ES2299477T3 - PROCEDURE TO ESTABLISH A FOUNDATION IN A MARINE FUND FOR AN OFF-COAST INSTALLATION AND FOUNDATION ACCORDING TO THE PROCEDURE. - Google Patents

Abstract

A foundation according to the invention for offshore facilities, preferably at shallow depths, has a substantially bell-shaped bottom part (1) of steel plate with a central cavity (21) and peripheral pressure tight chambers (25) which are separated by bulk-heads (23). The foundation is brought down into seabed or lake bottom (10) consisting of sand. After placing on the bed, according to the invention there is created underpressure in the chambers (25) so that a water current occurs through the bed layer around lower rims (17, 19) of the bottom part. Thereby the bed material is fluidised under the rims (17, 19), and the foundation sinks down into the bed by its dead load. The fluidization may be varied along different parts of the rims (17, 19) by providing different pressures in different chambers (25) whereby the foundation may be righted during the bringing down and thereby compensate for uneven bed etc. By the invention, the use of heavy concrete caissons may be avoided, and allowance is made for environmental demands regarding removal of the foundations after use as the foundation according to the invention weighs considerably less and is easier to remove.

Description

Procedure to establish a foundation in seabed for offshore installation and foundation according to the procedure

State of the art

The present invention relates to a procedure to establish a foundation on a seabed for an offshore installation, which includes a foundation bottom to place on the seabed and carry a structure to support an installation above sea level. further The invention relates to a foundation for use in the process. Offshore installation means offshore constructions in the broadest sense, including both open sea facilities and lakes, including both oil and gas platforms and wind turbines based at sea like the wharf facilities and the bridges. By background or seabed is also understood coastal areas as in the ports and the bottom of the lakes. The invention finds application at depths below 50 meters.

Normally, the background foundations are constituted in the form of drawers that are towed out of site and then submerged with water and supplied possibly with ballast. The constructions are made of concrete They are appropriate in case of large constructions. The weight of the drawer guarantees stability on the seabed but at the same time it is a relatively expensive way to establish the foundation. In some cases it is also desirable to remove the foundation again, which is particularly difficult when talking about drawers.

From WO 9406970 a known construction where the legs of the offshore installation are placed on the seabed having a lower structure on the legs and that is appropriate to be swallowed inside the background. The structure consists of a series of cameras. When place the installation offshore, it sinks a little inside the seabed because of static charge. Then it they activate the pumps in each chamber, evacuating the water inducing from this way a subpressure that makes the cameras gobbled within the seabed. For particularly hard layers at the bottom marine spray nozzles can be mounted on the edges lower of the structure that by water pressure can drop these hard layers and thereby boost the sinking of The structure inside the fund.

The construction presented in WO document 9406970, and other related foundations found engulfed within the seabed, they have all the disadvantage of what pressure of the pumps (underpressure) inside the foundation structure has to be controlled very carefully to prevent the layers of soil in the background sailors are released and disintegrated, leading to the structure lose your fixation on the seabed. Another problem arises in the initial phase when the pumps have to create underpressure in the interior of the structure. By sucking too hard, a strong water pressure through the top layers at the bottom marine, eroding them and inducing a collapse with which provides a clear path to seawater between the outside and the interior of the structure. In this situation, you cannot create the underpressure required for the structure to be engulfed within the seabed In addition, the seabed is damaged in an area around of the foundation with what the foundation or has to move to another location with an intact seabed, or the area around the structure in order to prevent A new pumping in the structure causes a new collapse.

From GB 2277547 a known foundation with a double wall skirt. In order to place the foundation at the bottom, the foundation structure is equipped with fluidization means at the lower edges of the skirt structure. These fluidization means are in the form of nozzles through which a water jet can be directed at high pressure against the bottom material under the structure of the skirt The material becomes brittle and due to the action of the high pressure water jet and allows the structuring of foundation penetrates the bottom materials.

From GB 288077 a system is known Basic placement of a drawer with open end on and partially inside a seabed using the drawer as a pressure-tight chamber. The drawer is placed in place desired, and the inside of the drawer is evacuated, whereby the drawer is It can sink into the seabed. The action of sinking due to the fact that it partially emptied inside the drawer, makes the water fragilize the material along the bottom flange of the drawer, so that the embrittled material and depressed facilitates the sinking of the drawer.

