Page 88 - Numerical Analysis and Modelling in Geomechanics

WAVE-SEABED-STRUCTURE INTERACTION 69
Application of GFEM-WSSI
This section, will demonstrate the application of the proposed general finite
element model on the wave-seabed-pipe interaction problem. The configuration
of the problem is depicted in Figure 3.2. In the problem, the boundary condition
at the seabed surface can be written as

(3.
21)

where p denotes the amplitude of the wave pressure at the surface of the seabed,
o
d is water depth, H is the wave height, k is the wave number and ω is the wave
frequency. In (3.21), “Re” represents the real part of the function in the brackets.
Since there is no flow through the pipeline wall, thus the pressure gradient on
the surface of the pipeline r=R, i.e.,

(3.22)

where x o and z o denote the co-ordinates of the centre of the pipe and n is the
normal direction to the surface of the pipeline. As mentioned previously, because
a concentration of stresses is expected, the local refinement of the finite element
mesh always has to be taken into account in the region near the structure, as
shown in Figure 3.3.
To verify the proposed finite element model for the wave-seabed-pipe
interaction, the experimental data from Turcotte et al. (1984) is used here
(Figure 3.4). The experimental study was conducted in the J.H. Depress
Hydraulic Laboratory at Cornell University. The input data of the experiment are
listed in Figure 3.4. As Turcotte et al. (1984) reported, the sediment was
considered to be uniform and isotropic. Although the case that Cheng and Liu
(1986) considered is slightly different from the present study, their results are
also included in the example. In Figure 3.4, the solid line represents the present
model, • denote the experimental data, and ≥ are the results of Cheng and Liu
(1986). As seen in the figure, the present model reasonably agrees with the
experimental data.
To enhance the current understanding of the mechanism of the wave-seabed
interaction in the vicinity of an offshore pipeline, several important parameters will
be investigated in this section. The input data for the numerical examples are
given in Table 3.1.

Effects of soil characteristics

Figure 3.5(a) demonstrates the difference in pore pressure (p/p ) between
o
isotropic and cross-anisotropic seabeds of coarse sand. There is a slight
difference between the two different soils. An examination around the pipeline