Note: Descriptions are shown in the official language in which they were submitted.
CA 02829206 2013-09-05
DEVICE FOR MEASURING DEFORMATIONS OF THE GROUND
The invention relates to devices for measuring strain distribution that use
optical fiber as
the sensing element.
The integrity and operability of distributed objects are mainly determined by
properties
and condition of a ground where they are laid. As a rule, damages to
distributed objects,
such as underground pipelines, roads, tunnels, etc., are caused by ground
movements or
by unauthorized digging. Problems relating to the integrity of underground
distributed
objects are most acute when such objects are laid under water, in mountain
regions (on
slopes) and in the conditions of thawing and freezing of a ground surrounding
them. In
order to prevent emergencies in distributed objects, continuous or periodic
monitoring of
ground movements (dislocations) and its temperature in a close proximity to an
object is
used. A optical detection cable is laid in the ground region subject to the
risk of
dislocations in such a way that ground movements can cause elastic tensile and
compression strains in segments of optical fibers constituting the cable.
Longitudinal
strain of an optical fiber is measured and used for analyzing ground
movements. An
optical fiber designed for measuring strain distributions is arranged in a
special detection
cable that, on one side, enables the fiber to deform (expand and compress)
under the
influence of external forces and, on the other side, protects it against
adverse external
effects in the processes of assembly and operation.
Optical fibers, as used in a detection cable, have a limited range of
allowable strains and
a corresponding range of cable tensile loads. If a maximum allowable tensile
load of a
detection cable that usually corresponds to extension of an optical fiber by
1% - 2% is
exceeded, the optical fiber breaks, which results in the impossibility of
using the whole
detection cable or a part thereof as a sensing element. Therefore, in order to
restore
operability, it is required to restore the integrity of the detection cable,
which is
associated with labor-intensive earth works for replacing its damaged section.
A device for measuring strain is known (see: RU Patent No. 2346235, published
on
27.07.2008), wherein a method is used that is based on the phenomenon of
stimulated
Brillouin scattering appearing in an optical fiber. According to this method,
an optical
fiber is used as a sensing element for detecting strain and/or temperature in
a medium
wherein the optical fiber is arranged. This device comprises an excitation
light radiation
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CA 02829206 2013-09-05
'
'
source 1, a sensitive optical fiber 2, an optical coupler 3, a sounding light
radiation
source 4 and a detector 5 (Figure 1). One end of the sensitive optical fiber 2
is connected
to the excitation light radiation source 1, and the second end is connected to
the sounding
light radiation source 4 and the detector 5 through the optical coupler 3.
At present, instruments are manufactured and commercially available wherein a
method
of measuring axial strain (tension or compression) distribution is used that
are based on
the phenomenon of stimulated Brillouin scattering. Examples of such devices
are the
Brillouin analyzer Ditest STA-R
manufactured by Omnisens SA [URL:
http://www.omnisens.ch/ditest/3521-ditest-sta-r.pbp, logging in date
11/02/111,
Switzerland, and the Brillouin reflectometer AQ8603 OPTICAL FIBER STRAIN
ANALYZER manufactured by Yokogawa Electric Corporation [URL:
http ://tmi.yokogawa.com/products/optical-measuring-instruments/optical-
sensing-
products/aq8603-optical-fiber-strain-analyzer/, logging in date 11/02/11].
A method and a device for monitoring a pipeline are also known [Long-distance
fiber
optic sensing solutions for pipeline leakage, intrusion and ground movement
detection.
Marc Nikles Omnisens S.A. "SPIE Defense, Security and Sensing Conference",
April
15-17, 2009, Orlando, Florida, USA, Proceedings of SPIE Vol. 7316, 7316-01
(2009)].
