a study of the parts supply of an automotive assembly...

Bernard Santens

lineA study of the parts supply of an automotive assembly

Academiejaar 2008-2009Faculteit IngenieurswetenschappenVoorzitter: prof. dr. ir. Hendrik Van LandeghemVakgroep Technische bedrijfsvoering

operationeel onderzoekMaster in de ingenieurswetenschappen: bedrijfskundige systeemtechnieken en

Masterproef ingediend tot het behalen van de academische graad van

Begeleiders: ir. Tim Govaert, Werner Grisouille (Volvo), Peter Ysebaert (DSV)Promotor: prof. dr. ir. Hendrik Van Landeghem

Preface - Word of gratitude

In the course of my Master studies of Industrial Engineering and Operations Research,it is strongly emphasized that optimization and high efficiency can make the differencebetween a good company and an excellent company. The objective of this thesis is touse all the knowledge I acquired during my studies and to apply it to a large factoryin Belgium.

A lot of people helped me in realizing this thesis, and therefore a lot of people deservethis word of gratitude. In a first instance I would like to thank my promoter, Prof.Dr. Ir. Hendrik Van Landeghem, for inspiring me to write about this subject and forhis critical opinion. This thesis gave me a lot of practical insight about enterprises ingeneral and about the automotive industry in particular.

A special thanks goes out to my mentor, Ir. Tim Govaert, for helping and supportingme throughout the whole year and for providing me with information from his previousproject at Volvo Cars Ghent.

I also would like to thank Werner Grysouille, material handling engineer at VolvoCars Ghent, and Peter Ysebaert, logistics manager at DSV, for providing me withvital information.

A final word of gratitude goes out to my family and friends for their never-endingsupport throughout the whole year.

Permission to loan - Toelating totbruikleen

The author hereby grants the permission to make this thesis available for consultationand to copy parts of the thesis for private use.Each other use falls under the restrictions of the copyright, in particular the obligationto explicitly mention the source when citing results from this thesis.

Bernard Santens, June 2009

De auteur geeft de toelating deze masterproef voor consultatie beschikbaar te stellenen delen van de masterproef te kopieren voor persoonlijk gebruik.Elk ander gebruik valt onder de beperkingen van het auteursrecht, in het bijzondermet betrekking tot de verplichting de bron uitdrukkelijk te vermelden bij het aanhalenvan resultaten uit deze masterproef.

Bernard Santens, juni 2009

A study of the parts supply of anautomotive assembly line

Bernard Santens

Masterproef ingediend tot het behalen van de academische graad van

Master in de ingenieurswetenschappen:

Bedrijfskundige Systeemtechnieken en Operationeel Onderzoek

Academiejaar 2008–2009

Promotor: Prof. Dr. Ir. H. Van Landeghem

Begeleiders: Ir. T. Govaert, W. Grysouille (Volvo), P. Ysebaert (DSV)

Faculteit Ingenieurswetenschappen

Universiteit Gent

Vakgroep Technische bedrijfsvoering

Voorzitter: Prof. Dr. Ir. H. Van Landeghem

Outline

The factory of Volvo Cars Ghent (VCG) is the largest industrial unit of the Volvo CarCorporation outside Sweden. For the just-in-sequence provision of parts to the assem-bly line, VCG makes an appeal to an external logistic partner, DSV Logistics. Thecurrent supply chain is showing evidence of too much handling and waste. Therefore,this paper will make four proposals for the future state of the supply chain. Each ofthese proposed situations is modeled using the simulation program Flexsim, in orderto determine the most optimal future state. Once determined, a sensitivity analysis ofthe most optimal situation is performed.

Key words

Supply chain, forklift free, Volvo Cars Ghent, simulation, Flexsim

A study of the parts supply of an automotiveassembly line

Bernard Santens

Supervisor(s): Prof. Dr. Ir. Hendrik Van Landeghem, Ir. Tim Govaert, Werner Grisouille (Volvo), PeterYsebaert (DSV)

Abstract—The factory of Volvo Cars Ghent (VCG) is the largest indus-trial unit of the Volvo Car Corporation outside Sweden. For the just-in-sequence provision of parts to the assembly line, VCG makes an appeal toan external logistic partner, DSV Logistics. The current supply chain isshowing evidence of too much handling and waste. Therefore, this paperwill make four proposals for the future state of the supply chain. Each ofthese proposed situations is modeled using the simulation program Flexsim,in order to determine the most optimal future state. Once determined, asensitivity analysis of the most optimal situation is performed.

Keywords— Supply chain, forklift free, Volvo Cars Ghent, simulation,Flexsim.

I. INTRODUCTION

WHEN parts are needed at the assembly line of Volvo CarsGhent (VCG), an order is sent to DSV. DSV has its logis-

tic center situated next to the VCG factory. At DSV, the neededparts are placed in racks and brought to the assembly line when-ever required. Currently, there are several disadvantages linkedto the parts supply. That is the reason why several possibilitiesfor an improved, forklift free supply chain will be investigated.

II. MEANS OF TRANSPORT

To have a better understanding of the future state, a short de-scription of the different means of transport is given first:

A. Dolly

To increase the mobility of the racks, a metallic frame onwheels, called a dolly, is welded underneath the racks.

B. Sulky

A sulky is a “C”-shaped, metallic frame that can be used toenclose and pull a standard rack. Due to the large distanceswithin DSV, it is more efficient to transport multiple racks atonce. In order to accomplish this, each tow tractor at DSV pullsa train of sulkies.

C. TOW unit

The tow tractors that are used to pull sulkies at DSV are calledTOW units. These tow tractors are also used within the VCGfactory.

D. TOWex unit

For the transport between DSV and VCG, a more powerfulkind of tow tractor is used, called a TOWex unit.

E. C-frame

A C-frame is used to transport racks between DSV and VCG.This is a “C”-shaped, metallic frame that can transport up to five

racks. In order to pull two C-frames at once, a TOWex unit canbe used. To load the C-frame, each rack is shoved over a pair offorks, which is lifted when driving.

F. Low loader

Since not all racks have standard dimensions, it is not al-ways possible to use C-frames and sulkies. For the non-standardracks, normal pulling carts are used, called low loaders.

