Page 106 - Pipelines and Risers
P. 106
79
Chapter 5
Soil and Pipe Interaction
5.1 General
An interaction model of the contact between the pipeline and the seabed are often referred to
as a pipelsoil interaction model. The pipelsoil interaction model consists of seabed stiffness
and equivalent friction definition to represent the soil resistance to movement of the pipe.
The equivalent friction is mainly based on coulumb friction (sand), cohesion (clay) or a
combination of the two (silty-, sand- clays), the soil density and the contact pressure between
the soil and pipe. It is therefore important to predict the soil contact pressure, equivalent
friction and soil stiffness accurately.
5.2 pipe Penetration in Soil
In a finite element model a non-linear pressurelpenetration relationship may be used. The
penetration of a statically loaded pipe into soil can be calculated as a function of pipe
diameter, vertical contact pressure, soil strength parameters (undrained shear strength for clay
and internal friction angle for friction materials such as sands) and submerged soil density.
This penetration is to some extent complicated by the circular form of the pipeline, which
leads to a combined effect of friction and bearing capacity resisting soil penetration.
In order to construct the pressurelpenetration relationship mentioned above, an approach
based on different methods for calculating the seabed penetration as a function of the static
ground pressure has been used. Two such methods for clayey soils are the Verley and Lund
(1995) method, and the buoyancy method (Hlland, 1997). It is also clear that this is an
approximation, since cyclic soil effects are ignored. In the following each method is given a
brief description.
5.2.1 Verley and Lund Method
The Verley and Lund method is based on back calculation of pipelines with external diameter
from 0.2 - 1.0 meters, resting on clays with undrained shear strength of 0.8 - 70 Wa. The
method presents the following formula for calculation of pipeline penetration:
= 0.007 1 ' (S . y" -t 0.062. (S . )".' (5.1)
D
