Dynamic analysis of girth welded pipes

Abstract

Pipes for pipeline and risers are subjected to fatigue loading during deployment. Additionally, in the case of free spanning pipelines and risers may be exited at their resonance frequency by vortex induced vibration. In some situations the pipes may assume a curved shape after unreeling aboard a pipe lying vessel and this may affect the dynamic behavior. Before installation the pipes are tested in a high cycle fatigue test to determine the life time of the pipe in fatigue.

In this master thesis the effect of a curved shape is studied. A finite element model was created in the finite element software Abaqus. It was important to make the model as similar to reality as possible; to obtain comparable results. The mesh put on the riser pipe, end masses and end caps is of the size 50 mm. The reason for using such fine mesh is to represent the geometry and the properties of the material used in a best possible way. The results obtained from the finite element model have been compared to the results obtained from the finite element model have been compared to the results obtained in the laboratory and the deviations were very small.The different sources of error are discussed due to the curved shape of the pipes. With this in mind two different solution of how to smoothen the stress distribution along the circumference of the pipe is obtained.

There are many different ways to smoothen the stress distribution. In chapter six and seven two different solutions arepresented. In the first solution it is tried to compensate for mass of the rising height by attaching a similar mass at the opposite side of the rising height. This solution did not function and the errors are commented on.

In chapter seven a parameter study for a passive mass damper is presented. Experiments using Abaqus is carried out to verify the assumptions.

The passive mass damper is a solution that can operate on pipe with various curvatures. It can be seen that the stress distribution along the circumference of the pipe smoothens when a passive mass damper is attached. The difficulties with this solution are to get the displacements of the mass damper limited to a reasonable value. With this in mind an alternative position for the passive mass damper were found. By placing the passive mass damper near the supports thedisplacements of the mass damper is limited and the functionality of the passive mass damper is extant.