III. THE SUPPLY CHAIN

A. The current state

The main disadvantage of the current supply chain is twofold:on the one hand, there is an abundant number of handling re-garding the transport of racks from DSV to VCG. On the otherhand, the use of wooden pallets does not only create a lot ofwaste but also involves several risks.

B. The future state

The proposed solution for the future state would solve bothdisadvantages of the current state: by combining dollies and theuse of sulkies and C-frames, the supply chain for standard rackswould be made forklift free. Of course, fork-lift trucks wouldstill be needed for loading and unloading the non-standard rackson the low loaders.

Four proposals for the future state will be simulated and dis-cussed, depending on two differences:

• in situations 1 and 2, a TOWex unit can depart from VCGback to DSV with the first empty frame that becomes available.However, in situations 3 and 4, a TOWex unit must return backto DSV with the same type of frame as the one it arrived with.

• in situations 1 and 3, the C-frames are unloaded immediatelyupon arrival at VCG. However, in situations 2 and 4, a full rackin only taken out of a C-frame at VCG when the rack has to betransported to the assembly line.

IV. THE MODEL

As can bee seen from figure 1, the supply chain can be sub-divided into five parts. VCG inbound, part four in figure 1, con-sists of three large entry halls, called GC01, GC03 and GC10.Each TOWex unit in the model can choose one of five differ-ent paths from DSV to VCG inbound: three paths representthe transport of low loaders to GC01, GC03 and GC10. The

other two paths represent the transport of C-frames to GC01 andGC10 (there are no C-frames going to GC03).

Fig. 1. Framework of the supply chain

Each of the five parts from figure 1 can be built up in thesimulation program Flexsim by using a combination of differentmodel objects and programming. Table I gives a summary ofwhat the most important Flexsim objects represent in reality.

Flexsim object in realitybox full rack containing partspallet C-frame or low loadertransporter TOWex unitoperator TOW unitprocessor at the end assembly line

TABLE ISUMMARY OF FLEXSIM OBJECTS

V. RESULTS

A. Situation 1

After the steady state period has been determined, the optimalnumber of TOWex units can be determined from simulations. Inorder to do this, the idle time of a TOWex unit is monitored,together with the mean number of fully loaded frames at DSV.Once done, the optimal number of C-frames and low loaderscan be simulated. The decisive variable here is the percentageof time that no frame was available at DSV outbound to place afull rack in.When taking the maximum number of parking spaces at DSVoutbound and VCG inbound into account, it can be concludedafter simulation that two extra parking spaces should becomeavailable at DSV outbound.To conclude situation 1, the optimal number of TOW units canbe determined. In order to do this, the response of the TOWunits to a sudden rise of the arrival rate of full racks at VCGinbound is studied.

B. Situation 2

The only difference between situations 1 and 2 is the fact thatin situation 2, C-frames are not unloaded immediately upon ar-rival at VCG. Therefore TOWex units have two extra activities

at GC01 and GC10:

• a TOWex unit arriving at VCG inbound with a full frame willhave to wait until an empty parking space becomes available.

• the TOWex unit will have to wait to go back to DSV until anempty frame becomes available.

Each of the above activities will take up a percentage of the time,depending on the number of parking spaces and the number ofextra C-frames provided at GC01 and GC10. The sum of thesefour percentages, two for GC01 and two for GC10, should beless than the TOWex unit’s mean idle time. By doing so, theoptimal supply from situation 1 will not be disturbed.

C. Situation 3

Situation 3 is also similar to situation 1, with the only dif-ference that the flow of empty frames back to DSV is divided:TOWex units arriving at VCG inbound with a full C-frame haveto take an empty C-frame back to DSV. The same applies forlow loaders. By doing so, there are two separate flows to DSV,one for C-frames and one for low loaders. This division willmake the model more susceptible to variability, and therefore aslightly higher number of C-frames and low loader is needed.

D. Situation 4

The approach taken in situation 2 is also applicable for sit-uation 4: in comparison to situation 3, two extra events occurin this situation. These events combined take up a percentageof the time that should be less than the mean percentage of idletime of a TOWex unit. However, taking the restriction on num-ber of parking spaces into account, the percentages for the twoextra events will be higher than in situation 2. Therefore themean percentage of idle time of a TOWex unit is increased bydeploying an extra TOWex unit.

The results of these four situations can be summarized in ta-ble II. It can be conluded that situation 1 is the most optimalsituation. Another conclusion is that in all four situations, it willbe the best to acquire an extra TOWex unit and an extra TOWunit, in order to be able to cope with unforeseen breakdowns.

number of ... sit. 1 sit. 2 sit. 3 sit. 4TOWex units 7 7 7 8C-frames (C-fr) 9 20 10 20low loaders (LL) 9 9 10 10parking DSV 10 10 10 10parking GC01 (C-fr) 2 7 2 7parking GC01 (LL) 2 2 2 2parking GC03 2 2 2 2parking GC10 3 9 4 9TOW units GC01 6 6 6 6TOW units GC03 1 1 1 1TOW units GC10 5 5 5 5

TABLE IIRESULTS OF THE FOUR SITUATIONS

VI. SENSITIVITY ANALYSIS

Now that the most optimal situation has been determined, it ispossible to run some “what-if” scenarios to determine the futurestate’s sensitivity to unforeseen circumstances.

The first scenario that can happen is that a TOWex unit is downfor an hour. This downtime will lead to a rise in the number offull frames at DSV. From the moment that the TOWex unit isagain available, it takes approximately 22 minutes for the pro-cess to recover from its breakdown.

The second possible scenario is the breakdown of a TOW unit.Simulations don’t show a direct link between the breakdown andmodel variables.

Nevertheless, calculations show that the maximal minimum leadtime for the whole supply chain is 31 minutes. The maximumminimal time to transport a full rack from VCG inbound to theassembly line is 6.5 minutes. Since Volvo stated that the maxi-mum lead time of a full rack could be at the highest 65 minutes,it means that there is still a large time buffer to outweigh thedisadvantages of the breakdown.

