Deep Ocean Corrosion Research In Support Of Oman India Gas Pipeline
Authors: Graham F.W. , McKeehan D.S. 
Published: October 2008

The increasing interest in deepwater exploration and roduction has motivated the development of technologies required to accomplish tasks heretofore possible only onshore and in shallow water. The tremendous expense of technology development and the cost of specialized equipmenthas created concerns that the design life of these facilities may be compromised by corrosion. The requirement ts to develop and prove design parameters to meet these demands will require an ongoing environmental testing and materials evaluation and development program.

This paper describes a two-fold corrosion testing program involving:the installation of two corrosion test devices installed in-situ, anda laboratory test conducted in simulated site-specific sea water.

These tests are expected to qualify key parameters necessary to design a cathodic protection system to protect the Oman-to-India pipeline.

INTRODUCTION

Oman Oil Company is implementing the Oman to pipeline (OIP) project, whose offshore section extends India 1139 km between Ra's al Jifan in Oman and Rapar Gadhwali in India. The pipeline route traverses water depths of up to 3500m. There is very little experience with pipelinecorrosion data and anode performance at these depths.

Aluminum alloy anodes of various compositions is well established for cathodic protection of pipelines installed in seawater and saline mud at ordinary depths. Relatively purealuminum alloyed with iridium as the activator element, with added silicon and zinc (A1-in-Zn) has proven to be the most reliable alloy.

The test program was designed to evaluate the ability of various anode alloys to activate, based on measurement of anode weight loss.

B Some factors known to affect anode performance, suchas current velocity and biofouling, were not considered to be deepwater issues and, therefore, are not addressed in this short-term test.

Anode performance and corrosion rates for structures buried below the mudline were not considered. The estimated cathodic protection requirements for the OIP pipeline sections which are self-buried below the seabed will be based on site specific soil resistivity measurements, and knowncurrent density relationships for both unburied and buried structures.

JUSTIFICATION

The design parameters of a cathodic protection system have not been established in 3500 meters of water. The objective of this test program was to quantify the parameters necessary to design the cathodic protection systemfor the deep ocean section of the Oman-India Pipeline.

ANODE PASSIVATION

Aluminum based anodes, well recognized as an efficient sacrificial material for cathodic protection of marine pipelines, require a small addition of iridium for activation, As temperature decreases, the amount of iridium must be increased to ensure proper galvanic performance. As the addition of this element increases anode activity, a balance must be achieved which prevents passivation but does not induce excessive self-corrosion, Reports of passivation of iridium activated anodes (A1-in-Zn) installed in cold seawater represent a major concern of deepwater facilities operators, Bottom temperatures on the Oman India pipeline route are 1.7 to 2.0 degrees C., whichis below temperatures of 4ºC previously tested for anode performance. While this is not a severe departure from known data, the anode.