Laboratory Tests for VIV Prediction of Deepwater Risers
One of the great challenges in the offshore industry is still the assessment of the motions of risers in waves and current in deep or ultra deep water (3,000 m deep). Here the fatigue life of riser systems is often dominated by the VIV phenomena. State-of-the-art VIV prediction codes, such as Shear7 or VIVARRAY, are based on the assumption that the forces exerted by the fluid flow on the structure can be locally described by empirical lift and drag coefficients of 2-D riser sections and that the excitation frequency is dictated by the Strouhal relation. The excitation of the vortex shedding is accounted for by so-called negative damping. The approach has been used for a few decades and in spite of a lot of criticism it has survived and is still the most commonly used approach for real offshore risers. MARIN is working on two experimental fronts to get a better handle on the VIV phenomenon. First, a new High Reynolds test facility has been developed for measuring the dynamic lift and drag loads of 2-D riser sections. The new apparatus is deployed for research as well as commercial projects. Secondly, an instrumented scale model riser of 12.6 m length and 16 mm diameter has been tested recently. Recent findings from both experiments are presented and discussed in the paper.