Abstract:
The invention concerns an assembly for installation of an underwater structure on the sea floor, said sea floor being covered with particles capable of being driven in suspension in the water above said sea floor, the mounting of said underwater installation capable of being implemented by an underwater robot driven by propulsion means, said propulsion means being capable of displacing the sea floor water and said suspended particles around said installation; said assembly comprising means ( 22, 26 ) for covering said sea floor surrounding said installation with a protective blanket ( 10 ) adapted to confine said particles on said sea floor.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   The present application is a 35 U.S.C. §§371 national phase conversion of PCT/FR2005/001411, filed 8 Jun. 2005, which claims priority of French Application No. 0406293, filed 10 Jun. 2004. The PCT International Application was published in the French language. 
   BACKGROUND OF THE INVENTION 
   The present invention relates to an assembly and to a method of installing a subsea structure on the seabed. 
   One envisaged field of application is in particular, but not exclusively, that of connecting subsea pipes on the seabed. 
   Systems for connecting two assemblies that can move with respect to each other are known. They generally require the intervention of a subsea robot or ROV (remotely operated vehicle), which includes its own propulsion means and is equipped with cameras, manipulators and a control system allowing an operator to remotely control it from the surface. 
   However, the seabed is very often covered with silt, sediment and various particles which are liable to produce clouds around the robot by said propulsion means, when the latter are directed toward the seabed. Such a cloud necessarily obscures the field view of the cameras and consequently the operator&#39;s visibility. This reduction in visibility impedes the progress of the connection operations and, sometimes, requires the intervention by the robot to be interrupted. Furthermore, the suspended particles may be deposited between the joints of the pipes to be connected, which is liable to impair the quality of the seal at the junction. 
   To reduce the visibility of clouds of particles appearing, it has been imagined to limit the tilt of the subsea robot. Thus, the propulsion means of said robot are not far from a horizontal direction approximately parallel to the seabed, thereby preventing layers of water lying close to the seabed from being moved and consequently generating clouds of particles. The reader may refer in particular to patent application FR 2 763 636 which describes such a device. 
   However, these devices designed to keep the subsea robot horizontal are relatively complex and limit the degrees of freedom of the robot. 
   Moreover, the height of the subsea structures above the seabed is generally considerable, so that the subsea robots operate some distance from the seabed and thus do not disturb the layers of water lying close to the seabed by the propulsion means. 
   However, such structures are necessarily bulkier and consequently require suitable installation means, which generally increases the installation costs. 
   SUMMARY OF THE INVENTION 
   The problem that arises, and it is this that the present invention is intended to solve, is therefore how to provide a method of installing a subsea structure on the seabed which prevents clouds of particles being formed close to the seabed, which consequently makes it possible not only to reduce the size of said structures, but also to reduce the time required to operate the subsea robot. 
   For this purpose, according to a first subject, the present invention proposes a method of installing a subsea structure on the seabed, said seabed being covered with particles liable to be brought into suspension in the water above said seabed, said subsea structure being capable of being installed by a subsea robot being driven by propulsion means, said propulsion means being capable of moving the seabed water and said suspended particles around said structure; according to said method, said seabed is covered around said subsea structure with a protective mat designed to confine said particles on said seabed. 
   Thus, one feature of the present invention lies in confining the silt or particles deposited on the seabed by covering the latter with a protective mat. One of the faces of the protective mat therefore bears on the seabed, whereas the opposite, external, face is turned toward the surface of the water. Thus, when the subsea robot propulsion means are oriented in a direction that encounters the seabed, the moving water flows over the surface of the external face of the protective mat without moving the particles located beneath it. Thus, not only can the subsea robot be guided to a point a short distance from the seabed, thereby making it possible to install low structures, but in addition the robot operating time is optimized since at no moment is the visibility reduced. 
   According to one particularly advantageous embodiment of the invention, in a first step, said protective mat is wound up, and in a second step, said protective mat is unwound in order to cover said seabed. Thus, the rolled-up mat is protected from surface sea currents during its descent, as will be explained in greater detail in the description. Furthermore, after said first step, certain elements of said structure can be installed on the seabed without a protective mat impeding the installation. Next, the protective mat is designed to be unwound around the subsea structure. 
   According to a preferred embodiment of the invention, in said second step, a pressurized fluid is injected into flexible impermeable ducts fastened to said protective mat, said ducts being drained and wound up beforehand. Thus, said flexible impermeable ducts, for example consisting of hoses made of plastic-coated fabric, of the fire-hose type, make it possible to form means of deployment that are relatively simple and easy to implement. 
