technical seminar for cathodic protection to gogc …. complex_structures_technical seminar... ·...

Technical Seminar for Cathodic Protection

to GOGC Design Unit Specialists

Dr. Nick Kioupis, Cathodic & Lightning Protection

Section Head, DESFA

Cathodic Protection for Complex Structures

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.


Eon ≤ - 1,2 Vcse

if the measuring point outside the area of influence of the large

cathode (e.g. reinforced concrete or copper) and if ρsoil < 100 Ω.m.

Eon < - 0,8 Vcse

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 1

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.


Structures

CRITERION 2

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


Structures

CRITERION 3

Thank you for your attendance


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