The present paper proposes a quantitative assessment method of safety margin when a ship approaches a berth. A ship travels for a considerable distance before an intended control result is achieved, when a ship encounters an unexpected situation during a manoeuvre and counters the emergency through the application of an emergency 'crash astern' manoeuvre or other similar action. The safety of a ship is guaranteed if there are no obstacles within such extra area of water needed to accommodate ship motions arising from her inertia. The larger the sea room lying between the edge of the extra water area and the obstacles the greater the safety margin, and vice versa. Based on the above consideration, the value of a ratio of B to D gives the magnitude of the safety margin when a ship approaches a berth, where the value of B is the distance between the berth and the edge of the extra water area after applying the emergency 'crash astern' monoeuvre and the value of B is the residual distance to the berth. The saferty margin therefore can be evaluated at every moment by calculating the value of this ratio at different point in time during the approach manoeuvre. The present paper also proposes the standardized deceleration patterns of approach speed which are capable of keeping the safety margin constant.