The method includes continuous monitoring of ground movements and temperatures
in a
close proximity to a pipeline 6 with the use of the device comprising a
monitoring unit 7
that includes a Brillouin analyzer, an optical switch and an optical cross-
connect and
may be located, e.g., in pipeline compressor stations, and detection cables 8
for
measuring temperatures 8 and detection cables 9 for measuring ground movements
that
are connected thereto (Figure 2). The monitoring unit 7 may be coupled via a
network
interface 10 to a distant control point 11. The pipeline monitoring device 5
fulfills the
requirements to pipeline integrity monitoring systems, measuring temperature
and strain
distributions along respective detection cables at distances typical for
pipelines, e.g.,
corresponding to a distance between pipeline compressor stations.
A device is known [DITEST SMARTEX SENSOR. - URL: http://www.smartec.ch
/PDF/SDS%2011.1050%20DiTeSt%20SMART Geo Tex% 20Fabric.pdf, logging in date
13.07.2010] that is intended for improving ground dislocation monitoring
accuracy and
that has an increased cable adhesion to ground surrounding it. The device is a
geotextile
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CA 02829206 2013-09-05
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material having a detection cable integrated therein for measuring strain. The
device
consists of non-woven material bands that are arranged on a cable and envelope
it with a
gap. This enables to achieve an increased area of contact between bands and
ground and,
hence, improved adhesion. But this device has the following disadvantages. It
does not
enable to accurately fix initial lateral dislocations of ground due to the gap
between the
cable and the bands as well as due to compliance of the band material.
Initially, when
lateral movements of ground are small, the bands move together with the ground
relative
to the cable within the limits of the gap, then, after the gap is closed, the
bands deform
and transfer a part of the load to the cable, and then, when ground movements
become
greater, the bands and the cable move jointly. All this leads to lower results
during
determination of initial lateral movements of the ground. Furthermore, since
the bands
are not attached to the cable longitudinally, axial slipping of the cable
occurs in regions
located on both sides from the area of a ground movement. Such cable slipping
introduces errors into accuracy of determining a location of a ground
movement. A
length of every region of cable longitudinal slipping is determined according
to a friction
force increasing along the cable length and required in order to keep it in a
stable part of
the ground.
The closest technical solution (prototype) is a device (Defining and
monitoring of
landslide boundaries using fiber optic systems. M. Iten, A. Schmid, D.
Hauswirth &
A.M. Puzrin. Prediction and Simulation Methods for Geohazard Mitigation - Oka,
Murakami & Kimoto (eds), 2009 Taylor & Francis Group, London, ISBN 978-0-415-
80482-0) comprising a Brillouin analyzer manufactured by Omnisens and a
sensing
system embedded into the ground. A Brillouin analyzer enables to measure
strain
distribution on a detection cable. The sensing system comprises anchors 12
rigidly fixed
on the detection cable for measuring strain 13 in pre-determined points
(Figure 3). The
dimensions of the anchors 12 are determined experimentally, according to a
measured
force of securing an anchor 12 in the ground. The design of each anchor 12
ensures
movement of the cable together with the surrounding ground, preventing the
ground
from flowing around the cable 13. This device has been used for defining
boundaries of
ground movements (landslides).
But this device has the following disadvantages. Since the anchor is rigidly
fixed on the
detection cable, a maximum load transferred by the anchor to the detection
cable is
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CA 02829206 2013-09-05
determined by strength of anchor fixation in the ground. Strength of anchor
fixation in
the ground depends on the anchor shape and ground properties that may change
during
operation, for example, when a ground density is changed over time, as a
result of
compaction, lowering of temperature or change of a ground moisture. If
dislocation of
the ground and the anchors together with the ground is significant (occurring
quickly or
slowly developing over time), a load transferred to the detection cable by the
anchor may
exceed an allowable tensile load of the cable, which will put it out of
operation
irreversibly. Secure engagement with the ground becomes a disadvantage in such
extreme operation conditions.
