The present invention relates to a method of installing large diameter piles at an offshore location, with the use of floating barges and temporary support structures which provide improved stability during installation.

The large diameter piles may for instance form the foundation of a large structure at sea, such as a wind turbine for instance. SUMMARY OF THE INVENTION

One embodiment of the invention relates to a method of installing a large diameter pile at an off-shore location, comprising the steps of:

- installing a temporary support structure above the final position of the pile foundation,

- installing a casing into a drilled borehole, which casing is supported by the temporary support structure,

- installing the pile into the casing, held vertical by the temporary support structure,

- pumping concrete into the borehole,

- grouting the pile, and

- retracting the casing.

Another embodiment of the invention relates to a method of installing a large diameter pile at an off-shore location, comprising the steps of:

- installing a temporary support structure in the form of a jacket above the final position of the pile foundation,

- installing a casing into a drilled borehole, which casing is supported by the temporary support structure,

- installing the pile into the casing, held vertical by the temporary support structure,

- pumping concrete into the borehole,

- grouting the pile, and

- retracting the casing.

Still another embodiment of the invention relates to a method of installing a large diameter pile at an off-shore location, comprising the steps of: - installing a temporary support structure in the form of a caisson above the final position of the pile foundation,

- installing a casing into a drilled borehole, which casing is supported by the temporary support structure,

- installing the pile into the casing, held vertical by the temporary support structure,

- pumping concrete into the borehole,

- grouting the pile, and

- retracting the casing. Still another embodiment of the invention relates to a method of installing a large diameter pile at an off-shore location, comprising the steps of:

- installing a temporary support structure in the form of a jack-up above the final position of the pile foundation,

- installing a casing into a drilled borehole, which casing is supported by the temporary support structure,

- installing the pile into the casing, held vertical by the temporary support structure,

- pumping concrete into the borehole,

- grouting the pile, and

- retracting the casing.

DESCRIPTION OF THE FIGURES

Figures 1 to 19 depict an embodiment of the method according to the invention, wherein the temporary support structure comprises a caisson. Figures 20 to 39 depict an embodiment of the method according to the invention, wherein the temporary support structure comprises a jacket. The figures are described in further detail below.

DETAILED DESCRIPTION OF THE INVENTION

A first part of the method according to the invention comprises the installation of a casing into which the large diameter pile is lowered. Prior to the installation of the casing by a drilling and hammering barge, a temporary support structure is installed above the final position of the pile foundation using, for instance, a crane barge. The temporary support structure preferably comprises a jack-up, and more preferably a jacket and/or a caisson. The drilling barge typically with casings on deck may be employed to install the casings inside the temporary support structure and lower the casing into the seabed. It is preferably equipped with a heavy lift crane, hydrohammer, vibrohammer, RCD (reverse circulation) drilling rig and/or de-sanding equipment. The temporary support structure acts as a guide during lowering of the casing to ensure its vertical and horizontal stability. Once the casing is properly positioned, the hydrohammer/vibrohammer is installed on top of the casing, and the casing is driven to final penetration depth or until refusal (i.e. further penetration hindered by rock layers) which ever comes first. In case of early refusal, the hydrohammer/vibrohammer is removed from the casing and the RCD drill rig will be installed for relief drilling after which the casing can be driven further until final penetration depth has been reached.

Drilling of the borehole is preferably performed with the RCD rig placed on top of the casing. A dumping barge may be positioned next to the pile and/or drilling and hammering barge. The dumping barge preferably comprises an open or split barge and is used for the transportation of remaining drilling cuttings for disposal or processing.

Drilling is performed to a final depth as determined by the size of the pile, but as a guidance, may be to a depth of 1 Im + 0.5m. It will be understood that the dimension of the dumping barge is sufficient in order to maintain sufficient pressure difference in the casing and drill string, so that a continuous flow of the drilling fluid and cuttings is maintained.

