The present invention relates to a cable protection assembly for a submerged structure, such as a foundation. The structure can be a foundation for an energy generator, for example, for an offshore wind turbine such as a fixed offshore wind turbine. In other examples, the cable can be adapted to connect to other submerged foundations, such as caissons, foundations for wave energy generators, or subsea substations used for power of signal transmission or relay etc.

Fixed offshore wind turbines are mounted on structures such as foundations in contact with the seabed. Foundations for fixed offshore wind turbines can comprise piles (e.g. monopiles) driven into the sea floor or jackets comprising frames supported on such piles, although gravity bases can also be used. The foundation supports the wind turbine throughout a lifespan lasting several decades, and normally houses cables and other equipment used to take off power from the wind turbine.

The seabed immediately surrounding the foundation is generally faced with one or more layers of “scour protection”, such as rock dumps and filter layers, to control and typically to reduce the extent of erosion of the seabed surface material from the vicinity of the foundation, for example, as a result of local current flowing around the foundation; such erosion can adversely affect the integrity of the substrate around the foundation, and therefore the stability of the foundation. The scour protection layers typically extend radially outward from the axis of the wind turbine for as far as is necessary, to stabilise the local seabed around the foundation.

The cable used for power take off connects equipment located inside the structure of the wind turbine with a power management and transmission equipment such as a transformer station or the like located outside of the wind turbine. Suitable cables are long, expensive and present challenges in handling and installation. The cable must pass through an aperture in the foundation from the outside of the structure to the inside, and to reduce damage to the cable as it passes through the aperture, it is known to use a foundation interface device such as is disclosed in EP3400638, which bridges the aperture between the outside and the inside of the foundation, and which has a bore through which the cable can more safely pass. Outside the wind turbine structure, the cable is generally trenched for protection of the cable between the wind turbine and the shore. The trench is generally initiated close to the outside edge of the scour protection layers. Also useful for understanding the invention are EP3812636; EP3967352; WO2015/071684; EP3319191; WO2020/174831 and DE102013208527.

Summary

The present invention provides a cable protection assembly for a cable connected to a submerged structure; the cable protection assembly comprising: a tube having a bore adapted to house the cable, the tube having an inner end and an outer end, the tube comprising at least two serially arranged tubular sections interconnected by joints; the tube having a first plane and a second plane, and wherein the tube has a differential between the maximum possible deflection of the tube in the first plane and the maximum possible deflection of the tube in the second plane.

Optionally the tube (e.g. the joints of the tube) resists deflection in the first plane (typically a horizontal plane) more than in the second plane (typically a vertical plane).

Optionally the cable protection assembly comprises first and second tubes each having a bore adapted to house the cable, an inner end and an outer end; each of the first and second tubes comprising at least two serially arranged tubular sections interconnected by joints, and each of the first and second tubes having a first plane and a second plane. Optionally an inner end of the first tube is adapted to connect to the submerged structure (optionally to an interface device on the submerged structure) and the outer end of the first tube is adapted to connect to an inner end of the second tube. Optionally the first tube is relatively more rigid than the second tube. Optionally the second tube is relatively more flexible than the first tube, at least in one of the first and second planes.

Whereas the second tube has a differential between the maximum possible deflection in the first plane and the maximum possible deflection in the second plane; in the first tube (typically in the joints in the first tube) the corresponding differential is smaller. Optionally the differential between the maximum possible deflection in the first and second planes in the first tube can be or can approach zero, or in other words, the maximum possible deflection in the first and second planes of the first tube can be similar or identical. In other words, optionally in the first tube there is substantially no difference between the maximum possible deflection in the first and second planes. Optionally the first tube can restrict deflection substantially equally in the first and second planes. Optionally the joints in the first tube can restrict deflection of the first tube substantially equally in the first and second planes.

Typically the second (flexible) tube is adapted to connect to the structure (e.g. to the foundation) via the first (rigid) tube. Optionally the bore in the second (flexible) tube connects to the structure via the bore of the first (rigid) tube. Optionally the first (rigid) tube is an inner tube, attached directly to the structure, and the second (flexible) tube is an outer tube, distally arranged in relation to the structure, spaced from the structure by the first (rigid) tube, and adapted to attach to the structure via the first (rigid inner) tube.

