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TRIple Energy Saving by Use of CRP, CLT and PODded Propulsion

Final Report Summary - TRIPOD (TRIple Energy Saving by Use of CRP, CLT and PODded Propulsion)

Executive Summary:

There is a need for fast ship transportation that is both efficient and non-polluting. Conventional propellers are known to have low efficiency. As an indicative example, most of ship propellers installed on cargo vessels waste about 40 percent of the energy in the form of rotational losses in the wake, vortex generation, noise production, cavitation, etc. The recovery of such losses is one of the major ways to contribute to a more rational, environmentally-friendly use of energy.

The main objective of the TRIPOD project is the development and validation of a new propulsion concept for improved energy efficiency of ships. The ship propulsion efficiency is optimized through the advanced combination of three existing propulsion technologies. In particular TRIPOD explores the feasibility of a novel propulsion concept resulting from the integration of two promising EU grown technologies (podded propulsion and tip loaded endplate propellers) in combination with energy recovery based on counter-rotating propeller (CRP) principle. The three existing technologies have been used separately and are known to improve the overall ship propulsion efficiency as compared to conventional propulsion. However, they have never been combined together in a single propulsion package.

TRIPOD contemplates two types of propulsive innovations, which have been tested for the first time:

- Using CLT propellers in combination with pods
- Using CLT propellers in connection with CRP propulsion and with pods

As a result of the investigation tools have been developed to assess the optimal use of propulsive energy from environmental and economic viewpoints both for new designs and for the retrofitting of existing ships with the novel propulsion concept.

Project Context and Objectives:

During the last two decades integral electric-driven “pod propulsor” units have been applied in increasing number to different types of vessels. As stated by the ITTC Specialist Committee on Azimuthing Podded Propulsion (2008), “looking back on the past years, podded propulsors have been treated as the newconcept of marine outboard propulsive device, which has opened a new page on the history of marine propulsor.” Conscious of the relevance of the matter, the EU has supported projects related to electric driven units, being “pod propulsion” considered as one of the most promising home grown technologies. Among others, projects like OPTIPOD, POD-in-Service, and FASTPOD were funded and completed during the past years within EU framework programmes, and their achievements have prepared the way to the next phase of R&D in podded propulsion. These projects have addressed issues dealing with noise reduction, efficiency increase, manoeuvrability improvement, and mechanical robustness among others. Some Far East countries have also been interested in the podded propulsion market. A fast Japanese ROPAX Ferry has been equipped for the first time with an ABB hybrid CRP-POD system and energy savings of 13 percent have been claimed by the Japanese, contributing to a reduction both of operating costs and of CO2 emissions.

In parallel the decade of the eighties witnessed a growing interest in unconventional propellers of tip loaded type. Blade tip loading (not allowed for conventional propellers without efficiency loss and high levels of noise) was made possible by placing an endplate at the blade outermost radial edge. After the first work of Gonzalo Perez on TVF propellers in the second half of the seventies, endplate propellers have evolved into CLT propellers during the eighties. Simultaneously other concepts of tip loaded propellers have appeared mainly in Europe and Japan promoted by several research groups. In the EU project Kapriccio a systematic study of Kappel propellers was made and three versions were manufactured. Recently the EU funded the LEADING EDGE project where scale effects on CLT propellers were numerically studied in fully turbulent flow using RANS solvers. The main interest has been in seeking more efficient ways of saving energy and additionally of developing environmentally-friendly propulsion systems by reducing propeller-radiated noise levels. As an example the super ferry Fortuny has been equipped with CLT propeller and a reduction of noise and improved efficiency has been reported

In Finland the ENVIROPAX project, investigated the Hybrid CRP-Podded propulsor concept including various hydrodynamic issues like power split, propeller design, powering performance evaluation. One of the major outcomes of this programme was the realization of the first two Hybrid CRP-Podded propulsor driven Ropax ferries built in 2004. In Japan, a domestic coast tanker of 4999GT, named “Shige Maru” was launched within the EcoShip project at Niigata Shipbuilding & Repair Corp. (NSR) in October 2007. She has two sets of podded drive with contra-rotating propeller each of which absorbs 1250 kW.

From the examples of the previous paragraphs it can be deduced that large energy savings and consequently, CO2 emission reductions are expected by a rational combination of the proposed technologies.

