WO2014096100A1

WO2014096100A1 - Turbine blade

Info

Publication number: WO2014096100A1
Application number: PCT/EP2013/077235
Authority: WO
Inventors: Luke MCEWEN
Original assignee: Gurit (Uk) Ltd
Priority date: 2012-12-19
Filing date: 2013-12-18
Publication date: 2014-06-26
Also published as: GB2509082B; EP2935870A1; GB201222943D0; GB2509082A

Abstract

A turbine blade (4) adapted for fitting to a turbine member (8), the turbine blade (4) comprising a root mounting portion which mounts an end of the turbine blade (4) to the turbine member (8) and is composed of fibre reinforced resin composite material, the root mounting portion having an end mounting surface for mating with a complementary mounting surface of the turbine member, a plurality of elongate bores (22) spaced around and extending inwardly from the end mounting surface of the blade, and a plurality of openings (24) in the root mounting portion, each opening (24) being located at an end, remote from the end mounting surface, of a respective one of the bores (22), each opening (24) being elongated in a direction extending longitudinally along a longitudinal axis of the respective elongate bore (22).

Description

TURBINE BLADE

The present invention relates to a turbine blade, such as a water turbine blade or a wind turbine blade. The present invention also relates to a turbine such as a water turbine or a wind turbine incorporating such a blade. The present invention particularly relates to a turbine blade for use in a tidal generator.

Wind and tidal turbine blades are typically of laminated composite material (for fatigue resistance), bolted to the turbine hub using steel bolts. A connection therefore needs to be made between the bolts and the composite blade. Tidal hubs are usually sealed, and hence there is no access inside the hub for tightening the nuts on the blade connecting bolts. Small wind turbines also have limited or no manual access due to the size of the hub.

There are a number of known techniques to affix such blades.

In one known technique, each hub spindle includes an enlarged circumferential flange to which the blade root is affixed using T-bolts. However, that structure exhibits the problem of poor hydrodynamics. There are further problems in that despite a heavy and expensive spindle, the spindle is susceptible to fatigue failure at the internal radius of the flange.

In another known technique, the blade root incorporates a radially outwardly directed circumferential flange, and studs are screwed into blind holes in a hub spindle. Nuts are fitted onto the outboard end of the studs. This arrangement suffers from the problem that the flanges are not strong enough when fabricated from fibre reinforced resin matrix composite material due to through thickness tensile (Brazier) loads at the tight radius, and so the flange tends to be made in steel or spheroidal graphite iron (SGI), which then needs to be connected to the composite blade, typically by an adhesive joint. The difference in stiffness between the composite material and steel/SGI can lead to the existence of stress concentrations in the adhesive joint. This in turn can result in a problem of the lowering of the fatigue resistance of the adhesive, particularly when subjected to underwater environments. So there is a need for an economically viable method of making blades for tidal turbines that are strong enough to work at the megawatt scale, are efficient in hydrodynamic performance and are cost effective to manufacture.

The present invention aims at least partially to meet that need.

Accordingly, the present invention provides a turbine blade adapted for fitting to a turbine member, the turbine blade comprising a root mounting portion which mounts an end of the turbine blade to the turbine member and is composed of fibre reinforced resin composite material, the root mounting portion having an end mounting surface for mating with a complementary mounting surface of the turbine member, a plurality of elongate bores spaced around and extending inwardly from the end mounting surface of the blade, and a plurality of openings in the root mounting portion, each opening being located at an end, remote from the end mounting surface, of a respective one of the bores, each opening being elongated in a direction extending longitudinally along a longitudinal axis of the respective elongate bore.

The present invention further provides a turbine including a turbine blade according to the invention fitted to a turbine member by a fitting system, the end mounting surface mating with a complementary mounting surface of the turbine member.

Preferred features are defined in the dependent claims.

