Dynamic analysis of geometrically imperfect pipes

Abstract

Manufactuering and installation processes for submarine pipes and risers result in geometrical imperfections. In operational conditions the pipes and risers experienced dynamic loading and fatigue failure is an issue. A pipe sample is tested in a laboratory for a fatigue-life assessment. The test exposes the welded pipe connections to rotational bending. Geometrical imperfections causes an uneven distribution of tensile stresses along the girth weld and the fatigue-life assesment test is non-conservative.

The parts of the fatigue-life test rig was modelled and assembled in the finite element analysis software Abaqus. The chosen assembely was intended comparable to a previous fatigue-lifetest configuration at the laboratory at NTNU, Trondheim. A solution to imitate the 'infinitly soft' supports in the laboratory was investigated. The numerical results were compared to theoretical calculations. Some deviations were identified and linked to the assumed mode shape and the assumed distribution of mass. Calibration of numerical input parameters were not prioritized.

The effect of introducing geometrical imperfections to simple pipe models in Abaqus were investigated. Ovality in the cross section was found to symmetrically effect the distribution of tensile stresses around the circumference of the welded connection. Spanwise curvature in the pipe specimen was found to introduce an asymmetric effect.

To effect the distribution of tensile stresses there were identified possible parameters to manipulate. Additional mass gave promising results on the natural frequencies for the system,

indicating a symmetric effect. The considered systems for adding mass to one mode were limitated by tenisle yielding and buckling capacity. Additional damping was investigated. The

demands for material properties disqualified the Stockbride damper. A self tuning ball damper offers a closed solution and is recommended for further investigation.

To effect  the distribution of tensile stresses there were identified possible parameters to manipulate. Additional mass gave promising results on the natural frequencies for the system, indicating a symmetric effect. The considered systems for adding mass to one mode were limitated by tenisle yielding and buckling capacity. Additional damping was investigated. The demands for material properties disqualified the Stockbride damper. A self tuning ball damper offers a closed solution and is recommended for further investigation.

Manipulations of the loadcase were investigated. A spring was introduced at the balanced end of the pipespecimen. The results showed an asymmetric effect. The possibilities to connect springs and dashpot dampers at the balanced end of the pipespecimen offers significant response and distance from the welded connection. A system adding a combination of stiffness and damping is recommended for further investigation.