Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
The invention relates to an underwater craft equipped for inspect-
ing pipelines for leaks by acoustic means.
A leak in a pipeline for gases or liquids, in particular a small
leak, causes a hissing sound. This sound comprises both audible and ultra-
sonic vibrations and it has long been known that acoustic means may be used
in detecting and locating such leaks. In particular for pipelines for
liquids - oil or oil products - use is made of listening and recording
apparatus accommodated in a pig which is carried along by the stream of
matter through the pipeline. However, this method cannot always be used.
In pipelines for the transport of gas it is far more difficult to detect the
sound of a leak inside the pipeline than in a pipeline for liquids. Again,
in the case of pipelines which are connected to producing fields offshore it
is by no means easy to provide facilities for introducing and removing a pig.
Acoustic vibrations, both audible and ultrasonic, are very well
propagated in water and thus the idea of using a craft as the basis for
detection readily suggests itself. This craft may be a surface vessel, a
submarine, or, alter~atively, a manned or unmanned underwater craft towed
by a ship. The certainty of detecting a leak, however, constitutes a problem
and the invention indicates how this can be improved.
The invention therefore provides an underwater craft equipped for
inspecting a pipeline for leaks by acoustic means while the craft is moving
along the pipeline, which craft is equipped with two or more directional
hydrophones, said hydrophones being sound-receiving only and being acoustical-
ly accessible from outside the craft, said hydrophones being adapted to re-
ceive acoustical vibrations emanating directly from a leak and being adapted ;
in use to be aimed at the pipeline from various positions on the craft.
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The acoustic equipment may be tuned into a frequency or
frequency range that has been cho~en as being de~initely signi-
ficant for a leak. In addition, acoustic generators which imitate
the sound of a leak may have been installed at various places.
Hereinafter the term source will consi~stently be used to indicate
both leaks and generators.
The underwater craft may be a submarine or a towed underwater
cr~ft, called a fish. The craft follows the pipeline, either
through guidance by a helmsman or automatically, for instance by
means of steering gear magnetically coupled to the steel of the
pipeline.
The hydrophones may be fitted outside the hull of the craft or
inside, but ~till acoustically accessible, for;in~tance via an
opening or via a suitable membrane. Directional sensitivity can
be obtained by placing a hood around the vibration pick-up, which
hood is coated both inside and outside with sound-reflecting material.
This will reduce the influence of interfering sounds and amplify
the reception of source sounds.
The use of at least two hydrophones promotes reliability in
operation. By installing them at various places on the craft, certain
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advantages may be achieved. If ~G~hydrophones are ~c~beii-~ in one row
parallel to the longitudinal axis of the craft at its underside, the
signal from a source is detected by all these hydrophones. In addition
to the increased reliability in operation mentioned hereinbefore
25 other advantages may be achieved. All hydrophones may be aimed ~;
roughly at the same point of the pipeline. This increases the
sensitivity of detection by increasing the signal-to-noise ratio,
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so that sounds are detected with more certainty. Moreover, the
detectability limit islshifted to include weaker source~. If in
the processing of the signals a correlation technique is applied
the signal from a source is detected as a sharp and high peak,
which increases the advantages mentioned ev~n further.
Another possibility is that the directions of the hydro-
phones arranged in a row are parallel to each other. In thi~ case
a larger area of the pipeline is covered. A source will he observed
for a longer period of time. An advantage is that sounds origin-
ating from an impact or a tap can be recognized as such and areconsequently not erroneously recorded as originating from a
source.
Another possibility is that at least one pair of hydrophones
is presentJ the two hydrophones of a pair being installed opposite
each other in a plane perpendicular to the longitudinal axis of
the craft, projecting outside the craft to the left and to the
right, respectively, and aimed at the pipeline. The hydrophones
forming a pair may be positioned on booms. A considerable advantage
of this configuration is that a much larger area o~ the circum-
ference of the pipeline can be monitored directly, the "dark side"is considerably decreased. The distance between the hydrophones
may be 3-5 times the diameter of the pipeline. There may be for
instance 2-5 pairs of hydrophones arranged in two rows on either
side of the craft spaced along the length, each row being parallel
to the longitudinal axis of the craft. These hydrophones may again
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be aimed at the pipeline and parallel to each other or roughly
aimed at the same cross-section of the pipeline. The respective
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advantages of the two ways Or positioning are analogous to those
described hereinbefore.
