Truss Spar VIM in Waves and Currents

The exciting force induced by vortex shedding on the hull of bluff body floaters, such as Spars, may cause response near the resonant period of any of the 6 rigid body modes of response of the floater [1]. Usually the sway vortex induced motion (VIM) is of particular concern for mooring and riser integrity. Most efforts to study this phenomenon to-date have focused on uniform current loading over the depth of the platform, such as one would find in the eddy/loop currents in the Gulf of Mexico. Under these conditions, tow tests with the model mounted horizontally or vertically and restrained with a spring system have been sufficient to characterize the VIM behavior [2, 3]. VIM responses of a Classic Spar measured in the field after the passage of a hurricane in what is referred to as the “hurricane inertial current” event has raised the awareness of the possibility of platform VIM response in sheared currents and in the presence of waves. In order to evaluate this condition, model scale Truss Spar VIM tests were conducted at the MARIN Offshore Basin in the summer of 2004. Uniform current, “shear” or two-layer current, and wave plus current conditions were simulated and compared with benchmark uniform current tow test results. The tests were done in current alone, in waves alone, and with waves in-line with the current, and, waves transverse and oblique to the current. Comparisons are made of VIM response in uniform and shear current conditions, and, response with and without waves. One of the key findings is that with waves in-line with the current, VIM is generally reduced; however, at some headings the response in waves plus current in transverse waves is larger than in current alone.