Method of building an offshore windmill

A method of building an offshore windmill includes, using a 3D-heave-compensated crane, placing on a windmill pedestal a lifting jack having a receiving region, and fixing the lifting jack to the windmill pedestal such that the lifting jack can be later removed, and such that a windmill column can be placed within the receiving region directly on the windmill pedestal. The windmill generator is installed using the 3D-heave-compensated crane. The windmill column is partially erected on the windmill pedestal using the 3D-heave-compensated crane and the lifting jack. Before the windmill is fully erected, windmill blades are placed on the windmill generator using the 3D-heave-compensated crane, and the erection of the windmill column on the windmill pedestal is completed using at least the lifting jack. Using the 3D-heave-compensated crane, the lifting jack is removed from the windmill pedestal.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a 35 U.S.C. § 371 national stage application of PCT/EP2017/071095 filed Aug. 22, 2017 and entitled “Method of Building an Offshore Windmill”, which claims priority to European Patent Application No. 16186996.1 filed Sep. 2, 2016, each of which is incorporated herein by reference in their entirety for all purposes.

Not applicable.

FIELD OF THE INVENTION

The invention relates to a method of building an offshore windmill on a windmill pedestal that is located offshore.

BACKGROUND

Offshore wind energy is becoming more and more important. In particular, in the last couple of years, there has been an enormous increase in the number of wind farms (a grid of windmills placed on the continental shelf in the seas). Not only is the number of offshore windmills increasing, but also their size, i.e. the windmills are getting bigger. The latter brings new challenges in terms of actually installing these offshore windmills. Dedicated huge ships have been built having very large heavy duty cranes placed on top of them. The general idea is to manufacture the windmills onshore in as large parts as possible and then transport these parts to the desired location, where they are placed using the ever-increasing heavy-duty cranes on an earlier prepared offshore windmill pedestal. There exists a huge variety of different technologies of preparing and building such windmill pedestals offshore, but that is beyond the scope of what is claimed herein as an invention. As the size of the offshore windmills increases further, also the size of these dedicated windmill installation ships is increasing. In order to facilitate the use of a huge crane these ships are provided with a jack-up system such that they lift themselves out of the sea, which makes the ships orientation independent of the waves. It is particularly the operational costs of these ships, which is of concern.

In the prior art some suggestions are reported to resolve this issue.

GB2,365,905A discloses an offshore structure comprising a watertight chamber having a base, which rests on the seabed and a telescopically extendable shaft, which extends upwardly from the chamber, with a hoist located at the shaft top. A wind turbine may be located at the top of the shaft, while a generator may be located in the base. The structure may be towed floating with the column retracted to its installation point, where the base is then ballasted to its resting position on the seabed. The column may then be extended and grouted in place, while the hoist may be used to raise the rotor hub and blades to the required position at the top of the column.

WO2010/151145A1 discloses a windmill comprising a generator house with a generator (also being referred to as “nacelle”) and a plurality of rotor blades at an upper part of an upright shaft. Said shaft comprises at least one telescopic joint for altering the height of said windmill, and further comprises a pivotal connection of the blades between a substantially vertical and a substantially horizontal position. The respective disclosure relates to a method of installation, intervention or decommissioning of said windmill. It finds application to fixed, bottom mounted offshore windmills or onshore windmills.

A first disadvantage of the above-presented windmills and installation methods is that they are not retrofit solutions, i.e. they cannot be applied to existing offshore windmill pedestals. In addition, said methods and windmills are not easily scaled up. Thus, there is a need for a further improved retrofit method of building offshore windmills.

SUMMARY OF THE DISCLOSED EMBODIMENTS

The present disclosure directed to remedying or reducing at least one of the drawbacks of the prior art, or at least provide a useful alternative to prior art.

In a first aspect this disclosure relates to a method of building an offshore windmill on a windmill pedestal that is located offshore. In some embodiments, the method comprises:placing a lifting jack having a receiving region on the windmill pedestal using a 3D-heave-compensated crane that is positioned on a floating vessel, and fixing the lifting jack to the windmill pedestal such that the lifting jack can be later removed, and such that a windmill column can be placed within the receiving region directly on the windmill pedestal;installing a windmill generator using the 3D-heave-compensated crane;partially erecting the windmill column on the windmill pedestal using the 3D-heave-compensated crane and the lifting jack;installing a plurality of windmill blades on the windmill generator using the 3D-heave-compensated crane at a stage where the windmill column has been partially, but not fully, erected;completing the erection of the windmill column on the windmill pedestal using at least the lifting jack;the lifting jack from the windmill pedestal using the 3D-heave-compensated crane.