An additional advantage of using pod propulsion for gas emission reduction can be explained as follows. Diesel engines are the main source of power in the vast majority of the world’s ships. From an environmental point of view, however, these engines are not the friendliest. Fortunately pollution levels are not equal across the working range of the engine. In the optimum operating range, fuel efficiency is considerably higher and pollution lower than at low speeds. Therefore, the solution is to keep engines operating in this optimum range in all situations. With traditional mechanical transmission this is not possible, as engine speed is rigidly coupled to propeller speed. Using electric transmission (generators and motors connected by cables), this is no longer the case. Additionally, power reserves can be shared with the vessel’s on-board service supply, decreasing the total power installed while raising reliability. Furthermore, cables are more flexible than shafts and permit greater freedom in the location of the engines. This can increase the vessel’s payload or permit more efficient loading and unloading. All these advantages translate into greater productivity and savings for the owner.

The main aim of TRIPOD is to combine these technologies (podded propulsors, CRP and endplate CLT propellers) and explore the feasibility and potential benefits to be gained by the use of such an innovative propulsion system.

The viability of the new propulsion solutions is being check not only from a technical standpoint but also by performing economical cost benefit analysis for the operation of the reference ships. The real economic criteria applied by a market leader are incorporated into the project. Elaborate procedures on how to determine yearly fuel savings and emission reductions based on the vessels operational profiles are applied and then the cost-benefit is assessed through a TCO (total cost of ownership) analysis. The use of CFD tools and model tests are combined to facilitate the design activities and the assessment on ship performance.

The project is organized into five interrelated technical WPs (WP1 to WP5), one devoted to dissemination (WP6) and one devoted to the management of the project (WP7).

The objective of WP1 defining a reference ship and reference propulsion system for the evaluation of the novel propulsion concept from the standpoint of energy savings. Additionally, WP1 collects technical and economic information concerning ship targets that can benefit from the project (cargo, cruisers …) and sets the hydrodynamic conditions to be applied to the test case of reference.

The ultimate goal of work-package WP2 of TRIPOD project is to produce new designs of conventional and CLT propellers to be used in a CRP-POD configuration as an alternative to the existing main propeller configuration. The design activities have been performed in two phases called “retrofit scenario” and “new building scenario”, respectively.

Retrofit Scenario. In simple retrofit scenario the reference ship hull form has been kept as it was. A first design of an equivalent CLT propeller has been developed (CLT1); afterwards the original horn rudder has been removed and Rudder-Pod designed and installed. The main conventional (CONV1) propeller is the same as in original vessel and its location related to hull remained the same; two new designs have been developed to work in Contra-Rotating Configuration (CRP) as POD propellers: one of conventional type (CONV3) and other of CLT type (CLT3).

New Building Scenario. This is the case of a new optimized hull design for CRP and developing new propellers designs and testing in full load and ballast conditions in order to be able to evaluate the possible advantages of this system. Both types of propellers are designed: conventional and CLT type propellers.

WP3 is conceived to help the propeller designer find the optimum geometry both for the pod housings and propellers using numerical tools. CFD tools evaluate the different design options from the standpoint of efficiency, scale effects, cavitation, etc. Additionally they reduce the number of cases to be experimentally tested that are needed in order to assess the performance of the new designs. RANS code FINFLO is mainly used in the computations.

The success of the propeller design by means of model tests is guaranteed in WP4. Complete propulsion tests of the ship models for the cases of retrofitting and new hull design are conducted. The tests allow evaluating the new propulsion concept from the point of view of energy saving. Cavitation and pressure fluctuation tests are conducted to assess the impact of the new propulsion concept on the induced vibrations transmitted to the hull. Additionally the test measurements are used to validate CFD some of the computations in WP3.

Estimation of energy savings on the CRP-CLT-pod concept as compared to the conventional propulsion is made in WP5 by a deep analysis of the model test data and computational data focused on the case studies selected in previous WPs. At the same time a feasibility study is made both for the introduction of the novel propulsion concept on new ship designs and for retrofitting CRP-CLT pods on existing vessels from a technical standpoint. Finally, an economic analysis of the viability of the new propulsion concept both for retrofitting and for new ship designs is made.

