WO2012030220A1

WO2012030220A1 - Method for introducing a hollow elongated structure into a water bottom

Info

Publication number: WO2012030220A1
Application number: PCT/NL2011/050596
Authority: WO
Inventors: Marinus Teunis Van Leeuwen, Jr
Original assignee: Ballast Nedam Offshore B.V.
Priority date: 2010-09-01
Filing date: 2011-09-01
Publication date: 2012-03-08
Also published as: EP2611970A1

Abstract

A method for introducing a hollow elongated structure into a water bottom of a body of water. A hollow foundation pile for a wind- or water turbine is provided. The provided structure has an inner space, a proximal end and an open distal end. The provided structure has to be introduced into the water bottom from its open distal end. In the method a device is provided for removing bottom parts out of the inner space. To remove the bottom parts out of the inner space the device is lowered from the proximal end of the structure into the inner space towards the distal end. A prefabricated integral structure is used. The structure comprises a base portion with a substantially constant base diameter and a transition portion with a varying diameter. The structure as a whole including the transition portion is lowered into the water bottom.

Description

Title: Method for introducing a hollow elongated structure into a water bottom.

The present invention relates to a method according to the preamble of claim 1 , a prefabricated structure and use of a structure.

US 2006/0185279 discloses a monopile for installation of a wind power installation. A so-called monopile (single-pile) foundation is disclosed which is already used for land-based wind power installations. For off-shore wind power installations, the monopile can be anchored in the seabed. Due to terrific technical development in recent years wind power installations have reached dimensions and weights which are increasingly at the limits of what is technologically feasible. The monopile must have substantial dimensions, for example, diameters of more than 5m and lengths of more than 30m. The wind power installation has a tower which can have a length of 130 m which is cemented to the mono pile. To obtain the cemented connection, a junction piece is provided which is aligned after placement on top of the monopile. The gap between the junction piece and the monopile is filled with a concrete-like mass (grout). It is noted, that in particular in the case of considerable water depths, it will become impossible to produce the monopile with the available limiting dimensions.

The cemented connection in between the junction piece and monopile or at other gaps between parts of the pile provide a drawback in that those cemented connections have a limited lifetime. At sea, the cemented connection is exposed to severe weather conditions and salt water. In practice, it has appeared that the cemented connection gets affected after some years. A periodic capital intensive service operation has to be performed to repair the cemented connection.

US 4.742.876 discloses a method and device for submarine foundation drilling. A drilling unit is disclosed which is linked by a series of boring rods to an operating vessel. The rod string is suspended from the vessel with the aid of a handling winch. The vessel has a hydraulic power station able to furnish hydraulic fluid by way of a control board. The lower end of the rod string is linked to the top of the drilling unit. The drilling unit is disposed inside a casing. The drilling unit is completely confined in the casing so that when the apparatus reaches the ocean bottom, the drilling unit lower part enters the earth where the drilling will take place. Clamps and breaks are subsequently used to link and unlink the drilling unit to the casing. Thus, the casing descends with the drilling unit until a desired depth has reached.

A problem to the disclosed method is that the method is only suitable to be applied in hard soils. The drilling unit excavates soil from beneath the casing which allows the casing to descend. However, if this method is applied in soft soils, too much bottom parts from a region around the distal end of the casing would be excavated which would result in an unstable foundation. The foundation would lack lateral stiffness which could result in a subsidence of the foundation. During its lifetime, a foundation may deviate from its initial upright position and may get into an inclined position. Thus, applying the method in soft soil may result in several stability problems.

It is an object of the present invention to at least partially eliminate the above mentioned drawbacks and/or to provide a useable alternative. In particular, it is an object of the invention to provide a method for introducing a hollow foundation pile in soft soil in a stable manner.

