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Recordings and occurrence of geomagnetically induced currents in the Finnish natural gas pipeline network
Journal of Applied Geophysics 48 (2001) 219 – 231 www.elsevier.com/locate/jappgeo
Recordings and occurrence of geomagnetically induced currents in the Finnish natural gas pipeline network Antti Pulkkinen*, Ari Viljanen, Kari Pajunpa¨a¨, Risto Pirjola Finnish Meteorological Institute, Geophysical Research Division, P.O. Box 503, FIN-00101, Helsinki, Finland Received 22 March 2000; accepted 31 October 2001
Abstract A project implemented to study the effects of space weather on the Finnish natural gas pipeline was started in August 1998. The aims of the project were (1) to derive a model for calculating geomagnetically induced currents (GIC) and pipe-to-soil (P/S) voltages in the Finnish natural gas pipeline, (2) to perform measurements of GIC and P/S voltages in the pipeline and (3) to derive statistical predictions for the occurrences of GIC and P/S voltages at different locations in the pipeline network. GIC and P/S voltage were recorded at a compressor station. The GIC measurement was made with two magnetometers, one right above the pipe, and another at the Nurmija¨rvi Geophysical Observatory about 30 km southwest. The largest GIC since November 1998 has been 30 A. The P/S voltage recording was stopped in May 1999, but GIC is still measured. GIC statistics were derived based on the recordings of the geomagnetic field at Nurmija¨rvi. The geoelectric field was calculated by using the plane wave model. This field was input to the general pipeline model resulting in the distribution of currents and P/S voltages at selected points in the pipeline. As could be expected, the largest P/S voltage variations occur at the ends of the pipeline network, while the largest GIC flow in the middle parts. D 2001 Elsevier Science B.V. All rights reserved. Keywords: Geophysical methods; Electromagnetic field; Induction; Magnetometers; Cathodic protection
1. Introduction Studies of geomagnetically induced currents (GIC) in the Finnish natural gas pipeline were started by Pirjola and Lehtinen (1985). The study was based on the method developed originally for the calculation of GIC in power systems (Lehtinen and Pirjola, 1985), which was applied to estimate GIC flowing through the cathodic protection stations. Due to the assumption of
*
Corresponding author. E-mail addresses: [email protected] (A. Pulkkinen), [email protected] (A. Viljanen), [email protected] (K. Pajunpa¨a¨), [email protected] (R. Pirjola).
the discrete groundings of the pipeline, the calculation did not correspond to reality, and thus the study was not able to give reliable estimations of GIC in the pipeline. Viljanen (1989) modeled the pipeline as an infinitely long cylinder buried in a homogenous medium. He computed the estimations for GIC along the pipeline and between the pipeline and the earth. Based on the model and the magnetic field measurements carried out at the Nurmija¨rvi Geophysical Observatory, he draws up statistical estimations of the occurrences of GIC. However, the assumption about an infinite length of the pipeline ignores the discontinuities of the pipeline, and thus the method cannot be regarded as completely satisfactory.
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A significant step forward was the application of the distributed source transmission line theory. The method was used for modeling AC induction in pipelines by Taflove and Dabkowski (1979). In geomagnetic induction studies, it was first used by Boteler and Cookson (1986) and was further developed by Pulkkinen et al. (submited for publication). The method is applicable to general, complex pipeline networks, and does not use any unrealistic assumptions. Because of this method, it was finally possible to study the Finnish pipeline network as a whole, and thus there was a good reason for starting GIC measurements to be used in parallel with our modeling efforts.
2. Measurements The Finnish pipeline network has a total length of 914 km (Fig. 1). The pipeline is composed of the main network, which has a length of 350 km between the eastern and the western ends, and of several shorter branches. The pipeline is electrically connected to the Russian pipeline. The exact length and the electrical characteristics of the Russian pipeline are not known but the length of the Russian part can be regarded as infinite. GIC measurements were started in November 1998. The measurement site is located at Ma¨ntsa¨la¨, at a branch that connects pipelines coming from Tampere,
Helsinki and Kouvola (Fig. 1). The pipeline is almost east-west directed at the measurement site. Measurements of the voltage between the pipeline and the earth (pipe-to-soil (P/S) voltage) were also started in November 1998. Measurements of GIC are still going on but the voltage measurements had to be interrupted during the summer 1999 because of possible lightning hazards. Because of interpretation problems due to the cathodic protection system of the pipeline, the voltage measurements were not continued. Data are collected with a 10-s time resolution to a computer at the compressor station at Ma¨ntsa¨la¨, from which they are transferred daily to the Nurmija¨rvi Geophysical Observatory. Daily updated data-plots can be seen at www. geo.fmi.fi/MAGN/GIC. The magnetic activity of the measurement period has been quite close to the average level. The average Ak index at Nurmija¨rvi has been 14, which is slightly over the average Ak of the period January 1953 –April 2001, which is 12.3. Until now, the largest Ak (268) during the measurement period was reached on March 31, 2001. 2.1. Measurements of GIC GIC in the pipeline is recorded following a similar method as Campbell’s (1980b), who measured GIC in the Alaska oil pipeline. The method was also used to record GIC in a Finnish 400 kV power transmission line (Ma¨kinen, 1993; Viljanen and Pirjola, 1994).
Fig. 1. Finnish natural gas pipeline network. GIC recordings are performed at Ma¨ntsa¨la¨.
