A new methodology developed by cetena to assess the seakeeping behaviour of marine vessels

The paper describes an analytical method to determine an operability index for a marine vehicle when its geometric characteristics, the geographical area in which the vessel is supposed to operate, and the limiting criteria for operations are known. The computer program based on strip theory provides reliable results not only for conventional hullforms, but also for catamarans, offshore vehicles, etc. The results are presented in the form of figures for different vessels, namely, crane barge, naval vessels and SWATH type catamarans. The authors are of the opinion that the proposed method would provide the designer with a valid tool to improve the seakeeping qualities of a vessel, while taking into account the limiting conditions imposed due to seaway operations.     

Dissolved carbon and 33-133-133-1anomalies in the water column caused by hydrocarbon seeps on the northwestern Gulf of Mexico slope

Alkalinity, dissolved inorganic carbon (DIC), and ¹³C profiles from seep sites on the northwestern Gulf of Mexico upper slope show anomalously negative ¹³C values of up to –4.5 PDB, increased levels of DIC of up to 2.45 mmole/liter, and slight alkalinity rises of up to 2.54 meq/liter, relative to water column profiles from a seep-free site (0.63, 2.04 mmole/liter, and 2.39 meq/liter). The observed DIC enrichments coupled with the¹³C-depletions are attributed to the release of CO₂ by microbial oxidation of crude oil in the seep environment, and its migration into the water column. The ¹³C composition of the migrating CO₂ is estimated to be –26.0 on the basis of dissolved carbon inventory. Manifestation of DIC and ¹³C anomalies in the water column caused by hydrocarbon seepage holds promise to be useful for hydrocarbon reconnaissance surveys over large offshore tracts on account of the simplicity of sampling acquisition, and rapidity of analytical techniques in the laboratory.     

Particulate flux in the water column overlying Santa Monica Basin

Fluxes of particulate carbon, nitrogen, phytoplankton pigments, biogenic silica and dry mass were measured using free-floating and moored sediment trap arrays in the Santa Monica Basin during the period from October 1985 to August 1990 as part of the California Basin Study (CaBS) Program. In field testing for potential sources of sediment trap biases, we found little significant or consistent difference in rate estimates between short-term drifting traps and long-term moored traps, between preserved and unpreserved traps in short-term experiments, between different preservatives (mercury or formalin) in long-term experiments, between different designs of small cylindrical traps, and between deep-moored cylindrical traps and large conical traps. We did, however, find that sediment trap samples collected and analyzed on 0.45 μm silver filters gave estimates of carbon and nitrogen fluxes about 25% higher than samples collected on GF/F glass-fiber filters. Concurrent trap deployments at two stations 18km apart revealed low mesoscale variability in flux estimates. Seasonal patterns in carbon and nitrogen flux were not evident in our data, but strong seasonality, with spring maxima and summer minima, were observed for fluxes of phaeopigments and biogenic silica out of the euphotic zone.Time-averaged rates of particulate flux for long-term trap deployments from January to August 1990, were 121, 18.8, 1.5, 67 and 633mg m⁻²d⁻¹ at 110–135m for carbon, nitrogen, phaeopigment, biogenic silica and mass, respectively. Flux estimates to the basin floor (835–860m) were 50, 6.5, 0.64, 41.6 and 575mg m⁻²d⁻¹ for the same parameters. The former estimates are constrained by and in good agreement with independent assessments of new production from nitrate uptake in the euphotic zone. The latter agree with rates previously inferred from the sedimentary record using ²¹⁰Pb as a tracer. In addition, the difference in carbon estimates in the water column between the euphotic zone and the basin floor is consistent with the requirements for bacterial growth and metabolism at intermediate depths as measured by the thymidine method.     

