RIASSUNTO
ABSTRACT
Many offshore operations, such as the dynamic positioning of a drilling vessel, require accurate data of the vessel location with respect to a referential point (drill hole). The Acoustic Measuring System (AMS) developed for this purpose by CIT-ALCATEL has been specially designed to ensure a highly reliable functioning in severe environmental conditions : drilling noise, propellers noise and disturbances, signal reflections, etc ..
This article describes the system and gives theorical accuracies and practical results obtained during drilling operations.
The equipment consists-of a redundant assembly of interrogation transducers and receiving hydrophones on the vessel, and transponders near the reference point(s). Accurate acoustic travel times are thus possible on two-way propagation paths, which makes it possible to completely solve the geometry in all cases and, above all, enables the use of a sophisticated monitoring procedure by a digital computer. Erroneous signals resulting from reflections are eliminated, as well as most After high level noise transients. In addition, the system uses coded signals and adapted signal processing that increases its noise rejection capabilities.
The equipment has been successfully employed in the dynamic anchoring of the ship ""LE PELICAN"" during drilling operations in the North and Labrador Seas.
INTRODUCTION
There are many instances in modern operations at sea where accurate and reliable data are required of the relative position between a reference point underwater and the sea surface vessel. This is the case in particular for the localization of a remote controlled fish (in pipe-line surveillance, submarine rescue, etc...) and for dynamic positioning purposes.
The use of acoustic devices for obtaining the desired coordinates is one of the most frequent. There are, however, a number of difficulties, especially in the case of dynamic positioning , because severe conditions of environment are encountered : drilling noises are strong, irregular, with high peak transients; propellers or thrusters generate somewhat more steady but still very loud noises. up to high frequencies, and moreover they are sources of water agitation with air bubbles that interfere with the propagation of acoustic signals; the surrounding waters are congested with all sorts of acoustic obstacles (well head structure, riser, diving bell) and can be disturbed by mud projections, etc... )