Continued

Robotic In-Service Inspections: The robotic process for tank inspection involves a robot tethered via an umbilical to an advanced control and monitoring system. The robot is lowered to the tank bottom, while the tank is in service, navigates across the tank floor using systems capable of mapping the tank floor and locations where high-density UT data is collected. Robot location accuracy is on the order of +/- 1 to 2 inches in absolute terms and down to mm in relative terms. A pump mounted behind the robot removes water and sediment from the tank floor, and a series of immersion transducers located under the robot takes UT thickness readings on the tank floor, measuring floor thickness in order to identify both top-side and bottom-side corrosion. A photo of the system is shown in Figure 1. In addition to the internal inspection process, inspectors can conduct a traditional API 653 external tank survey and combine the results in an inspection report that satisfies API 653 guidelines. If repair work is required, it can be scheduled in the future and competitively bid to minimize repair expenses and plant disruption.

The benefits of the robotic process include: • Reduced Project Planning – Since the tank will not be out of service during the inspection process there are no scheduling difficulties. The planning process also requires fewer internal resources and contractors since the project is simpler. • Reduced Safety and Environmental Risks – Since there is no necessary movement of product from the tank and no requirement to vapor-free the tank or place personnel inside of it. • Elimination of Supply Related Costs – There are no supply related costs associated with tank downtime. • Defer Tank Repairs – Tank repairs can be delayed until the optimal year and planned and administered for the lowest repair cost and disruption. • Maximize Asset Lifecycle – Establish a database on the lifecycle and reliability of the floor and monitor more frequently than is practical with conventional process.
 Conventional Inspection In-Service Robotic API 653 Bottom Inspection Tank must be drained and cleaned. Waste has to be collected, treated and disposed. Tank can remain operational and full of product. Significant turnaround and tank outage planning required. Scheduling is on an as-needed basis. Continuous oxygen, LEL, toxicity monitoring and a hole watch is required. Confined air space entry is 0 hours for fixed roofs and 4-8 man-hours for floating and internal floating roofs Alternate storage is a factor The need for alternate storage is eliminated. Tank must be degassed and vented. Tank air emissions are near –0-. Spot inspections and limited surveys are not cost effective. Spot inspections and routine limited surveys are possible. Typical tank survey consists of visual, MFL and a UT survey of discrete UT data points. Ultrasonic survey is automated and able to rec-ord as many as 15,000+ discrete UT data points per hour. Can save data in A, B, and C scan formats and differentiate and quantify top-side and bottom-side corrosion. Can save data in A, B, and C scan formats and, in most instances, differentiate and quantify top-side and bottom-side corrosion. Extent of repairs are unknown until outage. Based on the survey, tank repairs can be project-ed with materials and services pre-planned, thus reducing turnaround. Commissioning and refill required. No refill is required.   
Table 1 compares the two tank inspection strategies. Conventional Inspection In-Service Robotic API 653 Bottom Inspection Tank must be drained and cleaned. Waste has to be collected, treated and disposed. Tank can remain operational and full of product. Significant turnaround and tank outage planning required. Scheduling is on an as-needed basis. Continuous oxygen, LEL, toxicity monitoring and a hole watch is required. Confined air space entry is 0 hours for fixed roofs and 4-8 man-hours for floating and internal floating roofs Alternate storage is a factor The need for alternate storage is eliminated. Tank must be degassed and vented. Tank air emissions are near –0-. Spot inspections and limited surveys are not cost effective. Spot inspections and routine limited surveys are possible. Typical tank survey consists of visual, MFL and a UT survey of discrete UT data points. Ultrasonic survey is automated and able to record as many as 15,000+ discrete UT data points per hour. Can save data in A, B, and C scan formats and differentiate and quantify top-side and bottom-side corrosion. Can save data in A, B, and C scan formats and, in most instances, differentiate and quantify top-side and bottom-side corrosion. Extent of repairs are unknown until outage. Based on the survey, tank repairs can be projected with materials and services pre-planned, thus reducing turnaround. Commissioning and refill required. No refill is required. Robot Deployment Methodology: Typical robot deployment operations consist of locating the equipment control room and associated utilities adjacent to the tank within the berm area. The deployment process usually requires one or two crane lifts to the top of the tank. Equipment located at the top of the tank consists of the submersible vehicle and umbilical, pumping systems and in-tank deployment gear. Entry of the vehicle is completed through the roof’s top man way (20 inches in diameter or greater).