The technical effect of the invention is limitation of a load transferred to
the detection
cable by the anchor, when the anchors dislocate together with the ground
irrespective of
the ground conditions that may be known imprecisely or may change over time,
and, due
to this, an increase in the service life of the detection cable. An
accompanying particular
technical effect of the invention is maintenance of operability of the
deformable
mechanical safety device after its operation.
The said technical effect is achieved due to the fact that the known device
for measuring
ground strain, which comprises a strain-sensitive optical cable, a measuring
unit coupled
with the cable, anchors connected to the cable and to the ground, is provided,
according
to the claimed invention, with a system protecting the cable against breaking,
the system
including a safety device for each anchor.
Each anchor may be connected to the cable by a releasable clamp and to the
ground by a
thrust plate.
The safety device may include a fastening member provided with the possibility
of
fastening the thrust plate to the releasable clamp, the said fastening member
breaking
when a pre-determined load is applied to the fastening member, thus ensuring
free
movement of the cable relative to the thrust plate.
The fastening member may be made as latches located on the anchor thrust plate
and
being in engagement with the releasable clamp.
The safety device may include a fastening member provided with the possibility
of
securing the cable to the releasable clamp, the said fastening member
deforming when a
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CA 02829206 2013-09-05
pre-determined load is applied to the fastening member, thus ensuring free
movement of
the cable relative to the thrust plate.
The fastening member may be made as an longitudinal slot through which the
cable is
placed into the longitudinal channel, and the thrust plate is rigidly
connected to the
releasable clamp. The invention is illustrated on the drawings, wherein:
Figure 1 shows the device for measuring strain;
Figure 2 shows a diagram of a device for monitoring a pipeline;
Figure 3 shows a diagram of a sensing system embedded into the ground, which
comprises anchors arranged on a detection cable;
Figure 4 shows a sensing system comprising anchors arranged on a detection
cable;
Figure 5 shows a top view of a device embodiment with an anchor having a
breakable
mechanical safety device;
Figure 6 shows a front view of a device embodiment with an anchor having a
breakable
mechanical safety device;
Figure 7 shows a front view of one of the two identical parts 24 composing a
thrust plate
for the device embodiment with an anchor having a breakable mechanical safety
device;
Figure 8 shows a top view and a longitudinal section of the anchor in the
device
embodiment with the anchor having a deformable mechanical safety device;
Figure 9 shows a front view of the device embodiment with the anchor having
the
deformable mechanical safety device.
The claimed device comprises a measuring unit, which is a Brillouin analyzer
or another
similar device for measuring strain distribution of an optical fiber, and a
sensing system
embedded into the ground. The sensing system comprises a optical detection
cable 14
and anchors 15, 16, 17 rigidly arranged thereon in pre-determined points
(Figure 4). The
sensing system is arranged under the surface of the ground 18 at a certain
depth.
According to the claimed invention, the detection cable 14 senses a tensile
load along its
axis, and each anchor 15, 16, 17 has a thrust plate 19 perpendicular to the
cable axis and
secured to the detection cable 14 (Figures 5-8). The anchor thrust plate 19
cooperates
with the immovable ground and transfers a dislocation load of the detection
cable 14 and
CA 02829206 2013-09-05
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the anchors 15, 16 being in a movable and transitional ground regions. The
anchor thrust
plate 19 has a surface area sufficient for preventing the anchor from moving
in the
ground under influence of a load acting upon it from the side of the cable
along its axis.
If a ground movement (dislocation) 30 occurs, the anchors 15, 16 move together
with the
ground in the direction shown by the arrows in the area of the ground movement
(Figure
4). The anchor 17 located in the region of immovable ground is fixed therein.
Thus, the
cable rigidly secured to the anchors will deform (become longer) on the
movement area
boundary where a distance between the anchors changes (increases). A relative
elongation of the cable and, correspondingly, the optical fiber is measured
with a
Brillouin analyzer and is used for analyzing a position and parameters of
ground
movements. The relative elongation 0 (a dimensionless quantity) of a uniformly
elongated cable segment with a length L may be calculated according to the
following
formula:
El = D/L,
where: L ¨ a length of a segment in the non-deformed condition, in mm; and
0 - change in the segment length in the result of deformation, in mm.
