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Offshore Cathodic Protection
Submarine Pipelines Anodes (cathodic protection)
The Subsea Corrosive Environment
Corrosion of a submerged metal is an electrochemical reaction between the metal and its environment which results in wastage of the metal. Thus corrosion is a combination of chemical effects with an associated flow of electrical energy (corrosion current). In many practical situations where it is impossible to change the nature of the environment, corrosion may be prevented by employing cathodic protection. The natural tendency of the metal to react with its environment is prevented by the application of an appropriate direct current to the structure surface.
Two types of cathodic protection system are available:
- Galvanic anode systems, which employ buried or immersed metallic anodes which sacrifice themselves to provide the source of direct current for protection of the structure.
- Impressed current systems, which employ an external electrical power source of direct current for the protection of the structure.
Corrosion control for a structure should be considered at the conceptual design stage. The practices recommended relate to steps that need to be taken following a decision to apply cathodic protection to a structure. These steps are as follows:
- Decide whether the structure should be coated. If the decision is to coat, then decide what particular coating system should be employed. If the structure is already installed, a determination should then be made of whether the nature and quality of the coating are compatible with cathodic protection.
- Design the structure to be compatible with cathodic protection and to include cathodic protection facilities during construction. If the structure is already installed, determine the measures to be taken to apply cathodic protection effectively, and the facilities necessary for cathodic protection monitoring.
- If necessary, design the cathodic protection system to include provisions for the mitigation of stray current effects. If the structure is already installed, the design parameters may be measured and an optimum design provided for the mitigation of stray current effects. If the structure is not installed, a number of assumptions will be required for the estimation of design parameters, and an adequate design for the mitigation of stray current effects may not be possible to achieve. During the cathodic protection design stage, consideration should be given to the possibility that interference with foreign structures in the area may occur. In some circumstances this interference may only be resolved by field testing subsequent to installation.
- Install the cathodic protection system. NOTE: Legislation in some areas requires that a permit be obtained prior to the installation of a cathodic protection system.
- Commission the cathodic protection system after achieving a balance of cathodic protection current, to enable the entire structure to be protected with minimum current, and with as uniform a potential over its surface as is practicable. Equipment installed for the mitigation of stray current effects should be balanced for optimum performance. Carry out interference testing and satisfy all parties involved that any interference problems have been resolved, giving attention to regulatory requirements (if any) of the State in which the system is installed.
- Monitor cathodic protection at regular intervals, adjusting the conditions of operation as necessary, and maintain complete records of its operation.
This type of protection is usually made of sacrificial zinc and aluminum type of anodes. They are used in all types of offshore constructions, vessels, ships and submarine pipelines. Depending on the subsea environment corrosiveness properties, the anodes are engineered and the required weight and shape is calculated and fabricated.
For submarine pipelines, the anodes are engineered in order to protect the steel pipelines as long as the design life of the pipes (typically 15-20 years) because changing them every once in a while would be very expensive. During the pipe line installation from the offshore pipe lay vessel, anodes are installed at intervals of maximum 300 meters (as per the codes of practice - DNV), and their size and weight is based on the required anode mass for the total length of the subsea pipeline system.
Anodes are also attached on the offshore platforms, on the jacket legs. They are replaced from time to time when the old ones are corroded up to a degree that the steel jacket legs might be in danger of being exposed to the environment.
Many metals are protected from corrosion by the application of direct current which maintains the potential sufficiently negative with respect to its environment to prevent corrosion. Direct current is provided by the use of galvanic anodes, or by means of an impressed current system. The potential of a structure with respect to its environment can give a reliable indication of the degree of protection being provided. The potential criteria to provide protection are given by the engineering design of specific requirements. These criteria are with reference to either a copper/copper sulfate electrode or a silver/silver chloride electrode. This latter electrode is commonly used in marine and brackish water conditions, whereas the former is used on land and in fresh water conditions. Either electrode can be used in both situations; however, the copper/copper sulfate electrode can become rapidly contaminated by chloride ions whereas the silver/silver chloride electrode gives differing readings, depending on the concentration of the chloride in the environment.
Other reference electrodes may be used as alternatives to copper/copper sulfate and silver/silver chloride, provided that their relativity to these reference electrodes has been established. Criteria other than those listed may be used, if their efficacy has been established.
Typical Anode Arrangements
Anodes should be placed where they will achieve the desired spread of protection for all the
surfaces of the primary structure, and will cause minimal interference to secondary structures. The placement of anodes will depend upon a number of factors including the following:
- Site accessibility and availability.
- The electrolyte resistivity.
- The structure layout and location, and the presence of any foreign structures.
- The extent of structure coating.
- The geometric layout of the structure.
- The need to avoid excessive potential variations in the structure by maintaining appropriate anode separation.
- Whether any parts of the structure require a higher level of protection (e.g. the presence of more electro-positive metals).