Nocdap results: On the relationship of extreme waves and currents for design of offshore structures

Simultaneous wind, wave, and current data during 21 storms spanning four winters at Tromsøflaket (230 m depth) were analysed to determine joint probabilities of occurrence. Waves were measured with a Waverider Buoy, winds with a recording anemometer onboard a vessel and currents at up to five depths with Aandera RCM-4 current meters. Measured currents were filtered to separate tidal currents from residual currents. In most of the statistical analyses, the actual current profiles were replaced with a simplified “equivalent” profile (constant above 50 m depth) whose magnitude was chosen to give the same drag load on a single pile as the actual profile when both profiles were combined with wave orbital velocities. The data suggest that the equivalent current is weakly correlated with the wave height. Within the range of the data analysed, an in-line equivalent residual current of 30 cm sec⁻¹ is adequate for design drag force calculations.The data presented here are used to illustrate a procedure for data analysis and are not suitable for use as design criteria.     

Uptake of plutonium from seawater and sediment by a marine polychaete worm

The results of the uptake of plutonium 239 from sediment by a marine polychaete wormNereis diversicolor is reported in the present paper. A comparison is made of the relative importance of two possible uptake pathways, sediment and seawater by which this worm may obtain its plutonium body-burden. Under the laboratory conditions used for the comparison of those two routes it would appear thatNereis obtains greater than 98% of its body activity from seawater.     

Evaporative weathering of petroleum residues: The age of pelagic tar

The rate of evaporative weathering of a component of crude oil residue is assumed to be proportional to its equilibrium vapor pressure and to the amount remaining in the sample. This model is tested using literature data on normal paraffin profiles of crude oil weathered artificially and gas chromatographic data on crude oil residues weathered on rocky shores. Semiquantitative agreement over the time range from 1 to 400 days is demonstrated. Application of the model to gas chromatograms of pelagic tar lumps collected at Station S near Bermuda in 1972 implies that most of these lumps were formed by fragmentation of much larger and older masses, some within less than one day of the time they were collected. The weathering of pelagic tar thus involves a great deal more physical fragmentation than was previously supposed.     

Ship hull drag reduction using bottom air injection

The idea of bottom air injection to reduce ship hull resistance is not new. Early patents envisioned planing hull applications. Recent planing hull tests speed realized an increase of 7–12 knots. River barges and ship fitted with an air injection system results are presented to show a 10–15% reduction in the frictional resistance. Graphs for making initial estimates for displacement hulls with bottom air injection are presented. It is clear from these results that improvements in high speed planing catamarans and full form hull resistance can be realized by using bottom air injection.     

Kirsch problem and the lower natural frequencies of a clamped square-plate

A clamped square-plate is subjected to a uniform state of stress in the x-direction. The present study deals with the basic question of determining the variation of the lower natural frequencies of transverse vibration of a plate as a small diameter, central hole is made in it, introducing a stress concentration field.     

Transverse vibrations of a rectangular cantilever plate with thickness varying in a discontinuous fashion

A numerical solution is obtained for the title problem by means of the popular finite element method. An experimental investigation of the problem is also presented for plates of square planform. The agreement between theoretical and experimental values is very good in the case of the fundamental frequency, and reasonably good when the second and third normal modes of transverse vibration are considered. It is concluded that beneficial effects are obtained in the sense that one generates a lighter structural element with higher fundamental frequency of transverse vibration. Hence, dynamic stiffening is achieved.