The present invention relates to devices for installation on a seabed below a body of water. More specifically, the invention relates to a foundation base for installation on a seabed. The invention also relates to an autonomous flow control device for use with the foundation base.

Background of the invention

The principle of the skirted foundations has been developed and used in the offshore oil and gas industry for several decades, mainly in conjunction of the gravity based foundations, in particular for the concrete platforms. This principle ensures that the loads from the structure are transferred into deeper and as a rule stronger soils at the lower skirt tip compared to the soil at and close the mudline. Upon installation, these structures assume a position with respect to level that corresponds to the levelness of the seabed, unless the seabed has been prepared and leveled before the installation took place. I.e. these structures are not provided with the ability to rectify the levelness.

The principle of rectification of seabed based structures during installation has been introduced into practice in 1990 when a new type of skirted foundation was introduced. More specifically, the structure comprised a triple cylindrical skirt arrangement applied for four anchors for the Heidrun Tension Leg Platform in the Norwegian Sea. Compared to the former type of structures described above, this structure was characterized by substantially deeper skirts and a substantially lower weight. In sum these two circumstances resulted in the need to enhance the own weight of the anchor by suction in the skirt compartments in order to achieve the desired embedment depth of the skirt. This and similar applications where the suction in each of at least three compartments can be individually controlled, enable to rectify possible deviation from levelness during the suction embedment phase by applying different pressures in the compartments.

Till date no method for rectification of levelness has existed for structures provided with shallow skirts (compared to the extent of the structure in the horizontal plane) and/or weight exceeding the penetration resistance of the skirts into the seabed. Furthermore, the suction based method for achieving levelness of the structures is not applicable for permeable seabed materials, such as sand and gravel.

The invention provides a method and embodiment for foundations that overcomes all the above described shortcomings. Summary of the invention

It is provided a device for installation on a seabed below a body of water, comprising a first wall element having a second wall element extending from and along the periphery of the first wall element, the first and second wall elements thereby defining an open compartment, said second wall element being adapted for penetration into the seabed whereby the compartment is closed by said wall elements and the seabed, and at least one valve means for allowing fluid flow into and out of the compartment, characterized by a sealing means in the compartment and extending to the second wall element, thereby defining a membrane between the compartment and the seabed.

In one embodiment, the sealing means comprises a membrane which is impervious to water, whereby any water inside the compartment is prevented from entering the seabed in the vicinity of and enclosed by the second wall element, but may be evacuated via the valve means. The compartment comprises filling means and the sealing means comprises in a preferred embodiment a fluid material which does not penetrate substantially into the seabed. In one embodiment, the sealing means comprises a grout or similar substance which will subsequently cure and solidify. In one embodiment, the sealing means comprises a solid material, such as a plastic bag or rubber. It is provided a method of installing the device according to the invention, characterized by the steps of: a) placing the device on a seabed below a body of water and effecting a penetration of the second wall element a distance into the seabed, whereby the compartment is closed by said wall elements and the seabed; b) applying the sealing means onto the seabed inside the compartment and extending it to the second wall element, thereby defining a membrane between the compartment and the seabed; c) effecting a load onto the device, whereby the second wall element is driven further into the seabed and water entrapped in the compartment is expelled via the valve means.

In one embodiment, step b) is performed by injecting a fluidized membrane such as grout via the filling means, in which steps a portion of the water in the compartment is expelled via the valve means.

In one embodiment, step c) is maintained until substantially all of the water inside the compartment has been expelled and the first wall element is in contact with the sealing means. It is provided a foundation base, comprising a plurality of devices according to the invention, characterized in that the devices are connected via a central structure. In one embodiment, the central structure comprises a nodal element. In one embodiment, the central structure comprises a device according to the invention. The central structure may comprise a penetration limiting element extending into the compartment, whereby the skirt's penetration depth into the seabed may be controlled and at least a portion of the compartment and the seabed surface form a cavity when the penetration limiting element is in contact with the surface. The penetration limiting element may extend from the load-bearing element and comprise a surface for contact with the seabed surface, and be centrally located on the load-bearing element.

In one embodiment, the plurality of devices comprise three devices arranged with an angular spacing of 120° around the central structure.

