EP1728895A1 - Surveillance method for detecting the approach of a conductive body to a cathodically protected pipeline for fluids - Google Patents

Abstract

The pipeline (1), for gas or oil or water, carries a cathode protective current for voltage values (U1,U2) to be measured at given time intervals at least at two measurement points (3). A change in the difference between the measurements indicates the approach of a conductive body between the measurement points e.g. a construction vehicle and the like, which could damage the pipeline and also shows any pipeline damage and its location. The measurement points are =25 km apart and preferably at =5 km intervals.

Description

The invention relates to a monitoring method for detecting an approach of a conductive body to a loaded with a cathodic protection current fluid transport pipeline.

Fluid transport pipelines, e.g. Gas, oil or water pipelines are usually made of welded metal pipes laid underground or undersea. In general, a passive protection of the transport pipeline in the form of an insulating protective cover is provided. In addition, active corrosion protection of the transport pipeline is provided by applying a cathodic protection current to the fluid transport pipeline to avoid, as far as possible, electrochemical reactions affecting the metal of the transport pipeline.

The cathodic protection against corrosion is monitored by measuring the output voltage of the system supplying the cathodic protection current, the protection potential of the transport pipeline charged with the cathodic protection current and the protection or tube current via a pipe flow measuring section with the aid of appropriate sensors. The various measured values are transmitted once a day by SMS to a monitoring center for cathodic protection. The monitoring center monitors the long-term course of the measured values. "Gross" damage in cathodic protection (e.g., a bridging of an insulator between two line protection sections can be detected.

However, in addition to the long-term damage, a transport pipeline may be damaged by an accident, such as when a construction vehicle comes into contact with the transport pipeline or its protective cover. Since construction vehicles are conductive bodies, it is important to immediately detect an approach of a conductive body to the transport pipeline in order to prevent or at least minimize damage to the transport pipeline by suitable measures.

From the EP 0 411 689 , of the EP 0 495 259 and the EP 0 560 443 For example, monitoring methods are known for detecting an approach of a conductive body to a fluid transport pipeline loaded with a cathodic protection flow. For this purpose, the cathodic protection current is permanently modulated with rectangular pulses or sinusoidal pulses of different frequencies and evaluated the change in the modulation pulses. Due to the additionally required modulation pulses of the additional equipment technical effort to implement these methods is considerable. The costs are accordingly high.

The object of the invention is to provide a cost-effective monitoring method for detecting a sudden approach of a conductive body to a loaded with a cathodic protection current fluid transport pipeline.

• a) daß zu vorgegebenen Zeitpunkten an wenigstens zwei Meßstellen an der Fluid-Transportpipeline jeweils ein für den an der Meßstelle entlang der Fluid-Transportpipeline fließenden Schutzstrom repräsentativer Meßwert erfaßt wird,
• b) daß für jeden vorgegebenen Zeitpunkt aus den Meßwerten jeweils zweier beabstandeter Meßstellen ein für die Differenz der an den Meßstellen entlang der Pipeline fließenden Schutzströme repräsentativer Differenzwert gebildet wird,
• c) daß aus der zeitlichen Änderung der für die vorgegebenen Zeitpunkte gebildeten Differenzwerte wenigstens ein Wert abgeleitet wird, der eine Annäherung eines leitfähigen Körpers an die Fluid-Transportpipeline zwischen den beiden Meßstellen anzeigen kann.

This object is achieved according to the invention by a generic monitoring method, which is characterized

• a) that at predetermined times at at least two measuring points on the fluid transport pipeline in each case a representative for the current flowing at the measuring point along the fluid transport pipeline protective current is detected,
• b) that a difference value representative of the difference of the protective currents flowing at the measuring points along the pipeline is formed for each predetermined time from the measured values of two spaced measuring points,
• c) that from the temporal change of the difference values formed for the predetermined times at least one value is derived, which can indicate an approach of a conductive body to the fluid transport pipeline between the two measuring points.

