by Claudio Paschoa
29. September 2010 08:10
AUVs had only been used for a limited number of tasks dictated by the technology available until newer technologies were developed. With the development of more advanced processing capabilities and high yield power supplies, AUVs are now being used in Brazil for some new tasks, although its primary function is still seismic studies and environment analysis.
The players in the Brazilian O&G market use AUVs in conjunction with Survey ships to make detailed maps of the seafloor before they start building subsea infrastructure; pipelines and subsea completions can be installed in the most cost effective manner with minimum disruption to the environment. The AUV allows survey companies to conduct precise surveys on areas where traditional bathymetric surveys would be less effective or too costly. Also, post-lay pipe surveys are now possible and these will be of vital importance with the growing amount of pipelines and flow lines being installed on the seabed off the Brazilian coast. Just the pre-salt plays being discovered and beginning production will require thousands of miles of pipelines and flow lines, that will need to be constantly monitored.
O&G Researchers are extensively using AUVs to study the ocean and the ocean floor, including for purely scientific reasons such as providing vast environmental data to Biologists aboard rigs. A variety of sensors can be affixed to AUVs to measure the concentration of various elements or compounds, the absorption or reflection of light, and the presence of microscopic life.
Primarily oceanographic tools, AUVs carry sensors to navigate autonomously and map features of the ocean. Typical sensors include compasses, depth sensors, side scan and other sonars, magnetometers, thermistors and conductivity probes.
AUVs can navigate using an underwater acoustic positioning system. When operating within a net of sea floor deployed baseline transponders this is known as LBL navigation. When a surface reference such as a support ship is available, ultra-short baseline (USBL) or short-baseline (SBL) positioning is used to calculate where the subsea vehicle is relative to the known (GPS) position of the surface craft by means of acoustic range and bearing measurements. When it is operating completely autonomously, the AUV will surface and take its own GPS fix. Between position fixes and for precise maneuvering, an inertial navigation system on board the AUV measures the acceleration of the vehicle and Doppler velocity technology is used to measure rate of travel. A pressure sensor measures the vertical position. These observations are filtered to determine a final navigation solution. An emerging alternative is using an inertial navigation system in conjunction with either a GPS receive, or an additional magnetic compass for Dead Reckoning whenever the GPS signal is lost.
Most AUVs in use today in Brazil and around the world are powered by rechargeable batteries (lithium ion, lithium polymer, nickel metal hydride etc), and are implemented with some form of Battery Management System. Advances in computer systems, development of underwater navigation systems, acoustic modems and cameras made it possible to build vehicles which could be controlled precisely enough to execute an intervention mission requiring precise positioning & control and a level of reasoning about the environment. AUV task may soon include manipulating valves on a subsea tree or manifold. They also have the capability to be launched in storm conditions that negate the use of ROVs for intervention tasks. AUVs are being extensively used to map various possible pre-salt and post-salt basins all along the Brazilian coast and are proving to be very efficient in their tasking and increasingly more user friendly, and flexible in term of operation parameters.
Claudio Paschoa
photo courtesy of Kongsberg
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