Ocean Currents Offshore Northern Angola

Abstract

Abstract Large oil fields have recently been discovered in the deepwaters near the Congo River off northern Angola and Cabinda. Major facilities will soon follow. The metocean environment in this region is generally benign though little is known about ocean currents. Until 1990 there had only been a few synoptic measurements of currents but this is changing. Since 1990, Chevron has collected one year of moored current data, two years of wind data, three months of near-surface drifting buoy data, and two years of ADCP profiles from a drilling rig. All the measurements have been taken in roughly a 90-km square around the Congo Canyon, a deep east-west trench originating at the mouth of the Congo River. The paper shows a number of interesting processes that have important potential impact on deepwater drilling and facilities design and operation. These include 100 cm/s shelf waves, 150 cm/s river plume, 150 cm/s near-bottom jets, and turbidity currents. Some of the processes are no doubt observed in other low latitude sites, especially near large rivers. Introduction In the past two years, exploration in the deepwater off Northern Angola has discovered several billion barrels of oil in water depths greater then 400 m (Figure 1). Chevron is now designing major new facilities to produce these deepwater reserves. Metocean conditions are a key element in safely delineating and economically producing these frontier fields, as well as responding to oil spills. In general, the wind and wave environment in the region is benign and well understood. Cardone1 et. al report on an extensive hindcast of winds and waves in the region and indicate 100-yr design maximum wave heights of 6–7 m, roughly a factor of five less then the North Sea or Gulf of Mexico. These waves reach the region as swell, having been generated by stronger winds hundreds of km to the south. Local winds are dominated by the SE Trades with a mean speed of 4 m/s although these are occasionally interrupted by brief squalls. A strong land-sea breeze is evident within about 30 km of the coast. Astronomical tides in the region are small with a mean elevation range of 1.2 m generating mild currents of less then 10 cm/s on the slope and outer shelf. Much less is known about non-tidal currents in this region. Published work is limited to studies by Berrit2, Meulenbergh3, Wauthy4, and Eisma and Van Bennekom5. These studies involved synoptic CTD sections and limited synoptic current profiles within a few km of the Congo River mouth. Heezen6 et al. studied turbidity currents in the Congo Canyon and convincingly argued that they are responsible for the long deep trench at the mouth of the river (Figure 2). The limited historical measurements show a stronginfluence from the Congo River, not surprising given that it has an average discharge of 40 k m3/s making it second only to the Amazon. Larger-scale oceanographic studies indicate that there are no strong permanent currents - the Benguela Current breaks away from the southern African coast well south of northern Angola.