The present invention relates to a ground anchor for an off- shore foundation. In addition, the invention relates to an offshore foundation and to a method of establishing an off ^¬ shore foundation. More specifically, the invention relates to establishing and holding a foundation structure by means of one or more ground anchors .

It is known to establish mono piles, jacket constructions or tripod constructions on the seabed as foundations in particu ^¬ lar for offshore wind turbine installations. A mono pile structure is, for example, described in WO 2009/026933 Al . US 5,988,949 and EP 2 067 914 A2, for example, describe jacket constructions, and DE 10 2004 042 066 Al, for example, describes a tripod construction.

A typical method for establishing a foundation, in particular a foundation based on a a jacket construction or a tripod construction, is to prepare the seabed prior to establishing the foundation which includes levelling out variations of the heights of the seabed in order to achieve a construction in level. Furthermore, the method comprises establishing piles very accurately positioned in the seabed such as by means of a pre-fabricated and pre-positioned pile positioning template structure on the seabed. The piles function as anchor of the offshore foundation. The piles are established by using e.g. hydraulic drifting means which literally hammer the pile down into the seabed. When the pile is established, e. g. a jacket or a tripod con ^¬ struction is set over a part of the pile and the space be ^¬ tween the leg and the pile is grouted to establish a secure connection.

With respect to the mentioned state of the art it is an ob ^¬ jective of the present invention to provide an advantageous ground anchor for anchoring an offshore foundation. It is a further objective to provide an advantageous offshore founda ^¬ tion and a method of establishing an offshore foundation. The first objective is solved by a ground anchor for an off ^¬ shore foundation as claimed in claim 1. The further objective is solved by an offshore foundation as claimed in claim 5 and by a method of establishing an offshore foundation on the seabed as claimed in claim 8. The depending claims contain further developments of the invention.

An inventive ground anchor for an offshore foundation to be installed on the seabed, in particular for an offshore foun ^¬ dation for a wind turbine, comprises at least one foldable element which can be folded to have a minimum surface area and unfolded so as to be able to act like a barb. The ground anchor may, in particular, be used for a mono pile structure, a jacket structure or a tripod structure. Such a ground anchor performs an umbrella-like function, i.e. during a positioning procedure the anchor is folded so that it has a minimum surface area and a minimum resistance to ^¬ wards the material of the seabed and, as a consequence, to ^¬ wards being positioned. When the anchor is positioned in a correct position deep in the seabed it is unfolded and acts like a barb to provide a high resistance against a tension in the direction of an attached tension means and against being removed from the seabed. The ground anchor may either comprise a connecting means for connecting a tension means or it may be an integral part of such a tension means.

To facilitate unfolding of the foldable element the foldable element may be equipped with a sharp edge that shows in the direction in which the foldable element will move during the unfolding process. The inventive ground anchor offers a cheep and simple alter ^¬ native to the existing ground anchors and is also easy to in ^¬ stall . An inventive offshore foundation, in particular an offshore foundation for a wind turbine, is installed on the seabed and comprises an offshore foundation structure, e.g. a jacket or tripod structure, or a mono pile, and at least one ground an ^¬ chor which is placed in the ground and connected to the foun- dation structure.

By using the inventive ground anchor the offshore foundation can be anchored by a cheep and simple anchoring means which is, in addition, easy to install.

The ground anchor may be connected to the offshore foundation structure by a tension means to which a tension is applied. Such a tension means may be implemented by one or more wires, by one or more rods, by one or more chains, etc. The barb like structure of the anchor then withstands the tension so as to provide a secure fixing for the tensioned tension means and, hence, for the foundation.

In the inventive method of establishing an offshore founda- tion on the seabed, in particular of establishing an offshore foundation for a wind turbine installation, an offshore foundation structure is anchored in the seabed by positioning at least one ground anchor connected to a foot of the offshore foundation structure in the ground by means of an inventive ground anchor. When the ground anchor is being positioned in the ground the at least one foldable element of the ground anchor is folded. When the ground anchor is positioned in a correct position the at least one foldable element is un ^¬ folded and acts like a barb which secures the ground anchor against being pulled out of the seabed by a tension which may be applied from the offshore foundation. When the ground anchor is folded for being installed in the seabed it provides low resistance towards the material of the seabed because of its minimum surface area in the folded state. After the anchor has been positioned and unfolded it provides an increased resistance towards the material of the seabed. Hence, a tension can be applied to the ground anchor without removing it from the seabed so that a secure anchor ^¬ ing of the foundation can be achieved. Hence, when the connection between the foot and the ground anchor is realised by a tension means this tension means can be tensioned for securing the foundation after the ground anchor is unfolded. For positioning the ground anchor in the ground a channel may be drilled into the ground and the ground anchor may be posi ^¬ tioned through the channel. A drilled channel of low diameter can be used, which is cost saving. As an alternative, the ground anchor may be positioned in the ground by flushing it into the ground. Flushing the anchor into the seabed is fa ^¬ cilitated by its low cross section in the folded state and the resulting low resistance against the material of the sea ^¬ bed . Further features, properties and advantages of the present invention will become clear from the following description of an embodiment in conjunction with the accompanying drawings.

