Page 87 - Numerical Analysis and Modelling in Geomechanics

68 D.S.JENG

Figure 3.1 Principle of repeatability.

currents, it is common to use an appropriate covering layer with rock-fill or other
coarse material. This expenditure often accounts for a large proportion of the
total budget of a pipeline project.
Various numerical investigations for the wave-seabed-pipe interaction have
been carried out in the past. Cheng and Liu (1986) proposed a boundary element
model to examine the wave-induced pore pressure and effective stresses around a
buried pipeline. They discovered the great influence of the soil parameters on the
pore pressure concentration around the buried pipeline. Later, Magda (1996,
1997, 2000) carried out a series of studies on the wave-induced pore pressure and
lift forces on buried pipelines. These studies were carried out using finite
element modelling. Magda considered a similar case to Cheng and Liu (1986)
with a wider range in the degree of saturation. In both Cheng’s model and
Magda’s model, they considered the pipeline to be buried in a region surrounded
by impermeable walls. In fact, realistically, the buried pipeline is surrounded by
a cover layer, which consists of a coarser material. Thus, the lateral boundary
conditions for the wave-seabed-pipe interaction problem should be permeable,
rather than using impermeable walls.
Recently, Jeng and Cheng (2000) proposed a finite difference method in a
curvilinear coordinate system to investigate the wave-induced seabed instability
around a buried pipeline. They found that the soil particles always move
away from the pipe when the wave crest passes the centre of the pipeline.
However, their model did not work for finer materials (for example, permeability
smaller than 5×10 −4 m/sec) under a certain combination of wave and soil
conditions. Based on the numerical model proposed by Jeng et al. (1998), the
mechanism of the wave-seabed-pipe interaction has been investigated (Jeng and
Lin, 1999, 2000; Postma, 2000; Jeng, 2001; Jeng et al., 2001b). Some important
results will be presented in this section.