Experimental Study Into Vortex Induced Motion Response of Semi Submersibles With Square Columns

Abstract

Vortex Induced Motion (VIM) of a Deep Draft Semi® with four square columns has been observed in model tests and in a prototype configuration. The flow field around the columns causes the vessel to oscillate along one of the vessel’s main diagonals under particular conditions. The observations and conclusions of three series of VIM model programs are combined in this report. Each test program was executed with a unique objective in mind and each setup was unique. Many similarities in response characteristics have been identified. Some of the differences in observed response are identified and discussed. A discussion is provided regarding the scaling of VIM problems, and the importance of the Reynolds number under model test conditions is discussed. The VIM of a typical Gulf of Mexico Deep Draft Semi® may affect the fatigue life of the mooring system and risers. A formulation is developed that predicts the distribution of VIM amplitudes which then can be used to estimate VIM induced fatigue damage. Observations and conclusions are provided of several experiments where VIM response was evaluated in the presence of a fatigue sea state and for a system with increased sway damping.

Hydrodynamics Around a Deep-Draft Semi-Submersible With Various Corner Shapes

“…Most of the corner shape studies were focused on the 0 degree incidence. However, for square section shaped multi-columns structures, the most severe transverse motion which was induced by vortex shedding occurred at 45 degree incidence which was confirmed in both filed measurements and model tests [6][7][8][9][10][11][12]. Thus, the present numerical investigation about corner shape effects is focused on 45 degree incidence.…”

Experimental and numerical studies of the pontoon effect on vortex-induced motions of deep-draft semi-submersibles

“…Although the test was not specifically for the VIM of the DDS model, it was observed that a DDS with deeper draft experiences more significant VIM with predictable oscillation period and the VIM amplitude in strong current tends to be larger than that for the combination of currents and waves. Rijken and Leverette (2008) experimentally investigated the VIM responses of a DDS in the presence of a fatigue sea state and supported by a system with increased sway damping, which delayed the onset of VIM to a higher reduced velocity. Additionally, Rijken et al (2011) analyzed the effects of a SCR system and hull appurtenances on the VIM behavior of a DDS with square columns and observed the differences in VIM response between model tests and field observations.…”

Full-scale CFD investigations of helical strakes as a means of reducing the vortex induced forces on a semi-submersible

“…Conversely, as explained in Rijken and Leverette (2008) Reynolds number is still crucial in determining the other viscous effects i.e. drag load.…”

“…Their study explicitly showed the effect of the metacentric height of the rig on the motion responses of the platform. As clearly explained in Rijken and Leverette (2008), the crucial parameters determining the VIM responses in tank tests are the reduced velocity, Froude and Reynolds numbers. Of the three parameters, the reduced velocity is the most critical one which is used to assess the experiments.…”

“…Of the three parameters, the reduced velocity is the most critical one which is used to assess the experiments. Rijken and Leverette (2008) highlight the biggest issue associated with model experiments is the scaling such that the lower Reynolds number of the model compared to the prototype may undergo a different viscous flow regime which could lead to a different response between the model and prototype. They also state that vortex shedding location is independent of heading and almost independent of current speed.…”