According to the invention this object is achieved by a method as defined in claim 1 . According to the invention a method is provided for introducing a hollow elongated structure into a water bottom of a body of water. The body of water is e.g. a sea, river, lake, or ocean and the water bottom is e.g. a seabed, riverbed, etc. Wherever the wording seabed in this patent application is used, it is mend in a broad context of a floor of a body of water. The method comprises a step of providing a hollow elongated structure, in particular a hollow foundation pile for a wind- or water turbine. The provided structure has an inner space, a proximal end and an open distal end. The method comprises the step of supplying the structure to a predetermined location at the body of water. The predetermined location is in particular a location in a wind farm. In a step of the method, the structure is brought into an upstanding position. Subsequently, the open distal end of the structure is placed into the water bottom. The provided structure has to be introduced into the water bottom from its open distal end. The open distal end is to be introduced into the water bottom. As a result bottom parts of the water bottom start entering the inner space of the structure via the open distal end. In this step, the structure is introduced into the water bottom by its own weight. The hollow elongated structure will move into the water bottom over a certain distance. Due to occurring friction forces, the structure will finally stop moving. In the method a device is provided, in particular a drill-dredge device, for removing bottom parts out of the inner space. To remove the bottom parts out of the inner space the device is lowered from the proximal end of the structure into the inner space towards the distal end. By removing bottom parts, the resistance inside the inner space of the structure which is caused by friction forces will decrease, which will result in a further movement of the structure into the water bottom.

The method according to the invention provides an improvement by using an improved hollow elongated structure. The hollow elongated structure is prefabricated which means that the base portion and transition portion of the structure is assembled to one-piece before the structure is introduced into the water bottom. The hollow elongated structure which is lowered into the water bottom has an inner space formed by a through hole which extends from a proximal end to a distal end, said structure comprises a base portion with a substantially constant base diameter and a transition portion with a varying diameter. The structure as a whole including the transition portion is lowered into the water bottom.

In particular, the hollow elongated structure has a top portion on top of the transition portion including a substantially constant inner diameter of at most 4.5 m, more in particular at most 4m. In particular, the transition portion has an increasing, in particular tapered, inner diameter which increases from at most 4.5m, more in particular at most 4 m at the proximal end to at least 5m, in particular at least 7m at the distal end which is at the base portion. The transition portion fits at the proximal end to the top portion and at the distal end to the base portion. In particular, the hollow elongated structure has a base portion having a

substantially constant inner diameter of at least 5m, more in particular at least 7m. In particular, the hollow elongated structure has a top portion and/or transition portion having a length of at least 20 m, more in particular at least 30m. In particular, the hollow elongated structure has a base portion having a length of at least 20 m, more in particular at least 30m.

Preferably, the hollow elongated structure having a length of at least 20m is made of steel which advantageously allows the structure to be applied at deep waters having a depth of at least 20m. For a water body having a depth of at most 20m, the one-piece structure may be at least partially made of concrete.

In the method according to the invention a device is used which is dedicated to the improved hollow elongated structure with varying inner diameter. The device comprises a discharge unit, in particular a drill unit, which is extendable in radial direction, such that the device is able to pass through the through hole of the structure from the proximal to the distal end of the structure and is able to remove bottom parts over substantially the whole base diameter of the base portion.

The use of the improved hollow elongated structure provides an advantage in that it simplifies the installation of a foundation. The base portion of the structure which is also called a monopile is integral with the transition portion. In prior art structures, the structure is built up with several parts which requires a cemented connection. In the method according to the invention the cemented connection of the transition portion to the base portion is no longer required during an installation offshore. Herewith, not only the amount of steps to build the foundation are reduced, but also the quality of the connection of the transition portion to the base portion is increased.

The dedicated device which is used in the method according to the invention, allows a simple introduction of the structure into the water bottom. The device is lowered inside the structure from the proximal end to the distal end. The used device has outer dimensions which are adjustable such that the device is adjustable from a compact configuration to an extended configuration. The device may pass a smaller portion of the inner space of the structure in the compact configuration, while the device may operate in the extended configuration at the distal end. The extended configuration of the device may enable a clamping of the device to an inner wall portion of the inner space at the distal end. In particular, the outer dimensions of the device are adjustable from a first outer dimension in a first range of at least 3m to at most 5m to a second outer dimension in second range of at least 5m to at most 10m, more in particular from a first outer dimension in a first range of at least 3m to at most 4.5m to a second outer dimension in second range of at least 5m to at most 8m. Advantageously, the used device may further improve the introduction of structures which have a varying inner diameter. Additionally, the extended configuration of the device may enable an extended reach of a working head when removing bottom parts.

