WO2021063507A1

WO2021063507A1 - Marine salinity measuring arrangement and method

Info

Publication number: WO2021063507A1
Application number: PCT/EP2019/076790
Authority: WO
Inventors: Viktor RÄFTEGÅRD
Original assignee: Volvo Penta Corporation
Priority date: 2019-10-03
Filing date: 2019-10-03
Publication date: 2021-04-08
Also published as: CN114450435A; CN114450435B; US20220349065A1; EP4038219A1

Abstract

The invention relates to an arrangement for measuring salinity in water, which arrangement is part of an impressed current cathodic protection system having an electrical circuit comprising a source of electrical power (310); at least one first electrode (315) connected to a positive pole of the power source (310); at least one second electrode (317) connected to a negative pole of the power source (310); a voltage sensor (341); a current sensor (342); and a control unit (313). The control unit is arranged to initiate a measurement sequence at predetermined intervals, wherein the control unit is arranged to connect at least one first electrode (315) to the negative pole of the power source (310) to act as a cathode; connect at least one passive electrode (326) to the positive pole of the power source (310) to act as an active anode; register the output voltage; register the current; determine the circuit resistance using the output voltage and the current; and calculate the resistivity of the electrolyte based on the determined circuit resistance and at least one stored electrode property value, which resistivity is inversely proportional to the salinity. The invention further relates to a vessel provided with such a measuring arrangement and a method for its operation.

Description

MARINE SALINITY MEASURING ARRANGEMENT AND METHOD

TECHNICAL FIELD

The present invention relates to a salinity measuring arrangement for marine vessels, wherein the arrangement utilizes an on-board impressed current cathodic protection system. The invention also relates to a marine vessel with such an arrangement, and also to a method for operating such an arrangement.

BACKGROUND

Seawater is a corrosive environment and the parts used for marine propulsion units and other immersed metallic parts require some form of cathodic protection in order to eliminate or reduce corrosion of those parts. An efficient way of providing corrosion protection is the use of a method termed impressed current cathodic protection (ICCP). ICCP systems are often used on cargo carrying ships, tankers and larger pleasure craft. KR101066104B1 discloses the general principle for an ICCP system wherein a metal element and an anode are attached to a vessel and immersed in water. The metal element is connected to the negative terminal and the anode is connected to the positive terminal of a source DC electrical power to provide an electric de-passivation current through an electrical circuit including the anode, the metal element and the electrolyte. In this way, the anode provides corrosion protection for the metal parts. By maintain a predetermined potential in the electrical circuit, the ICCP system can provide a desired protection level for the metal parts to be protected.

Marine vessels moving in waterways such as river estuaries will be exposed to seawater, fresh water and brackish water comprising a mixture of these. With reduced salinity the resistivity of the water in which the vessel is immersed increases. At some point the electrical resistance in the electrical circuit of the corrosion protection system may increase to a level where the ICCP is unable to maintain the potential of the protected structure within an acceptable interval. Conventional ICCP system will interpret the reduce potential and the corresponding low protection level as an internal error or a malfunction caused by external factors. When this occurs, the ICCP system will shut down and switch to passive back-up galvanic anodes for protection. In fresh or brackish water galvanic anodes will provide little or no corrosion protection. The invention provides an improved impressed current corrosion protection system aiming to solve the above-mentioned problems.

SUMMARY

An object of the invention is to provide a method and an arrangement for measuring salinity in water, which when applied to an impressed current corrosion protection system solves the above-mentioned problems.

The object is achieved by a salinity measuring arrangement and a method for operating the arrangement according to the appended claims.

In the subsequent text, the cathodic protection system used by the invention is described for application to a marine propulsion system in the form of a stern drive mounted to a transom on the vessel. However, the inventive arrangement is also applicable to, for instance, azimuthing or pod drives and outboard drives. The cathodic protection system according to the invention involves an impressed current cathodic protection (ICCP) system which is operated using direct current (DC), wherein elements to be protected are connected to a negative terminal and an anode is connected to a positive terminal of a source DC electrical power. In the subsequent text, the power source used for supplying DC power to the system is not necessarily a battery, but the power source can be any suitable source of electrical power such as a fuel cell or a source of alternating current (AC) provided with an AC/DC rectifier.