In some embodiments, the method comprises:providing a floating vessel comprising a 3D-heave-compensated crane on a deck thereof;providing at least one offshore windmill assembly and a lifting jack, wherein the offshore windmill assembly comprises a windmill generator, a plurality of windmill blades and at least two windmill column parts for forming a windmill column at a later stage, wherein the lifting jack is configured for receiving the windmill column in a receiving region thereof;moving said floating vessel, the lifting jack and the at least one offshore windmill assembly in proximity of the windmill pedestal;placing the lifting jack directly on the windmill pedestal using the 3D-heave-compensated crane and fixing the lifting jack to the windmill pedestal such that it can be later removed, and wherein the lifting jack is fixed to the windmill pedestal such that the windmill column can be placed within the receiving region directly on the windmill pedestal;installing the windmill generator using the 3D-heave-compensated crane;partially erecting the windmill column on the windmill pedestal using the 3D-heave-compensated crane and the lifting jack;installing the windmill blades on the windmill generator using the 3D-heavecompensated crane at a stage where the windmill column has been partially erected;fully erecting the windmill column on the windmill pedestal using at least the lifting jack, andremoving the lifting jack from the windmill pedestal using the 3D-heave-compensated crane.

In order to facilitate understanding of this disclosure one or more expressions are further defined hereinafter.

The effects of the method in accordance with this disclosure are as follows.

A first feature of this disclosure is that a floating vessel is provided having a 3D-heave-compensated crane. Throughout this specification the wording “3D-heave-compensated crane” refers to a crane, which has a system in place for keeping the load of the crane substantially free of movement due to waves in all three dimensions. Expressed differently, it refers to a crane having heave-compensation for the three position degrees of freedom of the load. In addition, there might even be heave-compensation for one or more rotation degrees of freedom, but that is not essential to this disclosure. As mentioned in the background of this disclosure often very large dedicated jack-up ships having very large cranes are used. By providing a 3D-heave compensated crane, it is rendered possible to use much smaller mainstream vessels, such as standard offshore crane vessels.

Another feature of this disclosure is that the offshore windmill is shipped towards the windmill pedestal in parts. The offshore windmill parts and the lifting jack may be shipped on the same vessel (having the crane) or on (a) different vessel(s). Important to note is that the windmill pedestal does not need to be substantially altered for facilitating the building of the windmill, i.e. the solution is fully retrofit. In the prior art solutions discussed in the background section special pedestal constructions are required to enable the suggested solutions.

A third feature of this disclosure is that a temporary lifting jack is provided, which means that the lifting jack is first added to the structure (that is directly on the windmill pedestal, or on the pedestal adapter if this one has been installed on the windmill pedestal first) and later removed after the building of the offshore windmill is finished. The lifting jack plays an important role in the actual building of the offshore windmill as will be more obviated in the detailed description of the figures.

A further feature is that the respective parts (the windmill generator, the windmill column (parts) and the windmill blades) of the offshore windmill are brought to the windmill pedestal one after the other, wherein the order of these parts is cleverly chosen to facilitate the building of the windmill in an efficient way and using a crane, which is much smaller than the existing cranes that are used for a windmill of a similar size. One of the reasons is that parts like the windmill generator and the windmill blades are installed before the windmill column is fully erected. Expressed differently, the windmill column is effectively erected to its final height after installation of these parts. Another reason is that the windmill column is partially erected before the windmill blades are installed. This allows for vertical installation, without the need for special hinge constructions as is the case in one of the prior art solutions. Another reason is that the windmill column is built on top of the pedestal without altering it.

It must be stressed that the amount of variations concerning the order of method steps is very large and that by no means the order as mentioned in the claims below is to be construed as limiting the claims. All variations in order are considered to fall within the scope of the claimed invention, unless such order would result in something unfeasible.

