Power management of vessel propulsion system for thrust efficiency and emissions mitigation

Applied Energy 161 (2016) 124–132

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Applied Energy journal homepage: www.elsevier.com/locate/apenergy

Power management of vessel propulsion system for thrust efficiency and emissions mitigation Feiyang Zhao a, Wenming Yang a,b,⇑, Woei Wan Tan a,c, Wenbin Yu b, Jiasheng Yang a, Siaw Kiang Chou b a

Centre for Maritime Studies, National University of Singapore, Singapore 117576, Singapore Department of Mechanical Engineering, National University of Singapore, Singapore 117576, Singapore c Department of Electrical and Computer Engineering, National University of Singapore, Singapore 117576, Singapore b

h i g h l i g h t s
A mathematics model of ship propulsion system has so far been developed.
Involuntary ship speed drop was captured by propeller ventilation and resistance.
Power system step modulation was employed to save thrust loss and fuel efficiency.

a r t i c l e

i n f o

Article history: Received 8 June 2015 Received in revised form 22 September 2015 Accepted 1 October 2015

Keywords: Marine propulsion modeling Vessel voyage modeling Propeller ventilation Fuel consumption Pollutant gas emission

a b s t r a c t To meet the stringent gas emissions legislation in marine industry achieving green shipping, the ship operational behavior in actual sailing condition is one of the major concerns for designers and ship owners. In this study, the assessment of fuel consumption and pollutant gas emissions during a container ship operating scenarios was carried out by a hydrodynamic vessel movement model capable of representing the vessel propulsion behavior. The marine engine equipped with turbocharger as well as shafting system and fixed pitch propeller was included in vessel propulsion model by separated sub models connecting the required variables to each other. The propulsion system performance in calm water was well validated by a container ship seakeeping test published in 2003. When sailing encounters heavy weather, the severe ship motion induced by irregular waves bring the thruster very close to water surface, making propeller susceptible to ventilation and causing huge thrust loss. Step modulation strategy of power management system has been employed to save thrust loss and improve fuel efficiency in this study. It manages to save large thrust degeneration and along with benefit of thrust efficiency and emission mitigation, but at the expense of shortened sailing distance. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction Although shipping is already the most efficient form of bulk transportation, more pressure had been put forth toward the green shipping to limit environmental impact. Emission regulatory changes were motivated by International Maritime Organization (IMO) and national authorities for the limitation of nongreenhouse gas Nitrogen Oxides (NOx) and Sulfur Oxides (SOx), as well as greenhouse gas (GHG) Carbon dioxide (CO2), which is hoped to be reduced from existing vessels by 20–50% by 2050 [1]. To address this challenge, one mandatory scheme is the Energy ⇑ Corresponding author at: Centre for Maritime Studies, National University of Singapore, Singapore 117576, Singapore. E-mail address: [email protected] (W. Yang). http://dx.doi.org/10.1016/j.apenergy.2015.10.022 0306-2619/Ó 2015 Elsevier Ltd. All rights reserved.

Efficiency Design Index (EEDI) [2], which requires new designed ships to meet a certain level of energy efficiency from the outset. And the Energy Efficiency Operational Indicator (EEOI) [3] is a voluntary guideline allowing ship operators to monitor the operational carbon emissions per ton-mile. Thus a number of energy-efficiency improvement measures have been developed for marine vessels, complying with more stringent legislation. IMO suggested three basic approaches: the enlargement of vessel size, the reduction of voyage speed, and the application of new technologies. In the case of the Asia–Europe route, more Super Post-Panamax Ships (larger than 10,000 TEU) will be largely engaged via the Suez Canal or the trans-Pacific routes [4]. And it is expected the fleet should increase by 50% in terms of total capacity of this type of ships from 2014 to 2017

