A Survey of Offshore Oilfield Drilling Wastes and Disposal Techniques to Reduce the Ecological Impact of Sea Dumping

by Jonathan Wills, M.A., Ph.D., M.Inst.Pet., for Ekologicheskaya Vahkta Sakhalina (Sakhalin Environment Watch); 25th May 2000

Minimising Waste Discharges and Their Effects (continued)

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Re-injection Offshore (continued)

The Norwegian agency responsible for vetting and surveying offshore installations, Det Norske Veritas (DNV), published a report (Det Norske Veritas. 2000. Technical Report - Drill Cuttings Joint Industry Project. Phase I Summary Report. Revision 2: 20th January 2000. DNV doc. order No. 29003500. Oslo) in January 2000 on the best ways to deal with the accumulated piles of oil-contaminated drill cuttings on the seabed around most North Sea oil installations. The research was part of the Joint Industry Project set up by the OSPAR countries in 1998 and, while it deals with the problem of accumulations of cuttings and OBM wastes over more than 30 years, its conclusions are very relevant to the question of drill cuttings and waste disposal at new oilfields such as the Sakhalin Shelf, if long-term problems with drill cuttings piles are to be avoided.

DNV noted "re-injection of cuttings is currently the only established technique for offshore disposal" and, like Patin, complained of "a lack of information on the leaching of toxic contaminants and the toxic effects of the cuttings material as a whole and its components other than hydrocarbons". There was also "still a lack of understanding of natural degradation processes and rates".

A recent study for UKOOA (AMBIOS Environmental Consultants Ltd. 1999. Impact of Offshore Disposal Solutions. United Kingdom Offshore Operators' Association, London) agreed:

Currently, the only acceptable offshore technique for dealing with oily drill cuttings is re-injection into a suitable formation. However, there are a number of emerging technologies as well as established onshore techniques for cleaning cuttings with potential for offshore application. Onshore solids treatment techniques considered were grinding; direct thermal desorption; and indirect thermal desorption. Emerging solids treatment techniques considered were microemulsion; supercritical extraction using liquid natural gas; and supercritical extraction using liquid carbon dioxide.

Seven options were compared using sixteen criteria to address engineering feasibility, performance, cost and environmental issues.

Being the established technique for dealing with cuttings, re-injection scored reasonably against most criteria. However, where there is no suitable formation for re-injection, other options need to be considered.

Transport of solids to shore for treatment compared favourably especially in terms of solid and water treatment rates and the potential for re-use of recovered oil and cleaned cuttings...

Of the options for offshore solids treatment followed by discharge to sea, grinding and indirect thermal desorption systems scored well, both being able to clean cuttings to less than 0.5% residual oil content and having potential for good energy efficiency especially when the energy value of recovered oil is taken into account. However, both these techniques have high deckspace and deck weight requirements and are probably best suited to vessel rather than topside installation. In addition, the operating and capital costs are currently high in comparison to the other options but there is potential for reducing these through greater system integration and further discussions with suppliers.

The Norwegian state oil company, Statoil, describes shipping cuttings ashore for treatment as "a costly solution losing support from environmentally conscious countries like Norway, the UK, Canada and New Zealand". A technique developed for Statoil offers "an effective solution that is both definitive and economical".