This paper addresses the dynamic positioning of surface vessels moored to the seabed via a turret based spread mooring system, an operation referred to as the position mooring. While the mooring system keeps the surface vessel in place most of the time, thruster assistance is needed in severe weather conditions to avoid mooring line failure. Traditionally, this is done by keeping the vessel within a predefined geographical region. We present a conceptually new controller for position mooring operations. By using a structural reliability measure for the mooring lines, the new controller protects the mooring system whenever needed as a result of severe weather conditions and high environmental loads. This is done by maintaining the probability of mooring line failure below a preset value. In particular, the excessive use of thrusters caused by conservatively defined safety regions in conventional PM systems is avoided, giving a fuel optimal operation. The feasibility of our controller is successfully verified in laboratory experiments. (C) 2009 Elsevier Ltd. All rights reserved.