| New sandglass-type FPSO has excellent motion performance and wide application prospects.The heave motion response is an important index of the performance evaluation of sandglass-type FPSO,and viscous damping has a great influence on its heave motion response.In this paper,the heave damping of sandglass-type FPSO is studied by numerically simulation of free heave decay motion and forced heave motion.Firstly,an effective and feasible grid partition scheme and numerical model for sandglass-type FPSO heave motion simulation are established,and the correctness of the numerical model is verified by the experimental results.The scale effect of sandglass-type FPSO is further studied,and the results show that the numerical results of the scaling model can be used to study the heave motion damping of sandlass-type FPSO at practical engineering scale.Secondly,the relationship between the shape of sandglass-type FPSO and heave damping is studied based on free heave decay motion.By orthogonal test method,the influence of waterline radius,inclination angle,displacement,and their interaction on the free attenuation coefficient of heave is studied.The results show that the interaction of each parameter has little influence on the free attenuation coefficient of heave,and the influence of displacement,inclination angle and waterline radius on the free attenuation coefficient of heave is highly significant,significant and obvious,respectively.On this basis,the single factor analysis of the influence of shape on the free attenuation coefficient of heave is carried out.The results show that the free attenuation coefficient of heave decreases with the increase of inclination angle and displacement,and is more sensitive when the inclination angle is larger and the displacement is smaller.The heave attenuation coefficient increases with the increase of waterline radius,but it is not sensitive.After that,the formula for estimating the free attenuation coefficient of heave is obtained by fitting method,which can be used in the preliminary shape design of sandglass-type FPSO.The influence of moon pool and corner cutting on the heave free attenuation coefficient of sandglass-type FPSO is also studied.The results show that the moon pool,which added to the sandglass-type FPSO model in this paper,has little influence on the heave attenuation coefficient,and the corner cutting has little effect on the heave free attenuation coefficient.Then,the numerical study of forced heave motion characteristic of sandglass-type FPSO is carried out.The numerical results of different motion amplitudes show that the shape of sandglass-type FPSO has a strong nonlinearity for the heave radiation problem,and the larger the motion amplitude,the stronger the non-linearity.Comparing the radiation damping based on potential flow theory with that obtained by CFD method,the main source of the difference between them are the eddy generated at the bottom edge when sandglass-type FPSO change its direction of motion and the local flow field.Comparing the non-viscous and viscous damping,the results show that the viscosity of the fluid has little effect on the damping,that is,the damping component caused by the viscosity is small.When predicting the motion response based on potential flow theory,it is necessary to adopt forced motion method or free attenuation method to modify the damping.It is found that the modified value of the damping obtained by forced motion method increases with the increase of the motion amplitude.For the peak frequency of Typhoon spectrum that occur once in 10 years to once in 200 years in the South China Sea,the modified value obtained by forced motion method and free decay motion method are basically the same.In addition,for larger frequency,the modified value of damping obtained by forced motion method is less than the value obtained by the free decay motion method.Finally,the wave free characteristic of sandglass-type FPSO is studied by CFD method.At the wave free frequency,the FPSO heave motion radiates very small waves with little equivalent radiation damping. |
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