The filler material may comprise grout. By using of the method according to the invention in the installation of the foundation base according to the invention, the inclination of the foundation base with respect to the seabed may be controlled by selectively controlling one or more of the devices.

It is provided an autonomous fluid control device for controlling the flow of a first fluid and a second fluid, comprising a chamber having a first opening and a second opening, characterized by a closure element movable within the chamber and adapted for selectively closing and opening the first opening and the second opening, and comprising a material having a bulk density which is greater than the density of first fluid and less than the density of the second fluid. In one embodiment, when the valve is in use, the first opening is arranged above the second opening, whereby when the chamber predominantly comprises the first fluid, the closure element will move into a closing position against the second opening, and when the chamber predominantly comprises the second fluid, the closure element will move into a closing position against the first opening. The closure element comprises preferably a spherical element.

The inventive foundation base solves a set of typical issues associated with this type of structures, in particular:

• The base can be installed directly on an uneven and unleveled bottom

• The top soil layer, typically of reduced strength, does not need to be removed, replaced or reinforced to improve the bearing capacity of the base • The base can be applied for bottom soil with wide range of strength parameters, and permeable as well as impermeable soils

• The base can be applied for structures with any practical skirt depth to the horizontal extent of the base ratios and for a wide range of weight to penetration resistance ratios

For the use with offshore wind turbine supports the following advantageous features are inherent in its design:

• Excellent resistance to torque moments (i.e. rotation resistance in the horizontal plane) that for conventional foundations are size driving loads • Can adjust for soil strength variations within the footprint of the base that are often encountered in costal water where the wind turbines are typically installed

The invention relates both to design and installation of the foundation base, means for ensuring applicability for permeable seabed soils, and to an autonomous valve compatible with the requirements from the installation procedure.

Preferably, the foundation comprises a circumferential skirt extending downwards from the bottom slab, thereby defining at least one main compartment underneath the foundation. Preferably, in addition to the circumferential skirt a number of outer skirt compartments are provided. In addition to that, or as an option the main circumferential compartment may be subdivided into compartments by means of skirts extending downwards from the bottom slab and preferably extending radially from a center portion of the bottom slab to respective areas of the circumferential skirt.

In one embodiment, the foundation base is adapted for serving as a base for a subsea structure or a structure that protrudes the water level, and it is either a separate unit or it is an integrated part of the said structure. The purpose of the foundation base is twofold - provide support with a desirable inclination (most often horizontal) and transfer the loads from the structure and the base itself into the soil strata. Optionally, the foundation comprises the outer skirt compartments only, which can be clustered or more advantageously, separated from each other by means of structural members of the substructure.

Applicability to permeable soils is achieved by separating entrapped water in the skirt compartments actively participating in the level rectification tasks from water in the permeable seabed soil. In the preferred embodiment the separation is achieved by the use of suitable liquid, preferably standard grout, injected in the active skirt compartments or by confining the entrapped water in e.g. plastic bags, or by the use of membrane.

Brief description of the drawings

These and other characteristics of the invention will be clear from the following description of a preferential form of embodiments, given as a non-restrictive examples, with reference to the attached figures wherein:

Figure 1 is principle sketch of a top view of an embodiment of the inventive foundation base,

Figure 2 is a sectional view, along the section line A-A of figure 1, projected to a straight line,

Figure 3 is a sectional view similar to figure 2, and illustrates the invented foundation base in an installed state on a surface,

Figures 4-7 are sectional views, illustrating main steps in the installation procedure of the foundation base, on permeable seabed soil, Figure 8 is a sectional view, illustrating the last installation step of the foundation base on an impermeable seabed soil,

Figure 9 is principle sketch of a top view of an optional embodiment of the foundation base without the main skirt compartment,

Figure 10 is a sectional view, along the section line B-B of figure 9, projected to a straight line,

Figure 11 is a sketch illustrating a principle of the invention, and

Figures 12-14 are sectional views, illustrating an autonomous valve compatible for use during the installation procedure.