It is an important advantage that the reach of the release unit is extendible in radial direction. The outer dimensions of the submergible device can be kept small which allows the device to pass through smaller portions of an inner space of a structure. The

submergible device can be brought into a compact configuration to pass through a structure which has an inner space having a diameter which varies from about 4 to about 7 meters. After passing a smaller portion of an inner space, the reach of the release unit may be enlarged. After enlarging the reach of the release unit, bottom parts are released over a larger front surface of the water bottom.

In an embodiment according to the invention, the outer dimensions of the device are adjustable to and fro a compact configuration to change radial outer dimensions of the device, wherein a maximum radial outer dimension is variable from 100% in an extreme first configuration to at least 80%, in particular 60%, but preferably at least 50% in an extreme compact configuration. Herewith, the device may be adjustable to and fro outer dimensions from about 4 meters to about 7 meters. Advantageously, the device is suitable to introduce a foundation pile which has a tapered transition portion. It is no longer necessary to split up the implementation of a foundation pile, wherein in a first step a base portion with a constant diameter is introduced, after which a transition portion is mounted to the base portion. Now, it is possible to introduce a structure already including a base and transition portion.

In an embodiment of the method according to the invention, the device being used is a drill-dredge device which drill-dredge device comprises an open front drill head, such that during drilling released bottom parts freely pass the open front drill head. The device further comprises a dredge means for dredging the released bottom parts away to the proximal end of the structure. The drill-dredge device comprises a drill head and dredge means. The drill head is used to release bottom parts. The bottoms parts may be released by excavate members or by jet members which cut and flush the bottom parts from the water bottom. The dredge means of the device are positioned adjacent to the drill head. Released bottom parts are dredged and moved away from the distal end, through the inner space to the proximal end of the structure.

The drill head of the drill-dredge device may be of the type of an open front drill head. The drill head is designed such that during drilling released bottom parts freely pass the open front drill head. In contrast to a closed front drill head, the open front drill head provide hardly any resistance to released bottom parts to pass the drill head.

Advantageously, the released bottom parts can be transferred quickly towards the dredge means to quickly move the bottom parts out of the inner space. Advantageously, in particular in soft soil, the method including use of an open front drill head is operating much faster than a method using a closed front drill head. Advantageously, the use of the device including the open front drill head in the method according to the invention provides a quick method which is advantageously when installing large hollow structures, e.g. large foundation piles of a wind-or water turbine, wherein a large volume of bottom parts have to be released and removed. The method according to the invention is in particular suitable to be applied in a water bottom of a soft soil. Herewith, the method according to the invention is a valuable and competitive method in comparison with prior art methods.

In an embodiment according to the invention, the submergible device comprises a control platform, which control platform is mountable to a proximal end of the structure. The control platform may be provided with a winch to lower and lift the submergible device into the inner space of the structure. Further the control platform may comprise provisions to carry out the method of discharging bottom parts. Advantageously, the control platform which is installed at the proximal end of the structure provides a stationary location which is not influenced by swelling of the sea. Further the invention relates to a method for introducing a hollow elongated structure into a water bottom of a body of water.

In an embodiment of the method according to the invention, the hollow elongated structure which is being used is a prefab hollow elongated structure, in particular a foundation pile for supporting an off shore installation, wherein the structure has an inner space formed by a through hole which extends from a proximal end to a distal end, said structure comprises a base portion with a substantially constant inner base diameter and a transition portion having a varying inner diameter. In an embodiment, the prefab structure comprises at the proximal end a flange for mounting an off shore installation.

