A monopile for a wind turbine having an upper end and a lower end when positioned and comprising of a tubular lower part extending upwards from the lower end and provided with injection nozzles and wherein the monopile is provided with one or more openings at a higher elevation.

EP3464734B1 describes a monopile for a wind turbine wherein at the lower end of the tubular monopile a ring of moveable tips is present. By movement of these tips the soil is scraped away. Water may also be injected at the tip such to create a flow of water on the inside and outside of the walls of the monopile. These two measures make it possible to install the monopile with no or less use of a vibro hammer.

W02020/207903 describes a monopile for a wind turbine having a lower end which consists of a ring shaped element having outlets for waterjets directed upwards and downwards. The ring shaped element is further provided with vibration means.

W02019/206690 describes a monopile which is provided with nozzles directed under an angle of between 90 and 180 degrees with the insertion direction of the monopile. The publication states that by angling the nozzles upwards a movement of the soil within the monopile is enhanced. This results in that an inward bearing failure below the lower end, referred to as toe, is reduced. The toe has an inward tapered design to direct the soil to the lower opened end of the monopile. Pumping means are present to pump away the resulting suspension within the monopile to reduce vertical downward stresses which would otherwise increase the installation resistance. The pumping system may also evacuate air from a lower zone of the monopile. Because a lower zone of the monopile can be closed of from an upper zone a suction effect can be created to reduce the pressure of the suspension within the monopile.

WO2021/228510 is another examples of a monopile provided with nozzles at its lower end and wherein a pumping system is used to control a suspension pressure at the lower end of the monopile. Vibrohammers are known to drive foundation piles, such as monopiles, in an underwater bed as described in W003/100178. In this method a tubular foundation pile of a monopile penetrates the seabed using a vibration arrangement clamped to the upper end of the foundation pile. The vibration arrangement may weigh 40-50 tonnes and may be one as described in US5653556. A disadvantage of a vibrohammer is that too much noise is generated for the sea life and that the metal foundation pile may be damaged in terms of strain by the vibrohammer.

A problem with the state of the art monopiles is their complexity. For example the presence of the vibration means at the lower end of the monopile as described in EP3464734B1 and W02020/207903 result in a complex design. The monopile of W02019/206690 and WO2021/228510 are complex because of the pumping means. The object of this invention is to provide a more simple design and method to install a monopile on a seabed.

This is achieved with the following monopile for a wind turbine. A monopile for a wind turbine having an upper end and a lower end when positioned and comprising of a tubular lower part extending upwards from the lower end and wherein at the inner side of the tubular lower part injection nozzles are present, wherein the monopile is provided with one or more openings at a higher elevation and wherein the injection nozzles are positioned at a distance from the lower end and directed to inject a flow of water in at least a tangential and an axial direction towards the lower end.

Applicant found that when water is injected from a distance of the lower end and towards said lower end as claimed a swirl is created which enhances the insertion of the monopile in the soil of a seabed. By injecting the water from a distance from the lower end it is found that the majority or even all of the water remains in the monopile while simultaneously enhancing the penetration of the monopile into the soil. Escape of large volumes of water from the lower end to the surrounding soil is disadvantageous, especially in more sandy types of soil, because this water can negatively effect the structure of the soil surrounding the monopile. By keeping the water as injected within the monopile such an escape is avoided. Applicant found that no pumping means are required in order to achieve a desired penetration into the soil. Instead the excess water and soil is discharged via the openings which are present at a higher elevation. In this way a pressure of the suspension within the monopile will be similar to the soil pressure at the lower end. This avoids that water can flow to the soil or water can flow from the soil to the interior of the monopile. Having a lower suspension pressure as in the prior art methods has the disadvantage that too much soil enters the monopile and therefore negatively influencing the soil support of the monopile. Therefore the invention is also directed to a monopile wherein no pumping system is present for evacuating fluid from within the monopile. Thus a much simpler designed monopile is achieved. The simple design allows that the nozzles can remain in the installed monopile, thereby avoiding complex detachable structures like described in W02020/207903.

