Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Impressed Current Cathodic Protection
This invention relates to impressed current cathodic protection arrangements.
Impressed current cathodic protection arrangements are commonly used to
protect metallic structures against corrosion. The systems work by applying a
negative potential to the metallic structure which is to be protected such
that
corrosion processes are suppressed. This is achieved by the use of a DC power
supply which has one terminal connected to the structure to be protected and
another terminal connected to an anode. The system will only be effective in
preventing corrosion provided that the structure is held at a large enough
negative
potential relative to the surroundings. For practical reasons the impressed
current
will be applied to the protected structure at a finite number of locations. In
some
circumstances this may be a single location.
It will be clear that as one progresses along the metallic structure away from
the
connection point, the magnitude of the potential of the protected structure
relative
to the surroundings will decrease. If a point is reached where the effective
potential of this structure relative to the surroundings is less than a
required
threshold level, the cathodic protection will cease to be effective from that
point
onwards.
Impressed current cathodic protection systems are used to protect a range of
different metallic structures. One particular situation where they are used is
in the
protection of flow-lines, for example pipeline systems and well insulations in
the oil
and gas inriustry. in RnrnP such nirnurnstannPs nirPnt inRpPntinn dirAnt
measurement of the conditions at various points along a protected structure
can
be impractical or impossible.
This situation is likely to be particularly acute with impressed current
cathodic
protection systems which are to protect elongate metallic structures such as
pipelines where the impressed current needs to be effective over a significant
distance of metallic structure leading away from the connection point.
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This invention is directed at addressing these types of issues.
According to one aspect of the present invention there is provided an
impressed
current cathodic protection arrangement comprising an elongate metallic
structure
to be protected, cathodic protection apparatus which comprises a DC power
supply and an anode, one terminal of the power supply being connected to the
structure at a connection point and another terminal of the power supply being
connected to the anode, and monitoring apparatus for monitoring effectiveness
of
cathodic protection provided by the cathodic protection apparatus by
determining
the electrical potential of the structure relative to surroundings at at least
one
location which is spaced from the connection point.
The monitoring apparatus may comprise means for determining the electrical
potential of the structure relative to surroundings at a reference point on
the
structure to be protected which reference point is spaced from said one
location.
The reference point may be the connection point.
The means for determining the electrical potential of the structure relative
to
surroundings at the reference point may comprise a second structure which is
spaced from the structure to be protected and the anode, and means for
measuring the potential difference between the second structure and the
structure
to be protected to allow determination of the electrical potential of the
structure
relative to surroundings at the reference point.
The means for determining the electrical potential of the structure relative
to
surroundings at the reference point may comprise a memory in which is stored a
parameter which is representative of the impedance of the elongate structure
as
seen from the reference point, means for measuring the current supplied to the
structure at the reference point and means for calculating the electrical
potential of
the structure relative to surroundings at the reference point using said
parameter
and the measured current.
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The elongate structure to be protected may comprise tubing. Typically the
tubing
may be tubing provided in an oil and/or gas installation such as a well and/or
pipeline system. In such a case the reference structure may comprise another
well
or pipeline installation.
Note that here the expression tubing is used to refer to casings, liners and
any
other such tubular metallic structure found in a well installation as well as
production tubing, for which the term "tubing" is sometimes reserved in the
oil and
gas industries. The elongate structure may comprise two or more tubing
structures running at least partly within one another.
The monitoring apparatus may comprise a central station.
The central station may comprise part or all of the means for determining the
electrical potential of the structure relative to surroundings at the
reference point.
The central station may be arranged to determine the electrical potential of
the
structure relative to surroundings at at least one location which is spaced
from the
connection point in dependence on one or more inputs.
The monitoring apparatus may comprise a tool which is arranged to run within
the
tubing. The tool and central station may be arranged for communication
therebetween.