Measurement of stress effects (scope for growth) and contaminant levels in mussels (Mytilus edulis) collected from the Irish Sea

The objective of this research was to quantify the impact of pollution along the coastlines of the Irish Sea. Pollution assessment was based on the combined measurement of scope for growth (SFG), and chemical contaminants in the tissues of mussels (Mytilus edulis) collected from 38 coastal sites around the Irish Sea during June-July in 1996 and 1997. On the UK mainland coast, the SFG showed a general trend with a significant decline in water quality in the Liverpool and Morecambe Bay region. High water quality was recorded along the west coast of Wales, as well as southwest England and northwest Scotland (clean reference sites outside the Irish Sea). Along the coast of Ireland there was a similar trend with reduced SFG within the Irish Sea region. SFG was generally low north of Duncannon and then improved north of Belfast. The poor water quality on both sides of the Irish Sea is consistent with the prevailing hydrodynamics and the spatial distribution of contaminants associated with urban/ industrial development. The decline in SFG of mussels on both sides of the Irish Sea was associated with a general increase in contaminant levels in the mussels. Certain contaminants, including PAHs, TBT, sigmaDDT, Dieldrin, gamma-HCH, PCBs, and a few of the metals (Cd, Se, Ag, Pb), showed elevated concentrations. Many of these contaminants were particularly elevated in the coastal margins of Liverpool Bay, Morecambe Bay and Dublin Bay. A quantitative toxicological interpretation (QTI) of the combined tissue residue chemistry and SFG measurements indicated that at the majority of coastal sites, c. 50 to > 80% of the observed decline in SFG was due to PAHs as a result of fossil fuel combustion and oil spills. TBT levels were highest at major ports and harbours, but these concentrations only made a minor contribution to the overall reduction in SFG. At no sites were individual metals accumulated to concentrations that could cause a significant effect on SFG. The study identified many sites where the observed reduction in SFG was far greater than predicted from the limited number of chemical contaminants analysed, thus indicating the presence of additional 'unknown toxicants'. Sewage (containing domestic, agricultural and industrial components) appears to be an important contributor to reduced SFG and linear alkylbenzenes (LABs) and As may provide suitable 'sewage markers'. There was a highly significant positive correlation between SFG and As (P < 0.001). This relationship may be due to reduced As uptake by algal food material and mussels at sites with elevated P04 concentrations (e.g. at sites with sewage inputs). Phosphate is a known competitive inhibitor of As accumulation, at least in algae. The results highlight that further research is required on 'sewage markers' in mussels. The SFG approach therefore provides a rapid, cost-effective and quantitative measure of pollution impact, as well as a means of identifying the causes through a QTI of tissue contaminants levels. It also serves to identify the presence of unidentified toxicants and areas that require further study.     

Estimated intensity needed for sampling flatfish assemblages in reference areas of tidal mud flats systems may be disproportionately costly and deleterious

Beam trawl surveys were carried out in a tidal stream system of the German Wadden Sea on two to three successive days of comparable weather conditions in three summers. The variability in time and space detected for plaice (Pleuronectes platessa), flounder (Platichthys flesus), dab (Limanda limanda), sole (Solea solea) and lemon sole (Microstomus kitt) was considerable even for species with a very high abundance. Other flatfish species such as turbot (Scophthalmus maximus) and brill (Scophthalmus rhombus) occurred only occasionally in the catches. Sampling effort calculated to detect a 50 or 20% difference between the surveys caused by immigration, emigration or mortality appears to be disproportionate. It is argued that such a major sampling effort of reference areas in tidal mud flat systems is likely to influence the results, to harm the environment, and to be very expensive.     

Dynamics of a moored barge under regular and random waves

Developments in the study of wave forces and construction techniques in deep water by the offshore oil industry have increased the use of marine terminals at deep water locations. A thorough understanding of moored ship dynamics when subjected to waves, wind and current combined with the use of flexible mooring lines would help to design berthing terminals for exposed areas. In this paper, the three dimensional problem of wave interactions with a barge moored to a single point is dealt with, based on the finite element method. The effect of flexibility of the mooring line and the point of mooring on the response of the barge as well as the mooring line tension is investigated. The paper compares the numerical results with model tests carried out on a barge moored to a fixed support under regular and random waves in head sea. The effect of stiffness of the mooring line on the barge response for different mooring points is discussed, which would be useful for the designers. The effect of viscous damping is also considered. The analytical results are in good agreement with the experimental results in both regular and random waves.     