The invention relates to the attachment of a blade root to the rest of the turbine, typically to a central hub of the rotor in the case of an axial-flow turbine. The turbine may be a water turbine, for example a tidal turbine, or a wind turbine

In this specification, the turbine blades incorporating the embodiments of the present invention may be used in any type of turbine which generates electrical power from flow of water or air relative to the turbine blade, which causes motion of the turbine blade which in rum drives an electrical generator, either directly or via a gearbox or hydraulic pump. Depending upon the turbine design, the water turbine is most typically disposed in the sea, but may be disposed in a river, or even a lake or other body of moving water. Most typically, the turbine is a tidal turbine which is adapted to cause electrical power generation from tidal motion, either on a rising sea tide or a reverse falling sea tide, with the turbine moving by water flow relative thereto in either of two opposite directions.

The turbine is typically an axial flow turbine, in which plural radial turbine blades are fixed to a rotatable hub which is caused to rotate about an axis by water or wind movement over the turbine blade surfaces.

Embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings, in which:

Figure 1 is a schematic perspective side view, partly in phantom, of a blade root structure and fitting system for fitting a tidal turbine blade root end to a rotatable hub in accordance with a first embodiment of the present invention;

Figure 2 is a schematic side view, partly in phantom, of a blade root structure and fitting system for fitting a tidal turbine blade root end to a rotatable hub in accordance with a second embodiment of the present invention;

Figure 3 is a part-sectional schematic plan view of a blade root structure and fitting system for fitting a tidal turbine blade root end to a rotatable hub in accordance with a third embodiment of the present invention;

Figure 4 is a schematic longitudinal cross-section through a blade root structure and fitting system for fitting a tidal turbine blade root end to a rotatable hub in accordance with a fourth embodiment of the present invention; and

Figure 5 is a schematic longitudinal side view of a bolt-style tensioner for use in a modification of the previous embodiments of the present invention to provide a fifth embodiment of the present invention.

Referring to Figure 1 there is shown part of a water-driven turbine, in particular a tidal turbine, in accordance with a first embodiment of the present invention. However, alternatively the turbine may be a wind turbine.

The tidal turbine 2 includes a turbine blade 4 having a root end 5 comprising a root mounting portion fitted by a fitting system 6 to a turbine member in the form of a rotational hub 8. A portion of only one turbine blade 4 is shown, although plural turbine blades 4 are fitted annul arly around the rotational hub 8.

The turbine blade 4 is composed of fibre reinforced resin composite material, for example produced from prepreg or resin-infused plies. The hub 8 is typically formed of a metal.

As shown in Figure 1 , the turbine blade 4 and rotational hub 8 define opposite respective annular load bearing mounting surfaces 12, 14 which are complementary and engage in a mating relationship at an interface 16. The load bearing mounting surfaces 12, 14 and interface 16 are generally substantially annular and may be continuous or discontinuous. The fitting system 6 extends across the interface 16 to fix the turbine blade 4 to the rotational hub 8. The fitting system 6 comprises an annular array of a plurality of mutually spaced threaded studs 18.

A first helically threaded end 20 of each stud 18 is threadably received in a respective blind helically threaded hole 21 in the rotational hub 8 which extends into the hub 8 from the load bearing surface 14 of the hub 8. A hollow elongate bore 22 extends inwardly of the turbine blade 4 from the load bearing surface 12 of the turbine blade 4 and terminates in an opening 24 which is elongated in the axial direction of the stud 18 to receive the stud 18. In the illustrated embodiment, the opening 24 is oval or elliptical in shape, with the major axis being aligned along the blade axis and/or along the longitudinal axis of the elongate bore 22 and stud 18. Preferably, the edges of the opening 24 are curved, so as to avoid or minimise stress concentrations in the composite material of the blade 4.