Optionally the structure comprises a foundation, optionally a foundation for a power generator, such as a wind turbine or wave energy generator. Optionally the structure comprises a foundation for an offshore fixed wind turbine. Optionally the foundation comprises a caisson.

Optionally the cable is a power transmission cable, such as a take-off cable. Optionally the cable can comprise a power supply cable, or optionally a signal transmission cable.

Optionally the inner end of the first (rigid) tube is adapted to connect to a wind turbine foundation, and optionally a foundation interface device. The wind turbine foundation may comprise a monopile foundation, a jacket foundation or any other suitable form of wind turbine foundation. Optionally the joints between adjacent sections of the first (rigid) tube are rigid, typically having at least the same resistance to deflection as the axial sections of the first (rigid) tube between the joints, and optionally greater resistance to deflection. Optionally the rigidity of the first (rigid) tube is substantially uniform along its length. Typically the minimum permitted radius of deflection (i.e. the minimum bend radius or MBR) of the first (rigid) tube is within the range 2-6m, typically 4m, and typically with angular coverage of 45-90 degrees. Optionally the outer end of the first (rigid) tube terminates within 2m of a peripheral boundary of a scour protection element, for example, a scour protection layer disposed on the seabed and surrounding the foundation, optionally within 1m of the peripheral boundary and optionally before the peripheral boundary. The scour protection element optionally extends radially from an axis of the wind turbine foundation. Optionally a joint between the first and second tubes is disposed within the peripheral boundary. Optionally the scour protection element comprises one or more layers. Optionally the one or more layers comprise a rock layer and optionally a filter layer, and optionally the rock layer is disposed on top of the filter layer.

Optionally the scour protection element is adapted to resist erosion of an area of the seabed surrounding the foundation. Optionally the outer end of the second (flexible) tube extends beyond a scour protection layer of the foundation, optionally over or optionally within the seabed. Optionally at least a portion of the second (flexible) tube is disposed within the seabed in use.

Optionally the joints in the second (flexible) tube are articulated, optionally comprising pivotable connections, with pivot axes (optionally single axis connections such as pivot pins) between adjacent sections, permitting sections of the second (flexible) tube to pivot relative to one another in the second (e.g. vertical) plane around the pivot axes in the joints. Optionally the pivot axes in the joints of the second (flexible) tube are mutually parallel. Optionally the pivot axes are arranged to be substantially parallel with the seabed, and can typically be horizontal in use.

Typically the second plane is orthogonal to the first plane. Optionally the joints between adjacent sections of the second (flexible) tube resist movement in the first plane (for example a horizontal plane) and permit movement in the second plane (optionally a vertical plane). Optionally joints between adjacent sections of the second (flexible) tube deflect more under load than the axial lengths of the second (flexible) tube sections between the joints in the second (flexible) tube. Typically the MBR of the second (flexible) tube in the first (horizontal) plane similar to the MBR of the first tube - i.e. within the range 2-6m, e.g. 4m.

Optionally each tubular section in the first and second tubes comprises a socket at a first (e.g. inner) end and a pin at a second (e.g. outer) end. The socket of each tubular section is adapted to optionally receive and optionally connect to the pin of an adjacent tubular section. Optionally the joints are adapted to secure the socket of each tubular section to the pin of the adjacent tubular section, optionally with fixings. Optionally each joint comprises a bolted joint, typically with a single axis pivot connection. Optionally at least one or each tubular section comprises a straight tubular section. Optionally at least one or each tubular section comprises a curved tubular section. Optionally at least one or each tubular section of the rigid and flexible tubes can be formed in segments that are circumferentially divided and assembled around the cable.

Optionally an inner surface of at least one socket or an outer surface of at least one or each pin is lined with a liner material. Optionally the liner material is compliant, optionally more compliant than the material of the flexible tube pin and socket. Optionally the liner material has a Young’s modulus of at least 0.5% of the corresponding Young’s modulus of the material of the flexible tube, typically 25%- 0.5% , for example 15%-0.5%, 8%-0.5% or in some examples, 1%-3% of the corresponding Young’s modulus of the material of the flexible tube.

Optionally the liner material reduces wear of the pin and socket as they articulate within the joint, optionally distributing load within the joint, and typically reducing point loading within the joint. Optionally the liner material comprises polytetrafluoroethylene (PTFE), Polyurethane (Pll), or rubber. Optionally the liner material is compressed between the tubular sections.

