The present paper discusses structural damping in nonbonded flexible pipes based on experiments and numerical simulations. The main conclusions from the study are:
Structural damping in flexible pipes is high and influence dynamic response significantly.
Refined cross section models allowing for hysteretic behaviour improve the accuracy of the results from dynamic simulation of flexible risers.
The experiments are performed on a free hanging fullscale 4" internal diameter, 8 m long nonbonded pipe pressurized to different levels. Additionally, the influence on bending stiffness from excitation frequency and curvature amplitude are studied by cross-sectional tests. A hysteretic bending model is implemented in a computer program. Structural damping and bending stiffness description in flexible pipe simulations are discussed, and a consistent procedure for simulations is presented.
Several damping mechanisms are present in flexible pipes during dynamic excitation. The motivation for the present study was the reported high structural damping levels, especially in nonbonded pipes (e.g. Otteren and Hanson, 1990). The damping level is high compared to other offshore structures, and will thereby influence dynamic response significantly. It is therefore important to apply an adequate damping model in numerical simulations. There are several problems linked to the modelling of structural damping in flexible riser analysis:
Damping properties for flexible pipes are generally not specified by the producers, and may be difficult to obtain for specific pipe structures.
Damping may be characterized in several ways, and it is not obvious which model will be best suited for a particular method of analysis.
Characterization of damping properties should be compatible with the particular damping model and method of analysis.
There are no standard methods of measuring damping properties, and choice of experimental method may significantly influence the results.
