1 Copyright © 2005 by ASME MODELING VORTEX INDUCED MOTIONS OF SPARS IN UNIFORM AND STRATIFIED FLOWS Owen H. Oakley, Jr. ChevronTexaco Yiannis Constantinides ChevronTexaco Claudia Navarro Applied Research Associates, Inc. Samuel Holmes Applied Research Associates, Inc. ABSTRACT This paper examines the vortex induced motions (VIM) of a spar type floating production platform in uniform and sheared currents. The large draft of modern production platforms means that in some of the extreme current events the flow past the platform is highly non-uniform along the hull. We discuss the simulation of these stratified flows associated with hurricane events and loop currents and the implications for experiments and numerical simulations. Model testing options are reviewed along with the potential effects of buoyancy due to temperature and salinity variations in the current. Comparisons are made between experimental test results and numerical simulations of VIM at small scale and projections are made to full scale behavior using computational fluid dynamics (CFD) and detached eddy simulation (DES). INTRODUCTION Spar platforms have become an important design type for offshore deepwater operations. The typical spar platform is a long vertical cylinder about 40m in diameter with a draft of 200m, tethered to the ocean floor with a catenary mooring system. Having a small waterplane area compared to its displacement and a low center of gravity, the spar has excellent stability and exhibits small responses to waves. Unfortunately the spar shape is prone to vortex shedding in strong currents which can give rise to large horizontal plane motions - stressing the mooring system and risers. Because vortex shedding is expected in many if not all spar installations, the industry has devised methods to reduce vortex induced motions (VIM) to acceptable levels. The two main approaches are to use strakes to break up the coherence of vortex shedding along the length of the spar and the tuning of the mooring system to keep the natural frequencies of the spar away from the vortex shedding frequencies. Both methods depend on accurate prediction of the flow around the platform. There are two types of current events that govern this aspect of the design: a deep eddy or loop current and a hurricane inertial current. The loop current can reach the full platform depth and is often modeled as a high speed uniform current. The speed of a hurricane current can be somewhat higher than that of a loop current, but the hurricane current is typically shallower and confined to the upper third of the spar platform. Although the VIM might be expected to be smaller in this case, the combined action of the VIM, hurricane generated waves and high mooring line tensions make this an important design condition. At this time there is little full scale data on spar VIM under these conditions and there are many different current and hull design scenarios. Furthermore, neither numerical simulation nor tow tank test procedures are validated for sheared currents. Thus, our objective here is to develop effective predictive capabilities, both numerical and experimental. In the remainder of this paper we first examine some important aspects of the flow around spar platforms and how these might affect CFD simulations and model scale experiments. At issue are the large size and complex spar geometry of the spar and the nature of sheared currents. Size and geometry affect CFD problem size. Spar platforms are not streamlined cylinders but typically have strakes to reduce VIM and often have external pipes and mooring chains or “appurtenances” which can affect the flow near the platform. The detached nature of the flow, the high Reynolds number and complex geometry suggest very large CFD problem size, stressing computer resources. We investigate the problem of simulation from a practical point of view with regard to the computer resources, the time required and the effect of mesh refinement. Proceedings of OMAE2005 24th International Conference on Offshore Mechanics and Arctic Engineering (OMAE 2005) June 12-17, 2005, Halkidiki, Greece OMAE2005-67238