4 - Position Reference Systems and Equipment
4.1 - General
Accurate, reliable and continuous position information is essential for dynamic positioning. Some DP operations require better than 3m relative accuracy. A DP control system requires data at a rate of once per second to achieve high accuracy. Reliability is, of course, of vital importance, to operations where life and property may be put at extreme risk through incorrect position data10.
All DP vessels have position reference systems (PRS), (sometimes referred to as position monitoring equipment or PME), independent of the vessel’s normal navigation suite. Five types of PRS are in common use in DP vessels; Hydroacoustic Position Reference (HPR), Taut Wire, DGPS, Laser-based systems (Fanbeam and CyScan) and Artemis. A brief description will be given of each.
DP control systems ‘pool’, or combine, position reference data from two or more position reference systems. If only one position reference system is enabled into the DP then it is simply checked, filtered and used. If two or more are available, then the system needs to use both equally or according to their individual performance.
In all modern DP systems the weighted average option can be selected, whereby individual position references are weighted in inverse proportion to the variance or ‘spread’ of position data; the higher the weighting for an individual position reference system, the greater the influence of that system in the position calculation.
Early DP control systems did not have the capability to learn from the past performance other than by the integral terms of the controller. Modern systems are able to improve station keeping performance by using a Karman filter, which provides a model of recent performance to improve present performance.
For any operations requiring [U]DP redundancy (equipment Class 2 or 3 operations) it is necessary to utilise three position references.[/U] Two PRSs are not adequate, because if one has failed, contradictory reference data provides an impass, whereas three systems provide two-out-of-three voting to identify a rogue sensor.
Where three PRSs are required, the DPO should choose systems that are different. This reduces the probability of common-mode failure, where one event may result in a loss of position.
A brief description will be given of the five commonly used position reference systems.