The present invention relates to a method for estab¬ lishing a connection from the sea bed to a tunnel which is provided in advance by blasting or drilling. Such tunnel connections are necessary in so called ocean

5. thermical energy plants producing energy by utilizing the temperature difference between the surface water and water on great depth.

In contructing power plants of the conventional design it is e.g. usual practice to establish a connec-

10. tion between a sub-water-formation tunnel and a sea by blasting from the inside, which will say from the rear side, the last portion of the tunnel, which will say the front portion of the tunnel, and the blasted rock is taken up by a cave blasted out in the tunnel. The security of

15. this method decreases with the water depth and at a depth of several hundred meters the method is considered not sufficiently safe in performance, paricularly with re¬ gard to water leakage in the last part of the tunnelling operation. Therefore it is necessary to develop a method

20. wherein a conventional tunnelling operation can be finished at a safe distance from the sea bed surface of the rock formation.

In connection with crude oil exploiting from the sea bed it is formerly known to finish a tunnel at a

25. distance from the sea bed surface and seal the front portion of the tunnel from the remaining part thereof and subsequently drill a hole from the inside of the tunnel through the remaining portion of the sea bed formation. A pipeline is then extended from the outside

30. through the drilled hole. The diameter of the pipeline is insignificant in relation to the diameter of the tunnel.

The object of this invention is to provide a safe method of establishing a connection of the kind stated

introductory, said connection having a flow cross, section, in hydraulic respect substantially correspon¬ ding to the cross section of the tunnel. The purpose of this invention is obtained by drilling from the sea

5. bed surface a plurality of drilling holes ending in the tunnel and having a total flow cross section which in hydrodynamic respect is corresponding at least to the cross section of the tunnel. To release the large pressure difference between the water side and the air

10v side, the tunnel is filled with water before establish¬ ing the through-flow-connection. The drilling operation is carried out preferably from a floating or semi- submergeable drilling vessel or the like and drilling templates are used to line. The

15. method according to the invention is applicable for vertical drilling as well as for inclined drilling.

The invention solves the problems in a simple and safe manner in the previously known flow communication establishing methods. Instead of carrying out the

20. establishing operation from the inside, the operation by drilling is carried out from the outer side, which will say from the water side. In this way it has been obtained that the tunnelling operations can be finished at a sufficiently safe distance from the surface of the

25. rock formation.

The invention shall be explained more closely in the following by means of examples and with reference to the drawings, wherein

Fig. 1 is illustrating diagramatically a vertical

30. drilling operation for obtaining a tunnel connection in accordance with the invention, fig. 2 is illustrating inclined drilling, fig. 3 is illustrating mounting of the drilling line for a inclined drilling operation and fig. 4 is showing diagramatically a drilling template

35. with a guide funnel.

Fig. 1 shows a vertical cross section formation

3 . 00073

with a cold water tunnel for an ocean thermal energy plant. For such an energy plant, e.g. of 5 MW, it can be suitable to provide a tunnel for receiving cold water

2 having a cross section of ca. 20 m . In the figure, 1

5. is the rock surface,.(sea bottom surface), 2 is the vertical portion of the rock tunnel and 3 is the hori¬ zontal portion of the tunnel. An alternative embodiment with a curved tunnel is shown at 4. The drilling opera¬ tion in connection with the tunnelling is carried out by

10. means of a drill ship or a semi submergeable drilling platform 5. The platform is anchored above the drilling place, e.g. by means of dynamic positioning. The start¬ ing of the hole drilling is performed preferably with the help of a steel template frame 6 adjusted in advance

15. and having centring means for the drilling string 7.

The number of drilling holes 8 is determined by the necessary cross section. A cave for rock drilled out and blasting rock rest if any, is shown at 9. The normal maximum diameter in crude oil industry is at the present

20. 36", which will say that 25-30 holes are needed for a

2 cross section of 20 m . The hole length will depend on the geometry of the rock surface (sea bed) and the geography of the tunnel.

By a suitable placing of the contact location the

25. drilling operation of the holes can be performed quickly, effectively and economically and above all safely.

Fig. 2 is illustrating an alternative method. The drill holes 8 are drilled obliquely in order to obtain better flowing conditions at the inlet to the tunnel 3

30. or/and the cave or rock space 9, resp. It appears from the figure;.that the extension of the cave is relatively large in relation to the cross section of the tunnel. Thereby, a greater possibility is provided for a suit¬ able locating of a great number of drilling holes, and

35. thereby a better water supply and better hydraulic relations at the inlet to the tunnel. It also appears

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from the figure that the drilling template 6 is provided with a guide funnel 10 which is pivotably journaled at 11. The drilling string is extending along an arcuade path from the drilling vessel 5 to the guide funnel 10

5. of the template so that the lowest portion of the dril¬ ling string with the drilling head is registering (on line) with the intended drilling direction. The template can be made with as many guides as necessary for drilling holes in the sea bed, however, it can also be provided

10. with one or more guides which can be moved longitudinally and transversally on the drilling template frame. Such drilling templates are known from vertical drilling in a substantially horizontal sea bed. The guide funnel 10 may also be arranged for rotary movement so that ar..

15. circular drilling hole design is obtained.

Fig. 3 illustrates a method for adapting the drilling string for inclined drilling. The drilling template 6 is provided with a guide funnel 10 for accomodating the lowermost portion of the drilling string (see also fig.

20. 4) . In position I the drilling vessel is located straight above the guide funnel and the drilling head is guided into the funnel as a consequence of the weight of the drilling string. Subsequently, the drilling vessel is displaced to position II and at the same time new drilling

25. string sections are put into the string while the guide funnel 10 is provided to obtain a. correct drilling angle. An auxiliary vessel 12 helps to keep the drilling head in position when moving the drilling vessel.

Fig. 4 shows diagramatically a drilling template

30. frame 6 shaped as a sledge (sled) for moving across a sea bed and provided with a pivotable guide funnel 10 as also shown in fig. 2.