NL1043827B1 - Pile guide with suction caissons - Google Patents

Abstract

A guiding structure for guiding a monopile foundation during the installation in the seabed, which is standing on the seabed and which vertical angle can be corrected, characterized in that the correctie") of the vertical angle is clone by pumping water in- or out of at least tinree separated compartments of one or more suction caissons.

Description

Pile guide with suction caissons The invention concerns a pile guide for the installation of offshore monopile foundations, whereby unwanted deviations from the verticality can be corrected according to the beginning of conclusion 1.

Offshore monopile foundations are used for wind turbines or platforms. A monopile can be installed by driving it into the seabed e.g. by means of hammering or vibrating tools, which are operated from a floating vessel or jack-up vessel. During this process the verticality of the pile is to be controlled within specified tolerances of typically less than 0.5 degree.

For this purpose a pile guide can be used which is standing on the seabed by means of three or more supports, whereby the verticality of the guide can corrected by hydraulic means in the supports.

The support will penetrate to some depth into the seabed under the weight of the pile guide. A disadvantage of a guide which is standing on supports at the seabed is that the soil bearing capacity is not accurately known. Therefore the depth of soil penetration of the supports is not predictable and can be different for each support and the pile guide can become unstable, especially in case of soft soil.

Another disadvantage is a guide standing on the seabed with hydraulically moveable supports is that the hydraulic systems used in seawater are sensitive and vulnerable to salt and dirt.

The objective of the invention is to remove one or more of the above disadvantages and to provide a useful alternative for the hydraulically moveable supports of the pile guide, which are standing at the seabed. This goal is achieved by replacing the moveable supports by suction caissons. In most soil conditions where piles can be driven in the soil, suction caissons can also be applied successfully. Suction caissons can be penetrated into the seabed by pumping water out of the caisson, creating a low water pressure inside the caisson, which forces the caisson down into the soil. The depth of penetration can be controlled accurately by the volume of water pumped out. The control of the water volume inside at least three separated compartments allows control over the vertical position of each compartment, such that the verticality of the pile guide can be corrected.

An advantage of a pile guide according to the invention is that suction caissons provide a stable support for foundations on soft soil.

Another advantage of a pile guide according to the invention is that all control systems are placed in a protected location above the water on board of the installation vessel.

The force which is required to penetrate the suction caisson is related to the soil resistance at the pile tip and to the shear resistance at the area of the inside and outside walls of the pile. This force is generated by the reduced pressure inside the caissons and relative overpressure acting on the projected top area of the suction caisson. For very hard soils,

therefore a very low pressure and a large projected top area are required. On the other hand the soil resistance is reduced if the area the inside and outside walls of the suction caisson is reduced.

In a special embodiment of the invention the separate compartments are integrated into one suction caisson. An advantage is that the construction can be made of steel plate, which is relatively simple.

Another advantage is that the required deck space on board of the installation vessel during transport is smaller than in case three separate suction caissons are used.

Also an advantage of this special embodiment is that the total area of the inside and outside walls compared to the projected top area of the suction caisson is relatively small and therefore it will be capable to penetrate into harder soil.

Further explanation of the invention will be given by means of figures below, which will give a practical embodiment of the invention, but may not be considered in a limiting sense, wherein: Figure 1 shows a side view of a wind turbine with a monopile foundation according to prior art.

Figure 2a shows a top view pile guide with hydraulic support at the seabed.

Figure 2b shows a side view of the same pile guide as in figure 2a with hydraulic support at the seabed.

Figure 3a shows a top view pile guide with suction caissons according to the invention.

Figure 3b shows a side view pile guide with suction caissons according to the invention, Figure 4a shows a top view pile guide with a suction caisson with separated compartments according to the invention.

Figure 4b shows a side view of the same pile guide as in figure 4a with a single suction caisson with separated compartments according to the invention.

The foundation monopile (1) shown in figure 1 is penetrated into the seabed (2). The water level (3) and wind turbine (4) are indicated.

The sketch in figure 2a and 2b shows the monopile (1) placed inside the pile guide (7) before pile driving into the seabed (2). The pile guide (7) is standing on three hydraulic supports (8) penetrated into the seabed under the weight of the pile guide and the verticality can be corrected by variation of the stroke (s) of the hydraulic cylinders. On top of the monopile (1) a pile hammer (6) is shown.

In figure 3a and 3b shows the pile guide according to the invention, whereby the hydraulic supports (8) in figure 2a and 2b are replaced by suction caissons (9, 10, 11). The penetration depth (s) can be accurately controlled by pumping water in- or out of the suction caissons (9, 10, 11). For this purpose hoses or piping is installed between each suction caisson and a pump on deck of the installation vessel (not shown).

An alternative embodiment of the invention is shown in figure 4a and 4b.

The three suction caissons (9, 10, 11) in figure 3a and 3b are replaced by one suction caisson (12) with three compartments (13, 14, 15).

Claims (3)

CONCLUSIONS 1. A guide structure for guiding a monopile foundation during installation in the seabed, which stands on the seabed and whose vertical angle can be corrected, characterized in that the correction of the vertical angle is done by pumping water in- or from at least three separate compartments of one or more suction anchors. A guide structure according to claim 1 wherein separate compartments are provided in one suction anchor. A method of installing monopile foundations according to any one of the preceding claims. moons