As drilling moves into sufficiently deep waters, economics dictate the necessity of completing wells with the wellhead and Christmas trees installed on the ocean bottom. This means remotely controlled valves, etc. hence the need for tale control systems. This paper will cover the original concept and design of an underwater acoustic telemetry system. Included are original field studies of natural and man-made underwater noises and some typical curves. The final design and field operations are included along with problems, solutions, conclusions, and recommendations.
In the latter part of 1965, Rockwell initiated a project to study existing control systems and actual requirements, to develop a concept for a practical solution, and to build and field test a prototype. Several criteria were established; one of the most important being that all major components should be off-the-shelf equipment with a good record of dependable operation.
Very early in the project it became obvious that in the entire energy wave spectrum, sea water is usefully transparent only to acoustic waves. Furthermore, there are many factors in the ocean which affect the propagation of sound waves through water.
A survey of published literature revealed a certain amount of basic knowledge and much specialized knowledge, but relatively little applicable to oilfield hardware. As an example, it is(known that the propagation losses 1) become so great, as the frequency increases, as to limit the practical range very seriously (Fig. 1). Spectrum levels for narrow band noise (1) in various sea states are reasonably well known (Fig. 2). Experimental work done during project Mohole provided some indications of man-made noise around a floating drilling rig (Fig. 3), but this was not nearly sufficient to support the design of a communication system.
In general it is known that because of the distribution of acoustic velocities, sea water causes sound waves to travel in curved rather than straight paths. In certain circumstances this may cause "shadow zones" However, for a given situation, problems related to this phenomenon can be anticipated and avoided. Further, there are other basic acoustic problems:
This signal-to-noise ratio at the receiver must be greater than to avoid tlie1need to provide very high discrimination power to extract a low level signal from a high noise environment. Unfortunately, "offshore oilfield" ambient noises were not well known.
In the relatively shallow waters of offshore oilfield activities and with the addition of numerous man-made obstructions, the problem of multiple paths comes to the fore. The acoustic waves will reflect off the surface, the bottom, the subbottom at several levels, any local temperature "inversions" or sharp gradients, and any solid object which they may encounter. Sometimes a phase reversal will occur, sometimes not (2); however, various simple coding systems had been developed and used successfully at long range (up to 15 nautical miles) (2) (3). It was believed that it would be possible to build a successfully functional system if the problem of man-made noises could be resolved. This could be accomplished rather easily and at moderate cost by direct observation and measurement.