Figure 1 schematically shows a tripod or jacket structure of an offshore wind turbine foundation that is anchored by in ^¬ ventive ground anchors.

Figure 2 schematically shows an embodiment of the inventive ground anchor in the folded condition.

Figure 3 schematically shows the embodiment of Figure 2 in the unfolded condition. Figure 4 schematically shows a mono pile structure of an offshore wind turbine foundation anchored by inventive ground anchors . Typical offshore foundations, for example for wind turbines, are shown in Figures 1 and 4 in highly schematic views. While Figure 1 shows a foundation using a foundation structure in form of a jacket structure or tripod structure Figure 4 shows a foundation using a mono pile as a foundation structure.

In Figure 1, as an illustrative example, an offshore founda ^¬ tion using a jacket structure 1 as offshore foundation struc ^¬ ture is established on the seabed 3. The jacket structure comprises three or more legs 7 with feet 4 and is secured to the seabed 3, e.g., at the feet 4, by one or more ground an ^¬ chors 6 which are established deep in the seabed 3. The con ^¬ nection between the ground anchors 6 and the structure 1, e.g. between the ground anchors 6 and the feet 4 of the structure 1, may be carried out by tension means 5 which may, e.g., be one or more wires, steel or iron rods, one or more chains, etc. For various embodiments of the invention the tension means is connected to the ground anchor 6 during po ^¬ sitioning of the anchor in the seabed 3. However, as an alternative, the ground anchor 6 may be an integral part of the tension means 5.

Tensions is applied to the tension means 5 in order to hold the structure 1 in position even when the structure 1 is in ^¬ fluenced by loads from e.g. sea waves 2 or from an installed wind turbine on the structure 1.

As it is shown in Figures 2 and 3, the ground anchor 6 com ^¬ prises an anchor body 12 and at least one foldable element 8 which can be moved from a folded condition as shown in Figure 2 to an unfolded condition as shown in Figure 3. The foldable elements 8 may have a triangular cross-section with the sharp side 11 showing outwards from the anchor body 12 to reduce the resistance towards the material of the seabed 3 when the foldable element is being unfolded so as to facilitate the unfolding of the foldable element 8. In order so facilitate introducing the ground anchor 6 into the seabed 3 the ground anchor may have a tip 13 at its downside end.

The folding and unfolding of the foldable element 8 can be realized by means of an umbrella-like mechanism comprising stretchers 9 for spreading the foldable elements 8, which can be compared to the ribs of an umbrella.

The ground anchor 6 of the present embodiment is equipped with a connecting means 10, e.g. a hook, an eye, a thread, etc., for connecting it to one or more wires, steel or iron rods, one or more chains, etc. which can be used as the ten- sion means 5. Furthermore, the connecting means 10 may be connected to the umbrella-like mechanism by means of an ac ^¬ tuator arrangement that stretches the stretchers 9 when a tension is applied to the actuator arrangement so that the foldable elements 8 can be unfolded by applying the tension to the tension means 5. Alternatively, the umbrella-like mechanism may be actuated by drives integrated into the ground anchor 6 and controlled by an operator via control signals. These control signals may, e.g., be sent to the ground anchor by wires running through or along the tension means 5.

As mentioned above, the ground anchor 6 according to the shown embodiment has an umbrella-like function i.e. during a positioning procedure the anchor is folded so that it has a minimum surface area and a minimum resistance towards the ma ^¬ terial of the seabed 3 and towards being positioned, whereas when the anchor 6 is positioned in a correct position deep in the seabed 3, it is unfolded and acts like a barb and have maximal resistance against a tension in the direction of the attached tension means 5 and against being removed from the seabed 3. The one or more ground anchors 6 may be established deep in the seabed e.g. by drilling a channel for the anchors 6 to be positioned through, by flushing the anchor into the seabed or the like.

Consequently this embodiment of the invention is character ^¬ ized in that the foundation structure (jacket or tripod) 1 is secured to the seabed 3 by one or more ground anchors 6 in ^¬ stalled in the seabed 3 and connected to the foundation structure 1 by one or more tension means 5.

The embodiment is advantageous in that the ground anchor 6 represents an alternative to conventional securing of the structure 1 to the seabed 3. The embodiment is further advan- tageous in that it is a very cheep construction and it is easy to install. Even further, ground anchors are known and well tested technology from onshore constructions.

The legs 7 of the embodiment which has been described in con- junction with Figures 1 to 3 are tilted with respect to the vertical direction. However, alternatively the legs 7 may as well be oriented vertically.

Figure 4 shows a mono pile structure used as offshore founda- tion structure as an alternative embodiment of the inventive offshore foundation. The mono pile structure comprises as one of the main elements a pile 14 with a foot 15 at its longitu ^¬ dinal end 16 which rests on the seabed 3. Three or more ten ^¬ sion means 5 extend from an upper location of the pile 14 into the seabed 6 to a ground anchors 6 placed deep in the seabed. The anchors are preferably positioned around the pile 14 at the corners of a regular polygon, e.g. at the corners of an equilateral triangle. The ground anchors 6 used corre ^¬ spond to those described with respect to Figures 1 to 3.