DK2700750T3 - Pile FOR OFFSHORE STRUCTURES AND METHOD FOR CONSTRUCTION OF A pile FOR OFFSHORE STRUCTURES - Google Patents

Description

Foundation pile for offshore structures and method for constructing a foundation pile for offshore structures

This invention relates to a foundation pile for offshore structures, e.g. as drilling or extraction platforms or for accommodating wind power plants. The invention also relates to a method for producing a foundation pile.

For certain foundation types for offshore structures, in particular for offshore wind power plants, pile foundation in the subsoil is required. Such foundation types are, so-called monopiles, jackets, tripods or tripiles, for example. For this purpose, first one or more steel piles are driven into the subsoil, then, in a jacket foundation, for example, a steel grid/ steel tube structure is placed on the steel piles receiving a so-called transition piece above sea level. The transition piece then receives the actual structure, such as a steel tower, for example. At the top end, the tower receives the wind power generator with a rotor.

As is known, driving of the foundation piles is accompanied by noise emissions which are undesirable especially because they will cause damage and behavioral disorder in marine mammals. It is thus known and sometimes specified for deterrent and noise reduction measures to be taken before driving work is executed in order to ensure that no marine mammals are present near the driving works, and/or for noise to be reduced to the allowed limit.

As the penetrating depth of the steel piles driven in and the progress of such driving works are dependent on the firmness of the subsoil, placing steel piles presents the drawback that the piles of a foundation placed on one or more piles are not always anchored in the subsoil altogether at the same penetrating depth. Finally, it cannot be excluded that an oblique position will result for one or more piles when driving works are performed. Thereby, when the foundation is put in place or an adjacent construction is mounted, costly measures for compensating level differences are required.

In principle, it is known from prior art to sink steel piles into the subsoil by drilling out or water jet driving, or to anchor foundations as so-called caisson foundations in the subsoil, e.g. from DE 103 022 94 A1.

From DE 10 2007 000 328 A1 and also GB 231 516 A, a pile construction is known which includes a pipe made of glass fiber reinforced plastic, at the lower end of which a cutting shoe is arranged having an overlap. Producing said pile construction can be performed in a first step by pile driving, vibrating, or water jet driving. Once the bottom of the pipe has been sunk into the subsoil, the material located inside the pipe is drilled out and still further drilled out through the core of the pipe below the cutting shoe, and the drilled out cavity including the pipe is filled with concrete. The pipe used for this purpose made of glass fiber reinforced plastic acts as permanent formwork. However, the pile construction described above only allows for use of small size pipes having diameters on the order of 0.3 m.

The above-mentioned methods are advantageous in that pile-driving of steel piles into the subsoil is avoided.

However, the known foundation types mostly present the drawback that costly measures for connecting and leveling of the structure to be erected on the foundation are required.

The invention is based on the object to provide a foundation pile for offshore structures which can both be anchored in the subsoil while avoiding pile-driving methods and forms a very simple connection/transition with a structure to be erected, in particular as a tower structure for offshore wind power plants.

Moreover, the invention is based on the object to provide a suitable method for producing a foundation pile for offshore structures.

The object is achieved by an inventive foundation pile for offshore structures which is assembled from individual pipe sections in a modular manner, of which at least one pipe section is anchored in the subsoil as a permanent borehole casing, wherein the pipe sections are made as concrete pipes which are connected to one another via clamping elements in a longitudinally non-positive manner.

The idea underlying the invention can be summarized in that the principle of separate pile foundation by means of steel pipes and steel superstructures also created separately is abandoned, and instead, a foundation pile for offshore structures is provided which entirely consists of reinforced concrete components. This does not only present the advantage that for instance a foundation in the form of a monopile can be made entirely of reinforced concrete, and can be assembled continuously or largely over a great length from the foundation tip to the wind turbine generator from reinforced concrete pipe sections of approximately the same type, but also presents the advantage of relatively simple dismantability. If steel constructions are used, such dismantability cannot, or only with difficulty, be achieved as the connections of the pile section located in the seabed from a pile section above in the water body and air space at seabed level must be severed. This would be very costly, e.g., in a pile-driven monopile foundation, due to the large diameter and wall thickness of the steel pipe.

Within the meaning of the present application, the term seabed hereafter refers to the interface between the water body above and the rocks/sediments below which in the present application are designated as the subsoil.

