Subsea tiebacks are becomingincreasingly prevalent in oil and gas field developments. As the accessibilityof the production from wellheads becomes more difficult, the need for subseacompression and pumping increases. Compression and pumping require significantpower which can be distributed and controlled from a HVSS (High-Voltage SubseaSubstation). The viability of an Arctic field development will be determined bythe reliability of all elements in the tieback and in particular, thecentralized subsea power distribution system.
This paper presents the conceptual design and capabilities of a modular, expandable and highly reliable subsea HV (High Voltage, U > 1000V)and LV (Low Voltage, U £ 1000V) power distribution system. The assemblyis designed to operate without intervention for extended periods of time and isideally suited to emerging green field developments in the Arctic. The conceptis built from components and subsystems each with a technology readiness levelof six or seven and is therefore well positioned for a prototype build anddeployment with minimal additional qualification.
The core switchgear is built around proven 36kV CBs (Circuit-Breakers)connected together via an SF6-insulated bus bar. The use ofSF6 as a dielectric medium allows the size of the CBs and bus barpressure vessel to be minimized, reducing the mass and thus deploymentconstraints of the assembly. Connections to the switchgear assembly for boththe incoming power feed and switched outputs are via wet mate connectors. Theonly components in the main pressure vessel are the CBs, current transformers, voltage transformers and busbars.
To maximize the overall system availability, the control and protectionsystems for each 36kV CB are redundant and housed in a series ofrecoverable modules rather than inside the main HV switchgear pressure vessel. Should there be a fault in the control and protection system, the redundancy isdesigned such that a faulty module can be replaced without loss of control ofits associated 36kV CB, thus maintaining production availability.
A key and unique feature of this switchgear assembly design is the abilityto power all of the protection, control and communication equipment independentof the main HV AC power. This is achieved by including two high voltage directcurrent and fiber optic cables in the umbilical. At the switchgear module, theHV DC voltage is stepped down to LV DC via a pair of redundant nodes thatdistribute auxiliary power and communication capabilities to the subseaequipment. By independently powering the control and monitoring system, thestatus and position of each CB can be known during a black start without theneed for a subsea battery-supplied UPS (Uninterruptible Power System). Inkeeping with the simple and robust design philosophy, only critical telemetryis provided to monitor and control the system. Where telemetry is provided, itis also redundant
