EP2864523B1

EP2864523B1 - Cathodic protection system

Info

Publication number: EP2864523B1
Application number: EP13732606.2A
Authority: EP
Inventors: David MORTLOCK; Daniel CONSTANTINIS
Original assignee: EM&I Maritime Ltd
Current assignee: EM&I Maritime Ltd
Priority date: 2012-06-20
Filing date: 2013-06-20
Publication date: 2022-03-23
Anticipated expiration: 2033-06-20
Also published as: GB201210929D0; DK2864523T3; WO2013190314A1; EP2864523A1

Description

This invention relates to Floating Production, Storage and Offloading (FPSO) vessel anode and to a method of attaching a FPSO vessel anode to a hull of a FPSO vessel according to the claims. Cathodic protection or ICCP is a widely used and accepted form of corrosion prevention. The cathodic protection system is used to reduce the deterioration of a metal exposed to an aqueous electrolyte by lessening the thermodynamic driving force for corrosion. A properly maintained cathodic protection system can effectively eliminate metal dissolution and provide a long-term solution to many corrosion problems. Most ICCP systems are designed to control the potential of the metal at a fixed value where anodic dissolution of the metal effectively does not occur. This is known as controlled-potential cathodic protection and is the method used most extensively for ship hulls and seawater applications. Most FPSOs use a cathodic protection system to protect the hull from external corrosion as protective coatings applied to the hull deteriorate over time.
The two cathodic protection systems that are most commonly used in the marine industry are (i) a sacrificial anode and (ii) Impressed Current Cathodic Protection (ICCP). US3488274 describes an electrolytic composite anode and connector. GB1467894 describes an anode for cathodic protection system. GB902667 describes an anode for cathodic protection of ships. US4420382 discloses reducing 'necking' of elongated cathodic protection anodes by placing a nonconductive shield spaced away from the anode. GB1347469 discloses an anode assembly comprising a pipe for use in cathodic protection of underwater structures.
An ICCP system generally uses anodes connected to a Direct Current (DC) power supply. Cathodic protection transformer rectifier units are powered by Alternating Current (AC) and are commonly used to provide DC for the ICCP system. The transformer rectifier unit output DC negative terminal is normally connected to the ship or other structure to be protected. The transformer rectifier unit output DC positive terminal is normally connected to one or more external anodes.
On a ship the DC power supply is provided within the ship and the anodes mounted on the outside of the hull. The anodes are generally shaped to reduced drag in the water and are typically mounted flush to the hull.
The ICCP system presents an effective means of corrosion protection only if it is submerged in a conductive fluid medium. Anodes do not work in air due to their limited charge-carrying capability.
The ICCP method of cathodic protection involves the direct application of current from an external power source to surfaces prone to corrosion. The basic principle of the ICCP method is the vulnerable metal is supplied with a surplus of electrons. The excess electrons reduce the potential of the metal (cathodic polarization) and tend to drive the anodic corrosion reaction in reverse. This results in a reduced corrosion rate.
ICCP systems are often fitted during the floating production asset's (FPSO) construction or conversion period in a shipyard and are generally designed to meet the specifications required for trading ships. However, ship specifications do not take account of the additional complexities of a floating production unit. These include issues such as the mooring chains and risers and the constantly changing draft of the hull as it loads and discharges. Neither do the ship specifications take account of the deterioration of the coating that might occur over the extended life of the asset at sea compared to a trading ship.
Although ships may periodically be removed from the water for inspections and maintenance, this process of drydocking the ship is expensive. In the case of FPSOs the process is significantly more expensive and further loss is incurred because of deferred production. It may therefore be beneficial if the installation and/or maintenance of the ICCP system on a ship did not rely on the removal of the ship from the water.
An FPSO vessel may need to remain at sea for up to twenty-five years without coming into a drydock for repainting or renewal of the cathodic protection system. Subsea intervention to maintain or replace cathodic protection systems is costly and hazardous and involves operational disruption. There are also often regulatory reasons why welding underwater is difficult or not allowable.
It is an object of the present invention to provide a method of installing and/or maintaining and/or replacing an ICCP system for controlling the corrosion of the hull of a vessel whilst it is in the water.
According to a first aspect of the present invention there is provided a Floating Production, Storage and Offloading (FPSO) vessel anode for controlling the corrosion of a hull of a FPSO vessel, as defined in the claims. In at least some examples, the anode includes:

a first portion being electrically conductive;
a second portion being electrically insulative; and
a connector being electrically insulative;
wherein the connector is attached to an end of the second portion and the first portion is attached at an opposite end of the second portion, the first and second portions having a width of equal to or more than 5cm and a total combined length of equal to or more than 100cm.

