The state-of-the art in model testing for Vortex Induced Vibrations (VIV) of Spars is presented. Important issues related to Spar VIV model testing are highlighted. The parameters that need to be modeled including hull geometry, strake configuration, mass and mooring properties and, considerations of test set-up and instrumentation are discussed. Results are presented from model tests of an as-built Spar deployed in the Gulf of Mexico. It is shown that the model test results compare well with the VIV responses measured in the field.
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.
An extensive model test program was conducted to explore the effectiveness of alternate strake designs to reduce Truss Spar VIV response. Different strake configurations were tested to minimize VIV response. The paper presents results of the model tests. The model test set-up is described, important parameters that are modeled (including hull and truss geometry, strake configuration, mass and mooring properties) and considerations of instrumentation and test methodology are discussed. The paper also describes the analysis of the test results and shows the effectiveness of new strake design. The present results are compared with VIV response of existing Truss Spars with conventional strake design.
A model test program has been performed to study the dynamic ice loads on a jacket platform used in Bohai Bay, China. The tests were performed at a geometric scale factor of 1:26. The prototype structure/foundation compliance was modelled and the structure was subjected to scaled ice sheets corresponding to the ice conditions in the Bohai Bay. The loads, moments, displacements, and accelerations of the structure were measured. The test variables include the ice thickness, ice speed, elevation of ice cover, and structure orientation with respect to the ice motion. An analysis of the data shows good agreement with measured full-scale information. Key words: ice, loads, dynamic, vibration, jacket structure, foundation, China.
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