With the monitoring method according to the invention, a contact between, for example, a conductive construction vehicle and the conductive fluid transport pipeline can be recognized immediately. Provided that the resistance of the environment of the transport pipeline is not too high, for example in the case of undersea Arrangement in the water, an approach of the construction vehicle or in general a conductive body can be already detected before the conductive body touches the conductive transport pipeline. By forming the difference values representative of the difference in the protective currents flowing at the measuring points along the pipeline, it is possible to reduce interference. Changes in the protective current due, for example, to stray currents generated by DC tracks or to earth-magnetic variations called Tellurik can be largely eliminated. All that is necessary is that at the two measuring points of the fluid transport pipeline, a respective representative measured value for the protective current flowing at the measuring point be detected substantially simultaneously. If more than two measuring points are provided, the difference values should preferably be formed in each case from the measured values of directly adjacent measuring points.

The measurement value representative of the protection current flowing along the fluid transport pipeline may be both a voltage measurement and a current measurement determined, for example, from a voltage measurement. The difference values formed in step b) which represent the difference of the protective currents flowing at the measuring points can be both voltage difference values and protective current difference values in the same way. If protective current difference values are used, these can be determined in two ways, namely by first determining the protective currents for the individual measuring points and then forming the difference, or by first forming a voltage difference value and therefrom a protective current difference value.

The value indicative of at least one approach of a conductive body to the fluid transport pipeline may be both digital and analog. It is determined from the temporal change of the difference values formed for the given times, ie on the basis of the time course of the difference values formed. For example, if the difference values suddenly rise sharply or fall off sharply, it may indicate and cause an approach of a conductive body For example, a corresponding value indicating the approach of a conductive body can be generated, for example as an error indication or for its generation. Alternatively, an approximation of a conductive body can only be displayed when the edge of such an increase or decrease has a certain, possibly stored, shape. In a further variant, the value indicative of the approach of a conductive body may be representative of the fluctuation range of the difference value in a preceding time period, whereby individual "outliers" of the difference value may be disregarded depending on the application. On the other hand, if the difference values formed do not change over time or fluctuate only within the scope of the measuring accuracy, there is no approximation of a conductive body. This may be indicated by the value indicating the approach of a conductive body, for example by not displaying an error message.

A preferred embodiment is characterized in that a constant direct current is applied as a cathodic protection current to the fluid transport pipeline.

The accuracy in determining the location of the approach of a conductive body can be increased by providing the measuring points along the fluid transport pipeline at intervals of less than 20 km, preferably at intervals of less than 5 km, and the difference values from the measurements for immediately adjacent ones Measuring points are formed. The accuracy with which an approach of a conductive body can be determined, moreover depends on the nominal size and the wall thickness of the fluid transport pipeline and the ratio between the turn-on potential of the fluid transport pipeline loaded with the cathodic protection current and the rest potential of the approaching conductive body , However, since these sizes can not be changed easily, the desired accuracy can best be realized by the appropriate adjustment of the distances between the measuring points.

So that smaller and / or particularly short-term changes in the protective current can be detected, it is proposed that the measured values are recorded at regular time intervals of less than 1 s, preferably every 0.1-0.5 s. If the temporal resolution is 0.2 s, for example, short-term metallic contacts can be reliably detected as early as 0.6 s.

Since, according to the invention, the DC change of the flowing protective currents is evaluated, it is advantageous that AC components possibly present at the measuring points are filtered out in such a way that the measured values recorded in step a) for the DC components of the protective current flowing along the fluid transport pipeline are representative. In this way, the influences of technical AC voltages can be eliminated by means of filters. Due to the damping characteristic of these filters, DC changes can generally only be detected if they have a duration of at least 200 ms. For a short-term metallic foreign contact can be detected reliably, at least three measurement times must be available, i. the external contact must exist for at least 600 ms. Consequently, in an AC-voltage filtering arrangement, the highest resolution can be achieved when five readings representative of the protection current are detected per second.

A preferred embodiment is characterized in that a voltage measurement value is detected as the measured value in step a).

Advantageously, each measuring point is assigned a measuring section running along the pipeline, and the voltage between the two end points of the measuring section is measured to detect the voltage measuring value.

