A Methodology for In-Line VIV Analysis of Risers in Sheared Currents

Abstract

As exploration and production move to even deeper water and more severe environment, the need to have a methodology for analyzing risers for in-line VIV fatigue damage without undue conservatism increases. The methodology presented in this paper reduces the conservatism in available methods by accounting for (1) the power-in region, (2) the power-out region (hydrodynamic damping), (3) competing modal excitation in the case of multiple mode excitation, and (4) the multiple constraints, if available, in the riser that result in irregular modal shapes. This methodology requires the use of a cross-flow VIV code with sheared flow capability such as SHEAR7, VIVA, or VIVANA. In this methodology the riser over the current profile is split into sections of cross-flow excitation and sections that have potential for in-line VIV excitation only. The cross-flow VIV code defines the sections for cross-flow excitation. All sections are analyzed for in-line VIV with the cross-flow VIV code using the appropriate in-line VIV force coefficients and Strouhal numbers. The assumptions implicit in the cross-flow VIV code regarding power-in, power-out, etc., are assumed valid for the in-line VIV analysis. The in-line VIV coefficients used in the analysis reported in this paper have been obtained from laboratory data, and are functions of both the VIV response amplitude and reduced velocity. The coefficients have been modified to give in-line VIV response amplitudes with the methodology presented that are consistent with DNV-RP-F105. The fatigue damage along the riser represents the sum of the damages produced by in-line VIV excitation for each of the riser sections.