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One magnetometer is placed right above the pipeline. Due to the small frequencies of geomagnetic variations, the current flowing along the pipeline is distributed uniformly across the pipeline steel. Consequently, according to the Biot –Savart law: I¼
2pR B l0
ð1Þ
where R is the distance from the center of the pipeline and B is the axial magnetic field. The geomagnetic variation field is subtracted with the reference measurements carried out at the Nurmija¨rvi Geophysical Observatory. It is located 30 km to the south-west from the GIC measurement site, so it is justified to assume that the geomagnetic field is nearly identical at both. The positive direction of the current was chosen to be eastward. The method described above has some problems worth noting here. The Finnish natural gas pipeline is made of steel, which is a highly permeable material. Consequently, there is a good reason to believe that the permeability has an influence on the magnetic field, which is recorded by the magnetometer at the pipeline. The effect of the permeability can in principle be divided into three parts: (1) The static magnetization produced by the main geomagnetic field in the permeable material changes the field in the vicinity of the pipeline. In our recordings, this is seen as a change of the baseline. (2) The permeability affects the skin depth of the material, thus changing the distribution of the electric and magnetic fields. This implies that the current density varies with the permeability, and a change of the current also modifies the magnetic (variation) field around the pipeline. (3) The magnetization induced by temporal magnetic variations in the permeable material contributes to the variations measured in the vicinity of the pipeline. Effect 1 does not matter in our recordings since only the variation field is considered. Also Effect 2 is not of importance regarding the recordings because it is just the GIC that we are actually measuring. Effect 3, however, should be considered carefully while it refers to a contribution to magnetic variations not associated with GIC, and this contribution is not eliminated in the comparison with the recordings of the reference magnetometer. Regarding GIC recordings with magnetometers at the Alaska pipeline Campbell
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(1980b) points out that the permeability of the pipeline affects the baseline of magnetic recordings thus referring to Effect 1, but he does not study closer Effect 2, and in particular, Effect 3. To examine Effect 3, we made model computations by considering an infinitely long steel cylinder with a coating lying in a homogeneous medium (the earth). The permeability of the pipeline steel is not known exactly. Therefore, we have used permeability values in the range from 10l0 to 1000l0. Having the radii equal to those of the Finnish pipeline and periods characteristic to geomagnetic variations, i.e. 10 to 1000 s, we found that the axial magnetic field outside the pipeline, from which the initial magnetic field is subtracted, is the same as the Biot – Savart field caused by the DC current flowing along the pipeline. Thus, the permeability does not affect the variation field around the cylinder at all and Effect 3 is ignorable. This is an understandable result since the wall thickness of the pipeline is much smaller than the skin depth in the pipeline steel. In addition, because the change of the period (between 10 and 1000 s) did not change the amplitude of the axial magnetic field outside the pipeline, GIC along the pipeline is simply determined by the DC resistance of the pipeline. In conclusion, Effects 1, 2 and 3 are physical facts that should in principle be investigated when performing GIC recordings with the help of magnetometers. However, the periods associated with GIC and wall thicknesses used in pipelines prevent any distortion in practice. The inaccuracy of the GIC measurement due to uncertainty of the distance of the magnetometer from the buried pipeline was estimated to be 5%. The inaccuracy due to the deviation between the geomagnetic fields of the Nurmija¨rvi and Ma¨ntsa¨la¨ is larger. When studying the deviations of y- and z-components (not affected by GIC flowing along the pipeline) and when only field variations exceeding 100 nT were taken into account, it was estimated to be about 10%. When smaller variations were included, the inaccuracy increased (with the threshold of 30 nT, inaccuracy is 20%), which is the result of local non-geomagnetic disturbances at Ma¨ntsa¨la¨, creating noise to the magnetic recordings. Thus, in this respect, the GIC measurement is more reliable with larger GIC. The effect of the cathodic protection (CP) system (Gummow, in press) to the current flowing along the
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pipeline is naturally quite large. It is very difficult to estimate the total effect of the whole CP system on the Finnish pipeline, but it is clear that the CP current disturbs the determination of GIC. As a rough estimate, based on a comparison between the model calculations and the actual measurements, one can estimate that the inaccuracy due to CP is of about the same order as the inaccuracy due to the spatial inhomogeneity of the geomagnetic field. The largest GIC since November 1998 occurred on July 15, 2001, when during intense magnetic storm, it was measured to be 30 A. This is small compared to the GIC measured by Campbell (1980a), who recorded a GIC near 100 A in the Alaska oil pipeline located roughly at the same latitudes as the Finnish pipeline, but is 1280 km long. Campbell estimated that the largest GIC in the Alaska pipeline may rise up to 1000 A. This is an order of magnitude larger than estimations for the Finnish pipeline (see below). For
another reference, Barker and Skinner (1980) recorded a GIC in the 450-km-long Kenyan pipeline. Due to the location at the equator, the largest measured GIC during magnetic storm on September 29, 1978, was only 2 A. However, in spite of the small amplitude of the GIC, the polarity of the current remained for several hours, giving rise to possible corrosion problems. 2.2. Measurements of the voltage Measurements of the voltage between the pipeline and the earth are quite problematic. The CP system at Ma¨ntsa¨la¨ is based on a varying protection current, which is trying to keep the protection voltage constant. Thus, the effect of the CP system screens the voltage variations of geomagnetic origin. The variations due to geomagnetic disturbances have been seen only during large magnetic storms (Fig. 2). Even during these intense storms, the voltage stayed negative.
Fig. 2. GIC and P/S voltage measured at Ma¨ntsa¨la¨ and the time derivative of the north component of the magnetic field measured at the nearby Nurmija¨rvi Geophysical Observatory on January 13, 1999. The GIC is positive eastwards. The negative of dX/dt is intentionally plotted, since it has roughly the same shape as the GIC.
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In the future, if voltage measurements are planned, it is worth finding a location where the effect of the CP system is minimal. In practice, this means a site where the CP current is kept constant, because then variations due to geomagnetic origin can be seen directly in the variations of the P/S voltage. One must note that the measured P/S voltage (Fig. 2) is not directly the CP voltage. The so-called IRdrop, which is a result of a voltage loss due to the current flowing in the earth, is also seen in the measurements. The actual CP voltage is smaller, being typically f 1 V measured against a Cu/CuSO4 reference electrode. Gasum, the owner of the Finnish pipeline, is carrying out voltage measurements at several sites along the pipeline. However, unfortunately the data are not stored digitally and the sampling frequency is not fixed (the system is not designed for GIC-monitoring purposes). Thus, those measurements cannot be used for a scientific analysis. A new monitoring system, now being installed by Gasum, produces voltage data that can be used in future studies.