The invention is applicable to a marine vessel with a marine propulsion system provided with a cathodic protection system in the form of an ICCP system. The marine propulsion system comprises at least one driveline housing at least partially submerged in water, a torque transmitting drive shaft extending out of each driveline housing and at least one propeller mounted on the drive shaft. If a propeller is used as an anode, then the at least one propeller is electrically isolated from its drive shaft and each electrically isolated propeller is connected to a positive terminal of a direct current power source. The vessel can comprise one or more driveline housings comprising a single drive shaft with a single propeller or counter-rotating propellers with coaxial drive shafts. The system provides cathodic protection, wherein each metallic component to be protected against corrosion is connected to a negative terminal of the direct current power source. A control unit is arranged to regulate the voltage and/or the current output from the direct current power source. The cathodic protection system is an impressed current cathodic protection (ICCP) system comprising at least one hull mounted anode or where at least one propeller can be used as an anode. The at least one metallic component to be protected forms a cathode and can be the at least one driveline housing, at least one trim tab, seawater intake, swimming platform and/or at least a portion of the vessel hull. Note that this is a non exclusive list of metallic components suitable for corrosion protection. At the same time, the ICCP arrangement provides marine growth protection for the at least one anode. In the example where at least one propeller is used as an anode, the at least one propeller is electrically isolated from its drive shaft by a torque transmitting electrically isolating component mounted between the at least one propeller and its respective drive shaft. The electrically isolating component is mounted in a gap formed by the outer surface of the drive shaft and the inner surface of the propeller hub. The torque transmitting electrically isolating component can be made from an elastic material, such as a natural or synthetic rubber. The at least one propeller is made from an inert anode material, such as titanium, niobium or a similar suitable metal or metal alloy.

According to one aspect of the invention, an arrangement for measuring salinity in water is provided, which arrangement is part of an impressed current cathodic protection system. The impressed current cathodic protection system has an electrical circuit comprising:

- a source of electrical power from a direct current power source;

- at least one first electrode connected to a positive pole of the power source to act as an active anode;

- at least one second electrode connected to a negative pole of the power source to act as a cathode;

- at least one passive electrode normally disconnected from the circuit and arranged to act as back-up protection; - a voltage sensor detecting an output voltage impressed on the circuit;

- a current sensor detecting a current supplied to the circuit; and

- a control unit for controlling the impressed current cathodic protection system.

During normal ICCP operation, the control unit is arranged to control the ICCP system as described in the text above, wherein an anode such as a hull mounted anode or an electrically isolated propeller anode is connected to a positive terminal of a direct current power source and cathodes to be protected are connected to a negative terminal of a direct current power source.

In order to measure salinity, the control unit is arranged to interrupt the impressed cathodic protection operation and to initiate a measurement sequence at predetermined intervals. A suitable time interval can be selected from a few minutes up to 15 minutes or more, although every 5 or 10 minutes is sufficient for the purpose of the invention. The interval can be selected depending on the use of the vessel and the likelihood of the vessel encountering fresh water conditions. For instance, the time interval can be set to a default value of 10 minutes during operation in sea water. If the calculated resistivity shows a deviation from an expected value or range of values after a measurement sequence, then the time interval can be set to a shorter value, such as 5 minutes, until the resistivity values return to normal values for sea water.

During a measurement sequence the control unit is arranged to;

- connect at least one first electrode to the negative pole of the power source to act as a cathode;

- connect at least one passive electrode to the positive pole of the power source to act as an active anode;

- register the output voltage transmitted from the voltage sensor;

- register the current transmitted from the current sensor;

- determine the circuit resistance using the output voltage and the current; and

- calculate the resistivity of the electrolyte based on the determined circuit resistance and at least one stored electrode property value, which resistivity is inversely proportional to the salinity of the water.

In this way, the at least one first electrode normally acting as an active anode during ICCP operation is arranged to act as a cathode during the measurement sequence. At the same time, the at least one passive, or sacrificial electrode normally disconnected from the electrical circuit during ICCP operation is arranged to act as an active anode during the measurement sequence. In order to improve the accuracy of the measuring arrangement, each second electrode normally acting as a cathode during ICCP operation can be disconnected from the negative pole of the power source during the measurement sequence. According to a first example, the at least one stored electrode property value utilized to calculate the resistivity p of the electrolyte is the surface area A_a of the passive electrode, that is, the area of the sacrificial anode. The control unit is arranged to calculate the resistivity p using the formula:

wherein: R_c is the circuit resistance (W); k is a correlation factor (-); p is the resistivity of the electrolyte (Qcm); A_a is the surface area of the passive electrode (cm²).

According to a second example, the resistivity p of the electrolyte is calculated using the surface area A_a of the passive electrode and a further electrode property value comprising the surface area A_c of the first electrode connected to the negative pole. In this example the control unit is arranged to calculate the resistivity p using the formula:

wherein: R_c is the circuit resistance (W); k is a correlation factor (-); p is the resistivity of the electrolyte (WOGTI);

A_a is the surface area of the passive electrode (cm²);

A_c is the surface area of the electrode acting as a cathode (cm²).

As indicated above, the accuracy of the measuring arrangement can be improved by disconnecting each second electrode normally acting as a cathode during ICCP operation from the negative pole of the power source during the measurement sequence.

