Page 196 - Pipelines and Risers

Trawl Impact, Pullover and Hooking Loads I69

11.6.3 Analysis Methodology
A real 3-D seabed surface from in-site survey data, with real spans which have been
determined in in-place analyses, has been adopted for analyses of pullover on spans.

The pullover load is modeled as a dynamic transient analysis.

The pipeline has in general been assumed to be in its operating condition prior to pullover,
that is at its full design pressure and ambient temperature, and with operating content.
Additional sensitivity analyses on the influence of soil friction, pre-buckled pipe, empty
condition, different trawl board weight, low seabed stiffness and a packing condition with
different internal temperature has been undertaken for the flat seabed model.

Pipeline material stress-strain relationship is based on Ramberg-Osgood parametric curves
at the design temperature.

Added mass of the pipeline has been taken into account by attaching point mass elements
to the pipeline nodes. An added mass coefficient of 2.29 is assumed in the analysis.

Due to symmetry, only half of a pipeline section is modeled for the flat seabed cases and
thus only half of the total pullover load is applied to the symmetric plane.

The large-deflection option and material nonlinear option in ANSYS are activated. This
means that geometric nonlinearly and material nonlinearly are taken into account, i.e. the
change in overall structural stiffness due to geometrical changes of the structure as it
responds to loading.

A Coulomb friction model is assumed.

No additional damping effect has been included in the analysis model.

The pullover load for the flat seabed cases is applied as a force vs. time history at the model’s
second end node (symmetry plane). For the real seabed cases the pullover forces are applied
at the middle of the span investigated. Two force components are applied at the pullover
point: one acting in the horizontal (lateral) plane and one acting vertical downward which
tends to punch the pipe into the seabed and thus increasing the lateral restraint. The time-
history of the pullover loads applied to the model are presented in Figure 11.3.

As industrial practice, upheaval buckling and lateral buckling have been considered as
displacement-controlled situation, and strain-criteria are applied to check load effects.
However, free-spanning pipeline and pullover response have been cheated as load-controlled
structures and moment criteria are to be applied to check load effects.