In particular, the offshore installation is a wind power installation. The wind power installation may have a rotor which has a diameter of at least 100m. The wind power installation may have a tower shaft which may have a height of at least 100m.

In an embodiment of the method according to the invention, the method provides an improvement in that during removing of the bottom parts, a minimum volume of bottom parts is maintained in the inner space of the hollow elongated structure. The minimum volume seals the distal open end of the structure during the step of removing bottom parts from the inner space.

Advantageously, the provided sealing of the maintained bottom parts at the distal end prevents an uncontrolled flow of bottom parts around the distal end into the inner space. The sealing of bottom parts provides a resistance which prevents a too fast flowing of bottom parts into the inner space. Large cavities around an introduced structure may be prevented which provide an improved stability of an installed foundation.

In an embodiment of the method according to the invention, the minimum volume of bottom parts forms a stopper which seals the distal end of the structure. The stopper fills the distal end of the structure and has substantially the same cross section as the inner space of the structure. Generally, the stopper had a cylindrical shape. The height of the stopper defines a flow resistance which prevents bottom parts surrounding the distal end of the structure to enter the inner space.

In an embodiment of the method according to the invention, the minimum volume of bottom parts to be maintained during carrying out steps of the method equals at least 50% in particular at least 70%, but preferably at least 90% of the volume of bottom parts which has entered the inner space when the structure was placed into the water bottom, the so called pitching step. Advantageously, by maintaining the minimum volume in dependence of an initial volume of bottom parts determined at the pitching step, it may be assured that the flow resistance at the distal end of the structure is sufficient large to control the lowering of the structure in a stable manner. The initial volume of bottom parts which has entered the inner space of the structure depends on the composition of the bottom parts. When the bottom parts comprise relatively more sand than clay, the initial volume of bottom parts in the inner space will be larger. It depends on soil properties of the predetermined location of what percentage has to be maintained to introduce the structure. The initial volume of bottom parts provide a good indication of the minimum volume to be maintained.

In an embodiment of the method according to the invention, the device for removing bottom parts is lowered into the inner space to a maximum extent. The maximum extent is limited by the distal end of the structure, such that during the removal of bottom parts the device does not pass the distal end of the structure. The device has at least one release member for removing bottom parts. The maximum extent of a lowering prevents the at least one release member to remove bottom parts from beneath the structure. Only bottom parts which are enclosed by the distal open end of the structure are possibly released and removed. The release member does not reach out of the inner space, which assures that a minimum volume of bottom parts is always present at the distal end of the inner space.

In an embodiment of the method according to the invention, a structure is provided which has an overcut at the distal end. The overcut causes an external annular space in the water bottom around the structure during lowering of the structure. To prevent a cavity around the outer surface of the structure, a hardening substance is introduced from the distal end into the annular space. Preferably, the hardening substance is a grout mixture.

In an embodiment according to the invention, the hardening substance comprises a retard ingredient. The retard ingredient decelerates the hardening process until the lowering of the structure is finished. The retard ingredient provides a delay of the hardening such that the grout mixture remains soft during the lowering of the structure. The grout mixture may remain soft for at least 1 hour, in particular at least 2 hours, more in particular at least 3 hours, but preferably at least 5 hours to fulfil the lowering of the structure into the seabed. The soft grout mixture advantageously provides lubricating properties during lowering of the structure. After installing the structure at a desired depth, the grout mixture will harden and create a fixture of the structure in the seabed. Advantageously, the hardening substance e.g. a grout mixture provides in a first instance lubrication means, and provides in a second instance after hardening a fixation means. Advantageously, this double function of the hardening substance makes it unnecessary to use different fluids.

By introducing the grout mixture from the distal end during lowering, it may be better assured that the grout mixture distributes equally in the annular space over the outer surface of the structure, which will assure a more stable fixture of the structure in the seabed during the lifetime of the structure. By using only one substance, it may be assured that the whole annular space is finally filled with fixation means.

