Technical Field

The present invention relates to systems and methods for providing impressed current cathodic protection (ICCP) .

Backgroun d

Cathodic protection is used to control the corrosion of a metal surface by arranging the surface to act as a cathode of an electrochemical cell . For effective cathodic protection, the potential of the steel surface is polarized (pushed) more negative until the surface has a uniform potential. At that stage, the driving force for the corrosion reaction with the protected surface is removed. There are two type s of cathodic protection, i .e . sacrificial anode cathodic protection and impre ssed current cathodic protection .

Metallic obj ects having surfaces that are submerged in an electrolyte can b e protected using cathodic protection. Such obj ects include for example, buried and offshore pipeline s, fuel storage tanks, steel piles for supporting j etties and harbour structure s, hulls of ships and other floating structures, such as offshore oil and gas platforms, and oil well casings, both on and offshore . For large structure s such as these, the more traditional sacrificial anode cathodic protection may not economically deliver enough current.

In impressed current cathodic protection, an external direct current (DC) power source is used to impress an electrical potential at the metal structure to be protected. As oil and gas exploration delves into ever-greater depths, the size of the supporting structure s is increasing, leading to greater current demands from the cathodic protection system. Typical prior art systems on offshore platforms require the use of an array of six copper cables each having a cross-sectional area of 35 square millimeters to transmit power from a D C source at an upper region of the structure to an anode deep underwater. Installation of such cabling is very expensive . SUMMARY

The present invention provides an impre ssed current cathodic protection system for protecting a metallic obj ect against corrosion. The system comprises a power supply connected to conversion means for converting the output of the power supply to D C at a required level . The conversion means is connected to the power supply by an electrical connection . In use , the structure form s a cathode . The conversion means is disposed at a distal region of the electrical connection line . An anode is electrically connected to the conversion means .

The conversion means may comprise a converter.

The conversion means can be used to step up the current provided by the power supply to a level sufficient to prevent corrosion of the structure and to rectify the output to the DC that is needed for ICCP if the output is not already DC . By virtue of the system according to the invention, the current can be stepped up in the vicinity of the anode , since both the conversion means and the anode are located at the distal part of the line . High current is thereby only transmitted through a short distance from the conversion means to the anode rather than being transmitted all the way along the connection line . The electrical connection line that is used can thus be of lower capacity, i . e . reduced cable cross-sectional area, than prior art examples, making the system less expensive and easier to install than the prior art. The present invention advantageously minimizes the length of high current capacity cable required between the conversion means and the anode .

The pre sent invention further provides an impressed current cathodic protection method for protecting a stucture against corrosion . The method comprise s arranging a power supply on an obj ect to be protected and electrically connecting the power supply to the obj ect, whereby the obj ect comprise s a cathode . The method further comprises installing an electrical connection from the power supply to conversion means and an anode and disposing the conversion means next to the anode at a predetermined distance away from the obj ect. The conversion means is to be electrically connected to the structure . The invention may comprise a control means, or controller, to control the power supply to the anode s .

The invention may comprise one or more features as shown in the drawing and/or as detailed in the description .

Brief Description of the D rawin g

By way of example only there follows a detailed description of an embodiment of the invention with reference to the accompanying drawing, in which :

Figu re 1 is a schematic drawing of a partially submerged structure including an impre ssed current cathodic protection system embodying the present invention . Detailed Description

A steel supporting structure 1 suitable for use in oil and gas exploration and/or extraction is shown in Figure 1 . The structure re sts on the seabed 7 and is submerged in the sea except for an upper platform 14 on which equipment and personnel may be located. Connected to the structure is an impressed current cathodic protection system comprising a power supply 8 situated on the platform 14. The power supply 8 may comprise mains power or can be a generator. The supply can be AC or DC, and typically is AC . The system can be seen to act as a regulator to regulate the voltage and current of the power supply to a level required for effective cathodic protection of the structure . It is possible that the output of the power supply may be at the correct level for ICCP, in which case, the system can provide an output that is the same as the input from the supply .

