Abstract

Owners, operators, regulatory entities and non-governmental agencies focus their attention on pipeline Integrity Management Plans (IMP) to address the safety and reliability of hydrocarbon pipelines as they cross environmentally sensitive areas as well as densely populated areas.  IMP includes a base line assessment prioritizing the inspection and remediation of expected sensitive areas.  Direct Assessment (DA) along with internal inspection, hydro-test and other equivalent methods to ascertain the condition of the pipeline are used as IMP assessment methodologies.

The topic to be covered in this article is Internal Direct Assessment which includes Erosional Velocity (EV) in addition to Internal Corrosion Direct Assessment (ICDA).  This article utilizes the analysis of an onshore gas pipeline in the Gulf of Mexico to identify possible areas susceptible to corrosion.

Multiphase flow analysis simulates the characteristics and specific fluid behavior to determine erosional velocity and internal corrosion in pipelines. A multiphase flow simulator will be used to obtain the specific flow characteristics to evaluate EV and ICDA for the above mentioned pipeline system.  Internal corrosion is analyzed both as top of the line corrosion and corrosion at the bottom of the line.  In addition multiphase flow simulation provides accurate information related to fluids velocity and residence time. 

The internal corrosion rate can be related to residence time of water and other electrolytes to formulate effective mitigation processes.  This could serve as the basis for a mitigation program and significantly increase the useful life of a pipeline system.

Introduction

Among several components of an IMP are methods to directly evaluate a pipeline system internally and determine its integrity.  These methods could be internal inspection tools or hydro-testing.  In many occasions the availability of internal inspection tools, facilities to launch and receive inspections devices and environmental restrictions do not permit direct internal inspection of the pipelines systems.  Additionally, some systems due to the service they provide may not interrupt service for periods of time necessary to complete an in-line inspection.  In these and other cases Direct Assessment could be used to determine the pipeline susceptibility to internal corrosion.

ICDA is a planned and closed loop process designed to assess the internal integrity of pipeline systems transporting hydrocarbons.  The ICDA process described in this article uses existing proven technologies to develop risk-based internal corrosion susceptibility profiles for pipeline systems.  Once the critical locations have been determined then non-invasive and non-destructive techniques like radiography or ultrasonic thickness measurement could be used to confirm wall thinning; if no corrosion damage exists then other locations along the pipeline are less likely to be corroded.  As part of the direct assessment post-process results of the inspection findings and model outputs are compared, and then tuning of the model is done until agreement with measured results is reached.  In this way the loop is closed and it is ready for a new cycle.

Multiphase flow transient modeling allows determination of key flow characteristics to determine the overall risk-based internal corrosion susceptibility profile.  Main flow characteristics are profiles for: pressure, temperature, water film velocity, pH, fluid phase velocities, fluid phase superficial velocities, partial pressure of CO2, shear stress between water film and pipeline wall, flow regime indicator, and critical velocity.  All the mentioned parameters and additional information from the model is used in determining the probability of corrosion susceptibility at each segment of the pipeline system.  It is important to mention that sweet corrosion exists only if a water film is wetting the pipeline wall.