Cathodic Protection Criteria for the North Sea

Abstract

ABSTRACT Experiences from "the first generation" of offshore structures in the North Sea have shown that cathodic protection criteria that are sufficient in other offshore areas like the Gulf of Mexico cannot necessarily be applied to steel structures in the North Sea. This paper describes some of these experiences. It also gives some background information about the special environmental conditions in the North Sea and their influence on cathodic protection systems. The main part of the paper is. devoted to the installation of a field testing equipment for measuring current output of anodes and the polarization level of a steel structure in service. Some preliminary results are given. The results show that the necessary practical current density for the North Sea when using sacrificial anodes may be in the order of 100 to 130 mA/m2. Special attention should be paid to calculation of the correct current output and distribute the anodes well to get an even current distribution. At the end of the paper we have outlined some further developments that are necessary to establish the correct design of cathodic protection. A special item is the combination of a coating system and cathodic protection. Finally, a brief introduction to the DnV rules for cathodic protection systems is given. INTRODUCTION Corrosion and corrosion protection are the most important factors to be considered in the design, construction, and inspection of offshore structures and pipelines. If basic corrosion protection technology is neglected, this will result in expensive and serious mistakes, not only in terms of money, but in safety as well. Corrosion, in fact, has turned out to be a major problem for the operation of offshore platforms and production facilities. For DnV, being a certifying authority in the North Sea area, special interest has been paid to pipelines, risers, and structural members, which are important to the over-all integrity of the structure. Of special concern are members that are difficult to inspect or repair and members that are exposed to very aggressive environments, for instance, the splash zone. The submerged zone of the structures and pipelines is a part that may be defined as difficult to inspect or repair. This is especially true for the North Sea with its deep waters and rough sea conditions, which are making diving operations more complicated. The predominant way of protecting the submerged zone is by cathodic protection systems, which sometimes may be combined with a suitable coating. Thus, the performance of a cathodic protection system is a very important factor in obtaining reasonable life and safety of the structures. Cathodic protection can be achieved either by sacrificial anodes, by impressed current systems, or by a combination of both methods. For North Sea applications, most of the systems so far have been sacrificial anode systems. This paper deals mainly with sacrificial anode systems.