Page 193 - Pipelines and Risers

166 Chapter I1

The trawl board is modeled by two structural masses: board steel & added mass. Both masses
are connected in parallel by springs to the end of coating spring. These two springs have
stiffness of trawl board‘s in-plane and out-of-plane stiffness respectively.

The soil to pipe interaction is modeled as Chapter 5.

In fact, as a result of investigations into effect of soil model on dent depth it can be concluded
that for different soil - pipe models’ results are very close. This can be explained by the very
short time of impact. At the moment of time when impact force reaches maximum, pipe
displacements are very small, so soil reaction is negligible.

In the first moment (initial conditions) all springs and contact elements are not loaded. Both
masses have velocities in a direction of 45’ to the “soil” plane.

11.5 Pullover Loads

The maximum horizontal force applied to the pipe model, Fp, is given by (DNV, 1997):
112
F,=C,V(mk) y ( 1 1.19)

where:
m= the trawl board steel mass
*
35
[N
lo’
k= the warp line stiffness = - m], (for single 38mm @ wire)
I
3*d
d= water depth
V= the tow velocity
y= load factor = 1.3
The coefficient CF is calculated as below:
CF 6.6(1- e4.8ii)for polyvalent and rectangular boards (11.20)

CF 4.8(1- e-””) for v-shaped boards (11.21)

where:
-
H is a dimensionless height
- Hsp+D/2+02
H= (1 1.22)
B
Hsp= the span height
D= the pipe diameter