The tool may comprise a spaced pair of contacts for contacting with an
internal
surface of the tubing at axially spaced locations; and a control unit
comprising a
sensor for measuring the potential difference between the contacts. The
control
unit may comprise a transmitter for transmitting, to the central station, data
indicating a measured potential difference between the contacts.
Preferably the tool is arranged to transmit said data along the elongate
metallic
structure. Other transmission mechanisms may be used, for example a cable may
be provided along which said data may be transmitted, or non-electrical
techniques may be used.
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The monitoring apparatus, for example the central station, may be arranged to
determine the electrical potential of the structure relative to surroundings
at at
least one location which is spaced from said connection point in dependence on
the potential difference measured between the spaced contacts of the tool as
measured by the sensor of the tool.
Preferably the tool is arranged for movement within the tubing. This can allow
potential difference measurements to be taken at a plurality of positions
along the
structure. The monitoring apparatus may be arranged to use the tool to take
potential difference measurements continuously or at intervals as it is moved
along
the tubing. This can allow a curve of potential along the tubing to be
plotted.
The monitoring apparatus may comprise a memory in which is stored a model
based on parameters specific to the elongate structure and its surroundings
which
model predicts how the absolute value of a potential applied to the structure
at the
reference point will decay along the length of the structure away from the
reference point.
The parameters on which the model is based may comprise one of, or a
combination of: the resistivity of the material of the elongate structure, the
relative
permeability of the material of the elongate structure, the cross-sectional
dimensions of the elongate structure, the resistivity of material surrounding
the
elongate structure, the relative permeability of material surrounding the
elongate
structure. These parameters may he recorded as a function of position along
the
elongate structure. That is to say the value of these parameters need not be
constant along the whole length of the elongate structure. The model may take
into account the presence of multiple lengths of tubing arranged within one
another, some of which extend further than others. Thus, for example, the
model
may be based on parameters concerning lengths of casing as well as production
tubing in a well installation.
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The monitoring apparatus may be arranged to use the model based on
parameters specific to the elongate structure and its surroundings in
determining
the electrical potential of the structure relative to surroundings at at least
one
location which is spaced from said connection point.
5
The means for determining the electrical potential of the structure relative
to =
surroundings at the reference point may comprise a memory in which is stored a
model based on parameters specific to the elongate structure and its
surroundings
which model predicts how the absolute value of a potential applied to the
structure
at the reference point will decay along the length of the structure away from
the
reference point.
The monitoring apparatus may be arranged to determine the electrical potential
of
the structure relative to surroundings at at least one location which is
spaced from
said connection point in dependence on:
i) the electrical potential of the structure relative to surroundings at said
reference
point as determined by the means for determining the electrical potential of
the
structure relative to surroundings at the reference point;
and in dependence on at least one of:
ii) the potential difference measured between the spaced contacts of the tool
as
measured by the sensor of the tool; and
iii) the model based on parameters specific to the elongate structure and its
surroundings.
Potential difference measurements from the tool and/or made at the reference
point can be used to help verify and/or improve the accuracy of the potential
difference determined using the model at any point.
The monitoring apparatus may be arranged to indicate when the magnitude of the
potential of the structure relative to the surroundings falls below a
threshold level
at at least one point along the length of the structure. The threshold level
may be
a minimum acceptable level for cathodic protection purposes.
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The monitoring apparatus may be arranged to indicate at what distance from the
reference point the potential of the structure relative to the surroundings
falls
below a threshold level.
The monitoring apparatus may be arranged to indicate the potential of the
structure relative to the surroundings as a function of distance away from the
reference point.
According to another aspect of the present invention there is provided a well
installation comprising an impressed current cathodic protection arrangement
as
defined above.
In such a case the central station may be provided at the surface of the well
and
the tool may be provided to run within the tubing of the well. The central
station
may be arranged to determine the potential of the tubing of the well relative
to the
surrounding at the reference point and to determine the potential of the
tubing of
the well at locations away from the connection point in dependence on:
i) the potential difference measured between the spaced contacts of the tool
as
measured by the sensor of the tool; and
iii) the model based on parameters specific to the elongate structure and its
surroundings.
