The gas field "Ormen Lange" is located about 120 km outside the Norwegian cost line at about 850m water depth. Several subsea pipelines are to be installed from the field to shore. As the bottom bathymetry is very uneven the pipelines will have a large number of long free spans. The combination of long free spans and strong ocean current makes fatigue due to vortex-induced vibration (VIV) an important design consideration. For long free spans, the VIV response is more complicated than for short spans. Several eigen-modes of the pipeline may be excited. Further the interaction between in-line and cross-flow response is important. To avoid too long spans the pipeline is re-routed and rock supports installed. This removes the long spans but introduces neighbouring spans separated by short supports (shoulders) only. Such spans may interact dynamically. Also in these cases multimodal VIV response may occur. To improve the understanding of the VIV response of long free spans and multi-spanning pipelines, several model test campaigns as well as numerical analysis have been carried out. The tests are described and some major findings discussed. The findings are implemented in a design guideline for VIV response estimation as well as computer tools.
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