Oliver coxon

Full Length Research Paper

A study suggesting the need for validating the quoted power reduction figures for Energy Saving Devices on vessels around 45,000 DWT. Oliver Coxon Falmouth Marine School, Killigrew Street, Falmouth TR11 3QS, UK. E-mail: oliver.coxon153@live.cornwall.ac.uk.

Tel: 01326 310 310

Since the oil boom of the 1980’s, a need to increase the fuel efficiency of shipping vessels has increased, and research projects have produced various methods of doing this. The area of interest for this paper looked at Stern Appended Energy Saving Devices, in

particular

the Mewis

Duct™. Manufacturers of Energy Saving Devices tested the products using a variety of scaled down models, full scale models, and Computational Fluid Dynamics (CFD) computer software to produce power reduction figures, however it was found that ESD’s were usually designed for what is known as the Sea Trial rather than in working conditions. This paper covered the need to validate these figures in real working conditions on a variety of vessels in order to provide a more accurate depth of data on the energy saving devices potential.

Key Words: Mewis Duct™, Sea Trials, Energy-Saving Device Non Standard Abbreviations ESD

Energy Saving Device

CFD

Computational Fluid Dynamics

PSD

Pre- Swirl Duct

WED

Wake Equalising Duct

DWT

Dead Weight Tonnage

JIP

Joint Industry Project

SPA

Service Performance Analysis

INTRODUCTION

Oliver Coxon

Falmouth Marine School

oliver.coxon@live.cornwall.ac.uk

The Mewis Duct™ consisted of a Pre-Swirl Duct (PSD) combined with a WED (Wake Equalising Duct). The duct targeted rotational losses and tried improve the flow of water to the propeller.

Previous practical and theoretical experiments with the Schneekluth WED showed improvements in propulsion efficiency. It is assumed that the WED speeds up the flow of water to the upper region of the propeller as the velocity is less than the velocity of the lower region of the propeller; due to the block coefficient of the ship (i.e. the hull body is blocking the water flow). A WED designed in the correct way will not only reduce the flow separation after the propeller it can also act in a similar way to the kort nozzle producing thrust, and also reduces the vibrations from the propeller and also improves the steering as the water hitting the rudder is more directed towards it. (Korkut 2005)

Mewis F. states that well known devices for reducing wake losses are the WED and the SILD (Sumitomo Integrated Lammeren Duct). Energy saving devices that are designed specifically to reduce rotational energy losses are; 

SVA fin system (Mewis & Peters 1986),



Daewoo pre-swirl fin system (Lee et al 1992)



Hyundai Thrust fin system, which is fitted to the rudder.

These are the main popular energy saving devices used in the marine industry. By combining two or more components of already established principles new developments are possible. Mewis (2009).

The idea that Mewis proposed has allowed the Pre-swirl duct to tackle more than one type of energy loss making it a prime candidate for use in the industry. The types of energy loss the Mewis Duct™ targets are:



Equalising the velocities of the water as it hits the propeller to give the propeller a more equal plane to propel.



Reducing the amount of swirl aft of the main body by adding Pre-Swirl stators into the duct itself.



Improvement of propulsion efficiency from increased thrust from the duct.



Removing propeller tip vortices in the same way as endplates. (Energy saving device designed to reduce axial energy losses.)

Oliver Coxon

Falmouth Marine School

oliver.coxon@live.cornwall.ac.uk



Draws water in towards the propeller from a wider angle.

The Mewis Duct™ provided a range of energy saving properties, however when the designers of Energy Saving Devices tried to produce accurate “actual” energy saving figures in the form of Power Reduction (to achieve the same speed) as a percentage of the overall energy being used by the vessel, they were met with a variety of limitations. The methods used by these companies usually looked at a combination of computer programs designed to show the flow of fluid around the hull form (Computational Fluid Dynamics (CFD)), in this instance around the stern of the vessel and aft of the vessels wake, scaled down model tests and full scale models were then usually used to predict the stated power reduction figures. However these figures at best were only estimated and in most cases, referred to the maximum energy saving potential of the devices rather than the actual working energy saving level in working conditions.

