Subsea scour protection

Abstract

A scour protection unit comprises a barrier that is deployable over seabed soil beside a marine structure such as a pile. The barrier comprises a group of elongate arms that are pivotable relative to each other and flexible webs that interconnect neighbouring arms of the group. Concretion of at least one of the webs is promoted after deployment of the arms. Base arms are pivotably supported by a hub and base webs connect pairs of neighbouring base arms. The group of elongate arms further comprises an upright pillar arm, with an upright web extending between the pillar arm and one of the base arms to form an anti-scour vane. The base arms are pivotable downwardly and outwardly relative to the hub from a retracted state into a deployed state to form a base panel. In the base panel, the neighbouring base arms are splayed apart and the base webs extend between them to cover the seabed soil. Deployment of the base arms can also deploy the upright web.

Description

[0001] Subsea scour protection

[0002] This invention relates to the problem of scour due to erosion of submerged soil, and the consequential need to protect structures that rely upon submerged soil for support. The invention has particular benefit for protecting such structures in open sea locations, such as foundations of fixed offshore wind turbines.

[0003] Scour can occur where water flows around an object that stands above the bed of a body of water in marine, riverine and lacustrine environments. Examples of such objects include upright elongate structural elements such as bridge piles, suction piles, legs of offshore oil platforms and pile foundations of bottom-fixed offshore wind turbines. A lower end of such an object is typically embedded in the soil, in which case the stability of the object often relies upon the depth of its embedment into the soil and the uniformity of embedment around the periphery of the object.

[0004] In open seas, water flows could come from any direction. On encountering and dividing around a submerged stationary object, the divided water flow tends to generate eddies or vortices around and downstream of the object. Such vortices may entrain, transport and erode the soil in a process of excavation that, over time, decreases the embedded depth of the object and so could undermine its bearing capacity and stability.

[0005] The problem of scour is especially acute where fixed offshore wind turbines are mounted on a monopile that is driven into seabed soil. Monopiles have a generally smooth cylindrical surface of circular cross-section. As is well known, a cylindrical body presents challenges of vortex formation when exposed to fluid flowing in a direction transverse to the central longitudinal axis of the cylinder. Additionally, where the seabed intersects the cylindrical surface, a ‘horseshoe vortex’ can develop close to the interface between the monopile and the seabed. A horseshoe vortex follows an arc that wraps around the upstream side and approaches the downstream side of the monopile. In that vortex, water tumbles about a curved horizontal axis in a manner that strongly promotes scour.

[0006] Various approaches have been proposed to mitigate scour around objects underwater such as the foot or pile of a fixed offshore wind turbine. None of the various known anti-scour solutions are sufficiently optimised to prevent, mitigate or avoid the worst vortices, yet they are bulky, heavy and expensive. For example, anti-scour arrangements for offshore wind turbines can be more than 20m in diameter. It is therefore important to be able to install them as easily as possible. In this respect, anti-scour systems are either attached to a structure to be installed with it or are installed afterwards. Known systems greatly increase the bulk of the structure to be transported and installed, or require multiple lifts to be lowered to the seabed and assembled around a structure already installed offshore. The weight of such systems is also an issue: protection that is too heavy is difficult to install, whereas protection that is too light or too flexible requires extra rock dumping.

[0007] A first family of known anti-scour solutions involves shrouding, stabilising or armouring seabed soil by placing mats, rocks and / or other items upon the seabed around the turbine. For example, KR 10-0652171 discloses geotextile bags, GB 2529252 discloses mattresses of interlinked used vehicle tyres, and EP 3736382 discloses scour prevention units that each comprise a bag body housing block objects in a bag material. Other similar solutions involve placing frond mats or sediment-collecting mesh onto the soil.

[0008] All of the above solutions are time-consuming and expensive to install and for many of them, efficiency or practicality is not fully proven. For example, rock armour solutions could require up to 5000 tonnes of rock per installed foundation. Also, the under-pressure of a horseshoe vortex tends to draw seabed sediments up through a porous armour layer such as rock armour, causing the armour layer to sink into the seabed over time.

