Wheatstone Umbilical Stability

Background

Chevron’s Wheatstone Project off the coast of Western Australia, comprises of the Wheatstone and Lago offshore gas fields. Gas from these fields is flowed from subsea wells, through manifolds and via insulated flowlines to the Wheatstone Platform for processing. Umbilicals are laid between the platform and the wells to allow control and monitoring. The design analysis considered three umbilical cross sections:

Option Armour OD
(mm)
Mass in Air
(Tubes Flooded) (kg/m)
Submerged Weight
(N/m)
SG
1 None 156.6 29.6 117.3 1.61
2 2 x 4.0 mm 186.6 57.0 316.2 2.15
3 2 x 4.5 mm 188.6 62.5 364.2 2.29

Traditional on-bottom stability calculations showed that none of the umbilical options were stable for the design conditions and would require costly secondary stabilisation measures.

Work Performed by Atteris

With traditional on-bottom stability methods failing to take account of the effect of seabed mobility, Atteris developed a novel approach to this on-bottom stability problem which would take into account the benefits of seabed mobility. The methodology used a combination of engineering approaches, including results from stability testing performed in the Large O-Tube facility at the University of Western Australia, theoretical analysis based on soil mobility and self burial, and stability behavioural evidence from existing facilities in the vicinity.

Benefits

In-house methodologies for the design of umbilicals and pipelines on mobile seabeds resulted in the elimination of secondary stabilisation from the project base case schedule and cost estimate.