Application of structural reliability methods in relation to online control of dynamic systems is considered. The particular case in focus is dynamic positioning of marine vehicles as related to mechanical limit states for the riser system. The magnitudes of top and bottom angles of the marine risers are considered. These angles are of crucial importance during e.g. drilling and workover operations. The relationship between surface floater motion and angle responses is first considered. The possibility of reducing the maximum angular response levels by dynamic positioning of the floater is then investigated. Typically, minimization of one of the riser top and bottom angles by adjusting the vessel position can only take place at the cost of increasing the other one. Hence, an optimum position should be defined by applying different weights for the two angles. Three different alternatives are considered: (I) The relative weights are kept fixed (II) The relative weights are determined as functions of the respective reliability indices for each of the two angles. (III) A loss function which is purely expressed in terms of reliability indices is introduced. The viability of different schemes of this type is explored by numerical simulation for a specific riser configuration.

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