Dynamic positioning of surface vessels moored to the seabed via a spread mooring system are referred to as position mooring (PM), the main objective of which is to keep the vessel within a small radius from a given position while preventing mooring line breakage. When environmental loads become high, position mooring systems apply thruster forces to protect mooring lines and position accuracy may need be relaxed. This paper suggests a new position chasing algorithm that works entirely online, is optimal according to a criterion and can protect any number of mooring lines simultaneously. Tensions of all mooring lines are included in a cost function where the criticality for each mooring line determine individual weights. With this strategy, external environment effects are included directly by without needing predefined tabular settings of environmental loads as in earlier approaches. There is no limitation to the number of mooring lines that can be close to critical tension. A reliability index is used as weight to include the dynamic influence of mooring line tension. Detailed simulations illustrate the features and advantages of the new method and results are compared with those of a fixed weight algorithm.

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