Storage Tank Management Background Any Oil terminal, production or petrochemical company handling thousands of barrels of crude oil or other liquid product per day needs to ensure that there is continuity of production as well as protect the environment from unforeseen catastrophes as documented in various industry trade journals. The following questions constantly challenge the facility and inspection managers of any company involved in the storage and handling of large volumes of liquid product: 1- How can we prevent unplanned repair shutdowns to storage tanks due to unforeseen environmentally damaging leaks or failure of bottom plates? 2- How can we minimise the impact of planned shutdowns to : a. Restore production in the shortest possible timescale for the purpose of improving the Company operating profitability b. Reduce operating cost by minimising the medium to long term inspection maintenance budgets by eliminating conventional inspection techniques. c. Eliminate the sometimes wasted cost incurred during normal tank maintenance involving opening, cleaning, performing inspection, integrity evaluation and decommissioning as well as re-commissioning testing and consequential lost production downtime due to there being no detrimental issues with the tank bottom. d. Identify critical repair areas so that repair materials can be pre ordered to ensure any repair downtime is minimised? 3- And finally how can we manage and prioritise the maintenance, inspection and repair works of the most severely corroded tanks to ensure a cost effective programme for internal tank inspection?
It is obvious that conventional inspection methods such as MFL, SLOFEC and ECT alone can not give suitable answers to all of the above mentioned questions. Whereas IOOC experience is that the Acoustic Emission Technique used as a screening tool and supported by Magnetic Flux Leakage(MFL) can greatly reduce costs and assist the industry on a long term basis. Acoustic Emission Testing (AET) Acoustic Emission, according to ASTM, refers to the generation of transient elastic waves during the rapid release of energy from localized sources within a material. The source of these emissions in metals is closely associated with the dislocation movement accompanying plastic deformation and the initiation and extension of cracks in a structure under stress. Other sources of Acoustic Emission are: melting, phase transformation, thermal stresses cool down cracking, stress build up, active corrosion and product leakage. Acoustic Emission Testing (AET) Process The Acoustic Emission NDT technique is based on the detection of these high frequency elastic waves (vibrations) and conversion of the waves to electrical signals. When the acoustic Emission technique is used for the inspection of tank bottoms, this is accomplished by directly coupling piezoelectric transducers to the external surface of the tank. Sensors are coupled to the tank shell by means of a fluid couplant and are secured with tape, adhesive bonds or magnetic hold downs. The number of sensors is dictated by the tank diameter; normally one or two rows of sensors are used, with the sensors positioned at equal intervals around the tank shell. The tank must be loaded to its maximum capacity and allowed to rest for a period of 24 hours before the Acoustic Emission inspection takes place. The output of each piezoelectric sensor is amplified through a low-noise preamplifier, filtered to remove any extraneous noise and further processed by suitable electronic equipment. As the sensors are mounted in different positions around the shell of the tank, Acoustic Emissions from individual sources within the tank will generally be detected at different points in time. The approximate position of individual AE sources can be determined by analysing sound velocities and comparing the arrival times at each sensor.