Optionally the tubular sections of the first and second tubes are formed from a composite material comprising a bonding agent such as a resin and a reinforcing material typically comprise a fibre such as glass fibre or carbon fibre. The tubular sections of the first and second tubes may be formed from a metal such as steel or from any other suitable material.

The invention also provides a cable protection assembly for a cable connected to a submerged structure; the cable protection assembly comprising: a first tube having a bore adapted to house the cable, the first tube having an inner end adapted to connect to the submerged structure, and an outer end, the first tube comprising at least two serially arranged tubular sections interconnected by joints; the first tube having a first plane and a second plane; a second tube having a bore adapted to house the cable, the second tube having an inner end adapted to connect to the outer end of the rigid tube, and having an outer end, the second tube comprising at least two serially arranged tubular sections interconnected by joints; the second tube having a first plane and a second plane; wherein the first tube has a first differential between the maximum possible deflection of the first tube in the first plane and the maximum possible deflection of the first tube in the second plane, and wherein the second tube has a second differential between the maximum possible deflection of the second tube in the first plane and the maximum possible deflection of the second tube in the second plane; and wherein the first differential is smaller than the second differential.

The invention also provides a method of protecting a cable connected to a submerged structure, the method comprising: connecting an end of a first tube onto the submerged structure, the first tube having a bore adapted to house the cable, and comprising at least two serially arranged tubular sections interconnected by joints, the first tube having a first plane and a second plane; connecting an end of a second tube to the first tube, the second tube having a bore adapted to house the cable, the second tube comprising at least two serially arranged tubular sections interconnected by joints, the second tube having a first plane and a second plane; wherein the method comprises: housing the cable in the bores of the first and second tubes; connecting the cable to an installation in the submerged structure; lifting the second tube relative to the first tube (for example in a vertical plane); removing seabed material below the second tube to form a trench; lowering the second tube relative to the first tube (for example in a vertical plane) into the trench; and burying at least a portion of the second tube in the trench; wherein the first tube has a first differential between the maximum possible deflection of the flexible tube in the first plane and the maximum possible deflection of the flexible tube in the second plane, and wherein the second tube has a second differential between the maximum possible deflection of the rigid tube in the first plane and the maximum possible deflection of the rigid tube in the second plane, and wherein the first differential is smaller than the second differential.

Typically the second (flexible) tube (and typically the first relatively more rigid tube) resists axial compression and extension. Connections to the wind turbine or the foundation can be direct, or to a foundation interface device, such as a locking sleeve, or a J-tube which is adapted to connect to the wind turbine or its foundation.

Optionally the maximum possible deflection of the second tube in the first plane (e.g. the horizontal plane) is similar to the maximum possible deflection of the first tube in the same first plane, for example, within 10%, optionally within 5%, and optionally within 2%. Advantageously, as the two tubes deflect under lateral loads in the horizontal plane, the extent of deflection of the first and second tubes in the first (horizontal) plane is generally consistent along the length of the two tubes.

Typically the maximum possible deflection of the second tube in the second plane (e.g. the vertical plane) is greater than the maximum possible deflection of the first tube in the same second (or vertical) plane.

Optionally either or both of the first and second tubes can be formed from segments that are divided longitudinally along the axis of the tube, and connectable together around the cable.

The various aspects of the present invention can be practiced alone or in combination with one or more of the other aspects, as will be appreciated by those skilled in the relevant arts. The various aspects of the invention can optionally be provided in combination with one or more of the optional features of the other aspects of the invention. Also, optional features described in relation to one aspect can typically be combined alone or together with other features in different aspects of the invention. Any subject matter described in this specification can be combined with any other subject matter in the specification to form a novel combination.

Various aspects of the invention will now be described in detail with reference to the accompanying figures. Still other aspects, features, and advantages of the present invention are readily apparent from the entire description thereof, including the figures, which illustrates a number of exemplary aspects and implementations. The invention is also capable of other and different examples and aspects, and its several details can be modified in various respects, all without departing from the spirit and scope of the present invention. Accordingly, each example herein should be understood to have broad application, and is meant to illustrate one possible way of carrying out the invention, without intending to suggest that the scope of this disclosure, including the claims, is limited to that example. Furthermore, the terminology and phraseology used herein is solely used for descriptive purposes and should not be construed as limiting in scope. In particular, unless otherwise stated, dimensions and numerical values included herein are presented as examples illustrating one possible aspect of the claimed subject matter, without limiting the disclosure to the particular dimensions or values recited. All numerical values in this disclosure are understood as being modified by "about". All singular forms of elements, or any other components described herein are understood to include plural forms thereof and vice versa.

