Stochastic modelling of quadratic wave components

This paper deals with stochastic wave models and their influence on wave load modelling of marine structures. The need for more refined theories is underlined for some applications and an entirely new quadratic model for the wave elevation is outlined. Finally, a number of physical phenomena relevant to waves are illustrated which are likely to be important for marine structures and could, in the near future, require ad hoc modelling.     

On the background level of Tc, Sr and Cs in the North Atlantic

Mean concentrations of the anthropogenic radioactive oceanographic tracers ⁹⁹Tc, ⁹⁰Sr and ¹³⁷Cs have been measured as 0.005, 1.6 and 2.5 Bq m⁻³ in oceanic Northeast Atlantic surface water, east and northeast of the Azores, in 1992. This is, apparently, the first published value for fallout “background” ⁹⁹Tc in oceanic Atlantic water.Comparison with older data indicates an observed half life for ⁹⁰Sr and ¹³⁷Cs in the northeast Atlantic surface water of 20 yr corresponding to a mean residence time of 80–100 yr for the stable elements.The observed ⁹⁹Tc/⁹⁰Sr ratio (3 × 10⁻³) in the Azores samples is 10 times higher than the theoretical fission yield decay corrected to 1992. This is in agreement with published data on rain water samples and may be characteristic for 1960's global fallout. Furthermore, the measured ¹³⁷Cs/⁹⁰Sr ratio is not significantly different from that observed for global fallout. There do not appear to be any additional significant sources of artificial radionuclides in this region.     

The weldability of steels used in jack-up drilling platforms

Weldability problems in jack-up rigs arise principally in leg construction where particularly high-strength steels (690 N mm⁻² yield) are used for chords and racks. Attainment of mechanical properties is not usually difficult, although procedural trials are advisable.

Of potential fabrication problems, hydrogen cracking (heat-affected zones or weld metal) is of greatest concern, and consideration of solidification cracking is advisable. Lamellar tearing is not generally a problem with modern steels, and the risk of stress-relief cracking will be confined to the limited number of joints which can be heat-treated.

Fatigue cracking is probably the major cause of service failure of jack-up rigs, and the use of high-strength steels, which permits higher static stress limits, can exacerbate this problem. Hydrogen-induced stress corrosion can also occur, either due to cathodic polarisation or corrosion, particularly in the presence of H₂S in foul sea-water. For this reason weld hardness limits need to be maintained.     

A cost optimized steel bridge girder design for aashto - WSD and LFD

A cost optimized design method of steel bridge girders is studied by using the specifications of the American Association of State Highway and Transportation (AASHTO) Working Stress Design (WSD) method and Load Factor Design (LFD) alternates. Here, cost trade-offs for the steel plate (e.g., the base price plus extras for size, grade, testing and inspection, etc.), stiffeners and splicing are weighed throughout the entire span to minimize the objective function. The method is then applied to a set of over 30 steel bridge structures which collected from various state bridge inventories in order to obtain optimal cost configurations for both WSD and LFD methods. These results are analyzed by a linear regression model to define relationships between cost differences and bridge lengths. From these comparisons, conclusions are drawn to provide the guidelines. It is believed that effective application of these guidelines should lead to more economical designs, especially under the LFD method.     

The wind-forced response on a buoyant coastal current: Observations of the western Gulf of Maine plume

The freshwater plume in the western Gulf of Maine is being studied as part of an interdisciplinary investigation of the physical transport of a toxic alga. A field program was conducted in the springs of 1993 and 1994 to map the spatial and temporal patterns of salinity, currents and algal toxicity. The observations suggest that the plume's cross-shore structure varies markedly as a function of fluctuations in alongshore wind forcing. Consistent with Ekman drift dynamics, upwelling favorable winds spread the plume offshore, at times widening it to over 50 km in offshore extent, while downwelling favorable winds narrow the plume width to as little as 10 km.Using a simple slab model, we find qualitative agreement between the observed variations of plume width and those predicted by Ekman theory for short time scales of integration. Near surface current meters show significant correlations between cross-shore currents and alongshore wind stress, consistent with Ekman theory. Estimates of the terms in the alongshore momentum equation calculated from moored current meter arrays also indicate a dominant Ekman balance within the plume. A significant correlation between alongshore currents and winds suggests that interfacial drag may be important, although inclusion of a Raleigh drag term does not significantly improve the alongshore momentum balance.     