[0009] In US 4114394, a scour protection system is deployed by unfolding a mat from a marine structure to cover the seabed adjoining the structure. The mat is essentially rigid but may comprise sections that can be unfolded during deployment.

[0010] EP 3228754 discloses various arrangements in which mats unfold down or away from a suction pile onto the surrounding seabed in a petaloid arrangement. EP 3228754 and CN 11662372 also disclose arrangements in which umbrella-like frames unfold upwardly or downwardly when deploying mats outwardly from a pile, those unfolding movements being driven by progressive penetration of the pile into the seabed soil.

[0011] CN 110965569 and CN 211735479 propose a shallow frusto-conical mat that opens upwardly and outwardly around a pile, unfolding like an umbrella, before being lowered onto the seabed. Conversely, CN 210104812 proposes a mat mounted around a pile that is deployed downwardly and outwardly onto the surrounding seabed, unfolding like an inverted umbrella. In the examples above that employ a scour protection mat, the mat is a pliant material such as a geotextile. They all have the drawback of being too light and insufficiently rigid, hence requiring rock dumping to stabilise the soil.

[0012] A second family of solutions involves placing geometrically-shaped anti-scour structures near to or around the foot of a wind turbine to impede vortex formation or to disrupt vortices once formed. For example, CN 216156628 and CN 113186986 teach various blade arrangements that are mounted to or in the vicinity of a monopile. CN 109372006 discloses an annular bottom plate whose upper surface is surmounted by upstanding sills disposed between radial plates.

[0013] CN 217267593 discloses vertical triangular plates that are spaced angularly around a monopile in respective radial planes extending from a sleeve that is concentric with the monopile. The circumferential gaps or spaces between successive plates are filled with sand or gravel, presumably for armouring or ballasting purposes, and are then covered by respective curtains in an umbrella-like arrangement that is intended to combat scour by modifying water flow. The sleeve extends above the plates and the curtains to accommodate holes in the sleeve for dissipating energy of tidal currents.

[0014] DE 20 2013 012108 discloses a flexible scour protection mat comprising a series of spokes extending from a central retaining ring, which may form part of a foundation. The spokes are connected by chains, which are slightly longer than the distance between the points on the spokes that they connect to in order to hang slightly loosely and gradually embed into the seabed. The spokes may also be connected by a flat, flexible material. Joints at the inner end and the middle of the spokes allow articulation with respect to the retaining ring and for the spokes to conform better to the contours of the seabed.

[0015] CN 211257044 discloses a deployable scour protection mat for offshore wind pile foundations. The mat is carried in a retracted configuration with cloth panels, stiffened by a frame, raised up against the foundation. The panels are held in place against the action of weights and tensioned springs by hooks. When the hooks release the weights, the frame and cloth panels fall to the seabed around hinges. The cloth panels carry artificial grass to assist with the anti-scouring effect.

[0016] GB 2614898 describes a mat comprising a flexible web formed of circumferential frond lines that extend between arms extending radially from a collar member, the frond lines comprising artificial seagrass. The collar member is sized to allow a pile or subsea foundation to pass therethrough. The arms may be hingedly, or otherwise rotatably, attached to the collar member to allow for for easy installation of the mat on the seabed.

[0017] GB 1383012 discloses a scour protection apparatus comprising a flexible sheet supported by ribs that are hinged to a ring clamped to a marine structure. A rope is connected to the sheet at a radially outer edge thereof and connected to ends of the ribs to hold the sheet taut. The ribs hang downwardly from the ring to create a frusto-conically shaped skirt.