A 350 foot (107 meters) umbilical is used to support vehicle operation. While the system is readied for deployment from the top of the tank, the crew locates the tank navigation transducers at their proper locations around the. These locations, as well as the position of all tank appurtenances, are entered into a CAD system. During vehicle deployment, a video recording is made by the on-board camera to ensure the proper positioning of the vehicle onto the tank bottom. As with most tanks, accurate drawings are not always available. Consequently, special procedures need to be implemented in order to determine the location of various components within the tank. These objects consist of roof supports, inlet and discharge pipes, sumps, and related internals. Once these objects are annotated into the CAD drawing and the proper position of the vehicle is determined, then the vehicle is ready for floor scanning. A photograph of the robotic vehicle prepared for tank entry is provided in Figure 2. A roof man way camera system is also deployed through the manway to record the condition of the roof structures and degree of corrosion, if present. In-Service Inspection UT Methodology: A typical UT run consists of capturing data from all eight transducers every 0.16 inch (.4 cm) while the vehicle is driven in a straight path for 6 feet (2 meters). During each individual run, >6000 ‘A’ scans, converted into ‘B’ scans, are collected and stored. Data can also be collected with vehicle stationery. Figure 3 is a photo of the In-Service Robotic immersion transducers. Eight (8) 5 MHz transducers are typically used for most tank inspections. Figure 4 shows the distribution of UT data in a typical 100, foot tank without significant obstructions (other then legs, inlet and suction lines). The thickness measurements are then loaded into a spreadsheet where the ‘B’ scan data limits are checked. Any location with a measured thickness of less than a pre-determined value is highlighted by proprietary signal processing software and is then reviewed by the analyst. This review determines the cause of the low thickness measurement. These causes could include actual component thinning, a gate error, a loss of signal, sludge/sediment, and/or if the vehicle ran over a weld seam wide range of UT data points can be acquired using this technique and usually amount to between 50,000 and 6 million UT readings per tank, depending on tank size. These readings are taken throughout the tank including the critical zone around the shell. Figure 5 is a data analyst display that provides the automated results of UT A-Scan/B-Scans. The analyst is able to investigate minimum thickness thresholds and deterioration of coating and post process/revisit the data if required due to the full waveform capture and storage of A-Scans. UT and EVA Sampling Strategies: There are often several areas (sump, under heater coils, under floor fixed anodes etc.) where the robot cannot travel. Consequently, a statistical method known as Extreme Value Analysis (EVA) is utilized to extrapolate the acquired data and predict the minimum remaining floor thickness with less than 1% chance of overestimation. This technique is used throughout the medical, insurance and inspection industries and is often found during the inspection of pipelines, where “100%” coverage is impractical and unnecessary. There are some tank owners that use EVA statistics when they evaluate out-of-service floor UT data following MFL scans. The goal of an In-Service Inspection is to establish an acceptable time interval for the next internal floor inspection. Here, it is necessary to obtain an accurate assessment of the deepest pitting. It has been shown that EVA statistics are suitable for making this estimate - See references 2, 3 for further reading on this subject. Defect Identification: A key objective of on-stream inspections is to identify a range of plate defects in addition to plate thickness. Direct visual examination is often not possible (unless the clarity of the product allows for the close optical examination of a surface using the onboard camera). Therefore, an indirect classification of topside and bottom side corrosion is achieved through analysis of the UT data. Additionally, laminated plates can be identified, and coating failures can be detected. Topside v. Bottom side Corrosion: Corrosion between topside from bottom side corrosion can be accomplished by examining the series of returns prior to and following the “flagged” return. When the front face, the first discernable return from the plate, remains steady on the x-axis (time) while the second discernable return on the time axis moves toward the first return, the thinning is most likely due to bottom side pitting. On the other hand, if