIGS. 13 and 14  are partial simplified views illustrating two embodiments of the connection between a hydraulic turbine engine support platform and the base plate of the foundation structure of  FIG. 1 ; and 
         FIG. 15  is a view of a detail of  FIG. 2  illustrating the system for maintaining the arms of the foundation structure in folded position. 
     
    
    
     DETAILED DESCRIPTION 
     For clarity, the same elements have been designated with the same reference numerals in the different drawings and, further, only those elements which are useful to the understanding of the present invention will be described. 
       FIGS. 1 and 2  show an embodiment of a foundation structure  10  according to the invention. In  FIG. 1 , foundation structure  10  is shown in a configuration of use where it is arranged at the level of a sea or river bed, not shown in  FIGS. 1 and 2 . In  FIG. 2 , foundation structure  10  is shown in a storage configuration at the beginning of an operation of installation of foundation structure  10  on the sea or river bed. 
     Foundation structure  10  comprises a base plate  12  having parallel upper and lower surfaces  14  and  16 , with lower surface  16  facing the ground. Base plate  12  may be made of stainless steel or of aluminum. In the present embodiment, base plate  12  comprises a rectangular central planar plate  17  which extends at its four corners in elongated portions  18  coplanar with central plate  17 . As a variation, central plate  17  of base plate  12  may have a circular shape. A platform  19  is connected to upper surface  14  of base plate  12 . Platform  19  for example substantially entirely covers plate  17 . Platform  19  is intended to receive one or several hydraulic turbine engines, not shown. Call main axis of base plate  12  the axis Δ perpendicular to surfaces  14  and  16  and running through the center of gravity of base plate  12 . In the present embodiment, main axis Δ corresponds to an axis of symmetry of base plate  12 . As an example, central plate  17 , which, advantageously, may be strengthened by ribs radiating on lower surface  16 , is inscribed within a circle having a diameter of several meters, for example, a diameter of some ten meters, and has a thickness of several centimeters, for example, some ten centimeters. 
     A central mooring  20 , for example, made of concrete or of steel, is connected to lower surface  16  of base plate  12 . Central mooring  20  is intended to be placed against the sea or river bed and possibly to partially sink into the sea or river bed. Central mooring  20  has a shape with a symmetry of revolution having lateral dimensions smaller than those of central plate  17  of base plate  12 . As an example, for a base plate  12  having its central plate  17  inscribed within a circle of some ten meters, the lateral dimension of central mooring  20  is smaller than 2 or 3 meters. The weight of central mooring  20  especially depends on the weight and on the dimensions of the hydraulic turbine engine(s) intended to be connected to platform  19 . As an example, the weight of central mooring  20  may be on the order of several tons. Advantageously, the axis of revolution of central mooring  20  is substantially perpendicular to the ground. In the present embodiment, central mooring  20  comprises a hemispherical portion  22  which extends in a conical portion  24  having its tip directed towards the ground. When the ground can be assimilated to a planar surface perpendicular to the direction of Earth&#39;s gravity, the axis of revolution of mooring  20  coincides with the main axis of base plate  12 . As a variation, central mooring  20  may have a spherical shape, a tetrapod shape, or may be replaced with a suction anchor. 
     Foundation structure  10  comprises arms  26  on the periphery of base plate  12  which, in the operating configuration, extend radially with respect to base plate  12 , in line with elongated portions  18  of base plate  12 . Each arm  26  corresponds, for example, to a stainless steel or aluminum and may advantageously be strengthened by a lattice structure. The length of each arm  26  may vary between the value of a characteristic dimension of base plate  12 , for example, the radius of central plate  17 , and ten times this dimension. Each arm  26  is connected, at one end, to base plate  12  via a pin joint  28  provided on upper surface  14  of base plate  12  at the level of one of elongated portions  18 . Pin joint  28  enables the associated arm  26  to pivot in a plane perpendicular to upper surface  14  of base plate  12  and containing the main axis of base plate  12  between a folded position, shown in  FIG. 2 , and a deployed position, shown in  FIG. 1 . Each arm  26  is connected to a peripheral heavy body  30  at its end opposite to base plate  12 . More specifically, peripheral heavy body  30  is connected to the free end of the associated arm  26  by a positioning device  32  and by a strengthening device  34 , as will be discussed in further detail hereafter. Each peripheral heavy body  30  is, for example, made of concrete or of steel and has a shape which may be spherical. The weight of each peripheral heavy body  30  especially depends on the number and on the length of arms  26 , and on the dimensions and on the weight of the hydraulic turbine engine(s) intended to be connected to platform  19 . As an example, each peripheral heavy body  30  has a weight greater than 500 kilograms, preferably on the order of from one to two tons. Base plate  12  comprises, for each arm  26 , a locking device  36 , arranged on surface  14  of base plate  12  at the level of elongated portion  18  associated with arm  26 . Locking device  36  is capable of locking the associated arm  26  in the deployed position shown in  FIG. 1 . Further, as can be seen in  FIG. 2 , a damping device  38  connects each arm  26  to base plate  12 . 
