Lambert Deep Umbilical Stability

Background

Woodside’s development of the Lambert Deep field on the Nother West Shelf of Western Australia includes the installation of a flexible flowline and umbilical from the field to the Angel platform. The 14 km long flowline and umbilical traverse a seabed consisting primarily of carbonate silty sand, with localised calcarenite outcrops.

Work Performed by Atteris

Stability of the umbilical was initially assessed using traditional two‑dimensional stability analysis in accordance with DNV-RP-F109. The results of this analysis concluded that the umbilical was unstable when subject to 100 year return period cyclonic conditions, particularly in the shallower water near the platform. The 2D stability assessment indicated that 402 concrete mattresses would be required to ensure the absolute stability of the umbilical.

Given the cost associated with the installation of concrete mattresses, Atteris reassessed the stability using a combination of three-dimensional dynamic stability analysis – using Atteris’ in‑house and verified stability software CORUS 3D, and an analytical stability assessment, considering the following:

  • Traditional two-dimensional absolute stability design (DNV-RP-F109)
  • Seabed scour calculations considering hydrodynamic and geotechnical conditions
  • Atteris database and experience of physical model testing
  • Field evidence of lines with similar properties in the vicinity to Lambert Deep

Benefits

Through a combination of in-house dynamic stability software and a novel approach to stability analysis considering the beneficial effects of seabed scour, , Atteris reduced the number of concrete mattresses required to ensure umbilical stability from 402 down to 17, providing a significant cost reduction to the client.