33 kV Subsea Power Re-configuration Project

Marathon Oil

Project title:
Oil Field Decommissioning

Key Areas:
North Sea

September 2016

33 kV Subsea Power Re-configuration Project

Two of the three platforms generated power for to supply all three platforms. Marathon Oil proposed to decommission one of the platforms which operated with three Gas Turbine Generators and the remaining generating platform supporting the non generating platform. The power management system that manages the power flow between all three platforms was complex.

Marathon Oil requested EPS to carry out a number of transient motor starting scenarios to determine whether the existing 33 kV power network would enable a 4.35 MW motor to start on their East Brae platform. The main concern was whether the limited power generation would maintain a voltage within a transient starting voltage dip limitation of no less than 20%.

At the time Marathon Oil did not have a detailed work scope, EPS assisted in creating a work scope in the form of a project plan, the plan consisted of a structured logical plan to help Marathon achieve decommissioning of the Bravo within a certain time period. EPS set and achieved the realistic delivery dates for the following activities:


  • Dynamic motor and load model creation
  • Gas turbine generator dynamic model
  • Check and verify the existing power system model (originally constructed some time ago for an arc flash study)
  • Power system Transient stability model
  • Transient motor starting report
  • FEED study report
  • Offshore motor starting measurement recording
  • Determination of worst case events and scenarios
  • A new 33 kV topside ‘T’ connection solution incorporating new 33 kV switchgear to supply a decommissioning load
  • Interplatform protection schemes and designs
  • Transformer tap setting optimisation (once the Brae Bravo has been removed)
  • Statement of Requirements for 40 MVA transformer energisation studies
  • Transformer energisation studies
  • Worst case scenario transformer energisation scenarios and events
  • Platform neutral earthing schemes and solutions

Load flow, short circuit and protection analysis with an arc flash study revision followed.