Hot Risers in the North Sea: A Parametric Study of CP and Corrosion Characteristics of Hot Steel in Cold Seawater
Authors: Fischer K.P. , Mehdizadeh Parviz , Solheim P.S. , Hansen A.H. 
Published: October 2008

In recent years doubts have been expressed regarding the adequacy of the design criteria for cathodic protection of hot oil risers in the North Sea area 1,2. Therefore it was decided to study some of the fundamental aspects of corrosion and corrosion control of hot steel in cold seawater. For studying these phenomena a specially designed channel flow system was constructed. Seawater was pumped into the laboratory facilities and through the flow channel. Here the flow rate, the steel surface temperature and the polarisation characteristics of the mounted steel probes could be controlled. The progressive current density at potentiostatic polarisation was found to be a sensitive function of the development of the calcareous deposit on hot steel. Thus on hot steel in cold seawater the initial current density will be very high. Subsequently, due to the build up of the calcareous deposit, the current will decrease to a value in the range as found for steel in seawater at ambient temperature. The acquired data on the cathodic polarisation behaviour of hot steel in cold seawater should be applicable for the designers of protection systems of marine risers.

INTRODUCTION

At the present time the CP criteria for hot marine risers have been uncertain. Notably here is the influence of the steel temperature on both the potential criteria for protection and the current requirements for achieving protection. The high temperatures of the oil (70 -100 C) produced from many fields in the North Sea area necessitates a full understanding of the basic corrosion control requirement for a hot riser. Fundamental properties related to corrosion and polarisation behaviour are not well established. The interrelation between the kinetics of the anodic dissolution of the hot steel and the reactions related to the cathodic reduction of oxygen across the reaction boundary layer with great gradient encompass a multitude of different variables. In order to understand these basic properties it was necessary to perform a detailed parametric study. Such a detailed investigation program would have to be performed in a laboratory. The present paper is only concerned with the submerged area where CP can ensure full protection.

EXPERIMENTAL SYSTEMS

The testing of hot steel exposed to seawater was performed in a channel. Tests have mainly been done on bare steel. The steel samples (probes) are mounted in the side walls and seawater flows through the channel. All the samples were heated and potential statically controlled. The water was pumped into the main tank from a depth of 40 m in the Sandefjordsfjorden in Southern Norway. From the main tank water was pumped to an inlet tank and flowed through the channel to the outlet tank and was then again pumped back into the sea. There was no recirculation of seawater. The composition of seawater is given in table 1. As the water was to be held at 8± 2°C during the testing, a water cooling unit was used. To avoid contamination of the seawater, titanium heat exchangers were used.