A method is described which may improve the dynamic positioning of large ships in wind and waves, especially in case of non-parallel wind and wave directions. The proposed method applies the measurement of relative motions at two opposite side locations to estimate the wave direction and the wave drift force level. Combination with the wind measurement leads to an optimum heading set-point. The potential improvements are illustrated by results of tests and dynamic positioning simulations.
The offshore industry is making use of a growing number of dynamically positioned ships, e.g. for diving support, supply, drilling and in the near future for production (the BP SWOPS vessel). This growth reflects the appreciation of operational flexibility which is characteristic for dynamic positioning. Furthermore, the positioning accuracy which can be achieved by dynamic positioning is sufficient for activities like diving support, pipeline burial by means of gravel dumping etc.
The advantages of dynamic positioning have to be considered in view of the capital expenditure and fuel consumption of a dynamic positioning system. These costs are related to the fact that the complicated dynamic positioning control systems are still crude methods to estimate the thruster activity that is needed to stay on position. Two typical problems should be mentioned (see Fig. 1):
The position filtering system, which gives a "best estimate" of the position error with respect to the set-points, suffers from phase lag.
Due to the inertia of the ship, the position measuring system measures position changes due to forces on the ship some time ago.
Since the "best estimate" of position error is fed into the PID-controller (feed-back system) and thrust allocation is based on the "current" force estimate of the position filter model, the two above mentioned problems lead to the fact that the ship's DP system is always late in its reaction to environmental forces. In many cases the consequence is over-dimensioning of thrusters and high fuel consumption.
Research efforts in recent years are aimed at methods to obtain an earlier assessment of the forces acting on the ship and on methods to find optimum headings for the ship. In this respect it should be realized that the relative importance of the environmental forces depends on the size of the ship.
Whereas wind forces are the major environmental force on relatively small ships such as diving support vessels or supply boats, the wave drift forces are the dominant forces on large vessels. In open sea the currents are usually low and nearly constant. Wind and wave drift forces are slowly varying forces which require varying thruster response. Dynamic positioning will therefore improve if a good, real time, estimate of these forces can be incorporated in the controls.
The low frequency drift force variations are described in Ref. [1] and [2]. For large ships the magnitude of the variations may be considerable Ref. [3 J. If a large ship is dynamically positioned, it is expected that applying a method to incorporate the drift forces in the control system will improve the positioning. Especially because the phase lag and feed-back properties of a conventional DP system allow a large ship to build-up a large inertia momentum under the influence of wave drift forces before the position error is detected.
