The invention relates to a pile-driver, and more particularly to a pile-driver suitable for offshore operations.

In addition, the invention relates to a method for driving a pile downward into the ground using such a pile-driver.

Drawbacks of existing pile-drivers, particularly for offshore pile-driving, lie in the fact that such pile-drivers are very heavy structures. In offshore applications they are operated by large vessels with heavy cranes provided thereon. The piles are driven one by one into the ground.

The pile-driving itself usually takes place by dropping a ram forming part of a pile hammer onto the pile from some height via a striker plate. Typical properties of such a pile hammer for striking a monopile for the purpose of offshore wind turbines are a length of about 20 m and a mass of about 400 tons (with a ram of 200 tons), as well as an associated striker plate of about 200 tons. Piles are becoming increasingly larger in diameter and length, and piles of 1300 tons and having a base diameter of 8 m are currently already being driven. The impact of the falling ram drives a pile into the ground but is accompanied by a considerable noise production. This noise production is particularly undesirable in offshore operations, since sound carries very far in water and may thereby disrupt marine life a great distance away from the pile-driving location.

Proposed in NL 2008169 and the corresponding international application WO-A1- 2015/009144 of Applicant is a pile-driver wherein a flexible member enclosing a combustion space of variable volume is arranged above and close to the support member. A drawback of applying a flexible member is that this member can tear during use and is difficult to access for replacement purposes.

Proposed in NL 2011166 of Applicant in order to solve the problem of tearing of the flexible member is an improved pile-driver wherein the flexible member is replaced by one, or more than one pressure build-up chamber, comprising an upper part enclosing a combustion space and a lower part provided with one or more passage openings which are in fluid connection with a liquid chamber. The pressure build-up chamber preferably forms a rigid housing. NL 2011166 is prior art, and discloses a pile-driver according to the preamble of claim 1.

A similar pile-driver is known from NL 2008169 and the corresponding European application EP-AO-2 807 307, which is deemed the closest prior art. At least the measures of the characterizing part of claim 1 are new with respect to EP-AO-2 807 307.

Although the pile-drivers as described in NL 2008169 and NL 2011166 were already suitable for driving conventional monopiles into a seabed, there remains an increasing need for more powerful pile-drivers. This need is driven mainly by the growing tendency to develop monopiles for ever greater water depths. Conventional monopiles have been found to be particularly suitable for applications at a limited water depth, for instance 25 metres. There is however an ongoing trend to also develop monopiles for greater water depths, for instance water depths of 30-60 metres, i.e. a range in which so-called jackets are in wide use at the moment. Jackets are however framework constructions and thereby much more complex, whereby they lack many of the above stated advantages of monopiles.

For the application of monopiles in the range of 30-60 metres water depth so-called XL monopiles are being developed. These XL monopiles have a greater top diameter of up to 7 metres, while they have an even greater diameter close to the seabed, whereby the power required for pile- driving is also greater than in the case of conventional monopiles.

An object of the present invention is to provide a pile-driver and method for application thereof wherein the stated drawbacks do not occur, or at least do so to lesser extent.

Said objective is achieved with the pile-driver according to the invention, comprising:

- a support member arranged or arrangeable in transverse direction at or on a pile;

- a liquid chamber which is bounded on the underside by the support member and which further comprises one or more than one side wall and is configured to receive a liquid therein;

- one or more than one pressure build-up chamber, comprising:

- an upper part forming a combustion space;

- a lower part which is in fluid connection with the liquid chamber;

- an ignition mechanism configured to ignite a fuel present in the combustion space;

- wherein the combustion space is configured to expand fuel present therein during combustion such that a pressure build-up takes place above the support member and the liquid present above the support member in the liquid chamber is displaced at least in upward direction away from the support member, and a downward force is exerted on the pile via the support member;

- a fuel supply channel for carrying fuel into the combustion space;

- a combustion product discharge channel for discharging combustion products after combustion;

- wherein the one or more than one pressure build-up chamber is arranged close to the side wall of the liquid chamber; and

- wherein the combustion product discharge channel is in fluid connection with a passage opening arranged in the side wall.

The operating principle is based on Newton's first and third laws: "action = reaction". In other words: when an object A exerts a force on an object B, this force is accompanied by an equal but opposite force of B on A. During expansion the combustion space (object A) exerts a force on the medium located thereabove (reaction mass B). According to Newton's third law, the reaction mass (B) exerts an equal but opposite (so downward) force on the combustion space (A). Because the combustion space is located above and close to the support member, the reaction force exerted by the reaction mass on the combustion space will be transmitted via the support member to the pile. The pile hereby undergoes a downward force via the support member, which is utilized according to the invention for the purpose of driving the pile downward into the ground.

In addition, the medium displaced upward during expansion as a consequence of the combustion will drop downward again and transmit an impulse to the support member, where it once again exerts a downward force on the pile via the support member. This operating principle corresponds to the operation of conventional pile-drivers, wherein a ram is dropped from some height onto the pile.

Because of the measures that the one or more than one pressure build-up chamber is arranged close to the side wall of the liquid chamber, and that the combustion product discharge channel is in fluid connection with a passage opening arranged in the side wall, the possibility arises of discharging the combustion products formed by the combustion very quickly through the side wall of the liquid chamber to the surrounding area. Use can be made here of the pressure in the liquid chamber, which is higher than the atmospheric pressure of the surrounding area.

It is hereby possible to discharge the combustion products before the medium which is displaced upward during the expansion has dropped down again and transmits an impulse to the support member. Because the combustion gases are for at least the most part discharged before the transmission of impulse between the falling medium and the support member, the gas spring effect of the combustion gases is reduced or possibly even prevented. As a consequence of the gas spring effect, wherein the falling medium compresses the combustion gases, the impact of the falling medium on the support member would after all decrease.