As stated above, the determined resistivity is inversely proportional to the salinity of the water. In response to the determination of the resistivity, the control unit is arranged to maintain the impressed current cathodic protection system in operation if the determined resistivity is above a set threshold value when the measurement sequence is terminated. The threshold value is set at a level indicating that the vessel has entered a stretch of fresh water. The control unit can also be arranged to determine a current salinity value based on the determined resistivity and to generate an output signal indicating the salinity value to a user. This can be used to alert the user to changes in salinity, which changes can cause error messages to be generated by the ICCP system. The user can then choose to ignore such error messages, knowing that the cause is changes in salinity, or to monitor and possibly intervene in the operation of the ICCP system to ensure that corrosion protection is maintained. The control unit can at the same time be arranged to monitor changes in the determined resistivity; to compare an increase in the determined resistivity to stored values for resistivity; and to determine if the increase is indicative of an electrical circuit malfunction. Depending on recorded changes in resistivity over time, the control unit can either decide that the increase is indicative of an increase in salinity or that the increase is indicative of an electrical circuit malfunction. In the former case the ICCP operation is maintained, while the latter case will cause the control unit to terminate ICCP operation in favour of passive corrosion protection.

According to a second aspect of the invention, marine vessel is protected by an impressed current cathodic protection system controlled by a device as described above.

According to a third aspect of the invention, a method for measuring salinity in water is provided using an impressed current cathodic protection system onboard a marine vessel. The impressed current cathodic protection system has an electrical circuit comprising: - a source of electrical power from a direct current power source;

- at least one second electrode connected to a negative pole of the power source to act as a cathode; - at least one passive electrode normally disconnected from the circuit and arranged to act as back-up protection;

- a voltage sensor detecting an output voltage impressed on the circuit;

- a current sensor detecting a current supplied to the circuit; and

- a control unit for controlling the impressed current cathodic protection system; The method involves performing the steps of:

- initiating a measurement sequence at predetermined intervals;

- registering the output voltage;

- registering the current;

- determining the circuit resistance using said output voltage and current, and

- calculating the resistivity of the electrolyte based on the determined circuit resistance and at least one stored electrode property value, which resistivity is inversely proportional to the salinity.

According to a first example, calculation of the resistivity p of the electrolyte is based on the determined circuit resistance R_c and the surface area of the passive electrode A_a.

According to a second example, calculation of the resistivity p of the electrolyte is based on the determined circuit resistance R_c, the surface area of the passive electrode A_a and a further surface area A_c of the first electrode connected to the negative pole.

The method further involves maintaining the impressed current cathodic protection system in operation if the determined resistivity is above a set threshold value. The threshold value is set at a level indicating that the vessel has entered a stretch of fresh water.

The arrangement according to the invention solves at least in part the problem of maintain the function of a corrosion protection system, such as an ICCP system, when a vessel is operated in stretches of water where the salinity can change, such as river estuaries. With reduced salinity, the resistivity of the water increases and at some point the electrical resistance will be too high for the ICCP system to be able to maintain the potential of the protected structure within an acceptable interval. Current ICCP do not understand if the reason for the low protection is an internal error or if it is due to external factors. As a consequence it will shut down the ICCP system and switch to a back-up galvanic anode for protection, which results in a reduced protection status. The inventive system is able to measure the salinity and can determine that the limited capacity of the ICCP system is due to increased water resistivity. Instead of automatically shutting itself down, the ICCP system can continue to provide as good protection as possible when the vessel moves into less saline water. A further advantage is that the invention can determine the salinity of the water using on-board equipment already present, eliminating the need for a separate salinity meter. Further, the inventive arrangement can transmit a signal to a user indicating that the target potential is not reached, but also inform the user that the reason is a change in salinity and not an internal malfunction. The user can therefore be prevented from manually switching to the back-up galvanic anode for protection.

Further advantages and advantageous features of the invention are disclosed in the following description and in the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

With reference to the appended drawings, below follows a more detailed description of embodiments of the invention cited as examples. In the drawings:

Fig. 1 shows a schematically illustrated vessel comprising a marine anti-fouling arrangement / corrosion protection system according to the invention;

Fig. 2 shows a schematic a cross-section of the rear portion of the marine vessel; Fig. 3A shows a schematic first representation of an electrical circuit for the corrosion protection system of the vessel in Figure 2;

Fig. 3B shows schematic second representation of an electrical circuit in Figure 3A; and Fig. 4 shows a schematic diagram illustrating a method of operating a salinity measuring arrangement in an impressed cathodic corrosion protection system according to the invention. DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS OF THE INVENTION

