Alan Conci Kubrusly

A robot for offshore pipeline inspection

The robot presented in this paper was designed to find damages to the external sheath of offshore pipelines. The pipeline section subject to inspection is known as “riser” and connects the pipe running on the seabed to the production facility at the platform or “floating production, storage and offloading” ship (FPSO). When compared to the current tools used to perform this work, the main advantage of this robot is to be able to execute the inspection without umbilical cable or operator assistance. The vehicle, called Autonomous Underwater Riser Inspection robot (AURI), uses the pipeline for guidance. A closed feedback loop control system is used to maintain constant speed during the mission. Several sensors provides multiple return conditions for safe operation, such as maximum depth, maximum time or maximum distance. The robot is suited to carry different inspection devices (payloads). The payload for the first prototype consists of four cameras which provide 100% coverage of the external pipe surface. All robot vessels were designed to support up to 100 Bar of pressure, allowing the vehicle to reach a maximum depth of one thousand meters. This paper presents the general concepts of the robot as well as results from the tests in a pool, including some hydrodynamic parameters of the vehicle.

Strain sensitivity model for guided waves in plates using the time-reversal technique

The use of the time-reversal technique to detect variations in the external applied traction in a strip of aluminum plate is presented. Experiments have been performed in transmission-reception mode, using two fixed ultrasonic transducers near the ends of the plate. Two different parameters of the time-reversed signal are used for monitoring the strain state, the peak amplitude and the focusing time. The inverse filter technique was used in the transmitted signal to equalize the amplitude spectrum, enhancing the sensitivity in the focus amplitude by 5 times. The external traction sensitivity was experimentally verified by using an aluminum plate (800 × 100 × 3 mm) and two 5-MHz ultrasonic transducers spaced 700 mm apart. At the maximum strain state (180 μstrain), the peak value was reduced by about 10% in the conventional process and by 50% using the inverse filter. To evaluate the effects of the strain in the time-reversal signal, a theoretical model was constructed. This model is successful in predicting the changes in the group delay and, consequently, the focusing time using a linear equation. This relationship can also be used to determine the strain level quantitatively. The experimental results show that the time-reversal signal technique can be used in practical monitoring of changes at the strain level in mechanical structures.

Derivation of acoustoelastic Lamb wave dispersion curves in anisotropic plates at the initial and natural frames of reference

The propagation speed of ultrasonic waves in pre-stressed media can be evaluated either at the natural or initial frames of reference. In this paper general equations that can be applied to the partial wave technique are presented in order to obtain the dispersion spectra of acoustoelastic Lamb waves in anisotropic plates in either frame of reference. Employing these equations, dispersion curves for the fundamental modes in a pre-stressed transversely isotropic aluminum plate were numerically obtained in both reference frames under longitudinal and transverse loading with the material transverse axis along each of the Cartesian directions, as well as the propagation along a non-principal direction. Results confirm that due to the material natural anisotropy, the speed variation depends not only on the pre-stress direction but also on the material orientation as well as on the polarization of the propagating mode. Similar to bulk waves, the relationship between the speed at the natural and initial frames is a function of the load direction.

Development of a mechanical strain sensor based on time reversal of ultrasonic guided waves

The development of a strain sensor based on the time reversal focusing technique is presented. The sensor is composed by a strip of aluminum plate with two ultrasonic piezocomposite transducers bonded at the ends of the plate. The time reversal technique acts as a dispersion compensator of the guided waves propagating in the plate, allowing time recompression of the waves. When the plate is subjected to a longitudinal traction, a time reversal focusing is performed between the transducers in order to detect the change in the focus due to strain. The strain can be evaluated by measuring the change of the amplitude and shift in the time of flight, and comparing them with a reference signal obtained at zero strain state. In order to improve the systems sensitivity, 2-2 piezocomposite transducers designed to operate between 0.2 to 3.0 MHz are used. Experiments are conducted by applying strain up to 150 μ-strain. The results show an increase in sensitivity when compared with the results of the conventional mono-element transducer greater than 200%. Results presented here can be used in the project of stress monitoring transducers and structural health systems.

