Certain maritime operations require the accurate positioning of the vessel, and in order to accomplish that DP (dynamic positioning) systems were developed. It combines the information obtained from sensors with the expected dynamic of the ship to better estimate its actual position and the external forces, and with those information the controller allocates the forces among the available actuators so the vessel keeps a desired position.

In situations where drift of the vessel could cause great harm (human, material or environmental losses) it might be necessary to provide additional safeguards. One possible solution is to connect an AHTS (anchor handling tug supply) to the original DP vessel, in order to complement the forces generated by its thrusters. However as shown by Jensen (2008) and IMCA M 185 (2012), this connection could actually degrade the position keeping ability of the vessel, nullifying the purpose of improving the safety of the operation.

The objective of the present paper is to confirm the hypothesis that the use of hold-back vessels to support DP drilling rigs may degrade the performance of the DP system, causing dynamic instability, and to determine the boundaries of operation under which this phenomenon occurs: sea state, parameters of the vessels and force transmitted by the hold-back vessel.

Firstly, an analytical study of the system was done. It was considered a simplified model of two vessels connected by a cable with two degrees of freedom (one for each vessel), since the force applied by a cable is unidirectional. Using control theory, the limiting stiffness of the cable was determined by analyzing the poles of the system. Considering a catenary model for the connecting cable, it was possible to determine the maximum force that could be transmitted between the vessels without the system becoming unstable. The influence of the Kalman Filter in the stability of the system was also studied.

Those results were then compared and confirmed with fast time dynamic simulations of the system, in which the influence of different environmental conditions were also added to the analysis. To complete the study, real time simulations were done on a full mission simulator, equipped with the original Kongsberg DP system for the drilling rig.

The simplified model showed consistent results, validated by the simulations, demonstrating it can be a useful tool when analyzing the stability of two connected vessels.

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