During initial tests about twice the pressure peak was achieved with early discharge of the combustion gases with the construction according to the invention.

Preferred embodiments of the invention form the subject-matter of the dependent claims.

Preferred embodiments of the present invention are further elucidated in the following description with reference to the drawing, in which:

Figure 1 shows a view of an offshore wind turbine on a monopile construction;

Figure 2 shows a perspective view of a pile-driver according to the invention;

Figures 3 and 4 show detailed cross-sectional views of a combustion product discharge channel for discharging combustion products in a closed state (figure 3) and open state (figure 4);

Figures 5-9 show cross-sectional views of successive stages during use of the pile-driver according to figure 2.

The offshore wind turbine 36 shown in figure 1 is a so-called monopile construction and comprises a pile 2 which is driven fixedly into the ground 40, formed by the seabed, below water level 38. According to the invention this pile-driving is carried out with a pile-driver 1 as shown in figure 2. Pile-driver 1 comprises a support member 8 arranged or arrangeable in transverse direction at or on a pile 2, and a liquid chamber 5. Liquid chamber 5 is configured to receive a liquid therein and is bounded on the underside by support member 8 and further comprises one or more than one side wall 4. Pile-driver 1 further comprises one or more than one pressure build-up chamber 14, comprising an upper part 20 which forms a combustion space 22 and a lower part 24 which is in fluid connection with liquid chamber 5. An ignition mechanism 28 is configured to ignite a fuel present in combustion space 22, wherein combustion space 22 is configured to expand fuel present therein during combustion such that a pressure build-up takes place above support member 8 and the liquid present above support member 8 in liquid chamber 5 is displaced at least in upward direction away from support member 8, and a downward force is exerted on pile 2 via support member 8. Pile-driver 1 is further provided with a fuel supply channel 30 for carrying fuel into the combustion space, and a combustion product discharge channel 32 for discharging combustion products after combustion. The one or more than one pressure build-up chamber 14 of pile-driver 1 is arranged close to side wall 4 of liquid chamber 5, and combustion product discharge channel 32 is in fluid connection with a passage opening 6 arranged in side wall 4. An air supply channel 31 can further be provided, although it is also possible to envisage a combustible mixture of fuel and air being supplied to combustion space 22 through a fuel (mixture) supply channel 30.

Pressure build-up chamber 14 is formed by a one or more than one plate part 15 extending inward from the one or more than one side wall 4 into liquid chamber 5. In the embodiment of figure 2 pressure build-up chamber 14 is formed by an annular plate part 15 arranged around an inner periphery of side wall 4. Plate part 15 extends in the direction of support member 8 and thereby forms on the one hand a combustion space 22 and on the other a guide which deflects the downward falling medium in the direction of support member 8.

Passage opening 6 is arranged in side wall 4 between plate part 15 and support member 8 in order to enable discharge of the combustion products, i.e. combustion gases 33, from combustion space 22.

Combustion product discharge channel 32 is in fluid connection with this passage opening 6 arranged in side wall 4, and is further provided with a shut-off valve 35 which can be operated selectively by a controller 34. In figure 3 shut-off valve 35 of closing unit 27 is situated in a first state in which liquid chamber 5 is closed off from the surrounding area, while figure 4 shows a second state in which combustion products can be discharged via the shown arrow to the surrounding area through combustion product discharge channel 32.

Figure 5 shows a starting situation wherein a pile-driver 1 is arranged on a pile 2 and is filled with a medium, typically water. The connection between pile-driver 1 and pile 2 takes place in the shown embodiment with a sleeve 9. A fuel is then introduced via a fuel supply channel 30 into combustion space 22 (figure 6). The fuel spreads through combustion space 22 and is then ignited with an ignition mechanism 28.

During an initial test a 22 metre water column 42 was raised about 1.5 metres as a result of the combustion. The medium is then in raised state for about 1 second (figures 7 and 8). Controller 34 of closing unit 27 then opens shut-off valve 35 briefly: opening and closing each take about 0.1 second, and between the two actions shut-off valve 35 can be held open a further 0.5-0.6 second (figure 4). The overpressure of the combustion product, optionally assisted by the already downward moving medium, presses the combustion product to the surrounding atmosphere outside the pile-driver (figure 8). The medium can then fall onto the support member with greatly reduced 'gas spring effect' of a possible residue of combustion products, wherein it drives pile 2 further into ground 40 (figure 9).

Side wall 4 separates the liquid in liquid chamber 5 from the surrounding area, wherein the liquid present in liquid chamber 5 above support member 8 is preferably water.

It is noted that a plurality of fuel supply channels 30, combustion gas discharge channels

32 and/or a plurality of ignition mechanisms 28 can be provided around the peripery of pile-driver 1. It is possible here to envisage a greater number of combustion gas discharge channels 32 being provided than the number of arranged fuel supply channels 30, since the time available for discharging combustion gases is limited. Five times as many combustion gas discharge channels 32 as fuel supply channels 30 can for instance thus be provided, for instance twenty combustion gas discharge channels 32 compared to four fuel supply channels 30.

Although not shown, the pile-driver can be integrated into pile 2, which forms with its side wall a side wall 4 of liquid chamber 5.

Although it shows a preferred embodiment of the invention, the above described embodiment is intended solely to illustrate the present invention and not to limit the scope of the invention in any way. When measures in the claims are followed by reference numerals, such reference numerals serve only to contribute toward understanding of the claims, but are in no way limitative of the scope of protection. The described rights are defined by the following claims, within the scope of which many modifications can be envisaged.