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TRANSCRIPT
Report is a newsletter of MARIN September 2005 no. 86
CFD versus model tests
Blended wing body technology
Aft-body slamming
Twin-gondola LNG carrier design
Report is a newsletter of MARIN,
2, Haagsteeg, P.O.Box 28, 6700 AA Wageningen,
The Netherlands, Phone: +31 317 49 39 11,
Fax: +31 317 49 32 45
Printing 5.000
Editorial Board Arne Hubregtse, Jan Otto de Kat,
Ellen te Winkel ([email protected])
Cover CFD – streamlines and pressure distribution
around hopper dredger in oblique viscous flow in
shallow water.
Editorial consultant Helen Hill
Design & Production
Communicatie & Onderneming B.V.,
Bavel, The Netherlands
colophon
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CFD versus model tests,which one will be the winner?MARIN and IHC debate the issue ofCFD versus model testing.
Intercepting the InterceptorBoth MARIN and Deltamarin talk about theInterceptor on large, relatively-slow, merchant ships.
Cooperation helps wing the way to successFor a study of blended wing body technology MARIN made one of the most complex models ever tested.
Inboard noise from cavitatingpropeller tip vortices under scrutinyReport looks into one of the major discussion topics among clients.
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The editorial staff has made every attempt to ensure the accuracy of
the contents. However, experience has shown that, despite the best
intentions, occasional errors might have crept in. MARIN cannot,
therefore, accept responsibility for these errors or their consequences.
For remarks or questions, please contact Ellen te Winkel.
E-mail: [email protected]
For more information or a subscription to MARIN Report, please visit our
website: www.marin.nl.
editorial
Dear Reader,
I would once again like to welcomeyou to the latest edition of Report.This month you can really see thewide-range of work that MARINdoes and we would like to take thisopportunity to up-date you on someof our latest projects.
We are happy to share with you aninteresting debate with IHC aboutCFD versus model testing. Studying
the viscous-flow around a ship, sailing at a drift angle in shallow water,CFD turned out to be successful. In this same issue of Report we lookat the CFD code being used in LNG carrier design.
Involving one of the most complex models MARIN has ever tested, wehave been asked to examine the resistance, seakeeping and manoeuvringcharacteristics of blended wing body technology.
Larger, slower, vessels are the subject of a very interesting move by the Finnish consultant Deltamarin, which took the unusual step ofdeploying the Interceptor on large merchant ships. We talk toDeltamarin in this issue.
In other projects, we look at aft-body slamming worries and theincreasing concern of our clients about low-frequent, broadbandexcitation caused by cavitating propeller tip vortices. Simulations as away of improving profitability is another subject we touch upon.
This issue should certainly stimulate some interesting debate. But if youwould like to follow-up on any of the articles, there will be severalopportunities in the next few months. Visit us at the exhibitionsSNAME, Europort Maritime, METS or Marintec. Or attend theMARIN Open Day on October 22 when we again open our doors tothe public. See the news section for further details.
I certainly hope you will take up one of these opportunities to meet us,we very much look forward to welcoming you.
Arne HubregtsePresident of MARIN
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MARIN addresses aft-bodyslamming concernsMany designers face the possibility of slamming when
applying a relatively-flat aft-body. MARIN probes this issue.
Scenario simulations for profitableshippingThe balancing act between operational performance and
building costs is sometimes difficult to achieve. Simulations
may have the key.
PELS proves to be a “SMOOTH”operatorResearch on air-lubricated ships is close to being
converted into practical applications.
Major steps forward in twin-gondola LNG carrier design with the aid of CFDCFD tools are playing an important role in the development
of LNG carrier design.
New JIP brings sea trials up tospeedReports looks at what made leading shipowners
take a fresh look at sea trials.
Lashing@Sea This JIP aims to die down the problem of containers
lost at sea.
News/at your serviceNews flashes on exhibitions, courses, PIV and Simulators.
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Within the project, a study into the viscous-flow around theship, sailing at a drift angle in shallow water, was defined.IHC naval architect Arie de Jager and Serge Toxopeus, one of
the HOSWA project managers of MARIN, debate the results and tackle thethorny issue of whether computer calculations will ever replace model testing.