[0018] It is against this background that the present invention has been devised. From one aspect, the invention resides in a scour protection unit comprising a barrier that is deployable over soil beside a submerged structure. The barrier comprises a group of elongate arms that are movable relative to each other from a retracted state into a deployed state and flexible webs that extend between neighbouring arms of the group. The group of arms comprises a set of base arms and an upright pillar arm, and the webs comprise base webs that connect neighbouring base arms and an upright web that extends between the pillar arm and one of the base arms. At least one of the webs can be held in tension between neighbouring arms of the group when the arms are in the deployed state.

[0019] The unit further comprises a concretion facilitation system that acts on at least one of the webs to stiffen or to add weight to the at least one web after movement of the arms into the deployed state. The preferred concretion system functions in a way that is akin to how coral reefs are repaired, by a cathodic arrangement. Thus, the concretion facilitation system may comprise electrical conductors that are attached to or incorporated into the at least one web.

[0020] At least one arm of the group may be extensible in length during deployment of the barrier. Arms of the group may be, substantially, mutually parallel in the retracted state. For example, the arms can be substantially upright in the retracted state.

[0021] The base arms may be pivotably supported by a hub. The base arms may be pivotable downwardly and outwardly relative to the hub from their retracted state into their deployed state to form a base panel in which the neighbouring base arms are splayed apart and the base webs extend between them to cover the soil. The pillar arm may be in fixed orientation relative to the structure and may, for example, extend upwardly from the hub, which may in turn be in fixed relation to the structure. Conveniently, an actuation system can act on the arms via the hub.

[0022] The base panel may splay or widen in a direction away from the hub. For example, the base panel could comprise outward ly-divergent outer base arms, outwardly-divergent inner base arms disposed between the outer base arms, and outer base webs extending between the outer base arms and the inner base arms. The base panel may further comprise a central base arm between the inner base arms, and inner base webs extending between the central base arm and the inner base arms.

[0023] The base panel may be substantially planar when the base arms are in the deployed state. Nevertheless, at least one of the base webs could be slack when the base arms are in the deployed state. The base panel can extend substantially horizontally or downwardly from the hub when the base arms are deployed.

[0024] The inventive concept embraces a scour protection system comprising an array of scour protection units of the invention, disposed in side-by-side relation and potentially encircling or extending around a columnar submerged structure. When the base arms of the units are deployed, the base panels of the units suitably cooperate to form a petaloid shroud that substantially covers or encloses the seabed soil adjoining the structure. The inventive concept also embraces a submerged structure fitted with at least one scour protection unit of the invention or a scour protection system of the invention.

[0025] Correspondingly, the inventive concept extends to a method of mitigating scour, the method comprising: deploying a barrier over soil beside a submerged structure by moving a group of elongate arms relative to each other from a retracted state into a deployed state; extending flexible webs between neighbouring arms of the group; and promoting concretion of at least one of the webs after deployment of the barrier. The group of arms comprises a set of base arms and an upright pillar arm, and the webs comprise base webs that connect neighbouring base arms and an upright web that extends between the pillar arm and one of the base arms.

[0026] The arms may be substantially parallel, for example with upright orientation, before deployment of the barrier. At least one of the arms can extend in length when deploying the barrier. At least one of the webs can be held under tension between the deployed arms. Conversely, at least one of the webs can be draped onto the soil between the deployed arms.

[0027] By pivoting the set of base arms of the group downwardly and outwardly relative to the structure from the retracted state into the deployed state, a base panel can be formed in which the base arms are splayed apart and the base webs extend between neighbouring base arms to enclose the soil. Elegantly, pivoting the set of base arms can tension the upright web.

[0028] Thus, the invention provides a deployable and self-consolidating scour protection system. The system comprises a set of deployable arms that can be integrated onto a structure such as a monopile, the arms having a retracted position for installation operations such as lifting, lowering and hammering and a deployed position for in-place operation. The radial arms, and webs supported by the arms, can define a substantially horizontal and flat section on the seabed as a replacement for rock dumping or to minimise the need for rock dumping.

