Assessment of Potential Axial Resonance Due to VIV for Ultra-Deep Risers
- Richard I Harrison (2H Offshore Engineering Ltd) | Frank K Lim (2H Offshore Engineering Ltd)
- Document ID
- International Society of Offshore and Polar Engineers
- The 29th International Ocean and Polar Engineering Conference, 16-21 June, Honolulu, Hawaii, USA
- Publication Date
- Document Type
- Conference Paper
- 2019. International Society of Offshore and Polar Engineers
- VIV, Ultra-deep, Risers, Axial Resonance, Mining
- 1 in the last 30 days
- 27 since 2007
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As risers are designed for more deeper water applications, factors that previously were not considered to be important in determining the response of the system can become important. This is especially in the design of ultra-deep water risers, e.g. development of deepsea mining risers in around 5000m water depth. One such effect is the axial natural frequency which increases with depth and thus for deep water riser comes into the frequency zone that is excited by environmental conditions. It influences the dynamic response of the system due to wave motions, and this can be simulated via full beam element software that includes the axial stiffness. For the response under vortex shedding or VIV, industry software uses modal based methods, e.g. Shear7, these do not include any axial properties in the calculation, thus the potential for any axial stress and motion is missing. This paper addresses the impact of vortex shedding on riser axial resonance and performs basic assessment for the fatigue damage that could be caused due to the axial resonance. Reference is made to the EU funded Blue Mining project for which axial mode oscillation is detected to occur.
Axial motion in riser is typically not a concern, but with the increasing length of riser in deeper water depths the riser natural frequencies will come into frequency zones of both VIV (vortex induced vibration) and wave motions. This paper looks at the potential and the impact of axial resonance due primarily to VIV.
It is found that some ultra-deep drilling risers and EU deep mining riser have axial modal response in the 4 to 7 second range. At this range a typical deep-water drillship will have little vessel response, thus the potential for axial resonance is minimal. As the practice in the industry for wave simulation of riser is to use full beam element model and thus the axial resonance is captured in any event. But common industry VIV analysis tools, such as Shear7 and VIVA, use modal-based approach that consider only bending modes, and no information regarding any axial mode is entered, thus the effect of any VIV that causes axial resonance is missing.
|File Size||1 MB||Number of Pages||6|