SEABED SEDIMENT TRANSPORT AND OFFSHORE PIPELINE RISKS IN THE AUSTRALIAN SOUTHEAST

Abstract

The Australian seabed is influenced by extreme weather conditions of various types: cyclones, high tidal ranges, offshore currents and storm waves. In the past two centuries substantial progress in our understanding of the seabed and environmental conditions has been made by studies of the seabed sedimentology, hydrodynamics, and through habitat mapping. As part of the CSIRO Wealth from Oceans Flagship program the authors are involved in a new fiveyear study to investigate and predict the effect of possible climate change scenarios on the seabed in the next 50 years. Those changes will assess undersea infrastructure installations such as pipelines in a changed regime of seafloor stability, burial, erosion and abrasion.As an initial phase of this project the authors have extrapolated the current climatic conditions into the next 50 years. It was found that (under an extension of present day climatic conditions):the majority of terrigenous sediment carried down by major rivers (Murray River, Snowy River, Tamar River, etc) is trapped in the inland lakes or estuaries. Only a marginal fraction of fine grain sediment reaches the continental shelf;a high energy wave climate, significant tidal currents, and the frequent surges of wind-driven currents make the local seabed highly mobile and sensitive to the modern hydrodynamic changes. Both results imply that an exposed pipeline may suffer from local scour and fatigue damage due to oscillatory loads induced by vortex shedding behind the pipelines; andthere are several high risk zones in the region where turbidity current and submarine slope failure post a great threat to offshore pipelines.The next phase of the project will be to provide testable predictions of the changes under a variety of global warming scenarios.