Page 523 - Pipelines and Risers

490 Chapter 26

Trawl Impact and Pullover
It was found that leaving the flowlines inbopen trenches as protection against fishing gear
loads would not be viable as the high pressure and temperature would cause them to lift
upwards and out of the trenches. Protection by burial would have required a vast amount of
costly seabed intervention because upheaval buckling would also have to be prevented. There
was therefore a strong economic incentive to demonstrate that the flowlines can be left
unprotected on the seabed.

Freespans
When laid unprotected on the uneven seabed, the flowline will tend to span across the seabed
irregularities. The main problem associated with spanning was found to be the susceptibility
to Vortex Induced Vibrations, which may lead to fatigue damage.

Global Buckling
Small diameter, high temperature flowlines have a tendency to buckle laterally at local seabed
imperfections. As flowline buckling is associated with high stresses and strains seabed
intervention is required to control the buckling behavior of the flowline.

26.2.3 Design Approach
The project approach was to focus the design effort where the largest cost benefits to the
project could be made. These areas, which were focused on, were:

Trawl board protection
In order to demonstrate that the flowline coating can withstand the design impact load,
analyses were performed on trawl impact and pullover. In traditional impact analyses it is
assumed that the kinetic energy is totally absorbed by the steel and coating as deformation
energies. A more sophisticated approach is to account for kinetic energies absorbed by the
trawl board and by global pipe bending. Detailed time-domain dynamic FEM analysis showed
that the pipe wall will experience denting (plastic deformation) but this was within the
allowable limits.

A sophisticated FE analysis model was developed in order to study the global response to
pullover loads. The model considers a flowline lying on an uneven 3D seabed. From these
analyses it was demonstrated that a maximum span height of 0.5 m was allowed at full
operating conditions. For higher span heights, the flowline would have to be supported in the
operating conditions, but could be allowed to span during temperature phases.

Vortex Induced Vibration (VIV)
It was found that a large number of spans would occur, both in the empty as-laid and water
filled conditions. However, during operation a large number of spans will disappear as the
expanding flowline feeds into the depressions. The traditional approach would be to support
the spans in order to prevent VN. But if supports were placed in spans prior to reaching full
operating conditions, this would restrain the flowline from feeding into the spans during
heating.up which would be disadvantageous with respect to the overall behavior of the