Figure 1 shows a schematically illustrated marine vessel 100 comprising an anti-fouling arrangement. According to the invention this arrangement is adapted to provide an arrangement for measuring salinity in water. The vessel comprises a hull with a transom 104 to which a marine propulsion system is attached. The propulsion system in this example comprises a single driveline housing 101 at least partially submerged in water, a torque transmitting drive shaft 106 (not shown) extending out of the driveline housing 101 , and a pair of counter-rotating propellers 102, 103 mounted on the drive shaft 106. In the current example, both propellers 102, 103 are electrically isolated from its drive shaft 106. The drive shaft arrangement is shown in Figure 2 and will be described in further detail below. Each electrically isolated propeller 102, 103 to be protected against fouling is connected to a positive terminal 111 of a direct current (DC) power source 110, such as a battery, in order to form an anode. Further, each metallic component 101, 104, 105 to be protected against corrosion is connected to a negative terminal 112 of the direct current power source 110, in order to form cathodes. A control unit 113 is connected to the direct current power source 110 and distributes current to all component parts forming an electrical circuit. The control unit 113 is arranged to regulate the voltage and current output from the direct current power source 110. In order to assist regulation of the voltage and current output a reference electrode 124 is mounted on the hull remote from the anode and connected to the control unit 113 via an electrical wire 123. The reference electrode 124 measures a voltage difference between itself and the metallic components, which is directly related to the amount of protection received by the anode. The control unit 113 compares the voltage difference produced by the reference electrode 124 with a pre-set internal voltage. The output is then automatically adjusted to maintain the electrode voltage equal to the pre-set voltage.

Regulation of the voltage and current output from the direct current power source is controlled to automate the current output while the voltage output is varied, or to automate the voltage output while the current output is varied. This allows the corrosion protection level to be maintained under changing conditions, e.g. variations in water resistivity or water velocity. In a sacrificial anode system, increases in the seawater resistivity can cause a decrease in the anode output and a decrease in the amount of protection provided, while a change from stagnant conditions results in an increase in current demand to maintain the required protection level. With ICCP systems protection does not decrease in the range of standard seawater nor does it change due to moderate variations in current demand. An advantage of ICCP systems is that they can provide constant monitoring of the electrical potential at the water/hull interface and can adjust the output to the anodes in relation to this. An ICCP system comprising a reference electrode is more effective and reliable than sacrificial anode systems where the level of protection is unknown and uncontrollable.

The corrosion protection arrangement is an impressed current cathodic protection (ICCP) arrangement using the propellers 102, 103 as an anode 115. In Figure 1 , the metallic component to be protected against corrosion is the driveline housing 101 , the trim tabs 105 (one shown), and a metal portion of the hull, in this case the transom 104. Note that this is a non-exclusive list of metallic components suitable for corrosion protection. In order to achieve this, the positive terminal 111 and the negative terminal 112 of the battery 110 are connected to the control unit 113. The control unit 113 is arranged to connect the positive terminal 111 to the propellers 102, 103 via a first electrical wire 114. The control unit 113 is further arranged to connect the negative terminal 112 to an electrical connector 117 on the driveline housing 101 via a second electrical wire 116. The negative terminal 112 is also connected to an electrical connector 119 on the trim tab 105 via a third electrical wire 118, and connected to an electrical connector 121 on the transom 104 via a fourth electrical wire 120. The corrosion protection arrangement is further provided with a passive, sacrificial anode 126 that can provide protection if a failure occurs in the active corrosion protection arrangement. The sacrificial anode 126 can be located at any suitable location on the vessel and is connectable to the control unit 113 via a fifth electrical wire 125. According to the invention, the control unit 113 is further adapted to operate as an arrangement for measuring salinity in water. The arrangement for measuring salinity will be described in detail in connection with Figures 3A-3B below.

Figure 2 shows a cross-section of the rear portion of the marine vessel 100 of Figure 1, through a transom 204 and a driveline housing 201. The single driveline housing 201 is partially submerged in water and comprises torque transmitting drive shafts 232, 233 extending out of the driveline housing 201. A pair of counter-rotating propellers 202, 203 is mounted on their respective drive shafts 233, 232. In this example, the drive shafts 232, 233 are driven by an internal combustion engine ICE via a transmission 231. Transmissions for driving counter-rotating propellers are well known in the art and will not be described in detail here. Alternative drive units for driving the propellers are possible within the scope of the invention. For instance, drive units comprising one or more pushing or pulling propellers can be used within the scope of the invention. Although the described examples relate to drive units mounted on a transom, the invention can be applied to most drive installations, such as outboard/inboard installations, Z-drives and azimuthing pod installations. The invention is not dependent on the type of power source provided, but can be applied to marine vessels using ICE, hybrid or electric power sources for propulsion or power generation.

In the example shown in Figures 1 and 2, the at least one propeller is used as an anode, wherein the at least one propeller is electrically isolated from its respective drive shaft. This is not a requirement for examples where the propeller is not used as an anode. In the current example, both propellers 202, 203 are electrically isolated from its respective drive shaft 232, 233. The propellers are electrically isolated from their respective drive shafts by a torque transmitting electrically isolating component mounted between a propeller and its respective drive shaft. The electrically isolating component is mounted in a gap formed by the outer surface of the drive shaft and the inner surface of the propeller hub. The torque transmitting electrically isolating component can be made from an elastic material, such as a natural or synthetic rubber. The propellers are made from an inert anode material, such as titanium, niobium or a similar suitable metal or metal alloy. A dielectric shield can be provided on the drive shaft between each propeller hub and the drive shaft on which the propeller is mounted. A non-exclusive list of suitable materials for use in such a dielectric shield includes polymer or polymer-ceramic materials with suitable dielectric properties.