Nederlandse samenvatting

Inhoudsopgave

1 Inleiding 1

2 Transportmiddelen 22.1 Dollie . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22.2 Sulkie . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22.3 TOW unit bij DSV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22.4 TOW unit bij VCG . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22.5 TOWex unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32.6 C-frame . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32.7 Low loader . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

3 De supply chain 53.1 Huidige situatie . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53.2 Toekomstige situatie . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

4 Het model 74.1 DSV picking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74.2 DSV outbound . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84.3 Transport tussen DSV en VCG . . . . . . . . . . . . . . . . . . . . . . 84.4 VCG inbound . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84.5 Binnenin VCG . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

5 Resultaten 95.1 Scenario 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95.2 Scenario 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115.3 Scenario 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115.4 Scenario 4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

6 Aanbevelingen 13

7 Gevoeligheidsanalyse 14

Hoofdstuk 1

Inleiding

Deze scriptie beoogt de supply chain te optimaliseren in de productieomgeving van eenreeel bedrijf. Als bedrijf werd hiervoor de Volvo Car Corporation gekozen, een Zweedseautofabrikant die jaarlijks 24 000 mensen tewerkstelt.

Volvo Cars Gent (VCG) is Volvo’s grootste industriele vestiging buiten Zweden. VCGstelt 4 500 mensen tewerk en kan tot 270 000 auto’s per jaar bouwen, waaronder hetmeest recente model, de XC60.

De Volvo-fabriek in Gent kan opgedeeld worden in drie delen: de Lasfabriek, de Spuit-fabriek en de Eindassemblage. In de Lasfabriek worden staalplaten aan elkaar gelasttot een koetswerk. Dit koetswerk krijgt vervolgens verschillende verf- en beschermla-gen in de Spuitfabriek. Tenslotte wordt deze carrosserie in de Eindassemblage verderafgewerkt.

Voor de toelevering van auto-onderdelen werkt Volvo Cars Gent samen met DSV Lo-gistics. Het logistieke centrum van DSV is vlak naast de autofabriek gelegen. DSVbehandelt alle binnenkomende onderdelen volgens het “inbound-storage-outbound”-principe: de door VCG bestelde auto-onderdelen komen aan bij DSV, worden daaropgeslagen en worden, wanneer nodig, just-in-sequence naar VCG gebracht.

Deze scriptie beoogt de toevoer van onderdelen te optimaliseren vanuit DSV naarde Eindassemblage. Om dit te realiseren wordt eerst een duidelijke omschrijving vande toekomstige situatie opgesteld, alsook enkele alternatieven. Vervolgens wordt eensimulatiemodel opgebouwd van de verschillende toekomstige situaties, met behulp vanhet gespecialiseerd softwarepakket Flexsim. Door simulaties uit te voeren kan het bestealternatief gekozen worden.

Deze Nederlandse scriptie is een beknopte maar volledige samenvatting van de Engels-talige versie. Voor meer gedetailleerde informatie wordt u vriendelijk doorverwezennaar de Engelstalige versie.

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Hoofdstuk 2

Transportmiddelen

Om het vervolg van deze thesis beter te begrijpen wordt in dit hoofdstuk eerst een kortoverzicht gegeven van de verschillende transportmiddelen.

2.1 Dollie

Wanneer VCG een lijst met benodigde onderdelen doorstuurt naar DSV, worden dezeonderdelen eerst in rekken geplaatst in de DSV picking zone. Om de mobiliteit van hetrek te verhogen, wordt een frame met wielen onder het rek geplaatst. Dit frame wordteen dollie genoemd.

2.2 Sulkie

Wanneer de nodige onderdelen in de DSV picking zone in een rek geplaatst zijn, wordtdit rek vervolgens naar DSV outbound gebracht. DSV outbound is het deel van DSVvan waaruit de rekken naar VCG vertrekken. Vermits de afstand tussen de DSV pickingzones en DSV outbound redelijk groot is, worden meerdere volle rekken tegelijkertijdnaar DSV outbound gebracht. Hiervoor worden sulkies gebruikt. Een sulkie is eenC-vormig frame waarin een rek kan geschoven worden. Door meerdere sulkies achterelkaar te hangen, kunnen meerdere rekken tegelijkertijd getransporteerd worden.

2.3 TOW unit bij DSV

Een TOW unit is een driewielige elektro trekker, die binnen DSV gebruikt wordt omeen trein van sulkies te trekken. TOW units, en meer specifiek het model TOW310,worden gebouwd door de Nederlandse firma Spijkstaal en hebben een trekkracht van10 ton.

2.4 TOW unit bij VCG

Omdat bij VCG geen gebruik gemaakt wordt van sulkies kan het gebeuren dat een rekkantelt bij het nemen van een bocht in de autofabriek. Daarom is er geopteerd om eenander type TOW unit te kiezen voor het transport binnen Volvo, namelijk een TOW

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unit die in de bochten automatisch vertraagt van 10km/u naar 6 km/u. Uit testenis gebleken dat het onmogelijk is om aan deze verminderde snelheid een rek te doenkantelen.

2.5 TOWex unit

Voor het transport tussen DSV en VCG is gekozen voor een zwaarder model van deTOW unit: de TOWex unit. De TOWex unit, en meer bepaald het model 425, is eenvierwielige elektro trekker, eveneens vervaardigd door de Nederlandse Firma Spijkstaal,en met een trekkracht van 25 a 30 ton.

2.6 C-frame

Om het transport van DSV naar VCG efficienter te laten verlopen, worden meerdererekken tegelijk vervoerd. Vermits de ondergrond van de baan tussen de twee gebouwenniet optimaal is, is het onmogelijk om de rekken gedurende de hele afstand voort tetrekken op de wielen van de dollie. Daarom is een C-frame ontworpen. Een C-frame iseen C-vormig frame met vijf paar vorken dat in totaal vijf rekken kan dragen. Wanneerde rekken in DSV outbound over de vorken van het C-frame geschoven zijn, wordende vorken gelift waardoor de rekken van de grond getild worden. Vervolgens wordt hetvolle C-frame aan een TOWex unit gekoppeld, die dit C-frame naar VCG brengt.

De C-frames zijn ontworpen door de Duitse firma Fritz en elk C-frame heeft afme-tingen van 9275x2280x3000mm. Een TOWex unit kan aan een zijde gekoppeld wordenaan een C-frame.

In de toekomst zullen telkens twee C-frames aan elkaar gekoppeld worden om on-derstaande redenen:

• door twee C-frames te koppelen verdubbelt de capaciteit van het transport tussenDSV en VCG.

