Tensioning apparatus

Abstract

Apparatus 100 for maintaining tension in a shaft of a structure, the apparatus comprising a shaft 10 comprising a plurality of shaft indentations 15, a reaction block 20 configured to be positioned ar

Description

FIELD OF THE INVENTION The present application relates to tensioning apparatus. In particular, embodiments are directed to apparatus for tensioning a shaft to a structure, including a ratcheting mechanism. Tensioning tools for use with such apparatus may also be included. BACKGROUND OF THE INVENTION Many structures require pre-loading with tension to ensure strong joint forces and to reduce the effects of fatigue loading. For example, tensioning shafts of certain types of subsea anchors must be pretensioned against the rest of the anchor in order to correctly install the anchor and provide a secure anchor point for systems such as offshore wind farms, or for mooring tidal energy systems, FPSOs and so forth. Typically, pre-loading is achieved by tightening bolted joints using nuts or other threaded elements to set a torque value. However, this approach is prone to several issues. The relationship between torque and generated axial pretension is highly variable due to the specific friction coefficient in each joint. In addition, bolted joints by nature are prone to loosening under varying loads. Accordingly, existing approaches require joints to be inspected regularly and retightened to ensure sufficient pretension in the joint. Furthermore, in scenarios where access or visibility of a joint is compromised, such as in subsea environments, positioning a torque tool (e.g. at a specific orientation to engage a hex nut) can be time consuming, expensive or even impossible. SUMMARY OF THE INVENTION Aspects of the invention are set out in the independent claims and preferable features are set out in the dependent claims. There is described herein apparatus for maintaining tension in a shaft of a structure, the apparatus comprising: a shaft comprising a plurality of shaft indentations, wherein each shaft indentation comprises a restraining surface oriented transverse to a longitudinal axis of the shaft; a reaction block comprising a shoulder surface, the reaction block configured to be positioned around the shaft and constrained laterally relative to the shaft; and at least one locking collar comprising: at least one proximal engaging surface configured to engage the restraining surfaces of the shaft, and a distal engaging surface configured to engage the shoulder surface of the reaction block; wherein the locking collar is biased radially inwards towards the shaft to form a ratchet by virtue of the engagement between the locking collar and the shaft indentations. The arrangement of the shaft indentations, the shoulder surface and the engaging surfaces of the at least one locking collar allow a ratchet to be formed. Advantageously, this can prevent loosening or a decrease in the shaft tension over time e.g. under varying loads because the ratchet forms a one way lock which is highly resistant to loosening effects. The ratcheting effect for maintaining tension can enable greater control and precision when applying and maintaining (pre-)tension in the shaft, which can allow specific tension values to be achieved. The terms “tension” and “pre-tension” are used interchangeably herein, and generally refer to a tensile force to be maintained within the shaft, e.g. as part of an installation process prior to using the shaft and / or structure for their intended purpose (such as subsea anchoring of other structures which often make use of a tensioned stem to hold the anchor in a rigid configuration in which it grips the rock mass). The “longitudinal axis” of the shaft is an axis of the shaft extending in a lengthwise (e.g. distal-proximal) direction, and is typically parallel to the major dimension of the shaft (e.g. its length). The term “around” as used herein means at least partially around. For example, the reaction block (e.g. by virtue of its shape / geometry) may or may not be positioned fully around the shaft. The term “proximal” as used herein refers to a direction along a longitudinal axis of the shaft and away from the structure and / or towards a user operating / applying the apparatus to the structure. The term “distal” as used herein refers to a direction along the longitudinal axis of the shaft and towards the structure and / or away from a user operating / applying the apparatus to the structure. The term “inward” as used herein refers to a radially inward direction with respect to the longitudinal axis, and the term “outward” refers to a radially outward direction with respect to the longitudinal axis. The term “lateral” as used herein refers to a radial direction (inward or outward) with respect to the longitudinal axis. The term “transverse” as used herein means oriented at an oblique angle to the longitudinal axis of the shaft that provides sufficiently different functionality from a parallel or perpendicular direction. This includes but is not limited to angles relative to the longitudinal axis of at least around 5° (or at least 1°) and less than around 90°. For example, a restraining surface oriented transverse to the longitudinal axis may be oriented at an angle sufficient to hold, trap, restrain, constrain or otherwise inhibit outward movement of a locking collar against a reaction block, e.g. by providing a reaction force in a direction having sufficient components in both a parallel direction and a perpendicular direction (with respect to the longitudinal axis). The terms “bias” and “biased” as used herein refer to urging something in a predetermined direction, but not necessarily to the extent its motion is limited to that direction. For example, the locking collar being biased inwardly means that outward motion of the collar may still be permitted, but that it is nonetheless urged to return inwardly. A component being “biased” can be due to inherent properties of that component (such as its shape, geometry, structure, material etc.) or due to external factors, such as separate biasing means which can apply a biasing force to the component (such as using a spring, a compressible / tensionable material, magnets etc.), or indeed by virtue of the arrangement and orientation of the device, in the sense that gravity can be used to bias an element in a generally downward direction. The structure may be a subsea or underwater structure, such as a subsea anchor. The shaft may be a shaft for activating a subsea or underwater structure, such as a stem for activating a subsea anchor. Maintaining tension in the shaft may cause a mechanism within the structure to activate, such as flaring outwardly one or more cutting surfaces in a subsea anchor to anchor the structure within rock or other bulk material or applying tension to a stem of such an anchor to hold the cutting surfaces in their flared-out arrangement. The structure may be an upper surface (e.g. an upper taper) of a subsea anchor, particularly where the anchor comprises a single pile. Alternatively, the shaft may be that of a standalone rock anchor and thus the structure may be the rock or other material to which the anchor is to be secured. For example, the apparatus disclosed herein may be used to replace a nut (or other locking mechanism) on existing rock / subsea anchors. The locking collar may be shaped such that, upon engaging the restraining surface and shoulder surface, reaction forces from the restraining surface and shoulder surface bias the locking collar radially inwards. This can allow the locking collar(s) to be biased inwardly (to form a ratchet) by virtue of their geometry, removing the need for any external biasing. Longitudinal contraction forces in the shaft may cause each locking collar to be restrained between the restraining surface and the shoulder surface to inhibit contraction of the shaft. The term “restrained” as used herein refers to a component being held, trapped, constrained, confined, or otherwise prevented from moving or escaping the apparatus. Longitudinal contraction force in the shaft may be based on a (longitudinal) tension force applied to the shaft. For example, a longitudinal contraction force may be transmitted to each collar (causing each locking collar to be restrained) upon release of an (externally) applied tension force. The proximal and distal engaging surfaces may be arranged to lie in planes which are substantially parallel or which converge at a location radially outward from the locking collar. The term “substantially parallel” as used herein refers to an angle (between two planes or lines) in the range of 0° ± 5°. When surfaces are curved, they are considered “substantially parallel” if they are aligned (within ±5°) at least along a portion of the surfaces. The proximal and distal engaging surfaces may be arranged to be substantially parallel or, when projected, to converge at a location radially outward from the locking collar. The location of convergence of the proximal and distal engaging surfaces can be a line, such as a straight line or axis (if the engaging surfaces are planar) or a curve (if the engaging surfaces are curved). The location of convergence is radially outward from the locking collar in the sense that it can be found by moving in an outwardly radial direction (e.g. according to a cylindrical coordinate system centred around a longitudinal axis of the shaft) from the locking collar. The proximal and distal engaging surfaces may lie on surfaces (e.g. planar or curved surfaces) which are parallel or which converge at a location (e.g. along an axis or curve) separated from the longitudinal axis of the shaft by the locking collar. In other words, the proximal and distal engaging surfaces (and / or the restraining surfaces and shoulder surface) are oriented such that each locking collar is held in equilibrium or urged inwards towards the shaft. Each locking collar may be substantially wedge shaped such that, upon engaging the restraining surfaces of the shaft, each locking collar inhibits movement of the shaft in a distal direction. The term “wedge shaped” as used herein refers to a shape that is generally tapered along one or more axes, and is operable to prevent two surfaces (e.g. a restraining surface and the shoulder surface) from advancing towards each other when placed between said surfaces. The proximal and distal engaging surfaces may define an angle therebetween of less than around 90°, preferably less than around 85°. This means that, optionally in conjunction with frictional forces, the restraining surfaces, locking collar and the reaction block form a ratchet system. The shaft indentations may be formed along an outer surface of the shaft. The restraining surface may be configured to restrain the at least one locking collar against the reaction block. In use, the restraining surfaces and the shoulder surface may be configured to be substantially parallel to the proximal and distal engaging surfaces respectively. This can provide more reliable engagement between the surfaces. The restraining surfaces may define an acute angle (6) relative to the longitudinal axis (of up to 90°). The angle (0) may be at least around 5°. This can ensure the locking collars are driven outwardly when a tension force is applied to the shaft. The angle (6) may be between around 45°(± 5°) and 90° to the length of the shaft. This can provide a more stable equilibrium of the locking collars, meaning the apparatus can be more reliable. The angle (0) may be at least around 70° (± 5°). The reaction block may be configured such that, in use, the shoulder surface defines an acute angle (<p) to a direction perpendicular to the longitudinal axis (or parallel to a lateral direction). The angle ((p) may be less than around 85° (or greater than around 5° to the longitudinal axis). This can ensure the locking collars are not driven outwardly by a longitudinal contraction force from the shaft but can be driven outwardly