Similar Stern Appended Energy Saving Devices have derived from the Schneekluth (1986) Wake Equalising Duct. The Wake equalising duct was designed specifically to improve the flow of water to the propeller. Wake Equalising Ducts and other fixed shapes such as the Mewis Duct™, Kort Nozzle also, pre and post swirl stators have all been developed in the last 35 years. Scheekluth. H.,(1986) Mewis, F., (2009) Celik. F., (2006) With the Scheekluth W.E.D being first applied to a large vessel in March 1984. The reason for concentrating on the Mewis Duct™ came from the vast improvement in power reduction for vessels around 45,000 DWT using the device. Other devices for vessels around the same size were quoted to only be half as efficient as the Mewis Duct™. This was due to the Mewis Duct™ targeting more than one energy loss. These were to improve the flow of water to the top half of the propeller, (similarly to the Scheekluth WED) alongside efforts to reduce the rotational losses aft of the propeller and improve the wake flow aft off the vessel.

In order to validate these figures, measurements needed to be taken from the vessels with specific reference to the Energy Saving Devices that have been fitted to the vessels with a before and after style comparison. MARIN (Maritime Research Institute Netherlands) launched two joint industry projects (JIP) relating to the analysis and verification of different energy saving methods and ESD’s.

The first project was titled SPA standing for Service Performance Analysis. The product of this project

Oliver Coxon

Falmouth Marine School

oliver.coxon@live.cornwall.ac.uk

was a “New Standard Practice for Sea trials.” Hasselaar, T,. (2009). The problem being addressed by this JIP was that the ship efficiency was being designed for the Sea Trials, which in reality would rarely become an actual working condition, and therefore there was a need to be able to calculate how worse weather conditions would affect these results. The conclusions drawn by Hasselaar, T,.

(2009), in relation to Energy Saving Devices state that a range of challenges were faced when calculating the effectiveness of the ESD’s. These were that the ESD’s effectiveness varied greatly on the hull design on the ship. A badly designed hull form would benefit more from having a retrofitted ESD whereas a more hydrodynamic hull form would gain less from the same Energy Saving Device.

The Energy Saving Devices would be affected by variations in loading and speed and the location of the ESD on a scale model may not have produced the same results as a full scale replica. (This is due to the inability to downscale the size of the water molecules). Hasselaar also commented that the use of combined CFD calculations and model tests needed to be validated at full scale in a variety of service conditions.

The second JIP was titled REFIT2SAVE and was launched by MARIN specifically to measure the effectiveness of different Energy Saving Devices whilst adhering to the new specifications outlined by the SPA JIP. This project started in January 2011 and by April 2011 was in the process of testing six vessels, three vessels have been analysed using full scale CFD techniques and three identical vessels in sea conditions. Van Dem Boom, H,. (2011). This project has been set up to provide the industry will the all important accurate actual energy saving figures for different ESD’s on a variety of vessels, providing before and after figures to encourage refits to existing vessels and future builds.

Justification of this project came from the lack of available data on this topic. The difference between the quoted power reduction figures and the modelled power reduction figures showed the need to increase the accuracy of this data to ensure the energy saving potential was maximised on different types of vessels. In order for these results to be accurate the parameters had to be limited and therefore for this research paper, only vessels fitted with a Mewis Duct™ with a DWT of around 45,000 were analysed in detail.

Method

Oliver Coxon

Falmouth Marine School

oliver.coxon@live.cornwall.ac.uk

Collection of data: In order to find the data needed, varies searches were undertaken through online portals such as ‘MetaLib’ and ‘Science Direct’ provided academic papers on the topic. This data was then compared with more up-to-date data provided by Ship Yards, Universities and Model Basins. (Such as HSVA Hamburg Model Basin) A multitude of research papers and publications regarding energy saving devices have been found using online search engines, read and analysed. From these papers and publications the quoted power reduction figures were obtainable for a variety of vessels and energy saving devices.

