This paper reports the use of dispersive Higher Order Modes Guided Waves (HOM-GW) and non-dispersive Higher Order Modes Cluster Guided Waves (HOMC-GW) for the detection and sizing of corrosion defects in pipes and tank floors. Pin-hole and wall thinning defects of different sizes were artificially generated on the pipe and the tank floor specimens in order to simulate corrosion pits under pipe supports and tank floor annular plate regions respectively. Since, the nature of loss of material differs in each type of corrosion; the same mode excited cannot be effective in characterizing all the defect types, although they all may indicate the presence of defect. The effective modes are identified for each type of corrosion and their defect characterizing sensitivity was studied. The validity of this technique was tested with field samples. © 2008 American Institute of Physics.

Krishnan Balasubramaniam

Department of Mechanical Engineering

Jayaraman Chandrasekaran

Bruce W. Maxfield

IIT Madras is a public technical and research university located in Chennai, Tamil Nadu. Founded in 1959, it is recognised as an Institute of National Importance.

IIT Madras has been ranked as the top engineering institute in India for four years in a row by the National Institutional Ranking Framework of the MHRD

It currently offers undergraduate, postgraduate and research degrees across 16 disciplines in Engineering, Sciences, Humanities and Management. About 596 faculty belonging to science and engineering departments and centres of the Institute are engaged in teaching, research and industrial consultancy.

IIT Madras

Imaging hidden corrosion using ultrasonic non‐dispersive higher order guided wave modes

AIP Conference Proceedings

Finite Element Analysis (FEA) is an important tool that has been extensively used in wave propagation models for NDE applications. With the increase and diversification of Non Destructive Evaluation (NDE) applications, the complexity and size of FE models increase considerably. Also for modeling wave propagation models, the size of the FE problem depends on the frequency that is used and velocity of sound in the material. High ultrasonic frequency applications and materials with lower ultrasonic velocity contribute immensely to the increase in the FE problem size. Additionally, the requirements for 3D models are also increasing. This calls for higher computation resources to be available at the hands of the researcher at significant costs. We have developed a new approach to model such extremely large FE problems that involves splitting the entire model to be analyzed into smaller sub-models and by solving each sub-models sequentially, while transferring the outputs at the boundaries between the sub-models. Thus, the entire "large" model now consists of a few "smaller" sub-models. The size of these smaller models can be decided upon by the researcher depending on the computing capacity available. While this approach may not provide significant improvements in the time of computation, it allows for the computation of large models possible with relatively less computational resources. The model was tested on a few case studies and validated with the results obtained from a full "large" model. © 2014 AIP Publishing LLC.

Split approach for FEA simulations of very large wave propagation models

This paper reports on a new means of generating higher-order mode clusters of guided waves (HOMC-GW) using a meander-coil (MC) electromagnetic acoustic transducer (EMAT) in plates at frequencies significantly higher than the lower-order plate modes. These wave modes are considerably less dispersive and they occur at much higher frequency¿thickness (f x d) products. Our studies cover the f x d range of 13 to 20 MHzmm. Experimental measurements were carried out on Al plate samples of different thicknesses using three different EMAT coil periods. To understand the generation and propagation characteristics of HOMC-GW with EMATs, several simulations were carried out using 2-D finite element models at different f x d products. These simulations captured all features observed in the experiments. The time¿frequency smoothed pseudo Wigner-Ville distribution (SPWVD) was used to analyze the HOMC-GW modes. Defect detection measurements using HOMC-GW generated using EMATs were made on Al plates with machined defects. © 2012 IEEE.

IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control

Generation and detection of higher-order mode clusters of guided waves (HOMC-GW) using meander-coil EMATs

Corrosion detection in the annular plate region of above- ground storage tanks is a critical needfor tank farm operators in the oil and gas industry. The critical zone between the tank shell wall and the first few inches of the annular plate is difficult to inspect with conventional floor scanning methods due to the presence of coatings, uneven surfaces and a lack of access due to the presence of the weld toe. This region, however, is prone to accelerated corrosion due to the additional stresses caused by the weight of the tank wall and the increased possibility of water entrapment under the annular plate. Repairs in this region demand replacement of the entire annular plate and this leads to long shutdown of the tank, and often failures occur without any warning. A new method using the Higher Order Modes Cluster Guided Waves (HOMC*) to detect and screen defects in the critical inaccessible zones in the annular plate is discussed. This testing can be carried out online while the tank is in operation using HOMC guided waves generated on the outer exposed region of the annular plate. Robotic scanners allow for the inspection to be carried out in relatively short periods of time. Data collected in the laboratory as well as from the field trials are presented. The HOMC technique has potential for non-destructive inspection of inaccessible regions in structures for medium-range applications (2 to 3 m).

Insight: Non-Destructive Testing and Condition Monitoring

Higher order modes cluster (HOMC) guided waves for online defect detection in annular plate region of above-ground storage tanks

A higher order cylindrically guided ultrasonic wave was used for the detection and sizing of hidden pitting-type corrosion in the hidden crevice regions (between the pipe and the pipe supports) without lifting or disturbing the structural layout arrangement of the pipelines. The higher order circumferential guided waves were generated using a piezoelectric crystal based transducer, located at the accessible top region of the pipes, in a pulse-echo mode. By studying the experimental parameters such as dispersion, particle displacement, and wavelength of the ultrasonic guided wave modes, an appropriate higher order mode was selected for excitation using an appropriately designed acrylic angle wedge that conforms to the pipe's outer curvature. A manual pipe crawler was designed with a provision for holding the wedge, and the essential hardware such as data acquisition card, encoder, etc., was integrated with the system so that the corrosion was mapped in real time during the scanning of the pipes. The system was validated on pipes ranging from 6 in. to 24 in. outer diameters of wall thicknesses up to 12 mm, by mapping defects as small as 1.5 mm diameter and 25% penetration wall thickness. A 2D finite element model using ABAQUS® was used to understand the wave propagation in pipe wall and its interaction with pinhole-type defects. Copyright © 2008 by ASME.

Journal	AIP Conference Proceedings
ISSN	0094243X
Open Access	No