   Advantageously, in said first step, said mat and said ducts are wound up together by being rolled up in a defined direction and said ducts are fastened to said mat in said given direction. Thus, as soon as a pressurized fluid is injected into said ducts, they tend to extend longitudinally, stiffening up, and can then cause said protective mat to be unrolled in the opposite direction to the rolling-up direction. 
   According to another preferred embodiment of the invention, since said subsea robot includes controllable means for injecting a pressurized fluid, said ducts are connected to said controllable injection means in order to unwind said protective mat. Thus, thanks to the subsea robot which already has injection means consisting of a pump, the ducts can easily be unwound without further cost. 
   Particularly advantageously, since said structure comprises at least one structural element designed to be submerged from a surface installation and to rest on said seabed, said structural element is equipped with said protective mat to which said flexible impermeable ducts are fastened, and, before submersion of said structural element, said protective mat is wound up. Thus, the protective mat can be attached and wound up along the structural element in a particularly compact manner. This allows said element to be lowered onto the seabed without being impeded by the protective mat. 
   Furthermore, said protective mat is advantageously divided into portions in order to cover the seabed around the structure. Thus, for example, by dividing the protective mat into four equal portions and by placing said wound-up portions around the structure opposite each other in pairs, when they are deployed, the seabed around the structure can be at least partly covered. 
   According to another subject, the present invention proposes an assembly for implementing the aforementioned installation method, said assembly including means for covering said seabed around said structure with a protective mat designed to confine said particles on said seabed. 
   Advantageously, said protective mat consists of a flexible material suitable for being wound up and then unwound so as to cover said seabed. This flexible material consists for example of a resilient web of nonwoven fibers and is of course suitable for locking the particles against the seabed. 
   According to one particular method of implementing the invention, said means for covering said seabed include flexible impermeable ducts fastened to said protective mat, said ducts being capable of being brought from a first state in which they are wound up and drained to a second state in which they are filled with a pressurized fluid and in which they extend longitudinally. Thus, said impermeable ducts, formed for example from plastic-coated fabric hoses, are designed to be rolled up on themselves together with the protective mat. Thus, the air that they contain is completely expelled to the outside and as soon as a fluid is reinjected thereinto, they tend to extend longitudinally for unrolling said mat. 
   Moreover, according to one advantageous feature, said ducts are fastened to said mat in a given direction, which corresponds to the rolling-up direction of said mat. Thus, as soon as a fluid is injected into said ducts, they tend to extend in the rolling-out direction of said protective mat. This makes it possible to optimize the mechanical energy provided by said ducts. 
   Furthermore, since said subsea robot includes controllable means for injecting a pressurized fluid, said ducts are provided with connection means for connecting said controllable injection means to said ducts. Thus, thanks to the connection means for connecting the injection means and said ducts, and by controlling the subsea robot, the protective mat can be deployed. 
   Since said structure comprises at least one structural element designed to be submerged from a surface installation and for resting on said seabed, said structural element is advantageously equipped with said protective mat to which said ducts are fastened, said protective mat being wound up before submersion of said structural element. Thus, the structural element is designed to be lowered onto the seabed with the protective mat wound up. This avoids disturbing the descent and damaging the mat. It is only after the structural element has been deposited so as to rest on the seabed that the protective mat is unwound. 
   Moreover, said protective mat is preferably divided into portions distributed around said structural element so as to be able firstly to be easily wound up all around the structural element and subsequently for it to be unrolled radially around this structural element. In addition, the protective mat portions are not necessarily strictly rectangular—they may have parts that are rolled up in different directions. Thus, as will be explained in greater detail in the detailed description that follows, the entire perimeter of the region of the seabed on which the structural element rests can be covered by said protective mat. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other particular features and advantages of the invention will become apparent on reading the following description of particular embodiments of the invention, given by way of indication but employing no limitation, with reference to the appended drawings in which: 
       FIG. 1  is a perspective schematic view of an element of the invention; 
       FIG. 2  is a cross-sectional schematic view of the element illustrated in  FIG. 1 , as rolled up; 
       FIG. 3  is a perspective schematic view showing part of a subsea structure equipped with the element illustrated in  FIG. 2 , in a first state; 
       FIG. 4  is a perspective schematic view of a structure illustrated in  FIG. 3 , equipped with the element illustrated in  FIG. 2 , in a second state; and 
       FIG. 5  is a perspective schematic view of a structure identical to that of  FIGS. 3 and 4  above, equipped with an element according to one particular embodiment. 
