Development and Testing of New Offshore Cathodic Protection Criteria
Authors: Evans S. 
Published: October 2008

New sacrificial anode protection criteria were developed and applied to the cathodic protection system of Platform "JJ" which was installed in the Arabian Gulf in September 1982. The new design resulted in a 40 percent decrease in anode weight, and with 15 fewer anodes than required by conventional accepted design. Potential surveys carried out one month and nine months after installation showed potentials of the order of -1.0 volt with respect to the silver-silver chloride reference electrode. The calculated lifetime for this 20-year design is 26 years. The basis for the new design criteria will be presented and discussed.

INTRODUCTION

The cathodic protection design chain used for Platform "JJ" is primarily based on a high initial platform current density that insures the rapid development of a protective calcarious deposit. As a consequence the current requirement for protection is significantly reduced.

The design current density for cathodic protection in the Gulf of Mexico is 6 MA/ft. In sea water at Kure Beach, North Carolina, a current density of 3MA/ft was found to protect steel after the initial deposition of a calcareous coating. Subsequent work has confirmed that after the formation of a calcarious deposit, a current density between 3 and 3.5 MA/ft protects steel structures in the Gulf of Mexico. ' Utilizing present design criteria, it has been shown that platforms designed for a 20-year life will last for 33 years in the Gulf of Mexico. It is well established that high initial current densities result in the rapid development of a calcareous deposit.

In recent field experiments in the North sea,7 high initial current densities resulted in protective potentials within a few days after immersion. While the recommended design value for the Northern North Sea is 11 MA/ft, the current density determined after the calcareous deposit had formed was less than 5 MA/ft. Retrofitting of sacrificial anodes was recently completed on three platforms in the North Sea. These platforms were previously protected with an impressed current system. The recorded current densities on each of the three platforms were less than 5 MA/ft

DESIGN BASIS

Recently, a comprehensive study was made of the cathodic protection systems of seven platforms that have been installed in the Arabian Gulf within the last six years. The cathodic protection systems for these platforms were consistent with accepted design criteria. Field measurements clearly showed that the accepted cathodic protection design resulted in polarizing the platform beyond the minimum structural protective potential of -0.80 volt with respect to the silver-silver chloride electrode. Representative potentials determined one month and one year after installation are shown in Tables 1 and 2. It was concluded that, in the Arabian Gulf' an initial platform current density of 29 ± 2 MA/ft resulted in platform potentials of the order of -1 volt with respect to the silver-silver chloride electrode, within one month after installation. The lifetime of the 20-year design anodes was calculated to be 39 years.