Compared to other constructions, an inventive construction has the advantage, in particular, that due to the entirely modular design of the foundation pile the underwater length of the foundation pile including the penetration depth into the subsoil can be adapted, even during the founding operation, which is not the case when placing steel piles. The latter have to be provided at a specified length which cannot be changed. When pile-driving steel piles, first, exploration drillings have to be performed for the firmness of the subsoil to be determined. Once adequate calculations of firmness have been performed, the drive-in depth is determined, from which ultimately the length of the provided steel piles is measured the length of which cannot be adapted on site. Due to local heterogeneities of the subsoil, problems may occur during piledriving so that possibly the complete drive-in depth is not achieved. Moreover, calculation of the required pile length is complicated and prone to increased uncertainties due to cyclic loading of the piles because of waves and wind.

The foundation pile according to the invention being assembled from multiple reinforced concrete pipe sections, slanting of the foundation pile can already be compensated for during mounting thereof. For this purpose, different transition pieces can be supplied for use as the top module which compensate for slanting of the foundation pile so that a horizontal situation of the turbine generator shaft can be obtained within the production tolerances.

Further advantages of such a foundation pile construction assembled from concrete pipe sections are greater safety against the effects of undermining, a smaller proportion of steel, and last not least the possibility of low-noise preparation of the foundation pile. Another advantage of the inventive foundation pile construction is that the bond length in the subsoil can be easily increased so that even after later undermining secure anchoring in the subsoil is still given. In some instances, potholes may have a depth from 7 to 10 m below seabed level.

Finally, the risk of corrosion is also reduced as no unprotected steel construction is exposed to sea water.

In an advantageous appropriate variant of the foundation pile according to the invention, provision is made for the pipe section arranged at the very bottom in the borehole to be fitted with a circumferential cutting shoe which may form an overlap or projection with respect to the diameter of the relevant pipe section.

In an advantageous variant of the foundation pile according to the invention, provision is made for the foundation pile to extend from the seabed at least up to the water line, and preferably significantly above the water line, and to be assembled at least over this length from pipe sections which are approximately identical in construction.

Within the meaning of the invention, identical in construction means that the pipe sections are compatible in terms of joining ability or modularity and bracing ability, but that not all of the pipe sections above the seabed necessarily have to exhibit the same diameter over the entire height. I.e., the pipe sections are not necessarily made to be cylindrical, but rather several pipe sections may form one tower/mast or pile with a hyperboloid or trapezoidal profile.

The foundation pile may, but must not necessarily, be partially filled inside the borehole. Appropriately, the foundation pile is circumferentially anchored in the subsoil in a frictionally engaged or positive manner inside the borehole provided for this purpose. A particularly advantageous variant of the foundation pile according to the invention is characterized by at least one in situ concrete base extending in the subsoil below the cutting shoe in a borehole extension. Said in situ concrete base can be made like an elephant's foot and may have an overlap in relation to the concrete pipes. I.e., the in situ concrete base has at least locally a diameter which is greater than the diameter of the cutting shoe. The base may also have several overlaps or enlargements.

In addition, the object underlying the invention is achieved by a method for producing a foundation pile for offshore structures, comprising the following method steps: a) depositing on the seabed a plurality of concrete sections preferably of the same type into a vertical pipe string, b) installing a drill string into the pipe string, c) sinking a borehole inside the pipe string, d) feeding the pipe string into the borehole as boring advances while using the weight force of the pipe string, and e) stacking further pipe sections on top of the pipe string, and f) repeating steps c) to d) until an intended end depth of the pipe string is reached in the subsoil.

According to the invention, the pipe sections of the foundation pile are provided as a permanent borehole casing. At least in the beginning, the bore is sunk within the shelter of the casing/pipe string stabilizing the borehole wall. Once the intended end depth has been reached, the pipe string remains in the borehole as the actual support construction in the subsoil. Above the seabed, said casing forms an extension of identical construction which may reach at least up to just above the water surface or else significantly beyond. At the leading end, i.e. the base, the pipe string or casing is fitted with a cutting shoe, and is undercut by the drilling tool operating on the inside which breaks off the layer of rock of the subsoil in the shelter of the casing and transports it to the sea surface via conveying equipment, so that the pipe sections or the pipe string sink(s) into the borehole by dead weight. During drilling, pipe sections of defined length are respectively stacked upon the casing so that top edge of the pipe string is always located sufficiently above sea level, and thereby, on an installation platform, e.g. an auxiliary vessel, safe drilling operation is ensured including precise discharge of drill cuttings.

