Visual and Ultrasonic Inspection of Cranes
by Alex Bagarry III*
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The importance of regular NDT inspections in preventing catastrophic failures is well recognized. This month's article describes the application of ultrasonic technique for inspecting cranes. The capability of ultrasonics in detecting, characterizing, and determining the probable cause of cracks in structural studs used in these cranes is discussed. G.P.
Singh |
Problem
Our company was recently contracted to perform ultrasonic and visual inspections on a number of the 1.2 metric ton (40 ton) capacity "hammerhead" cranes used in Wilmington, California to load and unload the container ships plying the waters of the Pacific Rim. These cranes had been in service for 15 to 25 years and were periodically inspected for degradation of their structural elements.The ultrasonic portion of the examinations included contact angle beam scanning of the structural welds joining various tubular and "H" shapes of the boom assembly, and also straight beam examination of the structural studs that held the A-frame supports to the boom platform and machine house.
There were a total of 20 studs inspected on each of six cranes. The dimensions of each stud were 50 mm (2 in.) diameter and 246 mm (9.7 in.) length. Ultrasonic examination showed a reflection on several studs from 3.81 mm (1.5 in.) to 7.62 mm (3 in.) below the scanning surface and in the threaded area inside of each top nut (see Figure 1).
The graphic representation of the data helped us identify the cause of the cracking.
Additional scanning and evaluation allowed us to identify the shape of the crack through the partial cylinder of the threaded stud end. Comparing the data from stud to the next in the series was enlightening. We plotted the position of each crack on a single graphic report. The graphic representation of the data helped us identify the cause of the cracking (see Figure 2). The alignment of the ultrasonically plotted cracked areas indicated that the probable cause of these discontinuities was a side-impact load on the boom end occurring while in the boom-down position. The side leverage obtained by a large container ship impacting the side of the boom could more than exceed the shear resistance of these stud assemblies (see Figure 3).In summary, the value of regularly scheduled NDT inspections should be recognized for its true worth. The contribution to public safety - and perhaps lives saved - by this inspection far outweighs the relatively insignificant costs associated with these inspections. This early warning system allowed the maintenance crew to schedule repairs in a timely manner, thus reducing the local government's potential liability by the possible prevention of a catastrophic failure.
* ABC Inspection & Engineering Support Services, 4201 W. 5th St., No. 109, Santa Ana, CA 92703; (714) 265-2154.
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