Qualification Program for Subsea Batteries

Abstract

Cost reduction, deep water, long-step outs, HSE (Health, Safety and Environment) and reduction of carbon footprint drive the change from hydraulic to all-electric subsea control systems in the offshore oil and gas production. The main challenge of an all-electric control system is the safe and reliable actuation of the valves on the tree. A battery provides the power and energy to supply the electric actuators to close the valves in case of an ESD (Emergency Shutdown). The paper presents the qualification program developed to access the performance and safety over the design life of 25 years together with the results after a 3-year qualification program. The qualification program was based on a risk assessment taking into consideration the uncertainties associated to this novel subsea technology and the API 17F for qualification requirements. The qualification program developed covers the following main sections: Misuse (destructive testing), Cycle Testing, Environmental Testing and EMI (Electro-magnetic Interference) testing. To access the state of health a large signal EIS (Electrochemical Impedance Spectroscopy) has been developed and applied throughout the qualification program together with the DC-resistance and capacity measurements. For the cycle testing a DoE (Design of Experiment) methodology has been applied to develop a test plan which includes the parameters: temperature, cycles and idle time. The testing was performed on 30 cells in parallel. The qualification results show a degradation of the battery depending on the parameter sets. The EIS measurements correlate with the degradation of the battery performance. The testing parameters range from −4°C to 50°C to stimulate an increased degradation. A reference group was used to access the acceleration factor for the increased temperature. The DC (Direct Current) measurements show an increase of the internal resistance over time. The results provide details for a qualification program for subsea batteries in order to access the performance over the lifetime of 25 years. The program includes additional testing and monitoring compared to the API 17F requirements for PCBs (Printed Circuit Boards). The qualification program also provides information about the degradation kinematics of Li-Ion Batteries over the design life of 25 years. Based on the results of the qualification program a design life of 25 years can be concluded.