Stability of marine pipelines on unstable and liquefied seabed

TC Teh, Cambridge University
Geotechnical Engineering Group


The conventional approach for stability design of marine pipelines assumes that the seabed itself is always stable. Field experience, laboratory investigations and theory show that this assumption is incorrect during severe wave conditions, and that the seabed becomes unstable long before the design conditions for the pipeline are reached. It is therefore the objective of this thesis to understand the behaviour of a marine pipeline on a seabed that is mobile and liquefied, and ultimately to devise a rational design method.

An experimental investigation was carried out in a wave flume to study the stability of a pipeline on an unstable and liquefiable seabed. A wide range of commonly used, industrial pipeline specific gravities and wave conditions were examined. The results showed that for the given soil the seabed response is governed mainly by liquefaction, and also that the pipeline behaviour on an unstable seabed is strongly dependent on its specific gravity. During liquefaction, a light pipeline will float and a heavy pipeline will sink.

Density measurement in parallel with excess pore pressure measurement in the experiments shows that liquefied seabed behaviour is different from what has been widely presumed in the past. Deep soil layers are observed to progressively consolidate during cyclic shear at low stress ratios, expelling water due to an upward hydraulic gradient. Soil at shallower depths is observed to dilate under the influence of cyclic shear at large stress ratios, fed by water from below. As superficial soils cyclically dilate towards their critical state density at zero effective stress they become truly liquefied, even to the extent of going into suspension. This thesis examines the paradoxes in the conventional definition of soil liquefaction and proposes a new mechanism for wave-induced seabed liquefaction.

Finally, the thesis proposes a failure mechanism for a pipeline on a liquefied seabed and an alternative approach for designing the pipeline stability. This includes equations that govern the displacement of a pipeline on a liquefied seabed, which are used for the computation of new design charts. The new design method is verified against measurements from the wave flume experiments, and also compared with the conventional design method.

Keywords: waves; pipelines; stability; liquefaction; seabed; design