Language such as "including", "comprising", "having", "containing", or "involving" and variations thereof, is intended to be broad and encompass the subject matter listed thereafter, equivalents, and additional subject matter not recited, and is not intended to exclude other additives, components, integers or steps. Likewise, the term "comprising" is considered synonymous with the terms "including" or "containing" for applicable legal purposes. Thus, throughout the specification and claims unless the context requires otherwise, the word “comprise” or variations thereof such as “comprises” or “comprising” will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

Any discussion of documents, acts, materials, devices, articles and the like is included in the specification solely for the purpose of providing a context for the present invention. It is not suggested or represented that any or all of these matters formed part of the prior art base or were common general knowledge in the field relevant to the present invention.

In this disclosure, whenever a composition, an element or a group of elements is preceded with the transitional phrase "comprising", it is understood that we also contemplate the same composition, element or group of elements with transitional phrases "consisting essentially of’, "consisting", "selected from the group of consisting of”, “including”, or "is" preceding the recitation of the composition, element or group of elements and vice versa. In this disclosure, the words “typically” or “optionally” are to be understood as being intended to indicate optional or non- essential features of the invention which are present in certain examples but which can be omitted in others without departing from the scope of the invention.

References to directional and positional descriptions such as upper and lower and directions e.g. “up”, “down” etc. are to be interpreted by a skilled reader in the context of the examples described to refer to the orientation of features shown in the drawings, and are not to be interpreted as limiting the invention to the literal interpretation of the term, but instead should be as understood by the skilled addressee.

Brief description of the drawings

Figures 1 & 2 show schematic side views of a cable protection assembly;

Figure 3 shows a schematic side view in section of a tubular section of a first (rigid) tube in the Fig 1 cable protection assembly;

Figure 4 shows a perspective end view of a tubular section of a second (flexible) tube in the Fig 1 cable protection assembly;

Figure 5 shows a perspective side view of two tubular sections of the second tube connected by a pivotable joint;

Figure 6 shows a perspective section view of the Fig 5 arrangement;

Figure 7 shows a schematic plan view of a prior art cable protection assembly shown for comparative purposes, and a graph of cable bend radius against arc length for the same cable;

Figure 8 shows a schematic plan view and a graph of cable bend radius against arc length for a cable protection assembly according to the claimed subject matter; and Figs 9 and 10 show views of modified tubular sections of the first and second tubes .

Detailed description of one or more examples

A cable protection assembly 1 for protecting a power take off cable C connected to a submerged structure, which in this case comprises a foundation F of a fixed offshore wind turbine is shown in the drawings. The turbine foundation F has an aperture 4 passing through a wall of the foundation (the opposite wall is not shown for clarity), and an optional foundation interface device 5 disposed within the aperture. The foundation interface device 5 is a generally straight and generally rigid tube, with an outer end disposed outside the foundation F, an inner end inside the wall of the foundation F, and a bore connecting the inner and outer ends. Before cable installation, the foundation interface device 5 is passed through the aperture 4 and locked, resisting pull-out from the aperture and pivotal movement of the foundation interface device 5 relative to the foundation F. Once the foundation interface device 5 is locked in place in the aperture, the bore of the foundation interface device 5 then receives a cable C to guide the cable C through the aperture and into the foundation F during installation of the cable C. As shown in Fig 1, the foundation interface device typically extends through the aperture 4 at an acute angle with respect to the axis of the foundation, so that the outer end of the foundation interface device 5 is oriented towards the seabed 3, which is useful to minimise the bend radius necessary for the cable C as it transitions between the seabed 3 and the foundation interface device 5. A suitable interface device is shown in WO2022/129891.

The foundation F is surrounded with conventional scour protection S, which can take the form of one or more layers deposited on the seabed 3, such as a rock layer S1 and a filter layer S2. The layers of scour protection S extend radially around the axis of the foundation F, for a distance that in most cases depends on the material and condition of the underlying seabed 3. The scour protection S typically resists erosion of the material of the seabed from the local area, which helps to maintain the integrity of the pile(s) of the foundation F.