       FIGS. 3A to 3C  are partial side views of foundation structure  10  of  FIGS. 1 and 2  at three successive steps of an operation of installation of foundation structure  10  on a sea or river bed. 
     At the beginning of the installation, base plate  12  is held in a substantially horizontal configuration by chains  40  and lowered down to the sea or river bed. For an installation on a sea bed, foundation structure  10  may be transported to the site by boat and be lowered down to the sea bed from the boat. For an installation on a river bed, foundation structure  10  may also be lowered down to the river bed by a crane located on the bank when the installation site allows it. 
     During the transportation and the lowering of foundation structure  10  down to the sea or river bed, arms  26  are in folded position and the ends of arms  26 , provided with peripheral heavy bodies  30 , are joined and held in a “bunch”, as shown in  FIG. 2 , and as will be discussed in further detail hereafter. Once central mooring  20  approaches or reaches the ground, arms  26  are freed ( FIG. 3A ). Under the effect of peripheral heavy bodies  30 , arms  26  tilt and place themselves flat against the ground after having followed a circular trajectory, as illustrated in  FIGS. 3B and 3C . When arms  26  have opened all the way to their deployed position, that is, in a direction parallel to surface  14  of base plate  12 , they are definitively locked in this position by locking devices  36 , not shown in  FIGS. 3A to 3C . Arms  26  then extend radially along the circumference of base plate  12  along a general direction substantially parallel to surface  14  of base plate  12 . Chains  40  may be removed before or after the tilting of arms  26 . 
     The installation of foundation structure  10  on the ground then carries on with the setting of the positions of peripheral heavy bodies  30  via positioning devices  32 . Indeed, each positioning device  32  enables to modify the distance between the free end of arm  26  and the associated peripheral heavy body  30 . The actuating of positioning device  32  thus enables to position the end of arm  26  at the desired height when the associated peripheral heavy body  30  lies on the ground. By modifying the height of the ends of each of arms  26  of foundation structure  10 , the orientation of base plate  12  with respect to Earth&#39;s gravity, and thus with respect to the flow, can be set. When foundation structure  10  is placed on an uneven, non-horizontal ground, etc. positioning devices  32  provide for the horizontality of base plate  12  and thus for the right positioning of the hydraulic turbine engine which will be assembled on platform  19 . At the beginning of the installation of foundation structure  10 , positioning devices  32  are set so that each peripheral mooring  30  is at a smaller distance from base plate  12 , measured from main axis Δ, than the distance separating central heavy body  20  from base plate  12 . Once peripheral moorings  30  are placed against the ground and the orientation of base plate  12  has been adjusted, at least one hydraulic turbine engine may be connected to platform  19 . After the assembly of the hydraulic turbine engine, a new setting of the orientation of base plate  12  may be necessary. Further, the orientation of base plate  12  may be regularly measured and adjusted if necessary during the hydraulic turbine engine operation. During the operation of the hydraulic turbine engine, the stress exerted by locking devices  36  which prevent the pivoting of arms  26  opposes the tilting stress generated by the drag forces exerted on the turbine engine. 
     On installation of foundation structure  10 , a system for measuring the horizontality of base plate  12  may be provided. Such a system comprises, for example, placing one or several inclinometers on one of surfaces  14  or  16  of base plate  12 . These are, for example, inclinometers sold by Geomecanics and Sensorex companies. This system enables to avoid the need for a visual control, from the surface or by a diver, of the installation of foundation structure  10 . The installation of foundation structure  10  can thus be easily performed at significant depths. Such inclinometers may transmit signals to the surface via electric wires or a radio transmitter. Such signals can then be used to control positioning devices  32 . As a variation, an unattended orientation system may be placed on base plate  12  to automatically process the signals provided by the inclinometers and to actuate positioning devices  32  according to these signals. The unattended orientation system thus enables to automatically set the horizontality of base plate  12 , with no external intervention. 