The first portion may be connected to a source of an electrical current. The electrical current from the source may be transmitted from the source to the first portion and then to the vicinity of the anode. It may be an advantage of the present invention that the second portion has sufficient length so that the first portion is sufficiently far away from the hull of the vessel such that the electrical current is transmitted away from the hull and any electrical current damage to the hull in the vicinity of the anode is mitigated.
The first and second portions may have a width of from 5 to 100cm, optionally from 5 to 25cm and a total combined length of from 50 to 1000cm and optionally from 100 to 400cm. The length of the second portion may be such that electricity discharged from the first portion does not damage the hull coating or metal substrate. The length of the second portion may be equal to or more than 25cm, optionally equal to or more than 150cm, and may be equal to or more than 200cm in length.
The first portion may have an electrically conductive outer surface. The second portion may have an electrically insulative outer surface.
The electrically conductive first portion may comprise a material that has an electrical conductivity of more than or equal to 0.02 x 10⁶ / cm ohms (Ω). The electrically insulative second portion may comprise a material that has an electrical conductivity of less than or equal to 0.001 x 10⁶ / cm ohms (Ω).
The connector is typically electrically insulated from the first portion.
The connector may have a flange portion. The width of the flange portion may be greater than the width of the second portion. The flange may have an engagement surface facing the first portion. The engagement surface may include a seal. The seal may commonly be used to seal the anode to an internal surface of a vessel, for example the hull.
The first and/or second portions and/or connector may be cylindrical. The width may be the diameter of the cylinder. When the connector is cylindrical in shape the connector may include a threaded portion on its outer surface for cooperation with another threaded portion on a surface or component of the hull.
The second portion may perform the same function as a dielectric shield. The second portion may reduce the possibility of shorting of electrical current from the anode to the hull of the vessel adjacent to the anode. This may have the advantage of allowing the anodes to be used to distribute electrical charge more widely and/or may reduce the occurrence of the localised breakdown of coating(s) on the hull of the vessel in the proximity of the anode.
The second portion may be made of plastic. The second portion may be referred to as a boss.
The anode according to the present invention may be resistant to corrosion. The anode may have one or more of a low resistance to current flow, physical toughness, a low rate of consumption, and a low cost of production.
The anode may comprise platinum. In an alternative embodiment the anode may comprise a mixed metal oxide. The consumption of platinum, that is the degradation of the platinum over time is low compared to other metals, for example carbon steel, aluminium and zinc. The anode may alternatively comprise titanium. The titanium may be coated with a layer of platinum. The layer of platinum may be from 0.1 to 5mm, optionally from 0.5 to 5mm thick.
There may be provided an apparatus for controlling the corrosion of the hull of the vessel, the apparatus including:

the anode as defined in the claims; and
a pressure-tight container;
wherein the pressure-tight container has an engagement surface for engagement with the anode and an isolation valve and wherein in use, the pressure-tight container is attached to an inside surface of the hull.