the front return moves along the x-axis toward a fixed second return, then the thinning is probably due to topside pitting. The degree of thinning is always determined by measuring the distance between any two discernable peaks. The signal analysis software will assist in this classification. Laminations: Laminations are usually classified as defects that are inherent in tank floor steel created during the fabrication process. Usually, they appear in mid-wall and are characterized as UT ‘A’ scan returns that are 50% of the total peak-to-peak distance of the nominal plate. These mid-wall defects are usually difficult to detect using qualitative scanning techniques, but are commonly identified with high-density UT. They can be detected by monitoring abrupt changes from near nominal peak-to-peak values to values that are close to 50% of nominal. Laminated areas can occupy very small plate areas (i.e., only a few square cm) to large areas of the plate (i.e. one square meter). Such lamination detections are common using high density UT immersion transducers. However, in most cases, they do not pose any serious tank floor integrity threats unless they are located at a weld or in the critical zone adjacent to the shell-to-floor weld. Figure 7 illustrates an “A-Scan” image of laminated plate. Coating Failure: Unless the quality of the product allows for the direct inspection of the surface via a camera, the UT analyst must depend on his or her knowledge of UT to detect separations between the steel and the coating. Coatings in this instance refer to a direct layer of epoxy or a thick material liner (such as fiberglass-resin). Under circumstances where there is a non-ruptured coating detachment, the UT returns (pulse-echo modes) are scattered, and thickness measurements are not possible. This is usually due to the pockets of air or gas which occupy the space between the lifted coating (or liner) and the steel floor. However, when they are detected adjacent to plate characterized by clear UT returns with sufficient signal-to-noise ratios for peak-to-peak measurements, they are likely areas of coating failures. Estimated Service Life Calculations: The objective of the In-Service Inspection is to quantitatively establish the parameters that will set the next internal inspection. Documents such as API 653 typically use the bottom condition, corrosion rate and thinnest remaining steel as the governing parameters. The Robotic In-Service inspection provides that data. The minimum remaining metal estimate is provided as a direct result of the inspection and statistical data analysis. The corrosion rate is computed from this and historical information. With this information, the tank operator can be provided with an estimate of how much longer the tank can remain in service before a bottom leak. Some codes and regulations may specify a non-zero minimum remaining bottom metal thickness for operations. In either case, the operator can use the in-service data to make an informed, quantitative decision regarding the appropriate schedule for the out-of-service inspection and repair. The use of in-service floor UT data has recently been found to improve the effectiveness of Risk Based Inspection (RBI) inspection programs. RBI inspection philosophies have become widely used throughout the industry for the purpose of focusing resources on important components in process facilities. An RBI analysis assesses both the probability of failure and consequences of failure. The results may be used to prioritize inspections within a plant and to select appropriate inspection methods for the most probable modes of failure. The important feature of RBI analysis as it relates to UT scanning of tank floors is the added level of confidence now available for determining degrees of risk using accurate, quantifiable UT floor thickness data. This added degree of measurement responds to some of the ambiguity associated with risk-based assessments. That is, indirect, risk-based assessments of tank floor conditions can now be verified and augmented using reliable direct measurements. Figures 8 and 9 illustrate the results of a 35-tank analysis showing the RBI results with and without high density tank floor UT data. The results of this study support the benefit of using actual floor UT data in risk-based studies in order to improve their ability to better forecast the appropriate next internal inspection interval. Economic Impact: In addition to a variety of safety and environmental benefits such as carbon credits and reduced emissions, the In-Service approach to tank inspections has significant cost benefits compared to Out-Of-Service methods. With Out-Of-Service inspections, both visible and hidden costs are challenging to control. The visible