Detailed description of an embodiment Referring initially to figures 1 and 2, the illustrated embodiment of the foundation base 1 comprises a circular plate element 5 - in the following also referred to as a slab - which at its periphery is provided by skirt 2 extending from the slab 5 (in a downward direction when the foundation base is in use) thus defining a main compartment 9. Arranged with equal spacing around the slab 5 are a number of outer compartments, in the illustrated embodiment three outer compartments 3a-c arranged with an angular spacing of 120° and defined by respective plate elements 7a-c - which essentially are flush with the slab 5 - and by respective portions of the peripheral skirt 2 and respective outer skirts 4a-c each extending from the respective plate element 7a-c, in a downward direction when the foundation base is in use.

The main compartment 9 and the outer compartments 3a-c are in fact 'inverted cups' that are able to penetrate into a surface when the foundation base is being installed. This penetration is limited, however, by a landing element 18 - in this embodiment a plate-shaped element - on the lower face of the slab 5, extending a distance into the main compartment 9 as illustrated by figure 2. The surface area of the landing element 18 is dimensioned according to the overall design of the foundation base and in this particular embodiment serves to stop the penetration of the foundation base at a predetermined level, which ensures that the peripheral skirt 2 and the outer skirts 4a-c have penetrated to a target depth and leaving a sufficient space for grouting in the main compartment 9, which will be explained in the following. The landing element is optional and may be omitted if monitoring and control of target penetration depth can be done by measurements or by visual observation.

Figure 3 shows the foundation base 1 in an installed state on a seabed S below a body of water W, generally having an inclined and uneven surface (also referred to as mudline) M. It is seen that the lower face of the slab 5 is not touching the mudline M, but is - due to the interaction between the landing element 18 and the mudline M - lifted above the mudline M so much that there is a minimum distance in the main compartment 9 between these lower face of the slab 5 and the surface or mudline M in order to ensure that all voids of the main compartment 9 and the outer compartments 3a-c are eventually filled with a filler material F. This filler material may typically be a solid material in fluid or particulate form, e.g. such as grout used in the offshore industry and consisting of water mixed with cement and sodium silicate.

When the filler material F has hardened, it will be able to transfer loads from the slab 5 and plate elements 7a-c (including any structure supported by the foundation base) into the soil in the seabed S. Thus, this filler material F and the soil in the seabed confined by the peripheral skirt 2 and the outer skirts 4a-c serve to transfer vertical loads from the slab 5 and the plate elements 7a-c into the soil below these skirts' penetration depth P. Significant part of the loads from the structure are transferred by the skirts into the seabed layers below the skirt tip. Since the strength of the soil typically increases with the depth below mudline, therefore it may be advantageous to design the depth of the skirts according to each practical application.

Valves, etc. which are necessary in order to control the installation method, will be described and illustrated in the following. A typical installation sequence of the invented foundation base applied for a permeable seabed material will now be described with reference to figures 4 - 7. Although not shown in the drawings, the foundation base may be attached to or be an integral part of an offshore structure resting on it, or the foundation base may be a separate body preinstalled on the bottom and prepared for receiving a separate offshore structure. An example of such offshore structure is disclosed in the applicant's Norwegian patent application No. 20082860.

Figure 4 shows the first stage of the installation sequence, where the foundation base 1 has been lowered down to the seabed S and the peripheral skirt 2 and the outer skirts 4a-c have penetrated the surface (mudline) M and extend into the seabed, to a penetration depth P below the surface M. As the foundation base is being lowered, any water trapped in the main compartment 9 or the outer compartments 3a-c is displaced through valves lOa-c and lO'a-c. The valves 10'a-c connect respective ones of the outer compartments 3a-c with the surrounding water W, and fluid control devices lOa-c connect the main compartment 9 with the surrounding water W. Valve 10c is not shown in the figures. Preferably the valves lOa-c may be of the design described below with reference to figures 11-13.

Figure 5 indicates a next stage of the installation where the lowering of the foundation base 1 has been interrupted and the outer compartments 3a-c is being filled by a filler material F in an amount somewhat greater than the predicted volume of the cavities 3a-c above the mudline when embedded to the target depth, preferably a fluid filler F such as said grout, via fluid control devices 11 a-c, while the trapped water is being displaced through respective ones of the valves 10'a-c. The filling of grout and expulsion of water are indicated by respective arrows. When the outer compartments 3 a-c have been filled with the required amount of filler material F, the fluid control devices 1 la-c and 10'a-c close and the lowering - assisted by an application of additional vertical load onto the foundation base 1 - is resumed; leading to pressure increase in the filler material F and water trapped in the outer compartments 3a-c. Further penetration of the skirts 2, 4a-c into the bottom S is thus only possible when opening the fluid control devices 1 la-c and/or the valves 10'a-c, allowing the excessive water (if any) and the filler material F to escape from the respective outer compartment.

