Variability of ship’s electric signature during
the RIMPASSE trial
Marius Birsan
DRDC – Atlantic Research Centre
Defence Research and Development Canada
Scientific Report
DRDC-RDDC-2015-R272
December 2015
Template in use: (2010) SR Advanced Template_EN (051115).dotm
© Her Majesty the Queen in Right of Canada, as represented by the Minister of National Defence, 2015
© Sa Majesté la Reine (en droit du Canada), telle que représentée par le ministre de la Défense nationale,
2015
Abstract
Technology that incorporates electric sensors into influence mines has been already developed.
Thus, it is considered essential to invest more in electric signature research to better protect our
ships against this increasing threat. For this reason, the RIMPASSE trials were performed to
investigate the electric signatures of the research vessels RV Planet and CFAV Quest when
measured at various ranges. The trials revealed significant differences between the ranges for
both vessels.
The electric field beneath the ship is caused by metal corrosion and the related corrosion
protection system. Both vessels were protected against corrosion by a set of sacrificial anodes.
The measurements during the trial suggest that the cause of signature variability is the
modification of the propeller resistivity, which may be due to the precipitation/dissolution of the
calcareous deposits. This Scientific Report investigates this hypothesis. The goal is to predict the
ship’s electrics signature during the voyages in waters with different chemical composition.
The formation of calcareous (calcium carbonate) deposits depends on the seawater chemical
composition and temperature, the corrosion current density, and ship velocity. In principle, the
layer of calcareous deposits has a beneficial role on platforms subjected to corrosion in seawater
because it coats the cathodic areas, thus reducing the current. Using inorganic carbon chemistry in
the seawater, it is possible to predict which of the two processes, precipitation or dissolution of
the calcium carbonate deposit, takes place.
Using the water chemical composition available in the literature, the dissolution of the calcareous
deposits on propellers could not be predicted. Thus, the observed signature variability could not
be explained based on this hypothesis.
Significance to defence and security
This study is part of the effort to develop an onboard electric signature modelling module to be
integrated into a prototype Signature Management System. The results from this study will also
help to provide advice to RCN platforms with respect to electric signature vulnerability.
DRDC-RDDC-2015-R272
i
Résumé
La technologie qui intègre les capteurs électriques aux mines à influence a déjà été élaborée. Il est
donc jugé essentiel d’investir davantage dans la recherche sur les signatures électriques afin de
mieux protéger nos navires contre cette menace croissante. C’est pourquoi on a procédé aux
essais RIMPASSE dans le but d’examiner les signatures électriques, mesurées à diverses portées,
des navires de recherche RV Planet et NAFC Quest. Les essais ont révélé des différences
importantes entre la portée des deux navires.
Le champ électrique sous le navire se forme en raison de la corrosion et du système de protection
contre la corrosion. Dans ce cas-ci, les deux navires étaient protégés contre la corrosion par un
ensemble d’anodes sacrificielles. Les mesures obtenues lors de l’essai suggèrent que la variation
de la signature est causée par une modification de la résistivité de l’hélice en raison de la
précipitation/dissolution de dépôts calcaires. Ce rapport scientifique examine cette hypothèse.
L’objectif est de prédire la signature électrique du navire pendant les voyages dans des eaux de
compositions chimiques différentes.
La formation de dépôts calcaires (carbonate de calcium) dépend de la composition chimique et de
la température de l’eau de mer, de la densité du courant de corrosion et de la vitesse du navire. En
principe, la couche de dépôts calcaires joue un rôle bénéfique sur les plateformes soumises à la
corrosion dans l’eau de mer, car elle recouvre les zones cathodiques et réduit donc le courant. En
utilisant la chimie du carbone inorganique dans l’eau de mer, il est possible de prédire lequel des
deux processus, la précipitation ou la dissolution des dépôts de carbonate de calcium, aura lieu.
Il était impossible de prédire la dissolution des dépôts calcaires sur les hélices uniquement en
étudiant les diverses compositions chimiques de l’eau décrites dans la littérature. C’est pourquoi
la variabilité de la signature observée ne pouvait être expliquée en se basant sur cette hypothèse.
Importance pour la défense et la sécurité
L’étude fait partie des efforts visant à mettre au point un module embarqué de modélisation de la
signature électrique qui sera intégré à un prototype de système de gestion de la signature. Les
résultats obtenus aideront aussi à fournir des conseils en matière de vulnérabilité de la signature
électrique aux plateformes de la MRC.
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Table of contents
Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . i
Significance to defence and security . . . . . . . . . . . . . . . . . . . . . . i
Résumé . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
ii
Importance pour la défense et la sécurité . . . . . . . . . . . . . . . . . . . .
Table of contents . . . . . . . . . . . . . . . . . . . . . . . . . . . .
ii
iii
List of figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
iv
List of tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
v
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . .
vi
1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1
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Electric signature measurements . . . . . . . . . . .
2.1 Description of vessels and ranges . . . . . . . . .
2.2 The variability of the electric signature between ranges.
Modelling the electric signature . . . . . . . . . . .
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Calcareous deposits formation on propellers . .
4.1 Calcium carbonate and pH . . . . . .
4.2 Calcium carbonate and alkalinity . . . .
4.3 Calcium carbonate and water flow . . .
The solubility of calcareous chalk at the ranges .
5.1 Modelling the dissolved inorganic carbon .
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6 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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List of figures
Figure 1:
The longitudinal (left) and transverse (right) signatures (µV/m) of Quest at
Herdla with shaft grounding system ON (Condition A). . . . . . . . . . .
5
Figure 2:
Current source distribution on the hull and range quality plot for Herdla. . . .
5
Figure 3:
The longitudinal (left) and transverse (right) signatures (µV/m) of Quest at
Herdla without shaft grounding (Condition C). . . . . . . . . . . . . .
6
Current source distribution on the hull and range quality plot for Herdla
without shaft grounding. . . . . . . . . . . . . . . . . . . . . . .
6
The longitudinal (left) and transverse (right) signatures (µV/m) of Quest at
Aschau with shaft grounding system ON (Condition A). . . . . . . . . .
7
Figure 6:
Current source distribution on the hull and range quality plot for Aschau. . .
7
Figure 7:
The longitudinal (left) and transverse (right) signatures (µV/m) of Quest at
Aschau without shaft grounding (Condition C). . . . . . . . . . . . . .
8
Current source distribution on the hull and range quality plot for Aschau
without shaft grounding. . . . . . . . . . . . . . . . . . . . . . .
8
The longitudinal (left) and transverse (right) signatures (µV/m) of Quest at
Brest with shaft grounding system ON (Condition A). . . . . . . . . . .
9
Figure 10:
Current source distribution on the hull and range quality plot for Brest. . . .
