Page 129 - Pipelines and Risers

102 Chapter 7

inverted, and a set of simultaneous non-linear dynamic equilibrium equations must be solved
at each time increment. This solution is done iteratively using Newton’s method.

7.4 Element Types used in the Model

Three types of elements are used in the established finite-element model (Figure 7.5). These
are:

The rigid elements of type R3D4 used to model the seabed.
0 The PIPE3 1H pipe elements used to model the pipeline.
The GAPSPHER elements that are used as a winch when lowering the pipeline from its
initial position and down at the seabed (see Figure 7.1). These elements are removed from
the model when the pipeline has landed and gained equilibrium at the seabed.

Figure 7.5 Element types used in the model.

The PIPESlH element
The 3D finite pipe element (Figure 7.6) used in the established model is the two node twelve
degrees of freedom PIPE31H element. The element uses linear interpolation and therefore has
a lumped mass distribution. The hybrid formulation makes the element well suited for cases
with slender structures and contact problems, such as a pipe lying on the seabed.