F. Zhao et al. / Applied Energy 161 (2016) 124–132

125

Nomenclature AE , Ao D Eg=kW h F % kW Hl I J k KQ , KT l mf _ m M n Ncyl p P PkW Q r rev cy R th

propeller expanded area, propeller disk area diameter (m) power-based emission factor (g/kW h) engine load factor (–) low calorific value of the fuel (J/kg) moment of inertia (kg m2) advanced ratio (–) adiabatic coefficient (–) propeller non-dimensional torque and thrust coefficients (–) stroke length (m) fuel mass injected into each cylinder(kg) mass flow rate (kg/s) system inertia matrix including added mass (kg) rotational speed (r/min) number of engine cylinders (–) pressure (bar) pitch of propeller (m) total installed engine power (kW) torque (Nm) moment arms (m) revolutions per cycle (–) gas constant (J/kg K) engine running hours (h)

[5]. In addition, the ultra large vessels satisfy lower attained index which means they have huge potential to meet EEDI standards [6]. Since the total resistance force during the sailing increases with the square of the ship speed, thus becoming an important portion of the total power consumption of a ship. From mid-2010 onward, most liners began to adopt slow steaming as their main operational strategy, benefited with CO2 emissions mitigation, but not always to reduce the operating costs [7]. In order to control Ocean Going Vessel (OGV) emissions near the coastline, the ports of Long Beach, Los Angeles and California Air Resources Board (CARB) had proposed a voluntary Vessel Speed Reduction (VSR) program wherein OGVs were asked to reduce their speed to 12 knots on arrival to and departure from the ports [8]. It was noted that VSR to 12 knots or less resulted in approximately 61% and 56% reduction in CO2 and NOx emissions. On the other side, there is substantive potential for fuel saving by optimizing ship hull forms and propellers, reduction of 5–8% are anticipated [6]. Changes in trim and draft can dramatically influence the wave profile and resistance. More emphasis is placed on reducing wetted surface as viscous resistance is a major contributor to overall resistance. Large amount of developments dedicated to improving hull-fluid interaction, either by changing the way fluid behaves or the wetted surface texture. For a given ship hull, propulsion devices should be designed to suppress detrimental flow hydrodynamics effect such as propeller hub vortex or bilge vortex [9]. As a result, the need to match propeller and engine best performance was made to achieve high propulsion efficiency in the normal wake field behind the hull [10]. Furthermore, to achieve both environmental clean and economical operation of the ship propulsion plant, lots of new engine designs and retrofitting had been introduced by engine manufactories. Ultra-Long-Stroke design of two-stroke marine engine with low-load tuning had been proved to be capable of great fueloptimized [11]. Due to advent of electronically controlled engine, lower fuel consumption can be achieved at lower and medium loads of low speed engine, while without penalty of NOx levels required for the MARPOL Annex VI Tier II compliance and lower

T Tp

m

V cyl Z

temperature (K) propeller thrust force (N) vessel speed (m/s) displacement volume per cylinder (m3) Blades number (–)

Greek symbol q density of sea water si ignition delay (s) s force vector (Nm) g efficiency (–) subscripts a air e engine f fuel fr friction g exhaust gas i indicated in inlet manifold out outlet manifold p propeller vl volumetric

CO2 emission than its mechanical counterparts [12]. De-rating is a traditional method of reducing fuel oil consumption [6,12], selecting an engine with higher Maximum Continuous Rating (MCR) than required and de-rating it to a lower MCR power that meets the designed performance of a ship. And the extra expense of extra power will be paid back due to significant reduction of fuel consumption. EEDI impact of a de-rated engine should be favorable since the fuel consumption goes down for the same power and speed used for the vessel. In addition, engine cylinders cutoff, turbocharger isolation or with variable geometry turbines and have been presented [6,12,13], which bring forth more efficient engines. Regarding to the systemic analysis of engine and ship performance, accompanied by environmental effect and engine life prediction coupled with an economic feasibility study, the optimized propulsion system could achieve great success in both journey and economic scenarios [14]. More stringent NOx and SOx requirement by IMO will be implemented in Emission Control Area (ECA) zones: 0.1% fuel sulfur content limitation from 2015 onward and 80% NOx reduction limits in 2016. It appears that it is practical to add some treatment systems to clean emissions rather than solely making adjustments to the engines. The most likely used treatment systems to minimize NOx emissions are exhaust gas recirculation (EGR) which recirculates exhaust gas to the engine intake manifold, and selective catalytic reduction (SCR) reactor, in which exhausted NOx is decomposed by ammonia catalytically into nitrogen and water [15]. Operating low sulfur fuel like Marine Gas Oil (MGO) or Marine Diesel Oil (MDO) is an effective way to reduce SOx. Alternative clean fuels such as Liquefied Natural Gas (LNG) can be used for ship propulsion and auxiliary dual-fuel engines, thus significantly reducing CO2, NOx and SOx [16,17]. Even for modern efficient diesel engine, only about 50% of the energy generated by fuel combustion being converted to mechanical energy, there are still a large amount of waste heat and of which about 25% is discharged through the exhaust gas. Waste Heat Recovery System (WHRS) was proposed aiming to utilize the waste heat to generate steam for heating or electrical power

