Oliver C. Moghissi

Practical Applications of the Kramers-Kronig Relations

Abstract A technique is presented for using the Kramers-Kronig (KK) relations to evaluate the consistency of impedance data that do not exhibit a DC limit. The approach presented here differs from ...

Application of Measurement Models for Analysis of Impedance Spectra

Abstract Measurement models have been applied to electrochemical impedance data collected at the corrosion potential for a copper disk rotating in alkaline- and acid-aerated 0.5 M chloride solutions. Several applications of measurement models for the interpretation of experimental impedance data are demonstrated. Corrosion is a nonstationary process, and nonstationary behavior associated with corrosion frequently influences impedance data. If data can be shown to be consistent with the Kramers-Kronig relations, nonstationary influences can be neglected. Measurement models provide a quantitative method for identifying the portion of an impedance spectrum that is consistent with the Kramers-Kronig relations and, therefore, can be interpreted in terms of pseudo-steady-state models. Measurement models also provide a very sensitive means of interpreting impedance spectra in terms of the effects of physical parameters, such as immersion time and disk rotation speed.

Internal Corrosion Direct Assessment for Pipelines Carrying Wet Gas - Methodology

An Internal Corrosion Direct Assessment methodology is proposed for wet gas pipelines (WG-ICDA). Wet gas systems (i.e., those normally containing liquids) include storage and gathering systems with large gas-to-liquid volume ratios. Wet gas systems are not well represented by ICDA for normally dry gas, and existing corrosion models applied to wet gas systems are not sufficiently targeted at integrity verification. The essential focus of WG-ICDA compared to other internal corrosion models is the discrimination of conditions along the length of a pipeline so that possible local integrity threats with respect to internal corrosion are identified and mitigated. The basis of WG-ICDA is to prioritize locations along a pipeline segment by factors of traditional corrosion rate, flow effects, and other influencing factors. Corrosion rate depends on gas quality, liquid chemistry, pressure, and temperature. The corrosion rate can be normalized because WG-ICDA as integrity verification only concerns itself with corrosion distribution (i.e., the location along a pipeline segment where corrosion is more severe than other locations). Flow effects include possible flow regimes and the presence of water from condensation (at locations of heat loss). Expected possible flow regimes are stratified, slugging, and annular. The final term captures other factors influencing corrosion rate distribution. These factors include corrosion inhibition (batch and continuous, solubility and dispersibility in hydrocarbon and aqueous phases), biocide treatments, hydrocarbon condensates (including emulsion characteristics), maintenance pigging, bacteria, solids/scale, and other products. WG-ICDA follows the same four-step process as all other Direct Assessment (DA) methods: 1) Pre-Assessment: Data is collected, a feasibility analysis is performed, and the pipeline segment is divided into regions. 2) Indirect Inspections: Measurements are taken or calculations are performed to prioritize locations along a particular pipeline segment for susceptibility to corrosion. For WG-ICDA, the factors contributing to the distribution of corrosion will be included and an initial assumption about corrosion distribution will be made. WG-ICDA is sufficiently flexible to allow the use of existing wet gas models within the framework of the overall process. 3) Direct (or Detailed) Examinations: The pipe is excavated and examined at locations prioritized to have the highest likelihood of corrosion. The examination must have sufficient detail to determine the existence, extent, and severity of corrosion. Examination of the internal surface of a pipe can involve non-destructive examination methods sufficient to identify and characterize internal defects. 4) Post-Assessment: Analysis of the indirect and direct examination data is performed to determine overall pipeline integrity, prioritize repairs, and set the interval for the next assessment. If the results of excavations do not match the original assumption, the corrosion distribution model will be updated to guide the next excavations (i.e., the operator returns to step 2).© 2004 ASME