As schematically indicated in Figure 2, each electrically isolated propeller 202, 203 is connected to a positive terminal 211 of a direct current power source 210 at schematically indicated points 215 via electrical wiring 214. The electrical connection of the propellers will be described in further detail below. Further, each metallic component 201, 204, 205 to be protected against fouling is connected to a negative terminal 212 of the direct current power source 210. A control unit 213 is arranged to regulate the voltage and current output from the direct current power source 210. As described above, the positive terminal 211 and the negative terminal 212 of the battery 210 are connected to the control unit 213. The control unit 213 is arranged to connect the positive terminal 211 to the propellers 202, 203 via a first electrical wire 214. The control unit 213 is further arranged to connect the negative terminal 212 to an electrical connector 217 on the driveline housing 201 via a second electrical wire 216. The negative terminal 212 is also connected to an electrical connector 219 on the trim tab 205 (one shown) via a third electrical wire 218, and connected to an electrical connector 221 on the transom 204 via a fourth electrical wire 220. A reference electrode 224 is mounted on the hull remote from the propellers 202, 203 forming an anode and connected to the control unit 213 via an electrical wire 223. Regulation of the voltage and current output from the direct current power source using the control unit 213 has been described above. The ICCP arrangement is further provided with a passive, sacrificial anode 226 that can provide protection if a failure occurs in the active ICCP arrangement. The sacrificial anode 226 can be located at any suitable location on the vessel and is connectable to the control unit 213 via a fifth electrical wire 225. The corrosion protection systems described in Figures 1 and 2 are examples of such arrangements that can be adapted for salinity measurement, as will be described below.

Figure 3A shows a schematic first representation of an electrical circuit for the corrosion protection system of the vessel in Figure 2 in its normal, active operating mode. A battery 310 is connected to, and adapted to provide electrical power to an active anode 315 (A) and at least one cathode 317 (C) to be protected. This connection is provided via a control unit 313, which is adapted to vary and control the electrical power to the active anode 315 and the cathode 317, as indicated with an arrow adjacent the battery 310.

The control unit 313 is adapted to measure an electrical potential of the cathode 317 with a reference electrode 324 (R) as a ground reference. The electrical potential of the cathode 317 is measured using a voltage sensor 330. The electrical potential is indicative of the surface polarization at the interface between the cathode 317 and an electrolyte W; in this case water. The control unit 313 is further adapted to control the electrical power to the active anode 315 (A) and the cathode 317 (C) based at least partly on the measured electrical potential of the cathode 317 with the reference electrode 324 (R) as a ground reference. Through the control of the electrical power, a first electrical current (indicated in Fig.3A with an arrow l-i), through an electrical circuit comprising the active anode 315, the cathode 317 and the electrolyte W, is controlled.

More specifically, the parameter of interest for control of the corrosion protection of the cathode 317 is the electrical potential of the cathode 317 with the reference electrode as a ground reference, corresponding to the surface polarization at the interface between the cathode 317 and the water W, and the electrical power to the active anode 315 and the cathode 317 is subjected to a closed loop control so as for said surface polarization to assume a desired value.

Thus, the corrosion protection system for the cathode 317 comprises an ICCP system with the active anode 315, the reference electrode 324, the battery 310 and the control unit 313. In Figure 3A the schematic electrical circuit of the corrosion protection system is only shown to comprise a single cathode, in this case the drive 317. However, additional components to be protected, such as the trim tabs, the transom and other metallic components (see Fig.2) can be connected to the control unit 313 as cathodes in the same way as the drive 317.