Further, a tensile load of any cable segment secured on its both ends to
anchors is
associated with a cable specific elongation caused by shift of the anchors
relative to each
other in the result of a ground movement. In accordance with the Hooke's law,
they are
proportional to each other within the range of small strains (El 1).
F = kEID,
where: F ¨ tensile load, in newtons; and
k ¨ proportionality coefficient (rigidity), in newtons.
However, the cable resistance to a tensile load is limited by a quantity
typical for each
cable type, which is usually indicated in the specification (DiTeSt SMARTube
Sensor-
URL:
http://www.roctest-
group.com/sites/default/files/datasheets/products/SDS%2011.1040%20DiTeSt%20SMA
RT ube%20Sensor.pdf, , Logging in date 27.02.2011).
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CA 02829206 2013-09-05
In order to prevent a cable from breaking, the claimed device is provided with
a system
for protecting a cable against breaking, which system comprises a safety
device
embedded into each anchor, and the safety device operates in a case where a
load acting
upon the detection cable from the anchor exceeds a pre-determined value
(operation
threshold). As the safety device operates, the cable moves in the ground under
the action
of a tensile load, and, consequently, the tensile load in a dangerous region
lessens, thus
preventing the cable from breaking. Since a tensile load on the cable is
increased by the
value of the load under the action of the force acting upon the cable from the
side of the
anchor, the operation threshold should be significantly (depending on supposed
parameters of ground movements) lower than the cable resistance to the tensile
load. If it
is supposed that a movement is possible only in one point of the sensing
system, as
embedded into the ground, in a segment between two anchors, then it is
sufficient that
the operation threshold will be insignificantly lower than the cable
resistance to the
tensile load (by the value of the sum of errors in determination of such
parameters). The
setting of the operation threshold in the anchor design enables to avoid
uncertainties
associated with changeability of ground mechanical properties in different
places and
over time.
The claimed device embodiments comprise a mechanism for protecting the
detection
cable against tensile loads exceeding allowable values applied in the two
structural
embodiments of the mechanical safety devices.
According to the first embodiment of the structure with a breakable mechanical
safety
device (Figures 5, 6), an anchor comprises a thrust plate 19 and a releasable
clamp 21.
The anchor is symmetrical relative to the cable. Two identical halves of the
releasable
clamp 21 are fixed on the cable 14 with a screwed fastening member 22, and the
cable 14
is gripped in a slot 23. The thrust plate 19 consists of two identical parts
24 that are
attached to the releasable clamp 21 with latches 25, and the parts 24 are also
connected
therebetween with reinforcing rods 26 and latches 27. The reinforcing rods 26
and the
latches 27 make the structure of the thrust plate 19 more rigid, preventing
the parts 24
from bending and ensuring perpendicularity of the thrust plate 19 to the axis
of the cable
14. The structure of the thrust plate 19 is provided with receptacles 29 for
installing the
reinforcing rods 26.
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CA 02829206 2013-09-05
The function of a safety device in this structure is performed by the latches
25 (Figure 7).
Since the surface area of projection of the releasable clamp 21 on a plane
perpendicular
to the axis of the detection cable 14 is significantly less than the similar
surface area of
the thrust plate 19, a load acting upon the releasable clamp 21 from the side
of the thrust
plate 19 is approximately equal to a load acting upon the detection cable from
the side of
the anchor. In a case where a load acting upon the detection cable from the
side of the
anchor exceeds a pre-determined value, the mechanical safety device operates
by
breaking the latches 25. The latches 25 are broken by shearing along planes
28. In the
result of shearing the latches 25 along the planes 28 the thrust plate 19 is
mechanically
disconnected from the releasable clamp 21, whereupon the cable 14 moves, under
the
action of a tensile load, relative to the ground (and the thrust plate fixed
therein), which
causes a decrease in the cable relative elongation and, consequently, the
tensile load on
the dangerous segment, preventing the cable from breaking. An operation
threshold of
the mechanical safety device is selected by changing the strength of the
thrust plate
material or by changing the geometrical parameters of the latches 25 so as
their shearing
strength is equal to half a load acting upon the releasable clamp from the
side of the
thrust plate 19 at which the mechanical safety device should operate.