3. Statistical predictions 3.1. The method Statistical predictions of the occurrences of currents and voltages were calculated for seven different locations: Kyro¨skoski, Ha¨meenlinna, Ma¨ntsa¨la¨, Helsinki, Kouvola, Kotka and Imatra (Fig. 1). Corresponding predictions can be calculated also for other arbitrarily chosen sites. We assumed the geoelectric field to be homogeneous in the area of the Finnish pipeline. This assumption is reasonable since the deep geoelectric structure and the source field do not change very much in the scales (f300 km) of the pipeline. Furthermore, the geoelectric field affecting GIC is the integrated electric field, making very local field inhomogeneities non-important. GIC induced far in the Russian side of the network does not reach very far to the Finnish side. The basis used in the determination of the statistical occurrences was the method developed for the calculation of GIC and P/S voltages in a pipeline networks (Pulkkinen et al., 2001). Applying the method,
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we calculated theoretical coefficients an, bn, cn and dn (n = 1,2,. . .,7) (see Table 1) that give GIC and the P/S voltages at each studied points as a linear function of the geoelectric field: GICn ¼ an Ex þ bn Ey Un ¼ cn Ex þ dn Ey
ð2Þ
a, b, c and d are dependent on the geometry and electrical properties of the pipeline network. The geoelectric field was calculated with the plane wave method (Pirjola, 1982) from the geomagnetic data obtained from the Nurmija¨rvi Geophysical Observatory. In the plane wave method, the eastward electric field Ey at the earth’s surface can be determined from the rate of change of the northward component of the magnetic field Bx: Zt 1 1 dBx ðuÞ pffiffiffiffiffiffiffiffiffiffi du ð3Þ Ey ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pl0 reff t u dt 1
where l0 is the permeability of free space and reff is the effective conductivity of the earth. The latter was fitted in a lest squares sense so that Eq. (3) gives a geoelectric field, which combined with the model of the Finnish pipeline creates the observed GIC at Ma¨ntsa¨la¨. The obtained value is reff = 3.1 10 2 V 1 m 1. This is larger than the value used by Viljanen (1998) (he obtained reff = 0.9 10 2 V 1 m 1), when he studied GIC in the southern part of the Finnish power system. There are two main reasons for the deviation from Viljanen’s result: (1) The effect of the cathodic protection system of the Finnish pipeline. By keeping the protection voltage constant, the system is also trying to minimize the current flowing along the pipeline. This affects the measured GIC. (2)
Table 1 Coefficients for the linear relations between the geoelectric field and GIC and P/S voltages (see Eq. (2))
Kyro¨skoski Ha¨meenlinna Ma¨ntsa¨la¨ Helsinki Kouvola Kotka Imatra
a [Akm/V]
b [Akm/V]
c [km]
d [km]
0.4 10 5 74 70 0.4 10 5 24 0.2 10 5 36
0.2 10 5 43 88 0.1 10 5 190 0.03 10 5 180
41 7 9.2 44 1.5 19 11
20 9 21 11 1.5 2.7 2.3
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Fig. 3. Correlation between the y-component of the geoelectric field and Ak-index. The correlation coefficient is 0.86. Also a linear fit between two quantities is shown in the figure.
The incompleteness of the plane wave assumption used in the calculation of the geoelectric field and the assumption about homogenous geoelectric field. Es-
pecially during large geomagnetic disturbances, the field may be very inhomogeneous and greatly deviate from a plane wave. As presented by Pulkkinen et al.
Fig. 4. Modeled occurrence of GIC at Ha¨meenlinna.
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Fig. 5. Modeled occurrence of GIC at Imatra.
(submitted for publication), the deviation between the measured and modeled GIC increases with increasing GIC. However, the deviation is only in the amplitudes of GIC, reflecting the effect of the protection system,
and implying that the first of these two reasons probably plays a more important role here. Using the plane wave method (with the conductivity reff), we first calculated the x- and y-components
Fig. 6. Modeled occurrence of GIC at Kouvola.
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Fig. 7. Modeled occurrence of GIC at Ma¨ntsa¨la¨.
of the geoelectric field at Nurmija¨rvi during the period January 1993– April 2001. This is the period from which we have 10-s magnetic data. Then GIC and voltages were calculated using Eq. (2). From these,
we calculated the statistical occurrences of the quantities. Now, due to a linear relationship between the peak values of the x- and y-components of the geoelectric
Fig. 8. Modeled occurrence of the voltage at Helsinki.
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Fig. 9. Modeled occurrence of the voltage at Ha¨meenlinna.
field and the Nurmija¨rvi Ak-index (Fig. 3), and due to the known relationship between the geoelectric field and GIC, it is possible to normalize the statistics of the
period January 1993 – April 2001 to represent the statistics for geomagnetically different years. A similar linear relationship was found between the daily average
Fig. 10. Modeled occurrence of the voltage at Imatra.
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Fig. 11. Modeled occurrence of the voltage at Kotka.
geoelectric field and the Ak-index. Using Ak-indices of the period January 1953– April 2001 statistics were normalized to represent GIC and voltages of geomagnetically disturbed (mean Ak = 21.6), mean (mean
Ak = 12.3) and quiet (mean Ak = 6.0) year. The mean Ak of the period January 1993 –April 2001 was 12.3. The same normalizing method was used by Ma¨kinen (1993) to study GIC in the Finnish power system.
Fig. 12. Modeled occurrence of the voltage at Kouvola.