when a tension force is applied to the shaft. The angle (^) may be less than or equal to around 45° (± 5°). This can provide a more stable equilibrium of the locking collars, meaning the apparatus can be more reliable. The angle (<p) may be less than or equal to around 20° (± 5°). The angle (0) defined by the restraining surfaces and the angle (^) defined by the shoulder surface may sum to at least around 85°. The apparatus may comprise radial biasing means configured to apply a biasing force to the locking collar to inhibit radially outward movement of the locking collar. Upon the locking collar engaging the restraining surface and the shoulder surface, the biasing force may be configured to cause each locking collar to be restrained between the restraining surface and the shoulder surface to inhibit contraction of the shaft. The radial biasing means may be configured to constrain the locking collar to move within a predetermined radius. The radial biasing means may be configured to allow the locking collar to move beyond the predetermined radius resiliently upon application of a tension force to the shaft. This means that the biasing means can provide a biasing force sufficient to prevent contraction of the shaft, but which can be overcome (e.g. by applying a sufficiently large tension force to the shaft) to allow the locking collars to move outwardly and ratchet along the shaft indentations. The radial biasing means may be arranged to couple to the locking collar and the reaction block. The radial biasing means may be arranged to couple to two locking collars. The radial biasing means may comprise at least one of: a compression spring, a tension spring, a compressible stop, a magnetic component, a housing and a linkage. As used in this context, “radial” means that the biasing acts in a direction which has a component directed radially inward, and which is sufficiently large to bias the locking collar in the manner described above. The apparatus may comprise two or more locking collars. This can provide greater balance and improve the distribution of forces on the shaft and on each locking collar, reducing the chance of the apparatus failing. Each locking collar may comprise a plurality of proximal engaging surfaces configured to engage the restraining surfaces of the shaft. This can reduce the load on each proximal engaging surface, thus reducing the risk of locking collars breaking and improving their effective strength. The plurality of proximal engaging surfaces may form a plurality of protrusions. The proximal engaging surfaces may form a substantially corrugated or serrated surface of each locking collar. A cross-sectional shape of each proximal engaging surface may match a corresponding cross-sectional shape of each shaft indentation. The shaft indentations and / or the proximal engaging surface may comprise a curved surface. The shaft indentations may be formed by a single continuous curved surface. The curved surface may be curved in a plane perpendicular to a longitudinal axis of the shaft and / or in a plane parallel to the longitudinal axis. The curved surface can reduce localised shear forces within the shaft and / or the locking collars. The shaft indentations may be formed by one or more grooves in the shaft. The one or more grooves may comprise a helical groove. The helical groove enables multiple indentations to be provided using a single groove, which can improve the efficiency of manufacturing the shaft. The helical groove may comprise a threading of the shaft. This can enable the apparatus to be retrofit using an existing threaded shaft. The angle of the restraining surfaces to the longitudinal axis of the shaft may be defined by the pitch and / or depth of the threading. The one or more grooves may comprise a plurality of grooves perpendicular to a longitudinal axis of the shaft. This can ensure that the locking collars ratchet along the grooves at the same time. The plurality of grooves may comprise a plurality of circumferential grooves. The plurality of grooves may be elongate grooves. The grooves may be formed in a portion of an outer surface of the shaft. The grooves may be formed in a flat surface of the shaft. The reaction block may be configured to be coupled to the structure. The reaction block may be coupled to the structure using one or more bolts and / or clamps. Alternatively, the reaction block may be formed integrally with the structure, optionally machined into the structure or welded onto the structure. The structure may include a taper to further grip the rock mass, the taper being wider at the proximal end and narrowing toward the distal end of the structure. Upon engaging the at least one locking collar, the reaction block may be configured to self-brace to prevent lateral movement. The term “self-brace” as used herein means the reaction block prevents itself from moving laterally. For example, when there are multiple locking collars, by virtue of the geometry and positioning of the locking collars and the reaction block, forces exerted on the reaction block may cause it to self-brace. Alternatively, if an uneven distribution of one or more locking collars is used, the reaction block may brace against a surface of the shaft to prevent lateral movement. Upon application of a tension force to the shaft (in a proximal direction): each locking collar may be driven outwardly; and at least one of the locking collars advances distally relative to the shaft to engage at least one previously unengaged shaft indentation in a ratcheting fashion. Each shaft indentation may comprise a transverse release surface configured, upon application of the tension force to the shaft, to drive the locking collar outwardly. The transverse release surface may be configured, upon application of the tension force to the shaft, to drive the locking collar along the shoulder surface. Each locking collar being driven outwardly may comprise each locking collar sliding along the release surfaces to advance distally relative to the shaft. The apparatus may further comprise longitudinal biasing means configured to urge each locking collar distally relative to the shaft. This can ensure that the locking collars advance distally regardless of the orientation of the apparatus. Upon application of the tension force to the shaft, the longitudinal biasing means may cause each locking collar to advance distally relative to the shaft. Alternatively, the locking collars may be configured to be urged distally by gravity. The longitudinal biasing means may comprise a resilient biasing member. The resilient biasing member may be a biasing spring. Note that once the correct tension has been applied, there is no need to retain the longitudinal biasing means, since the ratchet effect holds the / each locking collar in place. The radial biasing means and the longitudinal biasing means may be provided by the same component(s) / mechanism and / or may be integrally formed. The radial biasing means and longitudinal biasing means may be provided by a biasing member which is transverse to the longitudinal axis, optionally parallel to the shoulder surface of the reaction block. A proximal end of the shaft may comprise a notch or a series of notches for engaging a tensioning tool. The reaction block may comprise a loading surface for engaging a tensioning tool. The notch and the loading surface of the reaction block may be configured to engage the tensioning tool for applying a tension force to the shaft. The notch for engaging the tensioning tool may extend around the shaft. This can enable the tensioning tool to be coupled (and thus the apparatus to be operated) from any orientation. The notch may comprise a circumferential notch. The apparatus may further comprise at least one compression spring disposed between the reaction block and the structure. The compression spring can enable greater precision in the tension maintained in the shaft. The compression spring may be configured to couple to the reaction block and / or to the structure. The compression spring may have a compressive stiffness less than a tensile stiffness of the shaft. The compressive stiffness of the compression spring may be less than a compressive stiffness of the reaction block. The compressive stiffness of the compression spring may be significantly less than a tensile stiffness of the shaft. This arrangement ensures that the spring is able to deform (in preference to the shaft) and alter the arrangement and alignment of the collar relative to the shaft, and thereby enables the correct operation of the apparatus as set out herein. The reaction block, shaft and / or locking collars may comprise a steel (e.g. stainless steel) and / or a plastic (e.g. a hard engineering plastic). The compression spring may be formed of a steel, polymer and / or plastic. The locking collar may further comprise an angled surface that is angled radially inwards towards the shaft and configured to receive a radially outward force from a detensioning tool. The angled surface may define an acute angle relative to the longitudinal axis of the shaft that is less than an acute angle defined by the shoulder surface of the reaction block relative to the longitudinal axis. The angled surface may be configured to receive an end of a linear force applicator, preferably a hydraulic cylinder, of the de-tensioning tool. There is also described herein a ratchet device for maintaining tension in a shaft of a structure, the shaft comprising a plurality of indentations, the ratchet device comprising: a reaction block comprising a shoulder surface, the reaction block configured to be positioned around the shaft and constrained laterally relative to the shaft; and at least one locking collar comprising: at least one proximal engaging surface configured to engage the shaft indentations, and a distal engaging surface configured to engage the shoulder surface of the reaction block; wherein the locking collar is biased radially inwards towards the shaft to form a ratchet by virtue of engagement between the locking collar and with the shaft indentations. The ratchet device can be used to maintain an applied tension in an existing shaft, for example as a retrofit installation. The ratchet device may comprise any reaction block and / or locking collar as described above. There is also described herein a kit of parts, comprising a ratchet device as described above, and a tensioning tool and / or a de-tensioning tool. The kit may further comprise a tensionable shaft having a plurality of indentations. The shaft may be any shaft as described above. A “tensionable shaft” is a shaft which can withstand repeated loading under tension, for example if the shaft can deform elastically (within a suitable operating range of tensions). The tensioning tool may comprise at least one linear force applicator configured to apply a tension to the shaft to successively ratchet the locking collars along the shaft indentations. Each linear force applicator may be a hydraulic cylinder. The de-tensioning tool may comprise at least one first linear force applicator configured to control tension applied to the shaft and at least one second linear force applicator configured to disengage the locking collar from the shaft indentations. The first and / or second linear force applicators may include a hydraulic cylinder. The second linear force applicator may be configured to contact an angled surface of the locking collar of the ratchet device and, upon application of force to the angled surface, drive the locking collar radially outward to a disengaged position and hold the locking collar in the disengaged position. There is also described herein a method of using any apparatus described above for maintaining tension in a shaft of a structure, the method comprising: positioning the reaction block around the shaft; and inserting each locking collar between the shaft and the reaction block to engage the shaft indentations. Positioning the reaction block may comprise: