A reliability-based control algorithm for dynamic positioning of floating vessels

The present paper is concerned with utilization of reliability methods in relation to on-line control of dynamic systems. The particular application is to dynamic positioning of marine vehicles in connection with reliability of mechanical subsystems. The present focus is on top and bottom angles of marine risers which are suspended between the seabed and the floating vessel. 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, and somewhat dependent of variation of current with depth, minimization of one of the riser top and bottom angles by adjusting the vessel position will take place at the cost of increasing the other angle. Hence, an optimum position should be defined by considering both angles but with different weight functions. An attractive approach is to determine these weights as functions of the respective reliability indices for each of the two angles. A further possibility is to apply an object function (loss function) which is purely expressed in terms of reliability indices. The viability of different schemes of this type is explored by numerical simulation for a specific riser configuration.