The opening 24 includes a near end portion 26 having a shaped support surface 28, typically a part-cylindrical, for example a semi-cylindrical, surface, adjacent to and surrounding the hollow bore 22. A bearing element 30 has a correspondingly shaped surface 32 which is shaped to mate complementarily with the shaped surface 28 of the end portion 26 of the opening 24. Typically, the bearing element 30 includes a non-threaded bore 34 extending therethrough, coaxial with the hollow bore 22, which has a larger diameter than the stud 18. Therefore the stud 18 can freely slide in the bearing element 30. Typically, the bearing element 30 includes an upper substantially planar surface 33 remote from the lower surface 32. The bearing element 30 is typically part-cylindrical, for example semi-cylindrical, and most typically comprises a semi-cylindrical barrel bearing. A nut 36 is threadably fitted to the second helically threaded end 38 of the stud 18. The nut 36 comprises an end member of the helically threaded element in the form of the stud 18, bearing directly or indirectly against the opening 24.

The free end 38 of the stud 18 extends into the opening 24 and is spaced from a far end portion 42 of the opening 24. A central portion 40 of the opening 24 has sufficient length in the axial direction of the stud 18 that the free end 38 of the stud 18 is spaced a sufficient distance, for example 10 to 50 mm, from the far end portion 42 of the opening 24 so that a nut tightening tool (not shown), such as a hexagonal socket and associated wrench, can be fitted into the opening 24 and over the free end 38 of the stud 18 to tighten the nut 36.

To allow sufficient space within the opening 24 for tool access to tighten the nut 36, the elongated opening 24 has a width greater than the width of the nut 36. In some embodiments where there are a large number of studs 18 and/or a small diameter of the blade root 5, the width of the openings 24 may leave only a narrow strip 44 of fibre-reinforced resin composite material between adjacent openings 24. A full blade bending load must be transmitted through these strips 44, and therefore minimising the width of the opening 24 is of paramount importance and drives the required thickness of the fibre-reinforced resin composite material at the root 5 of the blade 4.

In order to fit the turbine blade root 5 to the hub 8, each stud 18 is threaded into a respective one of the blind helically threaded holes 21. The free end 38 of the stud 18 is passed through a respective one of the hollow bores 22 and the respective bearing element 30 and into the opening 24. A nut 36 is threaded onto the end 38 and tightened to the required preset torque setting using a tool such as a socket torque wrench. Each of the pairs of stud 18/nut 36 is fitted in this manner annularly around the blade root 5.

Referring to Figure 2 there is shown a modification of the embodiment of Figure 1 in which like parts are identified with like reference numerals. In this embodiment, the nut 36 is replaced by a multi-jackbolt tensioner (MJT) 50. The MJT 50 comprises a cylindrical helically threaded annular body 52 which has an internal helical thread (not shown) which is coupled with the helical thread 54 of the second helically threaded end 38 of the stud 18. An annular array of tensioning bolts (jackbolts) 56 surrounds the stud 18 and each tensioning bolt 56 is threadably coupled with the body 52. The tensioning bolts 56 are parallel with the stud 18. Each tensioning bolt 56 has a head 58, for example a hexagonal head, located remote from the bearing element 30 and a free end 60 which bears directly or indirectly (for example using an intermediate washer, for example of hardened steel) against the outer surface 62 of the bearing element 30 or a washer 63 thereon.

In order to fit the turbine blade root 5 to the hub 8, each stud 18 is threaded into a respective one of the blind helically threaded holes 21. The free end 38 of the stud 18 is passed through a respective one of the hollow bores 22 and the respective bearing element 30 and into the opening 24. The body 52 of the MJT 50 is threaded onto the free end 38 of the stud 18 until the body 52 contacts or is slightly spaced from the bearing element 30. Then the tensioning bolts 56 are rotated so as to be urged downwardly against the bearing element 30. As the tensioning bolts 56 are rotated, they apply an upward force on the body 52 of the MJT 50 away from the bearing element 30, and thereby apply a tensioning force on the stud 18. Each head 58 of the tensioning bolts 56 is tightened to the required preset torque setting using a tool such as a socket torque wrench. Each of the stud 18/ MJT 50 pairs is fitted in this manner annularly around the blade root 5.