For small installations where you need columnar foundations, fixing of The foundation at the bottom. This implies that the foundation must be relatively heavy to ensure construction against moments tipping caused by the movement of waves and ice.

Explanation of the invention.

The purpose of the invention is to provide a procedure and a foundation as indicated above which is lighter than known constructions and that simultaneously it can be fixed in a simple and stable way in The seabed.

The invention addresses this purpose by providing a procedure to install a foundation of the type mentioned previously, but which is also characterized because the procedure includes additional features and processes in which at least one substantially tight cavity is formed at the pressure inside the foundation, after which, by pumping the contents out of the cavity, a underpressure of such magnitude inside the cavity in the double wall construction that produces a stream of water to through the seabed material around the flange from the surrounding parts around the foundation and inwards in the direction of the cavity and so that a fluidization and a removal of seabed material in the flange resulting in the foundation sinking into the seabed and the pumping continued until the foundation of seabed has reached a desired level on the seabed and that a device with a series of nozzles has been arranged evenly distributed along the lower flanges of the skirts, and that the nozzles are arranged to receive and distribute a lubricant, whereby the nozzles distribute the lubricant in the form of a lubricant film along the walls of the foundation during the laying of the foundation.

The peculiar feature according to the invention is that the foundation of the bottom is hollow, open down and provided with parallel or unidirectional sides and a flange directed downward surrounding the formed opening and extending mainly in the same plane.

The background foundation can be done in this way. build as a relatively light steel construction whose design looks a lot like a bell with building double wall and that is preferred to build in the form of a cylinder, although other geometries are possible, for example, with cross section elliptical or polygonal. The foundation, including the structure underwater, it is sent to the place and placed on the bottom that is leveled about as a plain. On the upper side of the background foundation the hoses are connected and taken to a water surface boat. With pumps on the boat the vacuum can be formed inside the cavity in the double wall construction, and the water found in the Inside the cavity can be pumped in a controlled manner. The water surrounding the background foundation will therefore sneak inside the cavity through the seabed material that in the practice consists of sand or sediment in a surface layer deep enough Therefore, a condition of type quicksand on the bottom edge of the foundation of bottom, after which the entire foundation sinks into the bottom Marine. The downward movement stops by stopping the operation of the pumps when the background foundation has been sunk deep enough into the seabed.

In a preferred embodiment, a underpressure pumping from the cavity formed inside the foundation inside the inner skirt that limits the cavity in the flange. In this way, we talk about an inner cavity and a outer cavity, coaxially arranged in the construction of double wall.

Likewise, creating underpressure in the cavity inside, increases the total suction force down. How has it mentioned earlier in the prior art, it has, however of limiting underpressure in the inner cavity so as not to destroy the seabed by disintegration.

In order to prevent local destruction of the seabed during aspiration, a layer can be deposited protective over the foundation area, for example, in the form of a layer of stones that can retain the seabed and allow The passage of water. Likewise, a filter can be arranged, either alone or in combination with other media inside the foundation to avoid the removal of material inside the flange. In the case of inclined seabed, a adrizante layer on the seabed.

When the seabed consists of a layer of sand at the top, which can possibly be displaced,  that is, carried by the current, the thickness of the layer can vary and it may be an advantage to first remove a part of The layer of quicksand. In this way diverse are achieved objectives. First, that the foundation can be placed on a more stable background. Apart from that, it is guaranteed to foundation a secure base since when removing complete or partially the quicksand ensures that the depth of the base is not reduced due to quicksand. Another advantage is that the layer of quicksand or layers of soft sediment and mud does not they can provide the necessary tightness around the foundation so that the necessary underpressure can be set so that the foundation can sink to the bottom Marine.

In another advantageous embodiment, there are units vibrations removably arranged on the foundation so that, when there are layers of hard soil under the location of the foundation, the combination of underpressure, fluidization in the flange and vibrations can cooperate in improving the foundation collapse. Likewise, when the foundation is inclines and when a rock floor is arranged as a layer of leveling, the foundation can be advantageously vibrated to through this layer since it is often not possible to establish the underpressure through the layer of porous stones.