9
Figure 11:
The Extremely Low Frequency (ELF) electric field recorded at different
ranges when the shaft grounding was ON. . . . . . . . . . . . . . . . 11
Figure 12:
Finite element model of Quest with anodes inside the hull. . . . . . . . . 13
Figure 13:
The modeled longitudinal (left) and transverse (right) signatures (µV/m) of
Quest and the corresponding current distribution. . . . . . . . . . . . . 14
Figure 14:
Calcareous deposits formation on a rotating disc, from [6]. . . . . . . . . 17
Figure 15:
Distribution of sea surface alkalinity (μmol/kg) at left, and salinity (psu), at
right, in the Baltic Sea from [9]. . . . . . . . . . . . . . . . . . . . 19
Figure 16:
Model predictions of Ω values. . . . . . . . . . . . . . . . . . . . 21
Figure 4:
Figure 5:
Figure 8:
Figure 9:
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List of tables
Table 1:
Measured [14] and calculated saturation states in central Baltic at different
temperatures and corresponding pH. Alkalinity AT = 1550 μmol/kg and
salinity = 7 psu. . . . . . . . . . . . . . . . . . . . . . . . . . . 20
DRDC-RDDC-2015-R272
v
Acknowledgements
 Bruno Lucas, DGA, France
 Jarle Selvag, FFI, Norway
 Sven Röschmann, Thomas Hopp, WTD 71, Germany
 Measurement teams at different ranges
 Center for Ship Signature Management, Kiel, Germany
 NATO Set-166
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1
Introduction
The static and extremely low frequency electric signatures of ships are easily detected with
modern sensors. This technology allowed the development of new influence sea mines which
respond to the electric field generated by the ships. We should therefore anticipate a more
complex future mine threat that includes the electric signature, as well as the magnetic and
acoustic signatures. Thus, it is essential to invest more in electric signature research to better
protect our naval platforms against this increasing threat.
In 2011 the magnetic and electric signatures of the Canadian Forces Auxiliary Vessel (CFAV)
Quest and of the German Research Vessel (RV) Planet were measured at different facilities in
Europe and Canada during the RIMPASSE (Radar Infrared electro-Magnetic Pressure Acoustic
Ship Signature Experiments) trials. The Underwater Electric Potential (UEP) signature was
measured at Herdla (Norway), Aschau (Germany), and Brest-Lanvéoc (France). These
experiments were under the auspices of the NATO SET-166 panel.
Both vessels were protected against corrosion by a set of sacrificial anodes. On RV Planet the
aluminum anodes were placed on the hull at selected positions so that the ship was fully
protected. The hull of Quest was protected at some level by zinc anodes placed inside the hull
openings. This was due to the malfunction of the Impressed Current Cathodic Protection (ICCP)
system.
To be able to compare the measured data from the three ranges, the UEP signature has to be
corrected for ship track relative to the sensors, different sensor depths, bottom conductivity, ship
crabbing over the sensors, and normalized to the same depth. The correction was performed using
an analytical model. The analysis of data from the RIMPASSE trial revealed significant
differences between the electric signatures at different ranges. The signature varied to a great
extent (up to four times) from one site to another, with the consequence that, contrary to the
magnetic signature, it would not be accurate to use the data obtained at one site to predict the
electric signature at another site. The prediction should be based on a different method.
The correlation of the signature measurements with the measurements of the shaft current and the
hull potentials suggests that the resistivity of the propeller-water interface (polarization curve)
varied when the ship was ranged in different environments. A possible cause of such changes
may be the precipitation/dissolution of the calcareous deposits on propellers. This Scientific
Report investigates this hypothesis which could explain the signature variability.
One of the characteristics associated with cathodic protection in marine environments is the
formation of calcareous (calcium and magnesium carbonate, or chalk) deposits. These deposits
form on cathodic surfaces and they reduce the cathodic currents requirements by creating a
resistive barrier between the metal and seawater. From the electric signature point of view, the
formation of calcareous deposits means a substantial reduction of the ship visibility to possible
threats.
The formation of the calcareous deposits depends on the seawater composition and temperature,
the current density at the cathode, and ship velocity. In general, the water at the ocean surface is
supersaturated in calcium carbonate, thus the normal reaction is the precipitation of this product
DRDC-RDDC-2015-R272
1
on metal surfaces. The effect of calcium carbonate precipitation on the ship’s electric signature is
beneficial. However, when the ship travels into brackish waters (the Baltic and Black Seas, for
example) the environmental conditions can change and the reverse reaction may happen, i.e., the
calcium carbonate layer dissolves. The consequence is an increase in the cathodic current demand
and, in turn, of the ship’s electric signature.
It was shown that the value of the electric current determined by the formation of the protective
layer can be up to three orders of magnitude lower in comparison with the bare metal surface
case. Because of the impact that the calcareous layer formation should have on the ship’s electric
signature, it is important to investigate the water conditions (composition and temperature) that
influence it. Using the inorganic carbon chemistry in seawater, it is possible to predict which one
of the two processes, precipitation or dissolution of calcium carbonate, takes place.
In this report, we analyze various aspects that influenced the variation of the electric signature
during the trials, including the water chemistry. The goal was to predict the ship’s electric
signature during the voyages based on the chemical reaction on the propellers. Unfortunately, the
data about the water chemistry available in the literature do not provide an explanation for the
observed signature variability.
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2
Electric signature measurements
A very detailed description of the localization and the geometry of the ranges, and of how the
measurements were performed is given in [1]. Here only limited information is repeated sufficient
to understand this report. All the presented signatures were corrected for tracking, crabbing,
sensor depth variations, normalized to a 16 m depth and an infinite bottom, and plotted on a
standard grid to facilitate the comparison between various ranges [2, 3].
2.1
Description of vessels and ranges
RV Planet is a (Small Water Plane Area Twin Hull) SWATH-type ship. Its length is 72 m, width
is 27.2 m and draught is 6.8 m. It has two main propellers, two bow and two stern thrusters. The
propellers are made of nickel-aluminum-bronze. RV Planet is protected against corrosion by a
system of aluminum sacrificial anodes [2]. These anodes provide the required current to protect
the hull and the propellers. The ship has electric pod drives where the shafts were not grounded,
so that it was not possible to measure the corrosion current going through the water into the shaft
and from the shaft to the hull. Also, the level of the Extremely Low Frequency (ELF) signal
generated by the shaft rotation is very low.
The length of CFAV Quest is 76 m, width is 12 m and draught is 4.8 m. Its ICCP system was not
functional during the trial and no sacrificial anodes were placed on the hull. However, the ship
was equipped with 16 zinc sacrificial anodes: four in the bow thruster compartment, eight in the
sea-bays, and four in the discharge sea-chest to provide corrosion protection to these areas. The
sacrificial anodes located in enclosed spaces within the hull are in contact with seawater through
four openings, two for the sea-bay, one for the sea-chest, and one for the bow thrust propeller.