An advantage of using a multi-jackbolt tensioner as in Figure 2 rather than a simple nut as in Figure 1 is that each tensioning bolt 56 requires a smaller torque wrench as compared to the torque wrench, or possibly a hydraulic tensioner, required to tighten the single nut 36. This means that in order to provide the same mounting force of the blade on the hub for the same number of studs the width of the opening 24 can be reduced for the embodiment of Figure 2 using the MJTs as compared to the embodiment of Figure 1 using the nuts. This in turn allows the width of the strips 44 of fibre-reinforced resin composite material between adjacent openings 24 to be increased, enhancing the blade strength. In Figure 2, the opening 24 extends inwardly from a radially outer surface 27 of the blade root 5 and terminates inwardly of a radially inner surface of the blade root 5 to define a rear wall 29 of the opening 24 on the radially inner side of the blade root 5.

Referring to Figure 3 there is shown a modification of the embodiment of Figure 2 in which like parts are identified with like reference numerals.

The opening 24, or at least a part thereof, may extend wholly or only partly through the thickness of the blade root 64. In this embodiment, the width of the opening 24 on the radially outer side 65 of the blade root 64 is larger than on the radially inner side 66, and the opposed sides 68, 70 of the central portion 40 of the opening 24 are not parallel but rather are mutually inclined to form a substantially angular segment in cross-section. The opposed sides 68, 70 may be radially oriented towards the blade axis or alternatively may be oriented so as to provide a more pronounced angular segment, with the opposed sides 68, 70 being oriented toward each other on the radially inner side 66 by an acute angle even greater than that provided by radially oriented sides 68, 70. This provides a relatively large opening width L on the radially outer side 65 and a relatively small opening width 1 on the radially inner side 66.

As can readily be understood by those skilled in the art, a conventional T-bolt mounting configuration for a substantially circular blade root provides minimum composite material width at the inside face of the blade root. In contrast, in accordance with this embodiment, by providing an increased width of composite material on the circumferential inside face of the blade root, the strength of the composite material between the holes, and correspondingly between the fittings of the fitting system, is enhanced. This in turn increases blade strength, because the minimum width of the composite material is increased.

Such an angular segment structure for the cross-section of the opening 24 also provides, for a given area of composite material in a transverse cross-section through the root 64 orthogonal to the longitudinal direction of the blade, a wider opening 24 for torque wrench 72 access on the radially outer side 65 of the blade root 64 than if the opening 24 had a substantially rectangular cross-section. This configuration permits the torque wrench 72 to have a wider arc of operation when tightening the tensioning bolts 56 of the MJT 50, and furthermore permits the tensioning bolts 56 on the radially inner side 66 of the root 64 to be effectively accessed by a torque wrench entering the opening 24 on the radially outer side 65 of the blade root 64. The result is that the blade root 64 can be effectively fitted to the hub 8 using the MJTs 50 without requiring any access to the inside of the blade root 64.

Referring to Figure 4 there is shown part of a water-driven turbine, in particular a tidal turbine, in accordance with a fourth embodiment of the present invention. However, alternatively the turbine may be a wind turbine.

The tidal turbine 102 includes a turbine blade 104 having a root end 105 fitted by a fitting system 106 to a turbine member in the form of a rotational hub 108. A portion of only one turbine blade 104 is shown, although plural turbine blades 104 are fitted annularly around the rotational hub 108.

The turbine blade 104 is composed of fibre reinforced resin composite material, for example produced from prepreg or resin-infused plies. The hub 108 is typically formed of a metal.

As shown in Figure 4, the turbine blade 104 and rotational hub 08 define opposite respective load bearing surfaces 1 12, 1 14 which engage at an interface 1 16. The fitting system 106 extends across the interface 1 16 to fix the turbine blade 4 to the rotational hub 108. The fitting system 106 comprises an annular array of a plurality of threaded studs 1 18, although only one stud 1 18 is illustrated in Figure 4.

