Metocean Measurements at Northern Santos Basin - Brazil

Abstract

Abstract Santos Basin is located on the western border of the South Atlantic Ocean, between latitudes of 23°S and 28°S along the Brazilian shelf. It will probably be one of the new deepwater frontiers of worldwide offshore oil exploration and production. An extensive two-year data acquisition program was conducted to measure meteorological and oceanographic conditions in the northern region of the basin near the 1500 m bathymetric contour. The program comprised a metocean wave buoy to measure directional waves, surface conditions and local meteorological parameters, a deepwater mooring line to obtain complete water column current profiles, and CTD/ADCP hydrographic transects to characterize the thermohaline and velocity structure of the region. The wave conditions are mainly described by a bimodal spectrum with a high-energy peak due to local easterly winds associated with fair weather conditions, and a low frequency peak associated with southern approaching swell due to extratropical cyclone activity. During the most severe storms, the wave spectrum is unimodal with its peak associated with local enhanced southwesterly storms due to propagation of intense cold fronts arriving at Santos Basin. The current regime clearly shows the Brazil Current flow in the upper 400 m of the water column. Below it, there is a northward counter-current flow associated with the Antarctic Intermediate Water (the Intermediate Western Boundary Current). The overall flow regime is complex, affected by eddies and meanders of the Brazil Current. Introduction During the last few years, the offshore oil and gas activity in Brazil has moved offshore past the continental shelf edge and down the continental slope towards increasingly deeper waters. In deep water, marine operations become more complicated and the spatial and temporal variability of the currents become more important in design and planning. In 2006-2008, atmospheric and oceanographic data were collected in the region of the BS-4 block at Santos Basin during a field program conducted by Fugro Global Environmental & Ocean Sciences (GEOS), in collaboration with Fugro Marsat/Geosolutions Brazil & Oceansatpeg JV, for Shell Brazil and partners Petrobras and ChevronTexaco. The site is placed in waters deeper than 1500 m, and the purpose of the measurement program was to characterize the behavior and variability of the Brazil Current and underlying current structures (including its meanders and eddies), as well as the concurrent surface processes related to wind and surface gravity waves. The meso- and large-scale oceanography at BS-4 block is dominated by the Brazil Current (BC) system, which is composed of the BC itself, the underlying Intermediate Western Boundary Current (IWBC) and the meanders and eddies that result from the coupled dynamics between these two currents. The Santos Basin is an important recurrent site for such meandering that may result in current reversal and intensification in the area. Both BC and IWBC form the western boundaries of large-scale ocean gyres that occur at different depths. The BC is a shallow, warm, and salty southward flowing Western Boundary Current as it passes adjacent to the Brazilian coastline between 20°S and 28°S. It transports two water masses: the Tropical Water at surface levels and South Atlantic Central Water (SACW) at pycnocline levels (Silveira et al., 2004). Most volume transport estimates for the BC are inferred from temperature-salinity profiles, varying from 5 Sv to 13 Sv within this latitude range. The reference level choice considered by several authors ranged mostly from 500 to 750 m. Those depths correspond essentially to the interface between the SACW and the Antarctic Intermediate Water (AAIW). Below 500 m, there is a direction reversion associated with the IWBC. It flows towards the north-northeast, transporting AAIW and Upper Circumpolar Water (Böebel et al., 1999). The BC-IWBC system develops intense mesoscale activity off SE Brazil.