During the drilling and extracting operation, the pipe string is filled with sea water by pumps so that the water level inside the pipe string is sufficiently above the sea water level. This ensures that hydrostatic pressure inside the pipe string or inside the borehole is so high that there is always an overpressure against possibly tapped aquifers in the subsoil, so that inflow of groundwater through the borehole wall and erosion of the borehole wall can be reliably avoided.

Preferably, the pipe sections are braced against one another in a longitudinally nonpositive manner, preferably already when a pipe string is assembled. Alternatively, with relatively large diameters or relatively large material cross-sections, it is possible to provide a so-called untensioned reinforcement instead of longitudinally non-positive bracing of the pipe sections.

Appropriately, once the borehole has been completed, at least part of the pipe string, preferably in the area of the base of the pipe string, is filled in.

Filling in, e.g. with drillings and/or in situ concrete, can be provided, wherein filling in does not have to meet any high strength requirements, and is not necessarily required. One advantage of this approach is that costly dumping of bored out drillings is not required.

In a particularly preferred variant, provision is made for sinking of the borehole to be performed by airlift drilling. This drilling technique does not require any casing stabilizing the borehole, in contrast to known wash drilling methods, no drilling fluid optimized in density and sealing of the borehole is used, rather above the drilling tool, compressed air is introduced into the extraction pipe or the drill pipes lifting and discharging drillings due to the difference in density with respect to the surrounding water-bearing borehole. Such a drilling method is suitable in particular for sinking large boreholes without any casing.

In the method according to the invention, provision can be made for a dismantable protective casing put in place before the concrete pipe strings are deposited, which casing may include e.g. steel pipe strings deposited on the seabed which have a larger diameter than the concrete pipes and which must not necessarily penetrate into the subsoil.

Appropriately, before and during sinking of the borehole, the concrete pipe string extends above sea water level. The individual concrete pipe strings can be sealed with respect to one another in order to ensure build-up of sufficiently high hydrostatic pressure inside the pipe string. In terms of the invention, sealing merely means that the individual pipe sections can be placed in register within one another so that a leak water current at the joints can be minimized. The pipe sections are appropriately made as concrete modular rings and stacked without joint mortaring and joint bonding.

In a variant of the method according to the invention, provision is made for an uncased extension of the borehole to be produced below the lowest pipe section after installation of the pipe sections until the intended end depth of the pipe string has been reached.

The extension of the borehole can be created at least locally with a larger diameter than the cased region of the borehole. This uncased region of the borehole can then be cast with in situ concrete and possibly with reinforcing elements.

Bracing of the pipe sections is appropriately performed via tendons in the form of steel cables which are guided either inside the pipe string or inside sleeves extending longitudinally in the pipe sections. The tendons can be locally connected together, e.g. via adequate couplings or adequate tension jacks.

Depositing individual pipe sections on the seabed into a pipe string can be done for instance so that the pipe sections are guided with respect to each other by the longitudinal tendons and deposited on the seabed by means of winches at a depositing device.

The pipe sections can be deposited on the seabed by means of winches, e.g. by means of guide cables applied externally to the pipe sections or pipe string. For this purpose, e.g. a total of three guide cables provided on the circumference of the pipe string can be applied which are fastened under the lowest pipe section, respectively, or reach below the same.

Alternatively, several pipe sections already pre-assembled into a pipe string can be deposited on the seabed.

Depositing the pipe string and performing drilling work is appropriately done from a floating or elevated installation platform.

Hereafter, the invention will be explained by means of a sample embodiment represented in the drawings.

Fig. 1 shows a first schematic representation of a foundation pile according to the invention as a monopile for accommodating a tower structure and a wind power generator according to a first variant,

Fig. 2 shows a sectional view through a pipe section according to the first variant of the foundation pile,

Fig. 3 shows a top view of the pipe section represented in Fig. 2,

Fig. 4 shows a top view of a pipe section according to a second variant of the inventive foundation pile, and

Fig. 5 shows a schematic representation showing the installation of the pipe sections inside a protective casing.

