Ocean Water Composition
About three-quarters of the earth’s surface is covered by oceans. The average ocean depth is 3,800 m, and the maximum depth is 11,524 m in the Mindanao Trench in the Pacific Ocean. In comparison, the average land elevation is 840 m. and the highest elevation is 8,840 m at the top of Mount Everest. Figure 7.3 is the cross-section of the ocean floor, depicting the general terminology of the shore and the continental shelf. About 8% of the ocean floor is shallower than 1,000 m and 5% is shallower than 200 m, where the majority of offshore oil and gas platforms are located. Most offshore wind farms in Europe have been installed in water less than 15 to 20 m in depth.
The ocean shore and shelf material generally consists of sand and gravel coming from land via rivers and blown in by w'ind. The ocean water density of seawater at atmospheric pressure and 10°C is 1027 kg/m3. It contains 35 g of salt per kilogram of seawater, which is expressed as the salinity of 35 ppt (parts per thousand). The salinity is measured by measuring the electrical conductivity of seawater, as the two are related. Seawater salt composition by percentage is shown in Figure 7.4: 55% chlorine, 31% sodium, and 14% all others.
FIGURE 7.3 Ocean-floor terminology.
FIGURE 7.4 Salt composition of ocean water (by percentage).
Wave Energy and Power
The primary wave-generating forces are the wind, storms, earthquakes, the moon, and the sun. Figure 7.5 ideally represents a typical ocean wave in shallow offshore water. It is characterized by the following expressions:
where d = water depth and T = wave period.
As shown in Figure 7.6, the water particles under the wave travel in orbits that are circular in deep water, gradually becoming horizontal-elliptical (flat-elliptical) near the surface. The kinetic energy in the wave motion is determined by integrating the incremental energy over the depth and averaging over the wavelength:
The potential energy of the wave is determined by integrating the incremental potential energy in the height of a small column width over one wavelength:
Note that the wave’s potential energy and kinetic energy are equal in magnitude, which is expected in an ideal wave with no energy loss. Therefore, the total energy per unit length of the wave, £,, is twice that value, i.e.:
FIGURE 7.5 Ideal representation of ocean waves in shallow offshore water.
FIGURE 7.6 Orbits of water particles in deep and shallow waters.
The total energy E in one wavelength L is given by the following:
The mechanical power in the wave is the energy per unit time. It is obtained by multiplying the energy in one wavelength by the frequency (number of waves per second) as follows:
The energy in a wave thus depends on the frequency, which is a random variable. Ocean wave energy is periodic, with the frequency distribution in ocean surface waves as shown in Figure 7.7. The actual waves may have long waves superimposed on short waves from different directions. A simple linear one-frequency wave theory may be superimposed for an approximate estimate of the total effect.
Ocean Structure Design
Wind-generated waves with a period of 1 to 30 sec are the most important in determining the wave power and force acting on ocean structures.
FIGURE 7.7 Approximate energy distribution in ocean surface waves.
Forces On Ocean Structures
The offshore structure must withstand mechanical forces exerted by the ocean waves, currents, wind, storms, and ice. The wave force is the most dominant of all. The structure must absorb, reflect, and dissipate wave energy without degradation in performance over a long lifetime.
Offshore platforms have been built around the world since the early 1950s to drill for oil and gas, along with undersea pipelines. Offshore mining is also being developed now, and the offshore wind farm is the latest addition to such structures installed to provide a means of producing energy resources and transporting energy to the shore.
The weight and cost of a fixed platform that will withstand wave forces, currents, and wind increase exponentially with the depth of water as seen in Figure 7.8. The offshore cable or pipelines carrying the cable must withstand forces due to inertia, drag, lift, and friction between the floor and the pipe. The water drag and lift forces for an underwater structure in a somewhat streamlined water current can be derived from classical hydrodynamic considerations. They depend on the water velocity near the floor, which follows the one-seventh power law, namely
where V and Va are the water velocities at heights Y and T0, respectively, above the seabed.
The friction coefficient between the pipe surface and the seabed varies with the sediment type. The concrete-coated pipes commonly used offshore have a friction coefficient of 0.3 to 0.6 in clay, about 0.5 in gravel, and 0.5 to 0.7 in sand.
FIGURE 7.8 Cost of offshore structures at various depths.
Corrosion is caused by the electrochemical reaction in which a metal anode corrodes by oxidation and a cathode undergoes reduction reaction. The seawater works as an electrolyte for the transfer of ions and electrons between the two electrodes. The corrosion rate must be accounted for in the design. Table 7.1 lists the corrosion rates of commonly used metals.
Two types of cathodic protection are widely used for corrosion protection of materials submerged in seawater. The impressed current system gives more permanent protection, but requires electric power. Galvanic protection employs aluminum, magnesium, or zinc anodes attached to the steel structure in seawater. Under the cathodic protection principle, a metal receiving electrons becomes a cathode, which can no longer corrode. The zinc anode is most widely used as a sacrificial material to protect steel hulls of ships. When it has deteriorated, it is replaced for continued protection. Zinc provides about 1000-Ah charge-transfer capacity per kilogram. The sacrificial anode design follows basic electrical circuit principles.4