Performance Comparison of Different Propulsion Schemes for a Polar Carrier

JI Shao-peng,TIAN Yu-kui

(Laboratory of Ice Tank,China Ship Scientific Research Center,Wuxi 214082,China)

Abstract:Polar ships eventually need the assessment of their performance both in open and ice-infested waters.The authors carried out performance evaluations of a polar carrier in open and ice-infested waters and icebreaking capability in the three cases of its propulsion scheme,i.e.fixed pitch propeller(FPP),controllable pitch propeller(CPP)and duct propeller(DP),and then compared their performances.Lifting surface theory was applied to estimate their optimum propeller parameters such as diameters and pitch ratios,etc.The delivered power in full scale was estimated by the balance of net thrust of propeller and resistance of the ship,where interactions between ice and propeller and suction effect of propeller behind the ship were taken into account.The icebreaking capability and power performance were derived from an interpolation method based on given powers of the main engine.The comparison of the performances indicates that DP scheme has the highest icebreaking capability.On the other hand,it presents the worst performance in open waters among the three propulsion schemes.

Key words:ship performance;propulsion type;icebreaking capability;open water performance;polar carrier

0 Introduction

Power performances are substantially different between the conventional ships and the polar ships,due to the marked difference of navigation conditions between them.At present,taking account of the case when a ship is trapped in ice,ship owners would prefer a pod type propulsor to conventional ones for polar navigation,as a result of its high manoeuvrability and excellent icebreaking capability.However,its high cost often decreases their desires for the pod propulsion,and as consequence,they adopt the conventional ones for polar ships.

The authors carried out performance evaluation of a polar carrier in open and polar waters and icebreaking capability in three cases of its propulsion scheme,i.e,fixed pitch propeller(FPP),controllable pitch propeller(CPP)and duct propeller(DP),and compared their performances.Firstly,the propulsion performances of these propulsion schemes were analyzed.Secondly,structure strength of the propulsion schemes was checked in evaluation according to the regulations of a classification society[1-3].Finally,comparisons of the power performances between three propulsion schemes were made,by carrying out calculations in detail,which will hopefully provide a good reference for ship owners.In this study,the effects of flexural strength of sea ice could not be examined.

1 Hull form and propulsion type

1.1 Development of hull form

Hull form characteristics of polar ships are considerably complex.With this kind of hull form ships can safely sail in polar water in summer and/or autumn season(s)through thin oneyear ice with old ice inclusions,and,at the same time,can do in ice free region,naturally.Double Acting Vessel(DAV)was developed for navigation in polar water,which was a brilliant success of innovation in icebreakers.It also brought about the new operation modes in polar region.Through a number of operations and applications of the large power pod propulsion[4],it was found that the pod propulsion had much higher propulsive efficiency when the stern with pod(s)moved forward in ice.Accordingly,DAVs gradually prevail in the commercial shipping market.

From 1990 to 2002,DAV technology was widely adopted by design companies onto ships various in kind[5],such as buoy tender,product tanker,icebreaker,icebreaker supply vessel,multi-purpose icebreaker and tanker,etc,and the power of the pod unit increased 1.3MW to 16MW up to the present.The pod might have attained a position of main propulsion scheme in the case of double and triple shafts as well.

1.1.1 Oil tanker of ice region developed by AARC of Finland

AARC designed a type of oil tanker,with ice class of IA SUPER.The DAV with two 360°azimuth units of 7.6 m diameter propeller,was designed.The displacement was approximately 106 k tones.This tanker had the length of 252 m,breadth of 44 m,design speed of 15.1 kns and total propulsive power of 38 940 kW,and was in service to transport oil in Baltic Sea and Pechora Sea.The two ships,the ‘Tempera’ and ‘Mastera’,were built by Sumitomo Heavy Industries Ltd.of Japan.

1.1.2 Oil tanker of ice region developed by Stena Bulk of Sweden

The company of Stena Bulk designed several types of oil tankers.For example,B-Max type with IA ice class,P-Max type with IB ice class,Aframax type with IA SUPER ice class and Panama type with IA ice class.

Oil tanker,the ‘Stena Arctica’,was constructed by HHI,with IA ice class,fixed pitch propeller of 7.382 m diameter and displacement of 117 k DWT.The ship has been in service to transport crude oil in Finland Bay.The main parameters are as follows：length overall of 249 m,breadth of 44 m,design speed of 15.0 kns and total propulsive power of 16 660 kW.

The oil tanker of B-Max type had the displacement of 200～266 k DWT,ship length of 325.5 m,breadth of 66 m,design draught of 15 m and icebreaking capability of ice thickness of 1 m at speed of 5 kns.