positioning the reaction block around a base of an exposed portion of the shaft; and / or securing the reaction block to the structure. Positioning the reaction block around the base of the exposed portion of the shaft may comprise sliding the reaction block distally along the shaft. Each locking collar may be inserted between the shaft and the reaction block after the reaction block is positioned. Each locking collar may be inserted in the reaction block prior to positioning the reaction block. The method may further comprise: coupling a tensioning tool to the shaft and the reaction block; applying tension to the shaft by activating the tensioning tool; releasing the applied tension by deactivating the tensioning tool; and decoupling the tensioning tool from the shaft and the reaction block. Coupling the tensioning tool to the shaft may comprise inserting one or more tool arms into at least one notch in the shaft. The tensioning tool may comprise at least one linear force applicator and coupling the tensioning tool to the shaft and the reaction block may comprise: retracting the linear force applicator; coupling the tensioning tool to the shaft; and extending the linear force applicator to contact the reaction block. This can allow the tensioning tool to be coupled to the apparatus when the size of the apparatus is not known exactly. Each linear force applicator may be a hydraulic cylinder. There is also described herein a de-tensioning tool for reducing the tension in a shaft, wherein tension is maintained in the shaft using one or more locking collars restrained between indentations in the shaft and a reaction block that is coupled to a structure, the detensioning tool comprising: a tool arm configured to couple to the shaft; a first linear force applicator configured to apply force against the reaction block; and a second linear force applicator configured to apply force against an angled surface of each locking collar; wherein the first linear force applicator is rigidly connected to the tool arm, such that the first linear force applicator is configured to control tension applied to the shaft; and wherein the second linear force applicator is configured to disengage the one or more locking collars from the shaft indentations. The first linear force applicator and / or the second linear force applicator may include a hydraulic cylinder. The second linear force applicator may be configured, upon application of a force to the angled surface of each locking collar, to drive the locking collar radially outward to a disengaged position and hold the locking collar in the disengaged position. There is also described herein a method of using any de-tensioning tool described above for reducing tension in a tensioned shaft, wherein tension is maintained in the shaft using one or more locking collars restrained between indentations in the shaft and a reaction block that is coupled to a structure, the method comprising: coupling a de-tensioning tool to the shaft and the reaction block; applying tension to the shaft; disengaging the locking collars from the shaft indentations and holding the locking collars in a disengaged position; and with the locking collars held in the disengaged position, reducing tension in the shaft. Tension may be initially maintained in the shaft using any apparatus or ratchet device described above. Applying tension to the shaft may comprise extending a first linear force applicator of the de-tensioning tool. Disengaging and / or holding the locking collars may comprise extending a second linear force applicator of the de-tensioning tool. Reducing tension in the shaft may comprise retracting the first linear force applicator. Any apparatus feature as described herein may also be provided as a method feature, and vice versa. As used herein, means plus function features may be expressed alternatively in terms of their corresponding structure. Any feature in one aspect of the invention may be applied to other aspects of the invention, in any appropriate combination. In particular, method aspects may be applied to system aspects, and vice versa. Furthermore, any, some and / or all features in one aspect can be applied to any, some and / or all features in any other aspect, in any appropriate combination. It should also be appreciated that particular combinations of the various features described and defined in any aspects of the invention can be implemented and / or supplied and / or used independently. BRIEF DESCRIPTION OF THE FIGURES Apparatus and methods for maintaining tension in the shaft of a structure are described by way of example only, in relation to the Figures, wherein: Figures 1a-1c show (a) a perspective view of an apparatus for maintaining tension in a shaft, (b) a cross-sectional diagram of the apparatus and (c) an exploded view of an interface of the apparatus; Figures 2a-2d show (a) a force body diagram of an apparatus for maintaining tension in a shaft, (b)-(c) side and plan view diagrams of the apparatus of Figure 2(a), and (d) a force body diagram of another apparatus for maintaining tension in a shaft; Figures 3a-3d show (a) a cross-sectional diagram of an apparatus for maintaining tension in a shaft and (b)-(d) operation of the apparatus with a tensioning tool; Figures 4a-4d illustrate a method of using apparatus for maintaining tension in a shaft; Figures 5a-5f show (a)-(c) &the cross-sectional diagrams of an apparatus for detensioning a shaft, and (d) &(f) close-up detail views of the apparatus; and Figure 6 shows a force body diagram of an apparatus for de-tensioning a shaft. DETAILED DESCRIPTION Referring to Figures 1a-1c, an example apparatus 100 for maintaining tension in a shaft will now be described. The apparatus 100 includes a shaft 10, a reaction block 20 and at least one locking collar 30. In the example of Figures 1a and 1b, there are two locking collars 30, but other examples may have one locking collar or more than two locking collars. The shaft 10 extends at least partially into (e.g. through) a structure 5, for example into a suitably sized hole, in order to be coupled (e.g. fixed) to the structure 5. In some examples, the structure 5 is a flanged shaft, such as an outer shaft of a subsea device (e.g. subsea anchor). Accordingly, the shaft 10 may form part of a subsea device, such as an anchor stem configured to actuate the subsea anchor in order to grip and / or cut into the seabed 5. For example, in order to provide a secure coupling between a subsea anchor and the seabed, a pre-tensioning force is applied to and maintained in the anchor stem (e.g. shaft 10). For example, the structure 5 may comprise means (not shown in the figures) for gripping a bulk material, e.g. the seabed, for preventing the structure from being pulled out of the bulk material. The pre-tensioning force is applied via the shaft (e.g. anchor stem) in order to provide strong joint forces (e.g. for an anchoring effect) between the gripping means of the structure and the bulk material. As explained in greater detail below, the apparatus 100 reliably maintains pre-tension in the shaft 10. The shaft 10 has a plurality of indentations 15 in its outer surface, such that when the shaft 10 is coupled to the structure, the indentations 15 are located near a front surface 7 of the structure 5 (from which an exposed portion of the shaft 10 extends proximally). For example, as shown in Figure 1b (a cross-section of Figure 1a taken along line B-B), in use the indentations 15 are located near the surface 7 such that some of the indentations 15 are hidden within the structure 5 and some of the indentations 15 are located on an exposed portion of the shaft 10. In some examples, the indentations comprise grooves which extend around at least a portion of the circumference or outer surface of the shaft 10 (in a plane perpendicular to the length of the shaft). In some examples, the indentations 15 are formed by a single groove or by a threading of the shaft 10. As shown in Figure 1b, the shaft 10 has a longitudinal (or major) axis 11 extending in a distal / proximal direction. In the example of Figures 1b-1c, although cross-sections of the locking collars 30 and the reaction block 20 are shown, it is to be understood that the reaction block 20 extends (partially or fully) around the shaft 10 as shown in Figure 1a, and thus the two sections of the reaction block shown are sections of the same reaction block so are connected. On the other hand, the two sections of locking collars 30 shown are not connected and are sections of different locking collars 30. The locking collars 30 are configured to engage at least one of the indentations 15. For example, each locking collar may have one or more protrusions which have a shape (e.g. a cross-sectional shape) corresponding substantially to a shape (e.g. a cross-sectional shape) of the indentations 15, e.g. the locking collars 30 may be threaded to cooperate with a threading of the shaft 10. The reaction block 20 is configured to contact the outer surface 7 of the structure 5, preferably via a substantially flush surficial contact (as shown in Figure 1b). For example, the reaction block 20 may be welded onto or machined into the structure 5. The reaction block 20 includes a shoulder surface 22 such that when the reaction block 20 is placed against the structure 5, the shoulder surface 22 is oriented at an (e.g. oblique) angle to the surface 7 and / or the longitudinal axis 11. The angle of the shoulder surface 22 relative to the surface 7 is between around 0° and around 85° (e.g. 45°), preferably less than around 60°, more preferably less than around 30°. The locking collars 30, reaction block 20 and / or the shaft 10 may be made of steel and / or engineering plastic. In any case, the materials selected should be chosen to be suitable for use for extended periods of time underwater (salty or fresh), and resistant to damage by wildlife such as molluscs, plants, algae, etc. and / or bio-accumulation / fouling. As shown in Figures 1a-1c, the reaction block 20 is configured to support or engage the locking collars 30 (via the shoulder surface 22). In use, the reaction block 20 is positioned around the base of the exposed portion of the shaft 10. The reaction block 20 may be formed as an integral block of material, e.g. having a generally annular shape. This can allow the reaction block 20 to slide distally along the shaft 20 during assembly of the apparatus 100, which can be advantageous where lateral access to the base of the shaft is limited. In some examples, the reaction block is formed of multiple pieces, e.g. two or more portions which reversibly connect to form a generally annular shape. This can allow the reaction block to be positioned during assembly of the apparatus 100 from a lateral direction at the base of the shaft, which can be advantageous where access to a proximal end of the shaft is limited, or if sliding the block along the shaft is impractical. In either case, the shoulder surface 22 may form a chamfer or bevel on the reaction block 20 such that during use, the reaction block 20 comprises a countersunk or conical opening (in which the shaft 10 and locking collars 30 reside). Figure 1c shows a close-up exploded view at the interface between a locking collar 30, shaft 10 and reaction block 20. Each locking collar 30 has one or more proximal engaging surfaces 34 (e.g. forming one or more inward protrusions) and a distal engaging surface 32. For clarity, the example of Figure 1c only shows one proximal engaging surface 34, however, each locking collar 30 may have a plurality of proximal engaging surfaces. For example, the plurality of proximal engaging surfaces may form a corrugated or serrated inner surface of the locking collar that is configured to engage (e.g. mesh with) a matching corrugation or serration formed by the shaft indentations 15. Having a plurality of proximal engaging surfaces can distribute load and reduce shear forces at the interface between protrusions 15 and the engaging