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generation and it was possible to generate an electrical power output of up to 11% of the main engine power by utilizing the exhaust gas energy in a WHRS [18]. This system is attracting much attention from ship-owners by helping meet even tighter EEDI requirements in terms of saving fuel consumption and reducing CO2 emissions [19]. Besides improving the design of ship propulsion system, there are increasing concerns about ship operational control and optimization during actual sea conditions to minimize the fuel consumption and CO2 emission. Thruster allocation control [20], thruster toque and power anti-vibration control [21] have been drawn for fuel saving in waves. Ship weather-routing is defined as a procedure to determine an optimal route based on the weather forecasts and ship hydrodynamics motion [22,23], but generally ignoring the operational limits due to excessive ship’s motion and accompanying dangerous phenomena in the elaboration of ship performance curves. However, since the ship propulsion system will operate in a variety of conditions throughout the ship lifetime on the ship resistance variation and the engine degrading. From vessel maneuvering to full cruise, the main engine load changes from 10% up to 80%. Although the whole operating scenarios of diesel engine could be derived by numerical model [24,25] in aspects of fuel consumption and delivered power under varying loads, the dynamic response of engine in seakeeping voyage still under concerned about toward green shipping. When the ship encounters storm or heavy sea, the propeller works in a very hostile environment, the engine speed should be slowed down to reduce non-useful output work and ensuring the sailing safety. The propeller and engine form a strongly coupled system to determine vessel speed. Therefore the engine response in waves should not be separated while evaluating fuel consumption and emissions in the actual voyage. Basis of the ship’s hydrodynamics model is to a force balance equation relating ship resistance, ship dynamics and dynamic ocean environment. In the present study, a marine vessel voyage model was integrated to the ship hydrodynamics model with two-stoke marine engine and fixed pitch propeller system. The packed vessel voyage model was capable of representing vessel propulsion behavior, also with dynamic propeller ventilation effect induced by wave irregular motion. Interaction with dynamic wave motion, the involuntary drop of ship speed was detected due to ship resistance and propeller ventilation phenomenon, it caused severe thrust efficiency loss in heavy sea conditions. Therefore modulation of power management system was employed to save thrust loss and improve fuel efficiency in this study, which allowed fuel supply self-tuning by intelligent engine speed governor module. It provided feasible prediction of fuel consumption and discharged emission through the whole shipping network.

Fig. 1. Schematic of engine main components.

For a direct injection marine diesel engine, the degree of fuel/air mixture homogeneity is very important to determine the thermal efficiency and fuel consumption. In this cycle mean value model, the indicated thermal efficiency gi is expressed as a function of excess air ratio based on the measured data from MAN Diesel & Turbo Corporation [28]. The fuel mass flow rate is calculated by the variation of the mass of injected fuel per cylinder and per cycle controlled by fuel pump rack position. Thus the fuel mass rate m_ f and air mass flow rate m_ a are calculated:

m_ f ¼ mf Ncyl ne =ð60rev cy Þ m_ a ¼ gv l pin V cyl N cyl ne =ð60Rrev cy T in Þ

ð1Þ ð2Þ

The engine output torque Qi is derived using engine indicated thermal efficiency, fuel mass flow rate and rotational speed:

