Variability of hydrodynamic load predictions for a tension leg platform

A survey of diffraction/radiation analyses has been performed on behalf of the International Ship and Offshore Structures Congress, Committee I.2. This paper presents the results from the survey, including data from 17 organisations plotted in a common format. The variability of the computed hydrodynamic loads and responses is discussed, and it is concluded that there is a degree of uncertainty in results predicted on the basis of commonly used idealisations. Even in long period waves the variability between the computed results is considerable. To obtain reliable results at the shorter periods associated with heave, pitch and roll resonances of a deep water TLP, much finer meshes than those used by most of the participating organisations would be required.     

Effects of scattering and resonance on energy dissipation of an internal tide in a narrow shelf

The effects of scattering and resonance on the energy dissipation of an internal tide were investigated using a two-dimensional model which is a reassembled version of the theoretical generation model devised by Rattray et al. (1969) for internal tide. The basic character of the scattering process at the step bottom was first investigated with a wide shelf model. When the internal wave incited from a deep region (Region II) into the shallow shelf region (Region I), a passing wave into the shallow region, a reflected wave into the deep region, and a beam-like wave, i.e. a scattered wave (SW), emanated at the step bottom. The SW, which consists of the superposition of numerous internal modes, propagated upward/downward into both regions. The general properties of the SW were well expressed around the shelf edge, even in the present model with viscosity effect. The amplitude of the SW decreased dramatically when the depth of the velocity maximum of the incident internal wave in Region II corresponded with the depth of the shelf edge. In the narrow shelf model, where the decay distance of the internal wave in Region I is longer than the shelf width, the incident internal wave reflected at the coast to form a standing wave. When the internal wave in Region I is enhanced by the resonance, the energy of the SW in Region II is also intensified. Furthermore, the energy of the modes in Region II predominated when the velocity maximum is identical to that of the dominant mode in Region I. These results suggest that the spatial scale of shelf region is a very important factor governing the energy dissipation of the internal tide through reflection and scattering in a narrow shelf.     

Long-term Sensor Drift Found in Recovered Argo Profiling Floats

We recovered three Argo profiling floats after 2 to 2.5 years of operation, and recalibrated their temperature, conductivity, and pressure sensors. The results demonstrate that these floats exhibited a significant drift in salinity of −0.0074 to −0.0125, primarily due to the conductivity sensor drift. Combined with the recalibration result for another previously recovered float, the indication is that the negative salinity drift increases nearly in proportion to the operating period of floats. The increasing rate is −0.0041 (±0.0015) year⁻¹, which yields a salinity drift of −0.016 (±0.006) for the expected float lifetime of four years. The present result suggests that reducing the float surfacing time would improve the accuracy of the salinity measurements.     

A B-spline based higher order panel method for analysis of steady flow around marine propellers

A higher order panel method based on B-spline representation for both the geometry and the solution is developed for the analysis of steady flow around marine propellers. The self-influence functions due to the normal dipole and the source are desingularized through the quadratic transformation, and then shown to be evaluated using conventional numerical quadrature. By selecting a proper order for numerical quadrature, the accuracy of the present method can be increased to the machine limit. The far- and near-field influences are shown to be evaluated based on the same far-field approximation, but the near-field solution requires subdividing the panels into smaller subpanels continuously, which can be effectively implemented due to the B-spline representation of the geometry. A null pressure jump Kutta condition at the trailing edge is found to be effective in stabilizing the solution process and in predicting the correct solution. Numerical experiments indicate that the present method is robust and predicts the pressure distribution on the blade surface, including very close to the tip and trailing edge regions, with far fewer panels than existing low-order panel methods.     

Water vapor in the atmosphere over the northern Tien Shan

Refined data of systematic measurements of total water vapor in the atmosphere from May 1980 to April 2005 are presented. The data were obtained at the Issyk Kul atmospheric-monitoring station by the method of solar molecular-absorption spectroscopy. Over 25 years, the annual mean water-vapor content in the atmosphere increased by 4.5% at a mean rate of increase of 0.18% per year. However, the water-vapor content decreased in the last five years. The results of statistical processing of experimental data (general statistical characteristics, correlation coefficients, composite oscillations) are described. A refined model is proposed for forecasts of temporal variations in the monthly mean and annual mean water-vapor contents for the coming years. The model includes a linear trend and the sum of oscillations with periods close to the periods of a number of well-known geophysical phenomena. Regression equations are proposed to relate the water-vapor content in the atmospheric column to the surface temperature and absolute humidity.     

Chlorinated hydrocarbons in seawater: Analytical method and levels in the northeastern Pacific

A method is described for analyzing nanogram quantities of chlorinated hydrocarbons from 1-1 samples of seawater. Seawater samples are pumped through a copper column containing a mixture by weight of 5% activated carbon powder, 10% MgO and 85% refined diatomaceous earth. The chlorinated hydrocarbons in the seawater are adsorbed or trapped on the column and subsequently eluted with 30% benzene in acetone (v/v) for analysis by gas-liquid chromatography.This procedure was used to analyze chlorinated hydrocarbon levels in samples collected off the southern California coast. We suggest that anthropogenic chlorinated hydrocarbons can be used for the investigation of large-scale ocean currents and mixing processes.     

Effects of salinity downshock on dimethylsulfide production

During time-series observations in Sagami Bay, Japan, the concentration of dissolved dimethylsulfoniopropionate (DMSP_d), a precursor of dimethylsulfide (DMS), was negatively correlated with salinity. In the laboratory, low-salinity shock reduced DMS production rates of the natural bacterial community and induced rapid DMSP release from a dinophyte, Heterocapsa triquetra, suggesting that low-salinity shock reduced DMSP_d consumption but enhanced DMSP_d production, which agrees with the negative correlation between DMSP_d and salinity observed in Sagami bay. In addition, low-salinity shock did not affect DMSP lyase activity of H. triquetra. Low-salinity shock would increase the contribution from algae in DMS production, leading to an increase in potential DMS productivity in the environment.