“The dynamics of the oceanography in this region can be challenging,” explains Rosemary Smith, Senior Oceanographer. “The North Brazil Current (NBC) and its subsurface component, the North Brazil Undercurrent (NBUC), transport oceanic waters north westwards along the northern coast of Brazil . Once north of the equator these currents feed a system of zonal counter currents which flow eastwards. Large, anti cyclonic eddies (NBC rings) can form from the North Brazil Current and propagate northwards. They are thought to transport up to half of all the surface waters north westwards. The NBC rings are of concern to drilling operations, as they are known to cause strong currents that can lead to down time during drilling operations.
Devon identified a need to monitor these currents over a period of 12 months and the data provided by the two companies, Fugro Brasil Ltda (Rio de Janeiro and Macae, Brazil) and Fugro GEOS Inc (Houston, USA will primarily support operational planning for an exploration drilling program.
Current measurement configurations will also provide data to support the derivation of design criteria should the oil and gas prospect develop further. Knowledge and understanding of currents, the underlying water masses, and the interaction of wind and wave are all vital for the design and safe operation of production systems and key offshore oil and gas development components.
“To monitor these occurrences, a deep water mooring with multi oceanographic instrumentation has been deployed in 2,525 meters of water. To date, the mooring is one of our longest with service visits occurring once every four months,” adds Paul Williams, Operations Manager. “Our Brazilian and Houston-based companies, Fugro Brasil Ltda and Fugro GEOS Inc, provided the best possible vessel service options to Devon. While vessel requirements for oceanographic mooring deployment, servicing and recovery are of relatively short duration, vessel availability has been in short supply. Thanks to the efforts of Fugro Brasil Ltda a major issue of transportation was effectively eliminated.”
With the approval of Devon, Fugro GEOS Inc ran the ultra long mooring requirements through design software using a current profile based on existing data near the north of the BM-BAR-3 location. The analysis identified potential problems with mooring ‘knockdown’ due to the impact of the currents on the mooring string.
Moorings typically consist of instruments suspended on a taut wire, anchored to the seabed by a clump weight. Knockdown can affect the quality and integrity of the measured data if it is not minimized. In environments like the north of Brazil, it is impossible to design a mooring that will suffer no knockdown, but it is possible to minimize the effect.
To mitigate the chances of knockdown, the use of plasma rope was selected to support the mooring, rather than the more commonly used wire rope. Plasma rope is buoyant unlike wire rope and requires far less inline floatation thereby reducing the knockdown effect. It is also stronger and does not suffer corrosion.
An additional component to the study program requested by Devon Energy do Brasil was a vessel mounted ADCP (VMADCP) currents survey and CTD profiles over the study area. This survey, conducted following the mooring deployment, involved running a number of survey lines using an ADCP to map the current features in real-time. These data provide spatial detail of the current features in the upper layer of the water column. A deepwater rated CTD is used to collect full water column conductivity (salinity) and temperature profiles at key locations. These data provide valuable information on the water column structure and the various water masses that are causing the observed circulation patterns.
The rapid accomplishment of the first phase of the project has been significant. After the seamless mooring deployment the survey report was compiled within just five days. The graph below shows some of the observations.
“Fugro Brasil Ltda and Fugro GEOS Inc, understand the processes and circulation features involved at the location,” explains Paul Williams. “Jointly we continue to advise Devon scientifically of what to expect. Knowledge of observed situations from the ocean environment is vital to operations but so is teamwork between the two Fugro companies, to ensure total client satisfaction. We look forward to more successful collaboration with Devon Energy do Brasil in the future.”
Further information on Fugro Brasil Ltda is available www.fugro-br.com and on Fugro GEOS Inc fromwww.geos.com.