Durability of Composite Materials for Underwater Applications

Abstract

Deep sea applications of composite materials are increasing rapidly (Choqueuse D and Davies P in Ageing of Composites Woodhead, 2008). Light weight is critical for submarines structures, in order to facilitate their underwater deployment and increase pay-load, and various specific properties (buoyancy, thermal insulation, remarkable behavior with respect to contact with water,…) strongly favor the use of composite materials. Currently the three main sectors concerned are: the offshore oil and gas industry, the navy, and oceanographic equipment. Important developments are ongoing in the oil and gas offshore sector in particular for riser application (Airbone, 2012). In the oceanographic field, a composite pressure capsule was recently developed for a manned submarine to reach the deepest point of the oceans (Black S, High performance composites, 2010). For submarine housings the first point to be solved is the capability to withstand the hydrostatic pressure, which is the main design factor. Safety factors applied are generally high in order to take into account the difficulty in designing composite structures subjected to compression loads. Generally the parameters affecting durability (creep, fatigue, water absorption) can be neglected and are integrated in the static safety factor. Nevertheless, considering that long life times are expected (nominally 25 years) the durability of underwater composite structures has to be considered, and the main question marks, specific for underwater applications, still globally unsolved are the following: (1) Fatigue in sea water, for riser applications; (2) Creep-fatigue interactions and high compressive strains; (3) Effect of pressure on water uptake kinetics and resulting degradation.