Advantages and disadvantages of thruster allocation procedures in preliminary dynamic positioning predictions

Minimisation of power consumption is the primary target of the most commonly used procedures and strategies for the capability prediction of DP systems installed on floating objects and, therefore, dealing with an huge amount of limitations or restrictions that should be satisfied at the same time for different operating conditions. However, in the particular case of the “extreme limit prediction” for a considered system, other factors assume a prominent importance instead of fuel consumption, like the ability to use all the available power to keep the position in heavy weather conditions. When the maximum operating cases for a vessel are analysed, other effects like thruster–hull interaction and thruster–thruster interaction are rarely considered. To take into account the above-mentioned effects, different thrust allocation strategies were studied and implemented in a capability prediction suite. In this paper a new allocation philosophy is presented, with the aim of maximising the DP capability of a vessel operating in heavy whether conditions. An application of the procedure on two existing offshore vessels is also considered to highlight the effects of different allocation strategies and of the interaction effects on the resulting DP capability in terms of “operational rosettes”.