Aoka Mizu FPSO - Xodus Group
Client: Bluewater Energy
Location: UK Continental Shelf

Designing gas import facilities to overcome transition from gas exporter to gas deficient facility.

Xodus Group’s integrated team prepare a FEED package for importing gas from the SAGE pipeline system into the Aoka Mizu FPSO vessel.

The Aoka Mizu is the Bluewater owned and operated Floating Production, Storage and Offloading (FPSO) vessel, deployed at the Ettrick field for Nexen Petroleum UK in the UK North Sea.

Ettrick associated gas from oil production is processed on the Aoka Mizu and exported via the Scottish Area Gas Evacuation (SAGE) pipeline system to the St Fergus gas terminal in Scotland. Nexen established that the Aoka Mizu facilities would become gas deficient and be unable to supply sufficient gas to satisfy fuel demand before the end of the Ettrick contract / field life. In addition, as the reservoir pressure dropped and produced fluids became heavier, there would be an additional gas demand for the gas lift required to kick-off production after a shutdown.

As gas would need to be imported to meet the future fuel gas and start-up lift gas demand, Xodus Group were asked to prepare a Front End Engineering Design (FEED) package for importing up to 13 MMscfd of gas from the SAGE pipeline system into the existing Aoka Mizu facilities.

Assignment scope
We reviewed and modified an existing conceptual design before embarking on the brownfield FEED modification. The new import gas equipment consisted of a fiscal metering skid, electric heaters and a pressure let down facility. Additional facilities for pressurising sections of the topsides, pressure relief and shutdown would also be required.

One of the major challenges was the high pressure and low temperature of the import gas, causing very low temperatures to occur during relief or manual depressurisation. A detailed investigation into the effects of the possible import gas compositions was also necessary.

Further challenges arose from importing gas in between the first and second stage HP compressors that operate on a common driver. To accommodate the new import stream the compressor control scheme was modified. Moreover, the project needed to be completed within a short time period to enable Bluewater to install the facilities during the next shutdown, prior to gas deficiency.

The primary tasks Xodus performed were production of the FEED package deliverables, including the basis of design document, process flow diagrams, heat and material balance calculations, piping and instrumentation diagrams and philosophies. The main areas of design were the control scheme, low temperature flaring issues and the material corrosion concerns.

Other key features of the work we performed included:

  • A comprehensive site survey to assess feasibility of tie-in and skid locations
  • Dynamic simulation to assess low temperature challenges during relief and blowdown
  • Analysis of the effect of importing hydrogen sulfide, carbon dioxide, oxygen and water
  • Dynamic simulation to assess the compressor operation and new control scheme for all operating scenarios
  • Design of skid to allow installation with minimum disruption to facility production.


  • Rapid project delivery that met our client’s timescale
  • A FEED that would enable the import of up to 13 MMscfd gas to kick-off two wells simultaneously and provide enough fuel for the topsides
  • Robust design to allow for a wide variation of import gas composition
  • Control methodology to prevent problems when the FPSO transitions from exporting gas to becoming gas deficient
  • Robust compressor control design proven on a dynamic simulation package
  • Negation of the low temperature issues arising from the low temperature and high pressure of the import gas.