[0029] Each web may be an evolutive woven net that supports development of concretion processes such as marine growth, biofouling, calcification or solidification. This achieves the same functionality as existing scour protection systems that have solid walls, hence blocking or restricting water circulation and being heavy enough to be stable in operation.

[0030] Development of concretion can be promoted or controlled. The invention therefore improves existing deployed mat systems to leverage the possibility provided by controlled growth to generate substantially flat or rigid panels and upright walls, to define compartments between mat sections and rigid anti-scour vanes.

[0031] Embodiments of the invention provide a scour protection system that comprises: a mount on a subsea foundation; arms, articulated on the mount, that can be in a retracted configuration during installation and in an extended configuration for service life; an arm deployment system to change the configuration of the arms; fabric surfaces such as woven nets extending between the arms; and a concretion facilitation device.

[0032] To facilitate marine growth or calcite accretion, the concretion facilitation device may comprise electrical cables that are attached to or embedded into the fabric surfaces and are connected to a source of electrical power. The arms could be rigid, articulated or inflatable. The system may also comprise an inflation system, which may employ a gas or other fluid. More generally, the arm deployment system may comprise accumulators or tanks for air or other fluids held under elevated pressure.

[0033] The arms may be deployed from a substantially vertical retracted position to a substantially tilted, inclined or horizontal deployed position. The mount may be level with or at a level above the arms, and could be at a distance of, say, between 2m and 10m above the seabed.

[0034] The articulation of the arms on the mount may provide at least two axes of rotation. For example, the arms can pivot in a radial direction around respective tangential or circumferential axes and around respective radial axes during deployment.

[0035] After deployment, the fabric surfaces could have a catenary shape between two successive arms or could comprise a substantially horizontal area on the seabed and tilted or inclined areas near each arm.

[0036] In summary, a scour protection unit of the invention comprises a barrier that is deployable over seabed soil beside a marine structure such as a pile. The barrier comprises a group of elongate arms that are pivotable relative to each other and flexible webs that interconnect neighbouring arms of the group. Concretion of at least one of the webs may be promoted after deployment of the arms.

[0037] Base arms are pivotably supported by a hub, which may be fixed relative to the structure, and base webs interconnect pairs of neighbouring base arms. The base arms are pivotable downwardly and outwardly relative to the hub from a retracted state into a deployed state to form a base panel. In the base panel, the neighbouring base arms are splayed apart and the base webs extend between them to cover the seabed soil. Deployment of the base arms can also deploy an upright web that extends upwardly from the base panel to form an anti-scour vane.

[0038] In order that the invention may be more readily understood, reference will now be made, by way of example, to the accompanying drawings in which:

[0039] Figure 1 is a detail perspective view of a scour protection unit of the invention, mounted underwater on a submerged pile; Figure 2 corresponds to Figure 1 but shows a housing of the unit cut away to show components of a deployable scour protection barrier within the housing;

[0040] Figure 3 is a schematic perspective view of the barrier during deployment;

[0041] Figure 4 is a schematic perspective view of the barrier when fully deployed;

[0042] Figure 5 corresponds to Figure 4 but shows the barrier subjected to concretion;

[0043] Figure 6 is a schematic top plan view of a scour protection system comprising a circumferential array of scour protection units of the invention whose barriers are deployed and encircle a submerged pile shown here in cross-section;

[0044] Figure 7 corresponds to Figure 4 but shows another scour protection barrier of the invention; and

[0045] Figure 8 is a schematic side view that shows another scour protection system of the invention.

[0046] Referring firstly to Figures 1 and 2 of the drawings, a scour protection unit 10 of the invention is shown here mounted externally to a side wall 12 of a subsea structure 14 where the surrounding seabed 16 is to be protected from scour. The unit 10 is one of an array of such units 10 distributed side-by-side around or along the side wall of the structure 14. Together, the array of units 10 forms a scour protection system of the invention.