A first helically threaded end 120 of each stud 1 18 is threadably received in a respective blind helically threaded hole 121 in the rotational hub 108 which extends into the hub 108 from the load bearing surface 1 14 of the hub 108. A hollow bore 122 extends inwardly of the turbine blade 104 from the load bearing surface 1 12 of the turbine blade 104 and terminates in an opening 124 which is elongated in the axial direction of the stud 1 18 to receive the stud 1 18.

In the illustrated embodiment, the opening 124 is an open-ended recess 123 in a radially outer surface 125 of the blade root 5, the recess 123 being oriented along the longitudinal axis of the respective elongate bore 122. The opening/recess 124/123 has no material located above the hollow bore 122. The recess 123 has been cut, for example, by a rotating milling cutter moving downwardly in a single direction towards the hollow bore 122, the milling cutter comprising a cutting cylinder having an axis orthogonal to the axis of the hollow bore 122. As for the opening 24 of the previous embodiments, preferably the edges of the opening 124 are curved, so as to avoid or minimise stress concentrations in the composite material of the blade 4.

The opening 124 includes a near end portion 126 having a shaped surface 128, typically a part-cylindrical, for example a semi-cylindrical, surface, adjacent to the hollow bore 122. A bearing element 130 has a correspondingly shaped surface 132 which is shaped to mate complementarily with the shaped surface 128 of the near end portion 126 of the opening 124. Typically, the bearing element 130 includes a non-threaded bore 134 which has a larger diameter than the stud 1 18. Therefore the stud 1 18 can freely slide in the bearing element 130. The bearing element 130 is typically part-cylindrical, for example semi-cylindrical, and most typically comprises a semi-cylindrical barrel bearing. In alternative embodiments, the bearing element 130 is planar and bears against a planar surface adjacent to and surrounding the hollow bore 122.

As described for the previous embodiments, a MJT 150, or a nut (not illustrated), is threadably fitted to the second helically threaded end 138 of the stud 1 18.

The provision of the open-ended opening 124/recess 123 provides a number of advantages. For example, the open-ended opening 124 allows the MJT or nut freely to be applied onto, and removed from, the stud along the stud direction, as shown by the arrow in Figure 4, without any blocking or interference from an upper end of the opening. Also, the opening 124 is freely accessible on the radially outer side 125, allowing easier access for a torque wrench. The width of the opening 124 can be made even narrower as compared to the opening 24 of the previous embodiments, increasing blade strength. In addition, when a MJT 150 is employed, this provides the advantage that the opening 124 can be made narrower, and so the composite material of the blade root 105 can be made thinner, between the outer side 125 and an inner side 127 and so less expensive.

The first helically threaded end 120 of the stud 1 18 may have an enlarged diameter as compared to the remainder of the stud 1 18 so that the stud portion which is threadably received in the blind helically threaded hole 121 in the rotational hub 108 has greater thread diameter. This in turn provides a greater thread load-bearing area which extends into the hub 108 from the load bearing surface 1 14 of the hub 108. This strengthens the stud 1 18 and increases the fatigue resistance of the stud fittings.

A modified bolt for use in any of the embodiments is illustrated in Figure 5. The bolt 200 is a bolt-style tensioner rather than a stud plus nut-style tensioner as shown in Figures 1 to 4. In Figure 5, the bolt 200 has a fixed head 202 and a shaft 204 extending therefrom, with a radially enlarged threaded end 206 of the shaft 204. As discussed above, the enlarged thread diameter increases fatigue resistance. Tensioning bolts 208 in the head 202 extend through the head 202 aligned with the direction of the shaft 204 and have lower ends 210 which bear against a planar bearing 212 on or in an annular support 214 surrounding the shaft 204. The bolt-style tensioner of Figure 5 provides a more compact tension element which can permit a smaller hole and opening in the blade root. Also, the helical thread on the outer end of the stud can be omitted, thereby increasing the fatigue life.

