Biodegradation on the Seafloor - Science or Speculation?
Authors: Oswald R.J. , Hille M. 
Published: December 2007

This paper summarizes the activities in introducing biodegradable synthetic based mud systems in order to overcome ecological concerns like site contamination and heavy environmental impacts caused by the discharge of diesel/mineral oil contaminated drill cuttings during offshore drilling operations.

Based on an extensive literature study the biodegradation scenario of the sea is derived, with special attention to the elimination potential on the seafloor. Factors affecting biodegradation like the aqueous medium, bacterial community und number, light intensity, sample concentration, etc. are evaluated for one specific synthetic. However, this data show potential to be of general relevance.

Generally, the sea can be divided into three different zones. Two aerobic zones, i.e. photic zone at the surface and the water column followed by an anaerobic zone on the seafloor where definitely most of the biodegradation work has to be accomplished because the residence time of cuttings in the first two zones is rather short. These elimination scenarios are compared to the currently performed testing practice. Most of the today performed OECD tests use technology adopted from waste water purification plants, which obviously represent completely different systems compared to the anaerobic seafloor. Molecules that contain an oxygen-carbon-oxygen bonding within their structure, like acetals and esters, exhibit a distinct advantage over hydrocarbon types like olefins and paraffins, because this specific bonding represents a predetermined breakpoint resulting in an advanced elimination. The pre-determined breakpoint leads to an efficient cleavage of the molecule resulting in much smaller, water soluble and easily degradable material. Under the anaerobic conditions prevailing on the seafloor, where oxidation can not contribute to the overall elimination of organic matter, the incorporation of this pre-determined breakpoint is the key to success in mastering the complex requirements of the drilling industry today, both in performance as well as ecological respects.

Introduction

With the occurrence of the offshore exploration activities the interest of the authorities increased in order to minimise the environmental impacts. The massive introduction of invert-emulsion muds based on diesel resulted in heavy environmental damages triggering the special reason for this essential interest (Fig. 1). The fact that diesel-oil was substituted by less toxic so-called clean-oil (&lt; 0,5% aromatic compounds) could not significantly change or solve the problem. Up to now 15 year old cuttings sludges based on diesel and/or clean oil can be detected on the seafloor. These sludges, based on the molecular structure of their base oil, could not be degraded and therefore spread over the seafloor resulting in close to 8000 km of contaminated seafloor in the North Sea in 1990.

It is common sense that hydrocarbons are generally degradable under aerobic conditions but there is also an unanimous worldwide accepted opinion that hydrocarbons are stable under the anoxic conditions prevailing on the seafloor. This fact, however, represents the basis of the entire petroleum industry, where hydrocarbons, matured, trapped and stored for millions of years under anoxic conditions have been the primary target of all our activities.

The first attempts to drill with naturally occuring, biodegradable oils are stemming back into the 70-ies. But these glycerine-esters were much too viscous and the breakthrough was achieved by the optimization of the esters through cuts of natural fatty acids with a synthetically branched alcohol. P. 517^