Deep spill

By Anne Lise Aakervik

What happens if there is an oil or gas leak at a depth of 844 metres below the sea surface?

SINTEF has conducted the world's first simulated release of gas and oil in deep waters. The temporary conclusions suggest that the gas may not reach the water surface and that the oil rises to form an oil slick similar to those produced by an ordinary blow-out.

On Sunday, 26 June 2000, three ships were headed towards the Helland Hansen field 165 nautical miles off Kristiansund in western Norway. After one year of preparation, a team of 42 scientists and representatives of several oil companies are ready to spend the next three days simulating underwater blowouts of oil and gas. The tests are to be carried out at depths of more than 800 metres so tension is high. The weather is not looking good, but an improvement is predicted for Monday.

Ever deeper
The reason for this exceptional test is that oil companies are being forced into deeper waters to find oil. In the Gulf of Mexico, where American oil companies are actively searching for new oil deposits, they are already working at depths greater than 1000 metres. At present, most of the Norwegian oil production is in water less than 350 metres deep. In a few years, however, Norway may be recovering oil from depths perhaps as great as 2000 metres. The biggest challenges at such depths are the temperature and the pressure. Little is known about the immediate effects and long-term consequences if a blow-out should occur at such depths. Thus the three ships make their way to the Helland Hansen field on this Sunday in June.

Filled to the brim
The ships are loaded with custom-made equipment worth several million kroner. The deck of the supply ship Far Grip is packed full. The vessel is carrying 750 barrels of oil, 18 cubic metres of liquid natural gas (LGN) and some nitrogen. It is the command vessel and the base of operations from which the releases of oil and gas will be carried out. The research vessel Hĺkon Mosby from the University of Bergen is carrying hydro-acoustic equipment, a remotely operated mini-submarine (ROV) and tools for measuring currents in the water column. The third vessel, Johan Hjort, is carrying a workboat for collecting oil samples and equipment for temporary chemical analysis of the samples. At the closest airport, in Kristiansund, seven surveillance aircraft from the Norwegian Pollution Control Authority (SFT) and several Bonn Agreement countries including Germany and Denmark are also prepared to assist. The governments owning these aircraft have a strong interest in seeing how the spills develop since many countries around the North Sea may be at risk in the event of a major spill.

Extensive preparations
Such a large-scale experiment requires meticulous planning. A total of 23 different oil companies and Minerals Management Service, a US government agency, contributed to financing the USD 2 million "Deep Spill" project. SINTEF is the main contractor and project manager. On Monday the wind is calmer and the first, critical procedure could be carried out: deploying the platform from which the oil and gas will be released on the seabed. "The whole field campaign would be over if this operation fails," explains Řistein Johansen, senior researcher at SINTEF Applied Chemistry. Without the platform placed securely on the seabed, there would be no means of releasing the oil and gas. "What a wonderful sight it is to see the round platform with two parallel pipes extending out from it, everything perfectly positioned on the bottom." These are the pipes through which oil and gas will be transferred down to the platform. The scientists want to test their hypotheses through four experiments, all of which have been approved by the SFT. The first release is nitrogen, followed by a mixture of marine diesel and natural gas. The third test is a combination of natural gas and a North Sea oil and the final test, a release of natural gas alone.

Many questions
Since oil represents the most serious threat to the environment, the last two tests attract the greatest attention. Using an oil pump powered by a large V16 diesel engine and a cryogenic pump for the liquefied gas, the two products are forced down the pipes, raising many questions in the process: How widespread and which thickness will oil slicks have on the sea surface when they develop from oil released at a depth of 844 metres? How will the oil behave in water? Will it reach the surface or will it be dispersed in the water column? The pumping goes on for an hour. An ROV with video cameras is positioned on the sea floor, filming the release. After an hour, the first drops of oil reach the water surface, first the larger droplets with large buoyancy, then smaller ones. Johansen observes that they surfaced faster than he had anticipated. An oil slick develops, spreading out on the water. The small workboats have been launched and are sampling surface oil and water from under the slick. After eight hours, no more oil is surfacing. Overnight, the oil slick disappears, broken up and dispersed by the wave action and dissolved by the sea.

Some surprises
There is a one-day pause due to weather conditions. The expectations are high prior to the last test releasing only natural gas. This test also calls for special attention. No one considers gas leaks to be a direct threat to the environment, but if the gas rises to the surface there is a potential explosion hazard. The scientists want to see what happens with gas at these depths where the temperature and pressure are important factors. Most experts expect the gas to form hydrates, an icy slush that develops when gas reacts with cold water under high pressure. If hydrates form, the gas will not reach the water surface. The mini-submarine is posed, ready to record images and send them to the ship. There is tense excitement in the air as the gas escapes from the pipe so far below the surface. Due to the high pressure, the gas rises slowly, in clear, small, bubbles measuring about 1 cm in diameter. However, no hydrates are observed. - It is something of a mystery, says Johansen. I was convinced that hydrates would form. The test goes on for a long time, under constant surveillance. Since we don't see hydrates, it might be that conditions are not right for triggering the crystallization process. Nor is any gas observed at the surface. Without the crystallization process, the gas dissolves in the seawater.

Mission accomplished
The oil company Chevron, the project's initiator and coordinator, is satisfied with the tests, feeling they have achieved the desired results - a greater understanding of what to expect at such depths. - All the released oil has dissipated naturally, says Cortis Cooper at Chevron. From NOFO, the Norwegian Clean Seas Association, two vessels have been standing by with a complete offshore oil spill recovery system on site for backup. However, the Norwegian Pollution Control Authority has decided that no oil recovery is needed. The results of the tests are being analysed and will be used for validation of numerical models for oil and gas spills in deep water. These models are being developed at SINTEF and Clarkson University in the United States. The models will provide good indicators of what to expect following an accident at such depths, and provide the basis for effective precautions.

* Contact at SINTEF: Řistein Johansen
Tel: + 47 73 59 12 21
E-mail: Oistein.Johansen@chem.sintef.no