According to the second embodiment of the structure with a deformable
mechanical
safety device (Figures 8, 9), the structure has the elements similar to those
of the
structure with a breakable mechanical safety device, except for the following
differences
in the thrust plate 30 and the releasable clamp 31. The structure of the
thrust plate 30 is
different in that it is secured to the releasable clamp 31 reliably in the
whole range of
loads for which the anchor is intended. Dimensions and a material of the parts
of the
thrust plate 30 are selected so as to ensure their integrity when a load
acting upon the
detection cable from the anchor side reaches a pre-determined value at which
the
mechanical safety device operates.
The structure of the releasable clamp 31 is different in that the slot 32 of
the releasable
clamp 31 is made with internal recesses on each of the clamping plates 33 and
34. Each
recess has a broadened portion in the beginning and in the end of the slot 32.
An elastic
insert 35 with a pre-determined elastic coefficient is immovably installed in
the slot 32,
the said insert having the surface complementary to that of the recesses and
being
provided with an inner longitudinal channel with a semi-oval cross-section
which greater
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axis is oriented parallel to the release plane of the clamping plates 33 and
34. A rigid
calibration plate 36 consisting of two identical parts is arranged in the
splitting of the
clamping plates 33 and 34. The elastic insert 35 has at least one longitudinal
cut for the
detection cable 14.
The function of a mechanical safety device in this structure is performed by
the
releasable clamp 31. The anchor is held on the cable 14 due to a friction
force between it
and the elastic insert 35. In a case where a load acting upon the detection
cable from the
anchor side exceeds a pre-determined value, deformation of the elastic insert
35 occurs
that causes operation of the mechanical safety device because the cable 14
slips relative
to the anchor fixed in the ground, and such slipping is accompanied by a
decrease in the
said load. This process continues until the said load is equal to the
operation threshold of
the safety device. The said slipping of the cable relative to the ground
causes a reduction
in the cable relative elongation and, consequently, in the tensile load in the
dangerous
region, thus preventing the cable from breaking.
The operation threshold of the safety device is determined by the shape and
depth of the
detection cable outer surface, by the force of pressing the cable to the
anchor, and by the
coefficient of friction (static and sliding) of the anchor relative to the
detection cable.
The operation threshold Fc (in newtons) of the mechanical safety device is
determined
experimentally or calculated according to the formula: Fc = k1 .P, in newtons,
where k1 ¨
friction coefficient of the elastic insert relative to the detection cable, P,
in newtons ¨
force of pressing the elastic insert to the detection cable. The operation
threshold is
adjusted by replacing one elastic insert with another one having a different
coefficient of
elasticity and/or by selecting the thickness of the calibration plate 17.
The tensile load of the detection cable is limited at a level lower than a
maximum
allowable one by introducing the said safety devices to the device structure.
The load to
the detection cable is limited either by limiting the force of securing the
anchor thrust
plate at the releasable clamp (breakable mechanical safety device), or by
limiting the
force of securing the releasable clamp on the detection cable (deformable
mechanical
safety device). In the case of a deformable mechanical safety device the force
of securing
the anchor on the detection cable is limited by selecting elastic inserts with
a given
elasticity coefficient and by adjusting a distance between clamping plates 9,
10 by means
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of the calibration plate 17. In the case of breakable mechanical safety device
the force of
securing the anchor thrust plate on the releasable clamp is limited by
selecting materials
used for making anchors and by changing geometrical parameters of the anchor
parts.
