ABSTRACT
Not Under Command (NUC) drifting vessels may impose a threat to offshore assets and structures. A vessel at drift can be brought under control by tugs or Emergency Response and Towing Vessels. As the bollard pull and maneuvering characteristics of these vessels might be limited, salvaging a NUC vessel is not guaranteed. A study was conducted in order to assess adequacy of a hold-back tug of about 100 tons bollard pull to bring different NUC vessels under control in different environmental conditions. This work included a desk study as well as maneuvering simulations. The NUC vessels consisted of Qmax, ULCV and VLCC in ballast and loaded condition, whereas the tug used was an ASD-type. Given the sea state during storm conditions (resulting in Hs=3.4m~5.3m) thrust degradation of tugs was taken into account based on MARIN’s SafeTug JIP. The effect of mean wave drift forces, large relative motions, ventilation of thrusters and interaction between the wave and the wake of the tug were taken into account. Based on environmental forces acting on the vessel, expected changes in Course over Ground as well as optimal position of the tug with respect to the drifting vessel were derived. A distinction was made between pushing and pulling tugs. It was shown that a tug at a line length of 800m connected via center lead aft or fore is most effective due to the larger lever arm with respect to the pivot point of the vessel. The NUC vessels were set at drift for 60 minutes to include the time for arriving, connecting and paying out the towline. Real time maneuvering simulations were conducted by two experienced tow masters. Based on the simulations an optimal strategy in order to obtain largest change in drift direction was found. The found strategy does not necessarily correspond with ‘natural’ behavior of the tug crew, which would tow into the environment. Based on the optimal strategy conclusions and recommendations per type of NUC were given. Life on board of tugs during such operation was assessed as this could hamper operations.
