Improved strake design reduces spar VIV

Judy Maksoud
International Editor
Offshore

Offshore structure motions caused by vortex induced vibration (VIV) are of particular interest in the Gulf of Mexico. Eddy and Loop currents in the GoM exacerbate VIV, which contributes to fatigue. Dr. Merhernosh Irani, principle engineer at Technip Offshore Inc. addressed the issue of improving VIV for spars in the GoM at the Offshore Mechanics and Arctic Engineering conference in Greece.

Strakes were not standard on spars when the production units were first used in the GoM, Irani said. Developers began using strakes to deal with VIV nearly 10 years ago. Now, Irani said, research is targeting improved strake performance to minimize VIV.

Toward that end, Technip conducted model tests at Marin's test facility in the Netherlands, gathering data on the effects of exterior equipment such as chains, fairleads, pipes and caissons, and strake holes. The mooring systems were modeled using springs, Irani said.

Tests investigated both in-line and transverse strake effects, what Irani referred to as the "double whammy" in strake design.

Technip found that increasing strake width to 10-14% of the hard tank diameter, optimizing strake layout for minimum openings and discontinuities, and minimizing the size of the holes, improved motions. Test results show that VIV-induced motion is most sensitive to the amount of strake coverage on the hard tank surface.

06/15/05