Placing sand or sandy objects, it is the resistance at the end of the object to place the one to overcome, whereby when objects are to be placed in clay, it is the friction on the sides of the object which must be overcome and not the resistance at the end of the object.

When the foundation is to be placed in the seabed with a layer of clay, the preferred embodiment comprises a row of uniformly arranged nozzles distributed along bottom flanges of the foundation, both on the outer skirt and the inner skirt, so that the object or device, in which the nozzles are arranged, has a greater width than the thickness of the material in the wall materials of the skirts. The nozzles connect with a feeding device that an agent can supply sliding to the nozzles, for example, a suspension of Bentonite, drilling mud or similar non-polluting agent With lubricating properties. When the foundation has to penetrate fully or partially through a layer of clay, the device on which the nozzles are arranged will displace the clay. The device can be advantageously designed as a triangle, where a tip is oriented downwards from the foundation but the device can have any shape geometric, the important thing being that the device is projected on the foundation wall as mentioned above so that the lubricating film can be formed between the clay and the foundation wall. Adding lubricant and a continued descent, a gap will appear above the nozzle device along the foundation wall and it fill with the lubricant supplied, which forms a lubricant film between the foundation wall and the clay. In this way, the friction between the clay and the foundation, and continues the descent of the foundation.

When the foundation is to be placed on the seabed, sand / sediment and clay layers. They combine the different processes so that fluidization is used in the flange through the layers of sand / sediment as it has been previously described. Here, the subpressure is adjusted in the inside the foundation, that is inside the flange, so to avoid the disintegration of the fund. When the foundation has to cross layers of clay, these cannot be fluidized to it degree than sand / sediment layers, which is why it is added lubricant as described above. In addition, when they find large stones or foreign matter in the subsoil, by the removably mounted vibrating units, the invention provides a possibility to displace this matter strange outside the path of descent of the foundation.

When the foundation is used together with the placement of wind turbines at sea, the invention has some other advantages In connection with the operation of the wind turbines, there are situations in which wind turbines they have to stop, which may be due to maintenance reasons or to a technical stop due to speeds of seeing too much high. The stop can apply great dynamic forces, although very limited in time, to the foundation so that the construction will typically experience vibrations in connection with the natural oscillations of the construction in the range of natural frequencies The same happens when it is put back in the wind turbine runs. In such situations you can create a over aspiration (overconsolidation) inside and around the foundation. Creating an over aspiration, a major part of seabed around the foundation will become a active part in the foundation, that is the ability to lift / foundation fixing capacity increases momentarily. This implies that a foundation can be dimension for the use situation but that additional security due to overconsolidation makes it able to resist the tip of load, such as a storm, braking or putting on functioning. Likewise, the natural frequency can be changed of construction to a certain degree during braking / putting on March.

In a preferred embodiment of the process, the background foundation is divided into separate chambers of which various mutually watertight chambers are uniformly distributed to the pressure between the outer skirt and the inner skirt, and where the different pressures or subpressions are set to different cameras so as to induce a moment around an axis substantially horizontal. In a preferred embodiment of the foundation according to the invention, the bottom foundation is divided into  separate chambers of which are distributed evenly various mutually pressure-tight chambers along the side of the foundation foundation.

In this embodiment where 4 are typically used to 6 chambers along the side of the foundation foundation, although on the other hand the invention is not limited to a certain number of cameras, a valve is mounted on each chamber with each its hose connection to the pump equipment on the vessel in the surface. In combination with pressurized bulkheads between the cameras it is possible to choose different pressures in the chambers and therefore different degree of background fluidization sandy under different parts of the foundation flange of background since the greater fluidization or effect of quicksand is will take place where the differential pressure compared to the ambient water and therefore the water flow under the flange is older. Therefore you can establish moments of adrizamiento on the foundation during the bottom foundation descent inside the seabed so that the foundation is completely vertical when the descent is finished, for example if the seabed is not completely horizontal at the beginning or if fund conditions are irregular so that the sinking of the flange is not the same throughout the entire periphery with the same pressure difference in all sites.

