Traditionally; two basic approaches have been taken to the design of acoustic systems for determining the position of a surface vessel relative to a reference point on the sea floor. The first is known as the long-baseline system, which measures the slant range to three widely spaced transponding beacons that have been carefully surveyed into position on the sea floor. The second, called the short-baseline system, uses only a single free-running sea' floor beacon and measures the relative pulse arrival times at three widely spaced and precisely positioned receiving hydrophones on the surface vessel. Both techniques have been used successfully. Each, however, has a disadvantage which makes it less than practical for all vessel sizes and all types of offshore operations. The long-baseline system requires considerable time and equipment to place the sea-floor beacons in accurately surveyed positions, while the wide spacing of the ship-mounted hydrophones required in the short-baseline system precludes the use of that system on smaller vessels.
This paper describes an acoustic position referencing method which overcomes the primary disadvantages of the two traditional techniques, in that it requires only a single free-running hydrophone installation on the vessel. Details of the hardware implementation of this method are described. This includes
Unique multi-element hydrophone configuration
State-of-the-art, all-digital phase coherent receiver
Microprocessor implementation of an exact position-referencing algorithm
Field test results are summarized, verifying the viability of the technique and the effectiveness of the hardware implementation. Alternative modes of operation are described which fulfill special requirements of the offshore drilling operation, such as hole re-entry and marine riser angle monitoring.
The single-hydrophone configuration of this system significantly reduces system installation costs and even opens up the possibility of a portable and highly versatile system. It also makes installation on smaller vessels practical.
Two basic types of system are most commonly used for acoustic position referencing. There is the long-baseline system which measures ranges to three or more bottom-mounted transponding beacons, and the short-baseline system which has only a single bottom-mounted beacon, but requires three or more receiving sensors on the vessel.
The long-baseline system requires that the positions of three or more transponding beacons be precisely determined in terms of their ranges and true bearings from an established sea-floor reference point. Instantaneous ranges from the vessel to the transponders are determined by interrogating these acoustic transponders from the vessel and measuring the time delays of the responses. The range to each transponder is directly proportional to the measured two-way propagation delay. A special algorithm converts this range information into vessel position relative to the reference point on the sea floor.
The short-baseline system requires only a single free-running bottom-mounted beacon. The baseline in this system is aboard the vessel. Vessel position relative to the beacon is determined by measuring the relative times of the beacon pulse arrival at each of the three vessel mounted hydrophones.
