This paper describes the performance assessment programme initiated for the first operating DC all-electric control system. An overview of the applied method considering both model and data uncertainties is provided. In addition, the field experience gained within the successful installation, commissioning and operation of the system to date is discussed.
While presently obtaining operational experience subsea, significant data assessment and processing capabilities of the first operating DC all-electric control system can be utilised. Key parameters of interest and their context are investigated in a thorough-detail, permanent monitoring of the condition of the system. By using this approach, expert confidence level increases as additional decision support is provided. The systematic condition evaluation is based on a continuous probabilistic investigation which complex physical phenomena tend to impose. Preventive planning of maintenance leading to long-term cost reduction is enabled.
The central scope of an all-electric subsea control system consists of electric actuation allowing virtually zero response time, along with real-time reading and system feedback. The communication signal can be superimposed on the power transmission.
On topside the DC electric system consist of a master control station (MCS) utilising standard human machine interface (HMI) software and two independently operating electrical power and communication units (EPCU). Each channel (A and B) serves two subsea modules which are remotely operated vehicle (ROV)/diver retrievable. Prior to access to an electric subsea control module (eSCM), the power regulation control module (PRCM) regulates power depending on actual demand and demodulates the incoming communication signal.
The eSCM controls the subsea tree and manifold functions and also provides the interface for subsea instrumentation. More specifically, the PRCM converts and regulates the 3kV DC input from topside to 300V DC for each channel and ensures further transmission of the communication signal. The reduced voltage and independent communication signal are then transferred to the eSCM. The latter controls valve functions allowing the transmission of more than 250 different associated parameters, such as actuator parametric data, and communicates with surface control equipment. Sensor information is received and transmitted. A sketch of the system concept is shown in Figure 1.
In order to minimise energy losses over long distances, DC, rather than AC, was chosen to transport electric power. This choice also allows for a reduced cable cross-sectional area in comparison with AC transmission. In comparison with the umbilical of a conventional electrohydraulic (EH) actuation system, the all-electric umbilical cross-sectional area is reduced as all hydraulic lines (LP/HP/return) are removed.
An additional feature of a DC system is that seawater can be used as the electrical return path closing the electric circuit through the use of dedicated anodes and cathodes. The condition of the seawater return path is observed, and relevant data is transferred via the eSCM; however, this feature is not mandatory. The screens and shields of the umbilical itself can be used as a backup route, should the anodes or cathodes fail.