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431.
The paper deals with a numerical integrated procedure to calculate the effects of the interaction between sea waves and a tripod-type steel gravity platform. The main aspects to be taken into account for a reliable wave loading analysis of such a new type of platform are emphasized. Among them, the field perturbation effect due to the large bodies presence is analysed. Finally, some principal results in terms of structure global loads obtained by the numerical integrated procedure are presented. 相似文献
432.
P. Cobo C. Ranz A. Fernández M. Cuesta D.K. Anthony M. Siguero 《Marine Geophysical Researches》2005,26(2-4):87-95
Vertical resolution is of fundamental importance in sonar exploration and is directly related to the duration of the acoustic
pulse generated by the transducer. The shorter the radiated pulse, the higher the vertical resolution. Many sub-bottom profiling
sonar systems use piezoelectric transducers because they are reversible and well understood. Piezoelectric projectors are
normally resonant transducers, which are intrinsically narrowband. A piezoelectric transducer is usually driven by a tone-burst.
However, it is possible to use Fourier techniques to find a pre-compensated electrical driving function so that the transducer
radiates a prescribed wider band acoustic waveform. This technique can be applied to synthesize zero-phase cosine-magnitude,
Gaussian, and bionic pulses, with a conventional sandwich transducer. Zero-phase cosine-magnitude waveforms provide minimum
length pulses (and therefore maximum resolution) within a prescribed frequency band.The aim of this paper is to illustrate
the synthesis of wideband acoustic pulses using an underwater piezoelectric projector. The conventional acoustic waveform
radiated when a Tonpiltz transducer is transiently excited using a “click” and allows its frequency response function to be
measured. This function is used to design the electrical signal which then drives the transducer so that it radiates the shortest
pulse compatible with its mechanical response. The significant resolution enhancement of the waveform shaping process is illustrated
by its application to a sediment wedge model. 相似文献
433.
434.
This paper addresses an important problem in ship control application—the robust stabilization of underactuated ships on a linear course with comfort. Specifically, we develop a multivariable controller to stabilize ocean surface ships without a sway actuator on a linear course and to reduce roll and pitch simultaneously. The controller adapts to unknown parameters of the ship and constant environmental disturbances induced by wave, ocean current and wind. It is also robust to time-varying environmental disturbances, time-varying change in ship parameters and other motions of the ship such as surge and heave. The roll and pitch can be made arbitrarily small while the heading angle and sway are kept to be in reasonably small bounds. The controller development is based on Lyapunov’s direct method and backstepping technique. A Lipschitz continuous projection algorithm is used to update the estimate of the unknown parameters to avoid the parameters’ drift due to time-varying environmental disturbances. Simulations on a full-scale catamaran illustrate the effectiveness of our proposed controller. 相似文献
435.
Modeling bed-load transport of coarse sediments in the Great Bay Estuary, New Hampshire 总被引:1,自引:0,他引:1
A. Bilgili M. R. Swift D. R. Lynch J. T. C. Ip 《Estuarine, Coastal and Shelf Science》2003,58(4):937-950
Current, sea level and bed-load transport are investigated in the Lower Piscataqua River section of the Great Bay Estuary, New Hampshire, USA—a well-mixed and geometrically complex system with low freshwater input, having main channel tidal currents ranging between 0.5 and 2 m s−1. Current and sea level forced by the M2M4M6 tides at the estuarine mouth are simulated by a vertically averaged, non-linear, time-stepping finite element model. The hydrodynamic model uses a fixed boundary computational domain and accounts for flooding–drying of tidal flats by making use of a groundwater component. Inertia terms are neglected in comparison with pressure gradient and bottom friction terms, which is consistent with the observed principal dynamic balance for this section of the system. The accuracy of hydrodynamic predictions in the study area is demonstrated by comparison with four tidal elevation stations and two cross-section averaged current measurements. Simulated current is then used to model bed-load transport in the vicinity of a rapidly growing shoal located in the main channel of the lower system. Consisting of coarse sand and gravel, the shoal must be dredged every five to eight years. Two approaches are taken—an Eulerian parametric method in which nodal bed-load flux vectors are averaged over the tidal cycle and a Lagrangian particle tracking approach in which a finite number of sediment particles are released and tracked. Both methods yield pathways and accumulations in agreement with the observed shoal formation and the long-term rate of sediment accumulation in the shoal area. 相似文献
436.
Trends in biomass, density and diversity of North Sea macrofauna 总被引:6,自引:2,他引:6
437.
438.
Wave direction measurements using pitch-roll buoys 总被引:2,自引:0,他引:2
439.
B. T. G. Gowland A. D. McIntosh I. M. Davies C. F. Moffat L. Webster 《Marine environmental research》2002,54(3-5)
An aluminium smelter on the west coast of Scotland discharges an aqueous effluent containing polycyclic aromatic hydrocarbons (PAHs) at the head of Loch Leven. The loch also supports two mussel (Mytilus edulis) farms. Data are presented on burdens of PAHs in the soft tissues of mussels and the effect of these contaminants on glutathione S-transferase (GST) activity in mussel hepatopancreas. GST activity is shown to be correlated with total PAH burden and also with the concentrations of certain individual PAHs. These field data show that high molecular weight PAHs are closely correlated to GST activity, whereas low molecular weight PAHs are not. This suggests that 5- and 6-ring PAHs have a more pronounced role than 2- to 4-ring compounds in inducing GST activity in mussels from Loch Leven. It is proposed that it may be more appropriate to link GST activity with 5- and 6-ring compounds only, rather than with the total PAH burden. 相似文献
440.
A large number of 3D deep seismic surveys in the Faroe-Shetland Channel gives continuous coverage over most of the region. These surveys were designed primarily to image depths in excess of 4 km, use low frequency sources and are recorded at low temporal sample rates. However, commercial 3D data can generate highly detailed images of the seabed due to the high spatial sample rate, typically 12.5 m. This is particularly true in waters below 200 m. Despite geophysical artefacts, the images reveal that there are a number of sedimentary processes at work adjacent to and within this channel. On the West Shetland Shelf, iceberg scouring and moraines reflect the impact of glaciation. On the West Shetland slope there is clear evidence for down-slope processes, such as debris flows, linear erosion channels, basal fans and (one case) slope failure. Along-slope processes are also active as indicated by the presence of sediment waves and contourite mounds. On the floor of the basin, polygonal cracking can be observed. The most spectacular feature appears to be the Judd Deeps, a system of cliffs approximately 200 m high and 40 km across. Traditionally, seabed investigation has been performed using high-resolution surveys. This study shows that deep exploration data can also provide useful images of the seafloor. 相似文献