The present invention relates to an offshore foundation structure comprising a jacket or tripod structure.

It is known to establish jacket constructions or tripod con ^¬ structions on the seabed as foundations in particular for offshore wind turbine installations. A jacket construction is, for example, shown in EP 2 067 914 A2, a tripod construc- tion, for example, in DE 10 2004 042 066 Al .

A typical method for establishing a foundation based on a jacked construction or a tripod construction is to prepare the seabed prior to establishing the foundation which in- eludes levelling out variations of the height of the seabed in order to achieve a construction in level. Furthermore, the method comprises establishing piles very accurately posi ^¬ tioned in the seabed such as by means of a pre-fabricated and pre-positioned pile positioning template structure on the seabed.

The piles are established by using e.g. hydraulic driven means which literally hammer the pile down into the seabed. When the pile is established, a jacket or a tripod construc- tion is set over a part of the piles and the space between the leg and the pile is grouted to establish a secure connec ^¬ tion .

With respect to the mentioned state of the art it is an ob- jective of the present invention to provide an advantageous offshore foundation structure.

This objective is solved by an offshore foundation as claimed in claim 1. The depending claims contain further developments of the invention.

An inventive offshore foundation structure comprises a jacket or tripod structure. This structure includes legs with a foot section and a gravity foundation located at the foot section. The jacket or tripod structure may, in particular, comprise three or more legs . When using feet with gravity foundations for securing the structure on the seabed instead of anchoring the legs by use of piles establishing an offshore foundation is simplified since there is no need for accurately pre-positioning piles in the seabed and, hence, no need for a pre-fabricated and pre-positioned pile positioning template structure on the seabed .

According to the invention, two or more legs may share a gravity foundation. In particular, a single gravity founda- tion may be present where each leg is connected with its foot section to said gravity foundation. However, in an advantageous development of the invention, each leg comprises a gravity foundation of its own. In this case the gravity foun ^¬ dations used for the legs may be of limited size and weight which makes them easier to handle.

In a further development of the inventive offshore foundation each gravity foundation element comprises a bottom side by which it rests on the seabed and which is equipped with a skirt at its periphery. In particular, the skirt may extend around the whole periphery of the gravity foundation element. The skirt ensures that erosion around the foundation does not flush away seabed material beneath the foundation which in turn may alter the position of the foundation and thereby the position of the whole structure.

Further features, properties and advantages of the present invention will become clear from the following description of exemplary embodiments in conjunction with the accompanying drawings.

Figure 1 schematically shows an inventive offshore wind turbine foundation with a jacket structure. Figure 2 schematically shows an alternative gravity

foundation for the jacket structure shown in Figure 1.

Figure 3 schematically shows an inventive offshore wind turbine foundation with a tripod structure.

Figure 4 schematically shows an alternative gravity

foundation for the tripod structure shown Figure 3.

Figure 5 schematically shows a leg with a gravity foun ^¬ dation comprising a skirt.

The present invention will now be described by way of exem ^¬ plary embodiments. While Figures 1 and 2 show offshore foun ^¬ dations with a jacket structure, Figures 3 and 4 show off ^¬ shore foundations with a tripod structure. Figure 5 shows a detail of a gravity foundation.

A possible offshore foundation structure by which the inven ^¬ tion can be realised is a jacket structure 1 as it is shown in Figure 1 in highly schematic view. The embodiment of the jacket structure 1 shown in Figure 1 comprises a number of main legs 7 with foot sections 8 and an number of a gravity foundations 14. Here, as an illustrative example, the jacket structure 1 is established on the seabed 3. The main legs 7 of the structure 1 are connected to the gravitation founda- tions 14 which rest substantially on the surface of the sea ^¬ bed 3.

Positions for the gravitation foundations 14 on the seabed 3 must for various embodiments of the invention be prepared prior to positioning the foundations 14 in order to ensure a correct installation height and distance between the individ ual foundations 14. For various embodiments of the invention, the installation may comprise one big gravitation foundation 14' to which all legs 7 are connected as it is shown in Figure 2, or the in ^¬ stallation may comprise two or more separate gravitation foundations 14 whereto the legs are connected as it is shown in Figure 1.

Another possible offshore foundation structure by which the invention can be realised is the so called tripod structure 21 that is schematically shown in Figure 3. In a tripod structure, three legs 27 support a main tube 22 carrying an installation like a wind turbine. In the shown embodiment, each leg 27 comprises a foot section 28 which is connected to a gravity foundation 24 which rests substantially on the sur- face of the seabed 23.

In an alternative embodiment of the tripod structure, the in ^¬ stallation may comprise one big gravitation foundation 24' to which all legs 7 are connected as it is shown in Figure 4.

The gravitation foundations 14 may for various embodiments comprise a skirt 15 which is established in the seabed 3 as it is illustrated in Figures 5. The shirt 15 shown in Figure 5 fully surrounds the periphery of the gravity foundation element 14. The skirt 15 ensures that erosion 16 around the foundation 14 does not flush away seabed material beneath the foundation 14 which in turn may alter the position of the foundation 14 and thereby the position of the whole struc ^¬ ture. The skirt may be present in all embodiments described with respect to figures 1 through 4.

The invention is characterized in that the foundation struc ^¬ ture (jacket or tripod) is established on one or more gravi ^¬ tation foundations positioned on the seabed.

The embodiment is advantageous in that it is easy to estab ^¬ lish. Gravitation foundations can be manufactured onshore and may also be connected to the structure before transporting 0

the whole construction to off-shore site. Hereafter the construction can be placed one step. This in turn is time saving and cost effective.