Page 156 - Pipelines and Risers

Vortex-induced Vibrations (VIV) and Fatigue 129

Figure 9.4 The characteristic (maximum) amplitude response.

9.5 Modal Analysis

9.5.1 Introduction

In order to obtain natural frequencies, modal shapes and associated normalized stress ranges
for the possible modes of vibrations, a dedicated FEM analysis program should be used, and
as a minimum, the following aspects have to be considered

1) the flexural behavior of the pipeline is modeled considering both the bending stiffness and
the geometrical stiffness;

2) the effective axial force which governs the bending behavior of the span is taken into
account;

3) interaction between spanning pipe section and pipe lying on the seabed adjacent to the
span should be considered (multispan project ).

Due consideration to points 1 and 2 above is given in both the single span and multispan
modal analyses:

In the FEM analyses, the change (increase) in the overall stiffness due to the deflected
shape of the as-laid span is taken into account. This includes second order effects such as
stress stiffening due to the sagging of the span.