External dissemination of results by establishment of a public web site, participation in conferences and symposiums, writing technical papers, etc. is supported by WP6, which is intended to make TRIPOD known to the international community.

Finally, WP7 is devoted to the management of the consortium by the coordinator for administrative aspects.

Project Results:

The main results of the project are as follows,

- Designs of TRIPOD propulsion units have been made and tested for the pod housing and for the conventional and CLT propellers in the retrofit scenario in a cargo ship provided by Maersk during the first reporting period, and for the ship hull and new sets of conventional and CLT propellers for the second and final reporting period. Analysis of data obtained in model tests determined improvements reached in energy savings of up to about 5 percent for the retrofit scenario and 10 percent for the new building one.
- A new concept of non-rotatable podded propulsor called Rudderpod has been further developed and applied in the project in order to decrease installation and maintenance costs in the CRP units and make the TRIPOD concept feasible.
- A new extrapolation procedure of model test data for CRP POD propellers has been developed and presented to the scientific community. The procedure considers fixed rpm ratio between main and pod propeller. This procedure may be applied to any power/rpm combination.
- A more accurate way of computing effective wakes has been developed applicable also for the case in which propeller tangential induced velocities are relevant in the effective wake as it is the case in CRP units. The technique has been based on a correction factor approach, which allows cancelling errors derived from the coupling of RANS methods for the representation of the bulk flow around the ship hull and potential flow methods for the representation of the propeller action.
- The effect of endplate shape and location on efficiency and load has been investigated by CFD methods. Numerical validation/verification studies have been made.
- A new design tool has been developed allowing the estimation of the propulsive efficiency of the system resulting from the combination of the RudderPod in CRP with conventional or with CLT propellers. The data and analysis performed in D5.2 have been implemented obtaining a new procedure to estimate the expected improvement of a CRP-Pod system in comparison with a conventional propeller at an early design stage of the ship propulsion design. This procedure is applied to several types of ships and presented assessing the incidence in the three main world fleets of cargo ships.
- A study of the viability of the new propulsion solutions by performing economical cost benefit analysis for the operation of the reference ships has been made. The operational profile of the Gudrun class comprises the sailing behaviour in 2010. In order to analyse if any changes have been occurring in the vessels’ sailing behaviour since 2010, the operational profile of 2010 is compared with the profile in 2013.
- Dissemination activities have been made mainly in the form of attendance to valuable symposia (Symposium on Naval Hydrodynamics, Symposium on Marine Propellers, etc.): two peer-reviewed papers have been submitted and accepted, two non-peer reviewed papers were accepted, and another one is due for the TRA conference to be held next year in Paris. Two journal papers are in peer-review process. Press releases have been made in different magazines (MotorShip, InfoMarine, The Naval Architect, etc.)

Potential Impact:

The EU greening of surface transport is supported by TRIPOD from the standpoint that the technical developments proposed herein will enable more versatile and efficient propulsion systems. Significant energy savings and consequently lower emissions are predicted implying cleaner environment. Energy recovery concepts based on the counterrotating propeller principle and in advanced tip loaded propellers are the key factors to produce such effects.

Alternatively, the new propulsion system will make it possible to design propulsion units of lower noise and vibration levels. Two factors will contribute to the reductions. On the one hand propellers with smaller optimum diameter will have smaller velocities at the blade tip. Additionally, pods permit more flexibility in defining hull forms with more uniform wakes at the propeller plane. This will reduce in turn the danger/extent of cavitation. Noise/vibration attenuation both improves the quality of life on board and reduces harmful impacts on the environment.

Additionally the results of the investigation are used to validate existing tools for CLT propeller design and to further develop them in order to accurately estimate the propulsive energy consumption in the ship design phase. This need for advanced design tools was recognized by the 21st ITTC committee in the written discussion to the Propulsion Committee report for all types of tip loaded propellers. They suggested the use of CFD tools to clarify the tip fin/plate behaviour, and to facilitate the further development of design tools for unconventional propellers.

The proposed work and the derived output are expected to strengthen the competitiveness of the shipbuilding, marine equipment supply industry and consultancy. It will thus contribute to the overall employability in the sector. In particular the main focus of the project on advanced propulsion design will enable maritime industry sectors in the related field to promote their own technology developments within the ‘podded propulsion’ market.

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