surfaces, thus providing more reliable engagement between the shaft 10 and the locking collars 30. In addition, each shaft indentation 15 comprises a restraining surface 17 and a release surface 18. The restraining surfaces 17 are oriented at an (e.g. oblique) angle relative to the length of the shaft 10 such that in use the restraining surfaces 17 face a direction having a distal component. The angle of each restraining surface 17 relative to the length of the shaft 10 is between around 10° and around 90° (e.g. 45°), preferably greater than around 30°, more preferably at least around 60°. The release surfaces 18 are oriented at an (e.g. oblique) angle relative to the length of the shaft 10 such that in use the surfaces 18 face a direction having a proximal component. The angle of each release surface 18 relative to the length of the shaft 10 is between around 0° and around 80°, preferably between around 20° and around 60°, more preferably around 45°. In some examples, the angles of both the restraining surfaces 17 and the release surfaces 18 are in the range 45° to 60° (relative to the length of the shaft). Advantageously, these angles can increase the number of notches per unit length along the shaft which enables a shorter length of spring to be used to ensure that a target tension can be achieved, while also providing sufficiently angled indentations to provide a secure restraining effect on the locking collars. In some examples, the restraining and release surfaces 17, 18 are separate surfaces (having an edge therebetween) within the indentations 15. For example, the indentations 15 may form a “saw-tooth” pattern in cross-section (as shown in Figure 1c). In other examples, the restraining and release surfaces 17, 18 may be different portions of a single continuous (e.g. curved) surface of the indentations 15. For example, the indentations 15 may form a “wavy” or sinusoidal pattern in cross-section (as shown in Figures 3a-3d). The indentations 15 being formed of a single, curved surface can reduce localised stress at the interface between the shaft 10 and locking collars 30 and provide smoother engagement therebetween. The apparatus 100 has an equilibrium (or locked) state, as shown in Figures 1a and 1b. In the equilibrium state, the locking collars 30 engage at least one of the indentations 15 of the shaft 10 in order to maintain pre-tension in the shaft 10. Once a pre-tension is applied to the shaft 10 and external loading is released, the locking collars 30 are held between the shaft 10 and the reaction block 20, preventing the shaft 10 from contracting (by virtue of its elasticity) Specifically, the proximal engaging surface(s) 34 of the locking collars 30 engage restraining surface(s) 17 of the indentations 15, and the distal engaging surface 32 of each collar 30 engages the shoulder surface 22 of the reaction block 20. The reaction block 20 thus transmits a reaction force from the structure 5 to the shaft 10 via the locking collars 30. In some examples, the locking collars 30 are held between the shaft 10 and the reaction block 20 by virtue of their geometry (e.g. as explained in more detail below with reference to Figure 2a). For example, the locking collars 30 may be shaped such that they are self-biasing and wedge between the shaft 10 and the reaction block 20 (e.g. to be held in equilibrium or urged radially inwards). In this case, the reaction force from the reaction block 20 prevents the shaft 10 from contracting, since the geometry of the indentations 15 and locking collars 30 prevent the locking collars 30 from advancing proximally and / or outwardly relative to the shaft 10 (and thus prevent / inhibit contraction of the shaft 10). This therefore maintains the applied pre-tension in the shaft 10. Additionally or alternatively, the locking collars 30 may be held between the shaft 10 and the reaction block 20 in the equilibrium state using radial biasing means. As explained in further detail below with reference to Figure 2d, the apparatus can include biasing means which urge the locking collars inwards and / or inhibit motion of the locking collars 30 beyond a predetermined radius. This can allow the indentations 15 and locking collars 30 to have a wider range of geometries with any outwardly directed force being counteracted by the biasing means. In such examples, the radial biasing means acts against a tendency of the locking collars 30 to move radially outward under contraction forces of the shaft (e.g. due to the geometry of the indentations 15 and locking collars 30), and thus keeps the locking collars restrained between the shaft 10 and the reaction block 20 in the equilibrium state. As explained in further detail below in the example of Figures 3a-3d, the apparatus 100 also has a transition state in which the locking collars 30 can move relative to the shaft 10. In particular, when a tension force is applied to the shaft 10 (e.g. using a tensioning tool), the locking collars 30 are driven radially outwards (by one or more release surfaces of the indentations 15) as the shaft 10 extends due to the tension force. In examples having radial biasing means, the applied tension is sufficiently large such that the locking collars 30 act against (and overcome) a resilient inward biasing force imparted by the biasing means. Upon further application of tension to the shaft 10, the locking collars 30 then advance distally to engage a previously unengaged indentation. In some examples, when tension is applied to the shaft 10, in the transition state the locking collars are driven outwardly (and proximally) before advancing distally to engage at least one new indentation. In some examples, in the transition state, movement of the locking collars is limited to movement (e.g. sliding) along the shoulder surface 22 of the reaction block 20. In the transition state, the locking collars 30 can thus advance in a manner that provides a ratcheting effect, since relaxation or reduction of the applied tension would cause the collars 30 to be urged inwards (by virtue of their geometry or radial biasing means) and engage the indentations to enter the equilibrium state and prevent contraction of the shaft 10. Accordingly, once a desired tension is applied to the shaft 10 (e.g. using a tensioning tool), the external tension source can be removed and the locking collars and reaction block serve to maintain the applied tension in the shaft. Since the indentations, locking collars and reaction block maintain tension in the shaft by forming a ratchet, contraction of the shaft is not possible when the ratchet is installed, meaning the maintained tension does not decrease or loosen over time (e.g. under varying loads on the shaft). Typically, distal advancing of the locking collars 30 (to engage a new indentation 15) is driven by gravity. In this case, the shaft 10 may be oriented such that gravity acts in a primarily distal direction (e.g. the shaft is at no more than around 45° from a vertical direction), meaning gravity causes the locking collars 30 to slide distally (downwards) along the shoulder surface 22 once sufficient tension has been applied to the shaft 10. In some examples, the apparatus 100 also includes (longitudinal) biasing means for urging the locking collars 30 distally. Biasing means can include a spring or other resilient biasing member configured to apply a biasing force to urge the locking collars 30 in the distal direction. The biasing means may be (reversibly) fixed to the reaction block 20. This can ensure that, in the transition state, the locking collars 30 advance distally with a predetermined speed or force, or that they advance distally despite in spite of an orientation of the shaft 10, e.g. if the apparatus is arranged such that gravity act in a primarily proximal direction. In examples having radial biasing means (to urge the locking collars 30 radially inwards), the radial biasing means and longitudinal biasing means may be provided using the same component(s) or mechanism. For example, a single resilient biasing member positioned transversely to the longitudinal axis (e.g. parallel to the shoulder surface) may be configured to provide a sufficiently large distal and radially inward force to the locking collar so as to fulfil the function of both the radial and longitudinal biasing means. In some examples, the apparatus 100 includes a housing around the locking collars 30, which is preferably fixed to the reaction block 20. For example, the housing may be substantially dome-shaped or box-shaped and have a proximal opening through which the shaft 10 can pass. The housing can shield the locking collars 30, reaction block 20 and / or indentations 15 from unwanted debris which may otherwise clog or damage moving parts of the apparatus 100. Equally, in case one or more locking collars fails (e.g. breaks and / or is ejected outwardly), the housing can contain the failed collar(s) which for example can prevent them entering / damaging the nearby environment. Additionally or alternatively, the housing can restrain the motion of the locking collars, for example by including one or more grooves, rails or other structures that constrain the motion of each locking collar to a linear path along the reaction block. Figure 2a shows an example force body diagram for an example apparatus 100’ for tensioning a shaft. In some examples, the apparatus 100’ of Figure 2a is a simplified representation corresponding to the apparatus 100 of Figures 1a-1c. In particular, Figure 2a shows a shaft 10’ (having a longitudinal axis 1T) held in tension by n locking collars 30’ (only one of which is shown) against a reaction block 20’. The shaft 10’ has been pre-tensioned to a tension nT, and reaction forces Fi, F2 are exerted on the locking collar 30’ by the restraining surface of a shaft indentation and by the shoulder surface of the reaction block 20’ respectively. As a result of these reaction forces F1, F2, the locking collar 30’ experiences a net force R along the shoulder surface of the reaction block 20’. The restraining surfaces of the shaft indentations are angled relative to a length of the shaft 10’ at an angle 6. Similarly, the shoulder surface of the reaction block 20’ is angled relative to a front surface of the structure 5’ to which the shaft 10’ is being tensioned at an angle (p. It can be shown that the net (outward) force R exerted on the locking collar is given by the following equation (assuming the weight of the locking collar is insignificant): cos(<p) tan(0) sin(<p) (1) Accordingly, (assuming the shaft is positioned perpendicularly to the front surface of the structure in use) when 0 + <p is approximately equal to 90° the resulting outward force is approximately zero, meaning the locking collar is held in equilibrium. It has been discovered that the closer 0 is to 90° (and / or the closer <p is to 0°), the more stable this equilibrium is, thus providing a more stable engagement of the locking collar between the shaft and the reaction block. In other words, the closer 0 is to 90°, a given perturbation in the angle <p results in a smaller magnitude of resulting force R. In some examples (e.g. as shown in Figure 2b), the shaft 10’ and / or the reaction block 20’ (and / or the locking collar 30’) can be configured such that 0 + (p is greater than 90°. In other words, proximal and distal engaging surfaces of the locking collar 30’ (and / or the restraining surface and shoulder surface) may lie in planes which are substantially parallel or which are angled so as to trap the collar 30’ and impart an inward resultant force R. Therefore, tension in the shaft causes the locking collar(s) to be forced inwardly, which provides a firm grip on the shaft enabling a robust locking effect. Thus, increased tension in the shaft acts to improve the safety and security of the locking effect. If the proximal / distal engaging surfaces (and / or the restraining surface and shoulder surface) are curved in a plane perpendicular to the longitudinal axis of the shaft 