Effects of volumetric allocation on heave response of semisubmersible in deep sea

The configuration of semisubmersibles consisting of pontoons and columns and their corresponding heave motion response in incident progressive waves are examined. The purpose of the present study is to provide a theoretical approach to estimating the effects of volumetric allocation on natural period and response amplitude operator (RAO) in heave motion. We conclude that the amplitude of heave motion response can be considerably suppressed by appropriately adjusting volumetric allocation so that the natural heave period keeps away from the range of wave energy. The theoretical formulae are found in good agreement with the corresponding computational results by WAMIT

Hoop and axial plastic buckling modes of submerged cylindrical shells subjected to side-on underwater explosion shock wave

Cylindrical shells are widely used in marine structures from small-sized pipes to various immersed containers and large submarines. Dynamic plastic buckling of the shells could be caused by underwater explosion (UNEX) loads in industrial accidents or hostile attacks. Influential factors including non-dimensional hull shock factor reflecting the resilience of the material to shock intensity, slenderness ratio, localised ring stiffener and endcap are identified. Their impacts on the dynamic buckling modes are discussed relied on the finite element investigations. The results show that the global hoop and axial buckling modes are mainly determined by the hull shock factor and slenderness ratio, respectively. With the increase of hull shock factor, i) back-side buckling, ii) back- and front-side buckling with dominant buckles at the back-side, iii) backand front-side buckling with dominant buckles at the front side, and iv) overall buckling with very large buckles at the front-side originating from the priority buckling modes can be observed in the circumferential direction. The competition between the front- and back-sides buckling is attributed to their different buckling mechanisms. The former one is caused by shock transmitted from the explosion source through the fluid, whereas the later one is due to striking of the structural hoop stress waves propagated from the front-side via the shell itself. Axial buckling modes consisted of primary and potential buckles alternating in the axial direction could be triggered for a large slenderness ratio, but be concealed by localised buckling modes for small slenderness ratio. The localised buckling modes are controlled by the ring-stiffener and endcap, because they alter the continuity of compression potential of shells

Modelling windwave driven by typhoon Chan-Hom (201509) in the East China Sea

Typhoon-generated waves pose a serious threat to the development of offshore wind power; therefore typical wave parameters caused by typhoon near Donghai Bridge, a demonstration area of offshore wind farm, were analysed. We pay particular attention to the dissipation term which is one of the source terms of governing equation for windwave evolution in WAVEWATCH Ⅲ. Anisotropic energy dissipation in the wave propagation direction is considered and further applied in our model. A good agreement is observed by comparison with in situ data. Furthermore, the new improved model is used to simulate and forecast wave evolution caused by Chan-Hom (201509). The evolution of typical wave parameters i.e. significant wave height and mean wave period were discussed in the East China Sea, especially near Donghai Bridge

Analysis of current induced by long internal solitary waves in stratified ocean

An approximate theoretical expression for the current induced by long internal solitary waves is presented when the ocean is continuously or two-layer stratified. Particular attention is paid to characterizing velocity fields in terms of magnitude, flow components, and their temporal evolution/spatial distribution. For the two-layer case, the effects of the upper/lower layer depths and the relative layer density difference upon the induced current are further studied. The results show that the horizontal components are basically uniform in each layer with a shear at the interface. In contrast, the vertical counterparts vary monotonically in the direction of the water depth in each layer while they change sign across the interface or when the wave peak passes through. In addition, though the vertical components are generally one order of magnitude smaller than the horizontal ones, they can never be neglected in predicting the heave response of floating platforms in gravitationally neutral balance. Comparisons are made between the partial theoretical results and the observational field data. Future research directions regarding the internal wave induced flow field are also indicated

Wave loading on floating platforms by internal solitary waves

Morison's equation is used for estimating internal solitary wave-induced forces exerted on SPAR and semi-submersible platforms. And the results we got have also been compared to ocean surface wave loading. It is shown that Morison's equation is an appropriate approach to estimate internal wave loading even for SPAR and semi-submersible platforms, and the internal solitary wave load on floating platforms is comparable to surface wave counterpart. Moreover, the effects of the layers with different thickness on internal solitary wave force are investigated

Effects of incident wind/wave directions on dynamic response of a SPAR-type floating offshore wind turbine system

As a promising renewable energy, offshore wind energy currently is gaining more attention, by which the economic and efficient operation of floating wind turbine systems is a potential research direction. This study is primarily devoted to the analysis of dynamic response of the NREL-5 MW reference wind turbine supported by an OC3-Hywind SPAR-type platform using a recompiled code which combines FAST with WAMIT. To verify the reliability of the recompiled code, the free decay motions of a floating wind turbine system in still water are examined with satisfactory results. After that, thirteen scenarios with different angles between wind and wave from 0 degrees to 90 degrees are investigated. The dynamic responses of the turbine system in various degrees of freedom (DOFs) for different incident wind/wave directions are presented in both time and frequency domains via the fast Fourier transform

Interaction between Reflected Shock and Bubble in Near-wall Underwater Explosion

Shock and pulsating bubble, as two processes for structure damage in underwater explosion, are often studied separately due to their different time scales. In this study, we would pay particular attention to the interaction between them in underwater explosion nearby a wall by finite volume method. In order to capture the shock precisely, it is an issue at priority how to properly select grid density and artificial viscosity. Then a 鈥渢wo-step鈥?strategy is adopted to overcome the bottleneck of high computing requirements in CPU time and memory. The shock reflection, bubble oscillation and the interaction between them along with resultant load are presented and discussed