This chapter begins with some background on the role of dynamic positioning systems for vessels involved in static offshore operations before outlining the basic concept of a position mooring system that utilises DP techniques The main features of the system are described and the operation and simulation systems are examined The importance of state-estimation is then discussed Applications of the developed system to thruster assisted mooring and anchor line breakage are highlighted. Operational experience is discussed with the aid of an example.
The main purpose of a dynamic positioning system is to keep a floating vessel in a specified position by the action of its thrusters, propulsion system, etc. The commercial application of computer-based DP systems to deep sea vessels dates from the early 1960s The majority of the early systems depended on the utilisation of a PID-controller for each of the surge, sway and yaw motions, see Sargent and Cowgill.1 This, in combination with the use of notch filtering to eliminate the effects of the first order wave motions, resulted in limited positioning accuracy due to the slow integral action of the controller and to control loop phase lag Hence In the early 1970s, alternative control system configurations were investigated, and studies by the Norvegian Institute of Technology, Trondheim, Balchen et al,2 and others led to the commercial introduction to offshore vessels in 1977 of a DP system based on Kalman filtering and optimal control (mathematical modelling).
These concepts were, of course, well-known as part of modem control system theory, but had not until then been applied to the relatively complex model of a vessel in a practical wave pattern. From 1977 onwards, the commercial application of dynamic positioning systems utilising these principles was part of, and in some measure, perhaps, responsible for the increased number and variety of vessels m the 1977-85 period which were fitted with dynamic positioning. The increased accuracy and repeatability of vessel positioning that the system allowed has led to over 120 vessels to date being fitted with DP systems utilising these principles.
As part of the evolution of the control system capabilities over this period, additional features that have by now become regular requirements for the modem DP system were developed, over and above the basic capability of controlling the vessel to remain in one position. These included.
following a designated track, with a number of predetemined turn points manually inserted before commencing the manoeuvre,
following a moving position reference origin such as a submersible with pre-determined offsets in position from the origin
the capability to stop and retrace a track just completed,
operation of the DP system simultaneously with other systems which affected the vessel's position, such as large capacity firefighting water monitors, a gangway to an adjacent fixed structure, or an anchoring system.