Description of the drawings

The invention will now be described in more detail. with examples of preferred embodiments and with reference to drawings where

Figures 1 to 3 show a first realization of a foundation according to the invention seen from the side, in section along line II-II in figure 1, and in a lower perspective, respectively,

Figure 4 shows in a perspective view bottom a second embodiment of a foundation according to the invention,

Figures 5 to 7 show in principle three different stages during the execution of the procedure according to the invention

Figure 8 shows a detail of the edge bottom flange in a modified embodiment, and

Figure 9 shows a contoured section of the flange shown in figure 8.

Description of exemplary embodiments

The first realization of the foundation according to the invention shown in figures 1 to 3 consists of a part bottom 1 or bottom foundation that has a cylindrical shape with a flat top sheet 3. On top sheet 3 is build a columnar structure 5 that forms a connection between the bottom 1 and installation 6 to be placed in a marine area or of a lake; the water surface bears the reference 8 and the bottom marine reference 10. The structure has a conical transition 7 between the bottom 1 and a columnar section 9. In the part upper, there is a concave annular body 11 of columnar section 9 For use in ice-laden waters. The body 11 is known in Yes and it can be omitted in waters without ice.

In this embodiment, the entire foundation is manufactures with welded steel sheets, and therefore the construction can be relatively light; for constructions small at depths of up to 10 meters, construction It will often weigh less than 200 tons. The foundation can be manufacture also with other materials such as concrete, materials Compounds and the like.

The bottom 1 consists of a skirt outer 13 and an inner skirt 15 concentric with each other that together they constitute a double wall construction. Both skirts 13, 15 extend from its lower flanges 17 and 19 to the lower part of the upper sheet 3 whereby a cylindrical cavity 21 inside the skirt 13 and an annular cavity between the skirts 13 and 15, between the skirts 13, 15, are dividing walls or bulkheads 23 extending radially, sized to withstand differential pressures so that the different pressures can be maintained between cavities 25 mutually adjacent elongated formed therefore between the skirts 13 and 15. Here it is preferred to provide six cavities 25, but less are also possible, for example four, or more, by example eight. The inner cavity may be provided, in another embodiment, not shown, of mutually perpendicular sheets 27 contoured with dotted lines in figure 2 with respect to the resistance and stability of the bottom 1. These sheets 27 must not be sized or sealed along the edge with respect to differential pressure as well as not to be extended in the total height of the bottom 1.

A second embodiment shown in Figure 4 it has a cylindrical outer skirt 30 that is grooved and provided of four cylindrical chambers 32 that are open at the bottom and closed at the top by one or more upper sheets. The skirt it forms a cavity 21. The cameras 32 have the same function as the 25 cameras above in the description below. This embodiment can be varied with a different number of cameras 32 of the shown.

Another version of the embodiment of Figure 4 omit the grooving of the skirt 30 and arrange the cameras elongated on the outer side of the skirt 30 (not shown).

Finally, in an embodiment not shown and more simple, the bottom 1 is not provided with the inner skirt 13 and bulkhead 23 so that there is only one cavity. In such embodiment, instead of differential pressure control described above to straighten the foundation by cameras 25 you can use a stand not shown that can guide all the foundation to guide it vertically during the descent within the seabed.

A preferred embodiment of the procedure according to the invention is shown in figures 5 to 7 where it is used as an example the foundation according to figures 1 to 3.

In Figure 5, the foundation according to the The invention is sent to the site and placed at the bottom of the sea. The seabed 10 is leveled to obtain a surface approximately horizontal at the survey site. The seabed leveling or preparation can be carried out removing loose material or arranging a stone covering and / or a filter layer in the form of a geotextile or the like. The flanges 17 and 19 then rest on the seabed. In the Chambers 25 there is water and possibly more air. On top of each chamber 25 there is a valve opening not shown to which a suction hose 40 is connected. On a barge 42 from of which the operation is controlled, there is a pump unit 44 with a branch connected to, in this case, six hoses 40 for the shown embodiment of the bottom 1.

