Note: Descriptions are shown in the official language in which they were submitted.
2027~9
DRILL TUBE
Technical Field
This invention relates to the manufacture of drill
tubes of the type adapted to be progressively ~oined end to
end to prod-~ce a string of drill tubes rotatably driven on a
drilling rig to drill oil wells, bore holes and liXe
drilling applications.
Backqround Art
Drill tubes for such applications are conventionally
formed by a piercing process resulting in a seamless tube,
and can also be formed by rolling flat steel sheet into a
tube and welding the abutting longitudinal edges together to
form a seamed tube. In both cases the necessary internally
and externally threaded components at opposite ends of the
tube, which are used for the purposes of stringing thè tubes
together in a drilling rig, are subsequently inserted into
the ends of the tube and welded thereto.
2~7~69
The main disadvantages with such tube forming
techniques is their high cost due basically to the
relatively complex manufacturing techniques and associated
equipment, and the relatively small number of suppliers. It
is, therefore, an object of the present invention to provide
a method of manufacturing a drill tube for drilling
operations which is less costly than the piercing and
fabricating techniques referred to above, and which also
utilizes manufacturing facilities which are more readily
available. The problem with the prior art technique
involving a piercing process is likely to become greater as
general steel prices increase, particularly as such the
piercing technique requires as starting stock wrought round
steel rod.
Disclosure of the Invention
In accordance with the invention there is envisaged
a method of manufacturing a drill tube for drilling
operations, comprising centrifugally casting a plurality of
tube lengths and ~oining said tube lengths together end to
end to form a drill tube of the desired length.
Centrifugal casting of tube is generally cheaper
than the piercing and fabricating technigues discussed
previously, whilst the availability of manufacturing
facilities is greater. In addition, since the present
invention can utilize scrap steel, further significant cost
savings can be achieved. Furthermore, because the present
invention involves a casting technique, there is the
possibility to change the alloy composition of the steel
allowing for a lower wear rate and thus longer life.
Pierced tubes in particular have limitations with regard to
variations in the alloy composition as the piercing process
requires limitation on the alloy composition which may be
used. Currently tubes now in use are operational for
approximately three months, after which time they have to be
retracted from the drill hole and removed to a workshop
where their diameter is built up to replace worn material by
continuous welding process, involving further costs. Use of
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the correct high wear resistant alloys, made possible by the
present invention, leads to an increase in the life span of
the tube. If the costs envisaged can be reduced
sufficiently it may be possible to discard a worn tube
rather than rebuild its circumferential diameter.
The threaded end fittings for the tube may be
subsequently joined to the respective ends of the
manufactured drill tube at the same time as it is
manufactured, or connected later by the end user of the
drill tube. Alternatively, the production of the end
fittings could be incorporated as part of the centrifugal
casting operation and completed by appropriate machining and
thread forming techniques.
The invention also envisages a drill tube
manufactured in accordance with the method defined above.
Descri~tion of the Drawinq
One preferred embodiment of the invention will now
be described by way of example, with reference to the
accompanying drawing. The drawing represents a longitudinal
view, partially in section, of a completed drill tube
manufactured in accordance with the method of the present
invention.
Best Mode for CarrYinq Out the Invention
In the example of this preferred embodiment, the
main length of the drill tube designated as 10 is formed
from a plurality of thick wall tube lengths 11. Each tube
length 11 is centrifugally cast from a suitable metallic
material, using a permanent cylindrical mould which is spun
at high speed whilst molten metal is poured in, and which
results in the metal being flung to the inner surface of the
mould to produce a hollow cylinder or tube of uniform wall
thickness. The resulting tube lengths have a uniformly fine
grain outer surface, and in consequence can also be expected
to be sounder with fewer inclusions. The individual tube
lengths 11 are then welded end to end to produce welds 12
between their ends, and collectively the tube lengths make
up the desired length of the drill tube 10.
`-- 2027969
In order to complete the drilled tube for use in
drilling applications, a conventional externally threaded
end component 13 is welded at 14 to one end of the drill
tube, and a conventional internally threaded end component
15 welded at 16 to the other end of the drill tube. The end
components 13 and 15 may be welded to the drill tube at the
point of manufacture or alternatively connected later by the
end user of the tube. One other possible alternative is to
partially form the relevant end components as part of a
special centrifugally cast tube length 11. The technique
involves forming the relevant end of the final drill tube,
during the centrifugal casting of that tube length, with the
end components being finished by machining and thread
forming operations utilizing appropriate equipment.
In the example of the invention as described, a
typical drill tube 10 may be formed from four centrifugally
cast tube lengths 11 of 7.5 feet in length, which when
welded end to end result in a drill tube of 30 feet in
length.
The centrifugal casting of the tube lengths 11
should be controlled to produce uniform tube wall thickness
for each length to make up the drill tube. These wall
thicknesses may conform with the range of wall thicknesses
normally available in respect of conventional pierced
seamless and fabricated seamed drill tubes. The thicknesses
are dependent on the type and depth of the drilling
operation to which the drill tube will be applied.
It is expected that once full production of drill
tube in accordance with the method of the present invention
commences that the price (in 1990 dollar terms) of a drill
tube of conventional length will be as low as AS8,500.00.
This may be compared with a drill tube length manufactured
in accordance with known techniques which can be as high as
AS13,000.00.