     The fact for arms  26  to be pivotally connected to base plate  12  enables to maintain, before the installation, arms  26  in a folded position, in which the free ends of arms  26  are assembled in a bunch. Further, when arms  26  are in folded position, each positioning device  32  is in the configuration for which peripheral mooring  30  is at its closest to the end of the associated arm  26 . This decreases the total bulk of foundation structure  10  during its transportation  10 , for example, by boat, and during its lowering down to the installation site. Once arms  26  are deployed, peripheral moorings  30  are distributed around base plate  12  and distant from base plate  12  by the distance of arms  26 . This enables to obtain significant loads which efficiently oppose the loads which tend to tilt the hydraulic turbine engine, while decreasing the weight of peripheral moorings  30 . A foundation structure  10  of decreased weight is thus obtained, which decreases its cost of transportation and installation. 
       FIGS. 4 and 5  are simplified cross-section views only showing base plate  12  and central mooring  20  of two examples of foundation structure  10 . Conical portion  24  of central mooring  20  may have an elongated shape to ease a possible penetration into ground  50  on installation of foundation structure  10 . In  FIG. 4 , central mooring  20  is connected to base plate  12  by a rigid connection  52 . Such a rigid connection  52  is adapted to the case where central mooring  20  does not or only slightly penetrates into ground  50 , the latter being for example too rigid, or in the case where the axis of revolution of central mooring  20 , when it penetrates into ground  50 , remains aligned with the gravity direction. In  FIG. 5 , central mooring  20  is connected to base plate  12  by a ball joint  54  to enable to set the horizontality of base plate  12  independently from the orientation of central mooring  20 . This is advantageous in the case where central mooring  20  comes to a standstill with respect to ground  50  along a direction which does not correspond to the gravity direction. Ball joint  54  corresponds, for example, to the ball joint sold under trade name Eternum by Eternum France. Such a ball joint  54  has a stainless steel body and a composite spacer, which enables it to operate in (fresh or salt) water with no need to provide tightness means. 
       FIGS. 6A to 6D  show examples of foundation structures  10 A,  10 B,  10 C, and  10 D which differ from one another by the number of arms  26 . In these embodiments, central plate  17  of base plate  12  is circular. The foundation structure  10 A shown in  FIG. 6A  comprises three arms  26  which extend radially from base plate  12 , each arm  26  being, for example, angularly shifted by 120 degrees with respect to the other arms. Foundation structure  10 A is rather adapted to a one-way current, for example, to a stream current, two of arms  26  being advantageously placed symmetrically upstream of base plate  12 . The foundation structure  10 B shown in  FIG. 6B  comprises four arms  26 . Advantageously, foundation structure  10 B comprises at least one plane of symmetry perpendicular to surfaces  14 ,  16  of base plate  12 . Each arm  26  is, for example, angularly shifted by 90° with respect to the adjacent arms. Foundation structure  10 B is compatible with a two-way monodirectional tidal current. In this case, the plane of symmetry of foundation structure  10 B is advantageously arranged to be substantially parallel to the direction of the current. Foundation structures  10 C and  10 D respectively shown in  FIGS. 6C and 6D  respectively comprise five and six arms  26 . A number of arms greater than or equal to 5 enables to do away with the foundation structure orientation constraints and thus enables to place the foundation structure with a random positioning with respect to the current. 
       FIG. 7  is a perspective view of locking device  36  and  FIG. 8  is a cross-section view of the device of  FIG. 7  along a median plane of device  36  perpendicular to surface  14  of base plate  12 . Locking device  36  is, in the present embodiment, a “spring lock” system. It comprises a sub-plate  55 , assembled on upper surface  14  of base plate  12 , from which two blocks  56 ,  58  separated by an opening  60  are projecting. The portion of sub-plate  55  which forms the bottom of opening  60  is covered with a layer  62  of a flexible material. It for example is a foam, a rubber, a flexible polymer, etc., for example, of a synthetic polychloroprene-based rubber, for example, the product sold by Dupont Chemicals under trade name Neoprene. A “U”-shaped plate  64  comprising a base  65  and lateral walls  66  is arranged in opening  60 , with base  65  resting on layer  62  of the flexible material. The spacing between lateral walls  66  is slightly greater than the width of an arm  26 . Plate  64  is capable of sliding in opening  60 . A cylindrical hole  67  is provided in block  56  and emerges into opening  60 . A cylindrical hole  68  is provided in block  58  and emerges into opening  60 . Hole  68  is arranged to be coaxial to hole  67 . A cylindrical rod  70  is arranged in hole  67 . A spring  71  is interposed between cylindrical rod  70  and the bottom of hole  67 . In the absence of an external load on plate  64 , said plate is raised by layer  62  of the flexible material so that one of lateral plates  66  at least partly closes hole  67 . Cylindrical rod  70  is then maintained in hole  67  between lateral plate  66  and spring  71  that it compresses. 
