SOUE News Issue 9

Geotechnical Engineering for Offshore Pipelines

A 2009 Jenkin Day talk by Byron Byrne

Byron's subject was the flexible steel pipelines for oil and gas that are used to connect offshore installations to each other, to well-heads on the sea-bed, and to the shore. There are many hundreds of kilometres of these, for instance in the Pluto gas field off NW Australia, with which Byron was familiar, and in many other parts of the world.

The problem of interest was the tendency of these pipelines to buckle. When they are laid they are full of sea-water, and hence cold. In use, their contents may be quite hot, so they expand. This leads to two alternative forms of buckling:

"upheaval buckling", if they have been buried (we were shown a photograph of this happening to a pipe in Abu Dhabi);


"lateral buckling", if they have simply been laid on the sea-bed.

Lateral buckling can push the surrounding soil or sand into piled-up banks or "berms". This can constrain further movement, thus leading to excessive stresses in the pipe, hence possible fracture. It can be prevented by laying the pipe on cross-wise "sleepers", along which they can easily slide.

But there is a strong case for burying the pipe, especially in a site where it could be damaged by ships' anchors or fishing gear. In a soft or sandy bed this can be done with a trenching machine (an expensive item, e.g. $10m), which fluidises the sand with water jets, thus allowing the water-filled pipe to sink into it. The sand then settles back above the pipe, and, it is hoped, holds it down. But when the pipe is filled with gas it is much lighter, so any expansion will tend to lift it up. They had been conducting model experiments to measure how much force is necessary to lift a pipe out of its sand-filled trench. The force is quite high initially, but falls off, much as one would expect, as the pipe rises towards the surface. But if the sand gets fluidised it gets lighter, and the holding-down force is much reduced. There is some evidence that vibration in the pipe, e.g. from a travelling "pig" sent down to clean it, can initiate fluidisation, and hence cause "upheaval buckling".

If upheaval occurs, or is thought likely, one solution is to lay heavy rocks on top of the trench. But this may just transfer the upheaval to some other point, and we were shown pictures of where this had happened. One can end up having to lay rocks along the whole length - rather expensive!

Byron's group were also studying what happens in the pipelaying operation at the crucial point where the pipe coming down from the barge enters the trench.

<<   Previous article Contents Next article   >>