The measuring accuracy for the Spannungsmeßwert is particularly high when a test section of a length of at least 10 m, preferably of at least 30 m, is used.

In addition, the measurement accuracy can be increased in that the voltage measurements with a resolution of at least 1 μ V, are preferably detected by at least 0.1 V μ.

A development of the invention is characterized in that in step a) at the different measuring points measured values are transmitted to a central processing device and steps b) and c) are carried out by the central processing device. The measured value can be transmitted via a GPRS network. The transfer of steps b) and c) in the central processing device has the advantage that only a detection of the measured values and their transmission must be ensured at the different measuring points. The central processing of all measured data allows the use of less expensive hardware at the possibly numerous individual measuring points.

Preferably, steps b) and c) are performed in real time. This is necessary to detect sudden damage or, more generally, an approach of a conductive body also in real time. The data transmission to the central processing device is usually via an online connection. Existing pipe flow measuring points can be easily converted to the monitoring method according to the invention by installing monitoring sensors thereon which allow transmission of the measured values to the central processing device and the real-time execution of steps b) and c).

• i) Differenzwerte für unmittelbar aufeinanderfolgende Zeitpunkte verglichen werden,
• ii) festgestellt wird, ob sich die Differenzwerte für unmittelbar aufeinanderfolgende Zeitpunkte um mehr als einen vorgegebenen Schwellwert unterscheiden und
• iii) auf der Basis des Ergebnisses dieser Feststellung der eine Annäherung eines leitfähigen Körpers anzeigende Wert erzeugt wird.

An embodiment of the invention is characterized in that, in step c), the value indicative of an approach of a conductive body is derived from the temporal change of the difference values formed for the predetermined times by

• i) difference values are compared for immediately consecutive times,
• (ii) it is determined whether the difference values for immediately consecutive times differ by more than a predetermined threshold, and
• iii) is generated on the basis of the result of this determination of the indicative of a convergence of a conductive body value.

• i) die Differenzwerte für unmittelbar aufeinanderfolgende Zeitpunkte verglichen werden,
• ii) wenn sich die Differenzwerte für unmittelbar aufeinanderfolgende Zeitpunkte um mehr als einen vorgegebenen Schwellwert unterscheiden, zusätzlich die Differenzwerte für die Zeitpunkte unmittelbar vor und nach den im Schritt i) berücksichtigten Zeitpunkten verglichen werden,
• iii) festgestellt wird, ob sich auch die im Schritt ii) verglichenen Differenzwerte um mehr als den vorgegebenen Schwellwert unterscheiden und
• iv) auf der Basis des Ergebnisses dieser Feststellung der eine Annäherung eines leitfähigen Körpers anzeigende Wert erzeugt wird.

Alternatively, in step c), the value indicative of an approach of a conductive body may be obtained from the time change the difference values formed for the given times are derived by

• i) the difference values are compared for immediately consecutive times,
• ii) if the difference values for immediately consecutive times differ by more than a predetermined threshold, in addition the difference values for the times immediately before and after the times considered in step i) are compared,
• (iii) it is determined whether the difference values compared in step (ii) also differ by more than the predetermined threshold, and
• iv) is generated on the basis of the result of this determination of the indicative of a convergence of a conductive body value.

• i) der Verlauf der Differenzwerte mit der Zeit durch eine geglättete Kurve angenähert wird,
• ii) festgestellt wird, ob die zeitliche Änderung der geglätteten Kurve einen vorgegebenen Schwellwert überschreitet und
• iii) auf der Basis des Ergebnisses dieser Feststellung der eine Annäherung eines leitfähigen Körpers anzeigende Wert erzeugt wird.

Finally, another way of deriving the value indicative of an approach of a conductive body is to derive, in step c), the value indicative of an approach of a conductive body from the temporal change of the difference values formed for the given times by

• i) the course of the difference values is approximated by a smoothed curve over time,
• (ii) it is determined whether the temporal change of the smoothed curve exceeds a predetermined threshold, and
• iii) is generated on the basis of the result of this determination of the indicative of a convergence of a conductive body value.

