VIV Modeling of Bare and Suppressed Risers
Authors: Lee Li , Allen Don W. 
Published: October 2008

This paper presents results of a VIV modeling effort including calibration of VIV data that was obtained over the past 10-15 years. A significant portion of the data is from high Reynolds number tests of long riser models with and without VIV suppression devices. Shell's version of the SHEAR7 program has been calibrated with the experimental data. Examples on VIV modeling are used to demonstrate the importance of test data and model calibration to VIV analysis. An uncalibrated model can severely underestimate fatigue damage of both bare and suppressed risers.

Introduction

Vortex-induced vibration (VIV) analysis and design of offshore structures, such as marine risers and sea floor pipelines, has been a challenging problem for engineers. There are a number of reasons why this is the case. One major reason is that commercially available VIV predictive tools have not been extensively calibrated with valid experimental data. Very often, these tools provide drastically different answers among themselves for the same problem. As a result, there is less confidence in the outcome of VIV analyses, which makes it difficult to decide whether or not VIV suppression is needed. If suppression is needed, it is difficult to determine the amount of suppression required. VIV is an issue that often poses as an obstacle in deepwater project engineering. VIV analysis is a process that involves various phases, as is shown in Figure 1. It starts with testing to obtain data. Based on the understanding of data, a computer model is developed. This model must be calibrated with the data in order to be used in VIV analysis and design.

This paper presents results of a VIV modeling effort, including calibration of VIV data obtained over the past 10-15 years. A significant portion of the data pertains to high Reynolds number tests of riser models with and without VIV suppression devices. The tests were performed either in a current tank or on a rotating arm facility. The paper will establish or demonstrate the following:A methodology to analyze and model VIV of deepwater tubulars with and without suppression devices.The importance of test data and model calibration to VIV analysis.The physics behind VIV of tubulars with suppression devices.

This paper starts with a description of VIV test data. It then discusses key insights from the data, model calibration, and calibration results. This is followed by examples on VIV modeling of bare and suppressed risers to illustrate some of the findings. Finally, conclusions are presented. This paper will limit its discussions to modeling of isolated risers, with and without suppression, in steady-flows. Additional topics, such as modeling of riser transient VIV due to vessel motion, and simulation of riser interference, are not addressed in this publication.

Test Data Description

VIV test data can generally be classified as described in Table 1. Test cylinders may be rigid or flexible. A rigid cylinder does not have elastic bending modes. Its motion data is pertinent to spar-type structures. Flexible cylinder data is more relevant to risers, as the elastic transverse motion modes can generally be captured.