[0047] The structure 14 is embedded into or rests upon the seabed 16. In this example, the structure 14 is a submerged pile such as a suction pile or a monopile and so has a cylindrical contour defined by a tubular wall. A scour protection system of the invention can be used to protect other fully- or partially-submerged structures of other shapes, with curved or straight side walls.

[0048] In this example, the unit 10 is mounted to the structure 14 close to the seabed 16, with a lower end of the unit 10 close to, or substantially aligned with, the mudline being the interface between the seabed 16 and the water column above. The mudline intersects the curved side wall 12 of the structure 14. The unit 10 comprises a hollow housing 18, exemplified here by an upwardly-elongate cylinder, that contains a deployable scour protection barrier 20 as can be seen, before deployment, in the cut-away view of Figure 2. The barrier 20 is initially shrouded within the housing 18 in the compact retracted or folded state shown here and remains in that state to facilitate transportation and installation operations performed on the structure 14, such as towing, upending, lifting, lowering and hammering or pile-driving.

[0049] After the structure 14 has been installed, the housing 18 is opened underwater to deploy the barrier 20. The housing 18 could be opened remotely, or by an ROV or a diver, before deployment of the barrier 20 begins. Alternatively, the housing 18 could be opened during deployment, for example by the force of deployment of the barrier 20 breaking or otherwise moving aside a wall of the housing 18.

[0050] Deployment of the barrier 20 is driven by an actuation system 22 such as a hydraulic or pneumatic system that acts upon movable components of the barrier 20. The unit 10 shown here has an individual actuation system 22 comprising accumulators or reservoirs that hold a working fluid under high pressure. However, it would be possible instead for two or more units 10 of a scour protection system to share a common actuation system, for example by being connected to a manifold that distributes a high-pressure working fluid around the structure 14.

[0051] The barrier 20 comprises a group of arms 24 that are pivotable relative to each other and that are extensible in length during deployment. Flexible textile or fabric webs 26 shown in Figures 3 and 4 extend between neighbouring arms 24 of the group. When the barrier 20 is folded, the retracted arms 24 are, compactly, substantially parallel and upright as shown in Figure 2. The webs 26 are omitted from Figure 2 but may conveniently be folded or furled between or wrapped around the arms 24 when the barrier 20 is folded.

[0052] In this example, the arms 24 are all connected together at their lower ends by a mount or common hub 28 that defines horizontal pivot axes about which relative pivotal movements of the arms 24 take place. Being disposed near the bottom of the unit 10, the hub 28 is also close to, or substantially aligned with, the mudline.

[0053] The actuation system 22 acts upon the arms 24 of the barrier 20 via the hub 28, causing the arms 24 to pivot and extend during deployment. Those unfolding movements of the arms 24 will now be described with further reference to Figures 3 and 4, which show the barrier 20 in isolation without the housing 18 or the actuation system 22 of the unit 10 or the structure 14 to which the unit 10 is mounted.

[0054] The group of arms 24 comprises a pillar arm 24A that remains fixed in an upright orientation and a set of five base arms 24B-D, namely a central base arm 24B, a pair of inner base arms 24C and a pair of relatively short outer base arms 24D. The base arms 24B-D are joined by base webs 26A-B, namely a pair of inner base webs 26A that extend between the central base arm 24B and respective inner base arms 24C and a pair of outer base webs 26B that extend between respective inner base arms 24C and respective outer base arms 24D. The central base arm 24B is also joined to the pillar arm 24A by an upright web 26C. All of the webs 26 are generally triangular. The upright web 26C tapers outwardly away from the hub 28 whereas the inner and outer webs 26A-B taper inwardly toward the hub 28.

[0055] During deployment, the base arms 24B-D pivot down relative to the pillar arm 24A into a substantially horizontal orientation as shown in Figure 3. Thus, the base arms 24B-D pivot through nominally 90° about their respective horizontal pivot axes. The base arms 24B-D also pivot apart from each other or fan out about respective vertical axes. At this intermediate stage of deployment, the webs 26 extending between neighbouring arms 24 may remain slack as shown.