The type of smoothing can be varied as desired. For example, a single outlier can not be included in the difference values in the smoothing. The degree of smoothing can also be adapted individually to the measuring environment.

• Figur 1 eine schematische Darstellung der Stromverteilung an einer Gashochdruckleitung bei metallisch leitendem Fremdkontakt;
• Figur 2 ein Diagramm, das den Einfluß der Nennweite auf die Spannungsdifferenz zwischen zwei Rohrstrommeßstrecken bei metallisch leitendem Fremdkontakt darstellt;
• Figur 3 eine schematische Darstellung des Überwachungsprinzips für eine Gashochdruckleitung mit mehreren Schutzabschnitten;
• Figuren 4a und 4b jeweils Diagramme, die den zeitlichen Verlauf der Rohrstrommeßwerte an zwei Meßstellen zeigen, zwischen denen nacheinander drei metallische Fremdkontakte mit der Rohrleitung realisiert wurden;
• Figur 4c ein Diagramm, das die Differenz der in den Figuren 4a und 4b dargestellten Rohrströme zeigt, und zwar wiederum aufgetragen gegen die Zeit; und
• Figuren 4d und 4e jeweils Diagramme von Rohrstromdifferenzen für die den Leitungsabschnitt unmittelbar stromauf bzw. unmittelbar stromab dem Leitungsabschnitt mit metallischem Fremdkontakt.

In the following the invention will be explained in more detail with reference to exemplary embodiments. In the figures show:

• Figure 1 is a schematic representation of the current distribution at a high-pressure gas line at metallically conductive foreign contact;
• Figure 2 is a diagram showing the influence of the nominal diameter on the voltage difference between two Rohrstrommeßstrecken with metallically conductive foreign contact;
• Figure 3 is a schematic representation of the monitoring principle for a high-pressure gas line with multiple protection sections;
• Figures 4a and 4b are respectively diagrams showing the time course of the Rohrstrommeßwerte at two measuring points, between which successively three metallic foreign contacts were realized with the pipeline;
• Figure 4c is a graph showing the difference in the tube flows shown in Figures 4a and 4b, again plotted against time; and
• Figures 4d and 4e are each diagrams of pipe flow differences for the line section immediately upstream or immediately downstream of the line section with metallic foreign contact.

FIG. 1 shows schematically the current distribution on a pipeline 1 in the case of metallic conductive foreign contact. The pipeline 1 used as a gas high-pressure line is connected to a protection system 2 for cathodic corrosion protection, more precisely with its cathode (not shown). On the pipe 1 two measuring points 3 are provided for measuring the cathodic protection current. The measuring points 3 each have a measuring length of 30 meters for detecting the voltage measurement U1 or U2. The pipeline 1 has at a point 5 on a metallic foreign contact. This is a construction vehicle that has approached the pipeline 1 and now touches it. Since the construction vehicle, for example, by the excavator chains or the bucket having a finite propagation resistance to the soil, enters the construction vehicle during the contact time or during the damage an additional current, called fault current I _F in the pipe 1 a. When according to the invention Monitoring the protective or pipe flows are detected by the adjacent measuring points 3 at predetermined times, the additional power consumption can be detected from the difference of the flowing at the two measuring points 3 pipe streams and thus a sudden contact with the pipe 1 can be detected immediately in a simple manner.

