ABSTRACT

Evaluation of the performance of cathodic protection (CP) systems used on subsea structures once in service can be costly due the need for specialized personnel and ROV monitoring equipment. Often areas of a subsea structure can go unassessed in terms of CP coverage when the ability to assess the entire structure is not feasible. Improperly protected areas can lead to premature corrosion or structural integrity issues, or costly maintenance and retrofits. The use of computer software can aid in identifying improperly protected areas. Boundary element analysis (BEA) of a CP system, validated with past inspection data, can produce a computer model that will allow for predictive assessments of the present and future states of the CP system and subsea structure. Identification of under/over protection ‘hot spots’ by using a BEA model can guide the focus of the next inspection for a more thorough assessment of critical locations.

In this study, the CP system of an offshore complex consisting of Al-Zn-In galvanic anodes was modeled using BEA. The offshore complex consists of concrete and steel jackets, risers and pipework. Time-stepping, anode degradation and coating breakdown are incorporated to estimate system performance with time since installation. The results of the CP system simulation were validated against existing inspection data and used to optimize the established CP inspection program.

INTRODUCTION

An offshore complex consisting of concrete structures and steel jackets, risers and pipework operates in the North Sea. The subsea portion of the main platform is comprised of a concrete gravity oil storage tank, concrete tower nearly centered atop the concrete tank and a steel jacket and conductor frame. The concrete tank connects to the lower portion of a steel jacket which rises vertically to create the second connection point to the topside platform structure along with the concrete tower. The first of two ancillary steel jackets reside near the main platform and connects via subsea pipeline to the second ancillary steel jacket approximately 2 kilometers away.

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