Computational Fluid Dynamics (CFD) were fundamental to the project.“We wanted to know more about the physics, to get information on theflow field”, says Serge. “We had actually been using CFD for deep waterconditions but CFD for shallow water was something new for us. But thisfitted in with our long-term plans. The project came along at exactly theright time. Based on experience, we know how to change the manoeuvra-bility, but why it works is not fully understood.”Arie replies: “For us viscous-flow calculations are very important due to
the characteristics of our vessels and for manoeu-vring, turbulence is a determining factor. We onlyreally get the relation between speed and powerfrom model tests. Optimising a model is a timeconsuming and expensive procedure. CFD on theother hand, is more flexible and gives more insightinto the physics. Therefore, CFD plays a veryimportant role in optimising hulls nowadays.”They talk about their expectations from theproject. “I’m interested to hear what Serge has tosay here”, Arie grins. Serge seeks diplomatic words.“Some clients expect everything from CFD, Arie,but you are one of the few that realises that it canbe difficult. A mutual understanding facilitatesalternative approaches and then solutions can befound. We can try things, without having an angryclient!“However, since it was the first time that we hadused PARNASSOS – the viscous-flow solver of MARIN – to calculate the flow around a shipat a drift angle in shallow water we had somereservations.”Arie adds: “In former days there was the idea thatscientists live in Ivory Towers. But I think we needeach other from the very beginning. Nearly 30 years ago, IHC had a bad experience with asingle screw dredger. A new project forced us toinvestigate but we had no funding for an in-depthmodel test programme. Then I remembered theexistence of PARNASSOS and asked MARIN todo calculations. We were very lucky we took thatdecision because we delivered a successful ship.That was IHC’s first use of PARANASSOS.”“The beauty of CFD is that it allows IHC toactually use the information practically”, Ariestresses. “We used the information for propellerdesign, appendages were changed. We used it as atool for improvement.”Four vessels have now been optimised successfullyusing the code. The present results were in linewith practical experience, but MARIN waspositively surprised by the possibilities of the flow
IHC, Ballast HAM (now Van Oord), Boskalis and MARIN,
initiated the HOppers in Shallow WAter (HOSWA) project
when they realised there may be a glut of information
about manoeuvring in deep water but the impact of
shallow water is not yet fully understood.
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CFD versus model tests, which one will be the winner?
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solver, Serge adds. He explains that the increase of the speed on the“windward” side was expected but the increase ofthe cross-flow underneath the ship “was quitesubstantial and surprising”. Some unexpected flowseparation was also predicted and needs moreinvestigation, he says. Arie agrees: “Now we havethe information, we can do something with it.”Serge thinks it will be interesting to look at moredrift angles and Arie wants to use the results for dynamic tracking studies so that time andtherefore, money, can be saved during sea trials.
And the future?
“In the near future, CFD will be used more forresearch purposes but in daily practice, it is stilltoo complicated. But it definitely facilitates moreinsight into the flow around the ship and helpswith integrated design”, says Serge. Arie nods: “It is rather unpleasant to do calculationswith the same hull, one after each other, with different computational tools. Furthermore,PARNASSOS is especially good for the model-scale effects problem.”“In the future, calculations will be done earlier,design decisions made earlier but because somecalculations can never be 100% correct, modeltests will be carried out for final verification,” Sergesays to Arie. Vessels are becoming more complicated,making simulations more difficult. “I agree”, says Arie. “We believe we can trust
PARNASSOS. To convince clients however, weattempt to validate the investigations as often aspossible. One should realise that we always have tosatisfy our clients with good and reliable results,within a short time-span and with minimal costs.CFD will help us with this. But sometimes theyrequire model tests in the design stage to beconvinced of the performance of the vessel. So,calculations will not fully replace model tests butthey will be used more often.” Despite beingconvinced of the possibilities of viscous-flowcalculations, he adds: “I still like to do model testsbecause the performance data can be used foradjustment of our prediction tools but that’s me.”Both are pleased with the results but agree that itlooks like calculations and model tests are set towork alongside each other for a few more years tocome!
Courtesy IHC.
Streamlines and pressure distribution
around hopper dredger in oblique
viscous flow in shallow water.
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An Interceptor is a metal plate which isfitted vertically to the transom of a ship,covering the main breadth of the transom
and protruding with its lower edge severalcentimetres below the transom. The principle is tocreate a virtual trim wedge below the transom. Theprotruding section below the transom causes alocal pressure increase with a deadwater area infront of it. The flow over the aft-body of the shipthen bends downward when approaching this highpressure deadwater area, thus creating a similar lift effect as a conventional trim wedge on the flownear the transom (see illustration). An Interceptor on small craft can be moved up anddown by a hydraulic or electric system and can beadjusted to create the optimal flow deflection forevery ship speed. In this respect it has some advan-tages from adjustable trim flaps or tabs but flaps areprotruding behind the transom and have a smallwidth. For small, fast craft with stern drive or out-board propulsion, this creates additional drag andreduced lift. The Interceptor can be used over thefull transom width and with any type of propulsion.
Cruise ship hull design
Although the Interceptor was developed forrelatively small, fast craft, recently Royal CaribbeanCruise Line asked their Finnish consultant,Deltamarin and MARIN, to optimise a new cruiseship hull design. All known performance
Jaap [email protected]
Intercepting the Interceptor
Although more associated with semi-planing motor
yachts and other fast craft, such as patrol boats, the
Interceptor is now being deployed on several models of
large merchant vessels tested in MARIN basins. Report
plots the progress of the Interceptor as demand grows.
Ship model for RCI fitted with
adjustable Interceptor plate.
Effect of an Interceptor on the aft-body flow.
