Variability of ship’s electric signature during the ... · The electric field beneath the ship is...

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

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© 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

DRDC-RDDC-2015-R272 i

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.

ii DRDC-RDDC-2015-R272

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.

DRDC-RDDC-2015-R272 iii

Table of contents

Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . i

Significance to defence and security . . . . . . . . . . . . . . . . . . . . . . i

Résumé . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ii

Importance pour la défense et la sécurité . . . . . . . . . . . . . . . . . . . . ii

Table of contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . iii

List of figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iv

List of tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v

Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . vi

1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

2 Electric signature measurements . . . . . . . . . . . . . . . . . . . . . 3

2.1 Description of vessels and ranges . . . . . . . . . . . . . . . . . . . 3

2.2 The variability of the electric signature between ranges . . . . . . . . . . . 4

3 Modelling the electric signature . . . . . . . . . . . . . . . . . . . . . 12

4 Calcareous deposits formation on propellers . . . . . . . . . . . . . . . . . 15

4.1 Calcium carbonate and pH . . . . . . . . . . . . . . . . . . . . . 16

4.2 Calcium carbonate and alkalinity . . . . . . . . . . . . . . . . . . . 17

4.3 Calcium carbonate and water flow . . . . . . . . . . . . . . . . . . 17

5 The solubility of calcareous chalk at the ranges . . . . . . . . . . . . . . . . 18

5.1 Modelling the dissolved inorganic carbon . . . . . . . . . . . . . . . . 19

6 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

iv DRDC-RDDC-2015-R272

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


Herdla without shaft grounding (Condition C). . . . . . . . . . . . . . 6

Figure 4: Current source distribution on the hull and range quality plot for Herdla

without shaft grounding. . . . . . . . . . . . . . . . . . . . . . . 6


Aschau with shaft grounding system ON (Condition A). . . . . . . . . . 7

Figure 6: Current source distribution on the hull and range quality plot for Aschau. . . 7


Aschau without shaft grounding (Condition C). . . . . . . . . . . . . . 8

Figure 8: Current source distribution on the hull and range quality plot for Aschau

without shaft grounding. . . . . . . . . . . . . . . . . . . . . . . 8


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

DRDC-RDDC-2015-R272 v

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

vi DRDC-RDDC-2015-R272

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

DRDC-RDDC-2015-R272 1

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

2 DRDC-RDDC-2015-R272

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.


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


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


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.


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).


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.


Quest at Aschau with shaft grounding system ON (Condition A).

Figure 6: Current source distribution on the hull and range quality plot for Aschau.


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.



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.


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.


Figure 11: The Extremely Low Frequency (ELF) electric field recorded

at different ranges when the shaft grounding was ON.


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


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.

Figure 12: Finite element model of Quest with anodes inside the hull.

Zinc anodes


Figure 13: The modeled longitudinal (left) and transverse (right) signatures (µV/m)

of Quest and the corresponding current distribution.


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 (CO2), carbonic

acid (H2CO3), bicarbonate (

3HCO ), and carbonate (2

3CO ). When atmospheric CO2 comes in

contact with seawater, it dissolves and reacts chemically. Most of the atmospheric CO2 entering

the ocean reacts with carbonates to create bicarbonate:

32

2

32 2HCOOHCOCO (1)

while the rest remains as CO2. A small part of the resulting HCO3- dissociates further into

2

3CO 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.

3

2

3

2 CaCOCOCa (2)


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:

solutionsaturatedainionsdissolvedofionconcentrat

ionsdissolvedpresentcurrentlyofionconcentrat

The expression specific for calcium carbonate is:

saturatedCOionconcentratsaturatedCaionconcentrat

presentCOionconcentratpresentCaionconcentrat

2

3

2

2

3

2

(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 (2

3CO ) 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.


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 = [

3HCO ] + 2[2

3CO ] + [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

3HCO and 2

3CO 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].


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.

DRDC

Electrbetwetwo qcalcul35.7 pare coexampsalinit

5.1

A matbetwealkaliby calrelevathe eqcarbodissolprope

The Cthe pHthe cconstastoich

C-RDDC-2015

Fig

rical conductieen conductivquantities fromlation, the copsu at 20 m donsidered to ple, the correty as 15 psu,

Model

thematical moeen dissolvednity and salinlculating the ant bases (ionquation that dn dioxide, wlution of COerties of the w

CO32- concent

H, and the eqalcium-saliniants as funchiometric solu

5-R272

igure 15: Distand salin

ivity is a meavity and salinm one into theorrespondencepth, respectibe the real v

esponding conas shown on t

lling the d

odel has beend carbonic spnity. The modtheoretical pH

ns) for the condescribes howwater tempera

2 is negligiblwater.

trations were quilibrium (ioty relationshction on temubility produc

tribution of seity (psu), at r

asure for monnity but approe other as a fuces in salinitively. These vvalues becausnductivity at the map.

dissolved

n developed [pecies, CaCOdel determineH. To calculanditions in thew the concentature and sale and can th

calculated froonization) conhip in the Bmperature anct constants (

ea surface alkright, in the Ba

nitoring wateroximate formuunction of temty of the abovalues are diffse they were Aschau woul

d inorgan

11] to analyz3 in water an

es the saturatiate the pH, the Baltic Sea. Ttration of hydlinity. The c

herefore be tr

om the measunstants. The Caltic Sea [12nd salinity c(Ksp) of calci

kalinity (μmoaltic Sea from

r salinity. Thulae were devmperature andove conducti

fferent from thinferred from

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nic carbo

e the saturationd CO2 in aiion state of w

he practical alTherefore thedrogen ions dchange in alkreated as a c

urements of CCa2+ concentr2]. The equcan be founite and arago

l/kg) at left, m [9].

here is no exaveloped [10] d pressure. Acivities are 26he ones on thm local meas94 S/m if one

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water with reslkalinity (Eq. e alkalinity isdepends on thkalinity as a

constant only

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ccording to th6.6, 20.5, ane map but thesurements. Foe considers th

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4) includes a used to deriv

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estimated fromhe equilibriumliterature. Thendent strongl

19

ip he he nd ey or he

m of O3 all ve ric he he

2), m m he ly


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 A = 0.79 C = 1.03 A = 0.57 C = 4.94 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 A = 0.80 C = 0.80 A = 0.43 C = 4.73 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).


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 3C) and

lower in the summer (156 µatm at 19C) mainly due to the 16C 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 (14.4C 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.


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.


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.


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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.


[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.

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Variability of ship’s electric signature during the RIMPASSE trial

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Birsan, M.

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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.

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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

Variability of ship’s electric signature during the ... · The electric field beneath the ship is...

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