An Investigation Of Scale Effects In The Vortex-induced Vibration Model Test Of Of Risers

Vortex-induced vibration (VIV) is the main cause of fatigue damage for slendermarine riser. Currenly, model test is the most effective way for the investigation ofVIV. Predicting the VIV response of full scale risers according to the response ofscaled models is the objective of the investigation. Not all the similarity criteria couldbe applied in the model test. So the scale effects would exist in the model test. Givingthat to consideration, it is of great engineering significance for the VIV prediction offull scale risers to investigate the scale effects in the modal test of VIV.Based on the reseason above, this paper focuses on the investigation of the scaleeffects in the VIV model test. The main contents of this thesis can be summariszed asfollows:1. A comprehensive review of the approaches for VIV investigation is carriedout first. Numeric simulation and model test are introduced in this paper,respectively.2. Principle of similarity and the basic parameters related to VIV are set forthin the thesis. Numeric simulation of VIV response of top-tensioned risers(TTRs) with different size is conducted using the sofeware SHEAR7. Scaleeffects are investigated by analysising the difference of VIV responsebetween different TTRs. Futhermore, numeric simulation of a full scaleslender steel catenary riser (SCR) in sheared currents in and out of the planeis carried out. And a method is proposed by which the VIV response of SCRin sheared currents out of the plane can be calculated.3. VIV model test of flexible risers are designed and conducted in uniformcurrents. The modes are of three different sizes. Scale effects areinvestigated by analysising the difference of VIV response between themodel risers.4. The data processing and analysising approach is studied in this paper. Amethod is proposed in this thesis by which the influence of tension and the initial defomation can be dispelled. A careful investigation of modal analysisis carried on. A relable data processing approach forms based on the study.By using the approach, the characteristics of the initial deformation and VIVresponse are analysising. By comparing the diffence of the responsecharacteristics, scale effects are studied.The conclusions of this thesis can be summarized as below:1. The numeric simulation of the VIV response of SCR indicates that thedisplacement of SCR in currents in the plane is15%great than that in currentsout of plane.2. The investigation of scale effects in the initial deformation indicates that theaverage drag force coeffiecece of slender risers is great than1.2. The drag forcecoeffiecience is greater for the smaller dimeter riser.3. The investigation of scale effects in the VIV response indicates that there isn’t alinear relation between the displacement of scaled model risers and that of fullscale risers. The dominant frequency can be estimated by using the Strouhalfomula. The Strouhal number is smaller for the smaller dimeter riser.