US3108940A - Cathodic protection of ships - Google Patents

Description

1963 M. P. HOLDSWORTH 3,

CATHODIC PROTECTION OF SHIPS Filed April 12, 1.960

FIG. I

FIG.3 e

' INVENTOR:

FIG. 2

MAURICE P. HOLDSWORTH BYI f/- D. gAnP c/L) HIS ATTORNEY United States Patent 3,108,940 CATHODIC PROTECTIQN 0F SHIPS Maurice P. Holdsworth, Woking, England, assignor to Shell Oil Company, New York, N.Y., a corporation of Delaware Filed Apr. 12, 196i), Ser. No. 21,713 Claims priority, application Great Britain May 1, 1959 2 Claims. c1. 204-l96) the cathode and from which said wire is electrically insulated.

According to the present invention, a protective system for ships comprises a trailable wire, a winch fitted at the stern of a ship for carrying a supply of said wire, means in said winch for insulating said wire from the ships hull, a source of electric direct current for the passage of a current between the said wire as anode and the ships hull as cathode, means for driving said winch to pay out the wire carried by said winch over the ships stern, and means for actuating said driving means automatically and regularly at a controlled rate, soas to compensate for the consumption of said wire by passage of said current. It is found that the provision of means for feeding the wire automatically and regularly at a controlled rate not only minimizes the degree of personal attention called for in the operation of the system and diminishes the chance of personal error, but also, and unexpectedly, increases the material efiiciency of the system in terms of both wire and current consumption required for effective protection and enables nearoptimum conditions for protection to be steadily maintained. In addition, the provision of motorized drive for the winch and of the necessary controls for automatic operation make it easy to provide for remote control of the winch (egg. from the bridge) for the rapid hauling in of the wire in cases of emergency. The expression wire used herein means any metal element of great length and slenderness and includes bundles of wires, tapes, ribbons and the like.

While it is possible to arrange for the wrinch-driving means to drive the winch very slowly and continuously so as to pay out the wire continuously at the desired rate, it is preferable, in view of the extremely low speed at which the winch would need to be driven, to drive it at short regular intervals so as to rotate the drum of the winch, for example, one turn, or preferably a fraction of a turn (e.g. A to ,4,), at a time. For this purpose the system preferably comprises a process timer adapted to actuate the driving mechanism at regular intervals, which may be adjusted according to circumstances. Thus the winch drum, or the winch spindle, may be provided with one or more cams each adapted to actuate a limit switch for halting the driving means, the process timer being adapted to reconnect the motor at regular intervals, so that it will again drive the drum until the next cam surface reaches the limit switch. For this purpose it may be necessary to provide a delay mechanism to allow the motor to be started by the process timer while the limit switch is still engaged by the last operative cam surface.

The winch may be of any convenient size and capacity. A convenient size of winch is one having a drum diameter of about 2 ft., a flange diameter of 3 ft., and a space between the flanges of 18 inches; such a winch will accommodate about 500 lbs. of aluminum 3,1li8fi4h Patented Oct. 29, 1963 "ice wire which may be of any convenient diameter, preferably ot the order of .25.5 inch. The drum can be mounted on a suitable spindle carried in ballor rollerbeanings through which current is supplied to the Wire by way of a suitable terminal clamp on the drum; in this case, the bearings will be insulated from the body of the winch, itself grounded to the metal of the hull. Alternatively, the whole winch may be insulated from the hull or the drum may be made of, or lined with, insulating material, current being supplied to the terminal clamp by a suitable slip ring on the body of the drum itself. The material of the drum may be aluminum, galvanized iron, or compressed resin-impregnated fiber, as appropriate to the circumstances. The drum may be driven by an electric motor, preferably reversible, of appropriate horsepower, e.g. from A to 1 HF. Associated with the winch are suitable guide means adapted to lead the wire over the stern of the ship and clear of the hull, and to insulate the wire from the hull. The guide means, which may be of the nature of a chute, may be provided wtih safety switch means operated by the presence of the wire passing through the guide means and adapted to prevent the winch motor operating when the wire is absent. This provides an automatic stop effective on completion of the hauling-in operation, and also prevents the winch being driven in the pay-out direction unless the wire is properly led through the guide means over the stern of the ship.