The control unit 313 further comprises a number of controllable switches for controlling different functions of the corrosion protection system. A first switch 331 is arranged between the positive terminal of the battery 310 and the anode 315, which first switch 331 is normally closed to supply the anode with power during an active corrosion protection mode. When opened, the first switch 331 disconnects the active anode 315 from the positive terminal of the battery 310. A second switch 332 is arranged between the negative terminal of the battery 310 and the cathode 317, which second switch 332 is normally switched to a closed position to maintain a closed circuit including the active anode 315, the cathode 317 and the battery 310 during active corrosion protection mode, wherein a current \^ flows from the battery 310 to the active anode 315. When opened, the second switch 332 can disconnect the cathode 317 from the negative terminal of the battery 310. A third switch 333 is arranged between the negative terminal of the battery 310 and the anode 315, which third switch 333 is normally open during active corrosion protection mode. When closed, the third switch 333 can connect the active anode 317 to the negative terminal of the battery 310. A fourth switch 334 is arranged to connect or disconnect a sacrificial, or passive anode 326 (P) to or from the corrosion protection system. The fourth switch 334 is a three position switch that is normally in a first position (lower contactor in Fig.3A) during active corrosion protection mode, wherein the passive anode 326 is completely disconnected from the system. In a second position (upper contactor in Fig.3A), the passive anode 326 is connectable to the positive terminal of the battery 310, during a salinity measurement mode which will be described below. In a third position (central contactor in Fig.3A), the passive anode 326 is connectable to the cathode 317 to provide passive corrosion protection. The corrosion protection system for the cathode 317 comprises a passive corrosion protection system with the passive anode 326 and the control unit 313. Should a fault occur in the active corrosion protection system, then the fourth switch 334 is switched from its open position to a first closed position (central contactor in Fig.3A) to connect the passive anode 326 to the cathode 317. Prior to this action, or at least at the same time, the first switch 331 is controlled to its open position to disconnect the active anode 315 and the battery 310 from the cathode 317. This electrical circuit provides a passive back up corrosion protection system for the vessel. As indicated above, the control unit 313 is adapted to measure electrical potential of the cathode 317 with the reference electrode 324 as a ground reference. The electrical potential is indicative of the surface polarization at the interface between the cathode 317 and the water W. The control unit 313 is further adapted to control an adjustable resistance 335 in the electrical connection between the passive anode 326 and the cathode 317 based at least partly on the measured second electrical potential of the cathode 317 with the reference electrode 324 as a ground reference. Through control of the adjustable resistance 335 an electrical current between the passive anode 326 and the cathode 317, herein also referred to as a second electrical current (indicated in Fig. 3A with an arrow l₂), is controlled. Thus, the second electrical current I2 runs through an electrical circuit comprising the passive anode 326, the cathode 317 and the electrolyte W during passive corrosion protection mode.

Figure 3B shows a schematic second representation of the corrosion protection system of the vessel in Figure 2 in a salinity measurement mode. The electrical circuit indicated in Figure 3B has been described in connection with Figure 3A above in its normal, active operating mode the corrosion protection system comprises the cathode 317 (C), the active anode 315 (A), the reference electrode 324 (R), the battery 310 and the control unit 313.

However, in the salinity measurement mode, the switches in the electrical circuit are controlled by the control unit 313 so that the active anode 315 temporarily forms a cathode (C) and the normally disconnected passive anode 326 temporarily forms an active anode (A). While the corrosion protection system is in a salinity measurement mode, the cathode 317 which is normally protected by the corrosion protection system is temporarily disconnected from the circuit. The control unit 313 is arranged to interrupt the corrosion protection mode and switch to the salinity measurement mode at regular intervals to monitor the salinity of the water in which the vessel is operated. Any suitable time interval can be selected for this purpose, although an interval of 5-10 minutes is sufficient for the intended purpose.

In operation, when switching to the salinity measurement mode, the control unit 313 will actuate the controllable switches as follows. Initially, the first switch 331, arranged between the positive terminal of the battery 310 and the anode 315, is opened to disconnect the active anode 315 from the battery. Subsequently, the second switch 332, arranged between the negative terminal of the battery 310 and the cathode 317, is switched to disconnect the cathode 317 from the battery 310. The third switch 333, arranged between the negative terminal of the battery 310 and the active anode 315, is then closed to connect the active anode 315 to the negative terminal of the battery 310. The active anode 315 now forms a cathode for the duration of the measurement mode. Finally, the fourth switch 334, arranged to connect or disconnect the passive anode 326 to or from the corrosion protection system, is switched to a second position (upper contactor in Fig.3A) wherein the passive anode 326 is connected to the positive terminal of the battery 310. The passive anode 326 now forms an active anode for the duration of the measurement mode.

It should be noted that the electrical circuit described above is only one of a multitude of possible solutions allowing the circuit to be switched between a corrosion protection mode, a salinity measurement mode and a passive protection mode. Hence, the inventive concept is not limited to the electrical circuit shown in Figures 3A and 3B.

During the salinity measurement mode, the control unit 313 performs a measurement sequence. As indicated above, at least one first active electrode or anode 315 is disconnected from the positive pole and connected to the negative pole of the battery 310 to act as a cathode, at least one passive electrode 326 is connected to the positive pole of the battery 310 to act as an active anode and each second electrode 317 is disconnected from the negative pole of the power source. A temporary measurement circuit is then formed by the active anode 315 acting as a cathode, the passive anode 326 acting as an active anode and the battery 310, wherein a current l₂ flows from the battery 310 to the passive anode 326 acting as an active anode. During the measurement sequence the control unit 313 is arranged to register the output voltage to the measurement circuit using a voltage sensor 341. The control unit 313 is further arranged to register the current using a current sensor 342. Subsequently, the circuit resistance can be determined using the output voltage and the current, by applying Ohm^'s law. Based on the determined circuit resistance and at least one stored electrode property value, the resistivity of the electrolyte can be calculated. In this example, the stored electrode property values are the surface area A_a of the passive anode 326 acting as an active anode is used. Alternatively, the surface area A_a of the passive anode 326 and the surface area A_c of the active anode 315, acting as a cathode can be used.