According to another aspect of the present invention there is provided a
method of
monitoring an impressed current cathodic protection arrangement, the
arrangement comprising an elongate metallic structure to be protected, and
cathodic protection apparatus which comprises a DC power supply and an anode,
one terminal of the power supply being connected to the structure at a
connection
point and another terminal of the power supply being connected to the anode,
the
method comprising the step of:
monitoring effectiveness of the cathodic protection provided by the cathodic
protection apparatus by determining the electrical potential of the structure
relative
to surroundings at at least one location which is spaced from the connection
point.
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The method may include further specific steps corresponding to the features
defined above, with appropriate changes in wording where necessary. These
features are not restated here in method form in the interest of brevity.
Preferred embodiments of the present invention will now be described, by way
of
example only, with reference to the accompanying drawings, in which:
Figure 1 schematically shows a well installation including an impressed
current
cathodic protection arrangement;
Figure 2 shows an example plot of the absolute electrical potential of an
elongate
conductor protected by an impressed current cathodic protection system against
position along that conductor; and
Figure 3 is a flowchart showing steps in a method of monitoring an impressed
current cathodic protection arrangement.
Figure 1 shows a well installation including an impressed current cathodic
protection arrangement. The well installation comprises a wellhead 1 and
tubing 2
running away from the wellhead and down into the well. The tubing 2 is a
metallic
structure and it is this structure which is to be protected by impressed
current
cathodic protection in the present case.
Note that Figure 1 shows a simplified schematic view of the well installation,
in
=
particular of the metallic tubing running downhole. Only one run of tubing is
shown. In reality there will normally be additional casings (i.e. lengths of
metallic
tubing) running into the well as well as the central string of production
tubing. In
general terms these lengths of tubing contact against one another at multiple
locations and hence in practical terms will have the same potential as each
other.
Thus their presence does not interfere with the operation of the present
techniques
and the tubing 2 shown in Figure 1 can be considered to be representative of
the
castings as well as the production tubing of the well.
The impressed current cathodic protection arrangement shown in Figure 1
comprises cathodic protection apparatus 3 which is used to provide an
impressed
current onto the metallic tubing 2 of the well so as to protect the tubing
against
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corrosion. The cathodic protection apparatus 3 comprises a DC power supply 31
and an anode 32. A first terminal of the DC power supply 31 is connected to a
connection point 33 on the tubing 2 of the well. A second terminal of the DC
power supply 31 is connected to the anode 32. Thus the DC power supply is
arranged for applying cathodic protection currents. In this regard the
cathodic
protection apparatus 3 of the present arrangement is conventional and the
tubing
2 is protected against corrosion by virtue of the negative potential applied
to it
using the cathodic protection apparatus 3.
However the present impressed current cathodic protection arrangement further
comprises monitoring apparatus 4 for monitoring the effectiveness of the
cathodic
protection provided by the cathodic protection apparatus 3. In the present
embodiment the monitoring apparatus is distributed. That is to say, it has
various
components provided in different locations. In the present embodiment the
monitoring apparatus 4 includes a central station 5 provided at the surface of
the
well and in the same region as the power supply 31 of the cathodic protection
apparatus 3 and a downhole tool 6 which is provided within the tubing 2 of the
well. Also provided as part of the monitoring apparatus 4 is a remote
reference
earthing structure 1' which, in this embodiment, is in the form of a wellhead
of
another well. This other well is remote from the well having tubing 2 which is
to be
protected by the cathodic protection apparatus 3.
The central unit 5 comprises a monitoring unit 51 including a memory 52 and a
processor 53. The central unit 5 also comprises a meter 54 for detecting the
current applied by the power supply 31 of the cathodic protection apparatus 3
to
the connection point 33. Furthermore the central unit 5 comprises a meter 55
for
detecting the potential difference between the connection point 33 and the
reference wellhead 1'. The outputs of both of these meters 54,55 are fed into
the
monitoring unit 51.