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increasing appendages such as ducktails and trimwedges were tested but Deltamarin also proposedto investigate an Interceptor. And surprisingly, italso worked successfully for a large ship. After a series of trim wedge tests, the bestInterceptor position supplied 2% more powerreduction than the best wedge. Deltamarin thenarranged full-scale boundary layer flow calculationswith its alliance partner, Safety at Sea Ltd for theaftbody with this device. They found that the flow started to bend downwards when the flowapproached the plate at a distance equal to theheight of the Interceptor below the transom. So afinal propulsion test was done where theInterceptor plate was used to model a 45 degreewedge, with a length similar to the height of theInterceptor below the transom, thus filling in thedeadwater area. When this Interceptor wedge wastested, it appeared to yield a further 1% powerreduction. After this initial success, many more cruise shipsand ferries were tested with an Interceptor. But theInterceptor was not so successful with all ships. Itssuccess on large displacement merchant vessels wasnot so straight forward. Some tests showed onlysmall performance improvements and on someships there was no, or even a small negative effect.MARIN intends to undertake further investigationto be able to give the best advice on these type ofaft-body flow optimisation devices.
at MARIN
Juha Hanhinen, Deltamarin.
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Hanhinen admits that initially it was a desperate measure.“I was so frustrated
after testing so many trim wedges on this particular aft-body without being able
to fix the wake behind the transom.“I was expecting improved flow/wake behind
the transom but I wasn’t expecting improvements in powering. I was just hoping
that trying something completely different would bring fresh ideas.”The boundary
layer flow calculations more or less confirmed what Deltamarin was expecting
about the behaviour of the device.There were no major scale effects.
The Finnish company hopes that the Interceptor will be applicable to more or less
all transom stern ships.“It would definitely make life easier for designers as the
transom type of flow is quite difficult to control.“We are almost certain that it
works well on faster transom-type sterns. But we hope that after learning more
about the flow mechanics, it will also work well with slower transom sterns.”
“So far, the understanding is still very limited”, he points out.“Here, more RANS-
calculations will help and experimental (model scale) feedback is very important
at an early stage.”
The transom type of flow is fairly constant at speeds high enough (close to Fn 0.2
and above) despite the motions of the hull, so he is not expecting any nasty
surprises there. However, low speeds can cause some parasite drag. For large ships
the interceptor will have to be fixed but luckily the additional drag at low speeds
is quite small. Hanhinen expects that the performance improvements found in
model tests will be also be found for full-scale ships but the first full-scale trials
are yet to be made.“I remember the first time we tried a trim wedge on a
Panamax size newbuilding cruiser.The gain according to the first scale tests
was a bit questionable but in real life it turned out to be a major success.”
Deltamarin: firstInterceptor success
Reports asks Juha Hanhinen from the Finnish consultant Deltamarin,
what made the firm decide to deploy the Interceptor on large,
relatively-slow, merchant ships.
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There is no doubt that relatively-flat sternsections have definite advantages in calmwater, compared to V-shaped sections.
Fuel savings of up to 10% are not uncommon.But when a ship sails among waves, there is apossibility of stern emergence. Both the pitchmotions and the incident wave contribute to thisphenomenon which occurs mostly in following andhead seas. However, because a flat stern tends to“cling” to the water surface at higher speeds, sternemergence is most likely in the lower speed range. In the course of re-entry, the stern experiences animpulsive load which can be characterised by animpulse and a duration. If the sections are flat andthe entry-velocity is high, the duration of theimpulse is sufficiently short to excite the lowereigen modes of the ship structure. Discomfort and passenger concern related to thenoise and transient bending and torsional deflectionshave been reported from onboard observations.
Quantifying “stern slamming” is challenging andrelevant data is often hard to obtain.
Different techniques
MARIN applies two methods to quantify theflexural response. The first technique uses a largenumber of pressure gauges and a fast-samplingtechnique to quantify the pressures during an impactand impulsive loads are derived. These are then usedin a Finite Element Model of the ship to quantifythe transient flexural response.A second technique mimics the structural stiffness ofthe prototype in the model. The lowest mode shapesin bending can be realised relatively-easily with asegmented model. An important advantage of thistechnique is its simplicity, the fact that it accountsfor (possible) hydro-elastic interaction between thepressures and the flexural response and the direct testresults. A disadvantage is that the modelling of thehigher and more complex mode shapes, is difficult.A study that compares both techniques waspublished in cooperation with Ingalls ShipbuildingCorporation1. Generally, the results showed that the vertical accelerations at the vessel’s extremitiesincrease considerably in those cases where sternslamming occurs. The figure gives an example of amedium-size, fast, container vessel in head seas Intest conditions, the flexural response increases thevertical accelerations fore and aft, by some 20-40%.Aft-body slamming affects, through the related hullgirder vibrations, the operational performance ofships in certain wave conditions. The abovetechniques enable ship owners and yards to find anoptimum compromise between these problems and ahigh fuel economy in good weather.
1) Kapsenberg G.K, Veer A.P. van 't, Hackett J.P. andLevadou M.M.D., 2002, Whipping loads due to aftbody slamming, Proceedings, 24th Symposium onNaval Hydrodynamics, 8-13 July, Fukuoka, Japan.Kapsenberg G.K., Veer A.P. van 't, Hackett J.P. andLevadou M.M.D., 2003, Aft-body slamming andwhipping loads, SNAME 2003 Annual Meeting,October 2003, San Francisco, USA.
MARIN addresses aft-bodyslamming concernsHigh propulsive efficiency has motivated many
designers to apply a relatively-flat aft-body but in some
cases this shape has raised concerns about the possibility
of aft-body slamming.
