Polymers are widely used for passive thermal insulation coatings on steel pipe in offshore oil and gas production. In this industry, structures used in deep sea have to be reliable as they are in service for more than 20 years in a very severe environment: sea water, hydrostatic pressure, temperature. One of the main questions is how to test and predict the lifetime of such structures in the laboratory. This study presents one approach that has been developed to characterize and predict the degradation of polyurethanes used as thermal insulation materials. Based on results obtained during accelerated ageing of the PU in sea water, a prediction of degradation through the thickness has been set up taking into account the temperature profile in the coating, water absorption and hydrolysis. Validity of this model has been investigated by comparing predictions with experimental data obtained on a real structure that has been aged for more than a year with an internal temperature up to 125°C in water under hydrostatic pressure. Using this prediction, the effect of different parameters (such as coating thickness, internal and external temperature) on the degradation level of a structure has been examined.