The memory 52 of the monitoring unit 51 has stored in it a parameter which is
representative of the impedance of the elongate conductor to be protected
(i.e. the
tubing 2) as seen from a reference point, in this case the connection point
33.
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Using the supplied current value detected by the meter 54 and this stored
parameter relating to the impedance of the tubing 2 it is possible for the
monitoring
unit 51 to determine the potential at the connection point 33. Thus this is
one way
in which the present monitoring apparatus 4 is able to determine the
electrical
potential of the connection point 33 (reference point) relative to the
surroundings.
However the potential of the reference point/connection point 33 in the
present
embodiment may also be directly determined from the meter reading of the meter
55 arranged for measuring the potential difference between the connection
point
33 and the remote wellhead 1'. This is because it is reasonable to assume that
the potential of the remote wellhead 1' is the same as the potential of the
surroundings in the region of the connection point 33. This common potential
is
akin to "earth".
Thus the monitoring unit 51 may determine the potential of the connection
point 33
relative to the surroundings using either of these methods or both of these
methods.
However for the present purposes what is of primary interest is the potential
of the
metallic structure 2 at locations away from this connection point 33.
The memory 52 of the monitoring unit 51 also contains a model which is
representative of the way in which an absolute potential applied to the
metallic
structure at a reference point (i.e. in this case, the connection point 33)
will decay
as one progresses along the metallic structure (in this case, down into the
well).
This model is based on parameters relating to the resistivity of the material
of the
elongate structure (i.e. of the tubing 2¨ both casings and production tubing),
the
relative permeability of the material of the elongate structure, the cross-
sectional
dimensions of the elongate structure, the resistivity of the material
surrounding the
elongate structure and the relative permeability of the material surrounding
the
elongate structure. Furthermore these parameters may be recorded as a function
of position along the elongate structure. Using these parameters the model
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predicts how potential will decay along the metallic structure and a typical
plot of
how the potential will decay along the metallic structure is shown in Figure
2.
Using this model and the results of the determination of the potential at the
5 connection point 33, the monitoring unit 51 is able to predict the
potential of the
metallic structure 2 relative to its surroundings at different positions along
the
length of the metallic structure 2.
In particular it is possible to therefore determine if the magnitude of the
potential of
10 the metallic structure 2 falls below an acceptable threshold level for
providing
cathodic protection at any point along the metallic structure 2.
The monitoring apparatus 4 can be arranged to indicate if this occurs and/or
to
output a plot which is indicative of the potential of the tubing 2 along its
length.
Further it is then also possible to alter the current delivered by the DC
power
supply of the cathodic protection apparatus 3 so as to raise the magnitude of
the
cathodic protection current being supplied at the connection point 33 and
hence
hopefully raise the magnitude of the potential of the tubing 2 to acceptable
levels
along the entirety of its length (or the entirety of the length of the tubing
2 which is
to be protected). The monitoring apparatus 4 can be arranged to perform such a
function.
As mentioned above, in the present embodiment the monitoring apparatus 4 also
comprises a down hole tool 6. In the present embodiment the downhole tool 6 is
arranged for movement within the tubing 2. The tool 6 comprises axially spaced
contacts 61. A first of the contacts 61 is provided at one end of the tool 6
and a
second of the contact 61 is provided at the other end of the tool 6. These
contacts
are arranged for contacting with the internal surface of the tubing 2. The
tool 6
comprises control unit including a meter 62 for sensing the potential
difference
between this pair of contacts 61 and hence the potential difference between
two
axially spaced points on the tubing 2. Furthermore the control unit of the
tool 6
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comprises transmission means 63 for transmitting potential difference readings
back to the central unit 5.
Different forms of transmission between the tool 6 and central unit 5 may be
used.