Q i ¼ 30gi Hl mf =ðpne Þ

ð3Þ

In this study, the friction loss torque Qf due to piston reciprocating motion could be expressed as a function of friction force and engine speed:

Q fl ¼ 1000Ncyl V cyl ½75 þ 0:048ne þ 0:4ðne l=60Þ =4p 2

Therefore, the engine shaft rotation speed ne could be calculated by applying the angular momentum conservation in propulsion plant system:

dne 30 Q i ðt  si Þ  Q f  Q p  ¼ dt p I e þ Ip

si ¼ 60rev cy =Ncyl ne 2. Vessel main propulsion modeling 2.1. Marine engine modeling Fuel consumption is directly related to marine engine operation conditions. Normally, the larger container ships are equipped with two-stroke marine engine with speed lower than 130 rpm [26] as the main engine, delivering power to propeller. And a four-stroke marine engine is commonly used as auxiliary engine responsible for all onboard power or off-loading equipment. In this study, the marine engine is modeled using a cycle mean value model approach [27] in conjunction with differential equations for the fast transient power plant performance calculation of the engine crankshaft speed and delivered power. The thermodynamic and flow dynamic process in engine operation are taken into consideration. The main components of the engine are shown in Fig. 1.

ð4Þ

ð5Þ ð6Þ

The variance of flow mass, temperature and pressure through intake and exhaust manifold are expressed based on volume dynamic as follows:


 dT out dmg mg ¼ m_in kT in  m_out kT out  T out dt dt dmg ¼ m_in  m_out dt dpout kR ðm_in T in  m_out T out Þ ¼ Vg dt

ð7Þ ð8Þ ð9Þ

After the exhaust manifold, the exhaust gas is expanded in the turbine and then drives the compressor. The compressor and turbocharger is modeled using its steady-state performance map, given the compressor pressure ratio, the turbine mass flow rate and efficiency calculated using interpolation.

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2.2. Propeller modeling If there is no gearbox between the two-stroke marine engine and propeller, the propeller rotation speed equals to the engine speed. When the engine delivered power and operation speed are known, the propeller torque Qp and thrust Tp can be calculated using the dimensionless coefficients:

Q p ¼ K Q qn2p D5p

ð10Þ

T p ¼ K T qn2p D4p

ð11Þ

where KQ and KT are the non-dimensional thrust and torque coefficient which could be obtained using the interpolation polynomials for Wageningen B-screw series in first quadrant [29]. These coefficients are described by the following parameters:

K T;Q


 P AE ¼ f J; ; ; Z D Ao

ð12Þ

Then the mathematical modeling of the ship propulsion plant was implemented in Matlab/Simulink environment, as shown in Fig. 2. Incremental PID engine governor model is utilized to control the fuel injector rack position, according to the deviation of target engine speed and feedback calculated value. When the propeller load is changed suddenly due to the wave fluctuation or mechanical accident, the self-tuning of fuel supply could be achieved, maintaining the steady engine speed in order to protect the engine integrity during fast transients. 3. Ship-wave interaction modeling Fuel consumption prediction needs to be related to engine response, as well as the ship’s hydrodynamic behavior under different sailing conditions. Since a surface vessel may move in six directions, six independent coordinates are defined to determine the vessel’s position and orientation. The first three coordinates describes the position in terms of translational motions (surge, sway and heave), which roll, pitch and yaw in orientation and rotational motion are the last three coordinates, as shown in Fig. 3. The vessel speed v is given by the motion equation with fluid memory effects [30]:

M m0 þ C RB m þ C A mr þ Bmr þ Gg ¼ s þ sH

ð13Þ

[32] with the combination of back stepping control and approximation-based adaptive technique allowing the proposed controller to accommodate certain faults in the plant and the controller itself, thus handling time-varying hydrodynamic disturbances. The control forces and moments in six degree of freedom  T corresponding to the force vector f ¼ F x ; F y ; F z can be written as [30]:

