Device for guiding and vertically holding a monopile and method for installing a monopile using such a device

1 . A device for guiding and vertically maintaining a tubular monopile when it is placed in an excavation made in a seabed, filled with compacted granular materials and consolidated by a metal reinforcement tube, comprising:

a support structure intended to be positioned vertically to the excavation made in the seabed and mounted on at least four feet which are each adjustable in height;

a frame for positioning the support structure relative to the reinforcement tube, the positioning frame having a closed U shape, being mounted on a low portion of the support structure and being intended to ensure the positioning of the support structure relative to an upper end of the reinforcement tube;

a cage extending along a longitudinal axis (X-X) and mounted inside the support structure above the positioning frame in order to receive the monopile, the cage comprising a closed low portion and a high portion provided with a door able to open to allow lateral introduction of the monopile into the cage and able to close to maintain the monopile in the cage;

the high portion of the cage comprises a plurality of damping pads distributed around the longitudinal axis (X-X) of the cage, dimensioned to allow distributed contact with the monopile, and able to dampen the dynamics of the monopile and come radially into abutment against the monopile when it is introduced into the cage in order to stabilize and modify its inclination relative to the longitudinal axis of the cage; and

the low portion of the cage comprises a plurality of blades extending along the longitudinal axis (X-X) of the cage and distributed around it, said blades being dimensioned to allow distributed contact with the monopile, able to come radially into abutment against the monopile when it is introduced into the cage in order to guide it during its descent into the cage and to modify its inclination relative to the longitudinal axis of the cage until reaching a sufficient soil resistance to naturally maintain the positioning in angle and in translation.

2 . The device according to claim 1 , wherein at least some of the damping pads positioned at the level of the high portion of the cage are mounted on a cylinder so as to allow modifying their position along a radial direction.

3 . The device according to claim 2 , wherein the damping pads mounted on a cylinder are two in number and are associated with two diametrically opposite fixed damping pads so as to make sure to homogeneously maintain the monopile around the longitudinal axis of the cage.

4 . The device according to claim 1 , wherein each damping pad positioned at the level of the high portion of the cage comprises a rigid shield which is intended to come into abutment against the monopile to distribute the contact pressure and limit friction, and which is mounted on a fixed support by means of at least one fender made of elastomer.

5 . The device according to claim 1 , wherein the damping pads positioned at the level of the high portion of the cage are distributed over at least two rows longitudinally spaced from each other.

6 . The device according to claim 1 , wherein at least some of the blades positioned at the level of the low portion of the cage are mounted on slides so as to allow modifying their position along a radial direction.

7 . The device according to claim 1 , wherein the door of the cage comprises pads intended to absorb shocks and to ensure guidance of the monopile during its lateral introduction into the cage.

8 . The device according to claim 1 , wherein the positioning frame is movable in rotation around two axes (Y-Y, Z-Z) perpendicular to the longitudinal axis (X-X) of the cage and is movable in translation along said longitudinal axis of the cage.

9 . The device according to claim 1 , wherein each foot of the support structure comprises a plate articulated around an axis perpendicular to the longitudinal axis (X-X) of the cage.

10 . The device according to claim 1 , wherein one of the feet ends at a high end with a cone for receiving ballast.

11 . The device according to claim 1 , wherein the support structure comprises two central columns which end at a high end with a cone for receiving a hydraulic lifting tool.

12 . The device according to claim 1 , wherein the frame for positioning the support structure comprises a removable closing bar.

13 . A method for placing a tubular monopile in an excavation made in the seabed, successively comprising:

consolidating the excavation with a metal reinforcement tube,

drilling the seabed to the desired depth,

filling the excavation with granular materials then compacting the granular materials,

depositing on the seabed vertically to the excavation a device according to claim 1 , the frame for positioning the support structure being introduced around the upper end of the reinforcement tube;

lifting the monopile from above;

guiding the monopile to introduce it inside the cage by passing the lower end of the monopile laterally through the door of the cage;

lowering the monopile into the cage;

partially driving the monopile into the compacted granular materials while correcting its inclination relative to the longitudinal axis of the cage to correct verticality defects of the monopile; and

recovering the device.

14 . The method according to claim 13 , wherein the compaction of the granular materials is carried out by vibration and the partial driving of the monopile into the compacted granular materials is carried out by vibro-sinking.

15 . The method according to claim 13 , further comprising the adjustment of the cage to the diameter of the monopile by setting the pads positioned at the level of the high portion of the cage and the blades positioned at the level of the low portion of said cage.