The invention is also directed to a method for installing a monopile for a wind turbine according to the present invention into a soil covered by a mass of water by positioning the lower end on the soil surface and by ejecting water from the injection nozzles in a direction having at least a tangential and an axial direction towards the lower end resulting in that the ejected water penetrates the soil at the lower end resulting in a soil suspension of water and soil particles which soil suspension spirals upwardly within the monopile towards the one or more openings where it is discharged to the mass of water.

In this application reference will be made to terms like upper, lower, above, below, horizontally and vertically. These terms are used to more clearly describe the monopile in its installed orientation in for example a seabed. These terms should not be used to limit the claims to a monopile only in this orientation. Especially monopiles stored and/or transported which when installed would be described by the claims are understood to be part of this invention.

In order to achieve a optimal penetration into the soil it has been found that the majority of the water, preferably more than 80 vol%, more preferably more than 90 vol% and most preferred all of the water is injected in a direction having at least a tangential and an axial direction towards the lower end. It has been found that there is no additional benefit of having injection nozzles directed horizontally to upwardly as in the prior art designs.

In addition the monopile preferably does not have nozzles which direct water radially away from the monopile and/or does not have nozzles which direct water upwardly along the outer wall surface of the lower tubular end. As explained above supply of amounts of water to the soil surrounding the tubular lower part may negatively effect the supporting structure of the soil and thus its capacity to fix the monopile in the soil.

Preferably the injection nozzles are directed to the lower end under an angle of between zero and 90 degrees with the horizontal and preferably between 30 and 60 degrees with the horizontal. The direction may also have a small inwardly or outwardly radial component.

The method for installing a monopile into a soil covered by a mass of water may be performed for installing a monopile on the seabed. The mass of water may also be a lake. The depth of the water, i.e. the height of the mass of water is preferably between 5 and 100 M and more preferably between 10 and 50 m. When the height of the mass of water is lower than 5 m, preferably lower than 10 m, the column of suspension created within the monopile may exert a too high pressure to the soil as present at the lower end of the monopile when installing. The method may then include pumping the suspension from this interior. The soil may be sand silt, soft clay, very stiff clay or Boom clay or any combination of aforementioned. The monopile and method is especially suited for installation in sand.

The monopile for a wind turbine is suitably the part of the wind turbine which just extends above the water level. By just extending is suitable meant between 5 and 20 m extending above the water level when installed. On top of the monopile a mast is typically mounted and a wind turbine generator. The distance of penetration of the monopile in the soil is suitably between 20 and 50 m and preferably between 25 and 40 m.

It has been found that the optimal distance at which the nozzles are positioned above the lower end relates to the type of soil and the ability of a water jet exiting from a nozzle to penetrate the soil. Optimally the water jets penetrate the soil to the level of the lower end of the monopile. In this way an optimal disturbance and soil suspension formation is achieved in this region to enhance the penetration of the monopile into the soil while avoiding that significant amounts of water can escape to the surroundings of the monopile as explained above. This distance will be higher for easy to penetrate soil types such as sand and lower for difficult to penetrate soil types, like clay. The distance will also depend on the diameter of the lower end part of the monopile. The distance may be higher for larger monopiles because more water may have to be injected resulting in a higher penetration length. Further the angle at which the water is injected will influence this distance. When the direction is near zero degrees with the horizontal a relatively small distance will be chosen while when the direction becomes more downward a larger distance will be chosen. The distance can be easily determined by measuring a single penetration depth of a jet exiting a nozzle under design conditions in a soil type for which the monopile is to be designed. The distance will then be a function of the angle and the measured penetration depth. The monopile is thus preferably designed for a specific type of soil in which it is to be positioned wherein the distance between the nozzles and the lower end is determined by the type of soil, the direction of the nozzles and the diameter of the lower end part of the monopile.

In the method of this invention it is thus preferred that the water which is ejected does not substantially penetrate the soil to a level below the lower end of the monopile. Further it is preferred that more than 90 vol% of the water ejected from the injection nozzles is discharged via the one or more openings to the mass of water.

Preferably the pressure of the water as supplied to the water injection nozzles is at least 5 bar, and preferably between 20 and 300 bar. The pressure may depend on the type of soil. Suitably this pressure is between 5 and 50 bar for sand type of soil and between 50 and 300 bar for clay type of soil. The water pressure for mixtures of sand and clay may overlap these preferred ranges.

