RIASSUNTO
ABSTRACT
It is now essential to survey the sea-bed in great detail, to gather precise information at an exact location in order to minimize damage to large structures when working in great water depths. The only sure way to achieve this at present is with a manned submersible, but the next major advance must involve the unmanned vehicle. This would use the same techniques but with distinct advantages of working in severe weather, for long periods and with no risk to human life. This area of study has particular significance in the North Sea.
INTRODUCTION
The rapid expansion of the Offshore Industry has made great demands upon many associated activities. Not least among these is the field of Hydrographic Surveying, and here, the Industry has demanded accuracies hitherto unattainable.
In the North Sea, the majority of the oil strikes have been in the northern part and the nearest coast lines, those of the Orkneys and Shetlands, have very rocky sea-beds completely unsuitable for pipelines, without major modifications by way of blasting or rock dumping, Figure l. The selection and survey of the best route, the preparation of that route and the laying and final survey of the pipes, all require very accurate survey and navigation techniques. The consequences of not doing this properly can be very expensive, Figure 2.
The selection of a site for a production platform needs very accurate sea-bed contour and sub-bottom profile information, together with a sea-bed feature map of similar quality. Having chosen and surveyed such a site, the structure then has to be positioned accurately on the site. Of course, the pipe route survey and pipe lay operations into the site have to be conducted to similar accuracies.
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Until recently, all such surveys have been carried out from surface vessels, and although good accuracies can be achieved, the sensor accuracies - and in particular, those associated with towed fish operations are limited in respect of position and resolution. The accuracies of surface surveys are further down-graded by bad weather, and in deep water the power required for ship-fitted systems will prevent the achievement of the resolutions required.
These problems can be overcome by fitting the sensors to the submersible, and in this way,. the quality of the results from a survey in, say, 1000 meters of water, is the same as from one in, say, 100 meters of water. By operating the submersible close to the sea bed,(normal surveying altitude is about 10 meters) high resolution systems can be used to the greatest effect.
At the present time, the manned submersible is being used and this itself is limited in endurance .by battery life, human fatigue, and surface weather conditions for recovery. Submersible surveying techniques may be satisfying certain demands but economic pressures are demanding continuous, all-weather operation 24-hours-a-day.