Frequency Proximity for Multi-Mode VIV Interaction

Uneven seabed and seabed intervention may introduce free-spans in pipelines where interaction between VIV responses of several modes occurs, including longer single spans and moderate length multiple spans. A series of model tests were performed within the Ormen Lange project to study the phenomenon of multi-mode behaviour. While for a beam controlled single span the modes are well separated, the multi-span modes are closer in frequency, and a clear reduction of response amplitude is observed. For single mode response the maximum cross flow amplitude may be 1.3 times the pipeline diameter, whereas in the case of mode interaction the amplitude is restricted to below 0.8 of diameter. To benefit of the test results in detail design a general procedure is implemented into fatigue analysis where frequency proximity is implemented as a criterion for deciding cross flow amplitude. In the case of cross flow modes occurring with small frequency distance, as for multiple spans and for longer single spans, the response amplitude in cross flow direction is reduced from the maximum 1.3 of diameter. The use of frequency as a parameter instead of mode shape evaluation procedure has proven simple to implement into existing fatigue analysis schemes. The paper demonstrates the applicability of the updated analysis scheme and comparisons are made with the test cases where predicted curvatures are compared with measured data for cross flow responses. Similar analyses are also performed by procedures based on existing guideline procedures, and the effect on allowable free spans from the present update is illustrated.