wobei

R_Strom =

Widerstand der Rohrstrommeßstelle

ρ =

spezifischer elektrischer Widerstand von Stahl

l =

Länge der Rohrstrommeßstrecke

d_a =

Rohraußendurchmesser

d_i =

Rohrinnendurchmesser

d =

mittlerer Rohrdurchmesser

s =

Wanddicke der Rohrleitung

U_Ein =

Einschaltpotenzial

U_R =

Ruhepotenzial des Baufahrzeuges

R_Kontakt =

Ausbreitungswiderstand des Baufahrzeuges an der Kontaktstelle

I₁, I₂ =

Rohrstrom

I_F =

Fehlerstrom

ΔU =

Spannungsdifferenz zwischen den beiden Rohrstrommeßstrecken

ΔI =

Stromdifferenz zwischen den beiden Rohrstrommeßstrecken

Since the pipe flows can not be readily measured, the voltage measuring values U1 and U2 for the associated measuring sections are respectively detected at the measuring points 3. The resistance of a Rohrstrommeßstrecke is calculated as follows: $$R_{\mathit{electricity}}=\rho \frac{l}{\frac{\pi }{4}{d}_a^2-\frac{\pi }{4}{\mathrm{<figure-callout\; id="2"\; label="d\; i"\; filenames="imgf0001.png"\; state="\{\{state\}\}">d\; </figure-callout>}}_i^{}}=\rho \frac{l}{\frac{\pi }{4}\left(d_a^2-{\mathrm{<figure-callout\; id="2"\; label="d\; i"\; filenames="imgf0001.png"\; state="\{\{state\}\}">d\; </figure-callout>}}_i^{}\right)}=p\frac{4l}{\pi \left(d_a+d_i\right)\left(d_a-d_i\right)}=\rho \frac{4l}{\pi \left(d+s+d-s\right)\left(d+s-d+s\right)}=\rho \frac{4l}{\pi \left(2d2s\right)}=\rho \frac{l}{\pi ds}=\frac{l\rho }{\pi ds}$$

in which

R _current =

Resistance of the pipe flow measuring point

ρ =

specific electrical resistance of steel

l =

Length of Rohrstrommeßstrecke

d _a =

External pipe diameter

d _i =

Inside pipe diameter

d =

average pipe diameter

s =

Wall thickness of the pipeline

U _A =

Einschaltpotenzial

U _R =

Rest potential of the construction vehicle

R _contact =

Propagation resistance of the construction vehicle at the contact point

I ₁ , I ₂ =

tube current

I _F =

fault current

ΔU =

Voltage difference between the two Rohrstrommeßstrecken

ΔI =

Current difference between the two Rohrstrommeßstrecken

The pipe streams 11 and 12 are usually the same. It follows for the voltage difference when the pipe dimensions and the lengths of the two pipe flow measuring sections are constant: $$\Delta U=\frac{l\rho }{\pi ds}\left(I_1+I_F\right)-\frac{l\rho }{\pi ds}{I}_2=\frac{l\rho }{\pi ds}{I}_F$$

It is thus apparent under these conditions that in case of failure, the voltage difference between two Rohrstrommeßstrecken is proportional to the fault current. For the fault current itself: $${\mathrm{I}}_{\mathrm{F}}=\frac{{\mathrm{U}}_{\mathrm{One}}-{\mathrm{U}}_{\mathrm{R}}}{{\mathrm{R}}_{\mathrm{Contact}}}$$

In Figure 2, the influence of the nominal diameter of the pipe on the voltage difference between two Rohrstrommeßstrecken ΔU = U ₁ -U _{2 is} shown. Six different foreign contacts with a propagation resistance between 5 and 500 ohms were detected and displayed. The chart is based on the following additional parameters:
ρ = 0.13 Ohm * mm ² / m, l = 30 m, d = 250 to 1200 mm, s = 12 mm, U _A = -1.7 V, U _R = -0.5 V and R = _Contact 5 - 500 ohms

The diagram clearly shows that the voltage difference in the event of a fault is inversely proportional to the nominal diameter of the pipeline. Foreign contacts or sudden pipe damage can therefore be resolved better with smaller pipe diameters. In addition, the diagram shows that the voltage difference at low-resistance foreign contacts is much higher than for high-resistance foreign contacts. The reason for this is that the voltage difference in case of failure is inversely proportional to the propagation resistance of the foreign contact, such as the construction vehicle, at the contact point.

FIG. 3 shows a schematic representation of the monitoring principle on which the method according to the invention is based.

