TABLE OF CONTENTS Free-spanning subsea pipelines subject to oscillating environmental loads may fatigue at the welded joints. Remedial seabed intervention by trenching and rock dumping is intended to ensure that the span lengths are acceptable, but often at great cost. Therefore, the spans have to be carefully assessed with respect to fatigue due to vortex-induced vibrations and wave-induced oscillations.
A considerable amount of work has been performed to develop methods for assessment of vortex-induced vibrations, see Tura et al. (1994) and M rk et al. (1997). However, there is a lack of comprehensive mathematical formulations specifically dealing with wave-induced fatigue. In order to clearly present the theoretical background, a few equations available from reference books (e.g. Blevins (1994), Sumer and Freds e, (1997)) are presented. However, the rest of the paper is devoted to a methodology to assess wave-induced fatigue that the authors feel has not been given enough focus in the literature.
When calculating the fatigue damage due to transverse oscillations, it is first necessary to determine the stress amplitudes. This chapter describes the methodology used to calculate the stress amplitudes using a wave force model, based upon the well-known Morison Equation. Two different approaches are developed to solve the equation of the motion of the free-spanning pipeline:
to solve the non-linear equation of motion numerically. This solution is called a 'Time Domain Solution';
to linearize the Morison equation and solve it analytically. This is called a 'Frequency Domain Solution'.
Vortex-induced Vibrations and Wave-induced...
