Rock Berm Design for Pipeline Stability

Abstract

Stabilizing large diameter natural gas pipelines on the seabed against extreme hydrodynamic loading conditions has proven to be challenging in the northwest of Australia. Tropical storms, which affect the area annually between November and April, can generate wave heights exceeding 30 m and storm steady state currents of 2 m/s or more. Consequently, in shallow water depths, typically less than 40–60 m, subsea pipelines can be subjected to very high hydrodynamic loads, potentially causing significant lateral movement. To mitigate the risk of the pipeline suffering mechanical damage due to excessive lateral movement, quarried and graded rock is often dumped over the pipeline as a secondary stabilization solution. In order to satisfy functional requirements, the rock berm must comprise of a sufficiently large rock grading size and berm volume to withstand the design hydrodynamic loading such that the pipeline cannot break out of the berm. The design of rock berms for pipeline secondary stabilization has traditionally followed a deterministic approach that uses empirical equations for preliminary rock sizing, followed by small-scale physical modeling for design verification and optimization. Whilst the traditional approach can be effective in producing a robust rock berm design, opportunities for further optimization are inhibited by a lack of available data and an imperfect understanding of the failure mechanisms. This paper presents an overview of an improved approach for rock berm design optimization. A general overview of rock berms, the design principles, benefits and risks are also presented.