Furthermore,an oil tanker of P-Max type was also developed,with the displacement of 54 k DWT,length of 182.9 m,breadth of 40 m,draught of 11.3 m,design speed of 16.5 kns at scantling draught,and icebreaking capability of 0.6 m ice thickness.

1.1.3 Oil tanker of ice region built by Hyundai Heavy Industries(HHI)

The oil tanker of Aframax type,‘Moskovsky Prospect’was constructed by HHI,with IB ice class,displacement of 113.9k DWT and propeller of 7.2 m diameter,and operated by Sovcomflot Company,being in service in Baltic Sea and Russian Far East Region.The ship registered DNV Classification Society,as of Deice-C and Winterized cold from-20℃ to-30℃.The ship had length of 249.99 m,breadth of 44 m,design speed of 15kns and total propulsive power of 14 280 kW.

1.2 Propulsion type analysis for ice region environment

Conventional propulsion systems with FPP and CPP,and DP(in general and including DAV)were examined as the main propulsor of a ship,navigating in ice-infested waters.

For FPP,as its design load is fixed,the propulsive efficiency is poor in ice region,while CPP is able to adjust its load according to power of the main engine under various working conditions,which also help to extend the life of the main engine.In order to avoid damage of the pitch control mechanism from ice load,it is necessary to increase its overall strength.

DAV is a unique propulsion system.In ice-infested waters,broken ice pieces threaten the propeller.In the case of DAV,surrounded duct could effectively reduce ice-contact frequency and ice load as well,and improve the safety of the propulsion system.

Since conventional propeller-rudder system could not provide satisfactory maneuvrability in ice-infested waters,DAV has been adopted on this point of view,providing nicer steering and astern performance,even at the low speed range.

Rapid development and improvement in the electric propulsion system could provide a few versions of combination of pod(s)and conventional propeller(s)for the propulsion system of polar ships,for instance,combination of pod at the center shaft and CPPs at the wings.

A long time ago,some icebreakers installed a propeller at the bow.It can push water down under ice so that ice buoyancy against gravity force effectively breaks ice sheet,but when the ice is thick,the bow propeller rapidly loses its function and even seriously damaged.This design idea had a short history and has not been widely applied to polar ships.

2 Evaluation methods

Ice resistance owns expression form as below[6]：

where CBRis icebreaking resistance coefficient,CBis submersion resistance coefficient,CCis sliding resistance coefficient,B is breath of ship at draught,T is draught,hiis ice thickness,ρiis ice density, Δ ρ is density difference,g is gravity acceleration,V is ship speed,is strength coefficient,is Froude number of ice thickness.

It can be seen that the ice resistance is influenced by the ship speed,ice thickness and flexural strength,where the ice resistance is as a function of velocity,at constant ice thickness and flexural strength,and the effect of speed is nonlinear,it can be expressed quadratic polynomial equation.

where a0,a1,a2are polynomial coefficients,Viis ship speed in full scale.

Similarly,the nonlinear relationship[7]can be given between ice resistance and thickness,at constant ship speed and flexural strength.The ice resistance increases approximately as the square of the ice thickness.

where b0,b1,b2are polynomial coefficients,Hiis ice thickness in full scale.

The resistance at different speeds in open water condition can be expressed as a cubic polynomial equation.

where c0,c1,c2,c3are polynomial coefficients,Vjis ship speed in full scale.

The hydrodynamics performance of thruster can be predicted by the lift surface theory of propeller.It can be expressed as our degree relationship between thrust,torque and revolutions,at constant advance speed.

where d1,d2,d3,d4,d5,e1,e2,e3,e4,e5,g1,g2,g3,g4,g5,h1,h2,h3,h4,h5are polynomial coefficients.NSis propeller revolution in full scale.

In evaluation,it must consider the suction effect of propeller and interaction between ice and propeller when the thrust is used to calculate the delivered power.The thrust deduction t is utilized to assess thrust loss,which has different values at ice region and open water,and it mainly depends on experiences of the designer and statistical analysis of model test.

The thrust deduction t is estimated in calculation：

At ice-infested water At open water

t=0.06 for FPP t=0.26 for FPP

t=0.08 for CPP t=0.28 for CPP

t=0.12 for DP t=0.32 for DP

The net thrust of thruster in ice or open water is given by

According to balance of resistance and net thrust,the following equations are acquired by：

The Eqs.(10)-(12)are solved by the Newton iteration method to obtain revolutions of propeller in ice or open water.

The delivered power of propeller

Substituting Eqs.(6)and(8)into Eq.(13)：

The delivered power of thruster is obtained by actual propeller revolutions from Eqs.(10)-(12).

