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INTRODUCTION: NLNG Cathodic Protection Troubleshooting NLNG LTD owns and operates the Bonny Gas plant. This plant contains a network of pipes buried underground designed in separate piping circuits for cooling water, hot oil and amine. These lines are buried underground, but are coated and protected cathodically against corrosion, but due to the nature of different soil types around the submerged pipes, ground movement, difference in sub-soil temperature, varying soil resistances and
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    INTRODUCTION: NLNG Cathodic Protection Troubleshooting NLNG LTD owns and operates the Bonny Gas plant. This plant contains a network of pipes buried underground designed in separate piping circuits for cooling water, hot oil and amine. These lines are buried underground, but are coated and protected cathodically against corrosion, but due to the nature of different soil types around the submerged pipes, ground movement, difference in sub-soil temperature, varying soil resistances and a host of other factors there are always concerns as to the integrity of the pipeline coating and the CP system. As part of the strategy put in place by accepted standards to ascertain and maintain the integrity of pipelines, an ECDA (external corrosion direct assessment) is always carried out to ascertain the integrity of the cathodic protection system. From findings on taking pipe to soil potential readings it was discovered that there exist a problem with the system. As such, we (Polmaz Limited) were called in to trouble shoot and get exactly the real position of the total cathodic protection system for trains 1, 2, and 3. Most metallic structures have the tendency to corrode under certain conditions. Therefore, they should be protected against corrosion. The primary control of external corrosion on metallic structures is the application of integrity anti-corrosion external coating. Since no coating system/type can be perfectly applied to stop corrosion, all areas of buried pipeline which are unprotected other than the coating system and inclusive of damaged or scratched coatings are protected by the proper installation of a, well designed and properly maintained Cathodic Protection System . This is the accepted standard for protecting buried pipelines against the destructive effects of external corrosion and thereby enhancing the integrity of the operational life of the pipeline system. The function of the cathodic protection system is to protect the pipeline system by supplying a protective current to act against external metal loss particularly where the coating has been damaged. The idea is to make the potential of the pipeline total external surface area sufficiently negative with respect to the surrounding medium in which it is buried.   POLMAZ LIMITED  This is achieved by forcing an electric current (DC) to flow through the electrolyte (surrounding soil or water) towards the surface of the pipeline to be protected, thereby eliminating corrosion areas also called Anodic areas  thereby making the pipeline external surface Cathodic.  In the case of the NLNG piping network within the gas plant, the cathodic protection system, (the protective current), is obtained through an impressed current system comprising of TRANSFORMER RECTIFIERS and Anode   Ground Beds  located within the plant and close to the protected pipes. Therefore, for the adequate integrity of the buried pipeline section against external corrosion, the cathodic protection system must be in optimum working condition by supplying the needed protective current to the whole pipeline external surface throughout its entirely length. This is achieved only when the pipeline is maintained at a specified potential value provided by the cathodic protection system. This potential value is termed as the Cathodic Protection Criteria  which is not less than -0.850 VDC or -850mVDC . When the CP system is switched ‘’OFF’’  and the potential value measured instantly, the minimum value before depolarization occurs shall be no less than -0.850 VDC or -850mVDC. Anything below this value means that the pipeline at that point is not protected. In order to verify the integrity of the cathodic protection system and the external coating of the pipeline, a (Potential Survey)  is carried out.   POLMAZ LIMITED
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