ABSTRACT

Signal deterioration during heavy weather is a major problem in the use of acoustic techniques for determining the position of offshore vessels and floating platforms. Strong winds and heavy wave action during storms and wash from propellers create a near-surface layer of highly aerated water in the transmission path, resulting in excessive attenuation of the acoustic signal and poor reliability of the position-reference system.

The concept of spatial diversity through the use of redundant single-hydrophone phase-comparison acoustic position-reference systems can be shown to improve acoustic system reliability by an order of magnitude over the more conventional pulse-arrival-delay systems of the past.

This paper illustrates how spatial diversity techniques are implemented and predicts the reliability improvement that can be expected for various alternative configurations.

The performance predictions are validated by field data recorded aboard a dynamically positioned drillship during severe weather conditions. It is demonstrated that by use of the phase-comparison method of position determination, with the enhanced spatial-diversity capability that it makes possible, an array of four hydrophones experiencing signal-loss rates as high as 80 percent can produce a total acoustic system dropout rate (down time) of only 3.7 percent.

INTRODUCTION

Loss of-output information from the position reference system is a serious problem for a dynamically positioned vessel, if the loss is of significant duration. For this reason, the position-reference subsystem for a dynamic positioning system typically includes of more than one position-reference sensor.

One of the most commonly used position-reference sensors is an acoustic position indicator. In applications such as drilling, where loss of position information is critical, two completely independent acoustic sensors are used to provide total redundancy. Experience has shown that the redundancy is necessary, not only to overcome total system losses due to hardware failures, but also to overcome temporary loss of the acoustic transmission channel as a result of interference in the path between the subsea beacon and the hydrophones on the vessel, particularly during heavy weather. Experience has further demonstrated that the degree of redundancy provided by two independent acoustic position-reference systems of previous design was not sufficient to eliminate total system loss, nor even to reduce it to an acceptable level during extreme environmental conditions. This reality has led to the addition of nonacoustic position reference methods as a backup.

The concept of spatial diversity, achieved through the use of redundant single-hydrophone phase comparison acoustic systems, now offers an alternative to the addition of a nonacoustic backup method.

BACKGROUND
Spatial Diversity

The philosophy behind spatial diversity is to provide redundant information channels with independent, spatially separated signal transmission paths, so that interference in anyone path will affect the performance of only one information channel. By providing a number of redundant channels, the probability of always having at least one channel free of interference will be increased significantly. The ultimate goal, of course, is to improve total system reliability.

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