• door de achterzijden van de twee C-frames aan elkaar te koppelen, kan een TOWexunit gekoppeld worden aan de beide zijden van het duo C-frames. Daardoorkunnen de C-frames in twee richtingen getrokken worden, wat een groot voordeelmet zich meebrengt binnen DSV en VCG.

Voor het verdere verloop van deze thesis wordt een duo van C-frames gewoon eenC-frame genoemd.

2.7 Low loader

Wanneer alle rekken standaard afmetingen hebben, zijn sulkies en C-frames zeer nuttig.Er zijn echter ook verschillende rekken die geen standaard afmetingen hebben. Voordeze rekken is het onmogelijk om sulkies of C-frames te voorzien. Daarom worden deniet-standaard rekken vervoerd op een low loader. Een low loader heeft afmetingenvan 5400x2600x2500mm, eveneens voortgetrokken door een TOWex unit.

3

Net zoals C-frames worden ook low loaders altijd per twee voortgetrokken, waardoorde capaciteit naar zeven rekken wordt gebracht. Voor het verdere verloop van dezethesis wordt met een low loader steeds een duo van low loaders bedoeld.

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Hoofdstuk 3

De supply chain

Enkele uren voor er nieuwe onderdelen nodig zijn in de Eindassemblage stuurt VCGeen order door naar DSV. In de DSV picking zone worden deze onderdelen in rekkengeplaatst, waarna deze volle rekken naar DSV outbound gebracht worden. Ongeveer65 minuten voor ingebruikname worden deze rekken vanuit DSV outbound naar eenvan de drie ingangen van VCG gebracht. Er wordt naar deze drie ingangen verwezenmet GC01, GC03 en GC10. Een meer algemene benaming is VCG inbound.

Eenmaal aangekomen in VCG inbound worden de volle rekken naar de assemblagelijnin de fabriek gebracht. De lege rekken worden terug naar DSV gebracht om opnieuwte worden geladen.

3.1 Huidige situatie

De huidige situatie heeft twee grote nadelen. Enerzijds bevat de supply chain te veelhandelingen en anderzijds wordt er een teveel aan palletten gebruikt.

Enerzijds bestaat de huidige situatie uit te veel handelingen. Vermits de rekken mo-menteel nog niet van dollies voorzien zijn, wordt onder elk rek een houten palet vastge-maakt. Daardoor moet elke verplaatsing van een rek met een vorkheftruck gebeuren:nadat de onderdelen in een rek geplaatst zijn, moet dit rek met een vorkheftruck opeen low loader geplaatst worden. Aangekomen in DSV outbound moet het rek met eenvorkheftruck van de low loader genomen worden. Wanneer een vrachtwagen beschik-baar is, moet het volle rek opnieuw met een vorkheftruck op de vrachtwagen geplaatstworden. Deze vrachtwagen rijdt vervolgens naar VCG, waar het hele proces van ladenen ontladen opnieuw begint.

Het tweede grote nadeel is het overvloedig gebruik van houten palletten. Dit zorgtten eerste voor een grote berg afval: per uur worden meer dan 100 rekken naar VCGvervoerd. Een houten palet breekt gemiddeld na 55 transporten. Dit betekent dat elkuur gemiddeld twee palletten moeten vervangen worden. Deze overvloed aan pallettenbrengt anderzijds ook een zeker risico met zich mee: wanneer een palet zou brekentijdens het transport met een vorkheftruck kan het rek vallen en kunnen de onderdelenbeschadigd raken. Dit kan de toevoer van onderdelen naar de Eindassemblage, en dusnaar de assemblagelijn, ernstig verstoren.

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3.2 Toekomstige situatie

Nu de belangrijkste nadelen van de huidige situatie geıdentificeerd zijn, kan een toekom-stige situatie bedacht worden die deze nadelen verhelpt. De basis voor deze verbeterdesituatie is eenvoudig: de bevestiging van een dollie onder de rekken. Uit de volgendeparagrafen zal blijken dat deze verbetering de beide nadelen kan verhelpen.

Ten eerste zal het gebruik van een dollie het aantal handelingen sterk verminderen,en dan vooral het gebruik van vorkheftrucks: wanneer de nodige onderdelen in de rek-ken geplaatst zijn in de DSV picking zone, kunnen de rekken manueel in de sulkiesgeplaatst worden. Aangekomen in DSV outbound kunnen de volle rekken manueel uitde sulkies getrokken worden en in de C-frames geplaatst worden. In VCG inbound kaneen TOW unit rechtstreeks gekoppeld worden aan een vol rek in het C-frame en ermeenaar de assemblagelijn rijden. Omwille van de korte afstanden tussen VCG inbounden de assemblagelijn heeft Volvo namelijk beslist om de rekken een voor een naar deassemblagelijn te brengen.

Uit bovenstaande paragrafen kan besloten worden dat vorkheftrucks volledig overbo-dig worden in de supply chain. Hierdoor verdwijnen ook de houten palletten en eenheleboel afval.

Zoals reeds eerder vermeld hebben niet alle rekken standaard afmetingen. Omdathet onmogelijk is om voor ieder niet-standaard rek een C-frame of sulkie te ontwer-pen, geldt bovenstaande toekomstige situatie enkel voor standaard rekken, zijnde rek-ken met afmetingen 1600x1200mm en 1200x800mm. Niet-standaard rekken zullen nogsteeds met low loaders vervoerd worden, dus vorkheftrucks zullen nog steeds nodig zijn.

In het verdere verloop van deze thesis zullen vier verschillende scenario’s voor de toe-komstige situatie bestudeerd worden. Deze scenario’s zijn afhankelijk van twee ver-schillen in de toekomstige situatie:

Het eerste verschil heeft te maken met de stroom van lege rekken vanuit VCG in-bound terug naar DSV outbound. Wanneer een TOWex unit aankomt in VCG inboundwordt het volle C-frame of de volle low loader ontkoppeld van de TOWex unit. Daarnabestaan er twee mogelijkheden:

• in scenario’s 1 en 2 vertrekt de TOWex unit met het eerste lege frame dat be-schikbaar is terug naar DSV.

• in scenario’s 3 en 4 mag een TOWex unit enkel terug rijden naar DSV methetzelfde type frame als het type waarmee de TOWex unit VCG binnenkwam.

Het tweede verschil treedt op wanneer een TOWex unit VCG binnenrijdt met een volC-frame:

• in scenario’s 1 en 3 worden alle rekken onmiddellijk uit het C-frame gehaald.

