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 on-bottom steady-state currents of 2 m/s or more. Consequently, in shallow water depths, typically less than 40–60 m, subsea pipelines can experience very high hydrodynamic loads, potentially causing significant lateral movement. If the seabed is rugged, or at locations where the pipeline approaches a point of fixity, this can lead to the pipeline suffering mechanical damage, which is undesirable.
In many places on the Northwest Shelf of Australia, there is a layer of minimum 3 m deep marine sediments. The sediments predominantly comprise of relatively stable, fine to medium sized carbonate silts and sands, sometimes with some clay content. Traditionally, in Australia and other parts of the world, post-trenching techniques such as ploughing and jetting have been applied in such areas. These techniques can successfully lower the pipeline into the seabed. However, in many situations on the Northwest Shelf of Australia, post-trenching has had limited success. This has in part been due to the unpredictable levels of cementation of the carbonate sand, which has often resulted in an insufficient trench depth, with the need to implement costly and time consuming remedial works to ensure pipeline stability.
The uncertainties in the success of post-trenching tools lead to the development of the pre-trenching and sand backfill method, which was first applied in Australia in 2003 on a 42-inch diameter natural gas trunkline. This technique has several advantages compared to post-trenching and other conventional pipeline stabilization methods such as rubble mound pipeline covers or gravity anchors.
This paper presents an overview of the pre-trenching and sand backfill method, its design principles, benefits, and risks and opportunities.