A numerical wave tank based on the Harmonic Polynomial Cell (HPC) method is created to study the generation, propagation and interaction of solitary waves. The HPC method has been proven to be of high accuracy and efficiency in modelling of water waves, wave-wave and wave-structure interaction within the context of potential flow. An important feature of the present HPC method is that the free surface and solid boundaries are immersed in a stationary Cartesian grid. Solitary waves with , i.e. amplitude to water depth ratio, up to 0.6 are generated by different methods. We demonstrate that the results based on the first-, third-and ninth-order method are less satisfactory than the fully-nonlinear method in generating solitary waves with 0.4 . Additionally, both the head-on and overtaking collision between two solitary waves are studied. In the investigation of the phase shifts after the head-on collision, our window model successfully explain the main reason why Su & Mirie [1]"s third-order approximation of the uniform phase shifts is inconsistent with Chen & Yeh"s [2] experimental results and Craig et al."s [3] fully nonlinear numerical results. For the overtaking collision of solitary waves, the collision process and the phase shifts are numerically analyzed. Our present result also confirms Craig et al."s [3] category of the overtaking collision.
The convergence of drift force computation on a LNGC in shallow water is studied. It is found that the convergence of the result, especially for the pressure integration method, is largely dependent on the mesh quality. Furthermore, the irregular frequencies degenerate the results in the high frequency region. The LNGC analysis has numerical challenges related to the geometry and water depth. We also wanted to see if the convergence properties are due to this or if they are of more general nature. To investigate this, an additional analysis was done for a Wigley ship in deep water. The convergence properties were found to be even poorer in this case. This shows that the convergence problem seems to be quite general. It is also found that the vertical components, which have to be evaluated by pressure integration, converge much better than the horizontal components. Hence all six components can be found using a moderately dense mesh by using the far field integration for the horizontal components and only the vertical components from the pressure integration.
Fatigue of the pump tower structure is an important design aspect for spherical cargo tanks onboard liquid natural gas carriers. Current state-of-the art methodology for assessing these loads is based on a series of model tests carried out about 10 and 40 years ago. The last campaign recorded tower loads for several tank filling ratios, vessel headings and sea states. Long-term distributions of loads were established, and this required the development of a scaling methodology to estimate short-term load distributions for sea states outside the range of tested conditions. However, the scaling approach proved to be inadequate when put into use in projects. The present paper proposes a new and enhanced scaling method for short-term distribution of loads on the pump tower involving re-assessment of the model test results combined with new statistical treatment. The dynamic loads on the pump tower causing fatigue damage can be separated into loads due to sloshing and inertial loads. The sloshing loads are seen to correlate well with the amount of energy in the acceleration response spectrum of the tank in a frequency range in vicinity of the lowest natural sloshing frequency. A characteristic response variable is defined as the integrated energy in the acceleration response spectrum over one half octave band around the sloshing resonance frequency, and a functional relationship between the loads on the pump tower and this parameter is established. The results show that this model describes the observed pump tower loads well, especially for filling heights 30 to 70%. Some more scatter is observed for very low or high filling levels, but as these cause lower loads this is not of concern. The underlying model tests primarily focused on high sea states in order to reflect global trade. The proposed scaling methodology allows for robust extrapolation to lower sea states in a more accurate way than was previously possible. This is particularly useful to avoid overly conservative fatigue-load estimates for other applications such as FSRUs and FLNGs in more benign conditions.
In the quest for a numerical method for surface waves and wave-induced effects applicable when linear or weakly nonlinear methods are insufficient, a three-dimensional numerical wave tank assuming fully-nonlinear potential-flow theory is proposed. When viscous-flow effects, breaking waves or other violent flow-phenomena are not of primary importance, potential-flow methods may have similar capability in capturing the involved physics as Navier-Stokes solvers while being potentially more accurate in handling wave-propagation mechanism and more computationally efficient. If made sufficiently accurate, efficient and numerically robust, fully-nonlinear potential flow models can therefore represent a powerful tool in the study of ocean wavesThis is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.
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