The outer end of the foundation interface device 5 has a mating formation typically in the form of a pin or other male connector which is adapted to connect to a first tube in the form of rigid tube 10. The rigid tube 10 is formed in serially interconnected sections 11 , 12, 13, 14... n etc., which extend radially outwards from the foundation F toward the periphery of the scour protection S. Each section 11 , 12, 13, 14 of rigid tube 10 has a socket at one end and a pin at the other. The pin on tube section 11 is adapted to connect into the socket on the adjacent tube section 12, the pin and socket together forming a joint 11j. The other sections of rigid tube 12, 13, 14 etc. are similarly interconnected with joints 12j, 13j, 14j etc. Thus a joint is formed between adjacent rigid tube sections to serially connect the sections together to form the rigid tube 10. The rigid tube 10 extends radially from the foundation F and the outermost section of rigid tube (14 in this example, but the number of tube sections can be varied) terminates close to the outermost radial periphery of the scour protection S, in a pin.

The joints made by the pins and sockets between adjacent sections of rigid tube 10 are typically secured in this example by fixings extending radially through apertures which align when the pins are received in the sockets, thereby resisting axial disconnection and relative rotation of adjacent sections of the rigid tube 10. In the rigid tube 10, joints are typically formed by more than two fixings, for example four fixings such as bolts as best shown in Fig 3, which are regularly (optionally equally) spaced around the circumference of pin and socket in the joint. As well as resisting pull-out and rotation, the joints between the pins and sockets on the adjacent sections of rigid tube 10 resist lateral bending to a high degree, so that the rigid tube 10 is highly resistant to pivotal movement around any axis at the joints.

The sections of rigid tube 10 between the pin at one end and the socket at the other are also relatively stiff, and resist bending of the sections between the two ends of each section. This imparts a high degree of stiffness to the rigid tube 10. In the rigid tube 10, the maximum possible deflection of the rigid tube 10 is typically consistent in at least two different planes, for example, horizontal and vertical, and is typically highly consistent in all planes of the rigid tube. The rigid tube 10 typically deflects only slightly when subjected to a load, for example, a lateral load tending to deflect the rigid tube 10 sideways in a horizontal plane. Further, the extent of deflection of the rigid tube 10 in response to load (e.g. such as a lateral deflection in response to local current) is highly consistent in each direction. Thus, any differential between maximum possible deflection of the rigid tube in the vertical plane and horizontal planes is relatively low, and in this example, approaches zero, so that there is substantially no detectable difference or differential in the maximum possible deflection in the two planes. Typically while the differential is low and typically approaches or equals zero, there is some permitted deflection under load, and typically, the rigid tube 10 deflects to a small extent in a generally smooth arc.

The rigid tube 10 has a bore adapted to house the cable C. The inner diameter of the bore is typically slightly larger than the outer diameter of the cable C, and the cable C can typically move laterally and slide axially within the bore. Some sections of the rigid tube 10 can be straight, like rigid tube section 12, for example, and some can be curved, like rigid tube section 11 for example.

The rigid tube 10 is generally resistant to hoop strain, and maintains a generally constant inner diameter in the bore even when high forces area applied to the rigid tube 10, for example lateral forces urging the rigid tube 10 sideways.

The outer end of the first tube in the form of the rigid tube 10 is adapted to connect to an inner end of a second tube in the form of flexible tube 20. The flexible tube 20 is formed in serially connected sections 21 , 22, 23, 24...n in a similar manner to the serially connected sections of the rigid tube 10. Like the rigid tube sections, the flexible tube sections 21 , 22, 23, 24 each have a socket and a pin at respective inner and outer ends, and a bore adapted to receive the power take off cable C. The socket on the inner end of the first section 21 of the flexible tube 20 is adapted to connect to the pin on the outer end of the last section (section 14 in this example) of the rigid tube 10. The flexible tube 20 is connected via joints 21j, 22j, 23j etc. between the socket and pin on adjacent sections 21, 22, 23, 24 in a manner similar to the joints 11j, 12j, 13j, 14j etc. on the rigid tube 10, but instead of being rigid like the joints 11j, 12j, 13j between the adjacent sections of the rigid tube 10, the joints 21 j, 22j, 23j between the adjacent sections 21 , 22, 23 of the flexible tube 20 are articulated to permit relative pivotal movement of the adjacent sections of flexible tube 20.