Thus when the main engine power is given,the ship speed can be precisely predicted by the interpolation method.In calculation,70%MCR is utilized to interpolate the ship speed.For the purpose of detailed comparing power performance of the FPP,CPP and DP,the main engine power is divided into three grades.

where PMCRis power of main engine,the shaft efficiency is equal to 1.0,as shown above.

The relationship between icebreaking thickness and icebreaking speed at the same delivered power can be linearly expressed from

where m,n are coefficients of linear equation.

Thus when the main engine power is given,the icebreaking capability can be accurately predicted by the interpolation method.In calculation,100%MCR is utilized to interpolate the icebreaking speed and icebreaking thickness.In order to detailed compare the icebreaking capability of the FPP,CPP and DP,the main engine power is also divided into three grades.

PMCR=28 MW,PD=PMCR×1.0=28 MW

PMCR=30 MW,PD=PMCR×1.0=30 MW

PMCR=32 MW,PD=PMCR×1.0=32 MW

3 Performance comparison analysis for different propulsion schemes

In this study,the performance of a polar carrier,as an example,was evaluated at the conditions of the maximum continue rating(MCR)and continuous service rating(CSR)based on the balance principle of net thrust and resistance,for three different propulsive schemes,FPP,CPP and DP.

A wooden ship model was made for the polar carrier,with the twin screw propulsion,at a scale ratio of 35.The model tests were carried out in ice tank and towing tank,in the conditions of the level ice with the flexural strength of 500 kPa in full scale and open water,respectively.Under a range of speeds from 0 to 4.0 kns,the ice resistance tests were performed in the condition of the full load draught,with twoice thicknesses,1.25 m and 1.55 m.In addition,under a range of speeds from 10 to 18 kns,the resistance and propulsion tests in open water were carried out at its conventional towing tank,mounted a pair of stock propellers,at the full load draught,as shown in Fig.1.The main particulars were listed in Tab.1.

Tab.1 Hull form principals

Fig.1 Model test in ice and open water

The model test results were given in Fig.2.The predictions of ice resistance and open water resistance in full scale were utilized to design three new propellers for three different propulsive schemes,with the aid of lifting surface theory of propeller,and to assess the delivered power of the polar carrier.

Linear approximations would be applied to the relationships between the ship speed and the resistance,etc,within the relatively narrow range of the ship speed.

Fig.2 Results of model test

3.1 Evaluation results for Fixed Pitch Propeller(FPP)

For FPP scheme,for the given rotations of the main engine at 86 r/min and 82 r/min corresponding to the MCR and CSR,the optimum propeller parameters,including diameter of 7.55 m and pitch ratio of 0.910,were examined.And then,at several speeds of the ship,the delivered powers of the polar carrier were calculated,for two ice thicknesses of 1.25 m and 1.55 m,and in open water conditions.The evaluation results for the ice thickness of 1.25 m 1.55 m were shown in Fig.3.The evaluation results for open water condition were shown in Fig.3.The results of the performance prediction were listed in Tab.2.

Tab.2 Performance prediction with FPP scheme

Fig.3 Evaluation results for two different ice thicknesses and open water

3.2 Evaluation results for Controllable Pitch Propeller(CPP)

For the CPP scheme,for the given revolution of the main engine,at 86 r/min and 82 r/min corresponding to the MCR and the CSR,the optimum propeller diameter of 7.60 m was assessed.Besides,the optimum propeller pitch ratio at each speed was also evaluated.The performance evaluation at different ship speeds was carried out,for two ice thicknesses,1.25 m and 1.55 m,and open water condition.The evaluation results for the two different ice thickness and the open water condition were shown in Fig.4.The results of the performance prediction were listed in Tab.3.

Tab.3 Performance prediction with CPP scheme

Fig.4 Evaluation results for two different ice thicknesses and open water

3.3 Evaluation results for Ducted Propeller(DP)

For the DP scheme,for the given revolution of the main engine,at 86 r/min and 82 r/min corresponding to the MCR and CSR,the optimum propeller diameter of 6.75 m was gained.The performance evaluation for this polar carrier under different ship speeds was carried out,for two ice thicknesses,1.25 m and 1.55 m,and open water condition.The evaluation results for the two different ice thicknesses and the open water condition were shown in Fig.5.The results of the performance prediction were listed in Tab.4.

Tab.4 Performance prediction with DP scheme

Fig.5 Evaluation results for two different ice thicknesses and open water

3.4 Comparison and analysis

When the ice thickness capable to break is 1.25 m with the delivered power of 28 MW,the maximum speeds of the icebreaking ship with FPP,CPP,and DP schemes were found 2.16 kns,2.61 kns and 2.62 kns,respectively,as seen from Tabs.1-3.When the delivered power increases 30 MW,the maximum speeds of the ship were found 2.30 kns,2.87 kns and 2.82 kns,respectively.Further increase of the delivered power up to 32 MW gaves 2.44 kns,3.19 kns and 3.06 kns of the maximum speeds,respectively.

