Subsea Control System 

A UPS typically operates by rectifying and smoothing the incoming supply, converting it to direct current (DC), which can then be used to charge associated batteries. The output from the batteries is then converted back to alternating current (AC) ready for use to power the subsea system. In case of failure of the main incoming supply, the output from the batteries is quickly switched to power the DC to AC converter, thus ensuring a constant supply.

The technology behind modern UPS devices is varied and sophisticated, all aiming to improving the electrical efficiency of the conversion from AC to DC and back again, and for controlling the switchover from main power to standby power.

Constant-voltage battery chargers usually use thyristor^[1] technology to ensure a high level of reliability and switchmode rectifiers offer very high power to volume ratios. Designs are usually modular to allow for expansion and redundancy with ‘hot swap’ capability, including control and remote power management with full system and battery monitoring.

Usually the UPS is purchased from a specialist manufacture of such devices and is not built by a subsea control system supplier. The UPS is often procured as part of the overall Platform/FPSO supply and therefore not by those responsible for the control system.

The size of unit required depends on the existing and future field requirements and it is important to agree at the FEED stage the required maximum output required, especially if planning for future expansion. The field power requirement is never easy to predict and will vary with the different control system suppliers. It should be noted that space on a Platform or FPSO for such units is often at a premium, and adding a second unit later or additional batteries often causes design and implementation problems.

The equipment must eliminate mains borne electrical noise, sags, surges and transient spike and offer extremely reliable and economic computer grade power, else the specification of the subsequent EPU will need to deal with these problems. The unit will also provide isolation from the rest of the platform/FPSO systems and should not contravene any EMC regulations nor reflect electrical noise back into the Platform/FPSO supply.

Modern units provide interfaces to the Platform SCADA system, and it is not usual for the UPS to be controlled or monitored by the MCS in any way.

Models are available for single or three-phase outputs.

A UPS should be tested at all extremes of input and output power, temperature, and with injected mains-borne electrical noise to simulate a real Platform/FPSO environment. It is desirable to test it with the EPU and remaining subsea system as part of an SIT if possible, although, as mentioned above, it is often procured separately to the Control System and is therefore rarely available until commissioned offshore.