Dynamic positioning system is usually used to keep marine constructions at a desired position for offshore engineering operations. Traditionally, DP system only deals with horizontal motions regardless of vertical ones including roll and pitch. However, large roll and pitch motions may be induced by thruster actions, which obviously should be avoided. A new control strategy considering roll-pitch motion control is presented in this paper. The main idea in the new control strategy is to consider roll-pitch velocity and acceleration feedback in the horizontal control law in order to avoid large roll-pitch motions. The new control strategy can significantly suppress the roll and pitch motions. In addition its effects on horizontal motions are also discussed.

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