[0056] Next, or while pivoting as described above, the arms 24 extend in length to complete deployment as shown in Figure 4. For example, each arm 24 could comprise a piston that is movable longitudinally within a tubular cylinder. Alternatively, each arm 24 could comprise a longitudinally-inflatable bellows, bladder or balloon. By virtue of relative angular movement between the arms 24 and elongation of the arms 24 causing outer ends of the arms 24 to move further apart, the webs 26 extending between neighbouring arms 24 are tensioned to become taut when fully deployed.

[0057] The base arms 24B-D and the taut inner and outer base webs 26A-B disposed between them then lie in a common substantially horizontal plane, together forming a substantially rigid, generally planar base panel 30 that lies against the seabed 16, parallel with or aligned with the mudline. Conversely, the upright web 26C between the pillar arm 24A and the central base arm 24B lies in a substantially vertical plane that intersects the plane of the base panel 30 orthogonally. The webs 26 are of a foraminous material such as a woven or knitted net or mesh. Filaments forming the foraminous material or the structure of that material could be substantially inextensible or elastically extensible, hence enabling the material to stretch while remaining sufficiently taut.

[0058] By virtue of the porosity of their foraminous material, the webs 26 are apt to serve as a matrix or substrate that supports growth of marine organisms in a biofouling process or development of calcification due to accretion of calcites. As shown in Figure 5, the resulting concretion or solidification of the webs 26 stiffens and thickens the webs 26, closes pores that penetrate the webs 26 and increases the mass of the webs 26. Over time, this strengthens, stiffens and further stabilises the webs 26, and increases their ability to block and disrupt scouring water flows without requiring rock dumping or other additional protection or support.

[0059] Concretion of the webs 26 may be enhanced and accelerated after deployment by a concretion facilitation system 32 as shown schematically in Figure 5. The system 32 can employ known methods to facilitate concretion and marine growth formation, for example by electrolysis as described in US 2024 / 0035186. For this purpose, the system comprises a power source 34 that energises electrical cables or wires attached to or embedded into the webs 26. In the example shown, the wires of neighbouring webs 26 are interconnected by electrical connectors 36 that bridge the arms 24 between them. However, if reliance can instead be placed upon natural concretion processes, provisions such as these for facilitating or enhancing concretion may be optional.

[0060] Figure 6 shows a scour protection system comprising barriers 20 deployed in a circumferential array that encircles the side wall 12 of a circular-section structure 14 such as a monopile. In this example, each arm 24 of each barrier 20 comprises a piston that is movable longitudinally within a tubular cylinder.

[0061] The array of barriers 20 exemplified in Figure 6 comprises eight barriers 20 that are equi- angularly spaced around a vertical central longitudinal axis 38 of the structure 14. The base panel 30 of each barrier 20 abuts the base panels 30 of neighbouring barriers 20 and therefore extends around 45° of arc. The result of deployment of the barriers 20, as shown in Figure 6, is to form a substantially planar petaloid shroud defined by their abutting base panels 30 that cooperate annularly to cover the seabed 16 around the structure 14. In each barrier 20, the inner base arms 24C are longer than the outer base arms 24D so that both pairs of inner and outer base arms 24C-D span a similar range of arc despite being at different radial distances from the central longitudinal axis 38. Abutting side edges of the outer base webs 26B of neighbouring barriers 20 are nearly parallel, hence leaving negligible gaps between the base panels 30 of neighbouring barriers 20. This ensures that the base panels 30 of the barriers 20 combine to provide substantially continuous circumferential coverage of the seabed 16 around the structure 14.

[0062] It will be noted that the upright webs 26 of the barriers 20 lie in respective inwardly- converging vertical radial planes that contain the central longitudinal axis 38 of the structure 14. Those planes also contain the upright pillar arms 24A and the central base arms 24B of the respective barriers 20.