In some instances a direct cable connection may be provided, in other
instances
acoustic or other wireless transmission techniques may be used. A particularly
preferred wireless transmission technique is one which makes use of the
metallic
structure itself (in this case tubing 2) as a signal channel. The applicants
supply
commercially a tool under the name CATS, which can provide such a transmission
mechanism in an embodiment of the present kind. This tool works on the basis
of
supplying a very high current low frequency electrical dipole signal to the
metallic
structure 2 via the contacts 61 such that the signal propagates away from the
tool
6 and can be picked up from or in the region of the wellhead 1.
Because the spacing between the space contacts 61 is known, the potential
difference measurement which can be taken by the tool 6 can be used to
determine the slope of the potential curve of the tubing 2 in the region of
the tool.
Therefore provided that the position of the tool 6 within the tubing is known,
knowledge of this slope of the potential curve can be used to help calibrate
the
model and the plot of potential versus depth in the well (as shown in Figure
2) to
improve the accuracy of potential determination at points away from the
connection point 33.
As an alternative, where the tool 6 is arranged for movement within the tubing
2 it
75 is possible to take potential difference readings at multiple points
within the well,
for example, beginning from the region of the connection point 33. In such a
case
it is possible to plot a curve representing the potential of the tubing 2
relative to the
surroundings using the determined value for the absolute potential at the
connection point 33 as a start point and the potential difference readings
from the
tool 6 to plot the curve from there onwards. In such a case there is no
absolute
requirement to use the model contained in the memory 52, which describes the
decay of potential along the tubing.
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Figure 3 is a flowchart illustrating methods of monitoring an impressed
current
cathodic protection arrangement of the type shown in Figure 1. The steps of
these
methods broadly correspond to the techniques described above.
Step 1 of the method is to determine the potential of the tubing at a
reference point
(in the embodiment described above, the connection point 33) relative to the
surroundings.
Step 2 is to determine a potential decay curve along the tubing 2 as one moves
away from the reference point.
Step 3 is to determine whether the magnitude of potential falls below an
adequate
level for effective cathodic protection at some point along the length of the
protecting metallic structure (the tubing 2 in the above embodiment).
Step 4 is an optional step of determining the point at which the magnitude of
the
potential of the tubing falls below the acceptable level.
Step 5, which again is an optional step, is to adjust the current applied by
the
power supply of the cathodic protection apparatus in order to ensure that the
potential of the tubing does not fall below an acceptable level for cathodic
protection purposes.
In carrying out step 1, when operating the embodiment of Figure 1, it is
possible to
/5 use the meter 54 for measuring the current delivered by the power supply
31 of
the cathodic protection apparatus and the stored characteristic impedance of
the
tubing from the memory 52 to determine the potential at the reference point.
Similarly it is possible to use the potential difference measured by the meter
55
between the connection point 33 and the reference wellhead 1'. Furthermore it
is
possible to use a combination of both of these results to decide a value for
the
potential at the reference point 33. Thus, for example, an average of the two
values could be used.
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Note that it is not possible to directly measure the potential difference at
the
connection point 33, relative to the wellhead 1 or any other structure in the
region
of the anode 32, since when the cathodic protection apparatus is in operation
(which is when it is desired to measure the potential at the connection point
33)
the potential in the region surrounding the anode 32 will be heavily
influenced by
the cathodic protection system itself.
When conducting step 2 using the apparatus of the embodiment shown in Figure
1, it is possible to use the model of the decay of the potential along the
metallic
structure (tubing 2) stored in the memory 52 and similarly it is possible to
use the
results of the potential difference measurements made by the tool 6. Again,
furthermore it is possible to use a combination of these two techniques to
determine the potential decay curve.
As mentioned above, whilst the above description has been written in terms of
a
cathodic protection system provided in a well installation, the present
apparatus
and method is similarly applicable to any system with an elongate conductor
and
particularly so for an installation including a tubular elongate metallic
structure
within which a tool can pass. Thus another particular example are pipeline
systems, particularly those used in the oil and gas industry. Note, of course,
however that the present systems, where appropriate, can be used without the
inclusion of a tool 6 which is located within, or runs within the structure to
be
protected.
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