Inspecting the integrity of flexible pipelines in offshore installations requires the use of non invasive techniques, as the removal of the external sheath to allow inspection of the axial load carrying wires will increase the chance of structure degradation. Determining the strain in axial wires provides a significant indication of wire breakage and whether or not the structure has been compromised. This paper investigates the feasibility of employing a new technique based on acoustic resonance to monitor the mechanical stress in these wires. Results from experiments with flexible pipelines show that it is possible to obtain the wire’s resonance signals through the outer sheath in some circumstances. The technique, called Electromagnetic Acoustic Resonance (EMAR), produces ultrasonic waves on the wire using electromagnetic transducers (EMAT) and is able to estimate the mechanical stress by measuring the frequency of the acoustic resonances for different polarizations of the shear waves. Variations in coupling quality such as lift-off or even the unknown wire separation will not directly affect the estimated stress values. Laboratory experiments with the armor wires have shown that stress estimation is possible within a range of uncertainty of 18.8 MPa. Measurements on flexible pipelines through the external sheath, composed of 5 mm of polymer layer, two Kevlar layers and an adhesive tape, were made. Higher lift-offs, up to about 9–10 mm, are still possible depending on particular pipeline’s construction.

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