Model-Based Design of Energy Accumulators for Control of Subsea Wells

Hydraulic accumulators are vessels charged with inert gas used to store pressurized fluid to actuate specific functions. In particular, they are widely used as controls for remote system such as in deep water drilling. In this application, they assume a fundamental importance because they are responsible of the actuation of the blowout preventer valves (BOP), which have to be intrinsically safe and reliable. A direct method (DM) for the design of the subsea rapid discharge accumulators is presented and compared with the API 16D Method C, which is the primary international standard concerning the accumulators sizing. The design must ensure that the entire functional volume required (FVRtot) by all the functions will be delivered at or above the minimum operating pressure (MOPi). The DM presented is based on a fully mathematical model of the charging and discharging phases, which evaluates the pressure inside the accumulators during all the actuations. The actuator design includes physical representation of the processes, the influence of the operating conditions, and the effect of thermal uncertainties. A specific “failure plane” has been demonstrated, in a sequence of three actuations, where failure at specific condition of subsea and surface temperatures may occur.