On the short-term and long-term impact of drilling activities in the Dutch sector of the North Sea

Abstract

A research programme on the effects of drill-cutting discharges on the benthic system around platforms on the Dutch continental shelf has been running since 1985, with emphasis on locations where oil-based drilling muds (OBM) have been used and discharged. A few locations where only water-based muds (WBM) have been used were also investigated. At OBM locations, elevated total hydrocarbon concentrations in the sediment occurred up to 750–1000 m from well sites during the first year after discharges had stopped. Concentrations tended to decrease to natural background levels during the following years at distances ≥500 m from the platform, but remained far higher than background levels even after 8 yr within a few hundred metres. One year after drilling, biological effects of OBM discharges were detectable up to ≥1000 m by reduced abundances of a few very sensitive species (particularly Echinocardium cordatum and Montacuta ferruginosa). Closer to well sites, increasing numbers of species experienced adverse effects. In the longer term, the macrofauna seemed to recover at distances ≥500 m, but within that range the macrofauna was still affected after 8 yr. Possible effects of WBM discharges were investigated at a few single-well sites during surveys carried out within 2 mo to 1 yr after discharges were terminated. Adverse effects on the benthic community were not observed, even within 25 m of a discharge site.