Reint [email protected]
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MARIN has developed scenariosimulation methodology to quantifythe balance between investments,
operational revenues and costs. In these simulationsseveral years of operational service are mimicked,taking into account the weather on the route. Akey element in this analysis is the way the masterhandles the ship in adverse weather; prudentseamanship (risk-avoidance) and the impact ofspeed and reliability on short-term revenue.Most of the simulations that have been carried outare in the field of design verification or conceptdevelopment. The first work performed was in2001 for the Queen Mary 2. This addressed theissue of the required service margin needed toobtain a reliable transatlantic service. A very recent example was a contract to quantifythe impact of adverse weather on the logisticsoperations of Airbus A380 airplane parts, whichare transported by a specially-designed ro-rocarrier between manufacturing sites in Europe. Model tests were used to determine factors such asthe sustained speed and this was then used toestimate the reliability of sea transport in theAirbus logistic chain. The relationship between theencountered sea state and the risk of whipping andlocal accelerations on the cargo were obtained by
tests using a flexible segmented model. Scenariosimulations that covered several years of servicewere carried out. These then provided a solid basisfor determining the likelihood of encounteringextreme accelerations that may damage cargo.In the area of concept development, MARINinvestigated the reliability of a new inland/short-sea concept within the European projectInterModeShip. Issues examined included the tripduration and reliability, fuel consumption andextreme behaviour.
Opportunities
Voyage simulations also offer opportunities in thefield of fleet development. What would be themost efficient round-trip schedule and which(charter) vessel had the lowest fuel consumption,were some of the many questions answered bysimulation studies.Performing simulations at an early stage of thedevelopment of a design, enables shipyards andshipowners to get a detailed assessment. There isclearly a lot to gain from a preliminary evaluationof the future operational performance of a ship.But simulations need to be done at the preliminarystage in order to yield the right choices at the rightmoment.
Scenario simulations for profitable shipping
Within the realm of safety, the
shipping industry is continuously
searching for a competitive balance
between operational performance
and building costs. Report highlights
how MARIN helps the industry with
this complicated problem.
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Given the current trend in missionrequirements for high speed vessels,there is a growing demand to reduce
hydrodynamic resistance and expand maximumspeeds in a seaway on large displacement vessels.To this end, Navatek Ltd., a leader in design andconstruction of innovative, advanced ship hullsystems, patented a design to enhance the lift of avessel. The design incorporates blended wing bodysystems forward and aft. Each blended wing bodysystem consists of two lifting bodies connected bya hydrofoil. While the hydrofoil provides lift, thelarge underwater lifting bodies provide motiondamping to increase seakeeping capabilities. In order to prove the hydrodynamic advantages ofthe blended wing body system, the United StatesOffice of Naval Research (ONR) sponsored
computational studies and hydrodynamic modeltesting on a vessel containing the Navatek patenteddesign. The research group for this project includedNorthrop Grumman Ship Systems (NGSS),Navatek Ltd., and MARIN, and the project goalwas to quantitatively determine the resistance, seakeeping and manoeuvring characteristics of amonohull outfitted with the blended wing bodytechnology (identified as CHSV or CompositeHigh Speed Vessel) to an equivalent conventionalmonohull (identified as HSV or High SpeedVessel).
Optimisation through CFD
Prior to model fabrication both hulls were optimisedusing CFD tools. Using MARIN’s potential flowcode RAPID and viscous flow codePARNASSOS, the HSV hull form was optimisedfor wave making characteristics and chine alignment.The optimisation of the CHSV proceeded using the computational tools and expertise ofNAVATEK. For both the vessels 3D non lineartime domain seakeeping calculations were performedusing MARIN’s PANSHIP code. This code,which is specifically developed for high speedvessels, assisted in the optimisation of the ride
In a fascinating project MARIN has been asked to deter-
mine the resistance, seakeeping and manoeuvring
characteristics, of blended wing body technology which
involved one of the most complex models MARIN has
ever tested.
Cooperation helps wing the way to success
Panship pressure coefficients at 40 knots.
CHSV at 40 knots in head waves.
John Hackett & Jessica Calix (NGSS),Marc [email protected]
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control systems and gave an early indication of theseakeeping performance of both vessels.
Complex models a challenge
Models of the CHSV and HSV manufactured toa scale of 1:18 were used for seakeeping, manoeu-vring and powering tests. The HSV model was arelatively simple model with a double chine hullform propelled by two water jets and equippedwith active trim flaps and fin stabilisers. The morecomplex CHSV model consisted of a centre hullform with outriggers (ama’s) on the side. It wasequipped with three water jets and two blendedwing body systems, one fore and one aft. The crossfoils of the blended wing body systems wereequipped with two flaps, port and starboard, formotion control. In addition to these controlsurfaces, the aft struts of the blended wing bodysystem also had a vertical flap for directional control.In total the model was equipped with six servoactuated flaps. During the free running model testsall the flaps and water jets were active. With theactive flaps and jets and the need to measure mid-ship bending moment and loads in each blendedwing body, the CHSV model was one of the mostcomplex models ever tested at MARIN.