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Fig. 13. Modeled occurrence of the voltage at Kyro¨skoski.
3.2. Occurrence of GIC and voltages The absolute values of the occurrences of GIC and P/S voltage (Figs. 4 – 14) are given as minutes per year when the magnitude exceeds a given value.
The three horizontal axes represent the occurrences in a geomagnetically disturbed (the uppermost axis), mean and quiet year. Due to the location at the insulated end of the pipeline, GICs at Helsinki, Kotka and Kyro¨skoski are zero and thus GICs of
Fig. 14. Modeled occurrence of the voltage at Ma¨ntsa¨la¨.
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Fig. 15. Occurrence of the measured GIC at Ma¨ntsa¨la¨.
the corresponding sites are not shown here. It should also be noted that the statistical occurrences are given as absolute values, and therefore positive voltages that increase the corrosion risk of the pipeline are seen only half of the time, as given in Figs. 8– 14. The statistical distributions clearly show the most GIC sensitive sections of the Finnish pipeline network. The largest voltages are seen at Kyro¨skoski, which is one end of the network, while the smallest values occur at Imatra and Kouvola. This is seen roughly also from the Gasum’s voltage-plotter data of some events. The largest GIC are seen at Imatra and Kouvola. One can expect GIC over 90 A during very intense magnetic storms. The statistics of measured GIC (period November 13, 1998 – April 30, 2001) are shown in Fig. 15. They are normalized to represent the occurrences for 1 year. Also the measured GIC shows that the magnetic activity has been close to the average level. The largest measured GIC is 30 A, which is still about 15 A smaller than the expected largest GIC during a magnetically mean year (see Fig. 7). The most probable reason for the deviation between the measured and predicted distribution of GIC at Ma¨ntsa¨la¨ is the relatively short length of the measurement period: these two statistical distributions will most likely coincide as GIC measurements are continued.
4. Conclusions A project implemented to study the effects of space weather on the Finnish natural gas pipeline, was launched between the Finnish Meteorological Institute and Gasum Oy, the owner of the Finnish natural gas pipeline. In the first part of the project (described by Pulkkinen et al., 2001), we derived a theoretical model for calculating GIC and P/S voltages in the Finnish natural gas pipeline. In the second and third parts of the project, we carried out GIC and P/S voltage measurements in the pipeline, and with the help of the theoretical model and measurements, derived statistical pre-dictions for the occurrences of GIC and P/S voltages in the Finnish pipeline. Measurements are carried out near the center of the pipeline network, at Ma¨ntsa¨la¨. The GIC is obtained by measuring the magnetic field created by the quasi-DC-current flowing along the pipeline. The magnetization effect was found to be ignorable, which is the result of a thin pipeline wall. The only major problem in the measurements is that the cathodic protection system of the Finnish pipeline badly disturbs the voltage measurements. GIC at Ma¨ntsa¨la¨ is less affected by the protection system, and thus the effect is not critical from the point of view of the study.
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A linear dependence between the geoelectric field calculated using magnetic data from the Nurmija¨rvi Geophysical Observatory and GIC and P/S voltage at seven different sites was derived using the theoretical method. Coefficients of the linear model were then used to determine GIC and P/S voltages for the period 1993– 1998. These statistics were then extended to describe GIC and voltages of magnetically different years using a normalization based on the Ak-index of Nurmija¨rvi. Statistical predictions clearly points out the most GIC-sensitive sections of the Finnish pipeline: the largest P/S voltages are found at Kyro¨skoski, which is one end of the network, while the smallest variations are seen at Imatra and Kouvola. The most probable sites for large GIC are Ma¨ntsa¨la¨ and Imatra where GIC may exceed 100 A. From the point of view of the pipeline operation, P/ S voltages due to geomagnetic disturbances, especially at the end points of the network, seem to exceed the CP voltage during significant time periods. Thus, if the coating of the pipeline is for some reason damaged and the CP fails to keep the direction of the electric current from the soil to the pipeline, the risk of corrosion is increased.
Acknowledgements Our thanks go to Mr. Juha Vainikka, Senior Vice President of Gasum Oy, and to Mr. Matti Pitka¨nen for great support and interest in the study. We would like to express our gratitude to Dr. Larisa Trichtchenko and Dr. David Boteler of Geophysical Survey of Canada for their modeling efforts and advice concerning the magnetization effect, and for their useful comments on the paper in general. The measurements of induced currents in the Finnish pipeline were carried out with the help from Mr. Pentti Posio of the Nurmija¨rvi Geophysical Observatory (Finnish Meteorological Institute), which is gratefully acknowledged.