     Locking device  36  is placed on the trajectory of the associated arm  26  and is shifted outwards with respect to joint  28  of arm  26  so that when the axis of arm  26  is parallel to upper surface  14  of base plate  12 , arm  26  bears against base  65  of plate  64 . The weight of arm  26  compresses layer  62  of the flexible material and lowers plate  64  by a few centimeters. The displacement of plate  64  enables to release cylindrical rod  70 , which was blocked by plate  64  until then. Under the thrust of spring  71 , cylindrical rod  70  axially translates to eventually penetrate into hole  68 . Arm  26  is thus locked between base plate  12  and cylindrical rod  70 . 
       FIGS. 9 and 10  respectively are a side view and a lateral cross-section view of the free end of an arm  26 . Peripheral heavy body  30  is connected to the end of arm  26  by the associated positioning device  32  and strengthening device  34 . Positioning device  32  corresponds, for example, to a double-acting jack comprising a rod  72  attached to a piston  73  capable of sliding in a cylindrical tube  74 . Rod  72  is attached to the free end of arm  26  and cylindrical tube  74  is attached to heavy body  30 . As an example, the axis of jack  32  is oriented perpendicularly to the axis of arm  26 . Jack  32  may correspond to an electric, pneumatic, or hydraulic double-acting jack. The maximum length reachable by jack  32  is defined according to the relief of the site. Each double-acting jack  32  may be actuated by an actuating system, not shown. As an example, jacks  32  may be actuated from the surface by means of electric cables or ducts running from jacks  32  to the surface, or directly from an energy source present at the level of foundation structure  10 . When it is not actuated, jack  32  is immobilized in both displacement directions of rod  72  by a mechanical system requiring no power. This may for example be a locking system sold by Sitema Company under trade named Serra. 
     Strengthening device  34  corresponds, for example, to a jack with a pitch of approximately 45 degrees with respect to the axis of arm  26 . It comprises a rod  76  capable of sliding in a cylindrical tube  78 . Rod  76  is connected to arm  26  by a pin or ball joint  80  and cylindrical body  78  is connected to heavy body  30  by a pin or ball joint  82 . Jack  34  strengthens the end area of arm  26 . This enables to decrease the cross-section of the beam forming arm  26 . It is possible for jack  34  not to be a controlled jack. On setting of the spacing between the end of arm  26  and the associated peripheral mooring  30  by the control of double-acting jack  32 , it then keeps its liberty of translation. Jack  34  is then blocked, for example, by means of a Serra-type device. 
       FIGS. 11A and 11B  schematically illustrate embodiments of peripheral heavy body  30 . Strengthening devices  34  are not shown in these drawings. In  FIG. 11A , peripheral heavy body  30  comprises a hemispherical portion  83  which extends in a conical portion  84  having its tip directed towards the ground. This allows a possible partial penetration of peripheral heavy body  30  into the sea or river bed on setting of the position of peripheral heavy body  30  by the associated positioning device  32 . In  FIG. 11B , peripheral heavy body  30  is tetrapod-shaped. Generally, the shape of each peripheral heavy body  30  and the surface state of this body enables to increase, for a given weight, the friction with the ground to more efficiently oppose the drag in the flow direction. The surface of peripheral heavy body  30  may be rough, or covered with asperities, or again provided with one or several protrusions having a characteristic dimension that may be comparable to the dimension of the actual body, like the tetrapod shape of  FIG. 11B . These features also apply to central heavy body  20 . 
     According to another embodiment, peripheral heavy bodies  30  or at least some of them are replaced with suction anchors  31  (generically illustrated in  FIG. 1 ) to decrease the weight of foundation structure  10 . 