Model tests
For both vessels free sailing seakeeping andmanoeuvring, resistance, powering and captivemanoeuvring tests were performed. The purpose of
the model tests was threefold: to obtain data toquantify the hydrodynamic performance of bothvessels in calm water and motions in a seaway, toobtain engineering data to quantify such things asloads in the struts and midship bending moments,and to use the obtained model test results to finetune the designs. The test program started with seakeeping andmanoeuvring tests where issues which could not becalculated were investigated in the tank. Waterjetinlet ventilation was one such issue. Using underwater cameras, it was possible to detect if and howair bubbles were entering the water jets. Usingthese observations the forward strut/ama andspray rail configurations were optimised to give thelowest possible disturbances (air bubbles) in thewater. In addition, the effect of spray on the vesselperformance was quantified, leading to designchanges that proved to significantly improve thespray associated with vessel operations.
Cooperation a key to success
A comparison was made between the two optimisedconcepts using the model test data. The results ofthe seakeeping tests show that CHSV (lifting bodyconcept vessel) performed equal to or better thanthe monohull (HSV) design. In calm water a smallincrease in resistance was found over the HSVdesign. The results suggest that an improvementcan be achieved by further optimising the positionof the cross foil with respect to the hull, as inter-ference phenomena were observed between the hulland the two blended wing body systems. Themanoeuvring characteristics between the twovessels are quite different. This is due to the largevertical struts on the CHSV which make it morecourse stable than the HSV, but also increase theturning circles over HSV.The cooperation between all parties involvedresulted in a very successful project. The use ofCFD in the initial phase together with model testsin the later phase helped reduce development timeand costs while producing valuable engineeringinformation for the final design. Once again, thisapproach has proven indispensable in the develop-ment of new concept designs.
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CHSV model (above) and HSV model (below).
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Apresentation on PELS at the renownedSecond International Symposium onSeawater Drag Reduction, held in Busan
in May, received overwhelming positive feedback.The paper identified an important missing link –the validation that air bubbles reduce drag, evenwhen they are applied close to full-scale Reynoldsnumbers. The result was achieved by mounting a whole partof a ship’s bottom in the measurement section ofEurope’s largest cavitation tunnel, the UT2 inBerlin. Reynolds numbers close to 108 could betested this way in laboratory circumstances, evenallowing for correctly scaling the ambient pressure.
For PELS’ free sailing model experiments in calmwater, MARIN’s Depressurised Towing Tank wasused. In this way, the compressibility of air wasfound to be responsible for several measurablescale effects on the micro and macro scale ofair- lubrication techniques.
Considerable savings
Final calculations have shown that one singleinland shipping vessel could save as much as 130tonnes of diesel on an annual basis (15 to 18%)and about 400 tonnes of CO2, five tonnes ofNOx , plus about half a tonne of soot (PM10)particles. Given the fact that these results apply to one singleinland navigating craft, there has been a largeamount of interest from inland shipping companies.There might also be sufficient interest to start asecond Dutch research initiative on the subject.
All these positive results concerning air-lubricationare very important in both social and economicterms and research in this area is supported by theEuropean Commission. Consequently, a Europeanconsortium of well-known partners, withMARIN at the helm, is currently preparing a 6thframework EU STREP proposal “SMOOTH”(Sustainable Methods for Optimal design andOperation of ships with air lubricaTed Hulls) tocontinue PELS.The aim is to expand the fundamental researchdone in PELS 1 and to move a step closer to apractical application. There is certainly a great dealof interest in taking this project to the next stage. Interested potential partners are still welcome tojoin the consortia. The knowledge gained so farwill be of great benefit to those who undertakefurther research into lubrication. And it will bevery interesting to see if the first practical applicationvalidates the positive findings from laboratoryexperiments.
PELS proves to be a “SMOOTH” operator
The national Dutch research project PELS (Project
Energy-saving air-Lubricated Ships) was actually
completed in December 2004 after considerable
success, with net energy-savings resulting. MARIN
is keen to convert this research into practical
applications. Report explains.
Part of a ship’s bottom built in the large
tunnel of TU Berlin, tested at Re~108.
Cornel [email protected]
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Cavity dynamics in the vicinity of shippropeller blades cause pressure fluc-tuations which excite the hull structure
above the propeller. As these pressure fluctuationsact largely in phase over the aft-body surface,cavitation is very effective in generating inboardnoise and vibration. The last decades however, haveseen a considerable reduction in cavitation-inducedhull pressure forces, leading to lower inboard noiseand vibration levels.Unfortunately, an opposing trend of increasinglow-frequent broadband hull pressure fluctuationsis also witnessed within a range of about 20 to 70 Hz. This trend has received a lot of attentionas the ship’s aft-body structure is likely to beexcited at resonance in this range of frequencies.Such resonant vibrations often cause annoyance,despite the low magnitude of the excitation forcesinvolved.As a result of changes in propeller design philosophymany modern propellers show leading edge or tipvortex cavitation. As yet, there is no clear under-standing of the physical mechanisms underlyingthe type of broadband excitation they are causing.The lack of theoretical models has led to thedevelopment of empirical methods to relate tipvortex characteristics to inboard noise (e.g.Ræstad’s Tip Vortex Index). The notion of broadband noise being completelyrandom in nature is not confirmed when pressuretime traces are studied. It seems that bursts ofenergy in the frequency range of interest cause a
‘ringing’ effect, superimposed on the tonal components at blade passagefrequency. Such a phenomenon has been observed in time traces ofpressure signals measured with flush mounted pressure transducers onboard passenger vessels.At MARIN’s Depressurised Towing Tank detailed model scale studies are conducted, where high-speed video images and hull pressure data aresynchronised to study the character of the cavity dynamics and resultingpressure pulses. Combining information from such studies with resultsfrom wake flow experiments, CFD computations and experience, allowsfor the best trade-off between propulsive efficiency and low noise levels.Practical design studies need to backed up by background research (see references) as there is still much to be learned in this field.