The anodes inside these openings may provide some level of protection to the propellers and the
underwater hull.
On Quest, an Active Shaft Grounding (ASG) system was installed that provides a low resistivity
path for the current to flow from the shaft to the hull. When the ASG system was functioning, this
current was recorded. There are also two Ag/AgCl reference electrodes, one aft, and one near the
bow thruster, that monitor the hull potential and these values were recorded. Typically, the
reference electrodes provide the feedback potential to the ICCP control unit.
The Herdla range is located at the entrance of Bergen Harbour in Norway. The range is hidden
behind an island and therefore protected from the rough North Sea. The water conductivity was
3.3 S/m, as measured during the trial, and the bottom is rocky. The water depth at the Main
Sensor where the electric signature was measured is about 21 m, with a dependence on tidal
variations.
The Aschau range is located in the Bay of Eckernförde in Germany at the Baltic Sea with a
regular seawater conductivity of 2.6 S/m, and it has a sandy-muddy bottom with a conductivity of
0.73 S/m. The measurement depth in Aschau is about 23 m with only about a 10 cm tide.
The Brest range in France is located in the Celtic Sea with connection to the Atlantic Ocean. The
measured seawater salinity was 4.3 S/m, the ground is rocky and the bottom conductivity is about
DRDC-RDDC-2015-R272
3
0.5–0.6 S/m. The tidal variation is about 4 m, therefore the measurement depth varies from
24–28 m.
2.2
The variability of the electric signature between ranges
At the first glance, the electric signature measurements indicate that the electric current that flows
around the ships between various anodes and cathodes is low at Herdla and Brest, and high at
Aschau. The signature varied to a great extent (up to four times) from one site to another, with the
consequence that, contrary to the magnetic signature, it would not be accurate to use the data
obtained at one site to predict the electric signature at another site.
For Planet, the electric signature is the same at Herdla and Brest where it reaches a peak
magnitude of about 50 µV/m, and it is about six times higher at Aschau. Because the shaft current
and the hull potential were not measured, the interpretation of the data is not straightforward. Let
us assume the presence of calcareous layers on the propellers (cathodes) that reduces the value of
the corrosion current by decreasing the effective surface area, and thus limits the diffusion of
oxygen or water to the surface of material. The precipitation of the calcareous deposits on the
surface of materials does not take place when the water is moving over the surface [5].
Furthermore, the platform movement may be capable of reducing the already formed deposits.
To explain the variability of the electric signature of Planet by considering the protective
calcareous deposits, one has to assume that they were built up on propellers before the trials at
Herdla and Brest when the ship was stationary, and were removed somehow prior to the trial at
Aschau. Planet traveled from its base at Aschau (where normally it was without the protective
layer) to Herdla (where the protective layer formed) and back, then from Aschau to Brest. These
periodic transformations had a significant impact on the electric signature and ideally they should
be predictable.
The behaviour of Quest is similar but not identical. The following signature analysis is based on
the current source inversion [2, 3]. In the analytical model, 84 current sources (or sinks) were
placed symmetrical on the right and left sides of the hull, respectively. The source inversion plays
an important role in the analysis because it not only normalizes the signatures for easy
comparison but also reveals features otherwise difficult to observe. In particular, the current flow
around the ship is rather unusual: there is one stream from the hull to propellers, and another from
the right to the left side of the hull, both of comparable intensities. Also, the determination of the
current sources for the situation when the shafts were not grounded, thus the currents were not
measured, helped the understanding of signature variability.
The calculation of current sources from the electric field measurements at different ranges
revealed a general trend of their distribution: the current sources were placed in front and at the
middle of the ship, and the current sinks were the propellers and some unprotected surface at the
mid ship. The positioning on current sources suggests that they may be related to the sacrificial
anodes used to protect the inclusions represented by the sea-bay, the sea-chest, and the bow
thruster propeller.
At Herdla, the electric signature magnitude is the lowest (Fig. 1). The current distribution from
the measurements at Herdla is shown in Fig. 2, where the positive values correspond to current
4
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sources and the negative values to current sinks. The total shaft current is approximately given by
the summation of the current sink values that are at the stern (about -30 to -35 m) because the
right and left unbalance introduced by the tracking errors. From Fig. 2, the calculated current was
about 1.0A total current through both shafts, and it was equal to the measured one. Also, Fig. 2
presents the comparison between the calculated (from sources) and measured electric field as a
measure of the range quality (sensor calibration, range orientation and tracking correction, data
synchronization).
Figure 1: The longitudinal (left) and transverse (right) signatures (µV/m) of
Quest at Herdla with shaft grounding system ON (Condition A).
Figure 2: Current source distribution on the hull and range quality plot for Herdla.
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5
Figure 3: The longitudinal (left) and transverse (right) signatures (µV/m)
of Quest at Herdla without shaft grounding (Condition C).
The measurements at this range also offered the clue that the low current demand from the
propellers is due to a protective (resistive) layer: when the shaft grounding system was removed,
an additional (bearing) resistivity was introduced in the current path that did not significantly
change (Fig. 4) the signature. This indicates that the water-propeller resistivity caused by the
chalk layer is bigger than the shaft-hull resistivity caused by the bearings (shorted by the shaft
grounding). The hull potential varied little, from -0.71V (relative to Ag/AgCl), when the shaft
grounding system was functional, to -0.73V when it was removed. This variation may be caused
by the redistribution of the currents on the hull as illustrated in Fig. 2 and 4.
Figure 4: Current source distribution on the hull and range
quality plot for Herdla without shaft grounding.
Note that the corrosion potential of bare steel in natural seawater varies from -0.6V to -0.7V
(vs. Ag/AgCl). The hull potentials at Herdla, and for the rest of the trials, show that Quest was not
adequately protected from corrosion, when the potential should have been about -0.85V
(vs. Ag/AgCl).
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At the next range, in Aschau, the total shaft current increased to about 4.0A, and consequently the
signature (Fig. 5) was about four times higher than the one at Herdla. The current increase could
be caused by a lower water-propeller resistivity following the dissolution of the calcareous layer.
Again the shaft current measurements agreed with the calculation (Fig. 6). When the shaft
grounding system was removed, the current decreased (Fig. 7–8) to about 1.0A indicating that the
resistivity between the shaft and the hull is important relative to the overall water-hull resistivity,
opposite to the previous case. The variation of the hull potential was from -0.58V to -0.65V for
the shaft grounding system on and off, respectively. The increase of the cathodic current demand
means higher potential on the hull, thus less protection against corrosion.