In an embodiment of the method in accordance with this disclosure the method further comprises, before the step of placing the lifting jack, a step of preparing the windmill pedestal for receiving the lifting jack, for instance by placing a pedestal adaptor on the windmill pedestal. The lifting jack must be fixed to the windmill pedestal such that it hangs or stands on it, and in this embodiment that is done via the pedestal adaptor. However, it is also possible to modify the lifting jack such that it can be removably mounted to the pedestal, i.e. rendering the pedestal adaptor superfluous.

In an embodiment of the method in accordance with this disclosure, in the step of installing the windmill generator, the windmill generator is installed on the lifting jack. This embodiment is very advantageous, because it facilitates the provision of the windmill generator very early in the building process. The lifting jack may be in retracted or extended position, when placing the windmill generator. Each of these positions has their own advantages. More information will be given in the detailed description of the figures.

In an embodiment of the method in accordance with this disclosure, in the step of partially erecting the windmill column, at least a first one of the at least two windmill column parts is installed on the windmill pedestal, and, in the step of fully erecting the windmill column, the remaining ones of the windmill column parts are installed using the lifting jack for creating room between the windmill pedestal and the remaining part of the windmill column, as well as using the 3D-heave-compensated crane for moving the respective windmill column parts from the floating vessel to the pedestal. This embodiment constitutes a first main variant of building up the windmill column from bottom to top.

There is a great freedom in terms of the stage of the method, wherein the first windmill column part is installed. In a first variant, this first part of the windmill column may be installed even before placement of the lifting jack, such that the lifting jack is to be placed over or around the first column part. In a second variant, this first part of the windmill column may be installed between the placement of the lifting jack and the installing of the windmill generator, wherein the windmill generator is then to be placed on the first column part. In a third variant, this first part of the windmill column may be installed after installing of the windmill generator, but then the lifting jack must be extended to create the place between the windmill pedestal and the windmill generator. It is not unlikely that more variants are found by the person skilled in the art just using his normal routine and expertise.

In an embodiment of the method in accordance with this disclosure, in the step of providing the floating vessel, the at least two windmill column parts that are provided on the floating vessel are in the form of a telescopic windmill column in its retracted state. This embodiment facilitates faster building of the offshore windmill, but also the building of higher windmills, in particular when a plurality of telescopic windmill column parts are provided.

In an embodiment of the method in accordance with this disclosure, in the step of partially erecting the windmill column, the telescopic windmill column is installed in its retracted state on the windmill pedestal, and, in the step of fully erecting the windmill column, the lifting jack is used to extend the telescopic windmill column. This embodiment builds further on the previously-mentioned embodiment and constitutes a second main variant of building up the windmill column from bottom to top. In case the telescopic windmill column comprises more than two parts, the extension of the column can be done using multiple runs of the lifting jack. In each next run, the lifting jack is fixed to a different one of the respective parts of the telescopic column, until all respective parts of the telescopic column have been extended and the telescopic column has reached its maximum length. More information will be given in the detailed description of the figures.

It must be stressed that the first and second main variant of building up the windmill column may also be combined, in that the column is build up out of a telescopic part in combination with one or more further parts, or even one or more further telescopic parts. The lifting jack of this disclosure may be conveniently used to lift up/extend these telescopic parts and further telescopic parts one after the other.

In an embodiment of the method in accordance with this disclosure, in the step of installing the windmill blades, the windmill blades are installed sequentially, wherein the windmill generator is used to manipulate a position of a windmill hub of the windmill generator such that the respective windmill blade can be easily mounted thereto, preferably in its vertical downward-pointing direction. This embodiment facilitates the use of a smaller crane to mount the windmill blades or the building of larger windmills using the same crane size.

In an embodiment of the method in accordance with this disclosure, in the step of providing the floating vessel, the floating vessel is provided with an enhanced lifting jack that further comprises a cradle that is configured for i) receiving respective windmill parts, for ii) bringing said parts from a lower end of the lifting jack to an upper end of lifting jack for iii) manipulating, i.e. rotating, said parts to a vertical position, where necessary, and for iv) bringing said parts to said receiving region. This embodiment facilitates the building of even larger windmills as the enhanced lifting jack partially takes over the function of the crane. The main thing the crane needs to do in this embodiment is to put the respective parts on the cradle, while most of the other operations are carried out by the cradle. The enhanced lifting jack is explained in more detail in the detailed description of the figures.

