Page 195 - Pipelines and Risers

168 Chapter I1

11.6 Finite Element Model for Pullover Response Analyses

11.6.1 General
Pullover response analysis procedures are presented in this section. Pullover is the stage of the
trawl gear interaction in which the trawl board is held behind the pipeline and as the warp is
tensioned by the movement of the vessel, the trawl gear is pulled over the pipeline.

In general, two base cases have been considered:

the pipeline in full contact with a flat seabed, and
0 the pipeline suspended in a freespan on a real seabed.

In the first base case an ideally flat seabed profile has been assumed. In the latter base case a
real seabed profile giving freespanning is imported from the most up to date pipeline route
profiles and has been implemented to establish a span model. In the first base case, the seabed
profile has been expanded to create a quasi 3-D seabed surface while in the second base case a
certain width of real seabed has been used to represent the 3-D seabed.

The main objectives of these analyses are:

to establish whether a pipeline with continuous contact with the seabed can withstand a
pullover load;
to establish whether a pipeline in a freespan can withstand a pullover load or whether it
requires protection, or if a maximum allowable span length is required to ensure the
structural integrity of the span in the event of a pullover load;
to investigate the influence of different parameters such as vertical soil stiffness, seabed
friction etc. through sensitivity analyses.

In the analyses presented herein, the pipeline is subjected to the most severe pullover
loads.

11.6.2 Finite Element Models
The purpose of the analyses presented in this section is to study the pullover response. Non-
linear transient least-plastic finite element analyses using the model described in Chapter 7, is
performed to achieve this purpose.

The length of the model has been selected so that full axial anchoring is achieved at a distance
away from the pullover point, i.e. the effective force is unchanged after the time of impact.