Cathodic Protection Pre-Design Survey, Testing and Considerations
Existing Structures
This section is applicable for structures, which don’t have CP systems or their CP systems need to be upgarded.
Prior to CP design and application, appropriately trained and experienced engineers shall perform a condition survey of the structure to:
a. Determine the structure condition
c. Determine extent of coating deterioration.
d. Confirm suitability or upgradation of CP.
e. Provide system design input information.
Records
a. Prior to the start of site survey, available design drawings, specifications, records and previous survey reports shall be reviewed to assess the location, quantity, type of both CP components and the structures, and continuity of the structures.
b. Previously repaired areas shall be identified, together with their repair methods and materials.
c. The buried discontinuous strcutures in areas to be cathodically protected shall be identified.
d. Quality of the structure coating shall be assessed for suitability or upgaradtion of CP.
e. All nearby strctures, which are either protected by CP systems or not, shall be determined to avoid stray current corrosion.
The Contractor is required to make a site visit to perform the following:
a. Visual Inspection
b. Structure Continuity
c. Potential Measurement
d. Electrolyte Resistivity
e. The corrosivity estimation
f. A study of stray current interference
This is required to collect data for cathodic protection design calculations and select suitable location for installing anode ground beds.
Visual Inspection
a. All accessible parts/elements of the structure shall be visually inspected as closely as possible to determine the type, causes and extent of defects, and to identify any features of the structure or its surrounding environment which could influence the application and performance of CP system.
Structure Continuity
a. For successful operation of a CP system, the buried structure shall be electrically continuous, otherwise the discontinuous strucures will be subject to stray current corrosion. Therefore, structure layout drawings shall be checked for theoretical continuity. This shall then be proved on site by measuring the continuaty between different parts of the structure.
b. Continuity testing at this stage shall only be conducted for the purpose of confirming CP feasibility and providing design information, and confirming the locations of bonding when CP will be required.
Potential Measurement
a. Potential measurements shall be conducted along the structure using a poratble reference electrodes and digital multimeter.
b. Potential measurements shall be carried out at all soil access holes and all accessible areas. Also, all buried reference electrodes shall be tested.
c. Portable reference electrode shall be a saturated copper/copper sulphate (Cu/CuSO4 ) or silver/silver chlorode and shall be used directly over the structure surface.
d. Portable reference electrodes shall be supplied with a calibration certificate. They shall be stored, maintained and handled in accordance with the manufacturer’s instructions, and checked against a known laboratory standard reference electrode, or similar, at the beginning and end of each site application.
e. Potential measurement data shall be used to determine low protection areas. Some areas may then be selected for the placement of buried reference electrodes in the
detailed CP design.
Electolyte Resistivity
a. Resistivity of the structure in contact with soil or liquid in areas to be cathodically protected shall be measured using Wenner four pin method.
b. Soil/electrolyte box shall be used if the use of Wenner four pin method is not applicable.
c. Soil resistivity measurements shall be conducted at 20 m intervals over the full length of the proposed anode bed location. The Contractor shall take a minimum of ten (10) readings in each section of the facility to obtain an average of layer resistivity value for each section upon which the contractor shall base his design.
d. For ground bed location and design purposes, soil resistivity survey shall be measured with the terminals spaced at 1, 2, 3, 4 and 5 m intervals.
e. Resistivity and potential measurements on the area under investigation shall be conducted at the same time, to give an accurate basis for interpretation of potential measurement.
f. Measured soil or water resistivity shall be used in the design of the CP system.
Survey Report
The Contractor shall submit a written report documenting his site survey findings to SABIC STC. The written report shall include methodology, test conditions, observations, and resistivity values before proceeding with final design of system. The CP contractor shall also prepare an overall plan drawing of the facility showing layer resistivity values for each section, and his proposed location of equipment.
New Structures
When including a CP system in the original construction, to prevent corrosion of the structure to be protected, the CP specialist shall review the construction drawings to determine the feasibility and design requirements of the CP system, and its compatibility with the structure’s design and construction.
For successful operation of a CP system, the structure shall be electrically continuous, otherwise the discontinuous strucures will be subject to stray current corrosion. Therefore, structure layout drawings shall be checked for theoretical continuity. The discontinuous strcutures in areas to be cathodically protected shall be identified.
The design shall avoid the stray current interference with adjustent structures.
Electolyte Resistivity
a. Resistivity of the structure in contact with soil or liquid in areas to be cathodically protected shall be measured using Wenner four pin method.
b. Soil/electrolyte box shall be used if the use of Wenner four pin method is not applicable.
c. Soil resistivity measurements shall be conducted at 20 m intervals over the full length of the proposed anode bed location. The Contractor shall take a minimum of ten (10) readings in each section of the facility to obtain an average of layer resistivity value for each section upon which the contractor shall base his design.
d. For ground bed location and design purposes, soil resistivity survey shall be measured with the terminals spaced at 1, 2, 3, 4 and 5 m intervals.
e. Resistivity and potential measurements on the area under investigation shall be conducted at the same time, to give an accurate basis for interpretation of potential measurement.
f. Measured soil or water resistivity shall be used in the design of the CP system.
Survey Report
The Contractor shall submit a written report documenting his site survey findings to SABIC STC. The written report shall include methodology, test conditions, observations, and resistivity values before proceeding with final design of system. The CP contractor shall also prepare an overall plan drawing of the facility showing layer resistivity values for each section, and his proposed location of equipment.