Mechanical Strain Sensing by Broadband Time Reversal in Plates

In a previous work, the ultrasonic measurement of longitudinal strain in a plate using the time-reversal technique was proved. One drawback of this measurement is the low sensitivity of the signal against changes in strain. This problem can be solved using inverse filter signal processing. This technique increases the sensitivity but also reduces the energy of the signal and, consequently, the signal-to-noise ratio (SNR). Thus, a physical solution is presented to improve the sensitivity of the system. Additionally, the one-bit time reversal is introduced to simplify the hardware used in this technique. The strain sensing system is composed of a pair of piezocomposite transducers bonded to the surface of the tested plate and used to generate and sense the ultrasonic waves guided through the specimen. The use of time reversal provides phase compensation for dispersion and edge reflections in the propagation of the guided waves in the plate, allowing time recompression of the waves. The measurement principle is based on the detection of changes in the amplitude and time-of-flight of the focused signal when the plate is subjected to longitudinal strains. System sensitivity is improved using 2-2 piezocomposite transducers designed to operate between 0.2 and 3.0 MHz. In the signal processing, the one-bit time reversal is compared with the conventional time reversal in 12-bit resolution. A figure of merit is introduced to evaluate the influence of the transfer function on strain sensitivity. This figure of merit relates the energy concentrated at the time-reversal focus with the total energy of the signal. This value represents the ability of the time-reversal process to recompress the signal at the focus. The experiments were conducted by applying strains up to 150 μm/m. The results show a linear response in the change of the focus amplitude. The sensitivity depends on the transducers and it can be related to the proposed figure of merit. The focus quality is kept when one-bit time reversal is used, showing to be also feasible for the measuring technique. All the results agreed with the numerical time-reversal implementation.

Application of one-bit time reversal technique to mechanical strain monitoring in plates

This paper presents the application of the one-bit time reversal technique to a longitudinal strain sensor. The setup consists of a pair of piezoelectric transducers bonded in the extremities of a strip of aluminum plate. When the plate is subjected to traction, time reversal focalization is performed, the mismatch between the impulse response at initial and strained levels causes loss in the focusing quality. The strain can be evaluated by measuring either the time of flight shift or the amplitude decrease in the focused signal. One-bit time reversal can simplify the electronic device to perform the proposed technique. In this work, the results using one-bit and normal time reversal implementation were compared. Experiments were performed using three different 2-2 piezocomposite transducers pairs at 500, 1000 and 2250 kHz. The longitudinal strain was applied up to 150 μ-strain using a strain gauge as a reference. The time reversal energy efficiency was used as a spectrum figure of merit and obeys the sensitivity behavior. The one-bit time reversal variation provided good focused signal for all experiments and no significant loss in focus quality. Moreover, every configuration showed a higher sensitivity than its normal time reversal version, at least 10% depending on the transducer. The one-bit technique reveals an important enhancement for the method; it holds the natural advantage of being simpler and the benefit of higher sensitivity.

Strain monitoring in metallic plates using the time reversal focusing technique

This paper presents the use of the time reversal technique to detect variations on the external applied traction in a strip of aluminum plate. Experiments have been performed in transmission-reception mode, using two fixed ultrasonic transducers near the ends of the strip of aluminum plate. Two different parameters of the time reversed signal are used for monitoring the strain state, the peak amplitude and the focusing time. An inverse filter technique was used in the transmitted signal in order to equalize the amplitude spectrum, enhancing the sensitivity in the focus amplitude by five times. The external traction sensitivity was verified using an aluminum plate (800mm×100mm×3mm) and two 5 MHz ultrasonic transducers spaced by 700mm. At the maximum strain state (180μstrain), the peak value was reduced about 10 % in the conventional process and 50 % using the inverse filter. The changes in the group delay and consequently the focusing time was well predicted by a linear equation and it can be also used to monitor the strain level. The experimental results show that the time reversal signal technique can be used in practical monitoring of changes in the strain level in strips of plates.

Numerical analysis of the effect of longitudinal tensile stress on the time-reversal focusing of Lamb waves in plates

The evaluation of the strain state is a usual practice to health monitoring mechanical structures. In a previous work, the time-reversal signal processing was theoretically and experimentally evaluated to measure the strain in plates. In this work, the theoretical analysis is extended to include acoustoelastic effects and border conditions. The influence of high longitudinal tensile stress on the time-reversal process in plates is analyzed by means of finite element simulations using commercial software. The load effect is introduced in simulations by using the effective elastic constants as the material stiffness matrix according to the acoustoelastic theory. As an example to evaluate the simulations, a 3mm thick aluminum plate has been modeled subjected to longitudinal tensile loads from zero up to 120 MPa. A single emitter-receiver setup is used. The spectral content and the Lamb modes able to propagate are selected by the excitation signal, imposed as an out-of-plane displacement. Two scenarios are analyzed, an infinite plate and finite long plate considering reflections at the borders. Results show the ability of the commercial software to simulate the time reversal process in strained media using acoustoelastic constants. In the finite-length plate scenario, where end reflections occur, the time-reversal focus sensitivity to stress changes is much greater.