The pipeline 1 shown in Figure 3 consists of two protection sections 5 and 6, each having a protection system 21st or 22 is assigned. The protective sections 5 and 6 are insulated from each other by means of an insulating device 7. Further isolating means 7 are provided between the two protection sections 5 and 6 and adjacent protection sections (not shown). The protection section 5 comprises a protection system 21 for the cathodic protection of corrosion and six associated measuring points 31 to 36. The protection section 6 comprises a protection system 22 for cathodic protection and two associated measuring points 37 and 38. The voltage differences are always formed only between adjacent measuring points within a protection section , Measured values from measuring points that are assigned to electrically separate lines are not compared. Consequently, the measured values of the measuring points 31 and 32, 32 and 33, 33 and 34, 34 and 35, 34 and 36 as well as 37 and 38 are compared in FIG. If the resistance values of the individual pipe flow measuring sections are the same, the pipe flow difference is proportional to the voltage difference. Therefore, in this case, the measured voltage values of the individual measuring points can be directly compared as measured values. Otherwise, the voltage measurement values of the individual measuring points must be converted into pipe flow measured values and then compared with each other.

FIGS. 4a to 4d show measured values or difference values which were obtained in an experiment according to the method according to the invention. For the experiment, errors were simulated between the pipe flow measuring points 32 and 33 in accordance with FIG. 3 on the pipeline 1 by successively connecting appropriately sized ohmic resistors to the pipeline and the ground, each for a duration of 30 seconds. To ensure that the measurements at the different measuring points took place at the same time, the measured values were provided with a time stamp. In the experiment, the measured values were recorded with a time interval of 2 sec and a measurement resolution of the sensors of 1 μ V. Figures 4a and 4b show the time course of the Rohrstrommeßwerte at the measuring points 32 and 33 of Figure 3. The pipe 1 has an average pipe diameter of 1200 mm and is provided with a PE protective cover. The can not be seen in Figures 4a and 4b by the three simulated errors caused pipe flow changes. The reason is power fluctuations caused by Tellurik.

FIG. 4 c shows the difference between the measured tube flow values measured at the measuring points 32 and 33 and shown in FIGS. 4 a and 4b. By subtracting the three simulated errors are much clearer to recognize.

In comparison, the pipe flow differences for adjacent pipe sections outside the faulty portion in Figure 4d and 4e are shown, namely in Figure 4d, the pipe flow differences between the measuring points 31 and 32 and in Figure 4e, the pipe flow differences between the measuring points 33 and 34th

The three simulated errors can not be recognized in FIGS. 4d and 4e. The current differences of about 100 mA to 120 mA are also caused by Tellurik. The smaller fluctuations between the measured values are due to the limited measured value resolution of the measuring inputs.

Moreover, FIGS. 4d and 4e show that a sudden increase or decrease in the pipe flow difference, which is limited to one measured value, generally indicates no error. The pipe flow differences are therefore preferably to be evaluated so that individual outliers are not taken into account. This is possible for example by a suitable smoothing of the pipe flow difference curve. Accordingly, a sudden damage to the pipeline or a sudden approach of a conductive body can usually only be ascertained if not only the pipe flow difference values for immediately successive points in time differ by more than a predetermined threshold value but also immediately before the difference values for the points in time and also differ by more than a predetermined threshold after the previously considered times.

With the monitoring method according to the invention, it is possible with existing cathodic corrosion protection, without additional to superimposed signals sudden damage to a pipeline Reliable, without special technical effort and inexpensive to determine.

Numerous modifications are conceivable within the scope of the invention. The measured values representative of the protection current should preferably be detected at the same time at the at least two measuring points. Nevertheless, it may be sufficient in some environments if the measured values are only detected as simultaneously as possible. The distances of the measuring points along the fluid transport pipeline can of course be varied as desired, with distances as close as possible being preferred, since it is not the exact location of the damage or the approach of a conductive body that is determined, but only that somewhere between the compared measuring points conductive body of the pipeline approaches or suddenly touched. The time intervals between the individual measurements can be changed as desired. If voltage measuring values are detected at the individual measuring points, the pipe flow measured values could also be calculated in the individual measuring points instead of in the central processing device. Furthermore, the temporal change of the difference values can be arbitrarily evaluated in order to derive the value indicating an approach of a conductive body. In any case, however, it should be ensured that a sudden increase or decrease of the formed difference values, which persists across several difference values, generates a value indicating an approach of a conductive body. Additionally, the monitoring method of the present invention will typically be limited to detecting an approach of a conductive body to a fluid transport pipeline only upon electrical contact with the transport pipeline so that an approach can not be determined prior to touching the conductive transport pipeline. In this case, the value indicative of an approach of a conductive body is a value indicating a sudden electrical contact of a conductive body with the transportation pipeline.

