As seen in the January/February 2014 issue of Tank Storage Magazine
By Katherine Foster, ILI Solutions Executive, Diakont, San Diego, CA
With a large number of tank lines originally built without pig launchers and receivers, it falls upon current integrity engineers and risk managers to determine how best to address facility piping which may not have been inspected previously.
With tanks out of service, API 653 inspections provide a convenient opportunity to inspect and/or replace below ground tank lines. A thorough inspection of each tank line ensures environmental and human safety and eliminates blind replacement of piping.
With more emphasis being placed on pipe inspection rather than blind replacement, the industry has been faced with the challenge of finding inspection solutions which employ proven technologies that provide reliable, cost effective inspection results.
One proven technology for inspecting buried lines is Electromagnetic Acoustic Transducers (EMAT). Using a mobile robotic crawler equipped with multiple camera systems and EMAT transducers, inspection technicians can examine the condition of the entire pipeline. The main operating principle of the EMAT Ultrasonic (UT) method is the electromagnetic-acoustic transmitting and receiving of ultrasonic shear waves, with linear polarization. These ultrasonic waves are generated in the surface layer of an inspected metal by the tool’s EMAT transducers. A baseline magnetic field is applied to the coil and material by a permanent magnet within the transducers, fabricated from a Neodymium-Iron-Boron (Nd-Fe-B) alloy. Using 0 degree direct-beam and 30 degree angle-beam EMAT technology, inspection technicians detect internal and external corrosion with a minimum depth of 0.040”, with 90% probability of detection, and a measurement accuracy of ±0.020 inch, with 80% certainty.
US-based Diakont pipeline inspection services were recently selected by an interstate petroleum carrier to inspect an aging petrochemical storage tank line that lacked documentation pertaining to the pipeline characteristics and geometries.
The scope of this project included the inspection of a 24” diameter underground liquid pipeline, within a tank farm. The primary objective of the in-line inspection (ILI) was to determine the remaining pipe wall thickness, detect any volumetric defects, plus identify and characterize any internal or external corrosion in the inspection area of the pipe. The secondary objective was to perform a visual survey of pipeline as-built features.
The geometry and characteristics of the pipeline were not known before the inspection so a robotic crawler was selected for the inspection rather than flow driven inspection tools. Self-propelled robotic crawlers have the ability drive in both directions, which is imperative if something is found in the line that could impede the tool’s travel. This was the case with the storage tank line inspection; a vertical stinger was discovered in the pipeline that would have trapped a standard flow driven pig. To get integrity data on the entire line, Diakont deployed the robotic crawler through open valves on both sides of the stinger. The crawler demonstrated the ability to safely navigate multiple bends, accurately measure wall thickness and accurately detect corrosion on the interior and exterior of the pipe. Further, the ability to manipulate the robotic crawler in real-time was an essential functionality given the fact that the geometry of the line was unknown.
Knowing the pipeline had a 24” diameter, Diakont inspection engineers identified the RODIS-24 robotic crawler to complete the inspection. The in-line inspection process employed a Visual Measurement Inspection (VMI) module and direct-beam (0 degree) EMAT UT. Since EMAT scanning technology is effective through build-up of up to 0.040”, the decision was made not to clean the line in order to reduce the overall inspection time and cost. Prior to each ILI run, the EMAT and VMI sensors were calibrated to ensure the accuracy of inspection data.
Diakont first deployed the tool into the tank line with the Visual Measurement Instrumentation (VMI) module and conducted a visual survey of the entire inspection area, logging precise locations of pipe joints and fittings, and checking for any significant findings or deviations that could potentially impeded tool travel or dictate re-planning of the inspection procedure (e.g. foreign material, remaining product in the line, stinger).
Following the visual inspection, technicians then replaced the VMI module with an Ultrasonic EMAT module and operators conducted automated inspections (straight sections), and semi-automated ring-scan and/ or point scan inspections in elbows and fittings.
With the detection threshold dependent on the length of the beam aperture and the tool’s step-advancement, the step-advancement could be increased from 0.4” to 0.8” or greater. For example, an increase in the step advancement would result in only those anomalies being detected which are of sufficient length that they cross 2 (or 3) adjacent ‘rings’ (dependent on technology used). A larger step scan would result in a reduced total inspection time. In this project, straight pipe sections were scanned using a 0.8” step advancement. Even though the minimum length detection threshold may increase, the EMAT transducers continue to capture 365 degrees of data, thereby allowing the flaw width and depth detection threshold to remain at the tool’s maximum resolution.