Sensitivity to stress of the estuarine bivalve Macoma balthica from areas between the Netherlands and its southern limits (Gironde)

Variation in the sensitivity to stress of Macoma balthica was measured in several French and Dutch estuaries. For adult and juvenile Macoma balthica exposed to copper under conditions of starvation, differences in mortality rate, condition, glycogen, burrowing rate and copper content were assessed. No significant differences were observed between adults and juveniles; the influence of treatment and origin was always evident. Animals from the most southern estuaries, Loire and Gironde, near to the species's southern limit of distribution, showed, in the field, the strongest deviations for the ecophysiological traits measured, and were in the experiments the most sensitive to stress.     

The status of geological dredging techniques

Scientific sea-floor dredging is currently used in marine geology primarily by the hard-rock community interested in the recovery of basement rock samples from the unsedimented deep ocean floor. The technique has generally been eclipsed by ocean drilling for recovery of sedimentary rocks, because of perceived uncertainties in the location of sampling and in the representativeness of recovered material. This contribution reviews dredging equipment currently in use by marine geological institutions and refers to pinger attachments that allow precise information on the behaviour of the dredge to be telemetered back to the ship. We argue that improvements in ship navigation and transponder navigation at the seafloor, when used in conjunction with surface and/or deeply towed sidescan and swathemapping surveys, now allow for considerably less uncertainty on the location of dredge sampling. Refined sorting criteria for dredge hauls are now also available. Recent comparisons of regional sample recovery by ocean drilling and by dredge sampling indicate that the dredge hauls can usefully supplement the drilling data in the construction of sedimentary and tectonic histories of seafloor areas.     

Morphology and tectonics of the Galapagos Triple Junction

We describe the results of GLORIA and SEABEAM surveys, supplemented by other marine geophysical data, of the Galapagos Triple Junction where the Pacific, Cocos and Nazca plates meet. The data allowed detailed topographic and tectonic maps of the area to be produced. We located each spreading axis with a precision of about 1 km. All three plate boundaries change character as the triple junction is approached to take on morphologies typical of slower spreading axes: the fast-spreading East Pacific Rise develops the morphology of a medium-spreading rise, and the medium-spreading Cocos-Nazca Rise takes on the appearance of a slow-spreading ridge. The axis of the East Pacific Rise was found to be completely continuous throughout the survey area, where it runs along the 102°05 W meridian. The Cocos-Nazca axis, however, fails to meet it, leaving a 20-km-wide band of apparently normal East Pacific Rise crust between its tip and the East Pacific Rise axis. As a consequence there must be considerable intra-plate deformation within the Cocos and Nazca plates. A further 40 km of the Cocos-Nazca axis is characterised by oblique faulting that we interpret to be a sign of rifting of pre-existing East Pacific Rise crust. We infer that true sea-floor spreading on the Cocos-Nazca axis does not begin until 60 km east of the East Pacific Rise axis. Other areas of similar oblique faulting occur on the Pacific plate west of the triple junction and along the rough-smooth boundaries of the Galapagos Gore. We present a model involving intermittent rifting, rift propagation, and sea-floor spreading, to explain these observations.     

Wave drift enhancement effects in multi column structures

A theoretical assessment is made of mean wave drift forces on groups of vertical circular cylinders, such as the columns of a floating offshore platform. A complete analytical solution is obtained for two cylinders extending from seabed to free surface, and a long wave approximation is found to provide reliable predictions of the drift force in line with the waves at low frequencies. For moderate separation between the two cylinders, this force is found to tend at low frequencies to a value four times the force on an isolated cylinder.A numerical method is employed to study two surface piercing cylinders truncated below the free surface, and an arrangement of four vertical cylinders characteristic of a floating offshore platform. The mean vertical drift force is found to be reasonably well approximated, over the frequency range of practical interest, by the force on an individual cylinder considered in isolation multiplied by the number of cylinders in the group. Interaction effects, however, have a profound influence on the total horizontal drift force. At low frequencies this force is found to tend to the force on an isolated cylinder multiplied by the squate of the number of cylinders in the group.