The joints of the flexible tube 20 each comprise a pivot axis between adjacent sections, and typically the pivot axes in the flexible tube are mutually parallel. The sections 21 , 22, 23... etc. of the flexible tube 20 are typically relatively stiff between the ends of each section, and so relative movement of the flexible tube 20 sections occurs principally at the joints 21j, 22j, 23j... etc. between adjacent sections of the flexible tube 20, e.g. at the pivot axes in the joints. In this example, the pivot joint is formed by two fixings such as bolts passing through the socket and pin as best seen in Fig 4; the bolts are typically aligned on a single axis and in this example are diagonally arranged with respect to one another, and thus the joint 21 , for example, pivots around a single pivot axis passing through the diagonally opposed bolts in the joint 21j. In this example, at least one joint 21 j (and typically all joints in the flexible tube 20) incorporates a liner 211. The liner 211 in this example lines the inner surface of the socket of the tubular section 22, but it could be alternatively be on the outer surface of the pin of tubular section 21. The liner 211 in this example comprises a thin layer of PTFE, extending over the inner surface of the joint 21 j which contact the pin of the tubular section 21 , so that the pin on the tubular section 21 is isolated from the socket on the tubular section 22 by the liner. The liner distributes load between the pin and the socket and thereby reduces wear in the joint 21 j by reducing point loading on the pin and socket. Optionally the liner can be resilient. The liner material can typically have a lower Young’s modulus than the material of the pin and socket, and so is generally more compliant.

In this example, the tubular sections 11 , 12, 13, 14, 21 , 22, 23, 24 of the rigid and flexible tubes 10, 20 are formed from glass reinforced plastic/fibreglass but any other suitable composite material comprising a bonding agent (such as a resin) and a reinforcing material (i.e. a fibre such as glass fibre or carbon fibre) may be used instead. In other examples, the tubular sections 11 , 12, 13, 14, 21 , 22, 23, 24 of the rigid and flexible tube 10, 20 are optionally formed from a metal such as steel.

The internal detail of the joint 21 j is shown in Fig 6. The pin on the outer end of the tubular section 21 is generally straight and cylindrical, with a constant (typically circular) cross section along its length. The socket on the inner end of the tubular section 22 which receives the pin on the tubular section 21 comprises a bell. The bell has a partially frustoconical shape arranged at its outer end, and between the bell and its outer end, the tubular section 22 has an expanded section which tapers outwardly to a radial maximum and then tapers radially inwardly to the inner end of the bell.

The bell and the expanded section each have notional axes X, Y and Z shown in a schematic and isolated section of second tubular section 22 in Fig 4. Axis Z extends along the bore of the tubular section 22. Axes X and Y are perpendicular to each other and to axis Z. The bell (and typically the expanded section) tapers radially but only along the axis X, whereas the internal dimensions between the sides of the bell (and optionally the expanded section) along the Z axis remains generally constant, and all along the Z axis, the bell is typically a relatively close fit with the pin on the tubular section 21. This permits the pin of the tubular section 21 to pivot within the socket of the tubular section 22, but only around the axis Y, or in other words, only in the XZ plane (e.g. in a vertical plane in use), and no other pivotal movement is permitted by the joint.

The fixings connecting the tubular sections 21 , 22 are aligned on a single pivot axis which is within the bell and which is coaxial with axis Y, and outside the expanded section. The end of the pin of the tubular section 21 passes through the bell, and into the expanded section. The taper on the bell typically matches the radially inward taper between the radial maximum of the expanded section and the inner end of the bell, and the inner surfaces of the bell and the inward taper of the expanded section typically cooperate in this example to control and typically to limit the pivotal movement of the joint 21 j. For example, when pin on the tubular section 21 can freely pivot within the joint 21j relative to the socket on the tubular section 22, but when the outer end of the pin abuts the liner on the radially outwardly tapered surface of the bell, the inner end of the pin abuts the radially inwardly tapered surface of the expanded section, and can pivot no further around the fixings, thereby limiting the pivotal movement permitted by the joint 21j . Other joints in the flexible tube 20 typically operate in the same way.

