Acoustic emission analyses of the hygrothermal ageing of glass syntactic foams

In deepwater offshore production, thermal insulation of pipelines and risers is now commonly made with glass syntactic foams. These composite materials offer both good insulation properties and pressure resistance, and also provide buoyancy to the systems. But service conditions are increasingly severe, including pressure (up to 30 MPa at 3000 m water depth), water environment and temperature (up to 130 degrees C in contact with the steel tube). These coupled loadings induce complex effects on the material and make the prediction of its long-term behaviour difficult. Over the past 10 years, many studies have been performed to better understand the behaviour of the glass syntactic foams used as thermal insulation of pipes for deepwater production. Ageing tests in hot water revealed some specific behaviour of these materials with coupled chemical and mechanical phenomena of degradation. The experimental techniques included gravimetry, mechanical and physicochemical characterisations, impedance spectroscopy, X-ray tomography. As the rupture of glass microballoons is relatively emissive, acoustic emission technique has also been used. The contribution of this latter technique to the understanding of the syntactic foam behaviour is presented in this paper. Some ageing tests in water at several temperatures and on several syntactic foams have been conducted. Mechanical characterisation has been performed using a confined compression set-up instrumented with an acoustic emission sensor to allow the monitoring of glass microballoons degradation vs. time and temperature of ageing. These results are analysed for syntactic foam materials having various compositions. The comparison to the information given by other techniques is also discussed.