Telstra Cable Stability

Background

Telstra has been awarded a contract to install a subsea Fibre Optic Cable (FOC) network in the North West of Western Australia comprising of approximately 500 km of 1‑inch fibre optic cable. Water depth, cable heading, seabed conditions and metocean conditions vary along the cable route.

Work Performed by Atteris

Traditional on-bottom stability calculations as per DNV-RP-F109 indicate that the proposed fibre optic cable is highly unstable along the majority of the cable route and would  require the installation of secondary stabilisation.

Recognising that DNV-RP-F109 ignores the reduction of the wave velocity due to the effect of the seabed shear stress boundary layer, Atteris identified the opportunity to improve the analysis given that the diameter of the FOC is very small in comparison to conventional subsea pipelines. As the boundary layer height is only a few centimetres high, it has only a very minor effect on the average wave velocity acting on subsea pipelines. However, for the small diameter FOC the seabed boundary layer effect is far more significant.

Atteris developed a method to incorporate the beneficial effect of the wave boundary layer in the stability analysis and demonstrated that the cable in general will be self-stable.

Benefits

Through consideration of the beneficial effects of wave velocity reduction due to the seabed boundary layer effect, on-bottom stability calculations performed by Atteris have shown that the vast majority of the cable route will have adequate self-stability, effectively eliminating any requirement for secondary stabilisation.