In an embodiment of the method according to the invention, a structure is used which has an overcut at the distal end, which overcut causes an external annular space in the water bottom around the structure during lowering of the structure. Lubricating means, like bentonite, are supplied during lowering of the structure into the external occurring annular space to fill the annular space. The method may be improved by carrying out a step after lowering of the structure, wherein the lubricating means are pushed out the annular space by a supplied hardening substance which is supplied from the distal end of the structure. The hardening substance is equally distributed from the distal end and pressed upwardly. After hardening out, the hardening substance provide a fixation of the structure in the water bottom.

In particular, a ring element may be used which circumvents the structure. The ring element may be used to push the lubrication means out of the annular space. The ring element may have an outer diameter which is substantially equal to the diameter of the overcut of the structure. The lubricating means may be supplied to the annular space from outlets which are positioned above the ring element. To push the ring element through the annular space, the hardening substance may be supplied from outlets which are located under the ring element. Advantageously, the ring element may assure that the lubrication means are pushed away in a controlled manner and that the annular space heat subsequently filled with hardening substance to provide a stable fixation of the structure in the water bottom. The ring element may be used to obtain a proper separation of the lubricating means and the hardening substance. The ring element may assure that the hardening substance distributes equally over the outer surface of the structure. A hardening substance, in particular a grout mixture, may be supplied from the distal end of the structure at an underside of the ring element to push the ring element upwards through the annular space. The lubricating means are removed in that way and the annular space will be filled with the grout mixture. After hardening of the substance, the structure will be fixedly installed at the seabed in a stable manner.

Further, the invention relates to a use of a hollow elongated structure, in particular a foundation structure for a wind-or water turbine installation in a method, in particular the method according to the invention, for introducing the structure into a water bottom. The used structure has an inner space formed by a through hole which extends from a proximal end to a distal end of the structure. The structure comprises a base portion having a substantially constant base diameter e.g. of about 7m. Further the structure has a transition portion with a varying diameter, in particular a tapered diameter e.g tapered from about 4 to about 7m. In particular the varying inner diameter of the transition portion is decreasing from the distal end to the proximal end. The structure may further comprise a flange at the proximal end of the structure for mounting an offshore installation e.g. a wind turbine. The structure is introduced into the water bottom in one single piece as a whole. This means that the structure includes the base, transition and possibly also the flange portion when it is lowered into the water bottom. The method according to the invention provides

advantageously a stable introduction of structures such that such a large structure can be used and introduced as a whole. In prior art methods used structures only have a base portion with a constant diameter, in which the transition portion is connected later to the base portion after introducing the base portion in the water bottom. Advantageously, according to the invention the used structure is a prefab structure, wherein the transition portion is already included before lowering the structure into the water bottom. An important advantage of the prefab structure is that no longer a hardening substance is needed to mount the transition portion to the base portion. It had appeared that such grout connections which where used in prior art methods provide several stability problems. The life time of the grout connection has appeared to be limited which results in a lot of efforts to recover the grout connection. By using a prefab structure out of one piece, the grout connection problems are eliminated.

In an embodiment of the used structure, the prefab structure is completed with several features. The features may simplify or improve the carrying out of the method according to the invention or the mounting of an offshore installation. Features as a J-tube may be provided for a future electrical connection of a wind turbine at the top of the structure.

In an embodiment according to the invention, the used structure has an open proximal end. The distal end of the structure may be provided with an overcut. The overcut may have a cutting edge to improve the lowering of the structure into the water bottom. Preferably the distal end of the used structure is provided with at least one umbilical cable for supplying a fluid to an external annular space at the distal end. The umbilical cable comprises at least one outlet opening at an outer surface of the structure.

The prefab used structure may comprise a releasable ring element at the distal end which circumvents the used structure.

Further the invention relates to a prefab hollow elongated structure, in particular a foundation structure for an installation of a water-or wind turbine. The foundation structure has an inner space formed by a flue hole which extends from a proximal end to a distal end. Said structure comprises a base portion with a substantially constant base diameter and a transition portion having a varying inner diameter.