Means may be provided for preventing the winch motor operating in the pay-out direction if the drag on the wire, engendered by the passage of the ship through the water, becomes excessive. Any suitable torque limiting arrangement in the drive to the winch can be employed for this purpose. Thus the worm shaft of the motor may be allowed a degree of axial motion and be held in position by springs. When the drag imposed by the wire becomes excessive, the pressure of the springs is overcome and the movement of the Worm shaft can be made to actuate a limit micro-switch. This arrangement may be employed as an auxiliary control overriding the control of the process timer or it may be used as an alternative to the process timer, the pay-out of the wire thus being placed under the control of the drag upon the wire, and being actuated when the drag is small, indicating that the amount of wire payed-out is insufficient and stopped when the drag reaches a certain level, indicating that the appropriate amount is payed-out. In either case, when this control is operative, consumption of the wire by the passage of the electric current reduces the drag on the wire and releases the switch, so enabling the wire to be payed-out again. In addition to the automatic controls mentioned above, it is desirable to provide the winch and motor with manually operated switches in the neighborhood of the winch for .pay-out, haul-in and stop, and also with remote control manually operated switches on the bridge of the ship for haul-in and stop.

.A protective current is supplied to the wire by a suitable low-voltage high-amperage direct current source. A convenient source is a three-phase rectifier, supplied from the ships electric power supply through appropriate transformers, and adapted to give a continuopsly variable output current up to a maximum of the order of 500 to 1000 amps. depending on the size of the ship; suitable automatic overload, no-load and manual controls will be provided in the current supply circuit. The output voltages of the rectifier need not for this purpose exceed about 20 volts. The current supplied to the wire returns through the hull of the ship and then to the supply by way of an earthing point. An ammeter of appropriate rating can be included in the return from the hull of the power unit. It may be found desirable to provide separate returns for the hull in general and for the propeller, for which purpose heavy brush gear may be provided on the propeller shaft leading through a separate ammeter, if desired, to the power unit. By including variable low resistances in the return from the hull and/ or the propeller, the proportion of current taken by the propeller can be adjusted. However, it may be found in practice that the shaft is adequately grounded to the ships hull by way of its glands and bearings, rendering a brush connection unnecessary and ineffective.

It is desirable that the source of supply of protective current should be variable, because the magnitude of the current required to give eifective protection varies widely according to circumstances, e.g. the speed of the ship, the water temperature and the lapse of time as affecting the condition of the ships hull; to obtain the maximum protection it is desirable to maintain a predetermined voltage difference between the metal of the ships hull and the water immediately surrounding the hull. This can be measured by means of a submerged reference electrode either portably suspended over the ships side or, and preferably, inserted permanently through and suitably insulated from the metal of the hull. The voltage difference between such an electrode and a ships hull can be measured and the current adjusted to bring this voltage dilierence to a predetermined value, depending in part upon the material of the electrode which may be, for example, of silver coated with a deposit of silver chloride or of pure zinc. For silver/silver chloride electrode a suitable hull/electrode voltage is one of the order of 0.8 volt, while for a zinc electrode a voltage of +0.2 is suitable.

The adjustment of the anode current to achieve the appropriate hull/electrode voltage may be done manually or automatically. Thus the current may be controlled by hand from a visual observation of the hull/ electrode potential or of its difference from a standard potential. In any case, it is necessary that the standard potential itself should be adjustable since the current required for protective purposes may be found to vary in accordance with the history of the protective arrangement.

By way of example a protective system in accordance with the invention will now be described in greater detail in its application to tanker of the order of 32,000 d.w.t. The description refers to the accompanying drawing in which FIG. 1 is a profile view of the tanker showing the general layout of the protective system, and FIG. 2 is a diagrammatic representation of the winch and its asso ciated electric control circuits. FIG. 3 is a diagrammatic drawing showing in more detail the principle of the tension switch.

Referring to FIG. 1, the winch 3 is mounted at the stern of the hull 4 approximately on the fore and aft center line and comprises a drum 5 carryinga supply of aluminum wire 6 which is fed over the stern of the vessel to constitute a trailing anode 7. The wire is of a diameter of .45 inch and the drum 5 accommodates about 500 lbs. of wire, having a spool diameter of about 2 feet, a flange diameter of about 3 feet and a flange to flange distance of about 18 inches. The spool can be made of aluminum, galvanized iron or impregnated fibrous material. As indicated in FIG. 2, the drum 5 is mounted in bearings 8 through which current is passed by way of a lead 9 to a suitable terminal clamp on the drum and so to the Wire 6. The wire extends over the stern of the vessel by way of a chute 10, at which it is insulated from the hull. The current supplied to the wire returns from its submerged end 7 to the hull of the ship, and the magnitude of the current is controlled in accordance with the indications of a reference electrode, whose position is indicated in FIG. 1 at 11, this position being appropriate for a vessel having its engines towards the stern as shown.