While the invention will be described below by means of a so-called monopile, it is to be noted here that the inventive foundation pile 1 and the method for producing said foundation pile 1 are suitable for producing various types of foundations for any kind of offshore structure. E.g., there are jacket foundations, tripod foundations, or tripile foundations. Also, it is possible thereby to make foundations of offshore drilling platforms, offshore extraction platforms, or offshore supply platforms.

Foundation pile 1 represented in Fig. 1 includes a plurality of pipe sections 2 made of reinforced concrete assembled at the front into a pipe string 3. The pipe sections may have a diameter of 1 to 10 m, for example. The individual pipe sections 2 are braced in a longitudinally non-positive manner with respect to each other by tie beams 4, e.g. in the form of tendons or steel cables. The individual pipe sections 2 are provided with reinforcements. The pipe sections 2 can also be assembled from segments, respectively.

Appropriately, the joints 5 between the individual pipe sections 2 are neither bonded nor mortared, but simply stacked in register.

According to a first variant of the foundation pile 1 according to the invention, the tie beams 4 extend within the surface area of the pipe sections 2, e.g. inside cast-in sleeves.

In a second variant of the foundation pile 1 represented in Fig. 4, the tie beams 4 are placed with a clear span in the interior of the pipe sections 2, wherein the latter can be secured to the internal wall of the pipe sections 2 either via adequate fastening means, or else in case of a hyperboloid profile of the pipe string 3 provided above the seabed, for example, be applied totally freely against the internal wall of the pipe sections 2 or the internal wall.

When the pipe sections 2 were installed, the tie beams 4 were locally extended, respectively, and connected to one another via couplings or tension jacks, not shown.

Production of the foundation pile 1 according to the invention is performed as follows:

First, a protective casing 6 made of steel is deposited from an installation vessel. As already mentioned in the beginning, the protective casing is optional, the requirement thereof depending on flow conditions and nature of the seabed. The protective casing 6 consists of several steel pipe segments deposited on the seabed 7 and having a slightly larger diameter than a drilled hole to be sunk. The protective casing 6, as well as the permanent casing to be put in place thereafter, significantly extend above sea level 8.

Preferably, the protective casing 6 extends above sea level 8 exactly to the same extent as the pipe string 3 so that no hydrostatic imbalance results between the annular space of the protective casing 6 and the interior of the pipe string 3. At most, the pipe string 3 may extend in two parts above the protective casing 6.

Inside the protective casing 6, several pipe sections 2 made of reinforced concrete are stacked into a pipe string 3, said pipe string 3 extending significantly above sea level 8 and slightly above the protective casing 6, e.g. up to about 15 m above sea level 8. The individual pipe sections 2 are all interconnected via the tie beams 4, and in the erected state, braced in a longitudinally non-positive manner with respect to each other. The pipe section 2 which in the installation position is at the bottom is provided with a cutting shoe, not shown, as a special construction.

When the first section of pipe string 3 is mounted from the seabed 7 to above sea level 8 and above the protective casing 6, the individual pipe sections 2 can be held and stabilized, e.g. by external guide cables 11, so that the vertical loads occurring in the final version can be absorbed and transferred upwards via fastening of the steel cables. Depending on the diameter of the pipe sections 2, several guide cables 11 or cable groups can be distributed equally around the outside circumference of the pipe sections 2. They can be laterally deflected, secured, and operated via radially arranged deflection pulleys 12 and winches on the mounting platform. In the sample embodiment represented, a total of three guide cables 11 guided over deflection pulleys 12 are distributed along the outer circumference of the pipe string 3. The guide cables 11 reach under the lowest, or in the installation position leading, pipe section 2 or the cutting shoe arranged thereon, respectively. When the pipe string 3 has been released or mounted completely from the seabed 7 to the mounting platform above sea level 8, a drill string can be installed in the pipe string 3.

Fig. 2 shows the pipe string 3 in the position as already deposited on the seabed 7. Releasing the first pipe sections 2 to above sea level is preferably performed in suspension. In this position, a borehole 9 is sunk, e.g. by airlift drilling. Sinking the bore takes place while maintaining a hydrostatic overpressure inside the borehole 9, so that in principle other stabilizing measures can be omitted. However, depending on the consistence of the seabed or subsoil, upon initial sinking of the bore, conditioning of the subsoil or aspiration of the top layers may be required.

