Research On The Method Of Dynamic Trim Optimization Of Typical Oil Tanker And Bulk Carrier Sailing In Wind And Waves

To combat the climate change,the Paris Agreement was adopted in 2016,to limit the global warming to below 2℃.In response to this commitment,International Maritime Organization(IMO)adopted an initial strategy on the reduction of Greenhouse Gas(GHG)emission from ships in 2018,setting out a vision to reduce GHG emissions from international shipping and phase them out,as soon as possible in this century.Xi Jinping,President of PR.China announced at the general debate of the 75th United Nations General Assembly(2020)that China will strive to peak CO2 emissions before 2030 and achieve carbon neutrality before 2060.While as the main means of transportation in the world trade,ships have emitted over 100 million tons of CO2 annually in recent years.The 2019 CO2 emissions data shows that the global fleet emits about 800 million tons of CO2,among them,67%came from the ships built before January 1,2013 and tonnage above 400GT.In terms of the growth trend of GHG emissions from international shipping,only the CO2 emissions of container ships have been continuously reduced in 2019 and 2020,while those of oil tankers and bulk carriers are still on the increase.Based on Clarkson’s selection of benchmark ship types,40%of oil tanker and bulk carrier fleets currently in operation need to combine multiple methods to meet the Energy Efficiency Existing Ship Index(EEXI),otherwise they will be forced to withdraw from operations.Therefore,to meet the major international and national needs,it is of great significance to reduce the CO2 emissions of oil tankers and bulk carriers from the perspective of practical operations.Trim optimization has been widely used due to its easy implementation.The current research on trim optimization is mainly under calm water conditions.However,ships will encounter various sea conditions during operation,and static water condition is only ideal one,so it is very necessary to study the trim optimization of oil tankers and bulk carriers in wind and waves.The difficulty lies in the calculation of added resistance,which has been extensively studied in recent years.However the experimental methods are high-cost and long-period.The calculation based on empirical formula is less and the calculation accuracy is generally low under different trim angles.With the rapid development of computer technology,numerical calculation method has become the preferred method of trim optimization.Based on the theory of viscous Computational Fluid Dynamics(CFD)method,the calculation is higher accuracy,but time-consuming.And the other numerical calculation method is based on the theory of potential flow,in which the constant element method is not accurate in solving the velocity potential and velocity of non-smooth boundary elements,while the higher-order element method is more complicated to deal with elements with sharp corners.Meanwhile,the computational domain in numerical computation is finite,and how to eliminate the reflected waves in the numerical tank is also difficult in numerical calculation.To solve the above mentioned existing problems,we developed a matching method based on the Taylor Expanded Boundary Element Method(TEBEM)to solve added resistance problem.And this method divided the computational domain into inner domain and outer domain by introducing an artificial matching surface.The simple Green function and transient free-surface Green function are employed to construct the boundary integral equation(BIE)for the inner and outer domains respectively.The transient free-surface Green function satisfies the distant radiation and the free surface condition automatically,which can eliminate the waves from the inner.All the BIEs are solved by TEBEM,which can accurately obtain the velocity and velocity potential on the panels.Through the calculation and analysis of the motion and added resistance of 14000TEU container ship,it is found that the accuracy of the results are similar to that of CFD and experiments,but the computational efficiency is much higher than that of CFD.In the study of trim optimization,the shape of the ship underwater changes with the change of trim angles,so the panels of the underwater part of the hull need to be changed in the calculation.The panels generated by existing commercial software can complete the optimization calculation,but the efficiency is low,and human intervention is needed,which affects the calculation efficiency of optimization.To solve these problems,we improved the traditional method of hull surface partition,and developed a customized hull surface mesh automatic generator(CNT-Mesh)for added resistance calculation program,using Non-Uniform Rational B-spline(NURBS)technology.It has been verified that the panels provided by CNTMesh can meet the calculation requirements,and human intervention can be reduced,and the efficiency of the optimization process has been improved.With a complete set of numerical calculation tools,the trim optimization of VLCC oil tankers and bulk carriers on typical routes is calculated and analyzed based on the data of the global sea states in 2008 on the website of the European Centre for Medium-Range Weather Forecasts(ECMWF).It is found that for the same ship,the wind load and added resistance is dominant in the total resistance under sea state 7,sailing with calm water optimal trim alone can save energy but it is not optimal trim;Under sea state 6,it is related to the speed and sea state and wind load and added resistance should be considered to obtain the optimal trim position accurately in complex coupling conditions;Under sea state 5,the proportion of calm water resistance is much greater than that of wind load and added resistance in the total resistance,and it is the optimal trim to rely only on calm water trim optimization at most speeds.Meanwhile,we found that under the same sea state,the trim optimization results of ships of different sizes in wind and waves are similar by comparative study.Then we constructed the method of dynamic trim optimization based on the prediction of the sea states in wind and waves for oil tankers and bulk carriers by employing the results of trim optimization in calm water and wind load and added resistance under different trim conditions and the ratio(α)of wind load and added resistance to total resistance under floating state.α is quickly estimated by using empirical formulas.Compared with the numerical results in this paper,it is found that the estimation of a from empirical formula is reliable.Finally,taking VLCC oil tanker and bulk carrier with designed speed as examples,the fuel consumption is evaluated by using the theoretical framework of dynamic trim optimization based on the prediction of the sea states in wind and waves.It is found that the method of dynamic trim optimization based on the prediction of the sea states in wind and waves presented in this paper can reduce fuel consumption of ships in operation,which can save shipping costs and reduce GHG emissions.The method does have economic value and energy-saving effects on the operation of oil tankers and bulk carriers.