Returning back to Figure 4, the load bearing surfaces 1 12, 1 14 of the turbine blade 104 and rotational hub 108 are inclined at an acute angle a, typically from 2 to 30°, more typically from 5 to 10°, to the plane P which is transverse to the longitudinal axis of the blade root 105. Correspondingly, the interface 1 16 is similarly inclined, and is oriented towards the blade root 105 and away from the hub 108 so that the radially outer circumference of the interface 1 16 is located nearer to the blade tip than the radially inner circumference of the interface 1 16. The blade/hub interface 1 16 is therefore frusto-conical when the interface is annular.

In this embodiment the frusto-conical interface is directed upwardly and outwardly and the angle a may be regarded as a "positive" angle, but in an alternative embodiment the frusto- conical interface is directed downwardly and outwardly and the angle a (which may have the same values described above) may be regarded as a "negative" angle. In that alternative embodiment the interface is oriented away from the blade root and towards the hub so that the radially inner circumference of the interface is located nearer to the blade tip than the radially outer circumference of the interface.

Such a frasto-conical blade/hub interface 1 16 provides a number of advantages. The frusto- conical interface 116 permits the blade geometric taper to be initiated precisely at the free end of the blade root. When the root contact face is inclined or coned so that it is not perpendicular to the bolt axis or blade axis, the tendency of the blade to slide across the hub spindle contact surface is reduced. In a tidal turbine, such sliding forces are large due to tidal turbine blades generating large lift forces over a relatively short blade span (compared for instance to a wind turbine blade).

It is known to solve the problem of preventing such sliding forces by providing the hub spindle with a "spigot" in the form of a cylindrical extension protruding into the blade root and being received in an annular end part of the root. However, that known design requires additional material in the spindle and accurate manufacture of the inside surface of the blade to fit the spigot.

In contrast, in accordance with this embodiment, the provision of a "positive" or "negative" inclined angle for the contact surfaces of the blade root and hub, for example by making the inboard end of the blade slightly conical, or alternatively V-shaped or spherical, results in the reaction forces from the bending moment serving to counteract the sliding force and/or the bolt preload serving to counteract the sliding force.

For a tidal blade in particular, as its root diameter is relatively large for its span compared to a wind turbine blade, there is a need to taper the blade rapidly outboard of the root to avoid excessively thick sections along the blade span, which would cause too much drag. In a conventional T-bolt design the taper starts outboard of the barrel nut holes. In contrast, in the embodiment of Figure 4, for the elongated holes 124 it is advantageous to begin the taper further inboard, either at the inboard end of the hole 124 or even at the blade root contact face 1 12. This provides a number of technical advantages.

First, the geometric taper of the blade can be more rapid, hence reducing the drag of the blade, without excessive curvature of the laminate, which could otherwise lead to delamination of the composite material due to Brazier loads. Secondly, the blade is easier to laminate as the degree of double curvature in this area is reduced, i.e. the blade root can become more conical.

In the embodiment of Figure 4, the provision of the open-ended elongated holes 124 also provides a number of technical advantages. First, the studs can be removed from the outboard side without removing the blade, which can reduce the time taken to inspect or replace the studs. Secondly, the bolts can be tightened from directly outboard, allowing the open-ended elongated holes 124 to be made narrower because reduced width for torque wrench access is required. Thirdly, the machining of the open-ended elongated holes can be done in a single cutting operation in the blade axial direction with a milling cutter. Depending on the depth of cut, the open-ended hole can be left blind so that the blade remains watertight. This can be beneficial to avoid marine fouling inside the blade. Alternatively a watertight plate can be fitted to the inside face of the hole after machining if required.