When the bore is sunk, the pipe section 2 installed at the bottom is undercut, the pipe string 3 sinks into the borehole 9 as drilling advances, progressively, as drilling makes increased progress, further pipe sections 2 are stacked upon the pipe string, and the tie beams 4 are connected together.

Thereby, the pipe string 3 is guided as a permanent borehole casing up to a provided depth, as represented in Fig. 1.

The borehole is further drilled below the pipe section without casing installed at the bottom, flared at the end, and filled with in situ concrete so that the in situ concrete base 10 represented in Fig. 1 will result, which is also provided with encased concrete tie beams 4.

Once foundation pile 1 is completed, the protective casing 6 can be removed. The foundation pile 1 can be filled in completely or partially with the drilled out drillings, which may be compacted by cement, if required.

Now, a structure can be erected on the foundation pile. The structure may be for instance a monopile with pipe sections 2 of identical construction as those located underwater.

In Fig. 1,13 designates the thickening of the in situ concrete base.

REFERENCE LIST 1 foundation pile 2 pipe sections 3 pipe string 4 tie beams 5 joints 6 protective casing 7 seabed 8 sea level 9 borehole 10 in situ concrete base 11 guide cables 12 deflection pulleys

Claims (16)

PILOT PILOT FOR OFFSHORE CONSTRUCTIONS AND PROCEDURE FOR CONSTRUCTION OF A PILOT PILE FOR OFFSHORE CONSTRUCTION An offshore structure pilot pile (1) composed in a modular fashion of individual pile sections (2), of which at least one pile section (2) is anchored as a permanent borehole casting in the subsurface, where the tube sections (2) are formed as concrete pipes which are connected to each other via clamping elements in a longitudinal force-locked manner. Pilot pile according to claim 1, characterized in that the pipe section (2), which is arranged completely at the bottom of the borehole (9), is provided with a circumferential cutting shoe. Pilot pile according to one of claims 1 and 2, characterized in that the pilot pile (1) extends from the subsoil and at least as far as the water line and is composed at least over this length of substantially constructionally identical pipe sections (2). . Pilot pile according to one of claims 1 to 3, characterized in that it is filled at least partially in the borehole (9). Pilot pile according to one of claims 1 to 4, characterized in that the at least one in situ concrete foot (10) extends in a borehole extension into the subsurface beneath the cutting shoe. Pilot pile according to claim 5, characterized in that the in situ concrete foot (10) has, at least in certain areas, a diameter greater than the diameter of the cutting shoe. A method of manufacturing a pilot pole (1) for offshore structures comprising the following process steps: a) Placing a plurality of pipe sections (2), preferably in the form of concrete pipes, on the seabed (7) to form a vertical pipe string (3), b) mounting a drill string in the pipe string (3), c) lowering a borehole (9) into the pipe string (3), d) inserting the pipe string (3) into the borehole (9), in step with drilling progress, with simultaneous application of the weight force (3) of the pipe string, and e) stacking additional pipe sections on top of the pipe string (3), and f) repeating steps c) to g) until the pipe string (3) has reached a desired depth into the subsurface (7). Method according to claim 7, characterized in that the pipe sections (2) are supported against each other in a longitudinal force-locked manner, preferably also during assembly or immediately after assembly, to form a pipe string (3). Method according to one of Claims 7 or 8, characterized in that after completion of the borehole (9), at least part of the pipe string (3) is filled. Process according to claim 9, characterized in that a filling with drilling waste and / or in situ concrete is provided. Method according to one of claims 7 to 10, characterized in that the borehole is lowered while maintaining a hydrostatic overpressure in the borehole (9). Method according to one of claims 7 to 11, characterized in that the lowering of the borehole (9) takes place via the air lift drilling process. Method according to one of claims 7 to 12, characterized in that a removable protective casing (6) is placed before the pipe sections (2) are placed. Method according to one of claims 7 to 13, characterized in that the pipe string (3) extends above the sea level (8) before and during the lowering of the borehole (9). Method according to one of claims 7 to 14, characterized in that, after mounting the pipe sections (2) and until the pipe string (3) has reached the desired end depth, an uncoated extension of the borehole is produced below the bottom pipe section. Method according to claim 15, characterized in that the extension of the borehole (9) is produced, at least in certain parts, of a larger diameter than the coated area of the borehole (9).