Abstract

The North Sea environment is a harsh environment and cathodic protection is typically applied to offshore steel platforms to control degradation by corrosion. As offshore platforms near the end of their design life, the cathodic protection systems, either galvanic anodes or impressed current systems, may also be reaching the end of their life. Where the platform life is extended, it is necessary to consider whether the existing cathodic protection systems are fit-for-purpose for the intended life extension period.

Over the past few years the condition of a hybrid cathodic protection system, which comprises both galvanic anodes and seabed impressed current anodes, on one North Sea platform has been investigated and assessed. This has included regular performance assessment, specific assessment of anode wastage and estimates of remaining life. The aim of the assessment was to predict when extensive retrofit activity might be required, thus avoiding unnecessary corrosion degradation and enabling any retrofit work to be scheduled in advance.

Modeling processes can be used in the cathodic protection life assessment to provide a better understanding of the existing protection system and to predict future performance.

The assessment has demonstrated that the platform is satisfactorily protected from corrosion and that extensive retrofit work is not expected to be required in the short term.

Introduction

Offshore steel platforms installed in the North Sea require corrosion protection from the harsh environment. For the immersed area of the structure the corrosion protection is frequently solely provided by cathodic protection (CP). When the CP system is operating effectively the corrosion rate will be minimal, however if the system is not operating effectively degradation by corrosion should be expected.

Many platforms were installed in the late 1970's and early 1980's with a design life of 25 to 30 years, and the systems installed on the platform had a similar design life. Hence as offshore platforms approach the end of their design life, the CP systems will also be reaching the end of their life.

Where the platform life is extended, as is frequently the case, it is necessary to consider whether the existing CP systems are fit for purpose for the life extension period. This paper describes a process that has been undertaken to assess the present condition and to estimate the remaining life of a CP system, and discusses options for computer modeling.

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