The potential of vortex induced motion (VIM) in multi-column floating platforms such as semi-submersibles and tension leg platforms (TLPs) is well-acknowledged although the industry guidelines for design for VIM are not comprehensive and more research effort is required. Significant VIM in multi-column floating platforms will affect the fatigue life of the steel catenary risers and must be quantified and sometimes reduced. Industry-standard design tools used for drag estimation based on model tests of fixed structures may not accurately reflect the effects of drag augmentation due to VIM. Model tests and Computational Fluid Dynamics (CFD) analysis are feasible methods to investigate VIM, with the latter being more resource-efficient, provided sufficient benchmarking has been carried out to ensure reliable results. Subsequent to the model tests and preliminary Computational Fluid Dynamics (CFD) simulations done for a multi-column floating platform [1, 2], further CFD analyses for the VIM of the floating platform have been carried out using improved simulation techniques with a commercial software. Good agreement between model test results and CFD calculations for VIM of a multi-column floating platform is observed. Sensitivity of CFD results to the modeling assumptions such as mesh size and density, time-step size and different turbulence models is presented.
Offshore floating platform configurations often consist of geometrically simple and symmetrical shapes which are made complicated by the presence of appurtenances such as helical strakes, tendon porches, steel catenary riser (SCR) porches, pipes, chains, fairleads and anodes on the surface of the hull. Previous studies mainly on spars show that these hull external features affect the Vortex Induced Motion (VIM) performance of the platform significantly. This is to be expected since VIM is controlled by the flow separation on the hull surface and the resulting vortex shedding patterns. Scale effects may also play a role in model tests for bare cylinders or hulls with bare cylindrical columns, whereas previous studies have shown less Reynolds dependence when appurtenances are modelled. This study investigates the effect of hull appurtenances on VIM of a multi-column floating platform, i.e. a Tension Leg Platform (TLP) designed for Southeast Asian environment. Significant difference in VIM behaviors is expected between spars and TLPs since the column aspect ratios are very different and TLPs do not have helical strakes that are commonly fitted on spars. Model testing and Computational Fluid Dynamics (CFD) simulation are used in this VIM study, with the former being the emphasis of this paper. Descriptions of the respective experimental and numerical methodologies are presented and the comparison of the results is made. Further work required to improve the model test set-up and the CFD simulation are suggested. From this study, it is shown that the effect of appurtenances on TLP VIM simulation is important and must be taken into account to obtain realistic results.
In designing fixed offshore platforms located in regions of severe wave conditions, the potential resonant response of the hull structure due to wave loads must be checked. Since the natural frequency of vibration of the hull structure is typically much higher than the dominant design wave frequency, conventional wave load analysis based on linear wave theory does not show dynamic amplification. However, it is known that steep waves are nonlinear and may contain significant energy at higher harmonics of the fundamental frequency. When the forcing frequency of the higher-harmonic wave load is close to the natural frequency of the structural vibration, a resonance i.e. ringing will occur and the structural dynamic response will be significantly amplified. This paper describes an analysis procedure to estimate high-frequency dynamic load on a Gravity Based Structure (GBS) exposed to severe sea states using Computational Fluid Dynamics (CFD) analysis and modal analysis. To fill the statistical gap between the extreme values from short-duration CFD-modal analysis and that from 3-hour design sea states, an approximation method has been developed to estimate the global dynamic load from the measured quasi-static load in earlier model test and to obtain a calibration factor for the CFD-modal analysis results.
The main challenge in the hydrodynamic design of a dry tree semisubmersible is in limiting its motion responses particularly heave motions to enable the use of riser tensioners. Deep draft semisubmersibles have low heave motions but are more susceptible to vortex induced motions (VIM) due to high slenderness ratios of the columns. A novel in-house developed semisubmersible design named the Heave and VIM Suppressed (HVS) semisubmersible has been designed to possess low VIM and low heave responses required for dry tree applications. A case study of the feasibility of a dry tree HVS semisubmersible in South East Asian environment has been published separately [1]. This paper presents the VIM performance of the same hull, estimated using model testing and Computational Fluid Dynamics (CFD) analysis. From the model tests, VIM suppression is observed in the HVS semisubmersible due to the presence of the column steps. CFD simulations of the model tests show results comparable to the measured data for the HVS semisubmersible. Additional CFD analysis is performed to account for the external damping effect of the mooring lines and risers on the VIM performance of the HVS semisubmersible. This paper together with the earlier publication [1] shows the robustness of the HVS semisubmersible design concept in addressing both the heave and VIM issues in semisubmersibles for dry tree applications.
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