Test Methods For Cathodically Protected Marine Pipelines

Abstract

ABSTRACT Seven techniques are possible for the evaluation of the operation of cathodic protection systems on buried marine pipelines. The economics, theory, advantages, and the limitations of each are presented and contrasted. Results of the developmental efforts and field application of the promising techniques are presented. INTRODUCTION Cathodic protection is almost universally used to protect marine pipelines from corrosion. The effectiveness of onshore cathodic protection systems is typically measured annually by EMF measurements often at intervals of 5.00- feet or le5s. In contrast the level of marine pipeline cathodic protection is usually measured only at the two ends of the pipeline. The selection of this wide separation between measurements is due to practical difficulties in the measurements rather than due to technical justification. Although the cathodic protection has been generally effective in delaying corrosion, numerous documented corrosion failures have occurred on marine pipelines. Repair or replacement of corroded pipelines as well as the possible ecological damage makes the appropriate monitoring of these systems essential. The purpose of this paper is to emphasize the need for monitoring the corrosion prevention systems on marine pipelines and to summarize the procedures available to accomplish the surveys. Corrosion rates of unprotected steel in sea water are unacceptably large. Corrosion rates in excess of 10 mils per year are common. Pitting rates in excess of 25 mils per year have been observed in sea water. Most marine pipelines are coated. Bitumastic and thin film epoxy coatings are commonly used corrosion resistant coatings. These coatings alone are not adequate for corrosion prevention because of unavoidable holidays or holes in practical coatings. In fact, failures often occur sooner on coated pipeline with no additional protection than on the equivalent bare pipeline. The corrosion is concentrated at the holidays in the coating causing premature perforation. In addition to the corrosion resistant coating, a concrete weight coat is usually required on marine pipelines for negative buoyancy. Although the principle purpose of the mortar is not corrosion resistance, some corrosion protection is provided by the reduction of moisture permeation and raised pH. Cathodic protection is normally applied to provide protection to the holidays and areas of accidental coating damage. Two types of cathodic protection systems are used: galvanic and impressed current systems. The galvanic systems usually consist of a sacrificial metal bracelet (often zinc) placed on some sections of pipe. An impressed current system consists of a ground bed (often at the extremities of the pipeline) powered by a rectifier. Galvanic systems are relatively simple. No adjustment is required or usually possible. Virtually all of the cost of a galvanic system must be expended during the construction of the pipeline. Impressed current systems usually have a lower initial cost, but have a power cost and require more monitoring and maintenance. In the vast majority of cases cathodic protection systems have been very successful in mitigating corrosion.