In the modified embodiment of Figure 5, a bolt-style tensioner can be used instead of a stud plus nut-style tensioner as shown in Figure 4. Such a bolt-style tensioner can be made more compact and removes the need for a thread on the outboard end of the stud, which determines its fatigue life. Since the only limitation on the size of the inboard end of the stud is the size of the hole in the blade through which it must pass, the thread at the inboard end can be much larger, and hence is more resistant to fatigue. Alternatively for a given size of thread, a smaller bolt can be used, allowing the elongated hole 124 to be narrower and hence the blade laminate can be made thinner and be less expensive.

In the marine environment where tidal turbines are installed, the costs of vessels for installation and maintenance are relatively very high, which means that saving time during operations on board can confer a large financial benefit. The present invention has application in the manufacture of oscillating foil tidal turbine blades, cross-flow tidal turbine blades and axial flow tidal turbine blades, because all of these blade types have very high loading and relatively thin blade sections and their root connections can generate high shear loads.

The cost of maintenance and repair of submerged tidal turbines is high, for example much greater than for onshore wind turbines. The preferred embodiments of the present invention provide an advantage that the constructions can eliminate the use of highly-loaded adhesive joints in an underwater environment, which enhances the reliability of the tidal turbine incorporating the tidal turbine blade. The preferred embodiments of the present invention can also provide significant labour cost savings, efficiency and reliability improvements compared to other methods, particularly for an axial-flow turbine construction.

Various other modifications to the tidal and wind turbine blades of the present invention, and their manufacturing process, will be apparent to those skilled in the art.

Claims

1. A turbine blade adapted for fitting to a turbine member, the turbine blade comprising a root mounting portion which mounts an end of the turbine blade to the turbine member and is composed of fibre reinforced resin composite material, the root mounting portion having an end mounting surface for mating with a complementary mounting surface of the turbine member, a plurality of elongate bores spaced around and extending inwardly from the end mounting surface of the blade, and a plurality of openings in the root mounting portion, each opening being located at an end, remote from the end mounting surface, of a respective one of the bores, each opening being elongated in a direction extending longitudinally along a longitudinal axis of the respective elongate bore.

2. A turbine blade according to claim 1, wherein the opening has opposed closed ends and is oriented along the longitudinal axis of the respective elongate bore.

3. A turbine blade according to claim 2, wherein the opening is a substantially oval or elliptical opening with a major axis thereof oriented along the longitudinal axis of the respective elongate bore.

4. A turbine blade according to claim 1 , wherein the opening is an open-ended recess in a radially outer surface of the root mounting portion, the recess being oriented along the longitudinal axis of the respective elongate bore.

5. A turbine blade according to any one of claims 1 to 4, wherein at least a part of the opening extends wholly through the thickness of the root mounting portion.

6. A turbine blade according to any one of claims 1 to 4, wherein at least a part of the opening extends only partly through the thickness of the root mounting portion.

7. A turbine blade according to claim 6, wherein the opening extends inwardly from a radially outer surface of the root mounting portion and terminates inwardly of a radially outer surface of the root mounting portion.

8. A turbine blade according to any one of claims 1 to 7, wherein the width of the opening on a radially outer side of the root mounting portion is larger than the width of the opening on a radially inner side of the opening, and opposed sides of the opening are mutually inclined to form a substantially angular segment for the opening in cross-section.

9. A turbine blade according to claim 8, wherein the opposed sides are radially oriented towards a longitudinal axis of the root mounting portion.

10. A turbine blade according to claim 8, wherein the opposed sides are oriented toward each other on the radially inner side by an acute angle even greater than that provided by sides radially oriented towards a longitudinal axis of the root mounting portion.

1 1. A turbine blade according to any one of claims 1 to 10, wherein the opening includes a support surface for a bearing element surrounding the end of the respective elongate bore.

12. A turbine blade according to claim 11 , further comprising a bearing element mounted against the support surface, the bearing element comprising a bore extending therethrough which is coaxial with the elongate bore.