10’, the (parallel) planes in which they lie may be tangent to conical surfaces. Even in cases where R is directed outwardly, it is clear that frictional forces between the locking collar 30’ and the reaction block 20’ may nevertheless be sufficient to maintain the locking collar 30’ in place and implement the ratcheting effect. That is, the sum 0 + <p being less than 90° may still lead to stable arrangements, so long as the sum is close to 90° (for example is at least 85°). The shaft 10’ experiences a reaction force which is equal and opposite to the force Fi and is applied via the proximal engaging surface(s) of the locking collar 30’. In examples where more than one locking collar is used (and in particular where the collars are distributed evenly around the shaft circumference), the lateral components of such reaction forces on the shaft 10’ cancel out, resulting in a net proximal reaction force being exerted on the shaft 10’. This can make the apparatus 100’ more stable when in an equilibrium state. Where the shaft indentations are formed by a screw thread of the shaft, friction forces between the locking collars and the shaft and reaction block are sufficient to prevent the collars from being driven around the shaft in the equilibrium state (thus preventing the shaft from contracting and “unscrewing” itself). For example, the local gradient (pitch) of the screw thread may be relatively small such that the shaft imparts a relatively small tangential force (perpendicular to the longitudinal axis and radial direction) on the locking collars. In other cases (as discussed in more detail below), the locking collar 30’ may be constrained to move only in a linear (e.g. radial) direction with no tangential component, and / or to maintain surface contact with the reaction block through the use of grooves, rails, seats etc. Although in the example of Figure 2a the proximal and distal engaging surfaces of the collar 30’ (and the restraining surface and shoulder surface) are shown as approximately parallel, as shown in Figure 2b, the engaging surfaces 32’, 34’ may be angled relative to each other such that the locking collar 30’ is forced inwardly when engaging the shaft 10’ and the reaction block 20’. In other words, the proximal engaging surface 34’ and the distal engaging surface 32’ may lie in planes (or along curved surfaces e.g. conical surfaces) which converge at a location radially outward from the locking collar 30’. The location of convergence of these planes (or curved / conical surfaces) is radially outward from the locking collar 30’ in the sense that the line (or curve) of convergence is separated from the longitudinal axis 11’ of the shaft 10’ by the locking collar 30’. For example, as shown in Figure 2b, projections of the proximal engaging surface 34’ and the distal engaging surface 32’ extend along surfaces 42’, 44’ which converge at a location 46’. If the engaging surfaces 32’, 34’ are planar surfaces, the surfaces 42’, 44’ will be planes which are coincident along an axis 46’. If the engaging surfaces 32’, 34’ are curved (e.g. conical) surfaces, as shown in Figure 2c, then the surfaces 42’, 44’ will have corresponding curved (e.g. conical) shapes and will be coincident along a curve 46’. In either case, the axis or curve 46’ is located radially outward from the locking collar 30’. If the surfaces 32’, 34’ were oriented such that an outward force is applied to the locking collar, the projected planes / surfaces would converge either between the collar and the longitudinal axis or on an opposite side of the longitudinal axis from the collar in question. Figure 2d shows a force body diagram of another apparatus 100” for tensioning a shaft. In some examples, the apparatus 100” is a simplified representation corresponding to the apparatus 100 of Figures 1a-1c. In particular, Figure 2d shows a shaft 10” (having a longitudinal axis 11”) held in tension by n locking collars 30” (two of which are shown) against a reaction block 20”. The shaft 10” has been pre-tensioned to a tension nT, and reaction forces Fi, F2 are exerted on each locking collar 30” by the restraining surfaces of shaft indentations and by the shoulder surface of the reaction block 20” respectively. These reaction forces Fi, F2, produce a net force R on each locking collar 30” along the shoulder surface of the reaction block 20”. The apparatus 100” also includes radial biasing means. In the example shown in Figure 2d, the biasing means is a biasing member 50” connecting the two locking collars 30” using at least one spring (two tension springs and a linkage are shown). Although not expressly shown, there may be a second biasing member 50” located behind the shaft 11”, for symmetry and / or stability. The spring(s) of the biasing member 50” are configured to apply a radially inward force on the locking collars 30” to constrain the collars (e.g. to a predetermined radius) between the shaft and the reaction block 20” in the equilibrium state. When the shaft 10” is tensioned (in the transition state), the biasing member 50” will still apply an inward biasing force, but provided the applied tension is greater than existing tension within the shaft, the locking collars 30” will be driven further outwardly, allowing them to advance into another indentation and ratchet along the shaft. For example, the stiffness of the springs and / or the angles of the restraining and release surfaces may be tuned to provide an inward force sufficient to hold the collars in the equilibrium state yet to allow advancement of the collars in the transition state. In this example, the surfaces of the locking collars 30”, and the restraining surfaces and shoulder surface are free to be angled at any angle. Indeed, as shown, they are angled such that the force R (resulting from reaction forces) is directly outwardly. Thus, the inward force from the radial biasing means counteracts counteract the net outward force R resulting from reaction forces to keep the collars between the shaft and the reaction block. Accordingly, where a locking collar is connected to other locking collars around the shaft (via the biasing means), the collars can collectively self-brace, meaning any outward force R on one locking collar can be used to counteract a similar outward force R on other locking collars. In other examples (not shown), the radial biasing means may not connect multiple locking collars, but may act on the locking collars individually. For example, the apparatus may include one or more springs (e.g. compression springs) or other elastic / compressible materials and / or magnets for resiliently constraining each locking collar (within a predetermined radius). In one such example, the reaction block 20” may include a rail, groove, seat or other guide on the shoulder surface for each locking collar to move along in the transition state (e.g. along a radial direction), and optionally further constrained to move along the interface surface between the locking collar 30” and the reaction block 20”. The guide includes a resilient member at one end, for example a compression spring / compressible material or a stop at an outer end of the guide and / or a tension spring / tensionable material at an inner end of the guide, such that when the locking collar reaches a predetermined radius from the longitudinal axis 11”, the spring / material exerts a radially inward force on the locking collar to inhibit further outward movement of the locking collar. This development is beneficial because, contrary to the arrangement shown in Figure 2d which uses two collars 30” to brace against one another causing a mutual inwardly directed biasing force, the use of rails, grooves, seats etc. allows for a different number of collars 30” - anywhere from a single collar 30” upwards, and where multiple collars 30” are present, they can be spaced at any angle from each other, since each collar 30” is self-biased, and need not couple around the shaft 11 ” to brace against another collar 30”. Other examples of a similar arrangement may be provided to broaden the applicability yet further. For example, the collar(s) 30” may be coupled directly to the shaft 10” via a spring 50”. This would also allow a single collar 30” or any other number to be employed. Optionally, the coupling between the spring 50” and the shaft 10” may be able to travel along the length of the shaft so that the biasing force is directed perpendicular to the longitudinal axis 11”, thereby simplifying the analysis of the required forces to achieve the intended ratcheting effect. Radial biasing means may be used in addition to the arrangement of Figures 2a-2c in which the geometry produces a zero or inward net force on the locking collars. This can allow greater control of the motion of the locking collars (e.g. by selecting properties of biasing members). Note that in this example the biasing means can be selected to apply a sufficient force to the locking collars 30 to overcome any outward force. This in turn means that the apparatus for applying tension to the shaft need be configured to apply sufficient tension to overcome this force and enable the ratcheting, but importantly, the operation of the ratchet effect has been decoupled from the geometry of the collar, so this trade-off is usually worth considering. Referring to Figures 3a-3d, the operation of an apparatus 200 for tensioning a shaft will now be described. In some examples, the apparatus 200 corresponds to the apparatus 100 described above. Similarly to the apparatus 100, the apparatus 200 is for tensioning a shaft against a structure 205, and includes a shaft 210 with indentations 215, a reaction block 220 and one or more locking collars 230. Optionally, the apparatus 200 also includes a compression spring 225 configured to couple the reaction block 220 to the structure 205. In use, the compression spring 225 is fixed at one end to a surface 207 of the structure 205, and fixed an opposite end to the reaction block 220. The compression spring 225 serves to resist distal motion of the reaction block 220. In the equilibrium state, the apparatus functions as described above in relation to the apparatus 100, except the reaction force from the structure 205 is transmitted to the reaction block 220 via the compression spring 225, meaning the reaction block 220 can move relative to the structure 205. The shaft 210 includes an annular or circumferential notch 217 located proximally from the indentations and the reaction block 220. The notch 217 (or groove) is configured to receive (or interface with) one or more tensioning tools such as a linear loading tool. In the examples of Figures 3b-3d, a tensioning tool 300 including a hydraulic cylinder is shown. The notch 217 extends uniformly around a circumference of the shaft 210, and is preferably circular, such that the notch 217 has a constant depth. This allows a tensioning tool to be coupled to the shaft 210 independently of the lateral orientation of the tool, which can improve operation of the apparatus when visibility and / or access are restricted. However, in other examples, the notch may only extend around a portion of the shaft 10. In the example of Figure 3a (and of Figure 1a), the notch 217 is located near a proximal end of the shaft 210. In other examples, the notch 217 may not be near an end of the shaft 210, and the shaft 210 may extend proximally far beyond the notch 217. In other examples, the proximal end of the shaft 210 includes a series of notches for engaging a tensioning tool - for example, these notches can be arranged along a portion of the length of the shaft and the tool arm(s) can engage these notches sequentially (e.g. forming another ratcheting interface between the tensioning tool and the shaft) in order to apply tension to the shaft. Figure 3b shows the apparatus 200 in an equilibrium (or locked) state with a tensioning tool 300 attached. The tool 300 includes at least one tool arm 310 and at least one hydraulic cylinder 320. The tool 300 may also include