In an embodiment of the prefab structure according to the invention, the structure has a flange at the proximal end. The flange is suitable to mount an offshore installation.

Further preferred embodiments are defined in the subclaims.

The invention will be explained in more detail with reference to the appended drawings. The drawings show a practical embodiment according to the invention, which may not be interpreted as limiting the scope of the invention. Specific features may also be considered apart from the shown embodiment and may be taken into account in a broader context as a delimiting feature, not only for the shown embodiment but as a common feature for all embodiments falling within the scope of the appended claims, in which:

Fig. 1 shows in views l-V subsequent steps of the method according to the invention;

Fig. 2a shows in a cross sectional view a complete prefab foundation pile according to the invention, wherein the complete foundation pile is introduced into a water bottom;

Fig. 2b shows a view in more detail of Fig. 2A, wherein the used device is shown in a compact configuration;

Fig. 2c shows a view in more detail of Fig. 2A, wherein the used device is shown in an extended configuration. Fig. 1 shows subsequent steps of the method according to the invention. In a first step I a hollow elongated structure 1 is provided and supplied to a floating hoisting crane 30. The structure is dragged in a substantial horizontal position to the hoisting crane. The hoisting crane is positioned at two floating objects, wherein a free area is provided in between the two floating objects. During the first step I, the structure 1 is brought in upstanding position. The structure is connected to the hoisting crane which lifts the structure in upwards direction. The structure is a prefab structure, wherein hoisting elements are integrated at the outer surface of the structure. In particular, the structure is at least partially made out of concrete.

In a second step II the structure 1 is pitched into the water bottom. Due to its own weight, the structure lowers into the water bottom. As a result, bottom parts of the water bottom enter the open inner space of the structure. When the structure stops lowering, an initial volume of bottom parts has entered into the inner space.

In a third step III, the hoisting crane is used to place a device 20 onto the structure. The device has a control board and a submergible device. The control board is placed on top of the structure which may eliminate negative influences of tidal movements of the water body. The submergible device is lowered from the control board into the inner space of the structure until the submergible device reaches the distal end. In the third step III, the submergible device is depicted at three different positions in the inner space of the elongated structure. In a first position, the submergible device is at a start position and connected to the control board at a proximal end of the elongated structure. In a second position, the submergible device is lowered halfway down the inner space of the elongated structure. In the third position, the submergible device is lowered to the water bottom at the distal end of the elongated structure.

In a fourth step IV, the submergible device is used to release and discharge bottom parts out of the inner space. During operation, a minimum volume of bottom parts is maintained in the inner space. In particular, the submergible device does not pass the distal end of the structure, such that no bottom parts are released beneath the structure. The minimum volume of bottom parts provide a flow resistance which prevents an uncontrolled flow of bottom parts around the structure to enter the inner space.

In a fifth step V, the submergible device is lifted to the control board. Subsequently, the control board is demounted from the structure and the device is taken away from the structure. The hoisting crane can be moved to another location to restart the method for introducing a structure.

Fig. 2a shows a hollow elongated structure 1 which has been introduced into a water bottom 10. The elongated structure 1 is positioned in a water body 1 1 having a depth of at most 30 meters. The water bottom comprises soft soil bottom parts like sand and clay. The elongated structure is a foundation pile for a wind turbine. A tower shaft of a windmill can be positioned on top of the elongated structure. The elongated structure 1 has a base portion 2 and a transition portion 3. The base portion 2 has a constant diameter. The base portion 2 has an outer diameter of at least 5 to at most 7 meters. The base portion has been introduced into the water bottom over a distance of about 30 meters. The transition portion 3 provides a transition of a large diameter of the base portion to a smaller diameter at a top end of the foundation pile.

The hollow elongated structure 1 has an inner space which is formed by a through hole from a top end, the proximal end, to the distal end. Preferably, the elongated structure is made out of steel. The inner space of the elongated structure 1 has a varying diameter from the proximal end to the distal end. At the proximal end, the inner space has a diameter of about 4 meters and at the distal end the diameter of the inner space is about 5 to 7 meters.