DETAILED DESCRIPTION OF THE DISCLOSED EXEMPLARY EMBODIMENTS

FIGS. 1 to 25show different stages of an embodiment of the method of building an offshore windmill in accordance with this disclosure.FIG. 1shows a first stage of this method. In this stage of the method there is provided a floating vessel900, for instance an offshore crane ship. The vessel900has been provided with a 3D-heave compensated crane910on a deck901thereof. The 3D-heave compensated crane910is capable of lifting between 250 T or 400 T, for example. It is typically the windmill generator (nacelle), which is the heaviest part of the windmill assembly. A 6 MW nacelle/windmill generator (i.e. from RePower) typically weighs 316 T, so a 400 T crane will be able of installing most 6 MW nacelles and some 8 MW nacelles. A 5 MW nacelle (i.e. from Multibird) typically weighs 233 T, so a 250 T crane will be able to install most nacelle designs up to and including 5 MW.

The vessel900is provided with offshore windmill parts, including a plurality of windmill column parts110, a windmill generator120, and a plurality of windmill blades130. It must be noted that there may be a collection of other smaller parts, which is needed for mounting, fixing and installing the parts together, but all these details have been omitted in order not to obscure this disclosure. InFIG. 1the vessel900has transported the windmill parts close to an offshore windmill pedestal50in a sea1, so close that the pedestal is within reach of the crane910. In addition to the windmill parts the vessel900is also provided with a lifting jack90, which plays an important role in the method of this disclosure. In the embodiment shown ofFIGS. 1 to 25, the pedestal50has been provided with a so-called pedestal adaptor51, which serves to cooperate with the lifting jack90as will be explained later.

FIG. 2shows another stage of the method of building an offshore windmill. In this stage of the method, the lifting jack90is placed directly on the pedestal50using the crane910. The load on the crane is typically in the order of 200 T (depending on the size of the windmill that is built) and is lifted and moved to the pedestal50in 3D-advanced heave compensation (3D-AHC) mode. The lifting jack90comprises fixing members91, which facilitate placing and fixing the lifting jack90to the pedestal50via the pedestal adaptor51. It must be noted that there exist also other ways of temporarily mounting a lifting jack90to an existing pedestal50, which may render the pedestal adaptor51superfluous. In the figure there is two lifting legs90-1,90-2visible, but in this embodiment there are three, as will be discussed with respect toFIGS. 26-27. The lifting jack90further comprises a cradle95mounted to one of its lifting legs90-1. As will be further explained with reference to other figures this cradle95is used to lift windmill parts up to the upper end of the lifting jack90, but also to manipulate the orientation of said parts to facilitate the building of the windmill. The cradle95is configured to slide along the respective lifting leg90-1.

FIG. 3shows another stage of the method of building an offshore windmill. In this stage of the method, the windmill generator120is lifted and moved to the pedestal50using the crane910in 3D-AHC mode. The typical load on the crane910is 200-300 T, depending on the size of the windmill that is built. In this embodiment the windmill generator120is secured (i.e. bolted) to the top side of the lifting jack90as illustrated.

FIG. 4shows another stage of the method of building an offshore windmill. In this stage of the method a first one110-1of the three windmill column parts110-1,110-2,110-3is lifted with the crane910. The typical load on the crane910is 70-100 T, depending on the size of the windmill that is built.

FIG. 5shows another stage of the method of building an offshore windmill. In this stage of the method, the lifting jack90is run up to its top position. In addition, the cradle95is moved towards a position in between the two extreme positions (maximum extension and minimum extension) as illustrated.

The stages ofFIGS. 4 and 5can be easily exchanged, i.e. the lifting jack90can be run up before the first windmill column part110-1is lifted up by the crane910.

FIG. 6shows another stage of the method of building an offshore windmill. In this stage of the method the first windmill column part110-1is moved to and placed on the cradle95(also referred to as catwalk) of the lifting jack90using the crane910in 3D-AHC mode.

FIG. 7shows another stage of the method of building an offshore windmill. In this stage of the method the first windmill column part110-1is clamped by the cradle95and rotated to a vertical position. More details about the cradle95to facilitate this are given with reference toFIGS. 26-27.