The cable 10 connects the power supply 8 to conversion means 3 to convert the power supply to the most appropriate form for ICCP . A DC current is required through the anode and the structure . Typically, the potential difference to be overcome to prevent corrosion is not very high, e .g . a few Volts, but the amount of current required to maintain the protection for a large structure can be significant, e . g . as much as 1000 Amps or greater. The conversion means 3 comprise s a step-down transformer 1 1 , which reduces the voltage and increases the current provided to the anode and a rectifier 12 that rectifie s the AC to DC . The positive output of the rectifier is connected to the anode 13 whilst the negative output is connected to the structure 14 by a cable 9. Preferably, the rectifier is a full-wave rectifier.

The conversion means 3 is disposed at a distal end region of the electrical cable 10. An electrical connection 4 is provided between the conversion means 3 and an anode 13 which connection 4 is of a relatively high current carrying capacity compared to the connection 10 between the power supply 8 and the conversion means 3 . The electrical connection 4 between the conversion means 3 and the anode 13 is considerably shorter in length than the connection 10 between the power supply 8 and the conversion means 3 . The electrical connection 9 between the conversion means and the structure is also of a relatively high current-carrying capacity compared to the cable 1 0 and is considerably shorter than the high-voltage cable 10.

If the anode 13 is located too closely to the structure 1 , the effect of the ICCP may be localized to regions that are near to the anodes and hence near to the protecting current source . In a preferred embodiment, more than one anode can be connected to the conversion means 3 and suitably located to optimize the spread of the ICCP . Moreover, a plurality of conversion means 3 can b e provided, with one or more anodes 13 being connected to each conversion means . The anode 13 according to the embodiment shown in Figure 1 is disposed close to or in contact with the seabed 7. Various positionings of the anode with respect to vertical and horizontal axes (not shown) are possible . The anode(s) can be located in a number of other positions such as midway between the seabed 7 and the sea surface 15 . An array of anodes can b e positioned around the structure . The array is preferably arranged with symmetry and/or equidistant spacing .

In one embodiment, the power supply 8 is a 3 -phase, medium voltage i . e, 3 80 to 690V supply which operates at a frequency of 50 to 60 Hz . The power supply 8 can provide a variable voltage output up to or in excess of a maximum of the supply voltage . It is possible to vary the magnitude of the voltage or vary the frequency of the AC voltage , to optimise the transmission of power to the conversion means . The use of high voltage s and/or frequencie s enables the use of small cros s-section cable 10 to transmit the power down to the conversion means . The transformer of the conversion means may output low voltage i . e . le ss than 50V AC for example, which is then converted to D C by the rectifier.

The transformer 1 1 of the conversion means 3 comprises a fixed ratio transformer, typically having a step down ratio in the order of 10. Where the input to the transformer 1 1 is a variable AC voltage and the ratio is fixed, the output of the transformer is a variable, controlled voltage . This controlled voltage is rectified by the rectifier 12 as described above .

The system has a manual mode of operation, wherein it is controlled by reference to pre-set values of the potential difference and current required to protect a particular structure in a particular set of circumstance s .

Alternatively, the system can be automatically controlled based on reference electrode 15 , 16, 17 signals . The electrode signals are compared against a ' de sired ' input value and the output current is automatically regulated to achieve a desired electrode value from all signals . The reference electrode s are connected to the control means 5 which controls the power supply based on the reference electrode signals .

In one embodiment, the reference electrodes 15 , 16, 17 are made of Zinc . Without the ICCP system switched on, the potential at the reference electrodes would be typically 550 mV . With the system switched on the potential at the electrodes should be typically be 220 mV or less in order for the protection to be effective .

In an alternative embodiment, the ICCP system can be used to protect a ship ' s hull . Here, the anode may take the form of a plate mounted in a recess in the ship ' s hull and electrically isolated from the hull . Conversion means for producing DC at the required level can be disposed inside the ship in proximity to the rece ss, connected to the anode plate and connected to a central AC power source of the ship . In a further aspect, the control means 5 may be arranged to control the conversion means directly . For example, the transformer 1 1 can be a variable transformer, allowing control of the output current.