During drilling, the drill cuttings preferably pass a de-sanding plant, installed on the drilling barge or an additional barge, before it is pumped further into the dumping barge. The de-sanding plant is adapted to remove all large particles (cuttings) from the drilling fluid in order to be able to maintain the continuous closed circuit mentioned earlier. It is within the scope of the invention that the de-sanding plant is replaced or supplemented by an even larger dumping barge wherein the cuttings can be separated from the drilling fluid by gravitation. After the borehole has been drilled, the water inside the hole will have fine particles. This might be unacceptable as they will be released into sea when installing the concrete pile. Therefore, the water inside the borehole may be replaced by fresh water. This can be achieved by keeping the closed circuit running for a certain time, or any other suitable method. An alternative comprises introducing artificial flocculation of the particles into the water which has to be pumped from the bottom.

A second part of the method according to the invention comprises the installation of the piles, which are preferably pre-cast. The pre-cast piles are preferably transported and installed by a heavy lift barge disposed with a gantry crane. The floating barge will be able to position itself next to the casing - which is still held in position by the temporary support structure - and lower the pile into the casing. The pre-cast pile is preferably held some distance (e.g. 0.5m) above the bottom level of the borehole, which can be achieved by a number of methods, such as:

- maintaining the pre-cast pile in the hook of the gantry crane during further installation;

- transferring of the weight of the pre-cast pile onto footings at the base of the borehole. In this embodiment, the base of the borehole may be flattened or jacks installed below the pile to guarantee the correct pile tip level; and/or

- transferring the weight of the pre-cast pile on the casing as a vertical load.

The last two options are most preferred because they reduce the installation time of the heavy lift barge, which can be subsequently repositioned to the next location as soon as the weight of the pile has been transferred. In all cases, the pre-cast pile can be held in its (substantially) horizontal position by the temporary support structure, which is also still supporting the casing.

A third part of the method according to the invention comprises concrete pouring and grouting. After the pile has been placed inside the borehole, a concrete barge is positioned next to the pile and concrete is pumped to the bottom of the borehole with the aid of concrete pumps installed on the temporary support structure. As soon as the concrete has sufficient strength to prevent the grout from leaking, the grouting operation can start. During the waiting time for the curing, grouting lines and grout level indicators can be installed. The grouting may also be done from the temporary support structure on which a complete grouting plant is then installed. The grout materials will be transported in containers (e.g. bags) from the onshore base to the temporary support structure by means of, for example, a deck barge such as a multicat. Grouting is preferably achieved via pre-installed grouting inlets in the walls of the pile. The number of grouting inlets depends on the depth of the piles. During grouting, the grout level may be monitored by grout level indicators which are placed in the annulus prior to the grouting operation. After the grouting operation has been completed and the grout has reached sufficient strength, the annulus between the rest of the pile and the casing is preferably filled with sand to prevent creation of a large crater after retraction of the casing.

A fourth part of the method according to the invention comprises the retraction of the casing. Retracting the casing is preferably performed in two stages:

- a first relatively small lift of about 50 cm when the grout level has reached the top of the solid rock formation; and

- a full retraction of the casing after complete installation of the pile. Retraction of the casing may be performed by a crane barge. Two particularly preferred methods for retracting the casing comprise the use of a vibratory hammer and the pull force of a crane on a crane barge, and/or the use of a jacking system on the temporary structure. Lifting is then taken over by a crane barge as soon as the casing is completely free.

The required lift can be calculated by the skilled person. With the assumption of a friction of 1 ton/m ² , as a general guidance, a total friction force of approximately 200 tons is to be overcome. Together with the weight of the casing (± 100 ton) a total lifting force of 300 tons is required.

In case of difficult retraction, jetting alongside the casing is preferably applied or the casing can be greased with biological lubricant prior to installation. Figures 1 to 19 depict an embodiment of the method of the invention, wherein the temporary support structure comprises a caisson. In FIG. 1 a caisson 1 is transported to the installation site for the large diameter piles by trailing it with a tug 2 across the sea surface 3. Caisson 1 is subsequently sunk down by filling with water, as shown in FIG. 2. The water is supplied through a water feed pipe 5, installed on tug 2. A drilling and hammering barge 6 is then moved next to the caisson 1 (FIG 3). A plan view of the drilling and hammering barge 6 is shown in FIG. 4. Drilling and hammering barge 6 is typically provided with spud poles 7 that can be lowered onto and into the sea bottom 4 to support and stabilize the drilling and hammering barge 6. Barge 6 is further provided with a number of cranes 8, as well as drilling and hammering equipment, such as drill sets and vibrohammers.

