Technical Seminar for Cathodic Protection to GOGC Design Unit Specialists Dr. Nick Kioupis, Cathodic & Lightning Protection Section Head, DESFA
Cathodic protection of complex structures is protection applicable to structures which need to be cathodically protected but cannot be electrically isolated, whether for technical or safety reasons, from foreign metallic structures situated in the same electrolyte as the structure to be protected. Such a structure is referred to as a complex structure EN 14505:2005 Cathodic protection of complex protection structures covers these situations
EN 14505 is not applicable to structures covered by EN 12954 (and draft CEN ISO 15589) or where unwanted contacts with foreign structures or defective isolation exists. It is always better to follow the requirements as described in EN 12954 (and draft CEN ISO 15589) before applying this standard.
The design and installation of CP systems for complex buried structures presents a unique challenge to the cathodic protection engineer engineer. There are a multitude of different materials, processes, operating temperatures, coatings, buried and immersed structures that can suffer from both internal corrosion caused by the products contained within the system and external corrosion caused by the environment in which it is situated.
The following structures would normally be considered to require cathodic protection. Pressurized carbon steel hydrocarbon pipelines Bottom or soil side of above ground storage tanks Buried storage tanks and sumps Carbon steel/ductile iron drain lines Buried portions of process, gas, water, firewater or liquids pipelines Buried portions of steel valves, hydrants, monitors and fittings
These structures are constructed from a number of different materials, e.g. carbon steel, ductile iron, high tensile steels, stainless steels, corrosion resistant alloys steel in concrete, galvanized steel, copper etc.
Plant facilities usually require a considerable amount of cathodic protection current because of the large surface areas of buried, often bare, metalwork and are therefore normally protected with an impressed current cathodic protection system unless soil resistivities are very low.
Current densities to protect the various metals vary considerably as shown in the table below: STRUCTURE MATERIAL SURFACE CURRENT DENSITY (ma/m2) Uncoated copper 50.00 or more Uncoated steel 20.00 Poorly coated pipe 5.00 Tape or PE Wrap 1.25 Coal tar enamel 2.00 Epoxy wet applied 0.75 Fusion bonded epoxy (FBE) 0.10 Extruded Polyethylene 0.10
The ICCP systems utilize a variety of anode configurations, e.g. distributed anodes, local or remote surface groundbeds installed horizontally or vertically, local or remote deep anodes or a combination of the above. When choosing and/or combining anode bed types, the conceptual and the final designs must be coordinated with other plant operators to ensure all existing facilities are known, together with any planned extensions or new facilities.
Remote vs. close Groundbed Flow of current from an anode to a structure will be accompanied by a potential difference between the earth and the pipeline. The potential difference is used in certain criteria for determining the degree of cathodic protection being afforded to the pipeline. This potential difference can be accomplished in either of two ways: by making the pipeline negative with respect to remote earth, i.e. a remote groundbed, or by making the earth positive with respect to the pipe in local areas, i.e. close anodes anodes.
For Complex Structures, two basic cathodic protection principles may be employed: 1. Isolated systems 2. Non-isolated or total system
Isolated systems where every single piece where of buried pipework or structure that requires to be cathodically protected is isolated by means of insulating joints / flanges where it comes above ground. This can give rise to a very large number of insulated flanges which in turn leads to maintenance and reliability problems.
NON ISOLATED SYSTEMS With the issue of EN 14505:2005 Cathodic protection of complex structures, a whole new approach was introduced and backed up by an International standard. The main obstacles with the plant operators for a non isolated system are : 1. They do not understand it and consider it is dangerous to have cathodic protection current flowing through all the pipework 2. That there were no authoritative standards recommending this principle. EN 14505:2005 changed all that.
A complex structure is defined as a structure to be protected and one or more foreign electrodes, which, for safety or technical reasons, cannot be electrically separated from it. The term complex structure does not refer to the complexity of the structure or to the complexity of the cathodic protection system but more to the fact that there are different types of structure and metals used.
In a complex structure, all metallic parts of the structure included in the cathodic protection scheme must be electrically continuous. Isolating joints in incoming or outgoing pipelines are the only isolation required and these should be located outside the zone of influence of the cathodically protected complex structure so that unacceptable interference by the cathodically protected complex structure is avoided.
CRITERIA for Cathodic Protection of Complex Structures CRITERION 1 E on - 1,2 V cse if the measuring point outside the area of influence of the large cathode (e.g. reinforced concrete or copper) and if ρ soil < 100 Ω.m. E on < - 0,8 V cse is sufficient at entries to, and in the vicinity of, reinforced concrete structures and large copper earthing systems acting as foreign cathodes (i.e the formation of a galvanic cell with steel in concrete is prevented)
CRITERIA for Cathodic Protection of Complex Structures CRITERION 2 Demonstration that current is able to enter the structure either: a) directly (i.e. when the protection current is switched on, a negative shift of the free corrosion potential En by at least 0,3 V will indicate that current is entering the structure); or b) by means of test probes or coupons at critical locations of the structure.
CRITERIA for Cathodic Protection of Complex Structures CRITERION 3 A positive shift (depolarisation) on test probes or coupons of at least 0,1 V measured 1h after they have been disconnected from the structure to indicate that the structure is polarized. The test probes/coupons are temporarily disconnected only for measurements
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