Figure 5: The longitudinal (left) and transverse (right) signatures (µV/m) of
Quest at Aschau with shaft grounding system ON (Condition A).
Figure 6: Current source distribution on the hull and range quality plot for Aschau.
DRDC-RDDC-2015-R272
7
Figure 7: The longitudinal (left) and transverse (right) signatures (µV/m)
of Quest at Aschau without shaft grounding (Condition C).
Figure 8: Current source distribution on the hull and range
quality plot for Aschau without shaft grounding.
At Brest, the magnitude of the electric signature (Fig. 9–10) was in between the above, which
indicates a partial recovery of the calcareous layer. A misleading factor in the signature analysis
at this range was the measured total shaft current whose value was 4.8–4.9A, the biggest value of
this trial. This value is in discrepancy with the current obtained from the signature measurements
which correspond, as shown in Fig. 10, to a total shaft current of approximately 2.8A. This value
decreased to 0.8A when the shaft grounding system was removed. The corresponding values of
the hull potential were -0.64V and -0.69V.
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Figure 9: The longitudinal (left) and transverse (right) signatures (µV/m) of
Quest at Brest with shaft grounding system ON (Condition A).
Figure 10: Current source distribution on the hull and range quality plot for Brest.
The difference between the measured shaft current and the calculated one may be due to external
(short pathway currents flowing from the hull to the propellers) and internal (defective grounding
of electrical equipment onboard ship) causes. Modifying the present configuration of the
measurement system, it is possible to separate the internal and external contributions to the shaft
current.
One notices that the hull potential was well correlated with the calculated (not measured) current
flow to propellers when the shaft was grounded. This relationship was explained before.
Unfortunately, with the ICCP system functional, the hull potential is automatically forced to a
certain value and it will be the ICCP current that may provide information on the underwater
electrochemistry. When the shaft is not grounded, the additional resistivity produced a new
distribution of the currents around the ship but the hull potential is no longer correlated to the
shaft currents.
DRDC-RDDC-2015-R272
9
Further data analysis confirms the assumption that the measured shaft current does not represent
correctly the propeller current. The calculation of the current sources from the measured signature
suggests it, as well as the values of the hull potential, but the best evidence is given by the
magnitude of the time-varying cathodic current recorded when the shaft grounding system was
operational. In this case, the shaft current does not depend on the resistivity between the shaft and
the brushes, or on the bearings resistivity. Fig. 11 shows the longitudinal electric field produced
by the time-varying (Extremely Low Frequency) corrosion currents at different ranges when the
ship traveled over the sensor. In Brest, the ELF amplitude does not support the current
measurements. The conclusion is that the measured shaft current does not predict correctly the
signature level. A possible explanation for the discrepancy between the measured and the
calculated current is that the electronic circuit (10–100 µΩ) connecting the shaft to the hull
measured the current from the water through the propeller plus a current from inside the ship
flowing through engines to the hull. To eliminate this interference, a different setup for shaft
current measurements is required.
From these results, one may conclude that at Herdla the Quest propellers may have been covered
by a calcareous layer formed in the past, that the layer was possibly dissolved prior to the
measurements at Aschau, and then it may have been only partially recreated during the two days
port visit at Brest. Note that the precipitation of calcium carbonate on propellers takes place only
when the ship is at rest, so the two days of rest at Brest were not enough to fully cover the
propellers.
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Figure 11: The Extremely Low Frequency (ELF) electric field recorded
at different ranges when the shaft grounding was ON.
DRDC-RDDC-2015-R272
11
3
Modelling the electric signature
In general, electric signature measurement data are not available if we consider that a ship is
usually only ranged every few years. During this interval, the ship’s electrical condition could
change significantly along with its signature. Thus, without measurements, the temporal
variations of the electrical parameters of the ship cannot be included into a signature prediction
model. On the other hand, we should possess information about the ship parameters during the
construction stage which should be sufficient to reasonably predict its signature based on
numerical modelling. When building such models, we know the position of (active or passive)
anodes but make assumptions on the shape of polarization curves, hull paint degradation, water
chemical composition, nature of materials on the hull under the water, and position of cathodic
areas other than the propellers. As the result of these various assumptions, the model should
estimate the real signature. The RIMPASSE trial offered the opportunity to compare the signature
estimation with measurements.
A simplified finite element model of Planet was constructed [2] based on the known ship
geometry, and the location of anodes, thrusters, and propellers. The ship hull consists of a perfect
insulator material, so that no current flows into or out of the hull. The polarization curves of the
materials (Al and Ni-Al-Bronze) taken from [4] were implemented in the model.
When the results from the model were compared with the measurements, large discrepancies
were noticed. The model predicted a signature of different shape and much bigger in amplitude
(four to 20 times, depending on the range) than the measured one, which was a clear indication
that our a priori knowledge about the ship was inadequate. The electric signature of vessels with
electric pod drives is not well understood but, from the present measurements, it seems that the
propeller current is smaller than that predicted by the polarization curve.
Similarly, a finite element model of the Quest underwater hull was built to model its electric
signature. Based on the previous ship description, three zinc electrodes with an area of 0.5 m2
each were placed inside the hull which has openings at the approximate locations of the sea-chest
and sea-bay. The fourth electrode was placed approximately where the bow thruster propeller is
located (Fig. 12). A current sink at the middle of the ship has to be introduced because there is an
unprotected surface under the keel. The propellers were modeled as discs of equivalent area [5].
The entire underwater hull surface, whose paint quality could not be evaluated, was modeled as
painted iron that allows a small current to pass through.
The polarization curve of the Ni-Al-Bronze (NAB) propellers was taken from [4] for quiescent
flow, and the mid-ship current sink was assumed to be a bare metal iron surface along a portion of
the keel. This simple arrangement roughly predicts (Fig. 13) the signature of Quest at the Brest
range, which is a reasonable estimation, as no other information about the underwater chemical
reactions was used. The current distribution obtained from inversion reproduces very well the one
in the model.
Starting from the above “first approximation”, the numerical model can be adjusted to closely
accommodate a variety of range data, which may be done by adjusting the model parameters.
There are two aspects related to modelling the data: (i) the assumptions made on the model may
not reproduce the real thing, and (ii) whatever values for the model parameters are chosen, the
12
DRDC-RDDC-2015-R272
real signature cannot be reproduced in all details. The implication is that we cannot fully
understand how the signature was generated based on numerical modelling. Thus, instead of
trying to fit the data to some model, the purpose of this study is to predict the signature based on
the existing information.
Zinc anodes
Figure 12: Finite element model of Quest with anodes inside the hull.
DRDC-RDDC-2015-R272
13
Figure 13: The modeled longitudinal (left) and transverse (right) signatures (µV/m)
of Quest and the corresponding current distribution.
