Remote Monitoring Of Large CP System Saves Money In South America

Transportadora Brasileira Gasoduto Bolívia-Brasil S.A. (TBG) is saving about US$32,600/month in maintenance costs by employing a remote technique to monitor the cathodic protection system on the GASBOL pipeline.

Transportadora Brasileira Gasoduto Bolívia-Brasil S.A. (TBG) is saving about US$32,600/month in maintenance costs by employing a remote technique to monitor the cathodic protection system on GASBOL, the gas pipeline extending nearly 2,000 miles from Bolivia to Brazil.

Monitoring is conducted at a Central Station in Rio de Janeiro. The CP system uses 41 rectifiers mounted along the gas pipeline to control the electrochemical corrosion caused by the soil. Good performance by the system has allowed TBG to modify the inspection interval from once a week to once a month.

The GASBOL is a 32-inch pipeline running from the gas-producing fields of Rio Grande, Bolivia, to Canoas, Brazil. It distributes gas to five states in Brazil with a total length of 1,970 miles and a rated operating pressure of 1,420 psig. GASBOL is owned and operated by TBG S.A., a company whose shareholders are Petrobras, El Paso, Prisma, Shell, Total Fina, British Gas and YPFB.

Remote CP monitoring uses a remote terminal unit (RTU) to collect the information of a rectifier’s input/output voltage and output current. The RTU is able to store all the information available for 35 days and calculates soil resistance and system reliability. The RTU is fed by the rectifier. Communication with the control and supervision center (CSC) is accomplished using satellite telephony and can be initialized by either the RTU or the CSC.

Cathodic Protection
Corrosion is the result of an electromechanical reaction driven by a difference in electrical potential between two electrodes, an anode and a cathode, connected by an electronic path and immersed in the same electrolyte. In the case of uniform corrosion, a multitude of microscopic anodic and cathodic sites exist on the surface of the metal structure.

The rectifier is fundamental in an impressed current system as the rectifier provides the electrical current needed to polarize the entire pipeline to the potential electrolyte polarization equal to or more than -085 volts.

The concept of cathodic protection involves reducing the potential difference between the local anodic and cathodic sites to zero, resulting in zero corrosion current flow. This can be accomplished by impressing current onto the structure from an external electrode and polarizing the cathodic sites in an electronegative direction. As the potentials of the cathodic sites polarize toward the potentials of the anodic sites, corrosion current is reduced. When the potentials of all cathodic sites reach the open circuit potential of the most active anodic sites, corrosion is controlled at negligible rates on the structure. The structure is now the cathode of an intentional macroscopic corrosion cell.

The main components of an impressed current cathodic protection system are anodes, a power supplier (rectifier), wiring and connections. The anodes used in impressed current CP systems are different from those used in galvanic systems. Impressed current anodes are manufactured from materials that are consumed at low rates. Impressed current CP systems generally operate at higher current and driving voltage levels than galvanic anode CP systems.