costs are those payments made directly to contractors for cleaning, inspecting, waste disposal and repair. Unfortunately, these direct expenses are usually dwarfed by indirect, hidden costs. The traditional method involves extensive planning by the operator and multiple contractors before the job is ever scheduled. The planning process typically requires a significant amount of time from internal resources including engineering, supply, scheduling, safety, and tank operations. The process for product and vapor freeing the tank also has hidden costs. Frequently owners incur costs for transferring and downgrading products in preparation for cleaning. Sometimes temporary lines are constructed, and products must be moved out of the target tank using various systems. Once the tank is down and out of service, the operator may also incur supply-related costs associated with tank downtime: higher shipping costs for smaller inbound lots, transportation costs for two porting ships or rerouting trucks, and production impacts due to reduced storage capacity. Finally, if repairs are required, more hidden costs occur. The repairs become emergency and unplanned repairs since the operator is eager to return the tank to service. Frequently there is insufficient time to follow a normal bidding and contractor selection process. Instead, the first qualified contractor who can begin work immediately wins the work, completing the repairs typically at a higher cost due to the scheduling imperative. At most companies the full cost of tank cleaning and inspection spans multiple cost centers and there is no one person who accounts for all of the costs associated with the entire process for even one tank. As an example, the engineers know the costs for inspecting the tank but only the traders know the costs for securing an alternative source of product. Another example is that the operator is aware of the incremental trucking costs that are incurred while the tank is down but only the engineer knows the true incremental cost for emergency versus competitively bid tank repairs. Figure 10 depicts the total system cost associated with the tank cleaning, inspection and repair process. Waiving the standard bidding process is typical to avoid lengthy Out-Of-Service time for the tank. At the same time, vendors charge a premium to complete the work as quickly as possible. In addition to paying a premium for tank repair, most tank owners conduct tank repair work years before it is required (a phenomena known as “repair creep”) to prevent taking the tank out of service again. This is problematic because: • Most tanks require repairs an average of nine years in the future, • Immediate repairs effectively retire an asset early and shorten its useful life. • Significant capital expense is accelerated into the current budget cycle. In-service inspections allow tank operators to avoid emergency repairs during their API 653 inspection by: • Determining the scope of repairs prior to removing tank from service. • Competitively bidding the work and ordering the required materials. • Conveniently scheduling tank repairs. In addition, by waiting until the optimal year for tank repairs, the operator maximizes the assets, useful life and defers significant capital or expense dollars into a future year. Conclusion: Pressure is mounting worldwide for the development of AST inspection techniques that protect the environment, improve working conditions for the inspector, and maintain a critical level of performance for the AST product supply chain. The field of AST inspection is witnessing changes whereby the familiar and conventional approach to tank inspection is seen to be quite inappropriate for a large population of tanks that require frequent Out-Of-Service Inspections. The Out-Of-Service approach has several drawbacks that have a significant negative impact on the environment and personal safety as well as significant economic consequences. New technology is now available that provides the tank owner with an alternative solution that eliminates this negative impact and provides a high degree of confidence regarding the tank floor integrity. The number and type of tank where an in-service approach is appropriate is increasing. The operational consequences of this new development have far reached implications as they are applied to AST maintenance strategies. References: 1. American Petroleum Institute Standard 653, Tank Inspection, Repair, Alteration, and Reconstruction. 2. Introduction To Life Prediction of Industrial Plant Materials: Application of the Extreme Value Analysis Statistical Method for Corrosion Analysis; editor M. Kowaka, Allerton Press, 1994. 3. The Asymptotic Theory of Extreme Order Statistics, Janos Galambos, John Wiley & sons NY1978.