means (not shown) for controlling the hydraulic cylinder(s) 320. With the apparatus 200 in the equilibrium state (e.g. as shown in Figure 3a), the tensioning tool 300 is attached by inserting the tool arm(s) 310 into the notch 217 in the shaft 210, and positioning the end of each hydraulic cylinder 320 against (a proximal surface of) the reaction block 220. In some examples, in case the distance between the notch 217 and the reaction block 220 is not precisely known, the hydraulic cylinder(s) 320 can be fully retracted, the tool arms 310 attached to the notch 217, and the hydraulic cylinder(s) extended until they meet the reaction block 220. As shown in Figure 3c, since the tensioning tool 300 couples to the shaft (at the notch 217) and the reaction block 220, which itself is coupled to the structure 205 (optionally via a compression spring 225), further extension of the hydraulic cylinder(s) 320 causes tension to be applied to the shaft 210. As tension is applied, the apparatus 200 enters the transition state, in which the locking collars 230 are driven outward (radially outward from the shaft 210) by surfaces (e.g. release surfaces) of the shaft indentations 215. In particular, the locking collars 230 are driven along the shoulder surface of the reaction block 220 - the orientation of the shoulder surface is such that outward movement of the locking collars 230 also results in proximal movement of the collars 230 relative to the reaction block 220. Depending on the shape of the indentations 215, the locking collars 230 may simultaneously slide outwardly along one or more release surfaces (e.g. release surfaces 18) of the indentations and outwardly along the shoulder surface (e.g. shoulder surface 22) of the reaction block 220. When there is no compression spring positioned between the reaction block 220 and the structure 205, when the locking collars 230 move outwardly, they also move proximally in an absolute sense (relative to the structure, since the reaction block 220 does not move relative to the structure). Accordingly, the amount of proximal movement by the locking collars 230 is dependent on the extension of the shaft 210 due to the applied tension force and also on any movement available to the shaft internal to the structure, e.g. motion which takes up any “slack” by forcing internal components into as tight contact with each other as possible. When a compression spring 225 is positioned between the reaction block 220 and the structure 205, when the hydraulic cylinders 320 extend to apply tension to the shaft 210, the force they exert on the reaction block 220 causes the reaction block 220 to move distally. Therefore, the locking collars 230 still move proximally relative to the reaction block and outwardly, but their absolute motion (in a distal / proximal direction) will depend on the relative movement of the shaft 210 and the reaction block 220 in response to the applied tension. The distance by which the reaction block 220 moves (for a given force) depends on the stiffness of the compression spring 225. The stiffness of the compression spring 225 is lower (e.g. significantly lower) than an effective axial stiffness of the shaft 210. Accordingly, upon application of a force by the tensioning tool 300, the reaction block 220 will be displaced distally by a distance greater than the distance by which the shaft 210 extends. Thus, using the compression spring 225 increases the relative displacement between the shaft 210 and the reaction block 220. As explained below with reference to Figure 3d, this can allow the locking collars 230 to advance by a greater number of indentations 215 (ratcheting distally along the shaft) for a given applied tension, enabling a greater degree of control and precision over the tension applied to the shaft. The (compressive) stiffness of the compression spring 225 is significantly less than the effective (axial tensile) stiffness of the shaft 210. In some examples, the shaft 210 is made of a steel or engineering plastic. As shown in Figure 3d, further extension of the hydraulic cylinder(s) 320 creates further displacement between the shaft 210 and the reaction block 220 such that the locking collars 230 move distally along the shaft 210 to engage a previously unengaged indentation 215. In the example of Figure 3d, the further displacement between the shaft and reaction block causes the locking collars 230 to be driven outward until they lose contact with the indentations 215 (e.g. they lose contact with one or more release surfaces). This allows the locking collars 230 to move inwards (e.g. by sliding back along the shoulder surface) and engage a new indentation 215 (which was not previously engaged prior to applying the tension force). In this example, the apparatus is arranged substantially vertically such that inward movement of the collars 230 back towards the shaft 210 is gravity-driven. In other examples (e.g. when the apparatus is not oriented substantially vertically), the apparatus may include biasing means such as a spring to apply a biasing force to urge the locking collars 230 in a distal and / or inward direction. This biasing force thus causes the collars 230 to move inwards once they lose contact with the indentations 215. Figures 3b-3d illustrate a sequence of applying tension to the apparatus 200 in order to advance (ratchet) the locking collars 230 by one indentation 215. In particular, the sets of indentations engaged by the collars 230 prior to and after application of the tension force are shown respectively as sets 215a, 215b in Figure 3b-3d. After advancing the locking collars 230 by one indentation 215 in this manner, the tension force may be relaxed and the tensioning tool 300 removed such that the apparatus maintains the applied tension in the shaft 210 (in the equilibrium state) by the locking collars being wedged between the shaft and the reaction block to prevent contraction of the shaft as described above. Alternatively, the tension force applied by the tensioning tool 300 may be increased further to repeat the sequence of Figures 3b-3d and successively ratchet the collars 230 along the shaft 210 by a further one or more indentations 215. In some examples, the apparatus 200 may be calibrated such that ratcheting (advancing) the locking collars 230 by one indentation 215 corresponds to an increase in applied tension of a predetermined amount. Thus, a desired amount of tension to be maintained in the shaft 210 can be easily achieved. For example, if it is desired to load and maintain the shaft with a tension X, the apparatus may be calibrated such that each ratcheting progression of the locking collars 230 along one indentation corresponds to a tension increase of 0.1X. Thus, the tensioning tool 300 can be extended until 10 ratcheting progressions / sequences have occurred (corresponding to a total applied tension of X), at which point the tool is released and the desired tension is maintained by the apparatus 200. The tensioning tool 300 can be instrumented (e.g. using one or more motion sensors) to detect a travelled distance of the hydraulic cylinder(s) 320, which can be equated to a number of shaft indentations traversed by the locking collars based on a known / predetermined indentation spacing. Referring to Figures 4a-4d, a method of using apparatus 400 for maintaining tension in a shaft of a structure will now be described. The apparatus 400 is for retrofitting to an existing shaft 410, and includes a reaction block 420 and one or more locking collars 430. In some examples, the apparatus 400 and the shaft 410 operate in the same way as apparatus 100 or apparatus 200 as described above. The shaft 410 is an existing shaft of a structure 405, such as an anchor stem of a subsea anchor. As shown in Figure 4a, the shaft 410 includes indentations, which may be formed by a threading of the shaft 410. Alternatively, indentations may be formed by a plurality of grooves in an outer surface of the shaft. In some examples, the grooves do not extend all the way around the shaft 410, and as such may only be formed around a portion of the shaft 410 or on one (e.g. flat) side / surface of the shaft 410. As shown in Figure 4b, the method for using the apparatus 400 to apply and maintain tension in the shaft begins by positioning the reaction block 420 around the shaft 410. For example, the reaction block 420 may be positioned at or near the based of an exposed portion of the shaft. In some examples, the reaction block 420 is shaped such that is does not completely surround the shaft 410, e.g. it may have an incomplete annulus shape (e.g. a “horse-shoe” shape). In this case, positioning the reaction block 420 around the shaft 410 can include sliding the reaction block 420 onto the shaft 410 from a lateral direction. For example, the shaft 410 (or a portion thereof) may have a width that varies with orientation (e.g. it may be “D-shaped”), allowing the reaction block 420 to be positioned around the shaft 410 from a particular direction, but subsequent rotation of the block 420 (once positioned) prevents it from moving laterally away from the shaft 410. Alternatively or alternatively, the reaction block 420 can be positioned around the shaft 410 by sliding the reaction block 420 distally along the shaft into position (e.g. if the reaction block 420 forms a complete annulus around the shaft 410). Optionally, the apparatus 400 also includes a compression spring 425. The compression spring may correspond to the compression spring 225 described above. The compression spring 425 may be positioned around the shaft 410 in a similar manner to the reaction block 420. In some examples, the compression spring 425 is fixed to the reaction block 420 such that the block 420 and spring 425 are positioned together. Once positioned, the reaction block 420 (or the compression spring 425 if present) contacts the structure 405. In some examples, the reaction block 420 (and / or spring 425) selfbrace against the shaft 410 so there is no need to couple the reaction block 420 (and / or spring 425) to the structure 405. However, in some examples, for added security the reaction block 420 (or spring 425) is coupled (e.g. secured) to the structure 405 to prevent lateral movement of the reaction block 420. For example, a surface of the structure 405 may include one or more reliefs (e.g. grooves and / or protrusions) which engage corresponding reliefs (e.g. protrusions and / or grooves) in the reaction block 420 (or spring 425) once the reaction block 420 is positioned. The method also includes inserting the locking collars 430 between the shaft 410 and the reaction block 420 to engage the indentations in the shaft. In the example of Figures 4b-4c, the locking collars 430 are inserted after the reaction block 420 has been positioned. However, in other examples, the locking collars 430 may be placed within the reaction block 420 prior to positioning the reaction block 420, such that the locking collars 430 engage the indentations at the same time as positioning the reaction block 420. Once the locking collars 430 have been inserted, the method includes coupling a tensioning tool 300 to the shaft 410 and the reaction block 420 (as shown in Figure 4d). Tension is then applied to the shaft 410 by activating the tensioning tool 300 as described above. Once a desired amount of tension has been applied to the shaft 410, the apparatus maintains the applied tension (in an equilibrium state), and the applied tension can be released, e.g. by deactivating and / or decoupling the tensioning tool 300 from the shaft 410 and the reaction block 420. Accordingly, the apparatus 400 (reaction block 420 and locking collars 430) remains in place to maintain tension in the shaft 410. Although the shafts in Figures 3a-4d are shown as terminating just below the surface of the structure, this is simply for illustration purposes to indicate that the shaft is inserted into the structure, and these diagrams are not to scale. In reality, the portion of the