The joints 21 j , 22j, 23j of the flexible tube 20 resist pivotal movement in a first plane. In one schematic example of flexible tube section 22 shown in Fig 4, the first plane is a horizontal plane containing the Y and Z axes. The Y axis in this same schematic example is passing through the pivot axis of the joint 22j, and the Z axis is aligned with the axis of the tube 22. At the same time as resisting pivotal movement in the first horizontal plane, the joint 22j can allow relative pivotal movement of the two adjacent sections in a second plane. In this schematic example of tube section 22 in Fig 4, the second plane is a vertical plane containing the X and Z axes. Thus the flexible tube 20 is more resistant to bending movements in the first horizontal plane than in the second vertical plane, and there is a measurable differential between the maximum possible deflection in the vertical (XZ) plane and the maximum possible deflection in the horizontal (YZ) plane. Typically the flexible tube 20 only permits pivotal movement of adjacent sections in the second vertical (XZ) plane, and resists pivotal movement in all other planes. Typically the flexible tube 20 resists axial extension and compression also. The maximum possible deflection of the flexible tube 20 in the horizontal plane is typically similar to the maximum possible deflection of the rigid tube 10 in the horizontal plane, so that as the two tubes 10, 20 deflect under load applied by local currents acting laterally on the tubes 10, 20, the extent of deflection is generally consistent along the combined length of the rigid and flexible tubes 10, 20.

The end of the flexible tube 20 extends beyond the scour protection S. The pivot axes in the joints between adjacent sections of the flexible tube 20 are typically arranged horizontally, so that the sections of the flexible tube 20 can generally pivot relative to one another as permitted by the joints 21j, 22j, 23j...etc. in a generally vertical plane.

Relative pivotal movement of the flexible tube sections 21 , 22, 23... etc. in the vertical plane as described above permits assembly of the rigid and flexible tubes 10, 20 onto the foundation F and deployment of the cable C through the interconnected bores of the tubes 10, 20. The rigid tube 10 can be assembled in sections using appropriate curved sections such as section 11 to be chosen to suit the local conditions at the deployment site and the (often variable) distances from the foundation F to the edge of the scour protection S, so that the last section 14 of the rigid tube 10 terminates just above the scour protection (for example, around 1-2m above the uppermost layer of scour protection S and within around 1-2m of its peripheral edge). The first section 21 of the flexible tube 20 can then be attached to the last section 14 of the rigid tube 10 immediately, and the cable C then deployed. Trenching operations can then be carried out at any suitable time, by lifting the flexible tube 20 off the seabed 3 in the vertical plane, and forming the trench below the intended path of the flexible tube 20 and cable C, following which the flexible tube 20 can be lowered in the vertical plane into the trench, because of the articulated joints 21j, 22j, 23j...etc. in the flexible tube 20 which permit upward and downward movement of the flexible tube. Once the outer end of the flexible tube 20 is buried as shown in Fig 1, the exposed section of the flexible tube 20 and the rigid tube above the seabed 3 offer more protection to the cable C since the increased rigidity in the horizontal plane has increased resistance to lateral movement of the tubes 10, 20 and the cable C in response to local currents. Fig 7 shows test results of a prior art cable protection system (without the presently claimed rigid and flexible tubes 10, 20), which is deflected significantly by horizontal currents acting on the cable and its protective pipe during use. Fig 8 shows similar data for the assembly 1 shown in Figs 1-2, which undergoes much less deflection in response to the same lateral forces. This is indicated by the difference between a minimum and a maximum cable bend radius at a given arc length being smaller in the Fig 8 results than in the Fig 7 results.

Figs 9 & 10 shows a possible optional modification to the earlier examples, in which either the rigid tube 10 or the flexible tube 20 tube (or each of them) can be formed in at least two segments (optionally more than two segments) which are longitudinally divided along the axis of the tube, allowing assembly around the cable without pulling the cable through the bore of the tube. Fig 9 shows a segmented section of tubing 111 for a possible modification to a rigid tube, and Fig 10 shows a segmented section of tubing 121 for a possible modification to a flexible tube. The modified sections 111 , 121 can be used in the same way as the previous tubular sections of the rigid and flexible tube 10, 20, but can be assembled around the cable C instead of offering the cable C to the bore of the tubing sections. The edges of the segments in the modified sections 111 , 121 can be interconnected by fixings to make up the tubing.