Page 86 - Numerical Analysis and Modelling in Geomechanics

WAVE-SEABED-STRUCTURE INTERACTION 67
Numerical procedure
The present model is able to simulate the wave-seabed interaction problem as well
as the wave-seabed-structure interaction by the general finite element model
described in the previous section with different meshes.
The first step to solve the wave-seabed-structure interaction problem is to obtain
the lateral boundary conditions. To do so, the wave-seabed interaction can be
solved by employing the principle of repeatability (Zienkiewicz and Scott,
1972). As shown in Figure 3.1, the wave-induced pore pressure and soil
displacements at sections AA and BB should be identical, because the seabed is
under a periodical loading. That is,

(3.20)

This concept is particularly convenient for periodical loading such as the present
problem (Jeng et al., 1998).
Once the lateral boundary conditions are obtained, the whole wave-seabed-
structure problem can be solved with the boundary conditions for different types
of structures. Because a concentration of stresses is to be expressed, the local
refinement of the finite element mesh always has to be taken into account in the
region near a structure.
To ensure the accuracy of numerical calculations without increasing the finite
element mesh, an eight-node element is used in the presented model. An
isoparametric element is used near the structures, and a rectangular element is
used in the seabed.

Applications
To demonstrate the application of the proposed general finite element model, two
classic marine structures are used as examples in this study a buried pipeline and
a caisson-type breakwater. Based on the model, but with different finite element
meshes, the wave-seabed-structure interaction problem can be solved.

Wave-seabed-pipe interaction
Offshore pipelines are extensively used to transport hydrocarbons to shore. There
is also widespread application for ocean disposal of municipal waste. Design of
marine pipelines with respect to their stability is a complicated problem, and
improved design represents one of the main areas currently targeted by oil
majors for reducing the costs of offshore oil and gas developments. Wave-
induced failure of offshore pipelines has been well documented (Herbich, 1977).
Failure may occur as a result of wave-induced scour, liquefaction, or slope
instability. To protect the pipeline from possible damage caused by waves or