shaft embedded in the structure would be significantly longer than the exposed portion, and would typically include a mechanism for anchoring the shaft to the structure to provide a reaction load (e.g. a set of outwardly-flaring cutting surfaces). Referring to Figures 5a-5f, apparatus 500 for de-tensioning a shaft will now be described. The term “de-tensioning” refers to releasing, relaxing, reversing or otherwise reducing tension in a controlled manner. The apparatus 500 includes a de-tensioning tool 550 for reducing the tension in a tensioned shaft 510, where the shaft’s tension is maintained using one or more locking collars 530 that are restrained between indentations 515 in the shaft 510 and a reaction block 520 that is coupled to a structure 505. In some examples, the shaft 510, reaction block 520 and locking collars 530 correspond to those of the apparatus 100, 200 and / or 400, and / or the shaft 510 has been tensioned using the methods described above. In some examples, the tool 550 is configured to both tension and de-tension a shaft, for example the tool 550 may be configured to operate as the tensioning tool 300 described above or vice versa. Figure 5a shows the apparatus 500 in an equilibrium (or locked) state with an initial tension maintained by the locking collars 530 and reaction block 530, as well as a de-tensioning tool 550. The de-tensioning tool 550 includes at least one tool arm 555 and at least two hydraulic cylinders 560, 570. In the examples shown, the tool 550 includes a first pair of hydraulic cylinders 560 for engaging the reaction block 520, and a second pair of hydraulic cylinders 570 for engaging the locking collars 530. As shown in Figures 5a-5f, the tool arm(s) 555 may be implemented using hydraulic cylinder(s) which makes the tool arm(s) 555 extendable / retractable. The tool 550 may also include means (not shown) for controlling the hydraulic cylinder(s) 560, 570. With the apparatus 500 in the equilibrium state (e.g. as shown in Figure 5a), the de-tensioning tool 550 is attached by inserting the tool arm(s) 555 into a notch in the shaft 510, and positioning the end(s) of the first hydraulic cylinder(s) 560 against (a proximal surface of) the reaction block 520. In some examples, in case the distance between the notch and the reaction block 520 is not precisely known, the ends of the first hydraulic cylinder(s) 560 can first be placed against the reaction block 520 then extended to a length such that the tool arms 555 can then extend into (or otherwise attach to) the notch in the shaft 510. Alternatively, (as described above in relation to Figure 3b) the tool arm(s) 555 can be attached to the notch with the first hydraulic cylinders 560 retracted, then the first hydraulic cylinders 560 can be extended until they meet the reaction block 520. With the de-tensioning tool 550 configured as shown in Figure 5b coupled to the shaft (at the notch) and the reaction block 520 (which itself is coupled to the structure 505, optionally via a compression spring), further extension of the first hydraulic cylinders 560 causes tension to be applied to the shaft 510. As shown in Figure 5c, as tension is applied the apparatus 500 enters a transition state (e.g. as described above in relation to Figure 3a-3d) in which the locking collars 530 are driven radially outward from the shaft 510 by surfaces of the shaft indentations 515 and along a shoulder surface of the reaction block 520. This further extension of the first hydraulic cylinders 560 (in the direction indicated by the arrows in Figure 5c) effectively unloads the locking collars 530 such that they are able to move more freely and are no longer compressed by the shaft. Instead, the first hydraulic cylinders 560 of the detensioning tool 550 support the shaft 510 against the reaction block 520 in order to maintain tension and prevent it from freely contracting. Accordingly, the first hydraulic cylinders 560 are configured to control (e.g. increase, maintain or reduce) the tension applied to the shaft. As shown in the close-up view of Figure 5d, once the de-tensioning tool 550 has taken up the “slack” and unloaded the locking collars 530 (in the transition state), the locking collars 530 can be disengaged from the shaft indentations 515 and positioned further radially outward than when in the equilibrium state. Alternatively, a smaller or larger amount of tension may be applied by the de-tensioning tool 550 such that the locking collars 530 are in substantially the same position as in the equilibrium state or have advanced distally along one or more indentations 515, but still such that the locking collars 530 are no longer compressed or restrained between the shaft indentation(s) 515 and the reaction block 520. Once this (small) additional tension has been applied to the shaft 510 by the de-tensioning tool 550 and the locking collars 530 have been released, the second pair of hydraulic cylinders 570 are extended from an at least partially retracted state to contact the locking collars 530 in the direction indicated by the arrows in Figures 5e and 5f. the second hydraulic cylinders 570 are configured to disengage the locking collar(s) 530 from the shaft indentations 515. In particular, the end of each second hydraulic cylinder 570 is shaped to engage an upper angled surface 532 of the locking collar 530 such that, upon contact with the surface 532, the second hydraulic cylinder 570 applies a force to the locking collar 530 having a radially outward component (e.g. as explained in more detail below with reference to Figure 6). Thus, upon further extension of the second hydraulic cylinders 570 (in the direction indicated by the arrows), the released locking collars 530 are driven radially outwardly along the shoulder surface of the reaction block 520 and are prevented from returning radially inwards. In other examples, a surface for receiving the second hydraulic cylinders 570 may be provided on the locking collars in a different manner - for example, such a surface may be located on a different portion of the locking collars and / or form part of an attachment for attaching to the locking collars. Additionally or alternatively, the ends of the second hydraulic cylinders 570 and / or the surfaces on the locking collars (e.g. upper angled surface 532) may include one or more indentations, protrusions or other components for preventing slipping between the end of the hydraulic cylinders and the locking collars. Accordingly, in the configuration of Figures 5e and 5f, the locking collars 530 are held away from the shaft 510, so the shaft 510 can be retracted in a controlled manner. In particular, the first pair of hydraulic cylinders 560 can be retracted, which reduces the overall length of the shaft 510 and thus the tension within the shaft 510 (according to its elastic material properties). In one example, the first hydraulic cylinders 560 are retracted by a predetermined amount (e.g. corresponding to a predetermined number of shaft indentations and / or a predetermined tension reduction), such that the shaft 510 is under reduced tension. Then the second hydraulic cylinders 570 are retracted so as to disengage the locking collars 530 which return radially inwards and engage one or more shaft indentations 515. The first hydraulic cylinders 560 can then be retracted such that the locking collars 530 are restrained between the indentations and the reaction block, thus maintaining the reduced tension within the shaft 510. The de-tensioning tool 550 can then be removed. In another example, once the locking collars 530 are held radially outward from the shaft 510 by the second hydraulic cylinders 570, the first hydraulic cylinders 560 can be retracted and / or the tool arm(s) 555 can be removed from the notch in the shaft 510 such that the shaft 510 is allowed to contract fully (in a controlled manner). This can allow a tensioned shaft to be completely de-tensioned, e.g. enabling the shaft to be decommissioned for removal from the structure 505. Figure 6 shows an example force body diagram for an example apparatus 500’ for detensioning a shaft. In some examples, the apparatus 500’ of Figure 6 is a simplified representation corresponding to the apparatus 500 of Figures 5a-5f. In particular, Figure 6 shows a shaft 510’ (having a longitudinal axis 51T) held in a transition state by an applied tension T (e.g. from a de-tensioning tool). Locking collars 530’ rest against a reaction block 520’ and are held in a released state by a de-tensioning tool. In the example of Figure 6, the de-tensioning tool holds the locking collars 530’ away from the shaft 510’ in the released state using hydraulic cylinders 570’ (e.g. corresponding to second hydraulic cylinders 570 described above) which apply a force P in a distal direction parallel to the longitudinal axis 511’. In this example, each locking collar 530’ experiences reaction forces Fi, F2 exerted by the ends of hydraulic cylinders 570’ and by the shoulder surface of the reaction block 520’ respectively. In other examples, the reaction force Fi may be applied to the locking collars 530’ using alternative means. For example, the force may be applied by extending or retracting one or more hydraulic cylinders attached or coupled to other surfaces of the locking collars, or the locking collars may comprise ferrous material (such as stainless steel) and the force Fi is a magnetic force (e.g. applied by a controllable electromagnet disposed in the de-tensioning tool). In any case, the force Fi has a radially outward component to prevent the locking collars from returning towards the shaft 510’. As a result of reaction forces Fi, F2, each locking collar 530’ experiences a net force R along the shoulder surface of the reaction block 20’. By virtue of the angled upper surface 532’ of each locking collar 530’ (where the hydraulic cylinder 570’ makes contact) facing radially inwards (towards the shaft), the locking collars 530’ are held between the hydraulic cylinders and the reaction block. Thus, each collar 530’ experiences a net outward force (or at least zero outward force) without contacting the shaft 510’. Preferably, the upper surface 532’ of each locking collar is angled relative to the longitudinal axis 511’ at an angle a, that is less than an angle / ? of the shoulder surface of the reaction block 520’ relative to the longitudinal axis 51T. In this configuration, the locking collars 530’ form a wedge shape between the reaction block 520’ and the hydraulic cylinders 570’, so even without application of a force P at the hydraulic cylinders, the locking collars are prevented from moving radially inwards. Alternatively, if the angle a is equal to or greater than the angle p, the locking collars may nonetheless be restrained and prevented from moving inwardly by application of a sufficiently large hydraulic force P. As shown in Figure 6, with the locking collars displaced radially outwards and held in place by a de-tensioning tool, the shaft is no longer obstructed (in an axial / longitudinal direction) by the locking collars, meaning tension T can be reduced or released entirely. While specific apparatus and devices are shown, any appropriate hardware may be employed. For example, the shaft indentations and / or locking collars may have different cross-sectional shapes than those illustrated. The above embodiments and examples are to be understood as illustrative examples. 5 Further embodiments, aspects or examples are envisaged. It is to be understood that any feature described in relation to any one embodiment, aspect or example may be used alone, or in combination with other features described, and may also be used in combination with one or more features of any other of the embodiments, aspects or examples, or any combination of any other of the embodiments, aspects or examples. Furthermore, equivalents 10 and modifications not described above may also be employed without departing from the scope of the invention, which is defined in the accompanying claims.