The control of the winch 5, with which the present invention is principally concerned, is shown in detail in FIG. 2. The drum 5 of the winch is driven from a /2 HP. electric motor 12 by way of a worm 13 and worm gear 14. The worm shaft is capable of sliding axially for a short distance, but is restrained from doing so by means of a strong adjustable spring 15. If the tension in the wire 6 becomes excessive, the shaft moves towards the right compressing the spring 15 and breaking contact at a switch 16 for a purpose to be described hereafter.

The motor 12 is supplied with electric power by leads 17 from 440 v. 3 phase mains 18. The connection is made by way of two solenoid-controlled multiple ganged switches 19 and 20, the former causing the motor to be driven in the haul-in direction for hauling in the wire 6, and the latter in the pay-out direction for feedingthe wire over the stern of the ship. Control current is supplied to the solenoids of the switches, 19 and 20, from a transformer 21, whose primary is connected across two of the main leads 18 and whose secondary is connected in series with a switch 22 associated with the chute 10. The wire 6, in passing through a wire guide 24, forming part of the chute 10, passes under a tension lever 23 which is maintained in a raised position by the presence of the wire. In the absence of the wire, the lever 23 drops, the switch 22 is opened, and no actuating current can be supplied to either of the switches 19 and 20.

The present invention is not primarilyconcerned with the operation of the switch 19, but it may be stated that of its four pairs of contacts, the first two pairs (counting from the left side of the FIG. 1) supply current to the motor 12, the third constitutes a holding switch supplying the actuating current to its solenoid and the fourth, which is open when the other three are closed, controls the supply of actuating current to the other switch 20. An initial actuating current is supplied to switch 19 by a distant haul-in switch 25 (disposed, for example, on the ships bridge) or a local haul-in switch 26, both of which are connected in parallel with the holding contacts of the switch 19. When the switch 25 or 26 has been closed and then released, current continues to be supplied to the solenoid of the switch 19 by the holding contacts. However, this solenoid is also connected in series with a control switch forming part of switch 20 (the first pair of contacts thereof) and with two stop switches 27, 28, one local and one remote, by which the circuit can be broken at will.

The operation of paying out can be effected at will by means of a pay-on switch 29, which supplies the current to the solenoid of the switch 20 by Way of the last pair of contacts of switch 19 (assuming the switch 19 to be tie-energized) and the stop switches 27, 28. However, in normal operation, the switch 20 is operated in accordance with the present invention by means of a process timer 31 and the limit switch 16. The process timer 3]. contains the switch 32 which is normally open but is closed at regular intervals, measured by the process timer 31. The timer 31 is supplied with actuating. current by way of the switch 16 and the last pair of contacts of the switch 20, which are closed when the switch 20 is de-energized. In due course the switch 32 of the process timer is closed by the action of the timer, and current is then supplied to the solenoid of the switch 20 through the switch 32. The actuation of the switch 20 breaks the connection between its first pair of contacts thereby making it impossible to operate the winch in the haul-in direction by means of the switch 19. The second and third pair of contacts of the switch 20 supply driving current to the motor 12 The fourth pair of contacts supply holding current to the solenoid of the switch 20 by way of a switch 33. The last pair of contacts, already mentioned, are disconnected but only after a short delay occasioned by the provision of a delay mechanism 34. This enables the timer to run on for a short period after the switch 32 is closed. When, after this delay, the contacts open, the timer 3 1 is de-energized and its switch 32 returns to the open position; current is now supplied to the solenoid of the switch '20 by way of the fourth pair of contacts and the switch 33.

The switch 33 is normally closed, but is arranged to be opened by any one of a series of earns 35 mounted on the winch drum three such cams are shown in FIG. 2, but any desired number, e.g. up to twelve, may be employed. The delay action of the mechanism 34 enables the drum 5 to be driven until the contacting cam 35 is disengaged from the switch 33. The drum thus continues to be driven until the next cam 35 reaches the switch 33, in this case after /3 of a revolution. The switch 33 is then opened, the solenoid of the switch 20 is de-energized, and the drive of the motor 12 is stopped. At the same time, the process timer 31 is re-energized by the closing of the last pair of contacts of the switch 20 and continues to run until, after is set period, the switch 32 is again closed.

