Page 130 - Pipelines and Risers

Finite Element Analysis of In-situ Behavior 103

Figure 7.6 Two node twelve degrees of freedom 3D finite pipe element.
The hybrid elements are provided by ABAQUS for use in cases where it is numerically
difficult to compute the axial and shear forces in the beam by the usual finite element
displacement method. The problem in such cases is that slight differences in nodal positions
can cause very large forces in some parts of the model, which, in turn cause large motions in
other directions. The hybrid elements overcome this difficulty by using a more general
formulation in which the axial and transverse shear forces in the elements are included, along
with the nodal displacements and rotations, as primary variables. Although this formulation
makes these elements more calculation intensive, they generally converge much faster when
the pipe rotations are large and are more efficient overall in such cases.

The PPE31H element is available with a hollow thin-walled circular section and supports the
possibility for the user to specify external and/or internal pressure. The element can also
account for changes in the pipe section area due to high axial straining of the pipe.

The R3D4 element
The four-node R3D4 rigid element (Figure 7.7) makes it possible to model complex surfaces
with arbitrary geometry’s and has been chosen when modeling the seabed topography. A very
important feature of ABAQUS when modeling the seabed has been the possibility to smooth
surfaces generated with the rigid elements, this leads to a much better representation of the
seabed than the initial faceted surface.

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Figure 7.7 R3D4 rigid element, and example of smoothing of surface created with rigid elements.