Determination of Minimum Propulsion Power to Maintain the Manoeuvrability of Ships in Adverse Conditions

Abstract

The Energy Efficiency Design Index (EEDI), quantifying the amount of carbon dioxide emitted per gram fuel consumed, is to be fully implemented in the coming years. Compliance with the EEDI requirements can be achieved in a number of ways. One is to reduce the vessel s installed power and thus adopt a lower ship speed. This raises concerns as it might result in inability to control the ship in heavy weather.The International Maritime Organization (IMO) has recognized this, and has implemented an interim guideline for determining the minimum propulsion power required to maintain manoeuvrability in adverse conditions. Here a procedure is presented for calculation of the power required to maintain a speed of 4 knots in adverse conditions. The aim of this thesis has been to evaluate the IMO guideline and hence suggest a method for computing the required minimum propulsion power.The evaluation of the guideline indicates that a vessel s ballast condition may be dimensioning for the requirement to installed power. An investigation of accidents that have occurred in adverse weather conditions confirms this. This is due to the increased likelihood of propeller ventilation causing thrust loss.A method considering the effect of ventilation has hence been developed. A hull resistance calculation is performed, in which calm water resistance and added resistance in wind and waves are considered. This gives the basis for calculation of required thrust. A mean thrust loss factor is calculated in the time domain by considering a realization of the relative motion between propeller and waves. The corresponding power required to maintain vessel speed is estimated in the frequency domain. If ventilation is occurring, the requirement to installed power will increase and become dimensioning.The results show that two of the vessels considered, the KVLCC2 and a chemical tanker, both experience ventilation effects in ballast condition. A third vessel, Norlines, experiences ventilation effects in its design condition. However, according to the calculations, the vessels have sufficiently power installed to maintain a speed of 4 knots in adverse weather conditions.A comparison between experimental and calculated results for Norlines shows deviating results. Reasons may be the limited duration of the experiment and an inaccurate estimate of the thrust deduction factor.The results are questionable. The main reason is that increasing the power does not necessarily lead to increased thrust. This is because increased propeller loading may increase the effect of ventilation further. It is thus recommended to expand the model to include the effect of propeller loading.