The Effect of Trim on Tanker, Container and Bulk Carrier Ship Toward the Reduction of Ship’s Exhaust Gas Emission

Emission is one of the few environmental problems, and ships are one of the modes of transportation that produce it. This study aims to define the impact of using optimal trim during the cruising phase, so it can decrease the resistance and the fuel consumption, which will lead to less emission produced by the ship. The type and amount of ships used in this study are three tanker ships, three container ships, and two bulk carrier ships. The methodology used in this study is by using Holtrop’s resistance calculation method with the help of Maxsurf software. The resistance, the power needed, and the fuel consumption is calculated on 22 trim variations and seven speed variations. This study determined that the average decrease in fuel consumption caused by trim optimization for tanker, container, and bulk carrier ships is 5.641%, 8.269%, and 15.704%. Furthermore, the average decrease of emissions produced by tanker, container, and bulk carrier is 6.494%, 11.317%, and 13.775%, respectively. These results are narrowed down to conclude that trim optimization can reduce fuel consumption by up to 9.871% and decrease the emission produced by up to 10.529% for the three types of ships used in this study.


Introduction
The shipping industry is responsible for 90% of the world trade currently helping and developing the global economy to date [1], As ships being responsible for a huge part of the exchange of goods, internationally and domestically, it's only natural for ships to emit a lot of exhaust gasses. During the year 2007 2012, it is estimated that ships produce 3,1% carbon dioxide (C02), 15% nitrogen oxides (NOX), and 13% sulfur oxides (SOX) [1], According to the International Maritime Organization (IMO), it is forecasted that the potential carbon dioxide (C02) emission from international shipping could grow as much as 50% to 250% by 2050 [1], Seeing the constant increase of emission, IMO, through the Marine Environment Protection Committee (MEPC), imposes regulations and recommendations regarding the means of reducing exhaust gas emissions from ships. In the IMO strategy on reducing greenhouse gas (GHG) at MEPC72, the target of reducing C02 emitted by ships at least reaches 40% by 2030 and pursuing 70% target by 2050 compared to 2008 [2], To reduce emissions, ships need to consume less fuel or operate in a fuel-efficient manner MEPC70 guides to decrease the ships' fuel consumption, starting from ship handling, voyage planning, improved fleet management, etc [3], The ship's handling could be from utilizing ship Turnaround Time (TRT) in port to the trimming of the ship. TRT could reduce exhaust gas emission in port by up to 10% compared to the Business as Usual (BAU) scenario [4], Specifically, about ship trimming, it could decrease the total exhaust gasses from ships by reducing the Wetted Surface Area (WSA), thus decreasing its resistance, fuel oil consumption, and in the end, decreasing the exhaust gasses emitted by the ship. A study finds the potential C02 reduction by adjusting a ship's trim ranging from 1%-10% [5], Research done on the S60 hull model determined the reduction of wave-making resistance and total resistance compared to the even keel condition, ranging from 9.7% 26.2% and 3.5% 7.2%, respectively [6], Another definite evidence from several researches conducted on a container ship modelled by the Korean Research Institute of Ships and Ocean Engineering (KRISO) conclude the impact of optimized trim on reducing container ship's total resistance ranging from 2.29% to up to 5. 13% [7], [8], Moreover, a trim optimization study on 4250-Twenty Equivalent Units (TEUs) container ship with the implementation of the study on the real 4250-TEUs ship finds the trim optimization could save energy by 5% 8%, which results in saving fuel consumption around 3.2 ton/day [9], Another study with three loading conditions at different speed ranges in a bulk carrier ship found the ship could reduce resistance by 14% with a slight trimming angle [10], Based on the studies mentioned, it can be concluded that all the research for trim optimization was done only by using a very little data model for the case study. Contrary to all the research that has been done, this paper will mainly use 3 models for tanker and container ship and 2 models for bulk carrier ships. This paper selects these 3 types of ships (i.e. tanker, container and bulk carrier ship) because these ships are the type of ships to use the most fuel oil out of other types of ships [1]. The purpose of this study is to prove the utilization of ship trim as the easiest and quickest alternative to decrease ships' exhaust gas emissions by reducing the fuel oil consumption, regardless of the type of the ship. 59

