Page 191 - Pipelines and Risers

164 Chapter I1

kb = 10 (MN In) ( 1 1.17)

Pipe Stiffness, Mass and Added Mass
The mass of the activated pipe, mp, is a function of time. The length of the activated pipe will
increase during the impact.

The mass of the pipe consists of

The mass of the content within the pipe
The mass of the steel pipe
0 The mass of the coating
The hydrodynamic added mass related to the pipe. According to DNV'81 pipeline rules,
the added mass is 2.29 times the mass of the displaced water for pipes resting on the
bottom, 1.12 times the mass of the displaced water for pipes with Im elevation.

Pipe added mass is calculated for every case using the following equation:

(1 1.18)

where:
C, = 2.29, (Added Mass Coefficient)
=
Ma Added Mass
OD= Pipe outside diameter, including all coatings
rp Water density

The bending stiffness of the pipe, kpb, is also a function of time and decreases over the time.

The pipe stiffness and mass are simulated using beam elements.

Seabed Soil Stiffness
The soil stiffness, kps, is a function of time and decreases over time. The soil stiffness will
only be of concern for impacts with a downward vertical component, or when the soil forms a
support to the pipeline in the opposite direction of the impact. No soil stiffness is assumed for
pipeline free spans. If the pipeline is laid freely on seabed, not trenched or buried. No sticking
effect is applied for pipe - soil interaction.

The soil is represented by a spring stiffness kps in the vertical direction and friction m in the
horizontal direction.