2

s¼



f rf



Fx

3

7 6 Fy 7 6 7 6 7 6 Fz 7 ¼6 6F l  F l 7 y z7 6 Zy 7 6 4 F x lz  F z lx 5

ð14Þ

F y lx  F x ly In our study, only the main propeller aft of the ship hull is employed as actuator, producing a longitudinal force F x ¼ F. Thrust propellers are used as maneuvering, dynamic positioning and main propulsion units as well and very often works in heavy sea conditions. When the ship operates in high sea with severe motions, it will cause high amplitude of propeller relative motions. If the propeller operates too close to the water surface, or the submergence of the propeller become small, the localized low pressure created by the blades can draw air under the water and then cause ventilation effects. If the propeller loading is sufficiently high with higher rotation speed, the low pressure on the propeller blades could also create a funnel through which air is drawn from the free surface, thus ventilating the propeller as well [33]. For a heavily loaded propeller, ventilation may lead to an abrupt thrust loss as high as 70–80%, and high thrust loss combined with wave-frequency cyclic variations in propeller loading may cause server mechanical wear and tear of the propulsion units. Fig. 4 shows the measured results [34] of the nondimensional propeller thrust force versus relative propeller submergence h/R and relative shaft speed n, where h is the propeller shaft submergence, R is the propeller radius and shaft speed n equals to engine operation speed assuming no transmission gear between the engine and the propeller. Based on these results, a generic thrust loss model capturing the main characteristics of ventilation may be formulated, with ship position and orientation simulation in MSS module determining propeller submergence.

66

where M 2 R is the sum of the system inertia matrix and the added mass matrix. CRB 2 R66 is the Coriolis–Centripetal matrix. CA 2 R66 is the constant infinite frequency added mass matrix. mr = m–mc 2 R66 is the relative velocity between vessel velocity m and sea current velocity mc. B 2 R66 is the constant infinite frequency potential damping matrix. G 2 R66 is the restoring matrix. s 2 R66 is the control force vector produced by the propeller system. sH 2 R66 is a vector of time-varying hydrodynamic forces. All the detailed definition and computation of above matrices and vectors could be found in reference [30]. The Marine System Simulator (MSS) as reviewed in Perez et al. [31], which is a Matlab/Simulink library developed by Norwegian University of Science and Technology, is employed as the platform for vessel hydrodynamic studies. In recent years, there has been a significant interest of ship hydrodynamics models in relation to the time-domain models for simulation and control system design based on data obtained from seakeeping programs, and unified models for maneuvering and seakeeping. Basis of the ship’s hydrodynamics model is a force balance equation relating ship resistance, ship dynamics and dynamic ocean environment. It treats environmental loads such as wave, current, and other hydrodynamic forces and moments as timevarying hydrodynamic disturbances acting on the vessels. In this study, the ship hydrodynamics model has so far been developed

4. Vessel propulsion model validation Based on the full seakeeping tests carried out on a 23,400 dwt Hollandia container vessel by the Ship Hydromechanics Laboratory of the Delft University of Technology and Lloyd’s Register of Shipping in London [35], the numerical results from the vessel propulsion model in terms of engine output power and specific fuel consumption rate in calm water are validated with measured results. In this section, the attainable ship moving speed is calculated from the equilibrium of ship inertia, calculated total resistance by Holtrop method [36] and required propulsion. Fairly well agreements between predictions and experiments have been found in Fig. 5. 5. Results and discussions In this study, a typical feeder container ship S175, with length of 175 m and weight of 24,610 ton is simulated along a desired trajectory in the presence of time-varying hydrodynamic disturbances using the model described above. The MAN 6S60ME two-stroke marine engine is equipped for this vessel as the main power plant [10,37] with one turbocharger unit. The maximum output power is 14,280 kW, considering 15% sea margin and 10% engine margin for

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Fig. 2. Layout of marine propulsion model.

