Drilling activity discharges exceeding 1% of oil content have been outlawed in the UK North Sea since 1998. The government's target is zero discharge by the end of 2000, making seabed disposal of drill cuttings impossible.
Cuttings can be reinjected, but the technology has not been long applied, so there is uncertainty over the impact on the reservoir and the potential long-term costs. Another option is to set up a plant on the drilling rig to remove oil from cuttings prior to disposal. The problem here is the capital cost of the plant, plus associated deck space constraints.
A third option is to transfer the cuttings to shore for disposal, using supply boats returning to base. If logistical problems concerning vessel movements can be resolved, this may prove the best solution in financial and environmental terms, according to Andrew Yule, Technical Manager of Scotoil Group.
In Aberdeen, Scotoil has just invested £ 450,000 in the city's first oily cuttings recycling facility. The company, also a specialist in de-scaling, ore grinding, and chemicals, was formed in 1995 following a buy-out from previous owner ICI.
The recycling process was developed for use offshore early this decade by RigTech. Progress was halted in 1992, due partly to the sheer size of the plant envisaged for the technique, and also due to technical constraints concerning cuttings size reduction.
Quayside plant
"However, process philosophies have moved forward since," says Yule.
Scotoil's 24 acre site is located 200 meters from the quayside at Aberdeen's harbor. There is also potential to develop a bulk-handling system to take oily cuttings direct from the supply vessel to the recycling facility. The plant comprises a mixer unit, evaporator, separator and recovery unit and is due to be in operation this month. However, it is unlikely to operate at full steam for some time until North Sea drilling recovers.
At peak, the plant should be able to process 7-10 tons/hour, equivalent to 45,000 tons/year. Startup and shutdown can be effected within eight minutes. Competitive processes take longer, Yule claims. Among the alternative cuttings treatment techniques are thermal processing, which demands intensive use of energy to inject the required heat. Also, the high temperatures can cause damage both to the rock and the oil, Yule claims. "Our technique only involves putting in heat to recover the solvent, so energy requirements are far lower."
The plant at Aberdeen is being configured to take into account evolving discharge legislation over the next 10-15 years. Solvents for cleaning cuttings themselves generate emissions to the atmosphere. "We've decided to get as close to zero emissions from our process as possible," says Yule. Initially, residual oil content will be 0.2-0.3%. The solvent chosen may be less fast-acting than others, but it is more environmentally acceptable.
Solvent extraction
Scotoil has developed the solvent extraction process over the past two years, part funded by the Scottish Office. In practice, once the cuttings have been delivered, they will be mixed with the solvent (hexane) to remove the oil. Cuttings and fluid are then separated in a centrifuge, after which the cuttings travel along a heated conveyor to be dried. Any residual oil is captured for recycling. Liquid is then distilled and bulk recovered oil sent for recycling, while the solvent is also reused to process future batches of cuttings.
Cuttings could be shipped across in a skip, as part of an operator's general waste management system. Alternatively, cuttings from various rigs could be transported in bulk, perhaps using a standby vessel. Should operators opt for this approach, Scotoil would employ a bigger storage tank on site, with capacity of 8,000 cu meters.
Under its contractual terms, Scotoil will also take ownership of the offloaded materials. It aims to offer the recycled cuttings to the construction industry, for less than half the rate of £5/ton normally charged for construction aggregate in the UK. Cuttings from other processes normally end up in landfill sites, Yule claims, but space for such sites is rapidly running out. Scotoil is also examining other potential re-use markets.
Diesel-driven process
Another emerging force in this field is an alliance between Burgess & Garrick Oil Services and Global Recycling Systems. They have invested £1.5 million in two new drill cuttings recycling plants in Aberdeen and to the north in Peterhead. At peak, both sites will be able to process up to 30,000 tons of cuttings annually.
According to Burgess' Business Development Manager Brian Norris, the process involves use of diesel electric power instead of applied heat, making it less energy-intensive. "We use a hammermill - oil and water is driven off the cuttings by friction control hammers inside the mill or process chamber. Oil can be re-used as low grade fuel, while the water is clean enough to be discharged to the sea."
The residual dry powder contains inert hydrocarbons of typically 0.07-0.10% at the last independently performed test", says Norris, "which is lower than anybody else's." Above 1% hydrocarbon content, cuttings waste has to be disposed of, usually in landfall sites. "Ours, in contrast, can be used for asphalt or filler for other end-products," Norris points out.
The alliance is also working on a transportable version of its cuttings plant that could operate offshore anywhere in the world. This would save on costs of regular rig to shore shipments, and would also allow the contractor or operator to re-use the recycled fuel oil for their own purposes.
