A unique multimode position reference system was developed to support the special requirements of the Ocean Drilling Program (ODP). This system combined the advantages of the pulse time-of-arrival and phase-comparison concepts of short-baseline position referencing into a single system. Special enhancements were added to improve accuracy and reliability.
The Ocean Drilling Program has the mission of conducting coring operations in ocean depths to 27, 000 feet. These depths impose special requirements on the acoustic position reference sensor, in that there is no longer a non acoustic backup method, such as taut wire, of receiving position data. Yet the application calls for the redundancy and reliability requirements that are normally associated with the position sensor of a dynamic positioning system.
The coring mission also requires that the vessel move much more frequently than during normal oil exploration drilling. The ability to quickly and easily set up at a new site is important because it is now a much larger percentage of the vessel's total operating time.
The choice of acoustic position referencing method for the Ocean Drilling Program had to consider these requirements. In view of the need to frequently move between locations, the long-baseline mode was ruled out as a possible position referencing method.
The offshore community traditionally divides acoustic position reference systems into two basic categories: long baseline and short baseline systems (LBS and SBS).
Long baseline refers to the system that uses multiple subsea transponders and a single vessel mounted projector Hydrophone transducer (Figure 1). The baseline is formed by the transponder separation on the sea floor. The measured slant range to each of three or more transponders uniquely determines the vessel's XYZ position relative to the transponder grid.
The short-baseline system incorporates a single subsea transponder, or free-running pinger, and three or more vessel-mounted hydrophones (Figure 2(a)). Direction to the beacon is determined by measuring the differential pulse time-of-arrival at the three hydrophones. Coupled with a knowledge of hydrophone-to-beacon vertical separation or a measured slant range, the SBS determines the beacon's XYZ position relative to the vessel, in vessel coordinates.
The ultra-short baseline is a popular variation of the short baseline concept wherein all sensors are located close together inside a single vessel mounted hydrophone assembly (Figure 2(b)). Differential phase, rather than differential pulse time of- arrival, is used to determine direction to the beacon.
The three system concepts just described are referred to in this paper as the long-baseline system (LBS), the intermediate-baseline system (IBS), and the short-baseline system (SBS).
ach system has unique advantages and disadvantages that limit its application. The most significant of these are summarized in Table 1. The LBS has the greatest potential accuracy, but requires multiple subsea transponders.
The IBS provides reasonable accuracy and requires only a single subsea beacon; however, it requires the installation of multiple vessel-mounted hydrophones
