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Chapter 7
Finite Element Analysis of In-situ Behavior
7.1 Introduction
The design of high-pressurehigh-temperature (HP/HT) pipelines on an uneven seabed has
become an important issue in the recent years. The need to gain further insight into how
expansion, seabed friction and free spans influence on the pipeline behavior through selected
load cases is the background for this chapter. The behavior of such pipelines is largely
characterized by the tendency to undergo global buckling, either vertically if trenched or
covered, or laterally if the pipeline is left fully exposed on the seabed. The main concern in
the design of slender pipelines operating under HP/HT conditions is to control global buckling
at some critical axial force. The large horizontal andor vertical displacements induced by
global buckling may result in high stresses and strains in the pipe wall, that exceed code
limits.
The simulation of the designed pipeline in a realistic three-dimensional environment obtained
by measurements of the seabed topography, allows the engineers to exploit any opportunities
that the pipeline behavior may offer to devclop both safe and cost-effective solutions. For
example, the designer can first analyze the pipeline behavior on the original seabed. If some
of the load cases result in unacceptably high stress or strain, seabed modification can be
simulated in the finite-element model and the analysis re-run to check that the modifications
have lead to the desired decrease in stress or strain.
The finite element model may be a tool for analyzing the in-situ behavior of a pipeline. By the
pipeline in-situ behavior it is here meant the pipeline behavior over its through-life load
history. This part of the pipeline load history can consist of several sequential load cases, for
example:
0 Installation
0 Pressure testing (water filling and hydrotest pressure).
0 Pipeline operation (content filling, design pressure, and temperature).
0 Shut dowdcool down cycles of pipeline.
0 Upheaval and lateral buckling.
Dynamic wave and/or current loading.
