S Nallayarasu

Linear damping models have been used in the past for solving floating body dynamics, especially for simple geometries such as spar. However, due to the addition of heave damping elements to spar such as heave plate, complex flow around these elements may change the relationship between damping and velocity of the body to nonlinear. The damping plays a major role in accurate determination of motion response of spars, especially the heave. Free decay tests have been carried out for spar with and without heave plate in calm water condition. The Computational Fluid Dynamics (CFD) simulation of heave decay is carried out using ANSYS FLUENT and validated by free decay test results using scale models. Mesh convergence study has been conducted to determine the optimum mesh size. The heave motion obtained from CFD are used to derive the damping terms by matching the heave motion obtained using equation of motion by changing the damping term with linear, quadratic and the combination of linear and quadratic. The heave motion obtained from linear damping model matches well with that obtained from measured motion and CFD simulation for spar without heave plate. However, the linear / quadratic damping models alone are not suitable for spar with heave plate. Hence a combination of linear and quadratic damping model is proposed for spar with heave plate. The heave motion computed using a combination of linear and quadratic damping model matches well with that obtained from experimental studies and CFD simulations thus indicating the complexity of flow around heave plate in comparison to the spar alone. Further, the vortices around the spar models obtained from CFD simulations are also presented and discussed with regard to the higher order damping.

Spar, tension leg platform, and semi-submersibles use heave plates to reduce heave response by increasing heave damping and added mass. Conventional practice is to use a linear damping ratio, which is typically obtained from free heave decay tests or Computational Fluid Dynamics (CFD) simulations. However, with the addition of heave plates, the system damping becomes nonlinear. Understanding such nonlinear damping behavior of spar with circular heave plates in various configurations could be useful in the design of heave compensation devices. Experimental and numerical investigation on heave damping and added mass of a scaled model of spar with a variety of heave plate configurations have been carried out using free heave decay for a range of initial heave displacements. Applicability of the linear and quadratic damping models have been assessed for all configurations. The effects of parameters such as heave plate diameter, location of heave plate above the keel, and spacing between two plates on damping and added mass have also been studied. Flow fields obtained from numerical simulations are presented, and their implication on damping discussed.

Heave plates are widely used in floating offshore structures to reduce the heave motion by increasing heave damping and heave added mass, and these parameters are sensitive to the amplitude and frequency of heave motion. It is important to obtain the hydrodynamic parameters, namely, damping and added mass of the floater not only at its natural frequency, which can be obtained from free decay tests but also at other frequencies because the floater response is of interest over a wide range of amplitudes and frequencies. Forced oscillation tests in calm water can aid the investigation of these parameters at various motion frequencies and amplitudes. Heave damping and added mass of classic spar with heave plate are investigated in this study using experiments and numerical simulations of forced heave oscillation of a 1:100 scale model in calm water for various frequency-amplitude combinations. The least square method was used to determine the added mass and damping using three damping models, namely, linear, quadratic, and linear-plus-quadratic, and their applicability assessed. The effect of amplitude and frequency of oscillation on the parameters are discussed for various heave plate configurations with the aid of flow visualization from numerical simulations. Added mass effect is examined using flow visualization of the fluid acceleration field. The scale effect on the parameters is also addressed.