In many areas in which drilling takes place on ocean beds with so-called loose formations, there are major challenges directed towards prevention of wash out.

Such a wash out occurs when large volumes of fluid flush out from the hole. Such volumes of fluid are normally between 2000 and 4500 litres per minute. As these ocean beds basically consist of sand, such wash outs can be compared with, for example, placing a hosepipe down into a sand dune and simply hoping that the flow channel will not be larger than the diameter of the hosepipe itself.

Holes that are drilled in top-hole drilling are normally of a size of 36" (914.4 mm) and have a drill depth of between 35 and 65 metres. The wash out can lead to sand from up to 20 metres down into the well starting to slip, and this can further lead to the disastrous result of that the outlet hole becomes up to 15 metres in diameter. In connection with drilling templates that are normally 3.5

(1) x 3.5 (w) metres in size being fitted on the ocean bed, this will constitute an element of risk for the oil operator.

Therefore, the object of the invention is to prevent such wash out of top holes at oil related drilling, or at least considerably reduce said wash out to a minimum.

Thus, the present invention relates to a device for safeguarding against wash out which will ensure that such top holes can be drilled without the risk of wash out. The safeguard against wash out can ensure that at least about 80% of the fluid stream in the well disappears without affecting the necessary supply of fluid to the drill bit.

The device according to the invention is characterised in that a drill stem is composed of a number of drill pipes which each comprises an inner centre pipe and an outer pipe arranged around the inner centre pipe so that an annular space between said inner and outer pipes is provided, and where the annular space between the inner and outer pipes is arranged to transport return fluid out'of the drill hole.

Alternative embodiments of the invention are characterised in that a number of braces can be arranged mutually spaced apart between the inner centre pipe and the outer pipe, where the braces are preferably arranged circularly around the inner pipe. The number of braces can be arranged in groups, and each group can be arranged mutually spaced apart in the longitudinal direction of the pipe.

A lower part of a lower drill pipe in the drill stem can comprise a fluid collector, where the fluid collector comprises a ring-formed skirt with a diameter that is larger than the outer pipe. A number of drill cutters can be arranged on an upper, outer edge section of the fluid collector. The outer edge section is preferentially tilted and ring-formed, and extends from an upper section of the skirt to the outer pipe, and a lower part of the skirt can encompass, at its outer circumference, an area coated with a hard metal.

The procedure according to the invention is characterised in that fluid that is produced during the drilling of the top hole is led out of the bore hole through an annular space'which is provided in the safeguard against wash out, where the wash out safeguard comprises a number of assembled together drill pipes.

Alternative embodiments of the procedure are characterised in that the annular space in the drill pipe is provided between an inner centre pipe and an outer pipe that surrounds the inner pipe. The drill bit can be arranged adjacent to a lower drill pipe at a distance preferentially less that 1 metre, and the drill bit can preferably be arranged adjacent to the lower drill pipe at a distance preferably around 40 cm. Furthermore, fluid is led into the annular space in the drill pipe through a fluid collector which is fitted to the lower part of the lower drill pipe. The top hole can be drilled to a depth of about 20-100 metres, preferably 30-80 metres and even more preferred 35-65 metres.

The invention shall now be described in more detail with the help of the enclosed figures, in which: Figure 1 shows an example of a formation in which drilling of a bore hole is carried out on the ocean bed, figure 2 shows corresponding to fig. 1 what can happen in wash out of the hole, figure 3 shows a section of an assembly of two drill pipes according to the invention, figure 4 shows a section of a lower part of a wash out safeguard according to the invention, and figure 5 shows a section of a fluid collector according to the invention A drill stem for drilling of a top hole on the ocean bed is shown in figures 1 and 2. The shaded area illustrates an area that can be subjected to a wash out.

