Environmental issues along with rising fuel prices has increased the necessity for better energy efficiency in all sectors, especially in shipping sector which is a stakeholder in environmental issues. Shipping industry, responsible for approximately 3% of global CO2 emissions, 14-15% of global NOX emissions and 16% of global SOX emissions, is analyzing its scope on reducing the fuel usage which started with trial and error through experimenting in the initial phase to the mathematical formulation of factors involved.
Being an effective operational measure for better energy efficiency to reduce emissions, Trim optimization for ships have gained huge importance in finding the optimum condition in which a vessel should operate.
1 Effect of trim
Ship trim optimization simply advocates selecting the trim condition for minimum resistance. The resistance through water is then proportional to the amount of required power and governs the required fuel burn rate. Hence, less resistance implies fewer emissions and more economical operation by fuel savings. The effect on varying trim, draft and speed of a ship shall significantly vary the power required for operation. Determining the most efficient condition from analyzing all these combinations through experimentation is impractical whereas through computational fluid dynamics (CFD) simulation methodology, hydrodynamics of complex structures can easily be obtained.
Ship management can further focus on creating awareness in the crew about the economic and ecological value of the fuel savings and emission reduction, through operation at optimum trim angle. The information can then be used to adjust the trim of the vessel. If extra ballast is required to re-trim the vessel, this can be considered to find the new optimum trim including the ballast weight.
2 Analysis Procedure
XShip Design & Analytics team underwent extensive experimental exploration over sea trial results to provide a baseline on experimental data of mathematical methodology to enhance understanding of flow, physics and CFD validation for ship design applications. Most trim optimization tools use operational log data to find the efficient case, which only provides a very vague idea of fuel efficiency of the respective case.
Our solution to this starts from the precise modelling of the hull form to evaluate different trim cases at different operational speeds and further by varying the drafts at each case.
Drawing expertise from diverse pool of disciplines like draughting, modelling, formulating and analyzing, XShip has developed an innovative simulation method to obtain resistance values in different trim conditions at different speeds. Based on attained results, optimum trim is suggested.
3 Optimum trim
Various trim cases were considered for find the condition in which power consumption could be minimized. This study was carried out for numerous cases by varying speed and draft ranges. A significant decrease of the ship resistance for the analyzed models were achieved and validated by the comparison with the well-known sea-trial and model test result database. This case, after considering all factors, where the power consumption be minimal is termed as the optimum trim. Hence, the optimum trim is the one at which the vessel needs least fuel to meet its desired performance standard.
4 Savings potential
Optimum trim for vessel may be different from the current trim of the vessel. Vessels can attain the optimum trim by two ways by adjusting the ballast or cargo without changing the displacement and by adding extra ballast. In the former case, there is a fuel savings which the ship can achieve by just re-arranging the ballast or cargo. In the latter case, a different savings is achieved by adding extra ballast.
In any case, it is evident that the ship can attain some fuel savings by changing the trim compared to the current operational conditions which is called Saving potential. Each trim different from the current operating trim has some saving potential which may not be optimum but may be the best achievable trim. Saving potential is calculated considering the improvement that can be made from the current operating trim.
A trim is advantageous over the current trim if saving potential is showing a positive value and a trim has more fuel consumption if the saving potential is showing a negative value.
5 Trim Curve plot
For Ship Crew, the present case of draft and speed can be inputted to the software, after which the software suggests how the draft at aft and forward be changed to arrive at the optimum trim. The amount of fuel saved, and CO2 emission reduced is indicated in percentage which helps in creating an awareness to the crew on fuel consumption for different cases. Power Saving in percentage is plotted along the vertical axis and trim in meter is plotted along x axis.