The invention relates to a pile driver system for installing or removing (decommissioning) foundation

elements, such as piles, anchors, and conductors, in a subsea ground formation, comprising a closed housing

defining a cavity, an impact weight accommodated inside the housing, i.e. in the cavity and enveloped by the housing, and a hydraulic circuit for reciprocating the impact weight. The circuit comprises one or more valves, a cylinder, and a piston accommodated in the cylinder and connected to the impact weight, e.g. forming an integral whole with the impact weight or rigidly connected to the impact weight. The system further comprises a pump for withdrawing water from the surroundings of the pile driver and providing

pressurized water to the hydraulic circuit. The invention further relates to a method of installing or removing foundation elements.

EP 675 233 relates to a hydraulic pile driver comprises a casing (denoted by numeral 1 in the Figures of EP 675 233) in which is mounted a striker (2) with an anvil block (3), a hydraulic power cylinder (4) whose rod (5) is connected with the striker (2), and a hydraulic distributor (12). "The proposed device is ecologically clean since used as a working fluid is water, sea water inclusive, but not mineral oil which is typical for the state-of-the art device."

A "patent proposal" available on the internet under the name Lerch Sea Water Hammer relates to a pile driver that "uses seawater as its hydraulic fluid, and having given up its energy, it is disposed of by returning it to the ocean surrounding the hammer. Thereby, eliminating the need for two power transmission hoses between the pump and Pile Driver. ... Further to enable this Pile Driver to work more efficiently an air environment is maintained within the Pile Driver Casing."

In the underwater pile driving apparatus according to US 4,367,800, the hammer is movable upwards and downwards in a housing which, in operation, is filled with a liquid which is present both above and below the hammer, the hammer being driven at least on the upwards direction by a driving liquid which is pressurized by a motor driven pump located on or adjacent the housing and which is the same as the liquid in which the hammer moves. "Conveniently the liquid with which the housing is filled is the ambient water in which the apparatus is submerged."

It is an object of the present invention to provide an improved hydraulic pile driver which uses water as working fluid.

To this end, the exhaust of the hydraulic circuit communicates with the cavity and one or more outlets are located in the wall of the housing. In an embodiment, the outlet is located near or in the bottom of the housing.

By returning the pressurized water to the surroundings via the cavity, preferably via the entire length of the cavity, ingress of water and dirt directly from the surroundings into the cavity, e.g. as a result of pumping action generated by the reciprocating impact weight, is reduced or prevented.

In an embodiment, the pile driver comprises an upper bearing and/or a lower bearing for guiding an upper and/or lower part of the impact weight and an outlet is located below the bearing (s), preferably below the lowest bearing. By locating the outlet (s) or at least some outlets for return water below the lowest bearing, (part of) the return water is directed through the bearing, and the bearing is continually flushed and/or lubricated with return water . In a further embodiment, the pile driver system comprises a filter located upstream from the valves, preferably upstream from the pump. The filter preferably removes particles in excess of 50 pm, preferably in excess of 25 pm, and more preferably in excess of 10 pm, i.e.

removes at least 90% of particles having an effective diameter larger than the limits specified. Filtering the driving water enables tighter tolerances in the components downstream, in particular in relatively delicate components, such as the pilot valves that are often employed to control the main hydraulic valves. This in turn enables more

efficient operation of the driver, i.e. at an the efficiency close or equal to that of pile drivers using oil as an hydraulic medium.

Typically, the pump(s) and filter are located on deck of a surface vessel. However, if the pump(s) and filter are located near or even on or in the submerged pile driver, a supply line for the pressurized water is no longer

required, simplifying e.g. the umbilical.

In an embodiment, the impact weight is driven upwards and then downwards by the pressurized water. In such an embodiment, the one or more valves are arranged to alternately direct pressurised water to opposite sides of the piston, in particular to the so-called lifting and acceleration surfaces of the piston. Such pile drivers can be operated with a smaller pump, compared to pile drivers having a hydraulic circuit comprising a gas spring.