These quoted figures had to be compared against the “Sea Trial” figures, which were labelled Modelled Power Reduction, due to the conditions tested in. These were more frequently found in publications released by the manufacturers of the leading energy saving device at the time.

The Measured Power Reduction figure was inserted as a continuation to the project in 2011.

Once all the data had been collected it was inserted into a spreadsheet, stating the Ship’s Names, Weight, type of ESD, quoted power reduction (as a % to reach the same speed), modelled power reduction and actual power reduction.

In order to reduce the parameters the data was then grouped into vessel weight and type categories and similar vessels with similar energy saving devices are analysed. Once all the similar vessels were selected the percentage difference between the quoted power and the modelled power were plotted in a graph to see the correlation.

Table 1. Shows the data needed to be collected and the format in which it has been laid out in excel.

The measured power column remained empty, as the results were not returned by the end date.

Table 1.

Energy

Vessel

Quoted

Modelled

Measured

% Difference

%Difference2 √%Difference2

Saving

Weight

Power

Power

Power

between

(%)

Device

(DWT)

Reduction

Reduction

Reduction (%)

Quoted and

(%)

(%)

(to be completed

Modelled

by MARIN

power

Oliver Coxon

Falmouth Marine School

(%)

oliver.coxon@live.cornwall.ac.uk

REFIT2SAVE JIP)

reduction (%)

Example: Mewis Duct

45,000

9

6

N/A

33.3

1111.111

33.3

Mewis Duct

45,000

6

9

N/A

-33.3

1111.111

33.3

Results

The results from the calculations showed that the Mewis Duct™ varied in effectiveness by up to 20% in the sea trials for the 6 vessels around 45,000 DWT.

25% of the results showed that the Mewis Duct™ was MORE effective during the sea trials than they had anticipated in the Model tests.

62.5% of the results showed that the Mewis Duct™ did not perform as well therefore were LESS effective during the sea trials than the Model tests had suggested.

12.5% of the results showed that the Mewis Duct™ performed EXACTLY as was predicted.

The range of data was from 0% to 20% with the median value at 16.6% All of the quoted power figures for vessels around 45,000 DWT with a fitted Mewis Duct™ fell between 5 and 6% with the mean value of 5.73%.

The modelled power reduction figures fell in a similar range of between 4.5 and 6% with a mean value of 5.28%.

The difference in mean values is 0.45%.

The results were inserted into graphs placed on pages 8 and 9.

Oliver Coxon

Falmouth Marine School

oliver.coxon@live.cornwall.ac.uk

Oliver Coxon

Falmouth Marine School

oliver.coxon@live.cornwall.ac.uk

Graph 1. shows the % Difference between the quoted power reduction figures against the modelled power reduction for each of the vessels

Oliver Coxon

Falmouth Marine School

oliver.coxon@live.cornwall.ac.uk

Graph 2. A comparison between the quoted power reduction against modelled power reduction.

Discussion

The results have shown clearly that there is a difference between the power reduction figures produced using a series of models and computer programs and those produced by sea trials. In order for the testing conditions to be limited in variables there were executed in conditions that could be replicated. These conditions as a consequence were calm with little wave activity. After the trials, the ESD’s were altered using the CFD programmes to optimise the performance of the device. As a result the ESD’s became designed specifically for the sea trial in order to get the most of the device.

However as stated in the introduction, the sea trials were carried out in conditions that would have rarely become a working condition for the vessels. The Sea trial produced what had been labelled the Modelled Power Reduction Figures. These figures have been taken from measurements made on the vessel and were the more accurate of the two sets of results, however were not accurate enough to prove the exact amount of power reduction achievable in working conditions.

The difference between the conditions the vessels were tested in and the actual working conditions of the vessel were huge. Whilst the sea trials had the advantage of visualising the actual path of the water around the ESD and the body of the aft of the vessel, the pitch, yaw and roll of the vessel remained relatively constant. Ships in service would rarely experience these conditions for extended periods of time and as a consequence the power saving values for the ESD’s cannot be verified. For example, a wave with a height of 15m could have a large effect on the ESD when pitching, however these figures have not been published and could not have been added. In order for these results to be accurate an average of service conditions must have first been concluded with the inclusion of bad weather conditions (high winds and heavy rain, big and small swells etc) and some extreme conditions to find out how the ESD’s would act. This figure needed to be combined with the figures for good weather conditions to either give a more accurate overall working average or two separate figures.

