Time Domain Computation of Riser VIV From Vessel Motions

Abstract

Intermittent riser VIV behavior caused by vessel motions can affect both riser strength and fatigue life. There are frequency domain codes available that are used routinely to calculate riser fatigue damage from VIV due to currents. These codes are often adapted to calculations of the vessel motion VIV and fatigue damage. The adaptations reduce the intermittent VIV to steady state VIV by assuming an appropriate time invariant velocity profile over the length of the riser. However, since vessel motions cause a relative velocity profile over the riser that varies with time, and the VIV response is intermittent, a time domain VIV code is best suited for such an analysis. The paper demonstrates the use of Technip’s time domain riser VIV code ABAVIV to calculate steel catenary riser VIV response and fatigue damage due to vessel motions. Since time domain analysis is computer time intensive, the paper also outlines an efficient methodology to perform these calculations. The analysis example in the paper is based on surge, pitch, and heave motions which are the most important vessel motions for the riser fatigue damage near the touch down region. The ABAVIV code accounts for the nonlinear structural characteristics of the SCR, and the unsteadiness of the VIV phenomenon for the present application.