It may be an advantage of the present disclosure that the pressure-tight container holds the anode against the inside surface of the hull so that the anode can be installed and maintained from inside the vessel whilst the vessel is in the water and/or in operation or at station. This may mitigate the need for diver or ROV intervention and/or taking the vessel into a drydock during installation or maintenance or replacement of the apparatus.
The engagement surface of the pressure-tight container may have a threaded portion. One end of the anode may have a threaded portion that may cooperate with the threaded portion of the pressure-tight container. The threaded portion of the anode may comprise a connector.
The anode of the present invention may mitigate the need for a dielectric shield. Using other ICCP systems the anode must be surrounded by a thick shielding material (often referred to as "capastic coating" or "capastic epoxy"). The coating typically comprises a high-solids epoxy with high dielectric strength. The shielding mitigates shorting of the anode electrical current to the hull near the anode and aids in wider current distribution to the hull. The dielectric shield includes an inner and outer shield and covers an area around the anode. The shield is top coated with anti-corrosive and anti-fouling coatings. The shielding deteriorates over time and eventually requires replacement. The anode of the present invention may mitigate these disadvantages.
The term pressure-tight refers to the ability of the pressure-tight container to withstand an internal pressure of from 0 to 21 bar, normally from 0 to 11 bar and typically from 0 to 3.5 bar without leakage therefrom.
The apparatus may include one or more reference electrodes. The reference electrodes may be used remotely monitor the electrical current supplied by the anode and can be used to control the electrical current delivered by the transformer rectifier to the anode.
The apparatus may include a power supply. The power supply is typically provided by an electrical power generator on-board the vessel. The apparatus may include a transformer rectifier to convert Alternative Current (AC) into Direct Current (DC). The electrical power generator will typically generate an AC signal and the anode will typically generate a DC signal.
The power supply may be transported from the electrical power generator to the anode by cable. The cable may pass through a valve mounted on an outer wall of the pressure-tight container. The valve may provide a pressure-tight seal and therefore may mitigate any escape of water or other fluid through the valve along the path of the cable from outside the hull of the vessel, through the pressure-tight container and into the vessel.
It may be an advantage of the present invention that the valve, through which the cable passes, can be shut or closed in the event of a leak or penetration of water into the pressure-tight container before the water leaks into the inside of the hull of the vessel.
The hull of the vessel may comprise steel. The hull may be susceptible to one or more of corrosion, erosion and any other form of degradation that weakens or damages the structural integrity of the hull.
The pressure-tight container may comprise a first and a second pressure-tight container. The first pressure-tight container may be a stub pipe. The first pressure-tight container may have a second mating surface for mating with a first mating surface of the second pressure-tight container. The second mating surface of the first pressure-tight container and the first mating surface of the second pressure-tight container may be a flange with a seal and may include apertures through which bolts can be passed to secure or hold the two flanges together, generating a pressure-tight seal there between.
The pressure-tight container may have a mating surface for mating with the hull of the vessel. The mating surface of the pressure-tight container may be welded to the inside surface of the hull.
The pressure-tight container may include one or more drain valves and/or pressure gauges. These allow the fluid pressure inside the pressure-tight container to be monitored and controlled by an operator, thereby ensuring safe operation of the apparatus and continuous safe operation of the vessel.
The threaded portion of the anode, the connector, may include a seal wherein in use there may be a pressure-tight seal between the threaded portion of the anode and the hull of the vessel, when the threaded portion of the anode is mated against the hull, the threaded portion of the anode holding the anode in place relative to the pressure-tight container.
The pressure-tight container(s) may be cylindrical. The seals may be o-rings.
The pressure-tight container may have a second mating surface. A blanking plate may be secured to the second mating surface of the pressure-tight container. The isolation valve and the blanking plate may provide two further barriers against potential water escape through the hull and apparatus and into the vessel.
The apparatus for controlling the corrosion of a hull of a vessel may be suitable for controlling the corrosion of subsea metal structures, including for example offshore oil and/or gas platforms.
Features of the second aspect of the present disclosure may be incorporated into the first aspect of the present invention and vice versa.
According to a second aspect of the present invention there is provided a method of attaching a FPSO vessel anode to the hull of a FPSO vessel as defined in the claims. In at least some examples, the method includes the steps of:

attaching a container to an inside surface of the hull of the vessel, the container having a through-bore and two open ends, one of the open ends being attached to the inside surface, the container including an isolation valve;
fitting a plug in the remaining open end of the container, the plug having an aperture;
pushing a cutting device in through the plug and through-bore of the container, one end the cutting device extending beyond the container and the other end being in contact with the hull of the vessel; and
cutting a hole in the hull of the vessel using the cutting device, operated from outside the container, and pushing the anode according to the first aspect out through the hole in the hull of the vessel.

It may be an advantage of the method according to the present invention that the cost of taking a vessel to a drydock can be saved without compromising on safety and/or the integrity of the hull.
The cutting device is typically operable through the isolation valve when the isolation valve is open.
The method may include the step of pulling the cutting device out through the through-bore of the container through the plug, such that the cutting device remains in the plug but is clear of the isolation valve.
The method may include the steps of closing the isolation valve; and removing the cutting device from the container.
The method may include the step of pushing a brushing device in through the plug; opening the isolation valve; and pushing the brushing device through the through-bore of the container, one end the brushing device extending beyond the container and the other end being in contact with the hole in the hull of the vessel. The brushing device may be used to remove swarf from around edges of the hole. The brushing device may be a wire brush.
The method may include the steps of pushing an anode through the plug; opening the isolation valve in the container; pushing the anode in through the through-bore of the container and out through the hole in the hull of the vessel.
The method may include the step of securing the anode to the container thereby sealing the anode to the hull of the vessel.
The method may include the steps of attaching an electrical cable through a valve mounted on an outer wall of the container and connecting the electrical cable to the anode to provide electrical communication between the anode and inside the hull of the vessel.
The method may include the steps of closing the isolation valve; removing the plug from the container; and fitting a blanking plate to the remaining open end of the container.
The cutting device may be a hollow cutting device. The cutting device may cut a core from the hull. The cutting device may cut a hole in the hull having a diameter of from 5 to 100 cm, optionally from 5 to 25cm.
The seal between the anode and the hull of the vessel is typically pressure-tight and therefore also water-tight.
The step of attaching the container to an inside surface of a hull of a vessel may include welding.
The method may include the step of one or more pressure tests to check that the container or other component thereof will withstand the water pressure exerted on them and the seals between the container and the hull by the water outside the hull of the vessel.
The method may include the step of draining water or another fluid from the container before the isolation valve is opened.
The container may comprise a first container and a second container. The first container may be attached to an inside surface of the hull of the vessel, the first container having a through-bore and two open ends, one of the open ends being attached to the inside surface. The second container may be attached to the remaining open end of the first container, the second container including the isolation valve and having a through-bore and two open ends, one of the open ends of the second container being attached to the remaining open end of the first container.
The plug may be a 'hot tap' type connection. The plug typically fits the remaining open end of the container, providing a pressure-tight seal against the container. The aperture in the plug is typically at its centre through which the cutting device, brushing device and/or anode can be pushed and pulled. The aperture may be sized such that there is a pressure-tight seal between the cutting device and anode and the plug.