13. A turbine blade according to claim 12, wherein the support surface, and a lower surface of the bearing element engaging the support surface, are complementary and part-cylindrical.

14. A turbine blade according to claim 12 or claim 13, wherein an upper surface of the bearing element remote from the support surface is substantially planar.

15. A turbine blade according to any foregoing claim, wherein the end mounting surface is a non-planar surface having at least a portion thereof inclined at an acute angle to a plane extending transversely to a longitudinal axis of the turbine blade.

16. A turbine blade according to claim 15, wherein the end mounting surface is a frusto- conical surface.

17. A turbine blade according to claim 15 or claim 16, wherein the non-planar end mounting surface is upwardly and outwardly tapered, with the radially outer edge thereof being located more toward a tip end of the blade than the radially inner edge.

18. A turbine blade according to claim 15 or claim 16, wherein the non-planar end mounting surface is downwardly and outwardly tapered, with the radially inner edge thereof being located more toward a tip end of the blade than the radially outer edge.

19. A turbine blade according to claim 17 or claim 18, wherein the non-planar end mounting surface is inclined at an acute angle of from 2 to 30°, optionally from 5 to 10°, relative to the plane extending transversely to the longitudinal axis of the turbine blade.

20. A turbine blade according to any foregoing claim, which is a water turbine blade.

21. A turbine blade according to any one of claims 1 to 19, which is a wind turbine blade.

22. A turbine including a turbine blade according to any foregoing claim fitted to a turbine member by a fitting system, the end mounting surface mating with a complementary mounting surface of the turbine member.

23. A turbine according to claim 22, wherein the fitting system comprises a plurality of helically threaded elements, each threaded element being threadably fitted into a respective helically threaded hole in the turbine member and extending through a respective elongate bore in the root mounting portion and having an end member bearing directly or indirectly against the opening.

24. A turbine according to claim 23 comprising a bearing element mounted against the support surface, the bearing element comprising a bore extending therethrough which is coaxial with the elongate bore, and wherein each end member bears against a respective bearing element.

25. A turbine according to claim 23 or claim 24, wherein at least some of the helically threaded elements comprise a stud having two helically threaded ends, a first end being threaded into the turbine member and a second end being threaded into a respective end member in the form of a nut.

26. A turbine according to claim 23 or claim 24, wherein at least some of the helically threaded elements comprise a stud having two helically threaded ends, a first end being threaded into the turbine member and a second end being threaded into a respective end member in the form of a multi-jackbolt tensioner.

27. A turbine according to claim 26, wherein the multi-jackbolt tensioner comprises a cylindrical helically threaded annular body which is threadably coupled with the second end of the stud and an annular array of tensioning bolts surrounding the stud, each tensioning bolt being threadably coupled with the body, each tensioning bolt having a head and an opposite free end which bears directly or indirectly against a bearing element to tension the stud.

28. A turbine according to claim 23 or claim 24, wherein at least some of the helically threaded elements comprise a bolt having a first helically threaded end threaded into the turbine member and a second end comprising a fixed head in the form of a multi- jackbolt tensioner.

29. A turbine according to claim 28, wherein the fixed head of the multi-jackbolt tensioner comprises a fixed head member which is fixed to the bolt and an annular array of tensioning bolts surrounding the bolt, each tensioning bolt being threadably coupled with the fixed head member, each tensioning bolt having a head and an opposite free end which bears directly or indirectly against a bearing element to tension the bolt.

30. A turbine according to any one of claims 25 to 29, wherein the first helically threaded end has a diameter larger than the diameter of the opposite end of the helically threaded element.

31. A turbine according to any one of claims 23 to 30, wherein the turbine member is a rotatable hub.

32. A turbine according to any one of claims 23 to 31 , which is a water-driven turbine.

33. A turbine according to claim 32, which is a tidal turbine.

34. A turbine according to any one of claims 23 to 31 , which is a wind-driven turbine.