Subsea Christma Trees decommissioning - Neutralising the potential energy of Belleville springs in the valves

This study investigates the behaviour of Belleville springs used in subsea tree valves. The study includes (i) a survey of the literature on the modelling of springs, (ii) mathematical and numerical models that predict the behaviour of Belleville springs in terms of force-deflection of springs, critical stresses, and stored energy (iii) and the use of locking substances to freeze the potential energy in deformed springs. The mathematical and numerical models proved to give excellent agreement and predicted the field data (provided by end-user) accurately. Therefore, they were instrumental to select appropriate substances that can freeze the potential energy of deformed springs. High performance concrete and high strength resin proved to perform well as locking substances. However, this study proved that ordinary concrete, self-compacting concrete and low-range resin are insufficient to contain the spring of a 5” valve.