An Acoustic Position Reference System Intergrating Long And Short - Baseline Operation
M.L. Neudoefer · Offshore Technology Conference · 1979
ABSTRACT This paper describes a new acoustic position indicator system that integrates ultra-short and long-baseline modes. Major design features are described, and results of field experience with long- and short-baseline modes are presented. The system has been designed to perform the full range of acoustic positioning tasks required in offshore operations. In its simplest form, positioning is done with free-running acoustic beacons. The complete system performs short baseline positioning with both pingers and transponders. It performs long-baseline positioning of both surface vessels and submersibles. A special feature of the complete system is the use of the short-baseline system to calibrate a long-baseline transponder grid. This flexibility makes it possible for one system to perform the full range of acoustic positioning tasks in offshore oil-field development. Applications of the system to geophysical surveys, exploratory drilling, platform construction, and pipeline installation are discussed. Special features that make the system adaptable to a wide variety of offshore applications are highlighted. A unique feature of the system is a conversational display console and keyboard. By changing labels on the variable-function keys and providing prompting messages, the system guides the operator through the steps necessary during system setup and operation. Field experience has shown that this technique reduces both operator training time and errors. Results of field experience in the calibration of long-baseline transponder arrays are presented for two water depths. The use of the short-baseline system to aid the long-baseline operation is also presented. INTRODUCTION Acoustic position reference systems are widely used in offshore operations to measure and display the position of surface and subsea vessels with respect to underwater acoustic beacons. Two basic types of positioning systems are used short-baseline and long-baseline. Long-baseline techniques determine the position of a vessel in relation to a fixed grid of transponders mounted on the sea floor. This approach provides high accuracy for operations carried out over broad areas or at great depths; however, careful calibration of the transponder grid is required to determine the transponder's precise positions with respect to one another. Short-baseline techniques, in contrast, require only a single beacon (either a free-running pinger or a transponder) and a single ultra short-baseline hydrophone or hydrophone/projector. This approach is convenient, since it does not involve grid calibration. In addition, it provides high accuracy for close-range work. Accuracy decreases, however, over broader areas and greater depths. Each of the two approaches is useful for specific types of offshore operations: longbaseline methods for far-reaching operations such as geodetic surveys or site inspections, and short-baseline methods for applications such as underwater construction, drilling, and pipe laying. Both have been used by the offshore industry for years-in separate systems. This paper presents a system that is field upgradable in three configurations:Short-baseline pinger operationShort-baseline pinger and transponder operationLong-baseline and short-baseline operation combined in one system The system, by combining long and short baseline capabilities, handles the complete spectrum of offshore applications, from the initial geophysical surveys through platform installation and pipe-line maintenance.