Such-wash outs arise because of the large volumes of return fluid out of the hole. Such volumes of fluid are normally between 2000 and 4500 litres per minute. As these ocean beds are basically comprised of sand, the large fluid

volumes can thereby"drag"'sand out from a conventional top hole. A catastrophic result is shown in figure 2, where the whole drilling template has partially fallen into the drill hole. As the wash out can occur all the way down to 20 metres from the ocean bed, it is obvious that this can lead to considerable economic losses for the operator if these mishaps occur. As mentioned, the present invention is therefore developed to prevent such wash out in the drill- ing of a top hole.

The safeguard against wash out according to the invention comprises an assembled drill stem 10 which is arranged to transport the large volumes of fluid out of the bore hole without the fluid being able to do damage to the wall of the bore hole. Each drill pipe 12 in the drill stem comprises an inner centre pipe 14, which, for example, can be of a standard drill pipe type. One example can be a standard drill pipe with a diameter of 9.5" (214.3 mm).

An outer pipe 16 is arranged around the inner centre pipe such that a longitudinal annular space 18 is provided between the inner and the outer pipes in the longitudinal direction of the drill pipe. The annular space 18 can extend the whole length of the drill stem 10. Furthermore the annular space 18 is arranged to transport the fluid which is produced out of the drill stem, so that wash out is prevented. The drill pipes 12 can be assembled in a traditional way in that a coupling part 20 is fitted at the top and bottom of the pipe and thereby make it possible for several drill pipes 12 to be connected. In the transition between two drill pipes, the outer pipe 16 of a drill pipe can, of course, comprise a coupling 24 that overlaps the outer pipe on the adjacent drill pipe in the drill stem.

Braces 22 are fitted in the annular space 18 to reinforce the drill pipe, where the braces can be comprised of, for example, intermediate plates. The braces 22 are welded, preferably by glow welding, to the outside of the inner centre pipe 14 and to the inside of the outer pipe 16, and will normally be placed mutually spaced circularly around the inner centre pipe. Each drill pipe 12 can

comprise several groups of braces 22, which can be placed in groups in the drill pipe 12. The braces will thereby not prevent the flow of fluid in the drill pipe and give also a large area for the flow of fluid in the drill stem.

A lower drill pipe 12'that is assembled as described above is shown in figure 3. In addition, this drill pipe 12'comprises a fluid collector 30 at its lower end. The fluid collector 30 is mounted at the bottom of the wash out safeguard and is preferably mounted directly above the largest drill bit (not shown) which is used in the drill stem. The fluid collector 30 can have a diameter of, for example, 34" (836.6 mm), which is 50 mm less that the largest drill bit. This is to prevent the fluid collector touching the well wall. It is also important to get a certain amount of drilling mud past so that this can lie against the well wall and form a protection. The fluid collector 30 can be mounted so that the distance down to the largest drill bit is only about 40 cm.

The fluid collector 30 preferably comprises a ring- formed skirt 34 with a diameter 32 that is larger than the outer pipe 16. Furthermore, drill cutters 32 can be fitted onto the fluid collector 30, where the drill cutters are arranged to function as cutting knives when the drill stem 10 is to be retracted from the hole. This enables the operator to drill himself out when retracting if solids have fallen into the well. As well as if large stones have fallen into the well. The drill cutters, for example, four cutters, can be fitted mutually spaced apart around the fluid collector and the drill cutters 32 are preferably arranged on an outer edge section of the fluid collector 30.

The outer edge section is preferably tilted and ring- formed, and extends from an upper section of the skirt 34 to the outer pipe 16, whereby the drill cutters 32 are also tilted. The lower part of the skirt 34 can also comprise an area 30 coated with a hard metal, in a length of, for example, 100 mm from the lower edge of the skirt. The aim of the coated area with hard metal is to protect the skirt

when the coated area with hard metal touches the well wall . during drilling or retraction.

It shall be noted that the dimensions shown and described in the given drawings and description, must not be regarded as absolute values, other dimensions can also be used dependent on formation, bore holes etc.