The elongated structure 1 has further a top portion 4 which has a constant diameter. The top portion 4 has a diameter of about 4 meters. At the top end of the top portion 4 a flange 5 is provided for mounting an offshore installation like a wind turbine. The flange 5 is mounted at the top end of the foundation pile at about 20 meters above water level.

Further the elongated structure 1 is provided with several components which are used for mounting or operating the offshore installation. The components of the prefab structure may e.g. be a stair at an outer wall, conduit for guiding cables or hoses, hoisting elements etc. Herewith, a prefab structure is provided which is in an earlier stage prefabricated at another location away from the location of installation.

The prefabricated elongated structure 1 can be introduced into the water bottom 10 as a single piece. It is not necessary to mount the transition portion 3, top portion 4 and flange portion 5 in a later step. When introducing the structure, the base 2, the transition 3, top 4 and flange portion 5 are installed all together at the same time. The prefab structure has a length of at least 50m. The prefab structure has a base diameter of at least 5m. The prefab structure comprises a conduit, in particular a J-tube which extends in length direction of the structure from the proximal end for an electrical connection of a wind turbine at the proximal end of the structure. The prefab structure further comprises an umbilical cable at the outer surface of the structure for use during the operation of the device.

The elongated structure 1 is introduced into the water bottom 10 by using a suitable submergible device which is lowered in the inner space. The submergible device can be introduced in the inner space at the proximal end. In figure 2a the device is shown at a top region of the elongated structure 1 and at the distal end of the elongated structure 1. At the top region, the device is passing through the smaller part of the inner space. For that reason the device is kept into a compact configuration. At the distal end, the device is brought into an operational configuration, wherein the device may have a broader operational reach.

Figure 2b and figure 2c show the submergible device of figure 2a in more detail. Figure 2b shows the device in the compact configuration. In the compact

configuration, the device can pass smaller portions of the inner space. The device 201 can pass a smaller top portion of a structure which has an inner diameter of at most 4meters.

Figure 2c shows the device in an extended configuration at the distal end of the structure. In the extended configuration, the outer dimensions of the submergible device are increased. The device is adjusted from the compact to the extending configuration at the distal end of the structure. The device has clamping means to clamp the device in rotation about a central axis of the device. The clamping means may prevent the submergible device from rotation about the central axis as a result of reacting forces from a drilling process. In the extended configuration, the clamping means may clamp the device to an inner wall portion of the inner space of the structure.

Although the invention has been disclosed with reference to particular embodiments, from reading this description those of skilled in the art may appreciate changes and modification that may be possible from a technical point of view but which do not depart from the scope of the invention as described above and claimed hereafter. It will be understood by those of skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. Modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiments disclosed in the above detailed description, but that the invention will include all embodiments falling within the scope of the appended claims.

Thus, the invention provides a fast and stable method for introducing a hollow elongated structure with a varying inner diameter into a water bottom.

Claims

1 . Method for introducing a hollow elongated structure into a water bottom of a body of water, wherein the method comprises the steps of:

- providing a hollow elongated structure, in particular a hollow foundation pile of a wind- or waterturbine, wherein the structure has an inner space, a proximal end and an open distal end, which open distal end is to be introduced into the water bottom;

- supplying the structure to a predetermined location at the body of water;

- bringing the structure in an upstanding position;

- placing the open distal end of the structure into the water bottom such that bottom parts of the water bottom start entering the inner space of the structure;

- providing a device, in particular a drill-dredge device, for removing bottom parts out of the inner space;

- lowering the device into the inner space;

- removing bottom parts from the inner space out of the structure to lower the structure into the water bottom,

characterized in that

- the hollow elongated structure which is lowered into the water bottom has an inner space formed by a through hole which extends from a proximal end to a distal end, said structure comprises a base portion with a substantially constant base diameter and a transition portion having a varying diameter, wherein the structure as a whole including the transition portion is lowered into the water bottom;

- the device being used comprises a discharge unit which is extendable in radial direction, such that the device is able to pass through the through hole of the structure from the proximal to the distal end of the structure and is able to remove bottom parts over substantially the whole base diameter of the base portion.