Fig. 3. The schematic diagram of 6 DOF motions for a ship. Fig. 4. Experimental results [29] showing non-dimensional propeller thrust force versus relative propeller submergence h/R and relative shaft speed n.

fouling ship hull and heavy weather, in order to satisfy the maximum serving speed of approximate 20 knot for this type of container ship. The main engine specifications are given in Table 1. 5.1. Vessel speed estimation in different sea states The total added resistance during the sailing can be further decomposed into many components, such as the added resistance due to the current, different fluid layers, ice loading, waves, wind, and so on. And in the present study, only anticipating the added resistance due to waves, since it is the main phenomena experienced at sea. As a consequence of the added resistance, the ship speed is often lower than the service speed in calm water, it is

so-called involuntary speed reduction. In this section, the speed loss is predicted not only taking into account the added resistance effect, but also the thrust loss effect due to propeller ventilation phenomenon. The mean value of ship speed is obtained as averaged value during the time spent on specific sea state. Prediction of vessel speed at several sea states are given in Fig. 6. The results with black squares are from Kwon’s regression model of speed loss in rough weather condition [38] .The blue1 and red dots present the numerical results by the vessel voyage

1 For interpretation of color in Fig. 6, the reader is referred to the web version of this article.

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Specific fuel consumption / (m3/kWh)

Engine output power / kW

20000

Measured data Simulation results 15000

10000

5000

0 0

5

10

15

20

25

Vessel speed / knot

0.0005

0.0004

0.0003

0.0002

0.0001

Measured data Simulation results

0.0000 0

5000

10000

15000

20000

Delivered engine power Pe / kW

Fig. 5. Validation results of engine power, fuel consumption rate and vessel speed in calm water condition.

Table 1 Specifications of the marine diesel engine. Engine Bore Stroke Number of cylinders Maximum continuous rating (MCR) Engine speed (100% of MCR) Specified fuel oil consumption (100% of MCR) Turbocharger type

6S60ME 600 mm 2400 mm 6 14,280 kW 105 rpm 168 g/kW h Conventional T/C

25

Vessel speed / knot

20

15

10 Regression model

5

Vessel voyage model without ventilation effect Vessel voyage model with ventilation effect

0

1

2

3

4

5

6

7

Significant wave height / m Fig. 6. Average ship speed at different sea states.

model established in our study with and without propeller ventilation effect, respectively. The involuntary drop of speed could be captured in several wave height sea states under the assumption that the equipped marine engine is able to provide constant power output under different weather conditions. It could be noticed that the vessel speed is sensitive to wave height. Significant speed drop happens due to propeller ventilation effect. It means ventilation strongly affect the attainable speed, and the results taking into account of ventilation gives good agreement in involuntary speed loss compared with regression model. 5.2. Fuel consumption estimation in dynamic irregular waves 5.2.1. Rough wave detection Rough sea is the main factor resulting in propeller ventilation phenomenon. When vessel runs into bad weather, the sever ship

motions induced by irregular waves bring the thruster very close to water surface, making propeller susceptible to ventilation and causing huge thrust loss. The ventilation is dynamic and is not probability repeatable from one revolution to the other. It is an random phenomenon. So an implantable solution is to use the thrust loss estimation and rough sea detection schemes as the criteria of ventilation happens. Once the negative impact is detected, the modulation of operation and sailing strategy would function to avoid deteriorated thrust efficiency. As described in ventilation test results performed in Fig. 4, when the propeller is fully submerged, increasing engine speed is conductive to produce more thrust force. However, if the propeller submergence becomes small, it leads to gradually decreasing thrust force in case of low propeller loading due to loss of effective disc area, but a sharp thrust loss for high propeller loading because of air suction ventilation. At that moment, booting speed cannot generate more thrust force due to huge loss, thus means much fuel is wasted rather than into useful work. In that case, only reducing shaft speed is helpful for thrust compensation instead of accelerating. Otherwise, it will cause non-useful output work with more fuel consumption. And as suggested in ref [39], the ventilation detection limits could be set at the value when thrust force below 60% of maximum force and end when thrust recovers to 90% of maximum value. In the ship-wave interaction model implemented in MSS platform, International Towing Tank Conference (ITTC) spectrum is used to describe waves. Based on hindcast statistical results of rough wave observations off the US west coast [40], if the wave height becomes twice higher than the significant wave height, then rough wave happens. Besides, wavelet transform approach was commonly used to analyze wave energy distribution, which helps to detect irregular wave. As shown in Fig. 7, the water level is one of 25 cases derived from a data buoy off the coast of Hualien in Taiwan [41], and wavelet scalogram of this wave period shows the energy distribution. Strong energy density in the spectrum appears to surge instantly at the onset of the freak/rough wave, with the red color line denoting the corresponding frequency at the maximum energy density.

