On the Use of Main Hoist Tension Measurement for Feed Forward in DP Systems During Offshore Installations

DP systems are nowadays widely used on installation and crane vessels. In recent years a few cases have been reported where the DP system becomes instable during the installation. Two main reasons can cause these instabilities. First of all the predictor model that is used in the Kalman filter has to accommodate the difference in the mass that occurs when its weight shifts from the barge to the crane. The second and more important reason is the tension in the main hoist wire which acts as an additional spring in the horizontal plane. Some DP-systems have a special heavy lift mode, which is activated when instabilities occur. The present paper describes a different method that uses the actual tension measurement in the hoist wire to enhance the DP system and correct for the additional forces in the horizontal plane due to the hoisting wire. The proposed method is investigated in a series of model tests and results are presented. It is shown that the hoist feed forward improves the overall behavior significantly.