Vortex induced vibrations of free span pipelines

Abstract

Pipelines from offshore petroleum fields must frequently pass overareas with uneven seafloor.  In such cases the pipeline may havefree spans when crossing depressions. Hence, if dynamic loads canoccur, the free span may oscillate and time varying stresses maygive unacceptable fatigue damage. A major source for dynamicstresses in free span pipelines is vortex induced vibrations (VIV)caused by steady current. This effect is in fact dominating ondeep water pipelines since wave induced velocities andaccelerations will decay with increasing water depth. Thechallenge for the industry is then to verify that such spans cansustain the influence from the environment throughout the lifetimeof the pipeline.\\\\The aim of the present project is to improve the understanding ofvortex induced vibrations (VIV) of free span pipelines, andthereby improve methods, existing computer programs and guidelinesneeded for design verification.  This will result in more costeffective and reliable offshore pipelines when laid on a veryrugged seafloor.\\\\VIV for multiple span pipeline is investigated and the dynamicalinteraction between adjacent spans has been shown. The interactionmay lead to increased or decreased response of each spansdepending on the current speed and the properties for the twospans. The extension of the contact zone between the spans andseafloor parameters will of course also be important for theinteraction effect.\\\\The influence from temperature variation on vortex inducedvibrations has been demonstrated. The response frequency isinfluenced through changes in pipe tension and sag. Both increaseand decrease of the response frequency may be experienced.Moreover, it is shown that the influence from snaking of the pipeon the temperature effect is small, at least for large diameterpipes.\\\\A free span pipeline will necessarily oscillate close to the seabed.The presence of the seabed will therefore have some influences onthe ambient flow profile and also on the flow pattern around thecylinder during oscillation. Hydrodynamic parameters may thereforevary when the pipe is close to the seabed. In the present work, theinfluence from spatial varying current profiles is investigated forboth single and multiple span pipeline. It is shown that thedifference between using uniform and spatial varying currentprofiles is significant for some current speeds. It is also shownthat use of spatial varying current profiles can be even moreimportant for multiple span pipeline.\\\\The comparison of VIVANA analysis results with MARINTEK testresults has been given. It shows VIVANA predicts the cross-flow responsegenerally much higher than the test measurements, especially for thehigher mode responses. To improve understanding of this phenomena, the VIVANA model was tuned tothe test model and results are compared in different cases. Attempts were made to obtain a better agreement by adjusting some of the input parameters to VIAVANA. The reference point is tuned by changing various hydrodynamic properties, i.e. $C_{L}$, $St$ and added mass. The response frequenciesare also tuned in order to have a better agreement on the results. It is been concluded that the method used here by VIVANA is not able to describe VIV for free spanning pipelines adequately. It is not possible to find a set of parameter in a rational way that will give reasonably correct results. The discrepancy between the analysis and test results are highlighted which confirms the interaction between the in-line and cross-flow vibrations.Discussions are given and addressed on different reasons which may cause this phenomena.\\\\An improved strategy for non-linear analysis of free span pipelineis outlined. Time domain analysis for free span pipeline hasbeen performed. The difference between time and frequency domainanalysis has also been investigated by varying boundary conditions,pipe properties and axial tension. A significant difference is shownbetween results from time and frequency domain analysis at each endof the span where the pipe is started to interact with the seafloor.Due to high fatigue at this point, the importance of usingnon-linear time domain analysis is therefor obvious and highlyrecommended.