This paper describes the electrical power infrastructure including high voltage and low voltage power transformers, switchboards, distribution units and their associated connection and protection systems required to power a subsea EHTF installation. It discusses the options considered for both topsides and subsea electrical power infrastructure, their integrity monitoring and methods of fault protection.
The low power EHTF PiP system, developed by Subsea 7 and ITP InTerPipe, typically requires a subsea supply operating voltage of 1kVrms phase to neutral at the flowline start and the requirement to supply multiple heating wires arranged in subsets of three phase circuits, typically 12. This voltage sits at the challenging intersection of normal industry low voltage (<1kV) and high voltage standards (>1.8kV) and thus relevant clauses have to be selected to fit the qualification purpose of the full EHTF PiP system.
This voltage can be supplied from either local or remote topsides facilities bringing with them the option of multiple power circuits direct from topsides to subsea or alternatively using a subsea distribution/splitting system. The electrical power system needs to be able to either adjust the applied voltage, the time applied or the number of heating elements in order to control the power required by the pipeline heating system for different production scenarios and redundancy purposes. It also needs full integrity monitoring and appropriate fault clearance facilities that can operate for the local and remote topsides solutions and operate successfully on long umbilicals and EHTF lengths up to ca. 30 km from a single power feed. There may be multiple power feeds on longer lines (Cherkaoui 2016).
Instead of complex phase control systems, a simple heating control, using contactors on individual circuits in the local topsides option provides sufficient EHTF heat control. Whereas the addition of subsea fuse modules and/or solid state relay modules (SSR) in the remote topsides option can provide individual subsea circuit isolation and protection.
The benefits and drawbacks of different power transformer arrangements are reviewed with available topsides electrical network (power balance / voltages / redundancy). Integrity monitoring and fault protection for local and remote topsides options are reviewed; specifically, the application of line insulation monitoring systems and residual current protection devices.