Methods
The process of selecting the best trim was carried out on varying types of ships with various trim and speed conditions. This research uses lines plans from ships that are currently active (cruising) as a reference, which was acquired from several sources, and then all the lines plans are redrawn to be analyzed further. The principal dimensions and data used for tanker, container and bulk carrier ships are shown in Table 1, Table 2 and Table 3, respectively. Furthermore, an example of the original source of lines plan and one of the redrawn lines plan are shown in Figure 1 and Figure 2, respectively.  As shown in Table 1, Table 2, and Table 3, there are three main engine types and two fuel types used by the ships in this study. The engine types are High-Speed Diesel (HSD), Medium Speed Diesel (MSD), and Slow Speed Diesel (SSD), while the fuel that is commonly used for all the subjects of this study is Heavy Fuel Oil (HFO) and Marine Diesel Oil (MDO). These two data (i.e., engine type and fuel type) will affect the emission factor that will be used to quantify the number of exhaust gasses produced by each ship. After redrawing all the lines plans with the help of Maxsurf software, it continues to analyze each of the ship's resistance while the ships are on an even keel condition and during the trim condition. The variety used to decide which is the best trim for each ship uses 22 trim conditions (trim by bow and trim by stern) and 7 speed variations. After selecting the most optimum condition for each ship, then the fuel consumption and the total estimated exhaust gas produced by each ship can be measured.

Trim Variations and Speed Range
The trim variations are constrained by 2-meter trim by stern and trim by bow, with the addition of a trim condition that is limited by an Eq. 1 as follows [11]: The speed range is needed to determine at which speed the ship could efficiently cruise, despite the there is an addition to the resistance. The speed range is decided upon the service speed of the models that are going to be used for the research with a bit of bit addition to the speed range. It is decided that the speed range starts from 8 knots to 20 knots with the interval of 2 knots between each of the speed ranges.

Ship Resistance
The method of calculating the total resistance (RT) by Holtrop, because the hull type is U, is used in this study with the help of Maxsurf Resistance software. The data used for calculating the ship's resistance is based on the redrawn lines plans of each model so that this study can estimate the resistance as accurately as possible. The resistance calculated will vary even with the same model because the WSA and the ship's speed will impact the resistance experienced by the ship s hull. The total resistance consists of form factor, frictional resistance (RF), wave-making resistance (Rw), appendage resistance (RAPP), correlation allowance resistance (-RA), and air resistance (RAA); the formula for total resistance can be seen in Eq. 2 (2)

Ship Trim
The trimming of a ship happens when the forward draught (dF) value is not the same as the after draught (dA), resulting in the ship longitudinally uneven. This condition could affect many things in a ship because it modifies one of the most important parameters, that is, the draught of the ship. Ship trimming could be achieved through the shift of weight inside the ship. Ship trim could be defined through the following convenient formula (Eq. 3): (3) If the trim value is positive (+) it indicates the ship is trimming by stern, whereas if the value is negative (-) it indicates the ship is trimming by bow.

Ship Engine Power and Fuel Consumption
The power for the prime mover has to be specified first before defining the total fuel oil consumption of the ship. Several formulas are needed to calculate the Brake Horse Power (BHP) of the ship, and the first one is the Effective Horse Power (EHP). EHP can be calculated by multiplying the RT with the ship speed (VS); the Eq. 4 is as follows: (4) After the value of EHP has been determined, then the Shaft Horse Power (SHP) can be calculated. There are 2 types of formulas to specify the SHP the first one is to calculate SHP for ships with a single screw propeller, and the other one is for ships with twin screw propellers. SHP for single screw and twin-screw can be seen in Eq. 5 and Eq. 6, respectively. The Propulsive Coefficient (PC) usually ranging from 50% 70%, and this study uses 70% as its PC. (5) Lastly, the BHP of the ship can be determined by adding the SHP with a percentage of Sea Margin (SM), which ranges from 15% 20%, and then dividing the BHP with added SM with 0,85 as the engine margin is usually around 15%. The order of calculating the BHP for the ship is as follows: As for the fuel consumption, this study uses Specific Fuel Oil Consumption (SFOC) from the existing main engine data used by each of the models. The following equation (Eq. 9) is the formula for calculating the total fuel consumption: Where P is the power estimated for the main engine, S is the distance travelled by ship, and C is the constant addition of fuel value, usually ranging around 1.3 1.5, this study uses 1.5 as its C value. This study assumed the value of S by all of the ships as 1000 nautical miles; the purpose of selecting that value is so that the comparison of the fuel consumption can be on a more equal standing.