In cameras 25, a subpress is provided then from the pump unit 44 reduced to an absolute pressure in the magnitude 1/25 bar since the contents of the chambers 25. The pumping pressure is adjusted according to the nature of the seabed, and the indicated values are thus only examples of a typical seabed of the type indicated. The air of the surface can initially be found in cameras 25 but It comes out quickly and water is pumped in place. The difference between the pressure in the water on the seabed 10 and the pressure in the inside the chambers 25 is then the atmospheric pressure plus water pressure determined by water depth minus the absolute pressure in the chambers. At 10 meters of water, the difference It is close to 2 bars. Therefore a stream of water is achieved through the relatively loose background material from outside to inside of the bottom as shown with arrow 46, see figure 6. Water flow circulates from the surface from the seabed, down through the bottom material, under the flange 17 and rises through the bottom material and within the chambers 25. Similarly, a current is formed through the background material in cavity 21 and shown with arrow 48. The water stream that has no significant dimension, typically about 300 l / min, it is enough to form to form a quicksand type condition under the flanges 17 and 19, after which the seabed yields to the flanges 17, 19 relatively sharp, and the foundation sinks into the bottom Marine due to its weight. In order to increase the weight, you can Fill the columnar structure 5 with water in this case.

If you can not or could not leave the fund sea level before setting the foundation on the bottom 10, by this embodiment of the invention it is possible to have different pressures in different chambers 25. For example taking lower pressure in three chambers, all located on one side of one diameter line in the cross section at the bottom (see Figure 2), which in the three other cameras 25, can be create a stream of water 46, 48 more important along the flanges 17, 19 in the previous half than in the later half of the lower part 1. Whereupon, a greater degree of fluidization or condition of quicksand in the previous part, and the lower part 1 will sink more than stelae. In this way induces a rotation of the entire foundation around an axis horizontal, whose direction is determined by the position of the different pressures in the chambers 25. By proper control of the suction for each chamber 25 in the pump unit 44, by for example, by a level control that records the foundation itself some foundation foiling may occur during the descent into the seabed 10 and therefore a compensation of a non-level bottom. You can use the same type of adjustment for irregularities in the background material caused by different types of sediments, stones, etc. A regulation is used controlled water flow to avoid reduction detrimental to the bearing capacity in the background material under the flanges 17, 19.

In the case of layers of types of hard clay on the seafloor 10 or large moving stones, to help the descent you can add and provide units vibratory 50, for example, on top sheet 3 of the part bottom 1. The 50 units can be hammer devices known hydraulic itself that are removably fixed on a tab on the bottom 1, and after use the fixation it is released by operating from surface 8, and units 50 are hoisted inside barge 42.

Figure 8 illustrates a detail of a wall of foundation where a lower edge is arranged device 60, in which a series of nozzles are arranged 61 evenly distributed along the entire periphery of the bottom flanges of the foundation 1. Figure 9 shows illustrates a cross section of the foundation edge in the part bottom with a device arranged according to an example of the invention.

When the foundation is to be placed in a seabed with layers of clay, as shown in figures 8 and 9 rows of nozzles 61 are uniformly distributed to along the bottom edges on the flange 17, 19 of the foundation 1, both in the outer skirt and in the skirt interior 13, 15 such that the device 60 in which the nozzles 61 have a width greater than the thickness of the material in the wall materials 13, 15. The nozzles 61 are connected to a power device 63 that can supply to the nozzles 61 a sliding agent 63, for example, a bentonite suspension, drilling mud or similarly a non-polluting agent with lubricating properties. When the foundation 1 must completely or partially penetrate a layer of clay, the device 60 in which the nozzles are arranged They set aside the clay. The device 60 can be designed advantageously in the form of a triangle where a tip is oriented downwards with respect to the foundation. Adding lubricant and a continued descent, a gap will appear above of the nozzle device 60 along the wall of foundation 13, 15 to be filled with the lubricant supplied, whereby a lubricant film 62 is formed between the wall of the foundation and clay. In this way, friction is eliminated between the clay and the foundation, and the descent of the foundation .. This is indicated in figure 8 where the way in which the nozzles 61 form a clay film 62 on the walls 13, 15 and the foundation.

When the bottom 1 of the foundation is has dropped to sufficient depth within the bottom 10, as shown in figure 7, pumping stops, and can be injected possibly concrete 49 (shown with shading) to fix the foundation in the remaining cavity between the bottom 10 and the sheet top 3.