REFERENCES
• “Aspects of the cavitating propeller tip vortex as a source of inboardnoise and vibration”; Erik van Wijngaarden, Johan Bosschers, Gert Kuiper;ASME Fluids Eng. Div. Summer Meeting and Exhibition; June 19-23,2005, Houston, TX, USA.
• “Recent developments in predicting propeller-induced hull pressurepulses”; Erik van Wijngaarden; The First International Ship Noise andVibration Conference June 20-21, 2005, London, UK.
An increasing number of MARIN’s
clients consider low-frequent, broad-
band excitation caused by cavitating
propeller tip vortices as the most
important topic in inboard noise and
vibration abatement.
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Propeller tip vortex cavitation
on scale model.
Inboard noise from cavitating propeller tip vortices
Typical hull pressure spectrum showing broadband
excitation in between spikes at multiples of BPF.
Erik van [email protected]
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In the overall design process of an LNGcarrier, the hull form and propulsors play animportant role from a hydrodynamic point of
view. The twin-gondola aft-body has proven to bean adequate design concept but due to thecomplexity of the flow around the aft-body, thedesign must be carried out with great care. Computational Fluid Dynamics (CFD) tools areextremely valuable in the hydrodynamic hull formoptimisation process. Both potential flow andviscous flow codes are used to obtain the optimumhull form.
Particular attention is always paid to the shape andthe orientation of the gondolas. Usually the designprocess starts with calculations carried out byMARIN’s non-linear potential flow code, RAPID.In such a pre-design phase the bulbous bow andthe shape of the hull can be optimised. In addition,the pressure distribution around the gondolas isoften studied to obtain a first impression of theoptimum gondola orientation.
PARNASSOS also plays vital role
Viscous effects play an important role in the flowaround the aft body and therefore, the shape of theaft-body and the orientation of the gondolas canbe further improved by using MARIN’s viscousflow code, PARNASSOS. With the results of thePARNASSOS calculations it is possible to makedecisions with regard to the horizontal angle andthe inclination of the gondolas and the slope ofthe buttocks in the area between the gondolas. The wake field in the propeller plane can also beimproved by optimising the shape of the gondolas.And these have to be oriented in such a way thatmaximum efficiency is achieved. Furthermore, theeffect of a working propeller can be investigated by
The world has enormous quantities of natural gas but
much of it is located in areas far from where the gas
is needed. As natural gas becomes an increasingly
important energy source a large fleet is needed to
transport it in liquefied form, hence the development
of specially-designed LNG carriers. Report outlines the
importance of CFD tools in LNG carrier design.
Major steps forward in LNG carrier design
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Henk Valkhof & Klaas [email protected]
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applying an axial force field in the propeller plane.Scale effects can be studied by conductingcalculations for both model and full scale.Although the CFD part is of great importanceduring the pre-design stage of the project,verification of the performance of the ship bymeans of model tests is still needed. The combina-tion of CFD calculations and model tests makes itpossible to compare the calculated and measuredresults. For the model measurement of the wakefield, a 5-hole Pitot tube is mounted in the propellerplane. The results of the axial velocity componentsshow a wake peak depth of about 60 percent. Thegradient of the velocity through the propeller planeis soft on the outer side of the gondola, while on theinner side the gradients are somewhat steeper.
At the right a comparison between the calculatedcircumferential axial velocity field (throughPARNASSOS) and the measured wake survey ispresented. Both the slopes and the depth of thewake peak compare well.
Several design approaches
From recent studies it became clear that the designof a twin gondola aft-body can be approached inseveral ways. Whether the priority is to minimiseresistance, or whether it is to maximise hullefficiency, both approaches can lead to gooddesigns. For both approaches a thorough under-standing of the flow over the aft body is needed.In this regard CFD tools like PARNASSOS arevital and it is believed that an even more extensiveuse of these tools will lead to better design inwhich the optimum combination between resistanceand hull efficiency can be found. A few recentdesigns following the approach described abovehave resulted in significantly (between 10 and 15 percent) improved powering performance,leading to significant reductions of fuel consump-tion, or in some cases to an increased payload atequal fuel consumption characteristics.