14
DRDC-RDDC-2015-R272
4
Calcareous deposits formation on propellers
The key element that influenced the electric signatures of both Planet and Quest during the
RIMPASSE trial was the cathodic reaction that is controlled primarily by oxygen transfer and the
buildup of calcareous deposits. This assumption is based on the fact that no modifications were
made to the ship hull between the trials. Because no variation of the shaft current was measured at
any range when the ship changed speed, for example from six to 12 knots, we can deduce that the
oxygen transfer had little to no contribution to the cathodic reaction.
It is possible then that the cathodic reaction was determined by the creation/removal of the
protective layer that represents calcareous deposits on propellers. Thus, to be able to predict the
signature, the first step is to analyze the conditions for precipitation/dissolution of the calcium
carbonate at the three locations based on the well-understood chemistry of carbon in seawater.
The calcareous (chalk) deposit on the ship propeller is produced as a by-product of the cathodic
protection system. Ships usually have sacrificial or impressed current anodes that generate
electrons that flow to areas of paint damage on the hull and the propeller to prevent corrosion.
The bare metal areas become cathodes where the oxygen and water are reduced to hydroxyl ions
that react with calcium, magnesium and carbon dioxide to form calcium and magnesium
carbonates (chalk). The chalk deposits decrease the corrosion protection current.
The amount, rate and type of deposit are dependent on cathodic current density and ambient
seawater conditions. The factors that lead to higher chalk formation are higher calcium
concentration, alkalinity and pH. The effect of pH is especially dramatic, with an increase of
0.3 pH units being equivalent to a doubling of calcium or alkalinity in terms of the driving force
for precipitation [9].
In seawater, carbon is present in four inorganic forms: aqueous carbon dioxide (CO 2), carbonic
acid (H2CO3), bicarbonate ( HCO3 ), and carbonate ( CO32 ). When atmospheric CO2 comes in
contact with seawater, it dissolves and reacts chemically. Most of the atmospheric CO 2 entering
the ocean reacts with carbonates to create bicarbonate:
CO2  CO32  H 2O  2HCO3
(1)
while the rest remains as CO2. A small part of the resulting HCO3- dissociates further into
CO32 and H+ and reduces pH. Hence, the pH reduction is reduced since the carbonate ions,
which are already present in seawater, take care of the CO2 and act as a buffer.
Calcium carbonate (CaCO3) is created through reactions involving calcium ions. Equation (2)
shows the reaction between the calcium and carbonate ions in the aqueous phase as well as the
formation of solid calcium carbonate.
Ca 2  CO32  CaCO3
DRDC-RDDC-2015-R272
(2)
15
A solution that contains the maximum possible dissolved amount of a salt (like CaCO3) is
saturated. If more is added, the salt will precipitate until the excess has been removed. A solution
can be supersaturated and contain more ions that can combine to form a salt than is ‘theoretically’
possible.
In general, surface seawater is supersaturated with CaCO3. This means it takes very little effort to
precipitate CaCO3 from seawater. However, those conditions can be changed so that the reverse
reaction happens, causing the calcium carbonate to dissolve. To predict what will happen in a
given situation we can write the ratio of the concentration of dissolved ions currently present in a
given solution to the concentration of dissolved ions in a saturated solution. This ratio is called
omega, Ω:

concentrat ion of currently present dissolved ions
concentrat ion of dissolved ions in a saturated solution
The expression specific for calcium carbonate is:
concentrat ion Ca 2 present  concentrat ion CO32 present

concentrat ion Ca 2 saturated  concentrat ion CO32 saturated
(3)
The denominator in Equation (3) is called the solubility product. The two most common types of
carbonate minerals are aragonite and calcite. These minerals have different saturation levels, and
aragonite always has a lower saturation level than calcite. Thus there is an Ω for calcite and
an Ω for aragonite.
If Ω < 1, the solution is under-saturated and dissolution occurs. If Ω > 1, the solution is
supersaturated and dissolution does not occur (i.e., precipitation can occur). If Ω = 1, the solution
is exactly saturated and nothing happens.
4.1
Calcium carbonate and pH
The solubility of calcium carbonate depends strongly on pH. pH is the measure of acidity in a
solution. In pure water the hydrogen ions are equal to the concentration of hydroxide ions and this
means that the pH level is 7.0.
The lower the pH, the more soluble is the calcium carbonate. The pH effect is driven by changes
of the carbonate concentration in the solution.
Dissolved carbon dioxide, bicarbonate and carbonate are referred to as Dissolved Inorganic
Carbon (DIC). More than 99% of the DIC is made up of bicarbonate and carbonate ions. When
CO2 concentration increases, the bicarbonate form (HCO3-) predominates and this corresponds to
a lower pH value. At higher pH, more and more of the carbonate form ( CO32 ) exists. Because it
is the carbonate ions concentration that drives the rate at which carbonate precipitates, then the
higher the pH, the faster carbonate is depositing on the surface.
16
DRDC-RDDC-2015-R272
4.2
Calcium carbonate and alkalinity
Calcium carbonate’s solubility also depends strongly on the water’s alkalinity. Alkalinity is the
ability for a solution to neutralize acids. The most common bases in the oceans are bicarbonate,
carbonate (e.g., carbonate ions, CO32-), boron and hydroxide. Alkalinity is additive and is
approximated by a simplified expression (in equivalents per liter):
AT = [ HCO3 ] + 2[ CO32 ] + [B(OH)4- ] + [OH-] − [H+]
(4)
The higher the alkalinity (at a fixed pH), the more carbonate is present. In fact, the amount of
carbonate present is directly proportional to the alkalinity. Also AT is closely related to salinity
because HCO3 and CO32 are major components of seawater.
The reason that calcium carbonate solubility changes with alkalinity is the changes in the
carbonate concentration. Lower calcium carbonate solubility at higher alkalinity implies that
precipitation of calcium carbonate can be more extensive. In other words, as the alkalinity rises,
the amount of calcium that can be kept in solution without precipitation decreases.
4.3
Calcium carbonate and water flow
The precipitation of carbonate chalk depends on water flow. The propeller must be at rest
(Fig. 14) for this deposit to form [6]. Any period of inaction is an opportunity for a chalk film to
form over the whole propeller, and the waters of some harbours are more favourable to film
formation than others. According to [7], the film deposits on the outer parts of the blades are
removed during the voyage and reformed on each stopover in port, while the chalk deposit nearer
the blade roots will build up. However, this scenario is not fully supported by the laboratory
experiments [6], but one has to consider that the sheer stress on the real propeller might well
exceed the one on the laboratory rotating disc.
Figure 14: Calcareous deposits formation on a rotating disc, from [6].