Finally, the recognition of an approach of a conductive body or a sudden damage with Depending on the application, the method according to the invention can be increased in that all construction vehicles in the area of the pipeline are grounded in a specifically low-resistance manner.

Claims (14)

A monitoring method for detecting an approach of a conductive body to a fluid transport pipeline loaded with a cathodic protection flow,
characterized a) that at predetermined times at at least two measuring points on the fluid transport pipeline in each case a representative for the current flowing at the measuring point along the fluid transport pipeline protective current is detected, b) that a difference value representative of the difference of the protective currents flowing at the measuring points along the pipeline is formed for each predetermined time from the measured values of two spaced measuring points, c) that from the temporal change of the difference values formed for the predetermined times at least one value is derived, which can indicate an approach of a conductive body to the fluid transport pipeline between the two measuring points. Monitoring method according to claim 1, characterized in that a constant direct current is applied as a cathodic protection current to the fluid transport pipeline. Monitoring method according to claim 1 or 2, characterized in that the measuring points along the fluid transport pipeline at intervals below 20 km, preferably at intervals below 5 km are provided. Monitoring method according to one of Claims 1 to 3, characterized in that the measured values are measured at regular intervals Time intervals less than 1 s, preferably every 0.1 to 0.5 s, are detected. Monitoring method according to one of Claims 1 to 4, characterized in that any alternating current components possibly present at the measuring points are filtered out in such a way that the measured values recorded in step a) for the direct current components of the protective current flowing along the fluid transport pipeline are representative Monitoring method according to one of Claims 1 to 5, characterized in that a voltage measurement value is detected as the measured value in step a). Monitoring method according to claim 6, characterized
that each measuring point is assigned a running along the pipeline measuring section and
in that the voltage between the two end points of the measuring path is measured to detect the voltage measurement value. Monitoring method according to claim 7, characterized in that a measuring path of a length of at least 10 m, preferably at least 30 m, is used. Monitoring method according to one of claims 6 to 8, characterized in that the voltage measurements with a resolution of at least 1 μ V, preferably of at least 0.1 μ V, detected. Monitoring method according to one of Claims 1 to 9, characterized in that the measured values acquired in step a) at the various measuring points are transmitted to a central processing device and steps b) and c) are carried out by the central processing device. Monitoring method according to one of Claims 1 to 10, characterized in that steps b) and c) are carried out in real time. Monitoring method according to one of Claims 1 to 11, characterized in that, in step c), the value indicative of an approach of a conductive body is derived from the temporal change of the difference values formed for the predetermined times, by i) difference values are compared for immediately consecutive times, (ii) it is determined whether the difference values for immediately consecutive times differ by more than a predetermined threshold, and iii) on the basis of the result of said determination, the value indicating the approach of a conductive body is generated. Monitoring method according to one of Claims 1 to 11, characterized in that, in step c), the value indicative of an approach of a conductive body is derived from the temporal change of the difference values formed for the predetermined times, by i) the difference values are compared for immediately consecutive times, ii) if the difference values for immediately consecutive times differ by more than a predetermined threshold, in addition the difference values for the times immediately before and after the times considered in step i) are compared, (iii) it is determined whether the difference values compared in step (ii) also differ by more than the predetermined threshold, and iv) is generated on the basis of the result of this determination of the indicative of a convergence of a conductive body value. Monitoring method according to one of Claims 1 to 11, characterized in that, in step c), the value indicative of an approach of a conductive body is derived from the temporal change of the difference values formed for the predetermined times, by i) the course of the difference values is approximated by a smoothed curve over time, (ii) it is determined whether the temporal change of the smoothed curve exceeds a predetermined threshold, and iii) is generated on the basis of the result of this determination of the indicative of a convergence of a conductive body value.

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