The inaccuracy of these results would relay in to the payback investment time provided by the manufacturers of the ESD. These figures estimated the reduction of fuel needed per day. This combined with the price of the fuel needed and the cost of the fully fitted ESD provided a payback

Oliver Coxon

Falmouth Marine School

oliver.coxon@live.cornwall.ac.uk

time for potential customers. The results showed that there was a large difference between the quoted power reduction of 6% and the unknown actual power reduction. This means that the figures provided by the manufacturers needed validation. However with the increase in oil prices the payback time will continue to reduce.

The Data provided by Mewis Hydrodynamics built up the vast majority of the Quoted power reduction figures for the vessels in this paper. These figures were calculated by HSVA basin and will have provided higher levels of energy saving due to the scaling effects and the ability to position the device hypothetically (without having to actually fit it). The shape of the vessels hull played a large part in the power reduction figures. Modern hydro-dynamically designed hull forms with a lower block coefficient already had a better flow of water to the upper section of the propeller, thus reduced the effectiveness of the Mewis Duct™ as a flow equalising duct and relied more on its pre-swirl properties to provide power reductions. This could explain some of the higher values seen in the results. The Grieg I-Class vessel was a new build project with a modern hull form. The quoted power reduction figures provided

by Mewis hydrodynamics were 5.6% however the sea trials executed by MARIN (Maritime Research Institute Netherlands) showed energy saving levels of around 4.5%. The results provided by Mewis hydrodynamics had a level of bias as part of their marketing scheme, hence the figures provided were “estimated” and reflected test done to only 3 vessels at the time. The tests undertaken by MARIN were funded by the industry, ship owners and builders to verify the values given by Mewis hydrodynamics. The accuracy of these results was increased from the previous data however as stated in the previously were undertaken under sea trial conditions. A case study undertaken on Scheekluths duct provided quoted power reduction figures of 5% but Mewis and MARIN cross examined this to find only 2.5% a huge 50% less than the quoted output. This was due to the updated method of testing. The energy saving figures provided by Scheekluths duct in 1986 were based on model testing and using Reynolds number to upscale the model the figures were produced. Modern methods of testing and modern hull shapes provided more accurate results and the figure reduced by half. Likewise a similar case study on PBCF (Energy Saving Device targeting Hub Vortex Losses) provided quoted power reduction figures of 4.5% for a similar sized vessel in 1985. Mewis and MARIN found this figure to be around 3% for similar reasons. In order for the data to be comparable only the Mewis Duct™ was analysed in detail other energy saving devices and their properties were added to

Oliver Coxon

Falmouth Marine School

oliver.coxon@live.cornwall.ac.uk

the appendix however were not fixed to a certain vessel size. The Mewis Duct™ provided energy saving properties of nearly double the nearest competitor for vessels of the same size. Mewis Hydrodynamics released a range of values for power reduction figures for a vessel of 12,000 DWT analysing different energy saving devices including Scheekluth WED, Daewoo pre-swirl fin system (Lee et al 1992), SILD (Sumitomo Integrated Lammeren Duct) and the SVA fin system (Mewis & Peters 1986). These have all been calculated by Mewis and could not be compared against any other of the same type of data to analyse the accuracy of the data. These figures were added to the appendix.

These results would have been more accurate if the testing had taken place on vessels at sea in working conditions. The vessels would have been identical with half fitted with the ESD half without the ESD undertaking similar trips at the same time. The fuel consumption, power reduction, engine revolution would be measured in both vessels over a typical working season (220 days). This would have provided accurate energy saving values for different vessels. The vessels would have been categorised in weight categories and hull form. Modern hydrodynamically designed hull forms would have to be separated from older less hydrodynamically designed vessels. These measurements were being undertaken by MARIN in May 2011 and were to be releasing the data towards the end of 2011.

