Dynamic Tension Analysis of Surface Towing System

Abstract

This study is concentrated on the emergency towing arrangements onboard tanker referred in SOLAS Chapter V. The necessity of designing the towing gear at the aft of the tanker is surfaced to facilitate salvage and emergency towing to reduce risk of pollution. Followed up on these requirements, a simulation study centred on the dynamics of tanker-tug tow system is taken up. Three tankers with each of 20000dwt, 100000dwt, and 200000dwt were considered. Four tugs of different pulling capacity of 100kN, 200kN, 300kN and 400kN respectively were considered for towing operation. The important towing parameters like line length, tow speed are mainly decided based on the sea conditions. In a calm sea shorter line length is preferred in general.As the tanker is large in size compared to the tug, the excitations of the tug will be dominant. The wave exciting forces, drag of towed vessels and the line tensions govern the system dynamics. The line is assumed to be finite number of lumped masses and linear springs without mass. Harmonic motions, with the amplitudes and frequencies of oscillation correspond to various Beaufort scales were considered initially. The maximum tension due to the excitation of the towing vessl during its drive to move forward in irregular sea is determined in various cases of sea states. Deformation of line is caused by current and the movement of tug towing the tanker. The towline tensions for various Beaufort scale were determined for the prescribed harmonic motions. The analysis carried out helps compare the maximum line tension to the breaking strength of the line. Lumped Mass method is relied upon for the analysis of extreme towline tensions in random sea. Two conventional types of lines with different diameters were considered for the analysis. ISSC sea spectrum is considered for simulation in irregular sea. This study may help the towing operaters take more care so as to avoid accidents, material loss and loss of life.