Controlling the Yawing of an Aframax Tanker During a Lightering Manoeuvre

Abstract

Lightering is the process where a larger ship (the ship to be lightered, STBL) transfers (parts of) its cargo to a smaller vessel, known as service ship. This transfer occurs at a slow sailing speed (about 4 knots) while both ships are moored to each other. A knowledge-building project with user involvement entitled "Investigating Hydrodynamic Aspects and Control Strategies for Ship-to-Ship Operations" was carried out in 2007-2011 to offer more insight in lightering operations. The actual forces acting on both vessels while preparing for lightering can be analysed based upon more than two thousand captive model tests carried out at the Towing tank for manoeuvres in shallow water (co-operation Flanders Hydraulics Research -Ghent University) in Antwerp, Belgium. The tests were executed with a scale model of a very large crude oil carrier (VLCC) attached to the main frame of the towing carriage and a scale model of an Aframax tanker attached to the computer controlled planar motion carriage. Forces, moments and vertical positions were measured on both models.In this article particular attention is given to the comparison, in deep and shallow water, of the yawing moment induced on the service ship during the lightering operation and the yawing moment induced by the rudder of the same ship during open water tests. The combination of both allows defining ranges of relative positions between both vessels with an equal degree of controllability of the service ship during the lightering operation. In shallow water, the acceptable meeting area appears to be reduced significantly.