[0063] When the barriers 20 are deployed and in service, water flowing horizontally across the seabed 16 from any direction impinges successively on their upright webs 26C. The upright webs 26C thereby serve as vanes to disrupt, slow and de-energise the flow. Moreover, as the base panel 30 defined by the base arms 24B-D and the inner and outer base webs 26A- B lies against the seabed 16, it effectively shields the soil of the seabed 16 beneath from turbulently-moving water.

[0064] Beneficially, the base arms 24B-D hold the base webs 26A-B in contact with or in close proximity to the seabed 16, without requiring rock dumping to stabilise the base webs 26A-B or the seabed 16 beneath. The invention thereby provides a lightweight anti-scour system that can be transported and installed conveniently with the structure 14 to be protected, and that is compact enough not to hinder those transportation and installation operations. The system can be deployed in a single operation without requiring the expense and complexity of additional installation operations such as rock dumping.

[0065] In a variant of the invention shown in Figure 7, the base webs 26A-B are not taut when the barriers 20 are deployed but instead hang slackly from base arms 24B-D that lie slightly above the seabed 16. Thus, major central portions 40 of the base webs 26A-B lie draped on the seabed 16 whereas catenary-curved edge portions 42 of the base webs 26A-B extend upwardly from the central portions to the respective base arms 24B-D. The base panel 30 defined by the base arms 24B-D and the inner and outer base webs 26A-B may therefore be regarded as semi-rigid in this instance. In contrast, in this example, the upright web 26C extending between the pillar arm 24A and the central base arm 24B remains taut and in a substantially vertical plane.

[0066] Finally, Figure 8 shows a further variant of the invention applied to an upwardly elongate structure 14, such as a monopile, whose lower end is embedded in soil of the seabed 16. In this variant, a circumferential array of scour protection units 10 is mounted to a structure 14 at an elevated level that is a substantial distance above the mudline 44. Thus, when barriers 20 are deployed from the respective units 10, the base arms 24B-D and the base webs 26A- B defining the base panels 30 pivot down by more than 90° about respective horizontal axes to form a downwardly-extending skirt. Downward pivoting continues until the outer tips of the base arms 24B-D reach the mudline 44 and encounter the seabed 16 and outer edges of the inner base webs 26A lie against and parallel to the mudline 44.

[0067] The result of deployment of the barriers 20 as shown in Figure 8 is to form a generally frusto- pyramidal hollow shroud that covers or encloses the seabed 16 around the structure 14. The shroud has sides, faces or facets that correspond in number to the number of scour protection units 10 that encircle the structure 14. In this respect, it will be noted that the example shown in Figure 8 employs six mutually-abutting barriers 20, each extending around 60° of arc, and so exemplifies how more or fewer units 10 can together encircle a structure 14.

[0068] Many other variations are possible within the inventive concept. For example, the upright web of each barrier could be omitted if not required. In another example, the base panel of each barrier need not be planar. For example, the base webs of the base panels could be inclined relative to each other to form an approximately frusto-conical skirt around a structure when the base panels are arranged in a continuous circumferential array.

[0069] The base panels of neighbouring barriers need not abut in edge-to-edge or side-to-side relation but could, for example, cooperate in overlapping relation instead. For this or other purposes, the barriers of a scour protection system of the invention need not all deploy simultaneously but could instead deploy in a sequence to facilitate their mutual cooperation.

Claims

Claims1. A scour protection unit comprising a barrier that is deployable over soil beside a submerged structure, the barrier comprising a group of elongate arms that are movable relative to each other from a retracted state into a deployed state and flexible webs that extend between neighbouring arms of the group, wherein the group of arms comprises a set of base arms and an upright pillar arm, and the webs comprise base webs that connect neighbouring base arms and an upright web that extends between the pillar arm and one of the base arms, the unit further comprising a concretion facilitation system that acts on at least one of the webs to stiffen the at least one web after movement of the arms into the deployed state.