FIG. 8shows another stage of the method of building an offshore windmill. In this stage of the method the cradle95rotates around the respective lifting leg90-1to which it is mounted swinging the respective first windmill column part110-1to the centre region of the lifting jack90in between the pedestal50and the windmill generator120. Furthermore, the windmill generator120has been landed on the first windmill column part110-1by letting the lifting jack90descend a little bit. Subsequently, the windmill generator120has been secured to the respective windmill tower part110-1.

FIG. 9shows another stage of the method of building an offshore windmill. In this stage of the method, the first windmill column part110-1has been released and the cradle95has been swung outward to the outside region as illustrated. Furthermore, the lifting jack90has been further lowered until the first windmill column part110-1has landed on the pedestal50. The first column part110-1has been temporarily secured to the pedestal50, i.e. it will be detached from it at a later stage of the method. Finally, the lifting jack90has been released from the windmill generator120and lowered as illustrated.

FIG. 10shows another stage of the method of building an offshore windmill. In this stage of the method, a first one130-1of three windmill blades130-1,130-2,130-3is lifted using the crane910in 3D-AHC mode. The load on the crane in this stage is typically in the order of 15-35 T depending on the size of the windmill.

FIG. 11shows another stage of the method of building an offshore windmill. In this stage of the method, the first windmill blade130-1is moved to and placed on the cradle95using the crane910in 3D-AHC mode.

FIG. 12shows another stage of the method of building an offshore windmill. In this stage of the method, the first windmill blade130-1has been secured to (clamped by) the cradle95and rotated to a vertical position as illustrated. Subsequently the cradle95is moved up so that the first windmill blade130-1is touching the hub121of the windmill generator120. The first windmill blade130-1is then mounted to the hub121.

FIG. 13shows another stage of the method of building an offshore windmill. In this stage of the method, the cradle95is opened (releasing the first windmill blade130-1) and rotated back to its horizontal position. Furthermore, a second one130-2of the three windmill blades is lifted using the crane910in 3D-AHC mode. The load on the crane in this stage is typically in the order of 15-35 T depending on the size of the windmill. In addition, the hub121of the windmill generator120is rotated 120 degrees.

FIG. 14shows another stage of the method of building an offshore windmill. In this stage of the method, the second windmill blade130-2is mounted to the hub121in a similar way as illustrated inFIGS. 11 to 13.

FIG. 15shows another stage of the method of building an offshore windmill. In this stage of the method, the third windmill blade130-3is mounted to the hub121in a similar way as illustrated inFIGS. 11 to 13.

FIG. 16shows another stage of the method of building an offshore windmill. In this stage of the method, the propeller is complete and the windmill generator120(also called Nacelle) is rotated 180 degrees around the pedestal axis50a,such that the propeller of the windmill generator120is facing away from the vessel900. This step creates the space for the further part of installation process, i.e. it substantially frees the cradle95of the lifting jack90for further lifting operations.

FIG. 17shows another stage of the method of building an offshore windmill. In this stage the second windmill column part110-2is lifted from the vessel900using the crane910in 3D-AHC mode. The typical load on the crane910is 70-100 T, depending on the size of the windmill that is built. The cradle95is opened (its clamping arm is released and moved away) and moved to a lower end of the lifting jack90.

FIG. 18shows another stage of the method of building an offshore windmill. In this stage of the method, the first windmill column part110-1is fixed to the lifting jack90and subsequently detached from the pedestal50. The lifting jack90has been subsequently lifted (extended) and the cradle95is moved to a location in between said two end positions, Finally, the second windmill column part110-2has been moved to and placed on the cradle95using the crane910in 3D-AHC mode, which is then locked and rotated 90 degrees to bring the second windmill column in a vertical position.

FIG. 19shows another stage of the method of building an offshore windmill. In this stage of the method, the second windmill column part110-2has been rotated with the cradle95towards the centre region of the lifting jack90in between the pedestal50and the first windmill column part110-1.

FIG. 20shows another stage of the method of building an offshore windmill. In this stage of the method, the second windmill column part110-2is secured to the first windmill column part110-1, and temporarily secured to the pedestal50. Subsequently, the lifting jack90is retracted.

FIG. 21shows another stage of the method of building an offshore windmill. In this stage of the method, the lifting jack90has been coupled to a lower end of the second windmill column part110-2, and the second windmill column part110-2has been subsequently detached from the pedestal50. Finally, the lifting jack90is lifted to its maximum height, the third windmill column part130-3has been lifted, moved to and placed in the cradle95(using the crane910in 3D-AHC mode), locked, and rotated, similar to as discussed with reference to previous figures.