The invention further relates to a method of installing or removing foundation elements, such as piles, anchors, and conductors, in a sub-sea ground formation, comprising the steps of mounting the pile driver on a foundation element, filling, e.g. flooding the cavity with water, driving the foundation element into respectively out of the ground formation by alternately lifting and

accelerating the impact weight respectively away from and towards the element using water taken from the surroundings as an hydraulic medium, and returning the water to the surroundings via the cavity. In an embodiment, at least part of the water is returned to the surroundings via the bottom of the housing, the water thus flowing around the impact weight and preferably through the upper and lower bearings.

For the sake of completeness, attention is drawn to the following background art.

GB 2 078 148 relates to a drop hammer apparatus, wherein a hammer (E) is interconnected with a piston (B) by means of a piston rod. An upright cylinder (A) is open at its upper end, the piston is slidable within the cylinder and the piston rod is slidable through the lower end of the cylinder. The space within the cylinder below the piston is selectively connected to a source (C) of pressurized liquid e.g. water and exhausted by means of a valve (D) . In a totally submerged situation the surrounding water is used as the hydraulic fluid and is pumped into the cylinder and discharged back into the surrounding water by the valve D.

GB 2 069 902 relates to a submersible hammer (21) for driving piles comprising a piston (36) and cylinder (35) assembly provided in conjunction with a ram (30) to move the same upwardly when the piston is lifted. Sea water is supplied as power medium at a pressure in excess of the ambient pressure. An exhaust valve (51) vents the sea water allowing the piston and ram to fall until the ram impacts the upper end of a pile to drive the same into the sea bed.

US 4,089,165 relates to a water pressure-powered pile driving hammer. The piston of the pile driving hammer is raised by hydraulic (water) pressure. Other prior art relating to underwater pile driving includes EP 301 114, EP 301 116 and US 4,043,405. US 5,662,175 relates to a pile hammer utilizing either sea or fresh water as an operating fluid and comprising a hollow (external) ram. US 4,601,349 relates to a hydraulic pile driver including a housing (1) having an impact weight (2) mounted therein for reciprocating movement and a cylinder (8), which is open at the upper side and communicates with a gas filled chamber (9), surrounding this cylinder (8). "In the

embodiment of FIG. 2 the chamber 9 in the housing 1 through a connecting passage 31 communicates with the chamber 32 in the housing 1, containing the impact weight 2. Thereby the volume of the second pressure medium is considerably

enlarged, which is of importance for applying an

acceleration force as constant as possible on the impact weight 2. "

US 4,333,492 discloses a liquid inertia tool in which the kinetic energy of flowing liquid is converted to useful work by decelerating the liquid flow and directing the inertia force in the water against a movable element in the liquid flow-containing structure which in turn moves against a work piece.

The invention will now be explained in more detail with reference to the Figure. It is noted that the Figure is schematic in nature and that details, which are not

necessary for understanding the present invention, may have been omitted.

The Figure shows an embodiment of a pile driver 1 according to the present invention for installing or

removing foundation elements, such as piles, anchors, and conductors, in a subsea ground formation, which comprises a closed housing 2 defining a cavity 3, an impact weight 4 accommodated inside the housing 2, i.e. in the cavity and enveloped by the housing, and slidingly mounted in upper and lower bearings 5,6, a hydraulic circuit for reciprocating the impact weight 4, and a pump unit for withdrawing and filtering water from the sea and providing pressurized water to the hydraulic circuit. In this example, the hydraulic circuit comprises two valves 7,8, a hydraulic cylinder 9, and a piston 10 accommodated in the cylinder and connected to the impact weight by means of a piston rod 10A guided in the upper bearing 5. It is generally preferred that the piston and piston rod form an integral part of the impact weight. The hydraulic cylinder is provided with openings 11 in its wall near its upper end, through which the cylinder communicates with one or more chambers, in this example an annular chamber 12 completely surrounding the cylinder.

A first conduit 13 directly connects the pump unit to the space in the hydraulic cylinder 9 beneath the piston 10, to supply pressurized water to the lower or lifting surface of the piston and to a high pressure accumulator 14 accommodated in the chamber 12. The accumulator, known in itself, suppresses extreme variations in pressure in the hydraulic circuit.

