ABSTRACT

DP4 Platform in Bouri Oil field is located in the Offshore Libyan Mediterranean Sea and operated by Mellitah Oil and Gas B.V. Libyan Branch. The Platform Jacket was launched in 1987, protected against corrosion by galvanic cathodic protection system, constituted of slender cylindrical anodes made from aluminium-zinc-indium alloy. The design life of the CP system is 35 years.

Owing to recent installation works of I and J tubes on the jacket, the removal of some anodes was foreseen in order to allow free space for the installation of clamps on bracing elements.

For these reasons, an assessment of the protection status of the platform jacket with and without anodes removal was required. The aim of the assessment was to verify that protection conditions can still be achieved in all areas of the jacket, despite of the modification of the original galvanic anode system.

Furthermore, with the platform approaching the end of its original design life, and the intent of the Operator to extend the life, it was fundamental that the actual protection status be established in order to ensure the integrity and durability of the structure. Investigation was carried out mainly with the use of Finite Element Method (FEM) modelling, applying various scenarios based on expected current density at different levels of cathode polarization and anode consumption. The model was also compared and validated through real CP inspection data collected during a recent inspection campaign after the installation of the "I" and "J" tubes.

Results of the modelling confirmed that protection conditions are still being achieved all over the jacket structure, with minor variation of polarization. Consequently, installation of additional galvanic anodes in the modified regions was not necessary, leading to significant cost saving.

INTRODUCTION

Platform DP4 is located in Block NC 41 (Bouri Field) offshore Libya in the Mediterranean Sea, at a water depth of 169 m. It was launched in 1987. Jacket of the DP4 is protected against corrosion by galvanic cathodic protection system, comprising of 2,344 slender cylindrical anodes, made of aluminium zinc indium alloy with initial net mass of 600kg. The design life of the system at the time of 1987 was 35 years.

New I-tube and J-tube have been installed in 2016 on the east side of the jacket of DP4 platform. For the fixing system, based on clamps, thirteen (13) existing galvanic anodes has been removed. The removal of 13 galvanic anodes in the zone where clamps of I/J-tube have been installed could result to a lack of protection current sources in that area, then less negative potentials, with simultaneous effect on the surrounding anodes, that shall provide protection current for a higher cathodic surface area.

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