Page 192 - Pipelines and Risers

Trawl Impact, Pullover and Hooking Loads 165

In this study a constant soil spring stiffness is assumed due to the very small time period of
the impact response.

11.4.5 Impact Response
General
In this section, detailed impact response analyses will be described.

The purpose of this section is to model a trawl board impact on a pipeline resting on the
seabed. The main acceptance criteria on assessing pipe behavior under dynamic load are pipe
shell dent, coating deflection and forces and stresses in pipe body. The objective is to identify
the minimum for coating characteristics to provide sufficient protection of the pipelines
against impact load of trawl board - pipeline interaction.

Conclusions of the Analyses
The acceptance criteria for trawl board impact response under accidental loads is no leak,
STATOIL (1996). This implies that:

- no direct contact between trawl board and steel pipe (no gouge should occur due to
impact) - this means deformation of coating should be sufficiently less than the coating
thickness.
- the dent depth of the steel pipe should be less than 5% of steel pipe diameter.
- the local equivalent stress in the pipe should be sufficiently small to avoid possible
bursting - this failure mode is not considered to govern in the impact analysis.
- the maximum tensile strain should be sufficiently small to avoid possible fracture at girth
weld - this failure mode is not considered to govern in the impact analysis.

Detailed modeling of local strain is also necessary in case we evaluate possible fracture at
girth welds during impact process. However, bursting due to over-stress and fracture due to
over-strain will be more critical during pullover process. These failure modes are therefore
not considered in the present impact response analysis and will be evaluated in the pullover
response analysis.

Finite Element Model
In the finite element calculations, it is assumed that:

- coating density r, = 1200 kg/m3
- coating indentation curve is 6, = a F2

A 60 meters long pipe section is fixed at both ends. Pipe shell and coating are modeled by two
nonlinear springs acting in direction of the impact and placed one after the other. The springs
do not provide any reaction force on tensile deformation and the springs are unloaded along
line parallel to slope at origin of loading curve (plastic behavior).