Case of Critical Surging of a Moored Ship

An analysis is made of the circumstances in which an oil tanker, berthed at a solid quay wall in a rectangular basin of a port subject to influences of long period waves, developed critical surging motions and broke adrift after rupture of its mooring lines. The response characteristics of the ship are examined in the light of the non-linear equation of motion for longitudinal surging and the best available information on the conditions of mooring, the state of the sea and the time of the tide. From the location of the ship in the dock it is inferred that the possible modes of oscillation of the water body in the harbor basin that could have influenced the ship would have been the uninodal, binodal, quinquinodal (and possibly sextinodal) seiches. It is then shown that with the rise of tide in the dock and the accompanying known and probable amplitude increases of the various modes of seiches, only the quinquinodal seiche with a period of 22 sec could have been responsible for the development of rope tensions sufficient to fracture the mooring lines.