A second conduit 15 connects the pump, via the first or supply pressure valve 7 to the space in the hydraulic cylinder 9 above the piston 10 or, in this example, a lumen inside the piston and piston rod to supply pressurized water to the (effective) upper surface of the piston, i.e. the cross-sectional area of the lumen. The (effective) upper surface of the piston is larger than lower surface of piston - which surface, incidentally, is defined by an annular notch 16 at the transition between the piston 10 and the piston rod 10A -, to generate a net downwards force when the pressure above the piston is equal to that beneath the piston.

A third conduit 17 connects the second conduit 15, via the second or return pressure valve 8 to the annular chamber 12 surrounding the hydraulic cylinder 9.

The annular chamber 12 communicates with the cavity 3 in the housing 2 accommodating the impact weight 4 through one or more passages 18 extending e.g. parallel to the upper bearing 5. Further passages 19 connect the cavity with the space beneath the lower bearing 6, which space, in this example, accommodates a shock absorber pack 20 and a shock plate 21, both known in themselves. In embodiments

comprising one or more bearings and passages arranged in parallel with the bearing (s), the flow through the

bearing (s) can be set and adjusted by means of throttles, e.g. throttle valves positioned in the bearing(s).

The lower end of the pile driver is provided with a sleeve 22 with which the driver 1 is mounted on a foundation element, in this case a monopile 23, with an anvil 24 interposed between the two. Proximity sensors 25 are positioned on the inner wall of the cavity 3 to establish position and speed of the impact weight 4. Finally, the driver comprises a plurality of exhaust openings 26, located in the shock plate, to return the driving water to the surroundings, in this case to the sleeve.

The pump unit is located on deck of a surface vessel and comprises a high pressure positive displacement pump 27, a filter 28 located upstream from the high pressure pump, and a low pressure feed pump 29 upstream from the filter. The filter comprises three stages, viz. an automatic filter removing particles in excess of 50 ym, an

intermediate filter removing particles in excess of 25 μπι, and an bag filter removing particles in excess of 10 μιη.

When the pile driver 1 is submerged, flooded and mounted on a pile 23, it is operated as follows. Initially, both valves 7,8 are open and the pumps 27,29 are started. Thus, water is withdrawn from the surroundings, filtered and pumped through the supply pressure valve 7, the annular chamber 12, the upper bearing 5 and the passages 18

connecting the annular chamber to the cavity 3, past the impact weight 4, and through the lower bearing 6, the corresponding passages 19, and the exhaust openings 26 to the surroundings. Both sides of the piston 10 are exposed to the pressure in the circuit, maintaining the impact weight in its lowermost position.

When the supply pressure valve 7 is closed, pressurized water is directed exclusively to the space in the hydraulic cylinder 9 beneath the piston 10 and to the accumulator 14, pressure increases and the impact weight 4 is lifted. Water contained in the hydraulic cylinder above the piston is expelled into the annular chamber and towards the cavity 3 causing a downward flow inside the latter and along the impact weight 4, which flow more than compensates the pumping action of the weight moving upwards. Upon lapse of a preset lift time or reaching a preset stroke, the supply pressure valve 7 is opened and the impact weight stops moving upwards. Next, the return pressure valve 8 is closed and pressurized water is directed to the (effective) upper surface of the piston 10 and the impact weight 4 is accelerated towards the pile 23 until is hits the anvil 24. Water contained in the hydraulic cylinder below the piston is expelled into the hydraulic circuit, mostly into the accumulator. After impact, the cycle is started anew.

By returning the pressurized water to the surroundings via the cavity, ingress of water and dirt directly from the surroundings into the cavity is reduced or prevented. Further, as the system of the present invention provides a controlled environment, results during actual operation more closely correspond the results achieved during testing in a laboratory. This in turn facilitates optimization of operating conditions and settings and further development of the driver. Finally, as the system continuously withdraws and returns water from respectively to the surroundings, it requires in principle no tank for a hydraulic medium low pressure or low pressure accumulator and no so-called scavenger for re-generating the hydraulic medium. As a matter of course, this disclosure is not restricted to the above-disclosed embodiments, which may be varied in different manners within the spirit and scope of the invention.