       FIG. 12  shows damping device  38  in further detail. It may be a telescopic device connected by a pin or ball joint  86  to base plate  12  and by another pin or ball joint  88  to the associated arm  26 . It may be a device based on the pressure loss of a fluid circulating in a closed enclosure. Damping devices  38  avoids excessively violent shocks when arms  26  tilt and come into contact with plate  64  of locking device  36  resting on base plate  12 . Curve C shows the trajectory followed by pin joint  88  in the tilting of arms  26 . 
       FIGS. 13 and 14  schematically illustrate two examples of connection between platform  19  and base plate  12 . Platform  19  supporting the turbine engine comprises male or female parts, not shown, enabling to attach one or several turbine engines by jointing. As shown in  FIG. 13 , platform  19  may be solidly attached to base plate  12 . In this case, platform  19  and base plate  12  may correspond to a same part. Platform  19  of  FIG. 13  is adapted to the case where the turbine engine to be installed does not comprise means facilitating its installation according to the direction of the current or where it is not sensitive to the orientation of the current. As shown in  FIG. 14 , platform  19  may be connected to base plate  12  via a connection  89  which allows, for example, a rotation of platform  19  around the central axis of base plate  12 . Connection  89  is for example comprised of an Eternum ball joint  90  connecting platform  19  to base plate  12 , located under base plate  12  above ball joint  54  of central mooring  20 , and by a planar connection element  92  between lower surface  94  of platform  19  and upper surface  14  of base plate  12 . Platform  19  of  FIG. 14  enables the hydraulic turbine engine assembled on platform  19  to be freely oriented with respect to the current. This is advantageous in the case where the turbine engine comprises means which facilitate its orientation according to the direction of the current. 
       FIG. 15  is a view of a detail of  FIG. 2  and shows the free ends of arms  26  of foundation structure  10  in a folded configuration at the beginning of an operation of installation of foundation structure  10  on a sea or river bed. Strengthening devices  34  are not shown in  FIG. 15 . An elastic hoop  96  surrounds the free ends of arms  26 . Hoop  96  comprises two semi-cylindrical portions  97 ,  98  connected at one end by a deformable connection  99 . Semi-cylindrical portions  97 ,  98  are attached to each other at the opposite end by a pin  100 . The folded position of arms  26  corresponds to a stable equilibrium position. However, in the transportation of foundation structure  10  and the lowering of foundation structure  10  down to the installation site, disturbances such as current variations, various shocks, etc. might cause an incidental deployment of arms  26 . Hoop  96  is provided as a security and holds arms  26  in folded position. For the lowering of foundation structure  10  down to the installation site, an inflatable balloon  101  is arranged between arms  26  under hoop  96  in a partially inflated state. A gas supply duct  102  is connected to balloon  101 . When central mooring  20  of foundation structure  10  approaches or reaches the sea or river bed, pin  100  is removed via a cable  104  and balloon  101  is inflated via duct  102 . Balloon  101  exerts a thrust on arms  26 , especially due to buoyancy. This results in spacing apart arms  26  which are no longer maintained by hoop  96 , all the way to an imbalance position at which peripheral moorings  30  cause the tilting of arms  26 . Balloon  101  is then released and can be recovered. As a variation, inflatable balloon  101  may be replaced with a rigid balloon, the spacing of arms  26  being obtained by pulling the rigid balloon upwards, for example, via a cable. 
     Specific embodiments of the present invention have been described. Various alterations and modifications will occur to those skilled in the art. In particular, although, in the previously-described examples, each arm  26  is formed by a “monoblock” beam jointed with respect to base plate  12 , it should be clear that the arm may have a different structure. For example, each arm may have a telescopic structure while being jointed to base plate  12 . Each arm then is in a configuration where it is folded and where its length is minimum for the transportation of the foundation structure and the lowering of the foundation structure down to the installation site and is brought to a configuration where its length is maximum on installation of the foundation structure just before the arm is tilted. This enables to still further decrease the bulk of the seat structure during its transportation.

Summary:
The instant disclosure relates to a seat portion structure for a ground-based hydraulic turbine engine. The seat portion structure includes a base plate. A first bearing element is connected to the base plate and contacts the ground. At least three arms are connected to the base plate by a pivoting link. The arms are adapted for pivoting relative to the base plate between a first position, in which the arms are placed near each other, and a second position, in which the arms radially extend from the base plate. A second bearing element is connected to one end of each arm and contacts the ground. A positioning device is adapted for changing the distance between the end of at least one arm and the associated second element. The structure includes, for each arm, a device for locking the arm in the second position.