A New System for On-Line Monitoring of Corrosion and Bacteria in Pipelines

Abstract

ABSTRACT A new system for on-line monitoring of 'internal corrosion and bacteria in pipelines has been developed which maintains till pigging capability in the pipeline. Fluids are withdrawn from the pipeline and passed through a test chamber specially designed to provide reliable corrosion rate measurements under pipeline operating conditions. The system duplicates and measures worst case pipeline corrosion conditions, provides evaluation of corrosion and bacteria control programs in an on-line manner, and can function in the absence or intermittent presence of water in the pipeline. Corrosion and bacteria probes are able to be replaced under atmospheric conditions, greatly simplifying operation, and the unit can be easily cleaned of accumulated solids. All equipment is built to API pipeline codes, and the system can be installed on new pipelines and/or retro-fitted to existing pipelines. INTRODUCTION Minerals Management Service data (Ref 1) indicates that corrosion was the cause of at least half of all pipeline failures in the US Gulf of Mexico for the period 1967–1987, as shown in Figure 1. This compares to 20% due to third party mechanical damage, and 12% for storm and mud slide induced damage. Of the total of 343 offshore corrosion failures in this period, 15% were caused by internal corrosion, 46% from external corrosion, and 39% from "general" corrosion. Even though gas pipelines have a larger population in the Gulf of Mexico than oil lines, half of the offshore failures were in oil pipelines. Of thebalance, 28% occurred in gas pipelines, 3% each in multi-phase and miscellaneous lines, and 16% not reported. Recently, the 169 KM (104 mile) Forties Field pipeline was replaced due to internal corrosion (Ref 2). Design pressure of this 32 inch pipeline was 2084 psi (142 bar), and it was fabricated of API 5L X65 pipe with a 19 mm wall thickness. Corrosion was described as "characteristic of individual corrosion pitting, general corrosion containing pitting, selective corrosion attack of girth welds, and areas of relatively uniform metal loss, which resembles general wall thinning but with a rough internal surface texture." Most oil and gas pipelines are well designed for corrosion control in their initial operation, but changing conditions frequently result in off-design operation after some period of time. System add-ons and partial abandonments change the type and volume of fluids inthe line and create dead spaces. Maturing of oil and gas fields results in greater amounts of water and smaller amounts of oil and/or gas. This results in lower linevelocity and allows water to stratify in the bottom of or at low spots in the line where it can cause corrosion. Ricca has also reported that internal corrosion has occurred in "dead legs" in the Trans Alaska Pipeline (Ref 3). Full Well Fluids Transmission (FWFT), has resulted insubstantial volumes of water being transmitted through offshore pipelines which did not previously transport water, and for which the pipelines were not designed.