This paper presents the application of a general vortex induced vibration model for catenary flexible risers. A time domain simulation is performed using a modal analysis approach. The numerical calculation procedure allows fluid and structural non-uniformities in time as well as in space. Two realistic configurations are considered, a Lazy-S in 350m water depth and a Lazy-wave in 110m water depth. Wave only and current only flows in and normal to the riser plane are investigated. The effects of physical parameters of the risers and flows are demonstrated as leading to very low amplitudes of vibration in service environments. The reasons for this are explained.
Catenary flexible risers have attracted increased attention in the offshore industry in recent years with their advantages over rigid tensioned risers. The main types of catenary flexible risers are shown in Figure 1.1. Vortex induced vibrations of catenary risers potentially pose a threat to their service lives. Model scale tests, on site tests and theoretical models are available to investigate this problem. Model scale tests reveal basic mechanisms and validate analysis methods. However, the results cannot be applied directly to full scale risers since it is very difficult to achieve correct scaling of the necessary parameters between the model and full scale risers, including similarities in geometry, separation of natural frequencies and structural damping. Field tests are necessary, but they are expensive to conduct. Tests results are difficult to interpret owing to the uncontrollable flow conditions. Very few observations or measurements of vortex shedding induced vibrations of catenary flexible risers in service have been published. They reported that the dynamic components of tension caused by vortex induced vibration were of the same order of magnitude as that caused by heave and roll motions of a completion vessel.