The amount of water as ejected at the lower end part of the monopile will suitably depend on the soil type and dimensions of the monopile. For sand type of soils a high flow and low water pressure is preferred while for clay type soils a low flow and high pressure is preferred. This results in that for a sand type of soil substantially all of the soil within the monopile is present as a volume of fluidised soil, ie a suspension. For a clay type of soil a suspension will be present in an annulus against the inner wall and a more dense inner core of clay will be present in the centre of the monopile. To give an indication for a monopile having a diameter of the lower end part of between 6 and 15 meter a flow of water is preferably between 10000 and 100000 l/min. For a 8 meter diameter monopile this flow may be between 10000 and 30000 l/min.

For the preferred soil type sand the distance at which the nozzles are positioned above the lower end is suitably at least 0.01 m, preferably 0.1 m and even more preferably 0.25 m. The optimal distance suitably increases for monopiles having a greater diameter. This because the required higher water flow will be higher than the increase in available water nozzles. Thus per nozzle more water will flow resulting in a larger penetration length and thus in a larger distance at which the nozzles are preferably positioned above the lower end. The distance at which the nozzles are preferably positioned above the lower end will suitably not exceed 5 m, preferably not exceed 3 m and more preferably not exceed 2 m for the larger diameter monopiles. The optimal distance will further be influenced by the penetration length for the specific soil type or types and the angle of the nozzles as described above. This may result in that per project the design of the monopile may differ. The distance is defined as the axial distance between the lower end and the outlet openings for the water of the nozzles.

The nozzles may be positioned in any manner at the inner side of the tubular lower part. Preferably the nozzles are positioned in a circle such that the distance of each nozzle to the lower end is about the same. Optionally two or more of such circular rows of nozzles may be positioned above each other. Preferably the nozzles are fluidly connected to a circular conduit connected to the internal wall of the tubular lower part. Such conduit functions as a common header for the multiple nozzles. The circular conduit is fluidly connected to one or more fluid supply conduits which run upwardly. The circular conduit may have any shape and cross-section, for example a circular, square or triangular cross- section. Preferably the conduit does not have a large dimension enabling that the remaining opening is large enough for the upwardly moving soil suspension.

The circular conduit is preferably welded to the internal wall of the tubular lower part. The supply conduits may be high pressure conduits which are suspended from an elevated location, preferably an upper end of the monopile. These conduits may run free of the wall of the monopile. Preferably one or more fluid supply conduits are welded to the internal wall of the tubular lower part.

The nozzles are preferably high pressure injection nozzles suitably connected to pumps via high pressure water conduits. These pumps are preferably positioned externally from the foundation pile, for example on a floating vessel. The high pressure conduits may for example run upwards to the upper end of the monopile where they are connected to the welded supply conduits.

The monopile preferably has a lower zone which when positioned in a mass of soil is surrounded by said mass of soil and an upper zone which extends above the mass of soil. The one or more openings are present in the upper zone. These openings may be an opening or openings which, when the monopile is installed, are also used to pass power cables and other connections required for the wind turbine. In order for the suspension of water and soil to flow from the lower end of the monopile to these openings a fluid connection is present between lower end and these one or more openings.

The upper zone may be comprised of a tubular part having a smaller diameter that the tubular part of the lower zone. A transition part having a frusto-conical shape may then be positioned below the tubular part having the smaller diameter. Such a shape for a monopile for a wind turbine is well known.

The monopile has a lower zone and wherein the lower zone preferably has a cylindrical and flush outer surface and a cylindrical and flush inner surface. This simplifies the design. It has been found that the penetration of the monopile according to this invention does not require the specific designed toe as in the prior art publications EP3464734B1, W02020/207903 and W02019/206690.

In the method of this invention it is preferred to also use a vibrohammer at the upper end of the monopile. Applicants found that by providing the water jets at the lower part of the monopile according to this invention and a vibration to the upper end of the monopile a favourable penetration of the monopile is achieved in many types of soil. The water jets achieve a 70% strain reduction as compared to when only a vibrohammer would be used to drive the monopile into the soil. The reduction of strain is also a measure of noise reduction. It is believed that the waterjets achieve a significant reduction or even removal of the friction at the inner side of the monopile. The vibrohammer is suitably present to reduce the friction between the soil and the outer surface of the monopile and the soil resistance underneath the tip of the pile wall.