DRDC-RDDC-2015-R272
17
5
The solubility of calcareous chalk at the ranges
In this section, we analyse the effect of the water chemical composition at different electric
ranges on the formation/dissolution of the calcareous deposits. During the Quest’s voyage
through the North and Baltic Seas, from Scotland to Bergen to Aschau to Brest, the water
chemical analysis was not part of the investigation, thus the following analysis is based on the
available data in the literature and the measurements of the local temperatures and conductivities.
The purpose is to predict the direction of reaction (2).
In Germany, Quest was visiting both Kiel Harbour and the Eckernförde Bay. While the chemical
condition of the water at these two places may be different, the next discussion refers to the
period of about three weeks when the ship was in Eckernförde Bay, active or still, for the trials at
Surendorf and Aschau. Considering that at Brest the chemical reaction that formed the calcareous
layer took place in a few days, the three weeks period in Eckernförde Bay can be consider long
enough to guarantee a stable environment for the reaction to occur in either direction.
The pH level of surface seawater varies around the globe between 7.8 and 8.5. Herdla and Brest
range locations, being close to the ocean, have an almost constant pH level with some seasonal
variations. For example, the pH value at Brest was measured over one year period to
be 8.1–8.2 [8].
In the Baltic Sea, the pH level has large regional, seasonal and inter-annual variability. This
parameter is affected by several processes, such as the fresh water runoff in the area, air-sea gas
exchange, physical mixing, and biological processes.
In the open ocean the alkalinity is above 2200 μmol/kg, as shown in Fig. 15, and this value is
assumed to correspond to Herdla and Brest locations. Actually, the measurements obtained during
one year period in the Southern Bight (near Brest) of the North Sea were between 2300 and
2700 μmol/kg [8].
The alkalinity (AT) of the Baltic Sea varies with the seasons and the different areas. These
differences are attributed to river runoff and geology in the drainage area. River runoff entering
the south-eastern Baltic Sea drains regions rich in limestone, which have been exposed to
long-term weathering. Weathering of limestone contributes to an increased AT. The regional
variations in alkalinity ranges [8] from 770 μmol/kg in the Bothnian Bay to more than
2000 μmol/kg in the Kattegat (Fig. 15). According to this picture, the alkalinity of the water
around Aschau can be evaluated at about 1850 μmol/kg.
Fig. 15 also presents the distribution of the surface salinity from where one can infer that at
Herdla the salinity is about 31–33 psu (practical salinity units), and at Aschau its value is about
14–16 psu. The salinity at Brest was measured to be 35–36 psu [8]. These values explain
qualitatively the differences in seawater conductivity at the three ranges which were measured to
be 3.3, 2.6, and 4.3 S/m, at 15.1 ºC, 14.3 ºC, and 16.1 ºC, respectively.
18
DRDC-RDDC-2015-R272
Fig
igure 15: Disttribution of seea surface alkkalinity (μmol/kg) at left,
and salinity (psu), at right,
r
in the Baaltic Sea from
m [9].
Electrrical conductiivity is a meaasure for mon
nitoring waterr salinity. Thhere is no exaact relationshiip
betweeen conductiv
vity and salin
nity but appro
oximate formuulae were devveloped [10] to convert thhe
two quantities
q
from
m one into thee other as a fu
unction of tem
mperature andd pressure. Acccording to thhe
calcullation, the co
orrespondencces in salinitty of the aboove conductiivities are 266.6, 20.5, annd
35.7 psu
p at 20 m depth, respectiively. These values
v
are diff
fferent from thhe ones on the map but theey
are co
onsidered to be the real values
v
becausse they were inferred from
m local meassurements. Foor
examp
ple, the correesponding con
nductivity at Aschau woulld be only 1.994 S/m if onee considers thhe
salinitty as 15 psu, as shown on the
t map.
5.1
Modellling the dissolved
d
d inorgan
nic carbon
A matthematical mo
odel has been
n developed [11] to analyz e the saturatioon state and tthe equilibrium
m
betweeen dissolved
d carbonic sp
pecies, CaCO3 in water annd CO2 in aiir, and measuured values oof
alkalinity and salin
nity. The mod
del determinees the saturatiion state of w
water with resspect to CaCO
O3
by callculating the theoretical pH
H. To calculaate the pH, thhe practical allkalinity (Eq. 4) includes aall
Therefore thee alkalinity is used to derivve
relevaant bases (ion
ns) for the con
nditions in thee Baltic Sea. T
the eq
quation that describes
d
how
w the concenttration of hyddrogen ions ddepends on thhe atmospherric
carbon dioxide, water
w
temperaature and salinity. The cchange in alkkalinity as a result of thhe
dissollution of CO2 is negligiblle and can th
herefore be trreated as a cconstant only related to thhe
propeerties of the water.
w
The CO
C 32- concenttrations were calculated fro
om the measuurements of C
CO2 partial prressure (pCO2),
the pH
H, and the eq
quilibrium (io
onization) con
nstants. The C
Ca2+ concentrrations were eestimated from
m
the calcium-salinity relationsh
hip in the Baltic Sea [122]. The equations for thhe equilibrium
m
constaants as funcction on tem
mperature an
nd salinity ccan be founnd in the lliterature. Thhe
stoich
hiometric solu
ubility producct constants (Ksp)
(
of calciite and aragoonite are depeendent stronglly
DRDC
C-RDDC-2015
5-R272
1
19
on temperature and salinity. These constants were calculated from the already existing
functions [13].
The model was coded in MATLAB™ to obtain the pH and the saturation state of calcium
carbonate in seawater. The inputs are, for the marine system, the temperature and salinity (or
electrical conductivity), and, for the carbon system, the total alkalinity and the partial pressure of
carbon dioxide at the site.
A four degree Equation (5) is derived from (4) and then is solved for a set range of
CO2 concentrations to obtain the H+ ion concentration. This is then converted to pH which gives a
relationship between CO2 and pH.
[H+]4 + (kB+AT )[H+]3 − (kB BT+ k1 HCO2 PCO2+ kw − kB AT )[H+]2 (kB k1 HCO2 PCO2+ 2k1 k2 HCO2 PCO2+ kw kB)[H+] − 2kB k1 k2 HCO2 PCO2 = 0
(5)
The equilibrium constants kB, k1, k2, kw, representing the law of mass action are well known in the
literature and have been calculated as a function on temperature and salinity. The total
concentration of boron, BT, is calculated as a function on salinity.
The model was validated on the measurements [14] performed in central Baltic (Table 1). The
model reproduces very well the measurements.
Table 1: Measured [14] and calculated saturation states in central Baltic at different
temperatures and corresponding pH. Alkalinity AT = 1550 μmol/kg and salinity = 7 psu.