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References Barker, R.H., Skinner, N.J., 1980. The flow of electric currents of telluric origin in a long metal pipeline and their effects in relation to corrosion control. Materials Performance, pp. 25 – 28, February. Boteler, D., Cookson, M.J., 1986. Telluric currents and their effects on pipelines in the Cook Strait region of New Zealand. Materials Performance, pp. 27 – 32, March. Campbell, W., 1980a. Observation of electric currents in the Alaska oil pipeline resulting from auroral electrojet current sources. Geophysical Journal of the Royal Astronomical Society 61, 437 – 449. Campbell, W., 1980b. Induced electric currents in the Alaska oil pipeline measured by gradient fluxgate and SQUID magnetometers. IEEE Transactions on Geoscience and Remote Sensing GE-18 (3), 244 – 250. Gummow, R.A., in press. GIC effects on pipeline corrosion and corrosion control systems. Paper presented at Session E2 of the XXVI General Assembly of the International Union of Radio Science, Toronto, Canada, August 13 – 21, 1999. Journal of Atmospheric and Solar-Terrestrial Physics. Lehtinen, M., Pirjola, R., 1985. Currents produced in earthed conductor networks by geomagnetically induced electric fields. Annales Geophysicae 3 (4), 479 – 484. Ma¨kinen, T., 1993. Geomagnetically induced currents in the Finnish power transmission system. Geophysical Publications 32, Finnish Meteorological Institute, 101 pp. Pirjola, R., 1982. Electromagnetic induction in the earth by plane wave or by fields of line currents harmonic in time and space. Geophysica 18, 1 – 161. Pirjola, R., Lehtinen, M., 1985. Currents produced in the Finnish 400 kV power transmission grid and in the Finnish natural gas pipeline by geomagnetically-induced electric fields. Annales Geophysicae 3 (4), 485 – 491. Pulkkinen, A., Pirjola, R., Boteler, D., Viljanen, A., Yegorov, I., 2001. Modelling of space weather effects on pipelines. Journal of Applied Geophysics 48, 233 – 256. Taflove, A., Dabkowski, J., 1979. Prediction method for buried pipeline voltages due to 60 Hz AC inductive coupling. IEEE Transactions on Power Apparatus and Systems PAS-98, 780 – 794. Viljanen, A., 1989. Geomagnetically induced currents in the Finnish natural gas pipeline. Geophysica 25 (1 and 2), 135 – 159. Viljanen, A., 1998. Relation of geomagnetically induced currents and local geomagnetic variations. IEEE Transactions on Power Delivery 13. Viljanen, A., Pirjola, R., 1994. Geomagnetically induced currents in the Finnish high-voltage power system—a geophysical review. Surveys in Geophysics 15, 383 – 408.
Recordings and occurrence of geomagnetically induced currents in the Finnish natural gas pipeline network Antti Pulkkinen*, Ari Viljanen, Kari Pajunpa¨a¨, Risto Pirjola Finnish Meteorological Institute, Geophysical Research Division, P.O. Box 503, FIN-00101, Helsinki, Finland Received 22 March 2000; accepted 31 October 2001
Abstract A project implemented to study the effects of space weather on the Finnish natural gas pipeline was started in August 1998. The aims of the project were (1) to derive a model for calculating geomagnetically induced currents (GIC) and pipe-to-soil (P/S) voltages in the Finnish natural gas pipeline, (2) to perform measurements of GIC and P/S voltages in the pipeline and (3) to derive statistical predictions for the occurrences of GIC and P/S voltages at different locations in the pipeline network. GIC and P/S voltage were recorded at a compressor station. The GIC measurement was made with two magnetometers, one right above the pipe, and another at the Nurmija¨rvi Geophysical Observatory about 30 km southwest. The largest GIC since November 1998 has been 30 A. The P/S voltage recording was stopped in May 1999, but GIC is still measured. GIC statistics were derived based on the recordings of the geomagnetic field at Nurmija¨rvi. The geoelectric field was calculated by using the plane wave model. This field was input to the general pipeline model resulting in the distribution of currents and P/S voltages at selected points in the pipeline. As could be expected, the largest P/S voltage variations occur at the ends of the pipeline network, while the largest GIC flow in the middle parts. D 2001 Elsevier Science B.V. All rights reserved. Keywords: Geophysical methods; Electromagnetic field; Induction; Magnetometers; Cathodic protection
1. Introduction Studies of geomagnetically induced currents (GIC) in the Finnish natural gas pipeline were started by Pirjola and Lehtinen (1985). The study was based on the method developed originally for the calculation of GIC in power systems (Lehtinen and Pirjola, 1985), which was applied to estimate GIC flowing through the cathodic protection stations. Due to the assumption of
*
Corresponding author. E-mail addresses: [email protected] (A. Pulkkinen), [email protected] (A. Viljanen), [email protected] (K. Pajunpa¨a¨), [email protected] (R. Pirjola).
the discrete groundings of the pipeline, the calculation did not correspond to reality, and thus the study was not able to give reliable estimations of GIC in the pipeline. Viljanen (1989) modeled the pipeline as an infinitely long cylinder buried in a homogenous medium. He computed the estimations for GIC along the pipeline and between the pipeline and the earth. Based on the model and the magnetic field measurements carried out at the Nurmija¨rvi Geophysical Observatory, he draws up statistical estimations of the occurrences of GIC. However, the assumption about an infinite length of the pipeline ignores the discontinuities of the pipeline, and thus the method cannot be regarded as completely satisfactory.
0926-9851/01/$ - see front matter D 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 9 8 5 1 ( 0 1 ) 0 0 1 0 8 - 2
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A significant step forward was the application of the distributed source transmission line theory. The method was used for modeling AC induction in pipelines by Taflove and Dabkowski (1979). In geomagnetic induction studies, it was first used by Boteler and Cookson (1986) and was further developed by Pulkkinen et al. (submited for publication). The method is applicable to general, complex pipeline networks, and does not use any unrealistic assumptions. Because of this method, it was finally possible to study the Finnish pipeline network as a whole, and thus there was a good reason for starting GIC measurements to be used in parallel with our modeling efforts.
2. Measurements The Finnish pipeline network has a total length of 914 km (Fig. 1). The pipeline is composed of the main network, which has a length of 350 km between the eastern and the western ends, and of several shorter branches. The pipeline is electrically connected to the Russian pipeline. The exact length and the electrical characteristics of the Russian pipeline are not known but the length of the Russian part can be regarded as infinite. GIC measurements were started in November 1998. The measurement site is located at Ma¨ntsa¨la¨, at a branch that connects pipelines coming from Tampere,
Helsinki and Kouvola (Fig. 1). The pipeline is almost east-west directed at the measurement site. Measurements of the voltage between the pipeline and the earth (pipe-to-soil (P/S) voltage) were also started in November 1998. Measurements of GIC are still going on but the voltage measurements had to be interrupted during the summer 1999 because of possible lightning hazards. Because of interpretation problems due to the cathodic protection system of the pipeline, the voltage measurements were not continued. Data are collected with a 10-s time resolution to a computer at the compressor station at Ma¨ntsa¨la¨, from which they are transferred daily to the Nurmija¨rvi Geophysical Observatory. Daily updated data-plots can be seen at www. geo.fmi.fi/MAGN/GIC. The magnetic activity of the measurement period has been quite close to the average level. The average Ak index at Nurmija¨rvi has been 14, which is slightly over the average Ak of the period January 1953 –April 2001, which is 12.3. Until now, the largest Ak (268) during the measurement period was reached on March 31, 2001. 2.1. Measurements of GIC GIC in the pipeline is recorded following a similar method as Campbell’s (1980b), who measured GIC in the Alaska oil pipeline. The method was also used to record GIC in a Finnish 400 kV power transmission line (Ma¨kinen, 1993; Viljanen and Pirjola, 1994).