Claims

1. Apparatus for maintaining tension in a shaft of a structure, the apparatus comprising:a shaft comprising a plurality of shaft indentations, wherein each shaft indentation comprises a restraining surface oriented transverse to a longitudinal axis of the shaft;a reaction block comprising a shoulder surface, the reaction block configured to be positioned around the shaft and constrained laterally relative to the shaft; andat least one locking collar comprising:at least one proximal engaging surface configured to engage the restraining surfaces of the shaft, anda distal engaging surface configured to engage the shoulder surface of the reaction block;wherein the locking collar is biased radially inwards towards the shaft to form a ratchet by virtue of the engagement between the locking collar and the shaft indentations.

2. The apparatus of any preceding claim, comprising two or more locking collars.

3. The apparatus of any preceding claim, wherein each locking collar comprises a plurality of proximal engaging surfaces, optionally forming a plurality of protrusions, configured to engage the restraining surfaces of the shaft.

4. The apparatus of claim 3, wherein the proximal engaging surfaces form a substantially corrugated or serrated surface of each locking collar.

5. The apparatus of any preceding claim, wherein a cross-sectional shape of each proximal engaging surface matches a corresponding cross-sectional shape of each shaft indentation.

6. The apparatus of any preceding claim, wherein the shaft indentations and / or the proximal engaging surface comprise a curved surface, preferably wherein the shaft indentations are formed by a single continuous curved surface.

7. The apparatus of any preceding claim, wherein the shaft indentations are formed by one or more grooves in the shaft.

8. The apparatus of claim 7, wherein the one or more grooves comprise:a helical groove, preferably a threading of the shaft;a plurality of grooves perpendicular to a longitudinal axis of the shaft, preferably a plurality of circumferential grooves.

9. The apparatus of any preceding claim, wherein the reaction block is configured to be coupled to the structure; or wherein, upon engaging the at least one locking collar, the reaction block is configured to self-brace to prevent lateral movement.

10. The apparatus of any preceding claim, wherein the locking collar is shaped such that, upon engaging the restraining surface and shoulder surface, reaction forces from the restraining surface and shoulder surface bias the locking collar radially inwards;optionally wherein longitudinal contraction force in the shaft causes each locking collar to be restrained between the restraining surface and the shoulder surface to inhibit contraction of the shaft.

11. The apparatus of claim 10, wherein the proximal and distal engaging surfaces are arranged to lie in planes which are substantially parallel or which converge at a location radially outward from the locking collar.

12. The apparatus of claim 10 or 11, wherein, in use, the restraining surfaces and the shoulder surface are configured to be substantially parallel to the proximal and distal engaging surfaces respectively; and / or wherein the restraining surfaces define an acute angle relative to the longitudinal axis greater than around 5°, preferably between around 45° and 90° to the length of the shaft, more preferably at least around 70°.

13. The apparatus of any of claims 1 to 9, further comprising radial biasing means configured to apply a biasing force to the locking collar to inhibit radially outward movement of the locking collar;optionally wherein, upon the locking collar engaging the restraining surface and the shoulder surface, the biasing force is configured to cause each locking collar to be restrained between the restraining surface and the shoulder surface to inhibit contraction of the shaft.

14. The apparatus of claim 13, wherein the radial biasing means is configured to constrain the locking collar to move within a predetermined radius, optionally wherein the radial biasing means is configured to allow the locking collar to move beyond the predetermined radius resiliently upon application of a tension force to the shaft.

15. The apparatus of claim 13 or 14, wherein the radial biasing means is arranged to coupleto the locking collar and the reaction block and / or to two locking collars.

16. The apparatus of any preceding claim, wherein, upon application of a tension force to the shaft: each locking collar is driven outwardly; and at least one of the locking collars advances distally relative to the shaft to engage at least one previously unengaged shaft indentation in a ratcheting fashion.

17. The apparatus of claim 16, wherein each shaft indentation comprises a transverse release surface configured, upon application of the tension force to the shaft, to drive the locking collar outwardly, preferably along the shoulder surface.

18. The apparatus of claim 16 or 17, further comprising longitudinal biasing means configured to urge each locking collar distally relative to the shaft, preferably such that, upon application of the tension force to the shaft, the longitudinal biasing means cause each locking collar to advance distally relative to the shaft.

19. The apparatus of claim 18, wherein the longitudinal biasing means comprise a resilient biasing member, preferably a biasing spring.

20. The apparatus of any preceding claim, wherein a proximal end of the shaft comprises a notch for engaging a tensioning tool; and / or wherein the reaction block comprises a loading surface for engaging a tensioning tool;preferably wherein the notch and the loading surface of the reaction block are configured to engage the tensioning tool for applying a tension force to the shaft.

21. The apparatus of claim 20, wherein the notch for engaging the tensioning tool extends around the shaft, and preferably comprises a circumferential notch.

22. The apparatus of any preceding claim, further comprising at least one compression spring disposed between the reaction block and the structure.

23. The apparatus of claim 22, wherein the compression spring has a compressive stiffness less than a tensile stiffness of the shaft.

24. The apparatus of any preceding claim, wherein the locking collar further comprises an angled surface that is angled radially inwards towards the shaft and configured to receive a radially outward force from a de-tensioning tool;optionally wherein the angled surface defines an acute angle relative to the longitudinal axis of the shaft that is less than an acute angle defined by the shoulder surface of the reaction block relative to the longitudinal axis.

25. The apparatus of claim 24, wherein the angled surface is configured to receive an end of a linear force applicator, preferably a hydraulic cylinder, of the de-tensioning tool.

26. A ratchet device for maintaining tension in a shaft of a structure, the shaft comprising a plurality of indentations, the ratchet device comprising:a reaction block comprising a shoulder surface, the reaction block configured to be positioned around the shaft and constrained laterally relative to the shaft; andat least one locking collar comprising:at least one proximal engaging surface configured to engage the shaft indentations, anda distal engaging surface configured to engage the shoulder surface of the reaction block;wherein the locking collar is biased radially inwards towards the shaft to form a ratchet by virtue of the engagement between the locking collar and the shaft indentations.

27. A kit of parts, comprising:a ratchet device according to claim 26; anda tensioning tool and / or a de-tensioning tool.

28. The kit of claim 27, further comprising a tensionable shaft having a plurality of shaft indentations.

29. The kit of claim 27 or 28, wherein the tensioning tool comprises at least one linear force applicator, preferably a hydraulic cylinder, configured to apply a tension to the shaft to successively ratchet the locking collars along the shaft indentations.

30. The kit of any of claims 27 to 29, wherein the de-tensioning tool comprises:at least one first linear force applicator configured to control tension applied to the shaft; andat least one second linear force applicator configured to disengage the locking collar from the shaft indentations;optionally wherein the first and / or second linear force applicators include a hydraulic cylinder.

31. The kit of claim 30, wherein the second linear force applicator is configured to contact an angled surface of the locking collar of the ratchet device and, upon application of a force to the angled surface, drive the locking collar radially outward to a disengaged position and hold the locking collar in the disengaged position.

32. A method of using the apparatus of any of claims 1 to 25 for maintaining tension in a shaft of a structure, the method comprising:positioning the reaction block around the shaft; andinserting each locking collar between the shaft and the reaction block to engage the shaft indentations.

33. The method of claim 32, wherein positioning the reaction block comprises:positioning the reaction block around a base of an exposed portion of the shaft, optionally comprising sliding the reaction block distally along the shaft; and / orsecuring the reaction block to the structure.

34. The method of claim 32 or 33, wherein: each locking collar is inserted between the shaft and the reaction block after the reaction block is positioned; or each locking collar is inserted in the reaction block prior to positioning the reaction block.

35. The method of any of claims 32 to 34, further comprising:coupling a tensioning tool to the shaft and the reaction block;applying tension to the shaft by activating the tensioning tool;releasing the applied tension by deactivating the tensioning tool; and decoupling the tensioning tool from the shaft and the reaction block.

36. The method of claim 35, wherein coupling the tensioning tool to the shaft comprises inserting one or more tool arms into at least one notch in the shaft.

37. The method of claim 35 or 36, wherein the tensioning tool comprises at least one linear force applicator, preferably a hydraulic cylinder, and wherein coupling the tensioning tool to the shaft and the reaction block comprises:retracting the linear force applicator;coupling the tensioning tool to the shaft; andextending the linear force applicator to contact the reaction block.

38. A de-tensioning tool for reducing the tension in a shaft, wherein tension is maintained in the shaft using one or more locking collars restrained between indentations in the shaftand a reaction block that is coupled to a structure, the de-tensioning tool comprising: a tool arm configured to couple to the shaft;a first linear force applicator configured to apply force against the reaction block; anda second linear force applicator configured to apply force against an angled surface of each locking collar;wherein the first linear force applicator is rigidly connected to the tool arm, such that the first linear force applicator is configured to control tension applied to the shaft; andwherein the second linear force applicator is configured to disengage the one or more locking collars from the shaft indentations.

39. The de-tensioning tool of claim 38, wherein the first linear force applicator and / or the second linear force applicator includes a hydraulic cylinder; and / orwherein the second linear force applicator is configured, upon application of a force to the angled surface of each locking collar, to drive the locking collar radially outward to a disengaged position and hold the locking collar in the disengaged position.

40. A method of using the de-tensioning tool of claim 38 or 39 for reducing tension in a tensioned shaft, wherein tension is maintained in the shaft using one or more locking collars restrained between indentations in the shaft and a reaction block that is coupled to a structure, the method comprising:coupling a de-tensioning tool to the shaft and the reaction block;applying a tension to the shaft;disengaging the locking collars from the shaft indentations and holding the locking collars in a disengaged position; andwith the locking collars held in the disengaged position, reducing tension in the shaft; optionally wherein tension is initially maintained in the shaft using the apparatus of any of claims 1 to 23 or the ratchet device of claim 26.

41. The method of claim 40, wherein:applying tension to the shaft comprises extending the first linear force applicator of the de-tensioning tool;disengaging and / or holding the locking collars comprises extending the second linear force applicator of the de-tensioning tool; and / orreducing tension in the shaft comprises retracting the first linear force applicator of the de-tensioning tool.

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