   

   DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1  shows a protective mat  10  according to the invention. This protective mat  10  has a nonwoven textile web  12  of the geotextile type, used especially in constructional applications, for separating or stabilizing various aggregates for example and/or for filtering rainwater without these various aggregates being mixed. 
   These nonwoven textile webs  12  consist of a relatively tough sheet of fibers, for example polypropylene fibers. They have a thickness of between 200 and 900 microns, for example 500 microns, and a weight per unit area of between 50 and 500 g/m 2 , for example 150 g/m 2 . The nonwoven textile web  12  here is of rectangular shape, having two opposed lateral edges  14 ,  16 , a fastening edge  18  and, opposite it, a free edge  20 . 
   Moreover, flexible impermeable ducts  22  are fastened by stitching or bonding to the textile web  12  and are spaced apart, so as to be parallel to one another and to the two lateral edges  14 ,  16 . These flexible impermeable ducts  22  consist for example of plastic-coated fabric hoses, used especially as fire hoses. They have the advantage of being able to be flattened, their two opposed walls bearing against each other, when they are empty and thus of being rolled up in a compact manner. However, as soon as a pressurized fluid is injected into these rolled-up flexible impermeable ducts  22 , they extend longitudinally and become relatively rigid. 
   Here, these ducts  22  are connected together at their ends  24  located near the fastening edge  18  by a manifold  26  whereas their opposite ends  28 , located near the free edge  20 , are blocked off. The manifold  26 , which extends along the fastening edge  18 , has two terminations  30 ,  32 , and the flexible impermeable ducts  22  connected to the manifold  26  form a closed circuit between the two terminations  30 ,  32 . 
   Furthermore, a relatively heavy rod or lath  34  is fastened to the free edge  20  of the textile web  12 . This rod may be replaced with a cable or a tube. 
     FIG. 2  illustrates the protective mat  10  in rolled-up form. This figure shows the lath  34  around which the textile web  12  and the flexible impermeable ducts  22  have been rolled up. These flexible impermeable ducts  22  are shown here as broken lines so as to better distinguish them. 
   The protective mat  10  thus rolled up in a direction D parallel to the opposed lateral edges  14 ,  16  and to the ducts  22  is relatively compact and the flexible impermeable ducts  22  are substantially rolled up with the textile web  12 , each in one and the same plane. 
   Referring now to  FIG. 3 , this illustrates in perspective a structural element  36  of a subsea structure, equipped with a protective mat  10  as shown in cross section in  FIG. 2 . This structural element  36 , which includes a rectangular support plate  33 , is fastened to a subsea pipe  37  and allows the latter to be connected, for example to a riser (not shown) via an elbow extending the subsea pipe  37 , which elbow terminates in a termination  35 . The protective mat  10  here is kept rolled up, along one edge  39  of the rectangular support plate  33  of the structural element  36  by fasteners  38  of the type used to keep the cables of a cable tray together. However, they differ therefrom by the presence of a fracture initiator, which allows them to break as soon as they are subjected to an elongating force above a specified threshold. 
   According to another embodiment (not shown) the protective mat  10  thus rolled up is held in place by Velcro-type loops and hooks, said strips being closed up on themselves. Thus, and in the same way as the aforementioned fasteners, as soon as these strips undergo an elongation, the hooks release the loops and the strips are detached. Moreover, the mat may also be protected by a flexible or rigid cover. 
   Thus equipped, the structural element  36  is designed to be submerged from a surface installation (not shown) down to the seabed  40  so as to rest thereon, said rectangular support plate  33  being against said seabed  40 . The termination  35  is then located at a certain distance, less than 10 m, for example 5 m, from the seabed  40 . 
     FIG. 3  also shows the two terminations  30 ,  32  that can be connected to pressurized fluid-injection means on a subsea robot (not shown). Subsea robots, remotely controlled from the surface and designed to connect subsea pipes, usually include water injection pumps capable of taking up seawater and discharging it under pressure. Thus, the subsea robot is designed to connect the two terminations  30 ,  32  to its water injection means in order to deliver pressurized seawater into the manifold  26  and into the flexible impermeable ducts  22 . This delivery pressure is between 3 and 10 bar, for example 6 bar. 