Fig. 1. Finnish natural gas pipeline network. GIC recordings are performed at Ma¨ntsa¨la¨.
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One magnetometer is placed right above the pipeline. Due to the small frequencies of geomagnetic variations, the current flowing along the pipeline is distributed uniformly across the pipeline steel. Consequently, according to the Biot –Savart law: I¼
2pR B l0
ð1Þ
where R is the distance from the center of the pipeline and B is the axial magnetic field. The geomagnetic variation field is subtracted with the reference measurements carried out at the Nurmija¨rvi Geophysical Observatory. It is located 30 km to the south-west from the GIC measurement site, so it is justified to assume that the geomagnetic field is nearly identical at both. The positive direction of the current was chosen to be eastward. The method described above has some problems worth noting here. The Finnish natural gas pipeline is made of steel, which is a highly permeable material. Consequently, there is a good reason to believe that the permeability has an influence on the magnetic field, which is recorded by the magnetometer at the pipeline. The effect of the permeability can in principle be divided into three parts: (1) The static magnetization produced by the main geomagnetic field in the permeable material changes the field in the vicinity of the pipeline. In our recordings, this is seen as a change of the baseline. (2) The permeability affects the skin depth of the material, thus changing the distribution of the electric and magnetic fields. This implies that the current density varies with the permeability, and a change of the current also modifies the magnetic (variation) field around the pipeline. (3) The magnetization induced by temporal magnetic variations in the permeable material contributes to the variations measured in the vicinity of the pipeline. Effect 1 does not matter in our recordings since only the variation field is considered. Also Effect 2 is not of importance regarding the recordings because it is just the GIC that we are actually measuring. Effect 3, however, should be considered carefully while it refers to a contribution to magnetic variations not associated with GIC, and this contribution is not eliminated in the comparison with the recordings of the reference magnetometer. Regarding GIC recordings with magnetometers at the Alaska pipeline Campbell
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(1980b) points out that the permeability of the pipeline affects the baseline of magnetic recordings thus referring to Effect 1, but he does not study closer Effect 2, and in particular, Effect 3. To examine Effect 3, we made model computations by considering an infinitely long steel cylinder with a coating lying in a homogeneous medium (the earth). The permeability of the pipeline steel is not known exactly. Therefore, we have used permeability values in the range from 10l0 to 1000l0. Having the radii equal to those of the Finnish pipeline and periods characteristic to geomagnetic variations, i.e. 10 to 1000 s, we found that the axial magnetic field outside the pipeline, from which the initial magnetic field is subtracted, is the same as the Biot – Savart field caused by the DC current flowing along the pipeline. Thus, the permeability does not affect the variation field around the cylinder at all and Effect 3 is ignorable. This is an understandable result since the wall thickness of the pipeline is much smaller than the skin depth in the pipeline steel. In addition, because the change of the period (between 10 and 1000 s) did not change the amplitude of the axial magnetic field outside the pipeline, GIC along the pipeline is simply determined by the DC resistance of the pipeline. In conclusion, Effects 1, 2 and 3 are physical facts that should in principle be investigated when performing GIC recordings with the help of magnetometers. However, the periods associated with GIC and wall thicknesses used in pipelines prevent any distortion in practice. The inaccuracy of the GIC measurement due to uncertainty of the distance of the magnetometer from the buried pipeline was estimated to be 5%. The inaccuracy due to the deviation between the geomagnetic fields of the Nurmija¨rvi and Ma¨ntsa¨la¨ is larger. When studying the deviations of y- and z-components (not affected by GIC flowing along the pipeline) and when only field variations exceeding 100 nT were taken into account, it was estimated to be about 10%. When smaller variations were included, the inaccuracy increased (with the threshold of 30 nT, inaccuracy is 20%), which is the result of local non-geomagnetic disturbances at Ma¨ntsa¨la¨, creating noise to the magnetic recordings. Thus, in this respect, the GIC measurement is more reliable with larger GIC. The effect of the cathodic protection (CP) system (Gummow, in press) to the current flowing along the
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pipeline is naturally quite large. It is very difficult to estimate the total effect of the whole CP system on the Finnish pipeline, but it is clear that the CP current disturbs the determination of GIC. As a rough estimate, based on a comparison between the model calculations and the actual measurements, one can estimate that the inaccuracy due to CP is of about the same order as the inaccuracy due to the spatial inhomogeneity of the geomagnetic field. The largest GIC since November 1998 occurred on July 15, 2001, when during intense magnetic storm, it was measured to be 30 A. This is small compared to the GIC measured by Campbell (1980a), who recorded a GIC near 100 A in the Alaska oil pipeline located roughly at the same latitudes as the Finnish pipeline, but is 1280 km long. Campbell estimated that the largest GIC in the Alaska pipeline may rise up to 1000 A. This is an order of magnitude larger than estimations for the Finnish pipeline (see below). For
another reference, Barker and Skinner (1980) recorded a GIC in the 450-km-long Kenyan pipeline. Due to the location at the equator, the largest measured GIC during magnetic storm on September 29, 1978, was only 2 A. However, in spite of the small amplitude of the GIC, the polarity of the current remained for several hours, giving rise to possible corrosion problems. 2.2. Measurements of the voltage Measurements of the voltage between the pipeline and the earth are quite problematic. The CP system at Ma¨ntsa¨la¨ is based on a varying protection current, which is trying to keep the protection voltage constant. Thus, the effect of the CP system screens the voltage variations of geomagnetic origin. The variations due to geomagnetic disturbances have been seen only during large magnetic storms (Fig. 2). Even during these intense storms, the voltage stayed negative.