The lower part 10a of the casing is lowered onto the sea bed 4 by the crane 8, as shown in FIG. 5. The lower part 10a of the casing is then hammered into the sea bed 4 until the first rock plate is reached by the action of a hammer 9, as shown in FIG. 6. An upper part 10b of the casing is then installed above the first part 10a in a similar fashion (FIG. 7), thus forming a complete casing (10a, 10b).

A drilling apparatus in the form of a drill string 11 is then lowered through the casing (10a, 10b), and a borehole 12 is drilled in the sea bed (FIG. 8). After the borehole 12 has been drilled to the desired depth, the upper part 10b of the casing is removed by the crane 8, as shown in FIG. 9. A heavy lift barge 13 is then provided next to the caisson as shown in FIG. 10. The heavy lift barge 13 comprises a lifting frame 14 and carries a number of pre-cast concrete piles 15 for installation. A pre-cast concrete pile 15 is taken up by the lifting frame 14 and subsequently lowered into the casing 10a in the direction of arrow 16, as shown in FIG. 11. As can be seen in FIG. 12, the frame takes control of the pile inclination.

A concrete/grout barge 17 is then provided next to the caisson (FIG. 13). Barge 17 may also be provided with spud poles 18 to stabilize it with respect to the sea bed 4, and further comprises equipment for pumping cement and grout. A tremie pipe 19 is installed in the pile 15 as shown in FIG. 14. The lowest plug 20 is grouted (FIG. 15), and grouting lines 21 are installed, connecting to inlets in the pile 15 as shown in FIG. 16. Grout is then further supplied to the space between the pile 15 and borehole 12 (FIG 17). After having grouted the space between the pile 15 and borehole 12 substantially completely, the lower part 10a of the casing is removed (FIG. 18). In a last step of the method, the caisson 1 is removed and this completes the installation of the pile 15 (FIG. 19).

Figures 20 to 39 depict an embodiment of the method of the invention, wherein the temporary support structure comprises a jacket 100. In FIG. 20 jacket 100 is transported to the installation site using a crane barge 101, provided with crane 102. Jacket 100 is subsequently lowered onto the seabed 4 using the crane 102 (FIG. 21). A nail 103 is placed in a hollow leg of the jacket 100 to stabilize it with respect to the sea bed 4 (FIG. 22). A drilling and hammering barge 6, as described above, is moved next to the jacket 100 (FIG 23). A plan view of the drilling and hammering barge 6 positioned against two jackets 100 is shown in FIG. 24. The lower part 10a of a casing is lowered onto the sea bed 4 (FIG. 25). The lower part 10a of the casing is then hammered into the sea bed 4 with the aid of hammer 9 until the first rock plate is reached (FIG. 26). An upper part

10b of the casing is then installed above the first part 10a (FIG. 27). Drilled apparatus in the form of drill string 11 is subsequently lowered through the casing (10a, 10b), and drilling commences to produce a borehole 12 (FIG. 28). After the borehole 12 has been drilled, the upper part 10b of the casing is removed (FIG. 29). A heavy lift barge, comprising a lifting frame 14 a at least one pre-cast concrete pile 15, is then provided close to the jacket 100 (FIG. 30). The pre-cast concrete pile 15 is lowered into the casing (10a) in the direction of arrow 16, as shown in FIG. 31. The pile 15 is hung off the jacket 100 as is shown in FIG. 32. A concrete barge 17 is then provided next to the jacket 100 (FIG. 33). A tremie pipe 19 is installed in the pile 15 (FIG. 34). The lowest plug is grouted (FIG. 35), and grouting lines 21 are installed, connecting to inlets in the pile 15 (FIG. 36). The grouting operating proceeds until the space between the pile 15 and the borehole 12 has been substantially filled with grout (FIG 37). The lower part 10a of the casing is then removed (FIG. 38), which, after removal of jacket 100, completes the installation of the pile 15 (FIG. 39).