November 1997
March 2001
July 2001
T = 9.89 ºC and pH = 8.01
T = 2.71 ºC and pH = 7.99
T = 18.9 ºC and pH = 8.42
C = 1.44
C = 1.03
C = 4.94
A = 0.79
A = 0.57
A = 2.78
Calculated values
T = 10 ºC and pH = 8.02
T = 0 ºC and pH = 7.97
T = 20 ºC and pH = 8.42
C = 1.44
C = 0.80
C = 4.73
A = 0.80
A = 0.43
A = 2.67
Using this model and the measurements of water temperature and conductivity, we can predict the
direction of reaction (2) at the three locations. First we have to notice that in the Baltic Sea the
concentrations of the chemical elements have large variability in comparison with the ocean
levels. For example, in the ocean the concentration of Ca2+ is relatively constant at about
12 mmol/kg, and the pCO2 is at equilibrium with the atmosphere (375 µatm).
20
DRDC-RDDC-2015-R272
With these values, the model predicts at Herdla and Brest ranges the values of omega for
aragonite and calcite: ΩA = 2.6 and 3.1, and ΩC = 4.2 and 4.8, respectively. Thus, at these two
places the CaCO3 will precipitate from seawater on the propellers provided that the ship is at rest.
In the Baltic Sea, we have a different situation: the concentration of Ca2+ varies with salinity (or
alkalinity); the partial pressure pCO2 also varies being higher in the winter (400 µatm at 3C) and
lower in the summer (156 µatm at 19C) mainly due to the 16C temperature difference in the
water.
At the Aschau range, the partial pressure, pCO2, value was estimated to be 280 µatm because the
measurements were taken in September (14.4C water temperature). With the alkalinity value
AT = 1850 μmol/kg, the model predicts for pH = 8.15, and for omega the values 2.15 and 3.54 for
aragonite and calcite, respectively. These omega values are lower when compared to the ones at
Herdla and Brest but still favour the precipitation of CaCO3 on propellers.
According to the model, to be able to dissolve the chalk, the total alkalinity has to be lower than
1400 μmol/kg, as shown in Fig. 16. This value is specific for the Bothnian Bay only. Another
variable, the CO2 concentration in atmosphere, has to be 800 ppm for an alkalinity of
AT = 1850 μmol/kg, to reverse the chalk precipitation. This value is not realistic.
Figure 16: Model predictions of Ω values.
However, the unknown quantity in the sense that it was not directly measured at the site, but
evaluated from a map, remains the water alkalinity. Because the alkalinity is mainly controlled by
fresh water addition or evaporation, there is a (non-linear) relationship between alkalinity and
salinity [15], but for the specific waters in the Baltic Sea the coefficients of the equation are not
know. A simplified [9] linear relation (at constant temperature) shows that at Aschau for a salinity
of 20.5 psu corresponds an alkalinity of 1850 μmol/kg. The actual measurements may vary
largely (within ± 400 μmol/kg) around this value. Even so, explaining the chemical reaction of
calcite/aragonite dissolution at Aschau is still a problem.
DRDC-RDDC-2015-R272
21
So far, only the inorganic carbon chemistry was investigated as the mechanism for chalk
precipitation on propellers. The process is complicated by the fact that the cathodic reduction of
oxygen results in the formation of OH- ions, thus the pH and the total alkalinity of the seawater
increases locally (and favours precipitation). The higher cathodic current at Aschau was working
in this direction.
It is certain that the cathodic current increased at Aschau in comparison to the other ranges.
Unfortunately, considering the local measurements of the water temperature and salinity, the
process responsible for the alteration could not be predicted and still has to be explained. The
brackish water in the east part of the Baltic Sea favours the dissolution of the calcareous deposits,
but this is not the case in the western part.
The conclusion of this section is that further investigation is needed to clarify the mechanism
responsible for the variation of corrosion current as a function on the local chemical properties of
seawater. One direction of investigation is to identify all conditions that determine the
formation/dissolution of the protective chalk layer on cathodic surfaces.
22
DRDC-RDDC-2015-R272
6
Conclusion
The electric signature of the ship can be estimated to some extent by using a numerical (finite
element) model and basic information about the ship, such as the hull geometry, the position of
anodes and propellers, and generic polarization curves. Combined with environmental
measurements, such as water depth and conductivity, the estimated signature could be of the same
order of magnitude as the measured one. This was the case for Quest which is equipped with a
classical propulsion system but not for Planet which has underwater electrical motors.
The real signature can be determined by measurements, when factors like the hull paint
degradation and coverage, the various materials present, and the real polarization curves as they
are modified by the underwater chemical reactions, prove to have a significant influence.
The range measurements done during RIMPASSE supported the assumption that the cathodic
reaction may have been determined by the creation/removal of the protective layer that represents
calcareous deposits on propellers. Using the inorganic carbon chemistry in the seawater, it is
theoretically possible to predict the tendency for the two processes, precipitation and dissolution
of calcium carbonate, to take place. Unfortunately, using the present available data, the variation
of the electric signature between ranges based on this hypothesis could not be predicted.
Another problem investigated was the monitoring of the ship’s electric signature. From the
measurements, it was concluded that the most sensitive onboard parameter that reflected the
signature variations was the hull potential. However, this parameter cannot be considered when
the ship has a functioning ICCP system. The measured shaft current was a misleading factor. It
demonstrated that it is necessary to modify the shaft current measurement so that the contribution
from internal sources can be eliminated.
Following the analysis, one is left with a series of questions that demonstrate our limited
knowledge regarding the origin of the ship’s electric signature:
1. Was the precipitation/dissolution of the calcareous deposits on the propellers that determined
the signature variability?
2. What was the chemical parameter that determined the dissolution of the calcareous deposits?
3. How can this reaction be predicted based on the present knowledge of ocean chemistry?
4. How should we characterize the hull paint quality? How should we introduce this factor into
a signature estimation model?
5. How does the ICCP system contribute to the calcareous reaction?
To answer these questions more experimental work is necessary, both at the range and in the
laboratory. The field measurements have to be made in conjunction with the analysis of the water
chemical composition, a factor that was neglected during the RIMPASSE trials. In this way, our
understanding in this research area will increase, which in turn will provide possibilities for
electric signature reduction.
DRDC-RDDC-2015-R272
23
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24
DRDC-RDDC-2015-R272
References
[1] J.B. Nelson, T.C. Richards, M. Birsan and C. Greene, RIMPASSE 2011 Electromagnetics
Trials Quick-Look Report, DRDC Atlantic TM 2011-305, 2011.
[2] K. Höfener and M. Birsan, Comparison of Predicted and Measured UEP Signatures of the
German Research Vessel “Planet”, International Conference MARELEC 2013, Hamburg,
Germany.
[3] M. Birsan, Measurement and model predicted corrosion related magnetic signature. Applied
on CFAV Quest. DRDC Atlantic TM 2009-253, 2009.
