Mathematical modelling of forces acting on ships during lightering operations

Abstract

The transfer of liquid cargo (crude oil and LNG) from a larger ship (the ship to be lightered, STBL) to a smaller vessel (service ship) when both ships are moored to each other and sail at a (slow) constant forward speed is known as lightering. These ship to ship operations are expected to increase in the near future and are expected to take place in harsher environmental conditions (polar regions). In order to better understand the hydrodynamic phenomena involved in this specific manoeuvre, 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. As a part of this project, more than two thousand captive model tests were carried out at the towing tank for manoeuvres in shallow water (co-operation Flanders Hydraulics Research — Ghent University) in Antwerp, Belgium. A model of a very large crude oil carrier (VLCC) was attached to the main frame of the towing carriage and a model of an Aframax tanker was attached to the computer controlled planar motion carriage. Forces, moments and vertical positions were measured on both models.This paper covers the analysis of the extensive model test data reported by Lataire et al. (2009a) and the influence of different parameters on the manoeuvre. Particular attention is paid to the forces and moments induced on the service ship by the proximity of the ship to be lightered. A mathematical model of lightering manoeuvres for both the service ship and the ship to be lightered is derived. This model can be implemented in a ship manoeuvring simulator for training purposes.