Introduction

Historically, cathodic protection system design for offshore structures using galvanic anodes was based on a single nominal maintenance current density intended to protect a structure over the system design life, after polarizing it to a protected potential within several months. This maintenance current density was identified from service experience and was used simply to determine the amount of anode material to be used.

Today, typical design practices incorporate three design current densities: initial, maintenance, and final. The reason behind using three design current densities compared to the earlier single maintenance current density approach is because of the technical and economic benefits derived from the rapid polarization resulting from application of an initially high current density.

Unless an effort is made to optimize anode size and shape, the use of three design criteria usually results in three different answers for the number of anodes required, indicating a fundamental inconsistency among the typical values. If the three criteria are viewed as minimum criteria, the design for bare structures is normally driven by the initial current density criterion.

For coated structures, the design is normally driven by the final or maintenance current density criteria. In this case, the actual amount of anode material calculated is greater than has been shown in the past to provide the needed design life. Therefore, the result is increased life, not reduced cost. The experimental data and example designs in this report are appropriate for uncoated structures.

Although the applicable standards^1,2 have been revised, published experimental data^3,4,5 and theoretical developments provide a simpler and more universal empirical option to describe the polarization process to cathodically protected steel in seawater. This description leads to a simplified design procedure that incorporates both the rapid polarization and long-term maintenance current concepts into a single equation. The final current density concept is also sometimes included in the framework of this new method. In addition, these concepts are often applied to analysis of in-service cathodic protection survey data.

Although polarization is a critical factor in the new offshore cathodic protection system design method, the details of the polarization process and deposition of calcareous films are beyond the scope of this report.