• in scenario’s 2 en 4 wordt een vol rek enkel uit het C-frame gehaald wanneer hetnaar de assemblagelijn gebracht wordt. In deze scenario’s is het C-frame dus eendeel van de buffer.

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Hoofdstuk 4

Het model

Nu de vier scenario’s voor de toekomstige situatie gedefinieerd zijn, moeten deze sce-nario’s tegen elkaar afgewogen worden om uiteindelijk een scenario te kiezen. Hiervoorworden de vier scenario’s eerst in een simulatiemodel opgebouwd met behulp van hetgespecialiseerd softwarepakket Flexsim.

Een vol rek dat naar de assemblagelijn gebracht wordt, doorloopt vijf zones. De op-bouw van deze vijf zones in het simulatiemodel zullen afzonderlijk besproken wordenin onderstaande paragrafen:

4.1 DSV picking

Zoals reeds eerder vermeld, worden onderdelen in rekken geplaatst in de DSV pickingzone. Twee veronderstellingen kunnen worden gemaakt wat betreft de onderdelen ineen rek:

• elk rek bevat slechts een type onderdelen.

• wanneer het eerste onderdeel uit een rek genomen wordt voor montage op deassemblagelijn, worden enkel onderdelen uit dit rek gebruikt tot het rek leeg is.Slechts daarna worden onderdelen uit een nieuw rek gebruikt.

Door deze twee veronderstellingen wordt niet een onderdeel maar een rek als kleinsteentiteit voor het model gekozen. Een rek wordt in Flexsim voorgesteld door een box.Elk type box wordt gecreeerd door een verschillende source. De sources zijn ingedeeldin vijf grote groepen: drie groepen creeren de boxen die met low loaders getranspor-teerd worden naar GC01, GC03 en GC10. De andere twee groepen sources creeren deboxen die via C-frames getransporteerd worden naar GC01 en GC10 (er worden geenboxen via C-frames naar GC03 getransporteerd).

Voor elke source is de tijd tussen twee gecreeerde boxen exponentieel verdeeld omrekening te houden met de variantie die ontstaat bij het vrijgeven van gevulde rekkenin de DSV picking zone.

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4.2 DSV outbound

De volle rekken worden vanuit de DSV picking zone naar DSV outbound gebracht,waar ze in C-frames of op low loaders geplaatst worden. In Flexsim worden deze C-frames en low loaders voorgesteld door palletten, en het laden van deze palletten wordtgemodelleerd door een processor, gevolgd door een combiner. De geladen pallettenkomen vervolgens terecht in queue1, waar ze wachten om getransporteerd te wordenrichting VCG.

4.3 Transport tussen DSV en VCG

Volle C-frames en low loaders worden vanuit DSV naar VCG vervoerd door middel vanTOWex units. Deze TOWex units worden in Flexsim voorgesteld door transporters. Detransporters laden een vol palet op in DSV outbound, brengen dit naar VCG inbounden zetten de volle palet af. Vervolgens nemen ze een lege palet terug mee naar DSV.

4.4 VCG inbound

Een TOWex unit kan aankomen in VCG inbound met een vol geladen C-frame of eenvolle low loader. Afhankelijk van het frame en het scenario zal het model er als volgtuitzien:

een volle low loader wordt bij aankomst in VCG inbound altijd ontladen. Dit wordt inFlexsim als volgt gemodeleerd: wanneer de transporter de volle palet afzet, gaat dezepalet door de separator. Hierdoor worden de boxen en de palet van elkaar gescheiden.De lege palet kan weer meegenomen worden naar DSV, terwijl de boxen kunnen ge-transporteerd worden naar de assemblagelijn.

In scenario’s 1 en 3 worden ook de C-frames onmiddellijk uitgeladen bij aankomstin VCG inbound. In het model worden deze volle palletten dus ook door een separa-tor gestuurd, waarbij de lege palet terug kan getransporteerd worden naar DSV en deboxen naar de assemblagelijn worden gebracht.

In scenario’s 2 en 4 worden de C-frames echter niet onmiddellijk ontladen bij aan-komst in VCG inbound. In het model van deze twee scenario’s komt de palet eerst vastte zitten in een afzonderlijke queue. Slechts wanneer al de bijbehorende boxen naarde assemblagelijn gebracht zijn, wordt de palet vrijgegeven voor transport terug naarDSV outbound.

4.5 Binnenin VCG

Vanuit VCG inbound worden de rekken een voor een aan een TOW unit gekoppelden getransporteerd naar de assemblagelijn. Dit wordt in Flexsim gemodelleerd dooroperators, die de boxen een voor een oppikken en naar een processor brengen. Deoperators stellen de TOW units voor, de processor staat model voor de assemblagelijn.

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Hoofdstuk 5

Resultaten

In dit hoofdstuk wordt voor elk scenario een antwoord gezocht op volgende vragen:

1. hoeveel TOWex units moeten voorzien worden?

2. hoeveel C-frames moeten voorzien worden?

3. hoeveel low loaders moeten voorzien worden?

4. hoeveel TOW units moeten voorzien worden in GC01, GC03 en GC10?

5. hoeveel parkeerplaatsen moeten voorzien worden in DSV?

6. hoeveel parkeerplaatsen moeten voorzien worden in GC01, GC03 en GC10?

7. wat is de doorlooptijd van een rek?

5.1 Scenario 1

In scenario 1 worden de C-frames direct uitgeladen bij aankomst in VCG. Een TO-Wex unit kan, bij terugkeer naar DSV, het eerste lege frame meenemen dat in VCGbeschikbaar komt, ongeacht het type.

Eerst en vooral is het belangrijk om het einde van de overgangsperiode, en dus hetbegin van de steady state periode, te bepalen. Alle data moet immers uit de steadystate period komen om correcte resultaten te bekomen. Gedurende de overgangsperio-de werkt DSV reeds op volle snelheid, maar VCG nog niet. Daarom wordt bijgehoudenhoeveel boxen reeds gecreeerd zijn in het model, en hoeveel boxen het model hebbenverlaten. Wanneer het verschil tussen deze twee grootheden nagenoeg constant blijft,is de steady state periode bereikt. Als begin van de steady state periode, en bijgevolgals starttijd voor de metingen, kan 10 000 seconden genomen worden. Het einde vaneen simulatie kan vastgelegd worden op 423 000 seconden. Hiermee wordt per simulatieeen volledige week gesimuleerd.

