he power of the oceans has always daunted humanity - whether through the sweep of its tides and currents, or the crash of its waves on the shore. Now we may be beginning to exploit it to run our homes and businesses. It is not easy, partly because the very power that makes it so attractive also makes it hard to design and construct strong enough devices. But, wave and tidal power have staggering worldwide potential - and produce clean energy, which does not cause pollution or accelerate climate change. Of course we already get oil and gas from under the seabed, from the Caspian to the South China Sea and the Caribbean, from the Arctic to the Bight of Benin. But these reserves are finite, and new sources of energy will be needed. 
Less than 0.1 per cent of the energy of waves and tides, research suggests, could supply more than five times the world's current demand for electricity. Structures to capture it are already installed on the shoreline where the waves break, and in estuaries where tides ebb and flow. In 2006, wave power devices will be placed up to 5 kilometres offshore.
Australia, China, France, India, Japan, Portugal, the Scandinavian countries and the United States are all developing ocean power technologies. The engineering is difficult as the machines - though relatively inexpensive to operate and maintain - must withstand storms and corrosion from salt. Wave devices must also be able to exploit both strong and weak waves.
Research and development for ways to harness the ocean's power may be costly. But as demands grow both for more electricity and for lower emissions of carbon dioxide - the main cause of global warming, emitted by burning fossil fuels - the attractions of clean energy from the seas are growing.
Barrier power
The most advanced way to exploit the tides is by building barriers across estuaries. In this technology, pioneered in the 1960s, tides push water through the barriers, turning turbines to generate electricity. The largest is at La Rance, northern France. But barrages have been criticized for disturbing habitats. China is now working on an artificial lagoon scheme at the mouth of the Yalu river, harnessing the energy of the tides as the water pours out of the lagoon.
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As 2006 began, the world had only one commercial wave-power generating station, on the Scottish island of Islay. The Limpet 500 (Land Installed Marine Powered Energy Transformer) produces power for Britain's national grid. The operator, Wavegen, and the developers of the technology - researchers from Queen's University, Belfast - believe there could be enough recoverable wave power around the United Kingdom to meet all the country's growing domestic electricity demands, and more. The same team is now developing wave power for the Faroe Islands.
It may have been just 90 metres off the Californian coast, but when it started production in 1897 it was the world's first offshore oil well. Now around 30 per cent of our oil supplies come from offshore wells, as does about half our natural gas. They are exploited from some 8,000 platforms scattered around the world's oceans.
It is much more expensive to extract oil and gas offshore than on land. But as demand grows and reserves dwindle, prices increase - ensuring that exploitation at sea remains attractive.
There have been surprisingly few massive spills considering the size of the industry. But it still poses threats to sea life, at least nearby. Wastes from drilling pollute the seabed and damage life, delicate habitats can be disturbed and there are concerns over disposing of the platforms, and the toxic substances they contain, after the oil fields they tap are exhausted.
When the wells run dry they may find a new planet-saving use. Technologies are being developed to capture emissions of carbon dioxide from power plants and the like, and pipe it into oil and gas wells deep under the seabed. This gives a double benefit as it can help to flush out the last oil and gas reserves: the United States is already disposing of some 32 million tonnes of carbon dioxide in this way every year.
Rigs have drilled in waters more than 2,000 metres deep and 300 kilometres from the shore. Submersibles are floated into shallow waters, then ballasted so they sit on the seabed. Jackups - used in waters up to 100 metres deep - are towed out to sea where their legs are lowered and their hull jacked up out of the water. Drill ships - anchored or kept in place by computer-controlled propellers - prospect for oil or drill in waters up to 1,500 metres deep. But semi-submersibles are the most common of all; their superstructures stand on columns rising from hulls or pontoons ballasted below the water surface, providing excellent stability even in rough seas as much as 300 metres deep.
Oil platforms are industrial towns suspended above the sea, complete with all the equipment and supplies needed by their workers, from drill operators to divers, engineers to cooks. Each platform typically extracts oil and gas from a number of reservoirs at the same time. The tall derrick hauls the drillstring in and out of the hole, cooled by fluid called drilling mud. Water is pumped into the reservoir at high pressure to force up the crude oil, a mixture of natural gas, water and oil. These are separated and the water removed, along with particles. This wastewater can be used in the drilling muds or to help extract more crude oil - or is returned to the ocean after testing for contaminants. The oil and gas are pumped ashore or loaded onto ships.
Related Links:
Wavegen Ocean Power Delivery MIT World Changing School Science Feasta Adventures in Energy Oil and Gas Forum PDF Version