Further improvement expected
A few aspects are still being studied through aresearch programme with internal MARINfunding. In this programme, PARNASSOScalculations for full scale Reynolds numbers arecarried out. Propulsive coefficients will bedetermined to study the sensitivity of the changeof the position of the gondola on the hullefficiency and other relevant parameters. In thenear future, several CFD calculations will be carriedout with gondolas perfectly oriented in the flowbut also with gondolas deliberately placed somewhatout of the flow to increase hull efficiency. It isexpected that these additional studies will lead to afurther improvement in performance.
twin-gondola with the aid of CFD
Comparison calculated (above) and measured (below) wake field.
16
Historically, the analysis of shipbuilders’speed trials utilises corrections to allow fordeviation between the conditions during the
trial and those defined in the contract. However, recent adverse experiences of several shipowners suggests that it is time to reconsider thesecorrections, as some have not been revised since theywere developed 30 or 40 years ago. The STA-JIP is aiming at transparent and accuratemethods for the speed/power tests of ships upondelivery. A rational review of the trial analysisprocedures will be conducted within the framework ofthe existing ISO Standard 15016. Shipowners andyards are invited to join this project and to worktowards a new industry standard in this field.
The STA-JIP started-off with case studies to investi-gate results of trials by various analysis methods, fromwhich recommended practice for trial procedures andmeasurements will be developed. Subsequently, theproject group aims to develop ship-type specific analysis
methods and software. The project focuses onLNG-carriers, tankers, bulk carriers, containerships and car carriers.Evaluation of the results of some 20 previous sea trials were made available by the owners and these have clearly indicated areas in need ofimprovement. Trial procedures and measurement techniques arespecified in detail in the recommended practiceguidelines. New correction methods for waves andwind for instance, are being developed and thesewill be included in the STA-analysis software to bedelivered to all participants.
Demonstration trials
As a final stage of the project, trials on selectedships are conducted according to the new practiceguidelines and analysed with the new software.One of these demonstration trials concerned theCOLOMBO EXPRESS. This 8,600 teu containervessel was constructed by Hyundai HeavyIndustries for Hapag Lloyd and is currently thelargest container vessel in operation. During thetrials in March 2005, the shaft power and shipspeed were measured, as well as the incident waveand wind conditions.
Interested?
The STA-JIP is still open to new participants,with shipowners and yards welcome. The project will be completed in 2006. For furtherinformation please contact Henk van den Boom([email protected]) or Ivo van der Hout([email protected]).
New JIP brings seatrials up to speed
As leading shipowners and shipyards co-operate in the
“Sea Trial Analysis” Joint Industry Project, Report takes
a closer look at the new initiative.
Companies currently participating in the STA-JIPClaus-Peter Offen Reederei, Daewoo Shipbuilding & Marine Engineering, ER
Schiffahrt, Hanjin, Hapag Lloyd Container Line, Hyundai Heavy Industries, Maersk,
MARIN, Norddeutsche Reederei H. Schuldt, NSB Reederei, Samsung Heavy
Industries, Shell Shipping, STX Shipbuilding, Sumitomo Heavy Industries,Teekay
Shipping, United European Car Carriers,Vroon
Courtesy BuNova Development B.V.
Henk van den [email protected]
17
Most lost containers go unnoticed andthe incidents that make it into theheadlines often relate to hazardous
and toxic chemicals in western waters. These eventsare often just the tip of the iceberg, therefore it isextremely worrying that at any one time, there arelikely to be thousands of containers floatingaround worldwide, close to shipping lanes,endangering traffic, the marine environment, localfishing industries and beach economies. Transport safety and efficiency is governed bycargo lashing and lashing procedures. Containers,cars and trucks are transported worldwide in heavywinds and high seas, while often knowledge con-cerning the actual cargo securing loads is lacking. The current economies of scale have led to thedevelopment of new generations of ultra large,deep sea container ships that stretch design limitsbeyond experience. Cargo securing loads mightchange drastically, while lashing material remainsthe same.
Pressure on schedules
Efficiency improvements in the ferry and shortseashipping sector has put additional pressure onschedules and consequently, time is of the essencefor lashing work in ports on trucks, cars andcontainers. The industry is calling for a reductionin lashing requirements in order to improveefficiency. This might jeopardise safety because itis unclear what the loads really are.
The tendency to reduce lashing efforts, in combination with unknownloads, the increasing scale of the ships and the large number of containerslost at sea, have been the major reasons for the start of the Lashing@Seaproject. Together with operators, suppliers, authorities and class societies,MARIN will investigate the mechanics, safety and efficiency of containerand ro-ro transport. The project focus will be on deepsea containershipping, the ferry industry, shortsea shipping and on feeder lines. Theoverall goals are to gain insight, improve safety, reduce the number ofdamaged and lost cargoes at sea and where possible, to increase efficiencyof lashing practice for transport companies. With a combination of a review of current practice, in-service monitoringcampaigns on three different vessels and extensive analysis, the loadmechanisms will be investigated. Based on this, guidelines for recommendedpractice for lashing procedures will be formulated in order to provide alevel playing field for all operators.
Interested?
New participants are welcome to join. Please contact Jos Koning ([email protected]).
Lashing@Sea JIP addresses the problem of containers lost at sea
Unofficial sources estimate that the number of containers
lost at sea worldwide is between 2,000 and 10,000 each
year. Lashing@Sea aims to tie down the problem.Jos [email protected]
18
A new underwater, three-component Particle
Image Velocimetry system (3C-PIV or stereo-PIV)
is available now at MARIN for detailed flow
measurements.
