Offshore industries

In the commercial sector, acoustic sensing methods have found numerous applications including acoustic Doppler current profilers (ADCPs) for measuring currents, compact sonars for obstacle location and avoidance by AUVs (e.g. Brutzman et al., 1992), fish-finding devices, underwater communications systems for divers, fathometers for bathymetric sounding and navigation and side-scanning sonars for topographic mapping of the sea-floor relief. Some point-source and non-point-source pollution studies now use acoustic backscatter measurements to monitor the marine environment.

Offshore industries, particularly oil and gas, have undergone profound changes over the past decade in response to global economic factors. Specifically, the contribution of offshore oil production to the total non-OPEC supply increased from about 25 percent in 1990 to about 30 percent in 1995. Approximately 80 percent of the significant growth in non-OPEC supply up to 2000 was offshore. An appreciable contribution to the growth in offshore production was made by new technologies such as 3D seismic evaluations, horizontal drilling, sub-sea completions, multi-phase pipelines and floating production, storage and off-loading (FPSO) vessels (International Energy Agency, 1996).

As the exploitation of offshore oil reserves has increased, exploration and production (E&P) operations have expanded into deeper waters. For example, recent Angolan oil-exploration concession areas comprised three water-depth bands: shallow blocks (<500m), deep blocks (500-1,500m) and ultra-deep blocks (1,500-2,500 m). In comparison, significant sub-sea oil production in the Gulf of Mexico has typically occurred in water depths approaching 1,200 m. At such depths, it is not possible to build fixed oil rigs. Instead, floating platforms are anchored to the seabed. Since the equipment needed to operate each well is too heavy to install on the floating platforms, the equipment is placed on the sea floor where it is maintained by remotely operated vehicles (ROVs) deployed from the floating platforms.

Because these formerly topside systems were designed for direct (human) intervention rather than for remote intervention, the tasks necessary to install and maintain these systems are difficult (if not impossible) to perform with traditional ROV-based tools and techniques. Automation of remote-intervention tasks can make use of commercial-off-the-shelf (COTS) technologies such as acoustic positioning, acoustic imaging and enhanced user interfaces integrated into a single system (Schilling, 1998). These technologies can also be used for inspection, maintenance and repair (IMR) operations.

Pipeline routes are planned to be as short as possible to reduce costs. Moreover, bottom slopes that could put stress on unsupported pipes are avoided, seabed sediments are mapped to identify unstable areas and pipe-burial options are assessed. Surveys of potential pipeline routes commonly utilize data derived from sidescan sonars.

Met/Ocean data collection efforts in support of offshore operations often employ ADCP sensors to measure surface and sub-surface ocean currents. These data are required to determine the vertical and horizontal current shears that can impact the siting and placement of offshore structures.

Employing volumetric (acoustic) plume detection to identify hydrocarbon seepages of natural or man-made origins can fulfill environmental monitoring mandates. Oil-spill tracking, prediction and containment operations, as well as disposal monitoring, can also employ volumetric acoustic methods.

Offshore work in marginal ice zones (MIZ) requires knowledge of ice thickness and under-ice features (especially keels). Information on under-ice features can be obtained from AUVs or ROVs equipped with upward-looking (acoustic) echo sounders. This information can complement independent surface (altimeter) measurements of ice ridges to obtain estimates of total ice thickness.

Acoustic systems are used widely in the offshore industry for ROV tracking, seismic-towfish tracking and drilling operations. These systems must perform in noisy, shallow-water environments. Acoustic transponders function as navigational beacons and as remote-control release mechanisms in the deployment and recovery of instrumentation packages. Moreover, sub-sea drilling-rig supply operations employ acoustic beacons for navigation and docking evolutions. Similarly, divers often rely on portable acoustic devices for communication and navigation.

Noise-control design of planned facilities and noise-control retrofit of existing plants entail environmental noise monitoring, modeling and simulation, and development of noise-control procurement specifications.

Offshore industries can benefit most from recent advances in modeling and simulation by integrating such technologies directly into ROV/AUV control software in order to improve responsiveness to changing environmental conditions. Furthermore, increasing the use of M&S in system design and operator-training functions may derive additional technical and economic benefits.

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