SEAKEEPING AND EXTREME TENSIONS IN OFFSHORE TOWING

The theory of seakeeping of a single ship is used as a basis to develop a twelve-degree-of-freedom theory for a towing system consisting of a tug coupled to a tow through a towline. Statistics of extreme towline tensions are of principal interest because of their influence on the planning and operation of towing at sea. As shown by experimental and theoretical results, the relationship between dynamic towline tension and motion is highly nonlinear for wire rope towlines. This necessitates a fully nonlinear analysis for predicting the statistics of the extreme tensions. On the other hand, the ship motions are shown to be only weakly nonlinear because the dynamic influences of the towline forces on the ships are small in comparison with the fluid and inertial forces. The theory and procedure that is developed is applied to examples of actual towing conditions to show how towline length and towline speed can be chosen to keep the risk of towline failure below a predetermined limit.