Fig. 2. GIC and P/S voltage measured at Ma¨ntsa¨la¨ and the time derivative of the north component of the magnetic field measured at the nearby Nurmija¨rvi Geophysical Observatory on January 13, 1999. The GIC is positive eastwards. The negative of dX/dt is intentionally plotted, since it has roughly the same shape as the GIC.
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In the future, if voltage measurements are planned, it is worth finding a location where the effect of the CP system is minimal. In practice, this means a site where the CP current is kept constant, because then variations due to geomagnetic origin can be seen directly in the variations of the P/S voltage. One must note that the measured P/S voltage (Fig. 2) is not directly the CP voltage. The so-called IRdrop, which is a result of a voltage loss due to the current flowing in the earth, is also seen in the measurements. The actual CP voltage is smaller, being typically f 1 V measured against a Cu/CuSO4 reference electrode. Gasum, the owner of the Finnish pipeline, is carrying out voltage measurements at several sites along the pipeline. However, unfortunately the data are not stored digitally and the sampling frequency is not fixed (the system is not designed for GIC-monitoring purposes). Thus, those measurements cannot be used for a scientific analysis. A new monitoring system, now being installed by Gasum, produces voltage data that can be used in future studies.
3. Statistical predictions 3.1. The method Statistical predictions of the occurrences of currents and voltages were calculated for seven different locations: Kyro¨skoski, Ha¨meenlinna, Ma¨ntsa¨la¨, Helsinki, Kouvola, Kotka and Imatra (Fig. 1). Corresponding predictions can be calculated also for other arbitrarily chosen sites. We assumed the geoelectric field to be homogeneous in the area of the Finnish pipeline. This assumption is reasonable since the deep geoelectric structure and the source field do not change very much in the scales (f300 km) of the pipeline. Furthermore, the geoelectric field affecting GIC is the integrated electric field, making very local field inhomogeneities non-important. GIC induced far in the Russian side of the network does not reach very far to the Finnish side. The basis used in the determination of the statistical occurrences was the method developed for the calculation of GIC and P/S voltages in a pipeline networks (Pulkkinen et al., 2001). Applying the method,
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we calculated theoretical coefficients an, bn, cn and dn (n = 1,2,. . .,7) (see Table 1) that give GIC and the P/S voltages at each studied points as a linear function of the geoelectric field: GICn ¼ an Ex þ bn Ey Un ¼ cn Ex þ dn Ey
ð2Þ
a, b, c and d are dependent on the geometry and electrical properties of the pipeline network. The geoelectric field was calculated with the plane wave method (Pirjola, 1982) from the geomagnetic data obtained from the Nurmija¨rvi Geophysical Observatory. In the plane wave method, the eastward electric field Ey at the earth’s surface can be determined from the rate of change of the northward component of the magnetic field Bx: Zt 1 1 dBx ðuÞ pffiffiffiffiffiffiffiffiffiffi du ð3Þ Ey ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pl0 reff t u dt 1
where l0 is the permeability of free space and reff is the effective conductivity of the earth. The latter was fitted in a lest squares sense so that Eq. (3) gives a geoelectric field, which combined with the model of the Finnish pipeline creates the observed GIC at Ma¨ntsa¨la¨. The obtained value is reff = 3.1 10 2 V 1 m 1. This is larger than the value used by Viljanen (1998) (he obtained reff = 0.9 10 2 V 1 m 1), when he studied GIC in the southern part of the Finnish power system. There are two main reasons for the deviation from Viljanen’s result: (1) The effect of the cathodic protection system of the Finnish pipeline. By keeping the protection voltage constant, the system is also trying to minimize the current flowing along the pipeline. This affects the measured GIC. (2)
Table 1 Coefficients for the linear relations between the geoelectric field and GIC and P/S voltages (see Eq. (2))
Kyro¨skoski Ha¨meenlinna Ma¨ntsa¨la¨ Helsinki Kouvola Kotka Imatra
a [Akm/V]
b [Akm/V]
c [km]
d [km]
0.4 10 5 74 70 0.4 10 5 24 0.2 10 5 36
0.2 10 5 43 88 0.1 10 5 190 0.03 10 5 180
41 7 9.2 44 1.5 19 11
20 9 21 11 1.5 2.7 2.3
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Fig. 3. Correlation between the y-component of the geoelectric field and Ak-index. The correlation coefficient is 0.86. Also a linear fit between two quantities is shown in the figure.
The incompleteness of the plane wave assumption used in the calculation of the geoelectric field and the assumption about homogenous geoelectric field. Es-
pecially during large geomagnetic disturbances, the field may be very inhomogeneous and greatly deviate from a plane wave. As presented by Pulkkinen et al.
Fig. 4. Modeled occurrence of GIC at Ha¨meenlinna.
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Fig. 5. Modeled occurrence of GIC at Imatra.
(submitted for publication), the deviation between the measured and modeled GIC increases with increasing GIC. However, the deviation is only in the amplitudes of GIC, reflecting the effect of the protection system,
and implying that the first of these two reasons probably plays a more important role here. Using the plane wave method (with the conductivity reff), we first calculated the x- and y-components
Fig. 6. Modeled occurrence of GIC at Kouvola.