Brief description of the drawings

Embodiments of the invention are further described hereinafter with reference to the accompanying drawing, in which:
Figure 1 is a schematic drawing of the apparatus for controlling the corrosion of a hull of a vessel with an anode according to the present invention.

Detailed description

Figure 1 shows an apparatus 10 for controlling the corrosion of a hull 12 of a vessel (not shown). The anode 30 protrudes through and outside 14 the hull 12 through a hole 18 in the hull 12. The first 50 and second 70 pressure-tight containers are inside 16 the hull 12. Figure 1 shows the apparatus 10 that is left attached to the hull 12 after installation of the anode 30.
The anode 30 has a first portion 32 that has an electrically conductive outer surface 33 comprising an electrically conductive material and an anode connection point 37 that is in electrical communication with a power source (not shown) inside 16 the hull 12. A second portion 34 (or anode standoff) of the anode 30 has an electrically insulative outer surface 35 comprising a non-conductive material. A connector 36 at the end of the second portion of the anode 34 has a threaded outer surface that is electrically insulative. The threaded outer surface of the connector 36 cooperates with a threaded surface on the inside 54 of the first pressure-tight container 50.
The threaded surfaces may be tapered and may therefore not require any additional means of sealing. In Figure 1 however, the anode 30 is sealed against the hull 12 using an o-ring 38 that seals around the hole 18 in the hull 12.
The first pressure-tight container 50 is welded 51 at one end to an inside surface 52 of the hull 12. At the other end of the first pressure-tight container 50 there is a flange 53. Inside 54 the first pressure-tight container 50 is hollow.
The second pressure-tight container 70 has flanges 71 and 72 at both ends. The flange 53 of the first pressure-tight container 50 is attached to the flange 71 of the second pressure-tight container 70. Inside 76 the second pressure-tight container 70 is hollow.
The second pressure-tight container 70 also has an isolation valve 73 that can be raised and lowered (opened and closed) in the container 70 to provide or prevent fluid communication between the flanges 71 and 72 at the two ends of the container 70. The isolation valve is moved using a wheel 74 and gearing system (not shown).
Figure 1 shows the isolation valve 73 in its closed position and a blanking plate 80 attached to the second pressure-tight container 70. The blanking plate 80 has a flange 81 that is attached to the flange 72 of the second pressure-tight container 70.
The first 50 and second 70 pressure-tight containers have drain valves 55, 75 respectively. These drain valves 55, 75 allow the user to drain water from the first 50 and second 70 pressure-tight containers when the isolation valve 73 is closed and the blanking plate 80 is attached to the second pressure-tight container 70.
The first 50 and second 70 pressure-tight containers are cylindrical. The first 50 and second 70 pressure-tight containers have an internal diameter of 20cm and a total combined length of 100cm. The first pressure-tight container 50 extends 50cm beyond the second pressure-tight container 70, away from the hull 12.
The first portion 32 of the anode 30 is made of titanium and is coated with a layer of platinum (not shown). The second portion 34 of the anode 30 is made of polyethylene. In an alternative embodiment the second portion 34 is made of plastic.
The first 32 and second 34 portions of the anode 30 make up a composite anode. The insulating stand-off arrangement, provided by the second portion 34, provides the required electrical field and reduces the occurrence of electrical charge shorting back from the first portion 32 of the anode 30 to the hull 12 in the proximity of the anode 30. The insulating stand-off or second portion 34 alleviates the need for a dielectric shield around the anode 30. This allows the anode 30 to be installed either during the construction phase, or whilst the vessel (not shown) is in the water. This is particularly important when fitting the system to a Floating Production, Storage and Offloading (FPSO) type vessel which is required to stay on station at sea for an extended period without going into drydock.
A valve 90 is mounted on the first pressure-tight container 50. The valve 90 provides a pressure-tight seal around the electrical cable 91a, 91b that passes from the anode connection point 37, outside the hull 12 of the vessel (not shown), through the first pressure-tight container 50 and on into the inside 16 of the hull 12.
The anode described above and method of installing the anode is designed such that the anode can be installed working from inside the vessel and whilst the vessel remains in the water. It will however be appreciated by the skilled reader that the anode could be installed and the method of installing the anode of the present invention could be used from outside the hull of the vessel, for example when the vessel is in a drydock or using a diver when the vessel is in water.
It will also be appreciated by the skilled reader that the method of attaching an anode to the hull of a vessel according to the present invention could likewise be used to attach other devices to the hull of the vessel, these devices including one or more of sensors, reference electrodes, cameras and sacrificial anodes.
Improvements and modifications may be made to the apparatus shown in Figure 1. For example, the stub pipe and isolation valve body may have a square rather than cylindrical cross-section.

