Underwater Thickness Measurement of Underwater Steel Structures

ABSTRACT The structural integrity of waterfront steel structures is adversely affected by deterioration from corrosion. Pitting caused by corrosion reduces the thickness of the steel structures such as sheet, pipe, and H-piles, and could cause catastrophic failure. Thickness measurement of these steel structures is difficult because of extremely rough pitted surfaces where conventional nondestructive measurement methods cannot be used. A new underwater nondestructive testing method for measuring thickness of steel waterfront structures is presented. The technique is based upon a focused ultrasonic transducer that acquires signals at certain locations on the steel plate where the ultrasonic beam is collimated in the steel specimen, resulting in a strong back-surface reflection. Modifications to the transducer assembly were made and signal-processing methods were developed to enhance method performance in the presence of spurious signals that are frequently encountered. A prototype system incorporating these elements was demonstrated by measuring the average thickness of corroded steel plates ranging in thickness from 2.5 mm or more with a nominal accuracy of 15 percent. INTRODUCTION Corrosion of naval waterfront steel structures is of serious concern as it can cause catastrophic failures resulting in direct dollar losses and reduced fleet operational readiness. The key to improved maintenance and repair management, readiness assessment, and catastrophic failure prevention is reliable facility inspection and condition assessment. The problem addressed in this paper is related to corrosion developing in steel underwater structures such as 'H' and sheet piles. Generally, corrosion on these structures is most severe in two areas: the splash zone and the region just below the mean low-water line. The corrosion pitting ranges from 1 to 4 mm in depth and 2 to 10 mm in diameter and is present on both sides of the plates. These pits are mostly interconnected causing a rough, pitted surface. Even though corrosion is present on both sides, only one side is accessible for inspection. Because of the pitted surface, conventional thickness measuring devices or techniques cannot be used. One of the most common methods for inspection of corroded structures is visual inspection. However, visual inspection is limited to information about the surface damage, and cannot provide information regarding the remaining thickness of the corroded steel plate, which is a direct measure of its strength. Two other methods, conventional ultrasonic and eddy current, can only be used on plates with flat surfaces and, hence, cannot be successfully used for this application. Several new techniques using Lamb waves, phase matching of ultrasonic pulse-echo signals, and electromagnetic methods have been suggested1,2,3 to solve this problem. Analytically, these methods look promising, but none have been found suitable4 at this stage as a practical thickness measuring tool of corroded plates. The objective of this paper is to describe and present the results of a new technique for determining the thickness of corrosion-pitted steel plates. The technique uses an ultrasonic focused transducer, which is aligned over a corrosion-formed pit.