CPP scheme could improve the maximum speeds capable to break ice,by 20.8%,24.8%and 30.7%,compared to FPP scheme,with increase of the delivered power,respectively.

DP scheme,in comparison with FPP scheme,could improve the maximum speeds by 21.1%,22.4%and 25.5%,with increase of the delivered power,respectively.It is evident that the maximum speed of capable to icebreaking of the CPP and DP schemes was higher than those of FPP scheme.Comparisons of the maximum speeds between CPP and DP schemes showed that DP scheme unable to improve the speeds further than CPP scheme,as their differences were 0.2%,-2.0%and-4.1%,with increase of the delivered power,respectively.It could be said that in high delivered power range,DP scheme was not competitive to CPP one.

Contrary to the cases when ice thickness capable to break was 1.25 m,considerable improvement by DP scheme was found in the maximum speeds,when the ice thickness capable to break was 1.55 m.Under the same conditions as the case of 1.25 m,DP scheme could improve the maximum speed by 7.6%,4.9%and 4.3%,with increase of the delivered power.However,the trend of losing competitiveness against CPP scheme in higher delivered power was found unchangeable,as DP scheme could generate bigger power only in the low speed range than CPP one.Therefore,in the conditions of the lower speeds and thicker ices,the DP scheme could be expected to improve the icebreaking performance.

Fig.6 Delivered power comparison for different propulsive schemes

In other words,when the icebreaking speed was 2 kns with the delivered power of 28 MW,DP scheme could increase the ice thickness capable to break by 4.6%,compared to CPP scheme.However,when the icebreaking speed increased to 2.5 kns,with two different delivered powers,30 MW and 32 MW,the scheme could increase small amount of ice thickness by 0.9%and 0.6%,respectively.It is obvious that with the increasing of the icebreaking speed,the advantage of the DP scheme is getting smaller and smaller.From the delivered power charts of the three schemes,as shown in Fig.6,when the icebreaking speed was lower than 2.53 kns,the delivered power of the DP scheme was going down to the minimum.However,the delivered power of the CPP scheme was slightly superior to DP one when the icebreaking speed was higher than 2.53 kns.Accordingly,it could be said that the CPP scheme has much better adaptability along with the icebreaking speed increasing.In other words,CPP scheme has broader propulsion capability,while DP scheme decreases the performance with the icebreaking speed increasing,mostly due to the design limitation of FPP in duct.

In comparison with FPP scheme,when the maximum ice thickness capable to break was 1.55 m,with three delivered powers,28,30 and 32 MW,CPP scheme improved the icebreaking speed by 43.6%,38.3%and 33.7%,respectively.In the same conditions,DP scheme improved the speed by 54.5%,45.1%and 39.4%,respectively.FPP scheme was mostly inferior to both of CPP and DP schemes.

In open water conditions,within a range of speeds,10-18 kns,with three delivered powers,19.6,21.0 and 22.4 MW,FPP scheme could improve the ship speed by 0.20,0.19 and 0.20 kns,respectively,compared to DP one.Similarly,CPP scheme also could increase the ship speed by 0.15,0.14 and 0.17 kns,respectively,compared to DP one.It is evident that the propulsion performance of the FPP and CPP schemes is better than that of DP one,and that the delivered power of the FPP scheme was the minimum among the three,as shown in Fig.6,where the performance of the FPP scheme was slightly superior to the CPP one,because CPP scheme has a bigger ratio of hub to diameter than the ratio of FPP,which resulted in light loss of propulsion efficiency.

4 Conclusions

The following conclusions can be drawn：

(1)The estimation of performance of FPP indicated that the performance differences are very large between open and ice-infested waters.In spite of the FPP has very good performance in open water,it has unsatisfactory performance in ice-infested waters.

(2)The CPP scheme would have the best comprehensive propulsion performance in both waters.It can be said CPP scheme well fulfills overall performance requirements of the open and ice-infested water.

(3)From the evaluated results of the DP scheme,if the polar carrier adopts technology of the duct and CPP,the main engine power required will be smaller than that of the duct and FPP.

(4)Compared to the CPP and FPP schemes,DP scheme shows superior performance for the icebreaking capacity.However,the performance of open waters is weaker than that of the other two type propulsion modes.

(5)If the ship owner can reasonably configure the shipping line and power of main engine,it will be very possible that the DP is to be used in the polar ship in the future.