2. Method according to claim 1 , wherein the hollow elongated structure which is being used is a prefab hollow elongated structure, in particular a foundation pile for supporting an off shore installation, in particular a wind power installation.

3. Method according to claim 1 or 2, wherein the transition portion has a tapered inner diameter which increases from at most 4 meters at the proximal end to at least 5 meters at the base portion.

4. Method according to claim 2 or 3, wherein the prefab structure comprises at the proximal end a flange for mounting an off shore installation.

5. Method according to any of the preceding claims, the radial outer dimensions of the device are adjustable to and fro a compact configuration to change radial outer dimensions of the device, wherein a maximum radial outer dimension is variable from 100% in an extreme first configuration to at least 80%, in particular 60%, but preferably at least 50% in an extreme compact configuration.

6. Method according to any of the preceding claims, wherein the device being used is a drill-dredge device which drill-dredge device comprises an open front drill head, such that during drilling released bottom parts freely pass the open front drill head, and further comprises a dredge means for dredging the released bottom parts away to the proximal end of the structure.

7. Method according to any of the preceding claims, wherein a minimum volume of bottom parts is maintained at the distal end in the inner space during removing of the bottom parts, such that the minimum volume provides a flow resistance in the distal open end of the structure to resist surrounding soil to enter the inner space of the structure during the removal of bottom parts.

8. Method according to claim 7, wherein the minimum volume of bottom parts forms a stopper which seals the distal end of the structure.

9. Method according to claim 7 or 8, wherein the minimum volume of bottom parts to be maintained during carrying out next steps of the method is determined in dependence of an initial volume of bottom parts which has entered the inner space during placing of the structure.

10. Method according to any of the claim 9, wherein the minimum volume of bottom parts equals at least 50%, in particular at least 70%, but preferably at least 90% of the initial volume of bottom parts.

1 1 . Method according to any of the claims 7-10, wherein the device is lowered into the inner space to a maximum extent which maximum extent is limited by the distal end of the structure, such that during the removal of bottom parts a release member for releasing bottom parts of the device does not pass the distal end of the structure to maintain the minimum volume of bottom parts in the inner space.

12. Method according to any of the claims 7-1 1 , wherein a structure is provided which has an overcut at the distal end, which overcut causes an external annular space in the water bottom around the structure during lowering of the structure, wherein during lowering of the structure a hardening substance is introduced from the distal end into the external

5 annular space which hardening substance comprises a retard ingredient which decelerates the hardening process until the lowering of the structure is finished.

13. Method according to any of the claims 7-12, wherein a structure is used which has an overcut at the distal end, which overcut causes an external annular space in the water

10 bottom around the structure during lowering of the structure, wherein lubricating means are supplied during lowering of the structure into the external occurring annular space to fill the annular space, wherein after lowering of the structure, the lubricating means are pushed out the annular space by a supplied hardening substance which is supplied from the distal end of the structure.

15

14. Method according to claim 13, wherein a ring element which circumvents the structure is used to push the lubricating means out of the annular space.

15. Method according to any of the claims 7-14, wherein the distal end of the structure is 20 provided with at least one umbilical cable for supplying a fluid to an external annular space at the distal end, wherein the umbilical cable comprises at least one outlet opening at an outer surface of the structure.

16. Prefab hollow elongated structure, in particular a foundation pile for supporting an off 25 shore installation, wherein the structure has an inner space formed by a through hole which extends from a proximal end to a distal end, said structure comprises a base portion with a substantially constant inner base diameter and a transition portion having a varying inner diameter.

30 17. Prefab hollow elongated structure according to claim 16, wherein the transition

portion has a tapered inner diameter which increases from at most 4 meters at the proximal end to at least 5 meters at the base portion.

18. Prefab structure according to claim 16 or 17 comprising at the proximal end a flange 35 for mounting an off shore installation.