Vervolgens kan het optimale aantal TOWex units onderzocht worden. Deze wordenin Flexsim voorgesteld door transporters. Hiervoor worden in het model voldoendepalletten en operatoren voorzien. Voor een verschillend aantal transporters wordt eenafweging gemaakt tussen:

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• het gemiddelde percentage van de totale tijd dat een transporter niets te doenheeft.

• het gemiddelde aantal en maximum aantal volle palletten dat in DSV outboundstaat te wachten op een transporter.

• de gemiddelde tijd en maximum tijd dat een volle palet in DSV outbound staatte wachten op een transporter.

Uit deze afweging kan beslist worden dat het optimaal aantal TOWex units gelijk isaan zeven.

De volgende stap in het onderzoek is het bepalen van het optimale aantal C-frames.Hiervoor wordt het aantal transporters vastgehouden op zeven en worden voldoendelow loader palletten voorzien. Tijdens de simulaties wordt gekeken naar het percen-tage van de totale tijd dat er geen lege C-frame pallet voorhanden was om boxen opte plaatsen in DSV outbound. Hetzelfde wordt nagegaan voor de low loader palletten.Hieruit kan geconcludeerd worden dat negen C-frames en negen low loaders het opti-male aantal is voor scenario 1.

Wanneer vervolgens rekening gehouden wordt met het aantal parkeerplaatsen dat voor-handen is, kan opgemerkt worden dat enkel het aantal parkeerplaatsen in DSV out-bound problemen geeft. Na onderzoek blijkt dat het aantal parkeerplaatsen bij DSVuitgebreid moet worden met twee, wat het totale aantal op tien parkeerplaatsen brengt.Simulatie toont ook aan dat de restricties op het aantal parkeerplaatsen niets veran-derd hebben aan het optimale aantal TOWex units, low loaders en C-frames.

Bij het bekijken van de doorlooptijden van de boxen in Flexsim kan geconstateerdworden dat deze zeer hoog zijn. Verschillende oorzaken hiervoor worden onderzocht:zowel de minimum doorlooptijd als het aantal transporters blijken niet de doorslag-gevende redenen te zijn. Het laden van boxen op palletten blijkt daarentegen eenklempunt te zijn: veronderstel dat een eerste box op een leeg pallet geladen wordt. Opdat moment loopt de meting voor de doorlooptijd van deze box. Wanneer het echterlang duurt vooraleer de pallet volgeladen is met boxen, zal de doorlooptijd van de eer-ste box zeer hoog zijn. Een goede samenwerking tussen DSV picking en DSV outboundzal dus onontbeerlijk zijn in de toekomst. Deze optimalisatie valt echter buiten hetbestek van deze thesis.

Als laatste deel van de optimalisatie van scenario 1 kan het nodige aantal TOW unitsbinnen VCG onderzocht worden. Deze staan in voor het transport van rekken tussenVCG inbound en de assemblagelijn. Dezelfde aanpak als bij de TOWex units zal ech-ter niet tot het optimale resultaat leiden, vermits het hier om een pull-situatie gaat.Daarom wordt in eerste instantie gekeken naar de buffergrootte van volle rekken inVCG inbound. Wanneer deze stabiel kan worden gehouden, wordt in tweede instantiegekeken naar de responsie van TOW units op plotse stijgingen van de arrival rate vanvolle rekken in VCG inbound. Wanneer deze plotse stijgingen op een vlotte manieropnieuw kunnen afgebouwd worden, betekent dit dat het aantal TOW units voldoendeis. Zo komen we tot het besluit dat in GC01, GC03 en GC10 respectievelijk zes, een

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en vijf TOW units moeten ingezet worden. Daarbij wordt best nog een extra TOWunit als reserve gehouden.

5.2 Scenario 2

Na scenario 1 is een optimalisatie van scenario 2 aan de beurt: in dit scenario vertrekteen TOWex unit nog steeds vanuit VCG met het eerste lege frame dat beschikbaar is.Het verschil met scenario 1 is echter dat het C-frame bij aankomst in VCG inboundniet onmiddellijk uitgeladen worden. In plaats daarvan wordt een vol rek enkel uit eenC-frame gehaald wanneer het rek naar de assemblagelijn gebracht wordt. Het resultaathiervan is dat er in VCG inbound continu veel meer C-frames geparkeerd zullen staanen dat er bijgevolg veel meer C-frames nodig zullen zijn. Vermits scenario’s 1 en 2niet veel van elkaar verschillen, zullen veel simulaties een gelijkaardige uitkomst heb-ben. In deze paragraaf worden enkel de verschillen tussen de twee scenario’s besproken.

In vergelijking met scenario 1 kunnen bij de TOWex units in scenario 2 twee extraactiviteiten aangetroffen worden:

1. een TOWex unit moet bij het binnenrijden van VCG inbound wachten op eenvrije parkeerplaats.

2. een TOWex unit moet in VCG inbound wachten op een leeg C-frame dat be-schikbaar is.

Deze twee activiteiten zullen een percentage van de totale tijd van een TOWex unitinnemen. Het doel is om het aantal extra C-frames en parkeerplaatsen voor GC01en GC10 dermate te kiezen dat dit percentage van de totale tijd kleiner is dan hetpercentage van de totale tijd dat een TOWex unit geen opdrachten heeft. Op diemanier wordt de optimale supply chain uit scenario 1 niet verstoord. Na simulatiesblijkt dat in GC01 vijf extra parkeerplaatsen en vijf extra C-frames voorzien moetenworden. In GC10 moeten zes extra C-frames en parkeerplaatsen voorzien worden.

5.3 Scenario 3

Scenario 3 beschrijft de toekomstige situatie waar C-frames onmiddelijk uitgeladenworden bij aankomst in VCG inbound, zoals in scenario 1. Een TOWex unit kan ech-ter niet vertrekken uit VCG inbound met het eerste lege frame dat beschikbaar is. Hetlege frame moet van hetzelfde type zijn als het frame waarmee de TOWex unit VCGbinnenreed. Als een TOWex unit bijgevolg met een vol C-frame aankomt in VCG in-bound moet de TOWex unit wachten op een leeg C-frame om het terug naar DSV tebrengen.

