Fishtailing instabilities in emergency towing of disabled tankers

SAFETOW (Strategic Aid For Escort and Salvage Tugs at Work) is a European project developed to provide masters of disabled vessels and masters of salvage and escort tugs with support tools which will enable them to make decisions in real-time with the best available information. This paper focuses on predicting the dynamics of a tug towing a disabled tanker. Of interest are the course stability, trajectories and time histories of trajectory parameters such as heading and drift angles. The calculation takes into account wind effects (using new generic equations), tow point, and towline length. Different fishtailing instabilities are also investigated, i.e. when tug and disabled tanker are both moving, and in bad weather conditions when tug is positionally fixed and the disabled tanker is weathervaning. For trajectory calculations, the velocity of the tug and the disabled tanker can be either constant or allowed to vary with respect to time. The authors also detail the new generic equations for estimating second order mean wave drift and current generic equations for current forces/moment.