Claims

A Floating Production, Storage and Offloading (FPSO) vessel anode (30) for controlling the corrosion of a hull (12) of a FPSO vessel, the anode including:
a first portion (32) being electrically conductive;

a second portion (34) being electrically insulative; and

a connector (36) being electrically insulative;

wherein the connector (36) is attached to an end of the second portion (34) and the first portion (32) is attached at an opposite end of the second portion (34) from the connector , the first and second portions (32, 34) each having a width of equal to or more than 5cm and a total combined length of equal to or more than 100cm; and

the anode (30) being configured for protruding through and outside the hull (12) through a hole (18) in the hull (12).
An FPSO vessel anode (30) according to claim 1, wherein the electrically conductive first portion (32) comprises a material that has an electrical conductivity of more than or equal to 0.02 x 10⁶ / cm ohms (Ω) and the electrically insulative second portion (34) comprises a material that has an electrical conductivity of less than or equal to 0.001 x 10⁶ / cm ohms (Ω).
A method of attaching a FPSO vessel anode (30) according to claim 1 or 2 to a hull (12) of a Floating Production, Storage and Offloading (FPSO) vessel, the method including the steps of:
attaching a container (50) to an inside surface (52) of the hull of the vessel, the container having a through-bore and two open ends, one of the open ends being attached to the inside surface of the hull, the container including an isolation valve (73);

fitting a plug in the remaining open end of the container, the plug having an aperture;

pushing a cutting device in through the plug and through-bore of the container, one end the cutting device extending beyond the container and the other end being in contact with the hull of the vessel;

cutting the hole (18) in the hull of the vessel using the cutting device, operated from outside the container; and

pushing the anode according to claim 1 or 2 out through the hole in the hull of the vessel.
A method according to claim 3, wherein the cutting device is operable through the isolation valve (73) when the isolation valve is open.
A method according to claim 4, wherein the method further includes the steps of removing the cutting device from the container (50); and closing the isolation valve (73).
A method according to any of claims 3 to 5, wherein the method further includes the steps of pushing a brushing device in through the aperture in the plug; opening the isolation valve (73); and pushing the brushing device through the through-bore of the container (50), one end the brushing device extending beyond the container and the other end being in contact with the hole (18) in the hull (12) of the vessel.
A method according to any of claims 3 to 6, wherein the method includes the steps of pushing the anode (30) in through the aperture in the plug; opening the isolation valve (73); and pushing the anode through the through-bore of the container (50).
A method according to claim 5, wherein the method includes the step of securing the anode (30) to the container (50) thereby sealing the anode to the hull (12) of the vessel.
A method according to any of claims 3 to 8, wherein the method includes the steps of attaching an electrical cable (91a, 91b) through a valve (90) mounted on an outer wall of the container (50) and connecting the electrical cable to the anode (30) to provide electrical communication between the anode and inside (16) the hull (12) of the vessel.
A method according to any of claims 3 to 9, wherein the method includes the steps of closing the isolation valve (73); removing the plug from the container (50); and fitting a blanking plate (80) to the remaining open end of the container.
A method according to any of claims 3 to 10, wherein the step of attaching the container (50) to the inside surface (52) of the hull (12) of the vessel includes welding (51).
A method according to any of claims 3 to 11, wherein the method includes the step of draining one or more of water and another fluid from the container (50) before the isolation valve (73) is opened.