The inclination of the foundation base with respect to the horizontal can be monitored and, at any level until final penetration has been reached, corrected if an undesirable deviation from horizontal (or from any other target inclination) has been detected. Such correction is achieved by reducing the outflow of fluid, i.e. water and thereafter filler F, from that or those outer compartment(s) that is (are) lowest compared to the target. In the event that the penetration resistance from the seabed S exerted on the skirts 2, 4a-c exceeds the weight of the foundation base, additional vertical loading is required to fulfill the installation. This loading may be provided in form of a clump weight or similar (not shown), placed on the slab 5, or it can very efficiently be generated by pressure reduction in water entrapped in the main compartment 9. The pressure reduction is achieved by a conventional technique involving pumping the entrapped water via a valve-controlled port 12 out of the cavity in the main compartment between the mudline and the lower face of the slab 5. During the procedure described above with reference to figure 5, the valves lOa-c in the main compartment 9 are closed, either manually or by using autonomous valves of a type which is described below with reference to figures 11- 13. Figure 6 shows the foundation base 1 with its peripheral skirt 2 and outer skirts 3a-c penetrated into desirable penetration depth P, ensuring the levelness (or desirable inclination) of the base. Now the foundation base is resting either partly on the embedded skirts and partly on the grout F, or mostly on the embedded skirts if the embedment was enhanced by the use of suction. The main compartment 9 is not yet filled with grout. The foundation base itself, or integrated in a structure as explained above, has sufficient stability and the final grouting of the main compartment can be postponed if necessary (e.g. due to adverse weather conditions). Gradually and within hours, typical grout mixtures will stiffen and solidify.

Figure 7 illustrates the position upon completion of penetration, ensuring contact of the landing element 18 with the bottom M, and the main compartment 9 being at least partially filled with filler material F (grout or similar). Grout F is added through the port 12 in with several outlets and water is displaced out of the compartment via valves lOa-c. The grout F propagates in radial direction towards the main compartment's 9 periphery. Figure 8 illustrates the foundation fully penetrated into soil S having a low permeability. Although the installation is simplified, the outfitting in this application is identical with that described for the foundation suitable for permeable soils under figures 4 to 7. Also the installation proceeds in the same steps as in the previous case, however with the exception that the step described in figure 5 can be omitted, i.e. the use of the filler to separate the entrapped water from the water in soil is not required. In this application the leveling is done by controlling the water pressure in the compartments 3a-c. As in the case of foundation on permeable soil also here the foundation has a substantial bearing capacity immediately after completion of the embedment provided mainly by the water in the compartments 3a-c that can resist short term loads, thus the final steps involving replacement of entrapped water by filler can be at later occasion. The figure illustrates filling of the filler into the main skirt compartment 9 through the orifice 12 and filling into the outer skirt compartments 3a-c through the fluid control devices 1 la-c. Water is being displaced in the cavities by the filled and flows out trough the valves 10a-c and lO'a-c. Description of an optional embodiment

Figure 9 is principle sketch of a top view of another embodiment of the foundation base where - compared to figures 1 - 7, the slab 5 and its peripheral skirt 2 have been omitted. In this embodiment, the foundation base comprises only the outer compartments 3a-c connected by a structure 13 into a central nodal element 13'. In an optional embodiment, the structures 13 can directly interconnect the outer compartments 3a with 3b, 3b with 3c, and 3c with 3a.

Figure 10 illustrates a sectional view, along the section line B-B of figure 9, projected to a straight line, of the finally installed foundation base into seabed S. Skirts 4a-c are embedded to the required depth and the cavity above mudline M inside the skirt compartments 3a-c filled with filler F. The installation is performed in the same manner as described for the embodiments in figures 1 to 8. For control of the levelness and displacement of entrapped water the skirt compartments 3a-c are fitted with control devices l la-c and valves lOa-c. The skilled person will understand that the operation of controlling devices and filling of filler F (e.g. grout) can be performed from the same control unit (not shown). In instances where the soil penetration resistance to the skirts is larger than the gravity loads from the foundation base and possible superstructure (not shown) supported by the base, the embedment into to soils can be facilitated in conventional manner by water pressure reduction ("suction") in the compartments 3a-c. Levelness is achieved by pressure control in individual compartments.

