Dynamic Analysis of Full Scale Testing of Steel Catenary Risers using the Finite Element Method

Abstract

Pipes for pipelines and risers subjected to fatigue loading during deployment. Additionally, in the case of free spanning pipelines and risers the pipes may be excited at their resonance frequency by vortex induced vibration (VIV). In some cases the pipes may assume a curved shape after unreeling aboard a pipe laying vessel and this may affect the dynamic behavior of the pipe. The pipe cross section may also have an oval shape due to fabrication tolerances resulting in variations in stiffness around the circumference.

In this master thesis the effect of varying the tuning parameters as frequency of rotating mass, inner pressure, position of bearings and magnitude of end masses is studied as well as the influence of oval shape of pipe and variations of the length of the pipe section to the stress distribution around the circumference of the pipe.

To be able to do this, an Abaqus model similar to the full scale testing in the lab at Materialteknisk, NTNU, was created.

The results in terms of stress distribution was relatively stable for forcing frequencies lower than 33.8 Hz, but when increasing the rotating frequency of the pendulum above this value, the unevenness of the stress distribution increased drastically, when no other parameters were varied. A small difference in natural frequencies between the two main modes, due to bearings and gravity, had a large impact on the response when operating close to resonance frequency.

An oval shape of the pipe had a large impact on the stress distribution around the circumference of the pipe. The length of the pipe has a great impact on the natural frequency of the pipe.