AE Sensors being coupled to a tank shell Summary of the Acoustic Emission Technique: - Technique is based on detection of high frequency vibrations (AE signals) - Extremely sensitive technique A 0.5 mm pencil point break is detectable 70 m from sensor - Tank is filled to maximum capacity and product allowed to rest for 24 hours before inspection. - AE sensors are placed on the tank shell approximately 1 metre above the tank bottom. - Signals are detected by AE sensors (50 khz -1 MHz) and converted to electrical signals. - The high speed data-acquisition unit collects data. - Data is analysed to characterise corrosion / cracks /potential leaks. - Tanks are graded to indicate severity of any damage. - The simple grading system allows management to prioritise which tanks to be taken out of service for MFL inspection and subsequent repair. Advantages of the Acoustic Emission method for inspection of tank bottom plates: - High sensitivity - Early and rapid detection of defects, flaws, cracks, leakages, etc. - Cost reduction - Minimizes plant downtime for inspection - no need to empty or clean the tank. - Inspects the whole tank bottom (including annular plates) - Identify tanks which need to be prioritised for inspection and repair. - Leaves good tanks on-line and saves on shut down and cleaning time
Acoustic Emission inspection reports for storage tank bottom plates consist of the following information: 1- Overall corrosion activity grading. 2- Potential leak activity grading. Extract from acoustic Emission report showing overall corrosion activity Extract from acoustic emission report showing potential leakage activity Acoustic Emission inspection results are usually categorised as follows: A - No damage (or minor damage), no repair required B - Damage, some repair required C - Severe damage, extensive repairs or new tank bottom required
Group A grading The tank can remain in service and can normally stay in service for periods in excess of the industry norms for conventional tank inspection frequencies (tanks in this category should be re inspected by AET in 3 to 5 years). Group B Grading Tanks are normally scheduled for shutdown / MFL inspection / repair at a planned time in the future. Group C grading Tanks are normally taken out of services on fast track basis for further evaluation and repair / replacement of bottom plates. The following table is a summary of Acoustic Emission test results from a crude oil production tank farm: Tank No. Potential leak activity grading Overall corrosion activity grading Results TK 1 B B Over lay patch repairing TK 2 A A Satisfactory TK 3 C C Need to Re bottoming TK 4 B A Over lay patch repairing TK 5 C B Need to Re bottoming TK 6 A B Satisfactory TK 7 A A Satisfactory TK 8 A A Satisfactory TK 9 C C Need to Re bottoming TK 10 C C Need to Re bottoming TK 11 A A Satisfactory As can be seen, the results are an excellent information tool for assisting company management in the decision making process.
Magnetic Flux Leakage (MFL) Inspection For the group of Grade B tanks, the location and severity of corroded areas will be confirmed using the Magnetic Flux Leakage (MFL) technique. The MFL results will also provide base line data for future Risk Based Inspection forecasts and maintenance planning. The latest generation FloormapVS2i MFL scanner being used to inspect an aviation fuel tank With the MFL method the entire tank bottom is scanned and the dedicated reporting software automatically produces a CAD drawing of the tank floor showing the position and severity of any corrosion defects. Defects are automatically colour coded according to severity and positioned within acceptable accuracy of ±3 mm on an 8M long scan.
Automatic CAD Drawing of Tank Bottom showing Discontinuities Colour Coded for Severity With recent progress made in permanent magnet technology used on modern MFL scanners it is now possible to induce localised magnetic fields of 1.90 Tesla in tank bottom plates (A). Previously, MFL floor scanners were typically only capable of inducing fields of up to 1.4 Tesla. This increase in induced field strength greatly improves the accuracy and repeatability of MFL results and allows inspection of thicker materials. Magnetic model showing magnetic saturation (1.9 Tesla) of a 6mm thick Plate
It is now clear that the MFL technique is not only capable of generating a detailed report of almost 100% of the tank bottom in a short timescale but the accuracy of the results are equal to or better than those obtained using conventional UT. The image below is an extract from a recent MFL inspection report carried out in Iran. It demonstrates both the powerful features of the dedicated reporting software and the accuracy of the MFL results. This section of the report highlights an area of top-side corrosion that was measured by thefloormapvs2i MFL scanner as 52% loss. The corroded area was physically measured using a pit depth gauge and the52% loss reported by the MFL scanner was confirmed. The defect was then photographed so the image of the corrosion could be linked to the specific area of the affected plate within the MFL inspection report. Extract from FloormapVS2i MFL Report Showing Corrosion Defects and Linked Digital Photograph of Corroded Area All data captured by the MFL scanner (normally over 20% loss of the nominal plate thickness due limitations of MFL technique) is initially analysed by the on-board data acquisition computer. This initial analysis is based on peak amplitude alone which can occasionally result in defect sizing errors due to variations in defect geometry. The data is then automatically re-analyzed in the off-line reporting software, using dynamic filters which identify the type of corrosion and classify it as pipe, conical or lake type. Depending on the defect classification, the reporting software then adjusts the estimated material loss either up or down as required. This combination of amplitude analysis plus dynamic filtering ensures enhanced defect sizing even on badly corroded floors where small diameter deep pits may otherwise have been undersized. The reporting software has several useful display tools to quickly filter non-critical discontinuities and identify critical discontinuities. This simplifies identification of repairable defects and is a useful aid to calculating remaining life of individual plates or even complete floors.