2. The unit of Claim 1 , wherein at least one arm of the group is extensible in length during deployment of the barrier.

3. The unit of Claim 1 or Claim 2, wherein the arms of the group are, substantially, mutually parallel in the retracted state.

4. The unit of Claim 3, wherein the arms of the group are substantially upright in the retracted state.

5. The unit of any preceding claim, wherein at least one of the webs is held in tension between neighbouring arms of the group when the arms are in the deployed state.

6. The unit of any preceding claim, wherein the concretion facilitation system comprises electrical conductors that are attached to incorporated into the at least one web.

7. The unit of any preceding claim, wherein the pillar arm is configured to be in fixed orientation relative to the structure.

8. The unit of any preceding claim, wherein: the base arms are pivotably supported by a hub and are pivotable downwardly and outwardly relative to the hub from their retracted state into their deployed state to form a base panel in which the neighbouring base arms are splayed apart and the base webs extend between them to cover the soil.

9. The unit of Claim 8, wherein the hub is configured to be in fixed relation to the structure.

10. The unit of Claim 8 or Claim 9, comprising an actuation system that acts on the arms via the hub.

11. The unit of any of Claims 8 to 10, wherein the pillar arm extends upwardly from the hub.

12. The unit of any of Claims 8 to 11 , wherein the base panel splays in a direction away from the hub.

13. The unit of Claim 12, wherein the base panel comprises outer base arms, inner base arms disposed between the outer base arms, and outer base webs extending between the outer base arms and the inner base arms.

14. The unit of Claim 13, wherein the base panel further comprises a central base arm between the inner base arms, and inner base webs extending between the central base arm and the inner base arms.

15. The unit of any of Claims 8 to 14, wherein the base panel is substantially planar when the base arms are in the deployed state.

16. The unit of any of Claims 8 to 15, wherein at least one of the base webs is slack when the base arms are in the deployed state.

17. The unit of any of Claims 8 to 16, wherein the base panel extends substantially horizontally from the hub when the base arms are deployed.

18. The unit of any of Claims 8 to 16, wherein the base panel extends downwardly from the hub when the base arms are deployed.

19. A scour protection system comprising an array of scour protection units of any preceding claim disposed in side-by-side relation.

20. The system of Claim 19, wherein when the base arms of the units are deployed, the base panels of the units cooperate to form a petaloid shroud.

21. The system of Claim 19 or Claim 20, wherein the array of units extends around a columnar submerged structure.

22. A submerged structure fitted with at least one scour protection unit of any of Claims 1 to 18 or a scour protection system of any of Claims 19 to 21.

23. A method of mitigating scour, the method comprising: deploying a barrier over soil beside a submerged structure by moving a group of elongate arms relative to each other from a retracted state into a deployed state and extending flexible webs between neighbouring arms of the group, wherein the group of arms comprises a set of base arms and an upright pillar arm, and the webs comprise base webs that connect neighbouring base arms and an upright web that extends between the pillar arm and one of the base arms; and promoting concretion of at least one of the webs after deployment of the barrier.

24. The method of Claim 23, wherein the arms are substantially parallel before deployment of the barrier.

25. The method of Claim 23 or Claim 24, wherein the arms have upright orientation before deployment of the barrier.

26. The method of any of Claims 23 to 25, further comprising elongating at least one of the arms when deploying the barrier.

27. The method of any of Claims 23 to 26, comprising holding at least one of the webs under tension between the deployed arms.

28. The method of any of Claims 23 to 27, comprising draping at least one of the webs onto the soil between the deployed arms.

29. The method of any of Claims 23 to 28, comprising pivoting the set of base arms downwardly and outwardly relative to the structure from the retracted state into the deployed state to form a base panel in which the base arms are splayed apart and the base webs extending between neighbouring base arms enclose the soil.

30. The method of Claim 29, wherein said pivoting of the set of base arms tensions the upright web.

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