According to one example, the control unit is arranged to calculate the resistivity (p) using the formula:

wherein: R_c is the circuit resistance (W); k is a correlation factor (-); p is the resistivity of the electrolyte (Qcm);

A_a is the surface area of the passive electrode acting as an anode (cm²).

In this example the only surface area A_a of the passive anode 326 acting as an active anode is used. This formula can be used if the surface area A_c of the active anode 315, acting as a cathode is relatively large, whereby the contribution of this surface area is negligible.

A_a is the surface area of the passive electrode (cm²);

A_c is the surface area of the electrode acting as a cathode (cm²). In this example both the surface area A_a of the passive anode 326 acting as an active anode and the surface area A_c of the active anode 315, acting as a cathode is used.

As the resistivity is inversely proportional to the salinity, a current value for electrolyte salinity can be obtained using a stored conversion table and stored in a memory. Stored salinity values can subsequently be retrieved for comparison with updated salinity values.

A standard value for the correlation factor k can be taken from the McCoy formula:

R_c = 0,315 * p * (-L) wherein the correlation factor k=0,315 is a standard applicable to anodes that are flush mounted onto a hull or a similar surface. Active anodes and passive anodes can have different shapes and sizes, depending on anode design, which will affect the surface area and thereby the correlation factor. The correlation factor can also be dependent on which component part, e.g. a propeller, that is used as an active anode in the corrosion protection mode. Consequently the value of the correlation factor can vary. Suitable values for the correlation factor can be determined by testing and calibration of each system or type of installation.

The reason for using the active anode and the passive anode for performing a salinity measurement during the salinity measurement mode is that the surface areas of these anodes are known and will only be marginally reduced over time. The cathode(-s) of the corrosion protection system is less suitable for this purpose as the useful surface area of the at least one protected metallic components to be protected can vary with the type of installation and the number of components connected to the system. The surface area can also vary depending on the amount of surface oxidation or whether one or more components have been fully or partially coated with an anti-corrosion coating subsequent to the installation of the system.

As long as the detected salinity has a value within a range representing normal variations for sea water, having approximately 3,5 % salinity, the corrosion protection system resumes normal operation after exiting the salinity measurement mode. The detected salinity values are registered for comparison with subsequently detected values. When a decrease in salinity is detected, the control unit will automatically attempt to compensate for this by regulating the voltage to maintain a desired potential. If the detected salinity value drops to a value at or near zero, the control unit will no longer be able to compensate for this to maintain the desired potential. However, by comparing a currently detected salinity value with previously registered values, the control unit 313 can determine that the reduction of the salinity value is caused by the vessel moving into a body of fresh or brackish water. By making this determination, the control unit can establish that the inability to compensate for the drop in potential is caused by a change in salinity value and not by a malfunction in the corrosion protection system. Consequently, the corrosion protection system will continue to operate, albeit at reduced efficiency level.

Figure 4 shows a schematic diagram illustrating a method of operating a salinity measuring arrangement in an impressed cathodic corrosion protection system according to the invention. In operation, the ICCP system is being operated for protecting a marine vessel with a marine propulsion system against corrosion of submerged metallic components. The ICCP system can be operated using an on-board source of DC power, as described in connection with Figures 1 and 2, or using DC power supplied from a shore facility, in order to conserve the on-board power source.

With reference to Figure 3A described above, the ICCP system comprises an electrical circuit comprising a source of electrical power from a direct current power source 310; at least one first electrode 315 connected to a positive pole of the power source 310 to act as an active anode A; at least one second electrode 317 connected to a negative pole of the power source 310 to act as a cathode C; a voltage sensor 341 detecting an output voltage impressed on the circuit; a current sensor 342 detecting a current supplied to the circuit; and a control unit 313 for controlling the impressed current cathodic protection system.

With reference to Figure 3B described above, the method for operating the salinity measuring arrangement according to the invention comprises the following method steps. The method comprises an initial step 400 when the control unit 313 is arranged to interrupt the impressed cathodic protection operation and to initiate a measurement sequence. Interruption of the ICCP can involve disconnecting each second electrode normally acting as a cathode during ICCP operation from the negative pole of the power source prior to and during the measurement sequence. The measurement sequence can be initiated at predetermined intervals, such as every 5 or 10 minutes. In a first step 401, the method involves connecting the first electrode 315 to the negative pole of the power source to act as a cathode C. In a second step 402, the method involves connecting the passive electrode 326 to the positive pole of the power source to act as an active anode A. In a third step 403, the method involves registering the output voltage transmitted from the voltage sensor 341 , which voltage represents the potential of the electrical circuit. In a fourth step 404, the method involves registering the current transmitted from the current sensor 342. In a fifth step 405, the method involves determining the circuit resistance using said output voltage and current. In a sixth step 406, the method involves calculating the resistivity p of the electrolyte based on the determined circuit resistance and at least one stored electrode property value, which resistivity is inversely proportional to the salinity. The electrode property value is preferably the surface area of the electrode, as described above. After a predetermined period of the measurement sequence has been completed and can be terminated. Subsequently, the first and second electrodes 315, 326 are reconnected to their original terminals on the power source 310 in a final step 407. The normal operation of the impressed cathodic corrosion protection can then be resumed.