Vermits er in scenario 3 twee verschillende flows zijn van lege frames die worden terug-gebracht naar DSV, in plaats van een in scenario 1, zal er een lichte stijging zijn vanhet aantal nodige C-frames en low loaders. In scenario 3 zijn tien C-frames en tien lowloaders nodig, in plaats van negen in scenario 3.

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Het aantal nodige parkeerplaatsen in GC10 stijgt ook naar vier. Dit leidt op zichniet tot problemen, vermits er negen parkeerplaatsen voorhanden zijn in GC10.

5.4 Scenario 4

Het laatste scenario, scenario 4, is een combinatie van scenario’s 2 en 3: C-framesworden niet onmiddellijk uitgeladen bij hun aankomst in VCG inbound, en de stroomvan lege frames terug naar DSV wordt opgesplitst in een stroom voor C-frames en eenstroom voor low loaders.

Zoals voor scenario 2 beroep werd gedaan op conclusies uit scenario 1, zal voor ditscenario beroep gedaan worden op scenario 3. Ten opzichte van scenario 3 zullen bijde TOWex units in scenario 4 opnieuw twee extra activiteiten aangetroffen worden:

1. een TOWex unit moet bij het binnenrijden van VCG inbound wachten op eenvrije parkeerplaats.

2. een TOWex unit moet in VCG inbound wachten op een leeg C-frame dat be-schikbaar wordt.

Deze twee activiteiten zullen ook hier een percentage van de totale tijd van een TOWexunit innemen. Er is daarentegen er nog een bijkomend probleem. Vermits in GC10slechts negen parkeerplaatsen beschikbaar zijn, en in scenario 4 reeds vier van dezeparkeerplaatsen nodig zijn, is er nog plaats voor slechts vijf extra parkeerplaatsen voorC-frames (in plaats van zes in scenario 2). Het bijbehorend percentage van de tijd dateen TOWex unit hierdoor zou moeten wachten op een parkeerplaats bij het binnenrij-den van VCG is bijgevolg veel te hoog.

Om die redenen wordt geopteerd om een extra TOWex unit in te schakelen, wat zorgtvoor een stijging van het percentage van de totale tijd dat een TOWex unit geen op-drachten heeft. Hierdoor kunnen de percentages van de twee extra activiteiten ookhoger zijn. Na simulaties blijkt dat vijf extra C-frames en parkeerplaatsen in GC01, enevenveel C-frames en parkeerplaatsen in GC10, optimaal is mits in totaal acht TOWexunits voorzien worden.

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Hoofdstuk 6

Aanbevelingen

Uit tabel 6.1 kan besloten worden dat scenario 1 op alle vlakken het meest optimalescenario blijkt.

optimale aantal ... scenario 1 scen. 2 scen. 3 scen. 4

TOWex units 7 7 7 8C-frames 9 20 10 20low loaders 9 9 10 10parkeerplaatsen in DSV 10 10 10 10parkeerplaatsen in GC01 (C-fr) 2 7 2 7parkeerplaatsen in GC01 (LL) 2 2 2 2parkeerplaatsen in GC03 2 2 2 2parkeerplaatsen in GC10 3 9 4 9TOW units in GC01 6 6 6 6TOW units in GC03 1 1 1 1TOW units in GC10 5 5 5 5

Tabel 6.1: Samenvatting

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Hoofdstuk 7

Gevoeligheidsanalyse

Na het bepalen van het optimale scenario moet van dit scenario een gevoeligheidsana-lyse opgesteld worden. Deze analyse gaat na hoe het scenario reageert op onvoorzieneomstandigheden. Concreet wordt er nagegaan welke invloed een gebeurtenis heeft ophet proces en hoe lang het duurt eer het proces zich weer hersteld heeft.

Eerst wordt het effect van het uitvallen van een TOWex unit gesimuleerd. Hierbijwordt een van de TOWex units een uur op non-actief gezet en wordt gekeken of deandere zes TOWex units de volle frames tijdig naar VCG inbound kunnen transpor-teren. Er wordt met andere woorden gekeken of het gemiddeld aantal volle frames inDSV outbound stijgt, en hoe lang het duurt vooraleer deze stijging weer weggewerkt is.

Uit deze simulaties blijkt dat er inderdaad een stijging plaatsvindt gedurende de werk-onderbreking van de TOWex unit. Nadien heeft het proces gemiddeld 22 minutennodig om te herstellen van deze onderbreking.

Ook het effect van het uitvallen van een TOW unit kan gesimuleerd worden. Ver-mits de TOW units zich echter in de pull-sectie van de supply chain bevinden, is hetniet eenvoudig de juiste variabele te vinden om op te volgen:

• een plotse stijging van het aantal volle rekken in VCG inbound is niet noodzakelijkte wijten aan het verminderde aantal TOW units. Deze stijging kan ook hetgevolg zijn van de aankomst van enkele TOWex units in VCG inbound.

• een tekort aan onderdelen kan te wijten zijn aan het feit dat de tijd tussen tweegecreeerde boxen in het Flexsim model exponentieel verdeeld is. Dit tekort is dusniet noodzakelijk te wijten aan het uitvallen van een TOW unit.

Het wordt al snel duidelijk dat het erg moeilijk is om het effect van het uitvallen vaneen TOW unit op te volgen. Er kan echter opgemerkt worden dat de maximale mini-mum transporttijd van een rek tussen VCG inbound en de assemblagelijn 6.5 minutenis. De maximale minimum doorlooptijd van een rek doorheen de volledige supply chainis slechts 31 minuten, terwijl deze maximum 65 minuten mag zijn. Wanneer de rek-ken voldoende op voorhand naar VCG inbound gebracht worden, kan er geconstateerdworden dat het uitvallen van een TOW unit opgevangen kan worden door het tijds-overschot van de rekken.

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Er kan besloten worden dat met scenario 1 een robuuste en optimale oplossing biedtvoor de toekomstige toevoer van onderdelen vanuit DSV naar de VCG Eindassembla-ge.

