New Oceanographic Challenges in Brazilian Deepwater Oil Fields

Abstract

Abstract As new oil fields are discovered downslope on the largest Brazilian oil region, Campos Basin, situated on the shelf break and continental slope of eastern South America, new oceanographic challenges are continuously encountered by oceanographers and engineers involved in the design, installation and production of oil field ranging from water depths between 500 to 2000 m. The surface flow in the region is dominated by the Brazil Current, the counterpart of the Gulf Stream as the western boundary current of the South Atlantic gyre. The deep water flow is very complex with different layers of counter currents associated with the rich structure of water masses on this region. This paper discusses the attempts made by PETROBRAS to study the complex flow in the region and produce oil under the harsh environment of deep continental slope regions. Results of oceanographic measurements are presented to build a reliable metocean design basis for this region of the South Atlantic Ocean where there is a lack of long term data measurements. Introduction PETROBRAS has been involved in the exploitation of oilfrom continental shelves since the late 1960's. Initially, it was the production of oil fields in the northeast Brazilian shelf in the Sergipe State (about 12° S) when Guaricema oil field started its production in 1969. Later on, the production was increased with exploitation of the continental shelf at Rio Grande do Norte State (5° S) in 1976 (Fig. 1). The oil fields were located in the inner shelf on water depths ranging from 10 to 30 m. The production systems were based on jacket type platforms designed to withstand the local mild environmental condition obtained from ship weather reports and global databases. The economic exploration of oil and gas in Campos Basin, located offshore the Rio de Janeiro State (about 22° S) started in the mid 1970's with the discovery of the Garoupa field. During the first phase, only oil fields situated in the outer continental shelf on water depths ranging from 90 to 170 m were exploited. Due to the lack of local metocean measurements, jacket platforms were first designed based upon environmental conditions slightly modified from conditions of northern hemisphere production sites, such as Gulf of Mexico. Additional reports from environmental consultant companies included wave and wind information from South Atlantic ship weather reports and Navy databases to improve design conditions. The main environmental forces on the southeast Brazilian continental shelf are due to the tidal and wind driven current components, and to the local wave and wind fields. Simple analytical models were used to estimate current profiles and extreme environmental conditions. The oceanographic turn point for the company occurred in the middle 1980's when the company started to operate floating production systems (FPS) on the upper continental slope (Ref. 1). It was clear that the current measurements from the rigs were completely different from the design current profiles previously used in the continental shelf. As observed in Figure 2, the jackets on the continental shelf were situated entirely on the coastal water mass, where the physical properties are homogeneous along any depth profile. On the other hand, the floating production systems on the continental slope were situated on a highly stratified water column with different water masses.