FIG. 22shows another stage of the method of building an offshore windmill. In this stage of the method, the third windmill column part110-3has been rotated with the cradle95towards the centre region of the lifting jack90in between the pedestal50and the second windmill column part110-2. The third windmill column part110-3is secured to the second windmill column part110-2, and to the pedestal50.

FIG. 23shows another stage of the method of building an offshore windmill. In this stage of the method, the lifting jack90has been retracted, detached from the windmill, and is about to be removed (lifted up) by the crane910in 3D-AHC mode. The load on the crane910is typically in the order of 200 T (depending on the size of the windmill that is built).

FIG. 24shows another stage of the method of building an offshore windmill. In this stage of the method, the lifting jack90has been removed and placed on the vessel900by the crane910. Subsequently, the lifting jack90is to be prepared for the next windmill.

FIG. 25shows another stage of the method of building an offshore windmill. In this stage of the method, the offshore windmill100has been completed.

It has already been mentioned that in an advantageous embodiment of the method of this disclosure an enhanced lifting jack90is provided, having extra functionality besides the basic jacking function. This enhanced lifting jack90comprises a cradle95that is configured for i) receiving respective windmill parts, for ii) bringing said parts (in a horizontal position) from a lower end of the lifting jack to an upper end of lifting jack for iii) manipulating, i.e. rotating, said parts to a vertical position, where necessary, and for iv) bringing said parts to said receiving region. It has to be noted that the cradle95is advantageous, but not essential to this disclosure. Its function could be taken up by the crane, albeit that the crane then needs to be modified such that it can handle, hold and manipulate parts. Expressed differently, the crane needs to be turned into some sort of robot arm.

FIG. 26ashows a top view of an embodiment of the advanced lifting jack90in accordance with this disclosure, wherein the lifting jack is in an open position.FIG. 26bshows a top view of the lifting jack ofFIG. 26a, wherein the lifting jack is in a closed position. In these figures it is illustrated how the lifting jack90can be folded around an existing tubular structure if necessary. These figures also serve to illustrate how the earlier discussed fixing members91can be placed on the pedestal adaptor51as illustrated. The cradle95in this embodiment comprises effectively two receiving members95-2in order to receive the parts in a stable manner.

FIG. 27shows a more detailed top view of the lifting jack ofFIG. 26b, wherein the cradle95is swung from an outer position to a centre position. A first feature that is illustrated inFIG. 27are the locking members96that cooperate with the receiving members95-1,95-2for holding/clamping respective parts. It must be noted that inFIG. 27the cradle95inFIG. 27has been rotated 90 degrees (for manipulating the respective part to a vertical position). Furthermore, the cradle95has been illustrated in four different swing positions, wherein it is visible that the cradle95may effectively swing in between said first90-1and third lifting leg90-3of the lifting jack90towards the centre region.

The method as illustrated inFIGS. 1 to 26is just one of the many possible embodiments. There are many variations on the embodiments possible, which has been extensively discussed above.

In a further variation there is no cradle95on the lifting jack90as discussed with regards toFIGS. 26a, 26band27earlier.

For instance, it must be noted that the lifting jack90inFIG. 2may be provided after provision of the first windmill column part110-1. As illustrated inFIGS. 26aand 26bthe lifting jack may be simply folded around the windmill column part and land on the pedestal adaptor51with its fixing members.

Furthermore, the windmill generator120inFIG. 3may also be placed later, i.e. after provision of the first windmill column part110-1. In that case it may be directly mounted on the first windmill column part110-1instead of on the lifting jack90. Alternatively, the windmill generator120inFIG. 3may be placed on an already lifted (extended) lifting jack90, even if no windmill column part110-1is placed underneath.

In addition, the windmill column parts110-1. . .110-3may be partially replaced by (a) telescopic windmill column (part(s). The lifting jack90may be conveniently used to sequentially extend the telescopic column parts.

Alternatively, the windmill blades130-1. . .130. . .3inFIGS. 10 to 16may be installed after installation of two or more windmill column parts110-1. .110-3. On one side this means that the crane may need to reach higher or be larger, while on the other side this allows for the installation of larger windmill blades.