Discontinuity Display Filter to Identify Critical Discontinuities A comprehensive statistics package which can be configured to show statistics the whole tank bottom or individual plates is included in the package as is a patch plate design function. Additional data from visual, ultrasonic, vacuum box and magnetic particle inspection can be added to the report generating a full fingerprint of the tank floor including the annular plates. MFL Statistics
The MFL data can either be displayed electronically or printed as a hard copy. Flexible software license terms allow the reporting software and MFL data to be freely distributed within an organisation so the same report can be viewed at several geographic locations simultaneously. Other benefits of the FloormapVS2i MFL unit used by IOOC include: Use of permanent sintered neodymium iron boron magnets which are mechanically stronger and guarantee uniform field strength across the magnet. Using permanent magnets also eliminates the need for an external power supply and trailing power cables which can reduce inspection time by as much as 25%. Use of Hall Effect sensors to detect the flux leakage path. This ensures that the MFL scanner records precise data rather than the averaged data which is recorded when using electro-magnetic coils. Use of a fixed speed DC motor eliminates sizing errors caused by variable scanning speed as referred to in Clause G 5 Appendix G of API 653. Other MFL scanners are manually operated (hand pushed) which can result in recording different amplitude signals for the same discontinuity depending on the speed the scanner is moved. Improved defect detection, sizing and positioning over previous MFL scanner designs. Enhanced calibration procedure to enable the operator to enter the exact % loss for each artificial defect on the calibration plate. This greatly improves inspection accuracy, particularly when calibration plates have not been manufactured to design tolerances. Ability to map Annular plates Digital data acquisition to improve resolution and capture data much faster than previous designs. Use of a ruggedized, touch screen computer fitted with an automotive-quality vibration resistant hard drive to cope with the often harsh environment within a storage tank. Uninterruptable Power Supply to prevent loss of data in event of low battery power condition. USB port to simplify data transfer and facilitate daily reports and 24 hour working if required Automatic CAD drawing of tank floor which ensures that the operator and client have a very clear report detailing the areas which were not automatically scanned due to access restrictions. This reduces shutdown cost by eliminating the need for a third party to witness the scanning. Patch plate design capability built into the reporting software. This powerful engineering tool automatically produces a schedule of all material required to repair the tank bottom - A MAJOR time and overhead saving for any organisation. Real time data acquisition and analysis giving the MFL operator immediate access to corrosion data as well as the ability to repeat selected scans which may be suspect. Full finger print reporting to support trend analysis and Risk Based Inspection scheduling. Several other types of inspection result can be added to the MFL report including: o MFL Handscan results o Digital photos o Visual comments o Manual pit gauge and UT reports comments o Manual or Auto UT B or C scan corrosion profiling reports o Vacuum box reports, if performed by Silverwing o MPI inspection report. if performed by Silverwing Dedicated read only version of the reporting software which allows users to access the powerful analysis and engineering tools, but locks the data to prevent
accidental modification. The read only reporting software is licensed so that the report data may be freely distributed and viewed at various local and worldwide locations within a terminal operator organisation. Ability to superimpose a recent set of MFL data on top of older MFL data from the same tank. The older MFL data can then be used as a baseline to calculate corrosion growth rates or identify new corrosion growth. Multi Language capability. The conventional method of inspection on 110 m diameter tank using 2008 cost base: Element Cost US Dollars Direct costs Tank cleaning 50,000.00 Waste processing products 16,000.00 45 000 gals @ $ 0.11 Waste Transportation 14,700.00 MFL only Inspection 31,000.00 Total direct costs 111,700 Indirect costs Planning & administration 5,000.00 Petroleum reclamation 80,250.00 Product storage & handling 30,000.00 Small lot fees 30,000.00 Truck loading excess costs 16,000.00 Premature tank repairs 160,000.00 Total in direct costs 316,250.00 Loss of goodwill and????? product sales, if small lot fees not pursued MINIMUM TOTAL COST TO OWNER $427,950.00 The combination of Acoustic Emission used as an initial screening tool and Magnetic Flux leakage to provide accurate corrosion mapping results provide Iranian offshore oil Company (IOOC)with a complete above ground storage tank maintenance and inspection program. Regards: Farzad Noumani Head, plant inspection Dep. (IOOC)