Exhaust Gas Emission
The emitted exhaust gas is only estimated during the cruising phase of the ships, that means during hoteling and maneuvering the exhaust gas is not calculated, because trim doesn t give a lot of impact during those two phases (i.e. hoteling and maneuvering). The following formula is used to estimate the total exhaust gas [12]: (10) Where FC is the total fuel consumption of the ship and EF is the emission factor of the pollutant. The emission factor used in this study can be seen inTable 4 and Table 5, as follow [12], [13]:  Table 4 and Table 5, all the emission factor unit is kg/ton. It can be seen that the pollutants that are going to be estimated are Nitrous Oxide (NOX), Non-Methane Volatile Organic Compound (NMVOC), Particulate Matter (PM), Carbon Monoxide (CO), Carbon Dioxide (C02), and Sulphur Oxide (SOX). As for SOX, in particular, the amount of SOX emitted is based on the Sulphur (S) content of the fuel. The Sulphur content for HFO is 3,5%, and 1,5% for MDO, are used in this study [14], [15],

Results and Discussion
The calculation for the ships' resistance, power, fuel, and exhaust gas are all correlated. Before going further into those calculations, the trim variations need to be established. The first constraint for the trim is that the variation will be limited to 2-meter trim by bow and 2-meter trim by stern, with the interval of 0.2-meter trim between each of the trim conditions. The second constraint, the limited trim, is limited based on the state of the ship (i.e., the LBP and GML), where Eq. 1 will be used. In Table 6, the LBP and GML for each ship used in this study are shown, which later can be used with the formula in Eq. 1 to get the second constraint known as the limited trim to get a more controlled and more narrow scope of discussion in this study.  Table 8 and Table 9 the total resistance of each ship for 3 types of vessel in every condition is specified with the help of Maxsurf, then the power needed for the ship to overcome the resistance to achieve the selected service speeds are calculated with Eq. 9. After that, the total fuel consumption in each condition is then measured for every model of the ship

Trim Optimization based on the Fuel Consumption
The results of using trim to modify the total fuel consumption are mostly generating fuel-saving conditions. However, results denoting fuel consumption increased compared to even keel condition when using some trim conditions. This study compared the total fuel oil consumption during trim with the even keel condition on every speed variation to obtain the most fuel-efficient condition for each ship. After that, the mean value of increasing/decreasing the fuel consumption in each trim condition with every speed variation is calculated.
As seen in Table 7, Table 8 and Table 9, the resistance of the ships analyzed are in every speed and trim variations, these result of resistance in each condition of the ship is then used to proceed to calculate the fuel consumption of each ship based on the resistance that the hull of the ship experienced. The result of the mean value of the fuel consumption in every condition can be seen in Table 10, Table 11, and Table 12, and those mean values are used to establish the curves in Figure 3,    Table 13 shows the most optimum trim to save fuel within the trim constraint shown in Table 6, and Table 14 shows the percentage of average decrease of fuel consumption by using the limited optimal trim with the ships service speed or the closest speed variations with the ships service speed. Ultimately, the average reduction of the fuel consumption for tanker, container and bulk carrier ships is 5.641%, 8.269% and 15.704%, respectively. As a result, the average reduction of the fuel consumption after trim optimization for the 3 types of ships is 9.871%.   The emitted exhaust gas by each ship is measured on the limited optimal trim condition, and then the results are compared to the emitted exhaust gas when the ships are in even keel condition. The percentage of the reduced exhaust gas by ships is shown in Table 15. The percentage decrease of NOX, NMVOC, PM, CO, and C02 is the same because it depends solely on fuel consumption and emission. Although SOX isn t the same as the other pollutant because it depends on the Sulphur content in the fuel, the amount of SOX emitted differs from the others. In the end, the total reduction of exhaust gas for tanker, container, and bulk carrier ships is 6.494%, 11.317%, and 13.775%, respectively. All in all, the average reduction of the exhaust gasses after trim optimization for the 3 types of ships is 10.529%.

Conclusion
By using 3 types of ships (i.e., tanker, container, and bulk carrier ships), this paper can conclude that trim optimization can give a good amount of reduction to fuel consumption and emitted exhaust gasses. Seeing from the results, there is no particular golden ratio for the trim of each ship, and that is caused by the hull form of each ship having differences between each other. Despite there are 3 types of ships, it can be concluded that a ship will likely have a decent fuel-saving condition by using the optimized trim condition, which will later impact the exhaust gasses produced by the ship. In this study, the average reduction in fuel consumption and exhaust gas for the 3 types of ships are 9.871% and 10.529%, respectively.
As the whole research is based on only certain types of ships with a limited variation of principal dimension, and at the time being there is no absolute benchmark for the result of this study it is advisable to wait for further study, specifically on the mean of decreasing fuel consumption and exhaust gas emission, so this study can be more reliable and well-versed as it can be more accurately verified by future works.