Method for Extending the Time Between Out-of-Service, In-Tank Inspections Using Ultrasonic Sensor

Abstract

A method and apparatuses to extend the time interval between out-of-service, in-tank inspections while insuring structural integrity using a risk-based, Bayesian statistical approach comprised of a passing leak detection test and the using the results from (1) tank floor thickness measurements, (2) prior out-of-service tank floor inspection results, and/or (3) acoustic emission corrosion maps of the tank floor to estimate the minimum thickness and maximum corrosion rate of the tank during the extension period. The present invention uses an in-tank, mass-based leak detection system to establish tank integrity, three or more ultrasonic (UT) thickness measurement sensors for measurements of the tank floor at one location, and to establish the spatial distribution of corrosion of the tank floor, one or more prior API 653/12R1 or STI SP001 tank floor thickness inspections and/or three or more in-tank AE sensors mounted inside the tank with vertical and horizontal locations in an oblique plane relative to the tank floor.

What is claimed is: 
     
         1 . A method for determining the integrity of a storage tank including a tank floor, comprising the steps of:
 (a) performing a leak-detection test;   (b) determining that the tank is not leaking from the results of the leak detection test;   (c) estimating the thickness of the tank floor in at least one location of the tank at one or more places, wherein the thickness of the tank floor over at least one region of the tank is measured in-tank and used to estimate the corrosion rate and the thickness of the tank floor as a function of the selected time period to determine if a minimum floor thickness can be maintained during said period, and wherein said in-tank floor thickness estimates are made using an ultrasonic thickness (UT) measurement sensor; and   (d) conducting a risk assessment using statistical data of tank failures including data related to tank corrosion conditions, thereby assessing the probability of structural failure or leaking of the tank within a selected time period.   
     
     
         2 - 3 . (canceled) 
     
     
         4 . The method of  claim 1 , wherein said in-tank floor thickness estimates are used to estimate the minimum thickness of said tank floor at any location of said tank floor 
     
     
         5 . The method of  claim 1 , wherein said in-tank floor thickness estimates are used to estimate the maximum corrosion rate of said tank floor at any location of the tank floor. 
     
     
         6 - 8 . (canceled) 
     
     
         9 . The method of  claim 1 , wherein an acoustic emission (AE) inspection of said tank floor for active corrosion is used to determine whether the thickness estimate of the tank floor in one vicinity of said tank floor is used to estimate at least one of the thickness and the corrosion rate of another location of said tank floor. 
     
     
         10 . The method of  claim 9 , wherein said thickness estimate of the tank floor in one vicinity of said tank floor is used to estimate at least one of the thickness and the corrosion rate for the entire tank floor. 
     
     
         11 . The method of  claim 9 , wherein said in-tank floor thickness estimate is used to estimate the minimum thickness of said tank floor at any location of said tank floor. 
     
     
         12 . The method of  claim 9 , wherein said in-tank floor thickness estimate is used to estimate the maximum corrosion rate of said tank floor at any location of the tank floor. 
     
     
         13 - 18 . (canceled) 
     
     
         19 . The method of  claim 1 , wherein a previous out-of-service inspection of the tank floor thickness is used to estimate the minimum thickness and the maximum corrosion rate from said in-tank floor thickness measurements. 
     
     
         20 . The method of  claim 19 , wherein said in-tank estimate is referenced to previous floor thickness estimates made during previous out-of-service inspection in the same region as said in-tank thickness measurements and used with said previous out-of-service inspection of said floor to estimate the floor thickness and corrosion rate at another location on said tank floor. 
     
     
         21 . The method of  claim 20 , wherein the percentage change in said in-tank measurements over those measured in said previous out-of-service tank floor inspection is used to compute the percentage change of all other previous measurements of said tank floor from said out-of-service tank floor inspection. 
     
     
         22 . The method of  claim 21 , wherein said thickness measurement of the tank floor in one vicinity of said tank floor can be used to estimate the thickness or corrosion rate of another location in said tank floor. 
     
     
         23 . The method of  claim 21 , wherein said thickness measurement of the tank floor in one vicinity of said tank floor can be used to estimate the thickness or corrosion rate for the entire tank floor. 
     
     
         24 . The method of  claim 21 , wherein said in-tank floor thickness measurements are used to estimate the minimum thickness of said tank floor at any location in said tank. 
     
     
         25 . The method of  claim 21 , wherein said in-tank floor thickness measurements are used to estimate the maximum corrosion rate of said tank floor at any location in the tank floor. 
     
     
         26 - 34 . (canceled) 
     
     
         35 . The method of  claim 1 , wherein an acoustic emission (AE) inspection of said tank floor for active corrosion is used with a previous out-of-service tank inspection of said floor thicknesses and corrosion rates to estimate the minimum thickness and the maximum corrosion rate of said tank floor. 
     
     
         36 . (canceled) 
     
     
         37 . The method of  claim 1 , wherein AE inspection measurements are used to categorize the condition of the floor of the tank in terms of no active corrosion, some active corrosion, active corrosion, moderate corrosion, or severe corrosion. 
     
     
         38 . The method of  claim 1 , wherein the results of an acoustic emission (AE) inspection of said tank floor for active corrosion is used to determine the magnitude of said corrosion in the tank floor. 
     
     
         39 . The method of  claim 1 , wherein measurements from a previous out-of-service internal inspection are used to categorize the condition of the floor of the tank in terms of no active corrosion, some active corrosion, active corrosion, moderate corrosion, or severe corrosion. 
     
     
         40 . The method of  claim 1 , wherein the results of a previous out-of-service internal inspection of said tank floor for active corrosion is used to determine the magnitude of said corrosion in the tank floor. 
     
     
         41 . The method of  claim 1 , wherein the measurements from an in-tank internal inspection are used to categorize the condition of the floor of the tank in terms of no active corrosion, some active corrosion, active corrosion, moderate corrosion, or severe corrosion. 
     
     
         42 . The method of  claim 1 , wherein the results of an in-tank internal inspection of said tank floor for active corrosion is used to determine the magnitude of said corrosion in the tank floor. 
     
     
         43 - 48 . (canceled) 
     
     
         49 . The method of  claim 1 , where said measured thickness is made using is an EM Flux thickness measurement sensor. 
     
     
         50 . The method of  claim 1 , where said thickness estimate is made using a LRUT thickness measurement sensor. 
     
     
         51 . The method of  claim 1 , where said thickness estimate is made using an EMAT thickness measurement sensor. 
     
     
         52 . The method of  claim 1 , where said thickness estimate is made using an ultrasonic thickness sensor mounted on a robotic crawler. 
     
     
         53 . The method of  claim 1 , where said thickness estimate is made using an ultrasonic thickness sensor mounted on the end of a pole, which is inserted through an opening at the top of a tank. 
     
     
         54 - 58 . (canceled) 
     
     
         59 . The method of  claim 1 , further comprising estimating the corrosion activity on said tank floor with coverage of the entire floor of said tank. 
     
     
         60 . The method of  claim 59 , further comprising extrapolating the thickness of the entire floor. 
     
     
         61 - 62 . (canceled) 
     
     
         63 . The method of  claim 1 , where said leak detection test is conducted with a mass-based leak detection system. 
     
     
         64 . The method of  claim 63 , where said leak detection test is conducted with a mass-based leak detection system using a long range differential pressure (LRDP) system.