Summer distribution of seabirds in the North-East Water polynya, Greenland

The distribution at sea of seabirds was studied in the North-East Water (NEW) polynya, Greenland, during transect counts in the summers of 1991, 1992 and 1993 on board the ice-breaking RVs Polarstern and Polar Sea. Data collected within the polynya ‘box’ (78–82°N; 5–18°W) concern observations of 8000 birds counted during 1350 half-hour counts. Distribution is presented as density (N/km²) and calculated daily food intake. Five bird species were selected for discussion, representing more than 95% of the total numbers encountered: Fulmar (Fulmarus glacialis), Ivory Gull (Pagophila eburnea), Kittiwake (Rissa tridactyla), Glaucous Gull (Larus hyperboreus) and Ross's Gull (Rhodostethia rosea). For these species, densities are comparable in the NE Greenland polynya and in other European Arctic seas. The main difference is the absence in NEW of the species playing the main role in Arctic seas: Brünnich's Guillemot (Uria lomvia) and Little Auk (Alle alle). In the absence of fish-eating birds and of birds consuming zooplankton in the water column, the NEW polynya ecosystem is thus dominated by surface feeders and, closer to the coast, by benthic feeders like eiders, Somateria mollissima and S. spectabilis, and walrus, Odobenus rosmarus. The density and daily food intake for all seabirds are one order of magnitude lower in the polynya than in the Arctic seas. The distribution and abundance of seabirds in the NEW polynya seems to reflect a very low density of pelagic fish and Zooplankton in the water column, while Zooplankton must be present at ‘normal’ concentrations in the upper layer.     

Benthic habitat quality assessment of an oxygen stressed fjord by surface and sediment profile images

In May and October 1994 the benthic habitats in Havstensfjord (26 km²), a seasonally oxygen stressed stratified fjord, were classified by taking sediment surface and sediment profile images (SPI) at 90 stations, randomized into nine strata. Qualitative grab samples for faunal inspection were taken at about of the stations in both May and October to help to identify objects in the images. The images give information on sediment characteristics (texture, oxic/anoxic conditions, lamination) which often can be related to functional properties of the macrofauna (burrows, tubes, feeding voids, reworked sediment) or to observations on benthic epifauna. In combination, such variables mirror the quality of the benthic habitat. Analysis of sediment profile images was done with multivariate methods [Benthic Habitat Quality (BHQ) index] and univariate statistical methods to describe differences between areas and depths. Variance analysis of BHQ indices indicated a significant interaction between area and depth. In both the northeast and northwest strata the oxygen stress had induced a habitat degradation at depths deeper than 25 m compared to shallower northwest and northeast strata and all south depth strata. No significant difference in mean BHQ index was found between May and October. We discuss the potential application of the SPI technique for efficient monitoring oxygen stressed marine coastal areas.     

Impact of freshwater on a subarctic coastal ecosystem under seasonal sea ice (southeastern Hudson Bay, Canada). III. Feeding success of marine fish larvae

We monitored the feeding success (percent feeding incidence at length and mean feeding ratio at length) of Arctic cod (Boreogadus saida) and sand lance (Ammodytes sp.) larvae in relation to prey density, light, temperature and potential predator density under the ice cover of southeastern Hudson Bay in the spring of 1988, 1989 and 1990. Both prey density and light limited larval fish feeding. The relationship between feeding success and actual food availability (nauplii density X irradiance) was adequately described by an Ivlev function which explained 64 and 76% of the variance in Arctic cod and sand lance feeding success respectively. By affecting both prey density and irradiance, the thickness of the Great Whale River plume (as defined by the depth of the 25 isohaline) was the main determinant of prey availability. Arctic cod and sand lance larvae stopped feeding when the depth of the 25 isohaline exceeded 9 m. Limitation of feeding success attributable to freshwater inputs occurred exclusively in 1988, the only time when the depth of the 25 isohaline exceeded the 9 m threshold. The close dependence of larval fish feeding success on the timing of the freshet and plume dynamics suggests a direct link between climate and survival of Arctic cod and sand lance larvae. The actual impact of climate fluctuations and/or hydro-electric developments on recruitment will depend on the fraction of the larval dispersal area of the two species that is affected by river plumes.     

Roll Plane Analysis of Articulated Tank Vehicles During Steady Turning

The rollover immunity levels of articulated tank vehicles with partial loads are investigated. A static roll plane model of the articulated vehicle employing partially filled cylindrical tank is developed. The vertical and lateral translation of the liquid cargo due to vehicle roll angle and lateral acceleration, encountered during steady turning, are evaluated. The roll moments arising from vertical and lateral translation of the liquid cargo are determined and incorporated in the roll plane model of the vehicle. The adverse influence of the unique interactions of the liquid within the tank vehicle, on the rollover limit of the articulated vehicle is demonstrated. The influence of compartmenting of the tank on the steady turning roll response of the vehicle is analyzed, and an optimal order of unloading the compartmented tank is discussed.     

An Investigation into Stick-Slip Vibrations on Vehicle/Track Systems

A model for the numerical simulation of vehicle/track interaction and stick-slip vibration is presented. A finite element model is developed to calculate vertical contact forces. These forces are then coupled through the contact patch into a non-linear time-domain model by which the stick-slip vibration behaviour of a wheel-rail system is analysed. The investigation suggests that stick-slip vibration may occur if a vehicle which has a maligned or an initial 'wind-up' wheeiset meets a vertical irregularity or contaminants on the track.