A complex cross section such as an umbilical or a flexible riser will have two sources of structural damping; damping due to the strain variation in the individual materials that make up the cross sections, and damping due to the different layers slipping against one another. The first may be denoted material damping and is present at all response levels, and will be particularly important at low response levels. The second may be denoted slip damping and will contribute when the curvature exceeds the initial slip curvature.
Ideally, accurate data for both the material and the slip damping are available. Unfortunately, this is not always the case and the damping parameters must then be estimated. The material damping may be estimated from the material properties of the various layers in the cross section, taking operating conditions such as temperature into account. The slip damping may be estimated from detailed cross-sectional analyses.
As the slip damping is dependent on the curvature, iterations are needed to ensure that the applied damping and the calculated response are consistent with each other. A procedure to include these iterations within a VIV response calculation is proposed.
A case study is presented demonstrating the use of the proposed procedure for a deep-water umbilical in a lazy wave configuration. For the case studied, the maximum curvatures caused by VIV are significantly reduced.