The vibrohammer may serve as the connection between the up flow end of the supply conduits and the high pressure conduits which run from the pumps positioned externally from the foundation pile, for example on a floating vessel or a jacked up vessel. Preferably such a vibrohammer and the connected high pressure conduits may be positioned on the upper end of the monopile resulting in a fluid connection between high pressure conduits and the supply conduits within the monopile. Also possible to have supply conduits run directly through one of the holes in the upper part of the pile.

The invention will be illustrated by the following Figures 1-3.

Figure 1 shows a cross-sectional view of a monopile for a wind turbine according to this invention. The monopile (1) has an upper end (2) and a lower end (3) when positioned. A tubular lower part (4) extends upwards from the lower end (3). At the inner side (5) of the tubular lower part (4) injection nozzles (6) are present. The monopile (1) is provided with an opening (7) at a higher elevation (8) in an upper zone (9). The injection nozzles (6) are positioned at a distance (10) from the lower end (3) and directed to inject a flow of water in at least a tangential and an axial direction to wards the lower end (3) as shown. The shown nozzles (6) are directed to the lower end (3) under an angle of 45 degrees with the horizontal (11). The lower tubular part (4) has a cylindrical and flush outer surface (12) extending from the lower end (3). Also the inner side (5) of the lower tubular part (4) is a cylindrical and flush surface apart from a circular shaped conduit (13) from which the nozzles (6) extend. Circular shaped conduit (13) is welded to inner side (5) of the lower tubular part (4). Circular shaped conduit (13) in Figure 1 has a triangular cross-section and acts as a header for supply of water to nozzles (6). The water is supplied by a fluid supply conduit (14) welded to inner wall (5). No pumping system is present for evacuating fluid from within the monopile.

Figure 2 shows a monopile (1) according to the invention installed on the seabed (15). The upper zone (9) of the monopile (1) is comprised of a tubular part (19) having a smaller diameter that the tubular part (20) of a lower zone (21) and a transition part (22) having a frusto-conical shape positioned below the tubular part (19) having the smaller diameter. A vibrohammer (23) is attached to the upper end (2) of the monopile (1). Pumps (24) are positioned externally from the monopile pile (1) on a floating vessel (25). High pressure conduits (26) transport high pressure water to the upper end (2) of the monopile (1) where they are connected to the welded supply conduits (14). Further hydraulic pressure conduits (27) are shown to operate the vibrohammer (23).

The monopile (1) is being installed in Figure 2 in a soil situated below a mass of water (18) consisting of a layer of sand (29), a clay layer (30) and a deeper layer of sand (31). At the seabed a scour protection (32) is present to avoid sand around the monopile to be washed away by local currents. At the inner side (5) of the tubular lower part (4) injection nozzles (6) are present which inject water into the soil at an injection distance which does not extend beyond the lower end (3) of the monopile as illustrated by the arrow and cross. In this way substantially all of the injected water remains within the monopile to form a suspension (28) consisting of sand and clay and water and having a suspension water level (16) which is just above the water level of the mass of water (18). Because of this difference in water level the suspension (28) will be continuously discharged via opening (7) which is located below the water level (17) of the mass of water (18). Figure 3 shows the experimental results of laboratory tests for the penetration speed of an experimental monopile according to Figure 1 in a sandy soil type bed. The length of the monopile was 2 m.

Version VI was provided with a ring of injection nozzles at a distance of 25 cm from the lower end of the monopile. The ring was provided with injection nozzles directed vertically downwards and upwards. The upwards directed nozzles had a tangential direction.

Version V2 was as version VI except that all the nozzles are directed to inject a flow of water in at least a tangential and an axial direction to wards the lower end under an angle of 30 degrees with the horizontal. Version V3 was as version V2 except that all the nozzles are directed to inject a flow of water under an angle of 45 degrees with the horizontal.

As can be seen in Figure 3 the penetration speed increases when all of the water is injected in at least a tangential and an axial direction to wards the lower end. The best results were achieved when the angle was 45 degrees. No vibrohammer was used such to measure only the effect of the water jetting. It may be appreciated that when a vibrohammer would be used the penetration speed would be significantly higher.