External Corrosion Direct Assessment (ECDA) is a valuable inspection technique for assessing the integrity of underground pipelines and pipeline facilities. When used to fulfill the inspection requirements of the U.S. pipeline safety regulations its application must follow a rigid protocol compliant with NACE standard practice. In addition to limiting ECDA application to situations where external corrosion is the principal threat normal practice is to further restrict ECDA use to merely understanding the condition of a single pipeline. A broader understanding of ECDA shows its usefulness to understanding the overall underground environment from soil conditions to the effectiveness of the pipeline coating and Cathodic Protection systems as well as its applicability to multiple pipelines pipeline systems and area-wide underground networks. Expanding ECDA’s use to underground process piping in the petrochemical plant and/or terminal environment will require this broader approach.ECDA entails four phases: Pre-assessment Indirect Inspection Direct Examination and Post-assessment. The Indirect Inspection phase uses a combination of complementary Indirect Inspection Techniques (IIT’s) which include: Alternating Current Voltage Gradient (ACVG) Direct Current Voltage Gradient (DCVG) Close Interval Survey (CIS) and Pipeline Current Mapper (PCM). Because most IIT’s measure electrical current leakage and/or electrical potential between the pipeline and the soil the Indirect Inspection Phase is the phase most affected by and sensitive to whether the ECDA application involves a single pipeline with limited bonds or multiple pipelines bonded together or an entire network of underground piping with a common potential. In a standard ECDA application where only one pipeline segment is being assessed all electrical bonds must be identified and broken and the analysis will be increasingly complicated if there are one or more off-takes. When an ECDA is executed on multiple pipeline segments or a piping network then significant modifications to the application protocol need to be made. An ECDA in congested areas such as the plant environment requires consideration of many different aspects as compared to an ECDA for a single pipeline: multiple segments with varying critical attributes such as diameter and coating potentially many unknown electrical bonds electrical interference in congested areas potential for header systems and multiple connections and potentially much more restricted surface conditions. Additionally the plant environment presents challenges to site access increases the chance of pavement (concrete and asphaltic) over the pipe (making core-drilling necessary) and frequently introduces vibrations from close proximity to rotating equipment and stray currents from AC and DC interference.All of these challenges were effectively analyzed and overcome during a recent ECDA inspection of an underground ethylene header system which was inspected in 2010. The header system was in a very congested area with both above and below ground piping. A grid was laid out with 2’ x 2’ squares over the entire ethylene header system. There were 20 columns and 11 rows of squares. On/Off potentials were recorded in each square. DCVG and CIS surveys were used to pinpoint anomalies. Results and conclusions of this methodology are presented. Additional examples of challenges solutions and lessons learned are also presented from other recent ECDA inspections of in-plant underground piping.
