During the connection of large offshore structures, substantial forces and associated moments arise when the relative movements in both the vertical plane and the horizontal plane are to be stopped as the transfer of weight from the equipment supply vessel to the substructure is carried out. Therefore, when a deck structure or the like is connected to a fixed substructure on the seabed, special shock absorbers that are secured to the substructure are normally used. In concrete substructures these shock absorbers are embedded in each leg of the substructure. In simple terms, these shock absorbers consist of a receptacle that is anchored in the substructure, in which receptacle there is provided another receptacle capable of moving in the vertical plane, and which is cushioned by, for example, large rubber elements. When subjected to a vertical load, this inner receptacle will thus be capable of moving downwards in the outer receptacle, and the impact in the vertical direction is dampened by the said rubber elements.

Movements in the vertical plane are absorbed in that the inner receptacle is capable of moving a given distance in the outer receptacle. In the prior art, the inner receptacle is supported in some way or other at the lower end, so that when the inner receptacle is subjected to horizontal loads, it describes a pendulum movement which is limited by rubber elements (shock absorbers) at the upper ends of the receptacles.

The inner and the outer receptacles, and the resilient elements, are of course dimensioned according to the size of the structures that are interconnectable and the associated moments that are to be absorbed. In the case of large structures, this may involve substantial dimensions as these shock absorbers should be able to absorb dynamic loads of 3,000 to 4,000 tonnes in the horizontal plane, and between 5,000 and 10,000 tonnes in the vertical plane. This requires both receptacles to be long and the outer receptacle to be sufficiently wide to allow the inner receptacle to move the required distance. Typical dimensions for known shock absorbers are a total height (for both receptacles) of about 12 metres, and an external diameter of the outer receptacle of about 4 metres. It is obvious that structures of such dimensions are relatively costly to produce, bulky to install and require a great deal of equipment for installation in the substructure. It is therefore desirable to have shock absorbers that are more compact,

but that solve the same problem during the connection operation. A compact shock absorber will be somewhat cheaper to produce, and will of course be lighter and require less space for installation in the substructure. As is well known, both size and weight are important parameters in the construction of offshore structures. It is desirable to minimise both.

Accordingly, such an apparatus is provided for use in connecting two structures, comprising an outer receptacle having at least one bottom and at least one wall, mountable to one of the structures and containing a first receptacle which in one area is adapted for receiving at least one part of the other structure, wherein the first receptacle is adapted for movement relative to the bottom of the outer receptacle, limited by at least one resilient element. The apparatus according to the invention is characterised in that the first receptacle is received in the second receptacle which is located in the outer receptacle and adapted, when subjected to a relative movement between the two structures, essentially by translation, to move in the first receptacle.

Preferred features of the apparatus according to the invention are disclosed in attached claims 2 to 9.

With the apparatus according to the invention, the central point of which being that the inner receptacle moves by translation with minimum friction, and not rotation, a substantial saving of weight and space is obtained. Whilst the prior art shock absorbers have a length of 12 metres or more under given conditions, a shock absorber based on the principles according to the invention, and adapted for the same load, will only require a total height of about 4 metres. The height of the inventive shock absorber is thus reduced by about 2/3 of the height of the known shock absorbers.

The present invention will now be described with reference to the attached drawings where like parts are identified by like reference numerals.

Figure 1 is a schematic diagram of the apparatus according to the invention in a neutral state.

Figure 2a, like Figure 1, is also a schematic diagram of the apparatus according to the invention, and shows the first receptacle in a slightly depressed state when subjected to a vertical load.

Figure 2b, like the above, is also a schematic diagram of the apparatus according to the invention where the first receptacle is slightly depressed under application of a vertical load, and the second receptacle is pressed slightly to the right under application of a horizontal load.

Figure 3 is yet another schematic diagram of the apparatus according to the invention, embedded in a platform substructure.

Figure 4 is a simplified diagram of the apparatus according to the invention, equipped with an anti-friction coating.

The apparatus 10 according to the invention consists of an outer receptacle 12 which, as mentioned, can be embedded in a substructure using, for example, a cementaceous substance 26. In practical cases, a steel cylinder 22 is often embedded in the substructure (the shafts) first, partly shown and indicated by the reference numeral 24 in Figure 3. The outer receptacle is then inserted into a hollow space in the steel cylinder 22 on top of the shafts and cemented in place as mentioned. In the case of other types of substructure, the outer receptacle is secured in an appropriate known manner.

Thus, the figures show a second receptacle 14, which in this case rests on the inner bottom of the outer receptacle. It can be seen from the figures how this second receptacle is able to move by translation (in the figures in the horizontal plane), and is limited by the resilient elements 16'. These resilient elements are, as a rule, rubber elements, sometimes laminated with steel plate for reasons of construction, but this is prior art and is not comprised by the invention.

When the connection operation is carried out and the apparatus according to the invention performs its function, a part of the second structure (not shown), which, for instance, may be a platform deck or the like, will engage with the receiving area 28 of the first receptacle 30. Usually, there are guide pins mounted on the platform deck which pass into the first receptacle 30 during connection. The receiving area 28 is designed in a known way with bevelled faces. The bevelling (a) may typically be 40°.

When the said guide pins engage with the first receptacle 30, this receptacle can be pressed into the second receptacle 14, as indicated in Figure 2a. The resilient element 16 is compressed. Horizontal forces cause the second receptacle 14 to move in the horizontal plane (translation) as indicated in Figure 2b. Similarly, the right-hand resilient element 16'is compressed, whilst the left-hand resilient element 16'moves

with the receptacle. To counteract any tendency to rotation, protrusions 20 are provided at the bottom part of the outer receptacle. These form a cavity into which a projection 18 (e. g. , in the form of a disc) on the bottom of the second receptacle 14 can slide.

To facilitate the translational movement of the second receptacle 14, a friction-reducing element 19 may be installed or applied between the second receptacle 14 and the outer receptacle 12, and between the projecting part 18 and the protrusions 20, e. g. , as shown in Figure 4.

The width (the diameter) of the outer receptacle, which thus defines the permitted translational distance for the first receptacle, must of course be dimensioned for the intended purpose. For a use that is comparable to the examples given above, the outer receptacle 12 could have a diameter of about 5 metres whilst the first receptacle 30 may have a diameter of about 2.5 metres. The height of the whole structure is, as mentioned, about 4 metres. The total weight of the whole structure, used for the purpose the parameters of which are mentioned above, is about 140 to 200 tonnes. The corresponding weight for a shock absorber according to the prior art is about twice as much, about 360 tonnes.

Although the apparatus according to the invention has been described for use in connecting a platform superstructure and a fixed platform substructure, the invention should not necessarily be limited to this. Nor should the invention be limited to apparatus having a circular cross-section, although these are the most expedient.

Furthermore, the skilled person will understand that the apparatus according to the invention will function independent of its orientation, i. e. , that the invention should not be limited to the horizontal and vertical planes which are mentioned above. It is also conceivable that the apparatus according to the invention could be mounted on the large structure that is in motion whilst the guide pins are in the stationary part. The skilled person will understand that it is the relative movement that is the essence of the invention.