Evaluation of Floor Defects in In-Service API 650 Storage Tanks Using Advanced Non-Destructive Submerged Robotic Tools

Introduction

Aboveground storage tanks (ASTs) are a key component of the oil and gas industry and present unique integrity challenges when it comes to their inspection and maintenance. Corrosion is one of the predominant degradation mechanisms for AST failures. Hydrocarbons are generally not regarded as corrosive, but impurities such as sulfur, organic acids, bacteria, dissolved gases, and water contribute to corrosion in steel tanks [1]. Without effective cycling, mixing, or chemical inhibitor programs, water, contaminants, and debris may settle out and products may solidify on the tank bottom, leading to increased product-side corrosion. As such, the tank material in contact with oil typically sees less corrosion, while the bottom material in contact with water has a higher corrosion rate [1]. Moreover, the moisture in the soil also contributes to soil-side corrosion. Different factors can influence soil-side corrosion, such as soil type, drainage and permeability, conductivity, salinity, pH, and effectiveness of corrosion prevention methods [1]. Over time, the corrosion decreases the tank’s integrity, ultimately leading to bottom perforation and causing loss of primary containment.

To avoid such a situation, tank operators are required by regulations, codes, and standards such as API 653 to perform periodic inspections and repairs. Periodic external in-service inspections are routinely performed on storage tanks, but the tank floor remains inaccessible during these assessments. Also, sediment and sludge may accumulate at the bottom of the tank, reducing valuable storage capacity. Periodic sludge removal is often required. Existing techniques, such as water wash methods and solvent-based and thermo-chemical cleaning methods, often fail to achieve the complete removal of sludge and provide limited effectiveness. Therefore, regular out-of-service (internal) cleaning and inspection of these storage tanks is critical for business operators to address tank bottom integrity, leak threats, and ensure optimal storage availability. However, there are many challenges associated with AST internal cleaning and inspection, including the health and safety risks posed by confined space entry and hazardous products, as well as extended operational downtimes, business disruption, and high costs associated with taking the AST out of service. 

Tank floor inspections typically use conventional non-destructive technologies (NDT), which are based on ultrasonic testing (UT) or magnetic flux leakage (MFL) that require the tanks to be emptied and cleaned. However, in recent years, there has been significant development of non-destructive technologies for on-stream inspections using remotely operated tools. To minimize the safety risks and downtime, submerged robotic inspection tools (SRITs) that can inspect tank bottoms while in service have been developed by the industry. The use of in-service robotic inspection tools is allowed by API 653 Paragraph 6.4.1.2 [2]. Some tools can work in water, gasoline, diesel, and other refined products but have limited capability to function in a viscous crude oil environment. There is a need to improve the technology capability to implement non-destructive examination to effectively capture floor defects in the challenging and high-risk environment of crude oil storage tanks. Further research and development are required to overcome the challenges of sludge removal, sensor data acquisition under sediments, tool navigation in a viscous product, and electrical hazards in a flammable and combustible environment. 

To accelerate industry innovation, Enbridge has designed and constructed a test tank environment, which is a small-scale version of a large-capacity API 650 crude oil storage tank with prefabricated floor defects for vendors to evaluate their robotic in-service inspection equipment.