It will be seen that the limit switch 16 constitutes an overriding switch which prevents wire being payed out if the tension in the wire is excessive, thus causing the switch '16 to be open. It may be convenient in some circumstances of sea and weather to place the winch under the control of the limit switch 16 alone. For this purpose the switch 32 can be closed by hand, whereupon the wire 6 is payed out until a tension sufficient to open the switch 16 is reached. As the wire is eroded by the passage of the protective current, this tension decreases until the switch 16 is again closed, when more wire is payed out and so on. This tension can be adjusted by adjusting the pressure of the spring 15.

The protective current supplied to the wire 6 by way of the lead 9 is supplied from a suitable direct current source 36 (FIG. 1), the current-return being effected by way of an adjustable resistance 37 and an ammeter 38 connected directly to the hull 4 of the ship, and also by way of a separate adjustable resistance 39 and ammeter 40 from a slip ring 41 connecting with the propeller shaft 42 of the vessel. By this means, if the shaft is imperfectly earthed to the hull by way of its bearings and otherwise, the proportion of current returning by way of the hull 4 and by way of the propeller 43 can be adjusted by means of the separate resistances 37, 39.

I claim as my invention:

1. In an apparatus for the electrolytic protection against corrosion of the submerged part of the hull of a moving metal ship in which a bare soft metal wire insulated from the hull is trailed as an electrode and connected to the positive side of a direct current source and the negative side of said direct current source is connected to the metal of the hull, the combination comprising a revolvable winch reel having said soft metal wire wound thereon mounted at the stern of the ship, a motor operably connected to rotate said winch and unreel wire into the water, an adjustable timing device operating said motor at adjustably timed interval-s to intermittently pay out wire from the reel at a determinable average rate, tension switch means controlled by the tension imparted to the wire as it is dragged behind the moving ship to stop the motor and reel when the tension on the wire increases above a certain value, and thus prevent an excess amount of wire from being unreeled.

2. In an apparatus for the electrolytic protection against corrosion of the submerged part of the hull of a moving metal ship in which a bare metal Wire insulated from the hull is trailed astern as an electrode and connected to the positive side of a direct current source and the negative side of said direct current source is connected to the metal of the hull, the combination comprising a revolvable winch reel having a soft metal wire Wound thereon mounted at the stern of the ship, a motor operably connected to rotate said winch and unreel wire into the water, tension switch means controlled by the tension imparted to the wire as it is dragged behind the moving ship to run the motor when the tension on the wire decreases below a certain value to pay out wire from the reel, and to stop the motor and reel when the tension increases above a certain value.

References Cited in the file of this patent UNITED STATES PATENTS 2,046,467 Krause July 7, 1936 2,863,819 Preiser Dec. 9, 1958 2,916,429 Vossmack et a1. Dec. 8, 1959 3,010,891 Anderson Nov. 28, 1961

Claims (1)

1. IN AN APPARATUS FOR THE ELECTROLYTIC PROTECTION AGAINST CORROSION OF THE SUBMERGED PART OF THE HULL OF A MOVING METAL SHIP IN WHICH A BARE SOFT METAL WIRE INSULATED FROM THE HULL IS TRAILED AS AN ELECTRODE AND CONNECTED TO THE POSITIVE SIDE OF A DIRECT CURRENT SOURCE AND THE NEGATIVE SIDE OF SAID CURRENT SOURCE IS CONNECTED TO THE METAL OF THE HULL, THE COMBINATION COMPRISING A REVOLVABLE WINCH REEL HAVING SAID SOFT METAL WIRE WOUND THEREON MOUNTED AT THE STERN OF THE SHIP, A MOTOR OPERABLY CONNECTED TO ROTATE SAID WINCH AND UNREEL WIRE INTO THE WATER, AN ADJUSTABLE TIMING DEVICE OPERATING SAID MOTOR AT ADJUSTABLY TIMED INTERVALS TO INTERMITTENTLY PAY OUT WIRE FROM THE REEL AT A DETERMINED AVERAGE RATE, TENSION SWITCH MEANS CONTROLLED BY THE TENSION IMPARTED TO THE WIRE AS IT AS DRAGGED BEHIND THE MOVING SHIP TO STOP THE MOTOR AND REEL WHEN THE TENSION ON THE WIRE INCREASES ABOVE A CERTAIN VALUE, AND THUS PREVENT AN EXCESS AMOUNT OF WIRE FROM BEING UNREELED.

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