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Fig. 7. Modeled occurrence of GIC at Ma¨ntsa¨la¨.
of the geoelectric field at Nurmija¨rvi during the period January 1993– April 2001. This is the period from which we have 10-s magnetic data. Then GIC and voltages were calculated using Eq. (2). From these,
we calculated the statistical occurrences of the quantities. Now, due to a linear relationship between the peak values of the x- and y-components of the geoelectric
Fig. 8. Modeled occurrence of the voltage at Helsinki.
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Fig. 9. Modeled occurrence of the voltage at Ha¨meenlinna.
field and the Nurmija¨rvi Ak-index (Fig. 3), and due to the known relationship between the geoelectric field and GIC, it is possible to normalize the statistics of the
period January 1993 – April 2001 to represent the statistics for geomagnetically different years. A similar linear relationship was found between the daily average
Fig. 10. Modeled occurrence of the voltage at Imatra.
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Fig. 11. Modeled occurrence of the voltage at Kotka.
geoelectric field and the Ak-index. Using Ak-indices of the period January 1953– April 2001 statistics were normalized to represent GIC and voltages of geomagnetically disturbed (mean Ak = 21.6), mean (mean
Ak = 12.3) and quiet (mean Ak = 6.0) year. The mean Ak of the period January 1993 –April 2001 was 12.3. The same normalizing method was used by Ma¨kinen (1993) to study GIC in the Finnish power system.
Fig. 12. Modeled occurrence of the voltage at Kouvola.
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Fig. 13. Modeled occurrence of the voltage at Kyro¨skoski.
3.2. Occurrence of GIC and voltages The absolute values of the occurrences of GIC and P/S voltage (Figs. 4 – 14) are given as minutes per year when the magnitude exceeds a given value.
The three horizontal axes represent the occurrences in a geomagnetically disturbed (the uppermost axis), mean and quiet year. Due to the location at the insulated end of the pipeline, GICs at Helsinki, Kotka and Kyro¨skoski are zero and thus GICs of
Fig. 14. Modeled occurrence of the voltage at Ma¨ntsa¨la¨.
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Fig. 15. Occurrence of the measured GIC at Ma¨ntsa¨la¨.
the corresponding sites are not shown here. It should also be noted that the statistical occurrences are given as absolute values, and therefore positive voltages that increase the corrosion risk of the pipeline are seen only half of the time, as given in Figs. 8– 14. The statistical distributions clearly show the most GIC sensitive sections of the Finnish pipeline network. The largest voltages are seen at Kyro¨skoski, which is one end of the network, while the smallest values occur at Imatra and Kouvola. This is seen roughly also from the Gasum’s voltage-plotter data of some events. The largest GIC are seen at Imatra and Kouvola. One can expect GIC over 90 A during very intense magnetic storms. The statistics of measured GIC (period November 13, 1998 – April 30, 2001) are shown in Fig. 15. They are normalized to represent the occurrences for 1 year. Also the measured GIC shows that the magnetic activity has been close to the average level. The largest measured GIC is 30 A, which is still about 15 A smaller than the expected largest GIC during a magnetically mean year (see Fig. 7). The most probable reason for the deviation between the measured and predicted distribution of GIC at Ma¨ntsa¨la¨ is the relatively short length of the measurement period: these two statistical distributions will most likely coincide as GIC measurements are continued.
4. Conclusions A project implemented to study the effects of space weather on the Finnish natural gas pipeline, was launched between the Finnish Meteorological Institute and Gasum Oy, the owner of the Finnish natural gas pipeline. In the first part of the project (described by Pulkkinen et al., 2001), we derived a theoretical model for calculating GIC and P/S voltages in the Finnish natural gas pipeline. In the second and third parts of the project, we carried out GIC and P/S voltage measurements in the pipeline, and with the help of the theoretical model and measurements, derived statistical pre-dictions for the occurrences of GIC and P/S voltages in the Finnish pipeline. Measurements are carried out near the center of the pipeline network, at Ma¨ntsa¨la¨. The GIC is obtained by measuring the magnetic field created by the quasi-DC-current flowing along the pipeline. The magnetization effect was found to be ignorable, which is the result of a thin pipeline wall. The only major problem in the measurements is that the cathodic protection system of the Finnish pipeline badly disturbs the voltage measurements. GIC at Ma¨ntsa¨la¨ is less affected by the protection system, and thus the effect is not critical from the point of view of the study.
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A linear dependence between the geoelectric field calculated using magnetic data from the Nurmija¨rvi Geophysical Observatory and GIC and P/S voltage at seven different sites was derived using the theoretical method. Coefficients of the linear model were then used to determine GIC and P/S voltages for the period 1993– 1998. These statistics were then extended to describe GIC and voltages of magnetically different years using a normalization based on the Ak-index of Nurmija¨rvi. Statistical predictions clearly points out the most GIC-sensitive sections of the Finnish pipeline: the largest P/S voltages are found at Kyro¨skoski, which is one end of the network, while the smallest variations are seen at Imatra and Kouvola. The most probable sites for large GIC are Ma¨ntsa¨la¨ and Imatra where GIC may exceed 100 A. From the point of view of the pipeline operation, P/ S voltages due to geomagnetic disturbances, especially at the end points of the network, seem to exceed the CP voltage during significant time periods. Thus, if the coating of the pipeline is for some reason damaged and the CP fails to keep the direction of the electric current from the soil to the pipeline, the risk of corrosion is increased.
Acknowledgements Our thanks go to Mr. Juha Vainikka, Senior Vice President of Gasum Oy, and to Mr. Matti Pitka¨nen for great support and interest in the study. We would like to express our gratitude to Dr. Larisa Trichtchenko and Dr. David Boteler of Geophysical Survey of Canada for their modeling efforts and advice concerning the magnetization effect, and for their useful comments on the paper in general. The measurements of induced currents in the Finnish pipeline were carried out with the help from Mr. Pentti Posio of the Nurmija¨rvi Geophysical Observatory (Finnish Meteorological Institute), which is gratefully acknowledged.
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