5.2.2. Voluntary ship speed loss in rough sea When the vessel comes across extreme weather or storm, the master will probably decide to change the engine power to avoid severe consequence by excessive motion. In most of cases the jeopardized excessive motion related to slamming, excessive gravity accelerations and propeller in-and-out-of water. The decision that

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Fig. 7. Wavelet scalogram of a period of freak wave.

one master will take under specific circumstances is very subjective and can greatly depend on experience and potential danger estimation. Once the waves reach a certain height, slowing down the speed can make less strain on the hull and keep the speed match the speed of the waves at an appropriate angle to the wind. As mentioned above, in order to save unnecessary fuel consumption due to the thrust loss by propeller ventilation, reducing shaft speed is more likely to booting thrust efficiency rather than accelerating. Otherwise, it will cause non-useful output work with more wasted fuel consumption. Hence, the slowing down process is so called voluntary speed reduction, which is motivated in terms of propeller ventilation. The voluntary ship reduction process is a quite subjective decision. Some master/captain tend to immediately slow down and to reach very low speed as soon as possible. Others would like to slow down step by step and monitor how the ship behaves. However, in this study it could only be taken into account as some average behavior like step modulation of vessel propulsion system when propeller ventilation is detected. As explained in [30], the sea state depends on the wave spectrum and significant wave height. To imitate rough sea with large waves, the ITTC wave spectrum with significant wave height of 5 m and peak frequency of 0.56 rad/s is used in this study. Step modulation of power management is utilized to reduce the thrust loss and improve fuel efficiency. Because the thrust loss combined with wave-frequency cyclic variations in propeller loading may cause server mechanical wear and tear of the propulsion units. When the freak wave and ventilation is detected, the engine operation speed is triggered to reduce 20%, then keeps at this speed for a certain time to observe the ship behavior. If the thrust loss is still lower than 0.6, the modulation is carried on further until the thrust loss becomes higher than 0.6. Fig. 8 gives the modulation process of the engine speed according to the thrust loss in a certain sea state. During calm water voyage, the engine speed is accelerated to reach full cruise state at the engine load of 75%, and the propeller is fully submerged with h/R ratio higher than 1.5 and there is no thrust loss observed in this period. When the ship severe motion begins, the propeller submergence became fluctuated in higher amplitude, sometimes even very close to water surface. At that time huge propeller thrust loss is detected and the engine speed is triggered to modulate automatically step by step to minimize the thrust loss. When the heavy sea is terminated, the engine speed is recovered to full cruise state. It is noted that after modulation, even for the case of propeller very

3.0

h/R 2.5 2.0 1.5 1.0

thrust loss

0.5 0.0

n/np 0

200

400

600

800

1000

Time (s) Fig. 8. Engine speed modulation process under certain sea state.

Fig. 9. The overall modulation of engine speed during a certain sea.

close to the water surface as the result of high fluctuation of vessel motion, the thrust loss is mitigated apparently. The overall modulation process of the engine speed during certain wave height is described based on the ventilation test results as described in

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h/R

3 2

Thrust loss

Thrust loss

1 0 1.0 0.5 no modulation

0.0 1.0 0.5 with modulation

0.0

0

200

400

600

800

1000

Time / s Fig. 10. Comparison of thrust loss for the cases with and without modulation.

10

w/o ventilation with ventilation+engine modulation 1.0

6

NOx

Ship speed / (m/s)

8

0.5

4

0.0 2

0

0

200

400

600

800

1000

Time / s

CO2

1.0

0.5

Fig. 11. Voluntary ship speed reduction process due to modulation.

