Dual-role vessel concept marks potential breakthrough in oilspill recovery
- External arrangement principle of the oil spill recovery unit.
- Artisit's impression of the OSRV in action, with booms fully extended.
A novel solution has been put forward for rescuing oil lost from tankers in heavy seas: another tanker. This joint Norwegian/British concept would not involve a large-scale conversion job, nor would the chosen vessel need to be dedicated to oil recovery. But it could be operational in its extra role within a couple of years.
The tanker approach, known as the Oil Spill Recovery Unit (OSRU), was conceived by engineer Thor Haavie, who also invented the DP diving vessels Uncle John and Stadive during the 1970s. He points out that although supply vessels deploying booms, skimmers and dispersants are fine in calm to moderate conditions, they are no match for storm-force waves, such as those which wrecked the Braer tanker off the Shetland Islands two years ago.
Wind and wave direction at the time lessened the immediate consequences of that spill (although the impact on the marine food-chain is less clear). But the odds on a repeat incident in this area of huge swells will also lessen as shuttle tanker traffic starts up from the West of Shetland fields.
A recent study for the islands' council concluded that any slicks would end up on the west coast of Shetland, because of the prevailing currents and waves. Foinavon Field operator BP apparently admits to having no contingency plans for such a situation.
Offloading to tankers in these arduous conditions could also be risky, with transfer hoses buoyed at mid-depth. In the North Sea there are at least ten other facilities that export via shuttle tanker, with another five planned to come onstream shortly: Fife, Harding, Captain, Heidrun and F3-FB-1.
Another potential source of spills is aging pipelines. Norway's Environment Department predicts that in the Norwegian Sector, one pipe will rupture every ten years, and one leakage involving more than 1,000 tonnes will occur every year.
The OSRU is currently a collaborative venture between Haavie's company Submarine Engineering, JP Kenny and Britain's National Engineering Laboratories. The aim is a system capable of working in Force 8 conditions - 36-37 knots of sustained wind and occasional waves up to nine metres high - with negligible roll movements in the vessel. The design and back-up arrangements would allow it to at least match the most successful oil recovery operation to date, from the American Trader, where 25% of the oil spilt was collected.
First, however, an in-use tanker must be found which could double for recovery duties; funding would then be required for the engineering phase. The project partners recently presented their case to a group of oil companies in London.
The oil spill recovery arrangement would be implemented in the forward structure of an in-use shuttle tanker, in such a way that the cargo-carrying capacity would only reduce by around 5%. Should an incident arise, the tanker would simply be disconnected from its host facility, or unload at the nearest port, before steaming to the scene of the spill.
In contrast to other approaches, the OSRU would not skim the spilt oil. Instead, large volumes of oily water would be gathered by suspended booms and taken onboard through two weir doors (two metres wide by 1.5m high) in the hull into a large separation tank. This oily fluid would be collected via a paddle shaft, then pumped to other tanks on the vessel for secondary settlement by normal gravitational methods.
The water would then be drained from the tank bottom's outer periphery and pumped overboard, with an oil content of less than 0.5%. That may not attain MARPOL purity limits, but the designers claim it would still be a lot cleaner than using dispersants, which are known to cause toxicity when combined with the hydrocarbons they were sent to disperse.
When the OSRU is not in action, the separation tank could be used as a cargo tank; the function of the wing tanks would not be impaired. Only lost cargo volume would be in the two pump rooms.
When the vessel approaches a spill, it would steam at a minimal speed (around 1-1.5 knots) along the middle of a windrow, aiming for the thickest part. The OSRU would ideally be stern to the sea, thereby minimizing any roll and improving the operation of the booms as the wind would help prevent any oil slopping over and being lost. For an 80,000 dwt vessel, pitch should not be excessive, the partners claim, in sea states where oil recovery is possible.
In heavy sea states, oil migrates down from the surface, forming emulsions. Depending also on the sea temperature and wind, the emulsions formed can increase the oil volume fourfold. The OSRU booms, suspended from cantilever arms either side of the tanker, would be designed with enough depth to capture these emulsions (for the Exxon Valdez clean-up, 1.5 metre deep booms were found to perform best).
Chemical changes to leaked oil are slower in cold waters. Assuming that after 12 hours a slick had extended 2,000 metres downwind by 600 metres across the wind, the OSRU, sweeping downwind at 1.5 knots, could undertake one run in 45 minutes. Subsequent circling and re-entering the slick upwind might then take 25 minutes.
Six runs lasting six and a half hours would sweep 70% of the total area, allowing the vessel to collect around 3,000 tonnes of oil. A daily rate could yield 12,000 tonnes, equivalent to a quarter of the Exxon Valdez cargo.
Other advantages of the OSRU include manoeuvrability, provided by the bow and stern thrusters for offloading operations; and familiarity of the crew with the vessel's operating characteristics and also with offshore laws and responsibilities. A helicopter operating from the tanker could also provide infrared scanned data on oil thickness.
One OSRU-tanker operating in the Beryl/Gryphon/Harding Fields area could cover most of the North Sea in 12 hours, and a couple more could cover northern Norway, the eastern Atlantic and the Irish Sea. But the system would also be applicable to the waters of Alaska, the Great Lakes, the Baltic or the Molucca Strait.
The project team envisages operators contributing to a single club that funds all the tankers, perhaps with government support. Haavie claims the OSRU could work at 80% of the time during the three worst winter months, as opposed to 40% currently for smaller state of the art systems.
Cost of converting an in-use tanker is put at £2.5-3.5 million. A 28-metre test tank must also be built at NEL, as trials in the rough out at sea are not feasible. Assuming finance can be secured this spring, the plan is to design the centrifugal, oil catchment, and separation systems this year followed by detailed design of the vessel in 1996.
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