If the foundation is to be removed after its use, it is considerably easier to lift or remove it on the bottom that the concrete foundations built in the form of drawers. Foundation removal can be facilitated using reverse techniques regarding the descent, in other words pumping water inside chambers 21, 25, whereby the foundation is propelled back from the bottom. The withdrawal can be also perform with the assistance of hammer devices that in principle they are mounted and work, although only towards above or laterally, such as 50 units. This is significant in waters where the authorities require the withdrawal of foundations after use.

Claims (9)

1. Procedure for establishing a foundation on the seabed for an offshore installation, comprising a bottom foundation for placing on the seabed and carrying a structure to support an installation above sea level, said structure is hollow, open downwards and provided with parallel or unidirectional side walls whose side comprises a double wall construction consisting of an outer skirt (13) and an inner skirt (15) concentric with each other and a flange (17, 19) in the lower part of each skirt that extends substantially in the same plane, is positioned with the flange (17, 19) on the seabed (10), characterized in that at least one substantially pressure-tight cavity (25) is formed inside the foundation (1), after which, by pumping the contents out of the cavity, a subpressure of such magnitude is established inside the cavity (25) in the double-pair construction ie that a stream of water is produced through the seabed material (10) around the flange (17, 19) from the surrounding parts around the foundation (1) and inwards in the direction of the cavity (25) and so that fluidization and removal of the seabed material on the flange occurs, resulting in the foundation (1) sinking into the seabed (10) and the pumping continues until the foundation (1) bottom marine has reached a desired level on the seabed (10) and that a device (60) with a series of nozzles (61) has been arranged evenly distributed along the lower flanges (17, 19) of the skirts (13 , 15), and that the nozzles (61) are arranged to receive and distribute a lubricant (62), whereby the nozzles (61) distribute the lubricant (62) in the form of a lubricating film (62) along the foundation walls (13, 15) during placement of the c imentation 2. Method according to claim 1, characterized in that the bottom foundation (1) is divided into separate chambers (25) of which various mutually pressure-tight chambers are evenly distributed along the side of the bottom foundation, and where different pressures or sub-pressures are established in different chambers (25) to straighten the foundation during establishment to induce a moment around a substantially horizontal axis. 3. Method according to claim 1 or 2, characterized in that vibrating joints (50) are removably mounted on the bottom foundation (1). 4. Method according to any preceding claim, characterized in that overconsolidation can be established on the seabed for limited periods, inside and around the foundation (1) by increasing the subpressure inside the chambers (25, 21), with which the foundation can withstand larger force actions. 5. Foundation for use in the method according to claim 1 and for offshore installations, comprising a bottom foundation to place on the seabed and carry a structure to support an installation above sea level, in which the bottom foundation (1) is hollow, open downwards and provided with parallel or unidirectional sides, the side of which comprises a double wall construction consisting of an outer skirt (13) and an inner skirt (15) concentric with each other, and a flange (17, 19) oriented downwards on each skirt that surrounds the formed opening that extends mainly in the same plane, so that at least one pressure-tight chamber (25) is formed, characterized in that there is a possibility of connection for a pump to at least one pressure-tight chamber (25) in the foundation, whereby fluidization of the background material can be created around and between the skirts (13, 15) and because on the lower edges of the flange (17, 19) there is arranged a device (60) with uniformly distributed nozzles (61) that communicate with a feeding device (63) for a lubricant (62). 6. Foundation according to claim 5, characterized in that the flange is divided into separate chambers (25), of which several mutually pressure-tight chambers (25) are uniformly distributed along the periphery of the bottom foundation. 7. Foundation according to claim 5 or 6, characterized in that vibrating units (50) are removably arranged on or around the foundation (1). 8. Foundation according to claims 5 to 7, characterized in that the device (60) has a dimension perpendicular to the flange (17, 19) higher than the thickness of the skirt material, whereby the nozzles (61) are free and directed opposite to the direction of laying the foundation (1). 9. Foundation according to claim 5, characterized in that the bottom foundation is designed with an annular sleeve and with the pressure-tight chambers arranged in or on the outside of the sleeve in the form of mutually separate sections that extend vertically.