This system has been successfully used for the
EU-project LEADING EDGE. The challenge within
this project was to measure the flow near a
rotating propeller in open-water condition. The
experimental PIV tool yields unsteady and
averaged 3D flow field and this data can be used
to gain insight into the dynamics of unsteady
flows and of unsteady flows with immersed
bodies. PIV is also a powerful tool to validate
CFD-tools and it is also possible to quickly
measure the mean ship wake.
This maritime PIV system has been developed
through strong cooperation between the system
operator, Sirehna of France and the manufacturer,
DANTEC of Denmark. This system is available from
any European maritime experimental research
facility on a rental basis, with operational support
provided from Sirehna.
For further information please contact Jan Tukker
MARIN’s Project Team Simulator (PTS)
started in January this year and it
focuses on the selling, developing and
support of simulators. The group is
currently active in a number of delivery
and development projects.
Within PTS, a Real-time Manoeuvring
Simulator can run either as a Full
Mission Simulator, like the combined
dredging and fishing manoeuvring one
for CMO Zeebrugge, or as a Special
Simulator (DNV classification) such as
the one owned by TESO. This simulator
became operational last November on
the island of Texel and many months in
advance, all TESO captains and officers
were trained to handle the new
Dr Wagemaker ferry that comes into
service this summer!
On the development side, very
interesting projects have been initiated
regarding enhanced modelling of ship
motions in multi-directional waves,
improved interaction with other vessels
(also bottom and banks), as well as a
full-scale upgrade of the visual system
to include the latest features from the
gaming industry.
Early June, PTS sent out a mailing for
a Compact Manoeuvring Simulator that
can be installed in your own offices. The
initial reactions are quite promising. We
will keep you up-dated on the team’s
progress.
For more information please contact
Noël Bovens ([email protected]).
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Project Team Simulator off to a good start
New PIV system available at MARIN
Combined dredging and
fishing manoeuvring simulator
for CMO Zeebrugge.
Visualisation of averaged flow fields
measured at 8 planes with 3C-PIV.
19
Last spring, MARIN organised
the very popular courses
“Hydrodynamics of Floating
Offshore Structures” and
“Ship Hydrodynamics I”.
For 2006 the course schedule
is as follows:
“Hydrodynamics of Floating
Offshore Structures”
March 6-10, 2006 (5 days)
This course will focus on
floating offshore structures.
Emphasis will be on hydro-
dynamic and aerodynamic
aspects relevant to optimi-
sing the design of monohulls,
semi-submersibles, mooring
layouts and DP systems.
Attention will also be paid to the statistical determination of design
values in general and in particular, as a function of the simulation or
test duration. The operability analysis of ships and offshore structures
will be explained. Case studies will be used to explore different
approaches and techniques.
“Ship Hydrodynamics I” March 20-24, 2006 (5 days)
Various hydrodynamic design aspects (resistance, propulsion,
manoeuvring and seakeeping) will be presented in a balanced and
integrated way. The physical background, as well as the techniques
and tools available today will be dealt with. Case studies will be
included to facilitate direct application of the acquired knowledge
to selected practical problems. An advanced course (Ship
Hydrodynamics II) will be organised in 2006.
These two courses are intended for both existing professional staff
and for newcomers in the maritime industry. Participants should have
a university degree in naval architecture or ocean engineering, or
equivalent education or experience.
Fee: 1 3,250 per course (excl. hotel accommodation).
Interested?
For registration and additional information please refer to www.marin.nl.
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New course scheduleSNAME, Houston, October 19-21
Traditionally, MARIN is present at the
2005 SNAME Maritime Technology
Conference & Expo and Ship Production
Symposium and this year is no exception.
Meet us at stand no. 426/428 to discuss the
latest developments in maritime research.
Europort Maritime,
Rotterdam, November 1-5
Europort Maritime, the
international maritime and inland shipping
exhibition in Rotterdam is a result of a merger
between Europort and Rotterdam Maritime. Visit us
at stand 1306A to discuss our latest technologies.
METS, Amsterdam,
November 15-17
Together with NEDCAM (shaping technology)
MARIN will present its latest technologies for the
yacht industry. Meet us at stand 01.610.
Marintec, Shanghai,
December 6-9
This year you can also meet
us in China at the All China
Maritime Conference & Exhibition, Marintec 2005.
Visit us at stand 2D65 for the latest maritime R&D.
Open day MARIN, October 22With more then 4,000 visitors during the open day
in 2003, MARIN will again open its doors to the
public. The event takes place at our Wageningen
headquarters on Saturday, October 22, from 10 am
until 3 pm. For more information please refer to
www.marin.nl.
3830
4
2005 Offshore participants in action.
2, Haagsteeg
P.O. Box 28
6700 AA Wageningen
The Netherlands
Phone +31 317 49 39 11
Fax +31 317 49 32 45
E-mail [email protected]
http://www.marin.nl
MARIN USA, Inc.
2500 City West Blvd Suite 300
TX 77042, Houston
USA
Phone +1 713 267 2234
Fax +1 713 267 2267