Decommissioning of FPSO Turret Mooring Buoy

Background

OrcaFlex analysis was performed to simulate the proposed decommissioning method for the FPSO turret mooring buoy which consisted of a 9-point mooring system and was located in 826 m water depth. The purpose of the analysis was to determine the response of the buoy as the supported risers and mooring lines are released in a predetermined sequence.

Work Performed by Atteris

The submerged depth of the disconnected FPSO turret mooring buoy and the configurations of the attached lazy wave risers and 9-point mooring lines were modelled in OrcaFlex to replicate the as-surveyed conditions. Based on the model, the following were determined:

  • The change in depth and stability of the spider buoy as the risers and mooring legs are released to complete the re-float for recovery procedure.
  • The number of risers and / or mooring lines that require to be released to allow the buoy to float to the surface.

Benefits

Atteris’ dynamic analysis demonstrated that the turret mooring buoy of a client’s FPSO mooring system could be brought to the surface safely via the sequential release of the supported risers and mooring lines. This operation was proven to be achieved without the need for any corrective ballasting, simplifying the operation to minimise risk and expense.

The analysis also showed that 3 mooring lines should remain as these were critical to the stability of the buoy. This conclusion resulted in the amendment of the decommissioning strategy and improved safety of the operation.