Figure 11 further illustrates the principle described above, where a compartment 20 is defined by a base plate 23 and a circumferential skirt 22 extending into a permeable seabed S below a body of water W. The permeable seabed S comprises a material such as sand, gravel, silt, etc. The compartment 20 has been filled by a filler material F in a manner described above, via a filler valve 26 (indicated by arrow/in figure 11, this filler material forming a water-impervious membrane between the seawater w inside the compartment and the seabed S. When a load L is applied to the compartment, the water w trapped inside the compartment is forced out through the valve 24 (which may be similar to the valves 10'a-c and lOa-c described above), indicated by the arrow e in figure 11. This load L may e.g. be a static load imposed by a structure situated above the compartment.

The membrane created by the grout F ensures that the water w inside the compartment is expelled in a controlled manner through the valve 24 and not being forced into the permeable seabed in an uncontrolled manner that would prevent pressure build-up needed for level rectification. Further, it might cause wash-out and/or liquefaction and hence compromise the structural integrity in the seabed around the skirts. In a practical application, the load L is applied until the skirt has been driven into the seabed to such an extent that all water w has been evacuated and the entire compartment above the mudline M is filled with grout F. Following this installation, the grout will subsequently cure and harden, thus forming a solid foundation element.

Instead of using a fluid filler material F such as grout, etc., a similar membrane could be generated by watertight plastic bags. Further, a similar membrane could be generated by a rubber element or a similar structure. However, such non-fluid membranes would be difficult to install and cumbersome to operate during installation, unlike the fluid grout of the invention.

An embodiment of the fluid control device 1 Oa-c- will now be described, with reference to figures 12 - 14, schematically illustrating an autonomous fluid control device designated by the reference numeral 11.

This fluid control device 11, i.e. a combined outlet and infusion valve 11, is particularly suitable for the functions required for installation of the foundation base as described above, when flow of two fluids of different densities (e.g. water and grout) may be controlled.

The fluid control device 11 comprises in the illustrated embodiment a chamber 14 having a first opening 15 and a second opening 16. In a practical application, as illustrated, the first opening 15 is an upper opening 15 and the second opening is a lower opening 16. The upper 15 and lower 16 openings may serve as inlets or outlets, depending on the stage of the process, as will be described in the following.

A closure element 17 is arranged within the chamber 14 and is free to move inside the chamber. The closure element 17 is in the illustrated embodiment shown as a spherical element, serving to selectively close one of the first 14 or second 15 openings. The spherical shape has been selected in order to obtain an efficient closure , but shall not be restricted to such shape.

The closure sphere 17 has a bulk density which is greater than the density of the first fluid (e.g. water), and less that the density of the second fluid (e.g. grout). This means that the closure sphere 17 will move downwards (i.e. sink) in the first fluid (water) and move upwards (i.e. float) in the second fluid (grout). In figure 12, the fluid control device 11 is shown in an idle state or in a state in which there is suction in the lower opening 16 (i.e. a pressure reduction in the fluid on the lower opening 16). In such condition, the closure sphere 17 is being forced into a closure position of the lower opening 16. This state represents e.g. the situation when the embedment of the foundation base is enhanced by suction, as described above.

Figure 13 shows the fluid control device 11 in operation and when it is desirable that water flows from the lower opening 16 towards the upper opening 15. The sphere 17 is then lifted by the fluid flow, allowing passage of water through the chamber 14. Referring to the installation procedure of the foundation base above, this is representative of the situation when the trapped water is escaping out of the main compartment 9 and the outer compartments 3a-c. Figure 14 shows the fluid control device 11 when used for the main compartment 9 from which the trapped water is being displaced by grout, all water has been pressed out and the grout F has filled the chamber 14. The sphere 17 has been lifted by buoyancy and is being pressed to the upper opening 15 and thus closing it for further flow.