Lecture02-The-Marine-Environment

Lecture02-The-Marine-Environment - TTE 6755 Port Planning...

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TTE- 6755 Port Planning and Development Lecture #02 The Marine Environment 1.- Wind forces 2.- Wave forces 3.- Currents and tides 4.- Ice 5.- Seismic 6.- Propeller wash, scour, siltation 7.- Bio-chemical attack 8.- Vessel impact (mooring) forces 9.- Crane and cargo loads. L. Prieto-Portar 2009
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Environmental Forces on Marine Structures.
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1. Seawater. Seawater contains a great number of elements. The concentration of these elements are typically given in parts per million (ppm) which is equivalent to milligrams per liter (mg/L). The combined concentration of these dissolved solids in sea water is defined as its salinity . The salinity of seawater ranges from 3.1% to 3.8%, with a mean of about 3.5% (or 35 ‰). Ports typically have smaller salinity due to the inflow of freshwater from rivers and groundwater. Chlorine (1.9x10 7 mg/L) and sodium (1.1x10 7 mg/L) are the most abundant elements in seawater. Seawater’s salinity is given by S (‰) = 0.03 + 1.805 chlorinity (‰). The density of seawater is slightly higher than freshwater, 64 pcf versus 62.4 pcf (or, 1025 kg/m 3 versus 1000 kg/m 3 ). The heat capacity of water is among the highest of all liquids and solids. This is the reason that oceans help maintain uniform temperatures.
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The thermal conductivity of seawater is slightly less than freshwater. Chemical reactions are governed by acidity and alkalinity through their pH, pH = - log 10 (H + ) where H + is the concentration of the hydrogen ions. Since the H + concentration of pure water is 10 -7 , its pH = 7. The range for pH is 0 to 14, with values less than 7 being acidic and greater than 7 being alkaline. The pH of seawater varies from 8.0 to 8.4. The local value of pH determines the level of biological activity and the degree of corrosion on structures. The levels of oxygen both in the air and at its seawater interface determine the degree of corrosion of steel: vessel and port engines, hulls, cranes, cables, etc. The levels of carbon dioxide CO 2 and hydrogen sulfide H 2 S that are dissolved in seawater tend to lower the pH, and large concentrations of H 2 S embrittles steel. Silts and clays suspended in the water lead to turbidity.
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Marine fouling is the accumulation of marine plants and animals on immersed surfaces. A ship’s hull, for example, not only looses speed but also increases its weight with fouling. The drag coefficient C D is usually increased by 20%, although medium fouling may increase it by 70%. Fouling also increases the rate of corrosion of metals. Fouling in warm seawater from hard mussels may be greater than 300 mm thick. Many sea microorganisms grow at rates of 50 mm/year. Some researchers (Heaf 1979) have studied fouling on North Sea oil platforms, and concluded that fouling could be more severe to the structure than wave loads. The density of fouling varies from 1.0 to 1.4 tonnes/m 3 . Tables for the potential fouling in specific areas of the World are published by the U.S. Navy World Atlas of Coastal Biological Fouling.
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