CP Inspection of Jacket Based on High Sensitive Field Gradient Sensors

Lauvstad, G. Ø. (FORCE Technology Norway) | Bj⊘rnstad, M. (FORCE Technology Norway) | Osvoll, H. (FORCE Technology Norway) | Blakset, T. (FORCE Technology Norway) | Str⊘mstad, S. (Aker BP ASA)

OnePetro 

ABSTRACT

Inspection and monitoring of offshore structures and pipelines protected by cathodic protection (CP) are important factors in maintaining the integrity. As the offshore structures and pipelines get older, sacrificial anodes become depleted and coating gets damaged or deteriorates. Hence, methods for quantifying the various parameters affecting the protection become increasingly important. The CP systems on offshore structures do naturally have a limited life, and if underprotection, and in worst case, free corrosion, develops, this will have a major impact on remaining life. In order to assess the status and ensure the integrity of these structures, it is necessary to perform evaluations of the CP systems at regular intervals. The results from these evaluations can then be used to check the CP systems capacity for life extension, or whether a CP retrofit will be required.

This paper describes the capability of a new field gradient tool for CP inspection. The new tool has been developed to measure electric fields in seawater very accurately. Based on the information provided, the current density on the surface can be quantified and the location of coating damages determined. The results obtained from inspection of three jackets, which have been in operation more than 30 years, are presented. Here, it was important to establish the best basis for life extension and anode retrofit, both regarding anode performance and current density. The inspection results demonstrated very low current density on steel, but also a large variation in anode performance. Irrespective of this, the need for CP retrofit was greatly reduced, from immediate retrofit of a large number of single anodes and anode sleds on all jackets, to no retrofit requirement for two jackets and a potential, but very limited, retrofit requirement for the third jacket.

INTRODUCTION

The main objective of integrity management of a facility, is to ensure safe operation during the entire lifetime from installation to abandonment. Corrosion control on subsea facilities is typically achieved with cathodic protection (CP), by installation of sacrificial (galvanic) or impressed current (IC) anodes, often in combination with application of coating. CP design should be performed in accordance with standards or recommended practices, such as DNV-RP-B401 or NACE- RP0176.1,2

The status of the CP system is typically investigated by performing potential measurements, preferably by making electrical contact with the structure by means of a stabber. The potential at the stab location is then measured with respect to a reference electrode, typically the Ag/AgCl/seawater reference electrode. Alternatively, proximity measurements can be performed, but now the potential drop between reference electrode and surface to be measured needs to be considered. This potential drop can be significant when measuring on a very active anode (measured potential too positive) or a steel area with high current density (measured potential too negative).