Ichthys Pipeline: laying a new offshore pipeline

In 2014 started the construction of almost 900 kilometres of pipeline  in the ocean north of Australia to transport gas and some condensate (a very light form of oil, by-product of gas extraction) to onshore processing facilities in Darwin. The gas and condensate is being extracted from a newly developed complex of fields. The complex of fieldsis named ‘Ichthys’, after the Greek for fish. But while water is good for fish, its environment poses various damage risks to the Ichthys pipeline. For a proper design it is important to know how these risks can be avoided or minimised.

model anchor in rock berm in GeoCentrifuge

Considerations

The closer you are to the coast, the greater the impact of waves, currents and human activity- like ships’ anchors, for example. At these locations, where the pipeline is more exposed to external threats, it is extra difficult to operate a pipeline with acceptable low risks of damage. The challenge is to find a balance between the risk and consequential damage costs on the one hand and the investment in protective measures on the other hand. To support careful decision making, Deltares helped in this project by providing hydraulic and geotechnical knowledge.

First scale models in the Atlantic Basin

First of all, the effects of current and waves on the design for the protective bank were studied. From the designs tests the protective covers turned out to be hydraulically stable. From a hydraulic perspective, there was even scope to optimise the size of the stones.

Matching the stress level in the GeoCentrifuge

Next it was important to know how well the protective rock cover dealt with the impact of dragged ships’ anchors. New  scale models with precisely scaled down ships’ anchors were investigated in our GeoCentrifuge and tested a stress levels of around 40 times the normal earth gravity. Doing this enabled Deltares obtain highly accurate test results since the GeoCentrifuge can create stresses in the model which are exact matches of the stresses in the much larger real-world pipeline and rock-berm.

Report

Deltares incorporated the results of the hydrodynamic and geotechnical tests into a comprehensive report setting out recommendations on the best design and structure for the protective barrier, with variations – in terms of the quantity, size and shape of the stones – for different types of seabed. It turned out that the required level of safety can be achieved with 40% less stone than the developer had originally thought.