[4] H.P. Hack, Atlas of polarization diagrams for naval materials in seawater,
CARDIVNSWC-TR-61-94/44, April 1995.
[5] V.G. DeGiorgi and S.A. Wimmer, Geometric details and modeling accuracy requirements for
shipboard impressed current cathodic protection system modeling, Engineering Analysis with
Boundary Elements 29, pp. 15–28, 2005.
[6] H.P. Hack and R.J. Guanti, Effect of high flow on calcareous deposits and cathodic protection
current density, DTRC/SME-87/82, April 1988.
[7] G.T. Callis, The Maintenance and Repair of Bronze Propellers, Naval Engineering Journal, p.
645, August 1963.
[8] M. Frankingnoulle et al., Distribution of surface water partial CO2 pressure in the English
channel and in Southern Bight of the North Sea, Continental Shelf Research, vol. 16(3),
pp. 381–395, 1996.
[9] K. Wesslander, The carbon dioxide system in the Baltic Sea surface water, University of
Gothenburg, Dept. of Earth Sciences, 2011 (doctoral thesis).
[10] P. Fofonoff and R.C. Millard, Algorithms for computation of fundamental properties of
seawater, UNESCO Technical Papers in Marine Sci., No. 44, pp. 58–61, 1983.
[11] D. Pierrot, E. Lewis and D.W.R. Wallace, MS Excel Program Developed for CO2 System
Calculations. ORNL/CDIAC-105a. Carbon Dioxide Information Analysis Center, Oak
Ridge National Laboratory, U.S. Department of Energy, Oak Ridge, TN, 2006.
[12] D. Dyrssen, The Baltic-Kattegat-Skagerrak Estuarine System, Estuaries, ISSN 0160-8347,
Volume 16, Issue 3, pp. 446–452, September 1993.
[13] A. Mucci, The solubility of calcite and aragonite in seawater at various salinities,
temperatures, and one atmosphere total pressure. American Journal of Science,
Volume 283, pp. 780–799, September 1983.
DRDC-RDDC-2015-R272
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[14] T. Tyrrell, Calcium carbonate cycling in future oceans and its influence on future climates.
Journal of Plankton Research, ISSN 0142-7873, Volume 30, Issue 2, pp. 141–156,
February 2008.
[15] K. Lee et al., Global relationship of total alkalinity with salinity and temperature in surface
waters of the world’s oceans, Geophysical Research Letters, vol. 33, L19605, 2006.
26
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Variability of ship’s electric signature during the RIMPASSE trial
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the abstract of classified documents be unclassified. Each paragraph of the abstract shall begin with an indication of the security classification of the
information in the paragraph (unless the document itself is unclassified) represented as (S), (C), (R), or (U). It is not necessary to include here abstracts in
both official languages unless the text is bilingual.)
Technology that incorporates electric sensors into influence mines has been already developed.
Thus, it is considered essential to invest more in electric signature research to better protect our
ships against this increasing threat. For this reason, the RIMPASSE trials were performed to
investigate the electric signatures of the research vessels RV Planet and CFAV Quest when
measured at various ranges. The trials revealed significant differences between the ranges for
both vessels.
The electric field beneath the ship is caused by metal corrosion and the related corrosion
protection system. Both vessels were protected against corrosion by a set of sacrificial anodes.
The measurements during the trial suggest that the cause of signature variability is the
modification of the propeller resistivity, which may be due to the precipitation/dissolution of the
calcareous deposits. The present report investigates this hypothesis. The goal is to predict the
ship’s electric signature during the voyages in waters with different chemical composition.
The formation of calcareous (calcium carbonate) deposits depends on the seawater chemical
composition and temperature, the corrosion current density, and ship velocity. In principle, the
layer of calcareous deposits has a beneficial role on platforms subjected to corrosion in seawater
because it coats the cathodic areas, thus reducing the current. Using inorganic carbon chemistry
in the seawater, it is possible to predict which of the two processes, precipitation or dissolution
of the calcium carbonate deposit, takes place.
Using the water chemical composition available in the literature, the dissolution of the
calcareous deposits on propellers could not be predicted. Thus, the observed signature
variability could not be explained based on this hypothesis.
--------------------------------------------------------------------------------------------------------------La technologie qui intègre les capteurs électriques aux mines à influence a déjà été élaborée. Il
est donc jugé essentiel d’investir davantage dans la recherche sur les signatures électriques afin
de mieux protéger nos navires contre cette menace croissante. C’est pourquoi on a procédé aux
essais RIMPASSE dans le but d’examiner les signatures électriques, mesurées à diverses
portées, des navires de recherche RV Planet et NAFC Quest. Les essais ont révélé des
différences importantes entre la portée des deux navires.
Le champ électrique sous le navire se forme en raison de la corrosion et du système de
protection contre la corrosion. Dans ce cas-ci, les deux navires étaient protégés contre la
corrosion par un ensemble d’anodes sacrificielles. Les mesures obtenues lors de l’essai
suggèrent que la variation de la signature est causée par une modification de la résistivité de
l’hélice en raison de la précipitation/dissolution de dépôts calcaires. Ce rapport scientifique
examine cette hypothèse. L’objectif est de prédire la signature électrique du navire pendant les
voyages dans des eaux de compositions chimiques différentes.
La formation de dépôts calcaires (carbonate de calcium) dépend de la composition chimique et
de la température de l’eau de mer, de la densité du courant de corrosion et de la vitesse du
navire. En principe, la couche de dépôts calcaires joue un rôle bénéfique sur les plateformes
soumises à la corrosion dans l’eau de mer, car elle recouvre les zones cathodiques et réduit donc
le courant. En utilisant la chimie du carbone inorganique dans l’eau de mer, il est possible de
prédire lequel des deux processus, la précipitation ou la dissolution des dépôts de carbonate de
calcium, aura lieu.
Il était impossible de prédire la dissolution des dépôts calcaires sur les hélices uniquement en
étudiant les diverses compositions chimiques de l’eau décrites dans la littérature. C’est pourquoi
la variabilité de la signature observée ne pouvait être expliquée en se basant sur cette hypothèse.
14. KEYWORDS, DESCRIPTORS or IDENTIFIERS (Technically meaningful terms or short phrases that characterize a document and could be helpful
in cataloguing the document. They should be selected so that no security classification is required. Identifiers, such as equipment model designation,
trade name, military project code name, geographic location may also be included. If possible keywords should be selected from a published thesaurus,
e.g., Thesaurus of Engineering and Scientific Terms (TEST) and that thesaurus identified. If it is not possible to select indexing terms which are
Unclassified, the classification of each should be indicated as with the title.)
electric signature; corrosion currents; calcareous deposits