An attractive combination of the various possibilities
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- mentioned is installati~ of three rows of hydrophones parallel
to the longitudinal axis of the craft, one row centrally at the
underside of the craft, one row projecting to the left and one
row to the right. In this way advantages already described separately
hereinbefore are combined.
The craft may be equipped with two fins slanting downwards9
and carrying the hydrophones that form pairs. This has great con-
structional advantages. Any degree of projection of the hydrophones
can be realized in a robust design and the stability of the craft
is increased.
The equipment for processing the signals received may be composed
of combinations, known in themselves, of selective amplifiers,
detectors, integrators, comparators and indicating/recording
apparatus. Correlators for two or more channels may be present
as well.
The invention will now be further explained with reference
to some ~igures.
Figs. 1, 2, 3 and 4 show some variants of an underwater
craft equipped with hydrophones. Figs. 5 and 6 show a hood con-
struction for hydrophones.
In Fig. 1 numeral 1 shows an underwater craft, in this case a
manned submarine. The craft may be 6 m long and 1.5 m wide. In
this example the submarine is at about 3 metres' distance from a
pipeline 2 having a diameter of 0.75 m. Affixed to the bottom of
the submarine are two directional hydrophones 3 and 4, one near
the stem and one near the stern of the submarine. Both hydrophones
are directed perpendicularly downwards and the submarine will
manoeuvre in such a way that the two hydrophones are adapted to be
aimed at pipeline 2. Broken lines 5 indicate the main direction of
sensitivity.
Figure 2 shows a submarine 10 over a pipeline 11, the sub-
marine being equipped with three hydrophones 12, 13 and 14 installed
in one row. These hydrophones are adapted to be aimed at one point
of the pipeline as indicated by broken line 15.
Figure 3 shows a submarine 20 in cross-section over a pipe-
line 21. Hydrophones 24 and 25 are attached to booms 22 and 23 pro-
jecting laterally. This pair of hydrophones is adapted to be aimed
at one and the same cross-section of pipeline 21, as indicated by
broken lines 26 and 27. A large area of the circumference of the
pipeline is thus brought within the more optimum directional sensi-
tivity of the hydrophones.
Figure 4 shows a bottom view of submarine 30. This sub- ;
marine is equipped with fins 31 and 32. Near the edges of these fins
hydrophones 33 and 3~ are located, each time four in a row parallel
to the longitudinal axis of the submarine. A third row of hydro-
phones 35 is affixed to the bottom of the submarine along the `~
centre line. Each pair of hydrophones 33 and 34, one opposite the
other li.e. in this case four pairs)~ is adapted to be aimed at one
and the same cross-section of the pipeline under inves~igation. So, ;~
in this case four pairs aimed at four different cross-sections.
~Iydrophones 35 are also aimed at that pipeline vertically downwards.
Figure 5 shows a cross-section of a hood for a hydrophone.
The hood
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consists of a basis 40 of strong material, for instance a metal or
a strong construction of reinforced polymeric material. The outside
is covered with a layer 41 of sound-reflecting material and the
inside with a layer 42 of the same material. Such a layer may con-
sist of a thin metal film applied to a layer of ~ound-insulating
material. Vibration-sensitive element 43 is located in a central
position. This may consist of a number of piezoelectric crystals.
If the apex angle of the hood is 90 all the sound waves entering
in the direction of the centre line will impinge on the crystals
in the same phase and thus be converted into a strong electric
signal. The directional sensitivity is then at its maximum in a
beam coinciding with the centre line of the hood, and having a
base equal to the bottom view of the hood.
Fig. 6 shows such a bottom view. The hood is provided with
one or more vents 44.
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