In this paper, an unconventional dynamic positioning (DP) system is introduced which is relatively simple from the point of view of the thruster layout and, by virtue of the same thruster layout, automatically positions the vessel in a favorable heading relative to the environment.
This system, which in principle is based on thrusters installed only at the forward (or aft) end of the vessel, possesses automatic weathervaning properties i.e. the DP system itself does not control the heading as this is governed by the environmental conditions. The stable equilibrium heading of the vessel is consistent with the minimum power heading.
The basic principles of the system are explained and results of an extensive series of model tests giving insight in the capabilities of this type of DP system applied to a 200,000 DWT tanker are presented and discussed. The model tests cover a range of realistic sea conditions likely to be regarded as limiting conditions for offshore loading operations.
Such conditions include irregular waves, wind and current. For one test irregular cross-sea conditions combined with wind and current were generated. A number of tests were devoted to investigating stand-by sea conditions and the effects of fast changes in wind direction. The latter tests give insight in the reactions of the DP system to sudden variations in the environmental conditions as can occur, for instance, in cyclone prone areas.
The results of the model tests confirm that it is possible to DP a large tanker in operational conditions using a simple thruster layout and a relatively small amount of installed power.
Dynamic positioning or dynamic stationing is a method whereby the position of a surface vessel is maintained in close proximity to a required position in the horizontal plane through the controlled application of forces and moments -generated by purposely installed thrusters. The required position generally applies to the co-ordinates of a point (the reference point) of the vessel in the horizontal plane while for most applications a specified heading of the vessel is also a requirement.
The offshore industry has been making increasing use of dynamic positioning systems to station vessels in a wave, wind and current environment since it was first introduced in the sixties. Initially DP systems were applied to relatively small vessels such as survey ships and to larger vessels such as drill ships. Today vessels with displacements in the region of 130,000 DWT are being stationed if not wholly then at least partly with the aid of dynamic positioning systems, see Ref. [1]. As the vessel size increases, however, the power required by the DP system, the number of thrusters and the complexity of the power generation system and of the thruster layout form considerable obstacles to a wider application of what is generally considered to be a versatile and useful way of station keeping.