0.0

w/o ventilation with ventilation+engine modulation

0.8

-22.2%

0.6

0.5

0.4 0.2 0.0

Fuel Consumption

1.0

SOx

Distance

1.0

0.0 3

6

5

3

6

5

7

Hs / m

7

Fig. 13. Comparison of emissions under different sea states for the cases with and without modulation.

1.0 fuel save

0.8

-45.5%

0.6 0.4 0.2 0.0

3

5

6

7

Hs / m Fig. 12. Comparison of fuel consumption under different sea states for the cases with and without modulation.

Fig. 9. After modulation, the engine keeps at relative low speed but less thrust loss is found. Fig. 10 compares the thrust loss with and without power management modulation. It can be seen that with power management

modulation a thrust loss reduction of as high as 40% could be achieved under this sea state. Meanwhile, when the engine speed is modulated to reduce, it means less power delivered to push the vessel which makes the vessel speed slowed down involuntarily. After the bad weather, the vessel power management system was modulated back to full cruise state, then the vessel speeds up again as seen in Fig. 11. 5.2.3. Fuel consumption and emission benefit due to voluntary ship speed reduction Modulation process is not only for propulsion efficiency benefit, but also favors for fuel consumption and emissions. When the predicted power delivered by the engine and the ship speed in a rough

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sea are known, the corresponding fuel consumption and sailing distance will be obtained. Fig. 12 compares the results of fuel consumption under the conditions with or without power management modulation. For the case of the most adverse weather condition with wave height of 7 m, due to the voluntary ship speed reduction by modulation, the sailing distance is shortened by 22.2% at a given period, but almost half fuel is saved. Fuel saving could be achieved at the expense of shipping liner delayed. This is the dilemma of maximum fuel saving and minimum delay facing optimization. Based on the emission inventory of main engine test during container ship at sea [42], the NOx, CO2 and SOx gas emissions could be predicted based on the engine power. The calculated emissions are given by the following equation [26]:

Z Emissionskg ¼ 0

th


PkW  F % kW  Eg=kW h 

 1 dt h 1000

ð15Þ

If ventilation effect is ignored and without power management modulation, the engine keeps at the same operation speed under different sea states, thus the same magnitude for each type of emission. When propeller ventilation and engine modulation are integrated, emission mitigation benefit is achieved especially for extreme rough sea condition as shown in Fig. 13. 6. Conclusion Aim to reduce the fuel consumption and gas emission during a container ship operating scenarios, a mathematical model of the overall ship propulsion plant, implemented in the hydrodynamic vessel movement model under the Matlab/Simulink platform was developed. The packed vessel voyage model was capable of representing vessel propulsion behavior, also with dynamic propeller ventilation effect induced by wave irregular motion. According to the thrust loss caused by propeller ventilation, modulation of power management was motivated in order to avoid deteriorated thrust efficiency. Self-tuning of fuel supply during modulation could be achieved by intelligent engine speed governor module. It was found that vessel speed involuntary reduction was more sensitive to propeller ventilation rather than added resistance especially for very rough sea cases. Because the thrust loss combined with wave-frequency cyclic variations in propeller loading may cause huge thrust loss and severe mechanical wear and tear of the propulsion units, so step modulation strategy of power management system was motivated to reduce the negative impact. In this study, it managed to save up to 40% thrust loss at a certain sea state. Benefits of thrust efficiency, fuel saving and pollutant emission mitigation were obvious by power modulation as well, but at the expense of shortened sailing distance at a given time due to voluntary vessel speed reduction. Acknowledgements This study is supported by the research project ‘‘Analysis of Energy Consumption and Emissions by Shipping Lines” funded by Singapore Maritime Institute. References [1] IMO. Second IMO GHG study 2009. London: International Maritime Organization; 2009. [2] IMO. MEPC.1/Circ.681. London: International Maritime Organization; 2009. [3] IMO. MEPC.1/Circ.684. London: International Maritime Organization; 2009. [4] Panama Canal Expansion Study. US department of transportation maritime administration report; 2013.

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