The following images below show how the tool’s step-advancement impacts the length detection threshold. The red areas highlight the log distance (or length) which the transducer scans the pipe wall. When the transducers are oriented axially, the EMAT beam aperture is 0.8” (length) by 0.4” (width). The tool may take 0.8” steps resulting in minimum length detection threshold of anomalies larger than 1.6” (length). For comparison, if the tool were to increase step advancement length and advance every 1”, the tool could detect anomalies larger than 2” (length). NDE technicians review the data to ensure any indication of an anomaly which passes over two back-to-back EMAT scans is reviewed.
The following areas were not scanned in automatic mode: areas within 4ft of the launch site, sections of pipe within fittings, girth and long seam welds, and sections with unacceptable levels of surface roughness or buildup. The sections that were not scanned using the EMAT sensor are examined visually or point scans were performed.
Within elbows, fittings, and 22” after each bend, point measurements were conducted every 3.9” along the pipe and at degree positions: 0, 30, 90, 150, 180, 210, 270, and 330.
After the automatic scanning of the EMAT module was conducted, the tool was taken out of the line and the module was replaced with the VMI module for laser profilometry scanning. Having previously logged the specific areas of internal corrosion detected with the EMAT module, the VMI module was used to cast a laser grid onto the ID surface of the pipe to capture images and size an internal anomaly in more detail. As a safety precaution, the tool remains unpowered at the point of entry until verification of a nonhazardous, inert environment. This is achieved by displacing oxygen (O2) with nitrogen (N2), an inert gas. To do this, nitrogen (N2) is pumped into the pipe equal to five times the pipe volume. Once air quality readings confirmed that O2 concentration is below 10%, the tool is powered on. A flange covering, modified with a 2” hole for the crawler’s tether, is then bolted to the end of the pipe to help prevent O2 from entering the line during the inspection.
Diakont technicians calibrated the tool’s sensors and set up the system near the removed valve at the start of the tank line. Before powering on the tool, the O2 levels in the pipe were checked. After safe levels were reached, the inspection robot was deployed into the pipe and a flange cover was bolted onto the pipe to maintain the nitrogen concentration.
Diakont NDE inspection technicians conducted a visual inspection of the entire pipeline. Light corrosion was found in several sections of the pipeline in the lower regions. The most significant area of corrosion consisted of three pin holes found within the pipe at 113.8ft.
As mentioned, a stinger was discovered in the middle of the pipeline. To inspect the rest of the pipeline, and complete the entire scope of the project, Diakont re-deployed the tool through an opening created from a removed spool piece on the other side of the stinger and inspected the remainder of the line.
EMAT Inspection Results After Diakont NDE technicians completed the visual inspections and laser profilometry scans of the pipe, the inspection tool was equipped with an EMAT sensor to measure the remaining wall thickness. The resultant data determined that the average remaining wall thickness was 66% of nominal thickness. The section with the most severe wall thinning from corrosion was located 235.2 feet into the pipe and was found to have 52% remaining wall thickness.
The inspection revealed that the storage tank line geometry consisted of (QTY 1) 45 degree (1.5D) bend, (QTY 1) 90 degree (1.5D) right bend, and over bend which with a pitch of 18 degrees downward from horizontal (the stinger was located between the two 1.5D bends.)
Use of the robotic crawler in conjunction with the VMI and EMAT modules proved to be valuable in assessing the tank line. Despite the line not being cleaned, the robotic crawler and the EMAT module successfully conducted a thorough inspection
Immediately following the inspection, the operator was notified of all anomalies detected and measured. Subsequently, the preliminary report (provided within two business days) and the final report (provided within 30 days) were used to perform risk analysis. The inspection results were saved as benchmark data for future risk analysis.
The crawler's bi-directional capability allowed both parties to reanalyze areas of concern and navigate multiple bends and elevation changes. Additionally, the ability to detect and measure anomalies in real-time allowed the operator and contractor to make changes to the inspection scope based on real-time findings.
Overall, the tank line inspection project was a success. The inspection validated the integrity of a majority of the tank line and identified the sections with pipe wall thinning and pin holes requiring repair. The tank line remained out of service after the inspection and the operator made plans to excavate the damaged sections for repair or replacement. The cost and downtime of repairing or replacing the small damaged sections of pipeline will be less than it would have been to replace the entire line.