The risers are designed to meet the following design requirements:
• Vortex Induced Vibration
• Equivalent Stress
The methods of the analyses are described in the following subsections.
Vortex Induced Vibration
The allowable span lengths for the vortex induced vibration criteria are calculated based on riser general arrangement drawings and DNV 1981, whereby the reduced velocity is defined as:
Another parameter controlling the dynamic vibration is the stability parameter (KS) defined as:
Based on the calculated stability parameter, the limiting reduced velocity can be obtained from Figure A.3 of DNV 1981. As per DNV 1981, the in-line oscillation of a free span are initiated at lower velocities than those required for the onset of cross flow motion. Therefore, the maximum allowable span length for the in-line motion criterion will automatically satisfy the cross-flow criterion. The equation for reduced velocity (Vr) can be re-arranged as follows:
Combining the two equations and solving for L:
The vortex shedding analysis is performed using in-house spreadsheet files. The spreadsheet calculates the allowable riser span length to avoid the onset of pipeline in-line and cross flow oscillations induced by vortex induced vibration, which complies with the DNV 1981 method. Based on the calculated span length, the riser clamp elevation is then identified such that the clamp elevation spacing is always lower than the riser maximum span length.
Steady current and wave velocity are considered in the riser vortex vibration analysis
Riser analysis have been analyzed using AUTOPIPE software