ABSTRACT

Cathodic Protection by galvanic anodes installed at construction stage on an offshore jacket structure or a complex of several jackets needs to be upgraded when lifetime extension is required. Marine operations and underwater works being very expensive, the choice of the rehabilitation method is often driven by the associated costs for such revamping works. This document presents the overall approach undertaken on an offshore field in the Gulf of Guinea to carry out the revamping of the cathodic protection of several jackets and the technical choice of the impressed current when compared to galvanic anodes, considering technical and economic aspects, and pointing out the various advantages and drawbacks.

INTRODUCTION: SACRIFICIAL OR GALVANIC ANODES CP SYSTEMS

Sacrificial or Galvanic Anodes Cathodic Protection System is an excellent anticorrosion solution which offers to immerse structures a long-term protection. In TotalEnergies and in the Oil & Gas Industry in general, such system provides confidence, efficiency together with little maintenance over years which is a very good point for the subsea integrity of jackets (or pipelines) when knowing all other operational constraints or corrosion issues that can be met at the surface level.

Thus, from TotalEnergies experience, it has been established that as long as:

1/ the CP system is correctly designed (taking into account the applied - or not - painting on the jacket) following the recognized international codes and standards (DNV RP-B-401 [1], ISO 15589-2 [2])

2/ the galvanic anodes are correctly manufactured, respecting suitable chemical compositions, alloy/core cohesion, and controlled by adequate QA/QC

3/ the galvanic anodes are correctly welded and distributed over the jacket structure (or the pipeline) to provide homogeneous current distribution,

such systems are particularly efficient even though CP potentials shall be regularly measured to confirm the protection in seawater.

However, after a service life of 20 to 40 years, sacrificial anodes welded to jackets are generally depleted (see pictures below) and need to be replaced when lifetime extensions are required to continue operating safely the platforms [3]. The following paragraphs give the feedback Total Gabon for retrofitting such offshore CP systems.

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