Hydraulic capacitance compensation for long umbilical lines in hydraulic control system of Offshore Subsea Christmas Trees

To optimize the umbilical line design of the Offshore Subsea Christmas Tree Control System (OSCTCS) and reduce its cost, a mathematical lumped parameter model of the umbilical is developed. Taking advantage of the intermittent flow-rate need of the OSCTCS and the hydraulic capacitance compensation effect of the long umbilical lines between the Hydraulic Power Unit (HPU) and the Subsea Control Unit (SDU) combining with a subsea accumulator to compensate the decrease of umbilical´s flow capacity, an offshore hydraulic system design method is presented. The study results show that the instantaneous equivalent hydraulic resistances of the umbilical are reduced and the combination of the hydraulic capacitance and the accumulator capacitance could enhance the umbilical´s flow capacity which would optimize the umbilical design.