This accurate data would have enabled shipyards and ship owners to accurately plot the energy saving levels of the devices for specific vessels. The oil prices were increasing steadily and the need to know which ESD was appropriate for the vessel in question was increasing.

Conclusion

The Measured Power Reduction acquired from the sea trials was usually a slightly lower percentage than the quoted power reduction due to the conditions in which the vessels had been monitored in.

The method for measuring energy saving figures needed to be changed from the typical sea trials to more realistic working conditions for the testing process. The REFIT2SAVE project undertaken by MARIN commenced in February 2011, and was set to tackle most of the issues covered in this paper and by the end of 2011 should have provided the all important actual energy saving levels for 6

Oliver Coxon

Falmouth Marine School

oliver.coxon@live.cornwall.ac.uk

vessels and more in the unforeseen future. The results taken from these tests will have also helped to improve the accuracy of CFD modelling techniques.

Appendix

Friedrich Mewis, MSH, “Mewis Duct”, Rostock University 2010-01-29

Mewis Duct®-Case study

Installation of a Mewis Duct® on a 45,000 DWT Multi Purpose Vessel Costs -Price, ship set price, based on three vessels

abt. 220,000 $

-Installation

abt. 30,000 $

-Capital costs

abt. 25,000 $

Sum of costs

abt. 275,000 $

Saving in costs by MD -6% power reduction

abt. 1,000 $ / day*

-220 days / year

abt. 220,000 $ / year

Saving after 15 month

abt. 275,000 $

Payback of investment

after abt. 1 /4 years!

1

* at an actual bunker price of 450 $/t

Oliver Coxon

Falmouth Marine School

oliver.coxon@live.cornwall.ac.uk

Appendix Chart 1. shows figures obtained through model and CFD testing methods for different Energy Saving Devices on four different vessel. Svardal, J,O., Mewis, F., (2009)

Appendix Chart 2. shows figures obtained through model and CFD testing methods for different Energy Saving Devices on a J-class vessel. Svardal, J,O., Mewis, F., (2009)

Oliver Coxon

Falmouth Marine School

oliver.coxon@live.cornwall.ac.uk

Appendix Table 1. showed figures obtained through model and CFD testing methods for different Energy Saving Devices on four different vessels. Svardal, J,O., Mewis, F., (2009)

Conclusions drawn up by the Service Performance Analysis Joint Industry Project Publicised

by MARIN. Haaselaar, L., (2009)

Conclusions 1. Ships are designed for Speed Trial, not for operation : Design for Service!

3. ESD may improve performance, but requires validation 4. Operational performance can be often improved by 10-15% 5. In-service performance monitoring is underway, and stimulated by new IMO regulations

Oliver Coxon

Falmouth Marine School

oliver.coxon@live.cornwall.ac.uk

References Celik. F., (2005) Energy Saving device of stator for marine propellers Fathom Shipping., (2009) Ship Efficiency, The Guide www.fathomshipping.com (accessed 15/04/2011) Haaselaar, T., (2009) MARIN Ship Service Performance, Ship efficiency Conference 2009 Kessler J (2004) Use of the wake equalising duct of Scheekluths design on fast container ships of medium size.

Lehmann D et al (2009), Becker Marine Systems, Product Overview Mewis, F., (2009) A Novel Power saving device for full-form vessels Petersen, B,O., (2010) Laurin Marine, Energy Management, Energitinget 2010 Scheekluth. H.,(1986) Wake Equalising Duct. The Naval Architect 103 Svardal, J,O., Mewis, F., (2009) Fuel- and Cost-Savings by hydrodynamic Measures, Ship Efficiency Conference Van Dem Boom, H.,(2011) MARIN REFIT2SAVE Joint Industry Project Van Dem Boom, H.,(2011) MARIN Pers-Com Email regarding the data provided by REFIT2SAVE Joint Industry Project

Oliver Coxon

Falmouth Marine School

oliver.coxon@live.cornwall.ac.uk