   Referring now to  FIG. 4 , this illustrates the structural element  36  and the protective mat  10  after pressurized water has been injected into the manifold  26 . By injecting pressurized water into the manifold  26 , the impermeable ducts  22  now extend longitudinally, unrolling the textile web  12  in the direction opposite to the rolling-up direction, causing the heavy lath  34  to rotate and move translationally. Thanks to this lath, which nevertheless offers a certain resistance to extension of the permeable ducts  22 , the textile web  12  is kept in place resting on the seabed  40 . Thus, the protective mat  10  remains resting on the seabed  40 , despite the seabed currents. 
   The structural element  36  shown in  FIGS. 3 and 4  is equipped with a protective mat  10  consisting of a single portion fitted along a single edge  39  so as to simplify the drawing. This portion, which is substantially rectangular, the fastening edge  18  measuring between 4 and 5 m, for example 4.5 m, and the lateral edges  14 ,  16  measuring between 3 and 4 m, for example 3.5 m, once it has been deployed covers only part of the seabed around the structural element  36 . Consequently, the other three edges of the rectangular support plate may be equipped in the same way with three other portions of protective mat  10 , allowing the entire perimeter of the structural element  36  to be covered. 
   Furthermore, since the other three portions are of rectangular symmetry and the edges of the rectangular support plate  33  are inclined relative to one another at angles of 90°, the portions are joined substantially in the corners  46  of the rectangular support plate  33 , leaving four spaces where the seabed  40  between each portion of protective mat  10  are left free. 
   To cover these square parts of the seabed, located in the extension of the diagonals of the rectangular support plate  33 , the portions of protective mat  10  are equipped with complementary attachable corner pieces. 
   Referring now to  FIG. 5 , this illustrates in another embodiment a protective mat portion equipped with two attachable triangular corner pieces. 
   This  FIG. 5  again shows the structural element  36  illustrated in the previous two figures, which is equipped with an identical portion of protective mat  10 . However, the textile web  12  has a right triangular corner piece  50 , articulated on one of the lateral edges  16 , and a left triangular corner piece  52 , articulated to the other lateral edge  14 . The longest sides of the triangular corner pieces correspond substantially to the two respective diagonals of the triangular support plate. 
   Furthermore, these triangular corner pieces  50 ,  52  are also equipped with flexible impermeable ducts, here secondary impermeable ducts  54  that extend substantially perpendicular to the opposed lateral edges  14 ,  16  and are connected to the respective two ducts  22  that run along these opposed lateral edges  14 ,  16 . However, in another embodiment (not shown), a secondary supply network is provided, this being directly connected to the secondary impermeable ducts  54 , independently of the manifold  26 . This secondary supply network can then be supplied independently of the manifold  26  and in particular after the rectangular part of the textile web  12  has been deployed, so as to simplify the deployment of these lateral triangular corner pieces  50 ,  52 . 
   Thus, by equipping the three other portions of protective mat (which are not shown here) with triangular corner pieces designed to cooperate together, the entire perimeter of the structural element  36  can be covered with a protective mat over a width of about 5 m. Furthermore, in order for the protective mat  10  and these various portions, including the attachable corner pieces, to be completely wound up, said corner pieces are firstly rolled up perpendicular to the opposed lateral edges  14 ,  16  in the direction of their secondary impermeable ducts  54  toward their portions to which they are attached and, thereafter, each of the portions is rolled up toward its respective fastening edge, as indicated in  FIG. 3 . 
   The structural element  36 , equipped with the protective mat  10  thus rolled up, can be deposited on the seabed  40 . This protective mat  10  can then be deployed by means of subsea robots, as described above. The rectangular portions of the protective mat  10  that run along the edges of the rectangular support plate begin by being unrolled and then, once they have been unrolled, it is the attachable triangular corner pieces that are unrolled, thanks to the secondary impermeable ducts  54  that extend along their longitudinal direction and in the direction opposite to the impermeable ducts  22  to which they are connected. 
   Thus, silt, particles or sediment covering the seabed  40  around the rectangular support plate  33  are entirely covered by the protective mat  10 . Thus, despite the relatively short distance that separates the termination  35  from the seabed  40 , when a subsea robot is used to connect, for example, a riser to the termination  35 , its propulsion means disturb essentially the water layers located on the surface of the protective mat  10 . This raises no particles and does not increase the cloudiness of the water near the structural element  36 . 
   Of course, the subject of the present invention can be applied to any type of subsea structure, for example, and according to a nonlimiting list, PLETs (pipeline end terminations), PLEMs (pipeline end modules), ILTs (in-line tees) or UDUs (umbilical distribution units).