It is to be understood that the present invention is not limited to the embodiments described above and illustrated in the drawings; rather, the skilled person will recognize that many changes and modifications may be made within the scope of the appended claims.

Claims

1. Arrangement for measuring salinity in water, which arrangement is part of an impressed current cathodic protection system having an electrical circuit comprising:

- a source of electrical power from a direct current power source (310)

- at least one first electrode (315) connected to a positive pole of the power source (310) to act as an active anode;

- at least one second electrode (317) connected to a negative pole of the power source (310) to act as a cathode;

- at least one passive electrode normally disconnected from the circuit and arranged to act as back-up protection;

- a voltage sensor (341 ) detecting an output voltage impressed on the circuit;

- a current sensor (342) detecting a current supplied to the circuit; and

- a control unit (313) for controlling the impressed current cathodic protection system; characterized in that the control unit (313) is arranged toinitiate a measurement sequence at predetermined intervals, during which measurement sequence the control unit is arranged to;

- connect at least one first electrode (315) to the negative pole of the power source (310) to act as a cathode;

- connect at least one passive electrode (326) to the positive pole of the power source (310) to act as an active anode;

- register the output voltage;

- register the current;

- calculate the resistivity of the electrolyte based on the determined circuit resistance and at least one stored electrode property value, which resistivity is inversely proportional to the salinity.

2. Arrangement according to claim 1, characterized in that the at least one stored electrode property value is the surface area of the passive electrode (A_a).

3. Arrangement according to claim 2, characterized in that the control unit (313) is arranged to calculate the resistivity (p) using the formula:

wherein: R_c is the circuit resistance; k is a correlation factor;

A_a is the surface area of the passive electrode.

4. Arrangement according to claim 2, characterized in that a further electrode property value is the surface area (A_c) of the first electrode (315) connected to the negative pole.

5. Arrangement according to claim 4, characterized in that the control unit (313) is arranged to calculate the resistivity (p) using the formula:

wherein: R_c is the circuit resistance; k is a correlation factor;

A_a is the surface area of the passive electrode;

A_c is the surface area of the electrode acting as a cathode.

6. Arrangement according to any one of claims 1-5, characterized in that the control unit is arranged to maintain the impressed current cathodic protection system in operation if the determined resistivity is above a set threshold value.

7. Arrangement according to any one of claims 1-6, characterized in that the control unit (313) is arranged to determine a current salinity value based on the determined resistivity and to generate an output signal indicating the salinity value to a user.

8. Arrangement according to any one of claims 1-7, characterized in that the control unit (313) is arranged to monitor changes in the determined resistivity; to compare an increase in the determined resistivity to stored values for resistivity; and to determine if the increase is indicative of an electrical circuit malfunction.

9. Arrangement according to any one of claims 1-8, characterized in that the control unit (313) is arranged to disconnect each second electrode (317) from the negative pole of the power source during the measuring sequence.

10. Marine vessel characterized in that the marine vessel (100) is provided with an impressed current cathodic protection system comprising an arrangement for measuring salinity according to claim 1.

11. A method for measuring salinity in water using an impressed current cathodic protection system onboard a marine vessel; the impressed current cathodic protection system having an electrical circuit comprising:

- a source of electrical power from a direct current power source (310) - at least one first electrode (315) connected to a positive pole of the power source (310) to act as an active anode;

- a voltage sensor (341) detecting an output voltage impressed on the circuit;

- a current sensor (342) detecting a current supplied to the circuit; and

- a control unit (313) for controlling the impressed current cathodic protection system; characterized by performing the steps of: - initiating a measurement sequence at predetermined intervals

- registering the output voltage;

- registering current;

- determining the circuit resistance using said output voltage and current, and

12. A method according to claim 11, characterized by calculating the resistivity of the electrolyte based on the determined circuit resistance and the surface area of the passive electrode (A_a).

13. A method according to claim 12, characterized by calculating the resistivity of the electrolyte based on the determined circuit resistance and the surface area (A_c) of the first electrode (315) connected to the negative pole.

14. A method according to claim 12, characterized by calculating the resistivity of the electrolyte based on the determined circuit resistance and the combined surface area (A_c) of each electrode connected to the negative pole.

15. Method according to any one of claims 11-14, characterized by maintaining the impressed current cathodic protection system in operation if the determined resistivity is above a set threshold value.