Integrated marine monitoring for drilling discharges under mesotidal forcing

Abstract

There is a range of hydrocarbon exploration and extraction industrial project activities taking place on the North West Shelf of the Australian Continent. The shelf is a diverse and important marine ecosystem, with a number of globally significant shallow coral reefs and marine protected areas, which are vital for survival of threatened and protected species. Some of the mentioned exploration and extraction activities, such as offshore drilling, may be permitted within few kilometres away from sensitive habitats extant near a project resource extraction area. The habitats may in some cases be presented by submerged shoal ecosystems of high biodiversity and thus high environmental/ecological value, due to the presence of either or both benthic biota and fish assemblages. As part of an environmental assessment program for the industrial activities, some of the offshore exploration operators examine the effects of drilling discharges on the marine ambience. This study describes such an assessment. For the assessment, a novel integrated marine monitoring approach was conceptually designed; the program was then executed in the field using bottom-mounted equipment and a remotely operated underwater vehicle with sensors mounted on it. This study focuses on assessing the wind conditions in the project area, sea level and current velocities, in situ water temperature, salinity, turbidity and acoustic echo intensity, over several stages of the drilling operations while the drilling cuttings and muds were discharged into the water column. The collected during the austral autumn months of April and May winds showed prevailing east-southeaserlies, which is aligned well with the general understanding of wind climatology of the area. Importantly, stronger than usual winds may have a defining impact on current speeds and directions, with the current speeds enhanced by such winds. On these occasions the water transporting a discharge material may pushed towards the sensitive habitat present in the area. Normally though the hydrodynamics in the project area are dominated by semi-diurnal tides. Both vertical current velocity and water temperature distributions indicated the presence of three layers within the water column over the project site. The vertical distribution of water salinity was rather homogenous throughout the entire water column. The monitored by an optical instrument turbidity and acoustic echo intensity from an ADCP (acoustic Doppler current profiler) showed that tracing discharge plumes with a remotely operated underwater vehicle is a viable technique, which could be implemented at other locations. Such independent parameters though need a thorough cross-calibration and inter-validation for the results to be fully understood.

Keywords

• drilling, cuttings, muds, turbidity, plume, remotely operated underwater vehicle, continental shelf, water column layering, acoustic doppler current profiler, echo intensity