The Murchison Cathodic Protection System

ABSTRACT The Cathodic Protection System installed on the Murchison Platform sets new standards for the operation of cathodic protection in the arduous conditions of the North Sea. It has demonstrated its effectiveness and flexibility since being commissioned in November 1980. Extensive data has been accumulated from the system monitors, comprising zinc reference potentials, current outputs from impressed and monitored sacrificial anodes, and from underwater surveys of potential and current density. An improved understanding of the dynamics of cathodic protection systems can only be achieved through analysis of data from actual structures. The nature of the Murchison cathodic protection data has enabled computer modelling1 of characteristic current density and potential optimum protection level is projected from this model and the hybrid system adjusted to satisfy the demands of the structure. This paper briefly describes the system, the stages to successful operation and indicates the developments with which this unique system has enhanced our understanding of cathodic protection parameters. INTRODUCTION The Murchison jacket was to be the largest ever launched by barge when in September 1979 the limited launch barge capacity required a weight saving alternative to the proposed 1200 Tonnes of sacrificial aluminium anodes. Various options were considered 2 and the system adopted for Murchison is an unique hybrid. The main impressed current system is designed to automatically maintain potentials within set limits using numerous impressed current anodes which are mounted directly on the structure and controlled in small groups by nearby reference cells. A sacrificial system designed to provide protection to sensitive areas during period when the impressed system is switched off is also installed. Sacrificial anodes modified to monitor current output are located strategically on the submerged structure. The system was designed to comply with the cathodic protection design criteria available in 1976/1977, i.e. that a current density of 130mA/m2 is necessary to ensure adequate initial polarisation of the structure. Basic parameters of the design comprise:-A sacrificial system to provide limited levels of protection at sensitive areas for the period between float out (Sept. 179) and commissioning (Nov. 180) and when de-energisation of the impressed current system is necessary.An adequate monitoring system to enable effective control and monitoring of the system.An automatically controlled impressed current system using suitable anodes located directly on the structure.Robust design of the underwater parts of the system to ensure maximum integrity for the life of the platform, together with facilities for retrofitting of additional anodes if required. The main parts of the system equipment are summarised in Table 1, and are more fully described elsewhere.2,3.