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English thesis

Contents

1 Introduction 1

2 Literature study 3

3 Means of transport 53.1 Dolly . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53.2 Sulky . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53.3 TOW unit at DSV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63.4 TOW unit at VCG . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63.5 TOWex unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63.6 C-frame . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73.7 Low loader . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

4 Description of the supply chain 104.1 Current state . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114.2 Future state . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

5 The simulation model 135.1 The model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

5.1.1 Within DSV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145.1.2 DSV outbound . . . . . . . . . . . . . . . . . . . . . . . . . . . 145.1.3 Between DSV and VCG . . . . . . . . . . . . . . . . . . . . . . 155.1.4 VCG inbound . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195.1.5 Within VCG . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205.1.6 Flexsim images . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

5.2 The programming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 235.2.1 Different paths . . . . . . . . . . . . . . . . . . . . . . . . . . . 235.2.2 Number of parking spaces . . . . . . . . . . . . . . . . . . . . . 285.2.3 Unloading a C-frame . . . . . . . . . . . . . . . . . . . . . . . . 285.2.4 Variable speed operators . . . . . . . . . . . . . . . . . . . . . . 295.2.5 Lead times . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 305.2.6 Steady state . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

6 Results 316.1 Situation 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

6.1.1 Steady state time . . . . . . . . . . . . . . . . . . . . . . . . . . 326.1.2 Number of TOWex units . . . . . . . . . . . . . . . . . . . . . . 326.1.3 Number of low loaders and C-frames . . . . . . . . . . . . . . . 34

6.1.4 Parking spaces . . . . . . . . . . . . . . . . . . . . . . . . . . . 366.1.5 Lead times . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 396.1.6 Number of TOW units at VCG . . . . . . . . . . . . . . . . . . 41

6.2 Situation 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 486.2.1 Steady state . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 486.2.2 Number of TOWex units . . . . . . . . . . . . . . . . . . . . . . 486.2.3 Number of C-frames, low loaders and parking spaces . . . . . . 516.2.4 Number of TOW units at VCG . . . . . . . . . . . . . . . . . . 53

6.3 Situation 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 546.3.1 Number of low loaders and C-frames . . . . . . . . . . . . . . . 546.3.2 Parking spaces . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

6.4 Situation 4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 556.4.1 Number of TOWex units, C-frames, low loaders and parking spaces 55

7 Recommendation 58

8 Sensitivity analysis 598.1 Failure of a TOWex unit . . . . . . . . . . . . . . . . . . . . . . . . . . 598.2 Failure of a TOW unit at VCG . . . . . . . . . . . . . . . . . . . . . . 61

9 Conclusion 62

A Product specifications of a TOW unit, model TOW310 67

B Product specifications of a TOWex unit, model TOW425 69

C Technical drawing of a C-frame 73

D Transport details of the required parts and racks 75

Chapter 1

Introduction

The company I chose to become the subject of this thesis is the Volvo Car Corporation,a Swedish car manufacturer consisting of 24 000 employees world-wide and with aproduction of 461 000 cars in 2007.

Volvo Cars Ghent (VCG) is the largest industrial unit of the Volvo Car Corporationoutside Sweden. VCG employs 4 500 workers and can produce up to 270 000 cars ayear, including the most recent model, the XC60.

The VCG factory can be subdivided into three chapters: the welding plant, the paintshop and the final assembly. At the start of a new car production process, steel platesare welded together in the welding plant to form the body of the car. Once the bodyis shaped, transportation takes place to the paint shop, where it receives painting andprotective coatings. In a last stage the body receives its finishing touch in the finalassembly. A floor plan of the VCG plant and its three sections is presented in figure1.1.

Figure 1.1: Floor plan of VCG and its three sections

To supply the different VCG sections with the required parts, VCG makes an appealto an external logistic partner, DSV Logistics. This company has a logistic centersituated next to the VCG building and processes all the incoming parts according totheir “inbound - storage - outbound” principle: the ordered parts for VCG arrive at

1

DSV, where they get stored in one of the large picking halls. When VCG sends anorder, the ordered parts are picked, placed in racks and brought just-in-sequence tothe assembly line at VCG. A floor plan of the DSV center is shown in figure 1.2.

Figure 1.2: Floor plan of DSV

In this thesis, the supply chain between DSV and the third section of VCG, called thefinal assembly, will be studied. Based on the current situation, some proposals will bemade for the future state. These proposals will afterwards be modeled in a powerfulsimulation program called Flexsim. By running simulations on these models, the dif-ferent proposed situations can be compared in order to make a final recommendation.

2

Chapter 2

Literature study

As already mentioned in the previous chapter, a few possibilities for the future statewill be proposed in this thesis. The different possible situations will be entered in asimulation program called Flexsim.

In the first part of this chapter, the reason why simulation is being used to find anoptimal solution, will be provided, whereby several papers have been consulted.

In the second part, the advantages will be outlined of having a supplier nearby.

The reason for simulation. According to paper [1], simulation increases the un-derstanding of the production process and assists the manager in making the rightdecisions. Furthermore, the impact of these decisions on the performances of the pro-cesses can be evaluated using simulation. Paper [4] lists some specific issues that canbe tackled using simulation:

• the quantity of equipment and personnel:

– the number of machines needed

– requirements for transporters, conveyors and other support equipment (e.g.pallets)

– the location and size of inventory buffers

– the amount of capital investments

– the effect of a new piece of equipment entering the system

• the performance:

– throughput analysis

– time-in-system analysis

– reliability analysis

Paper [4] also provides us with some performances measures often estimated by simu-lations:

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• throughput

• time in system for parts

• time a part spends in queues

• queue sizes

• utilization of equipment and personnel

Obviously, there are also alternatives to gather the above information. However, ac-cording to papers [3] and [1], simulation has quite a lot of advantages:

• it enables to optimize the performance of the future state long before it is imple-mented

• it detects and eliminates problems that otherwise require cost- and time-consumingcorrection measures during production

• it enables to run experiments and “what-if” scenario’s without disturbing anexisting production system

• it enables to make trade-offs between capacity, cycle time and capital investment

Finally, one of the most important challenges when building simulation models is todetermine whether the simulation model is an accurate representation of the systembeing studied, i.e. whether the model is valid [4]. To determine the validity of a futurestate, the model has to be broken down into basic units that are equal or similar tothe current state. Only if these basic units are valid and the links between these unitsis logical and correct, the model is valid.

The advantages of having a supplier nearby. In paper [2], David Levy states thatjust-in-time delivery and low inventories are at the heart of lean production systems. Inorder to acquire these low inventories, fast and frequent flows of goods and informationare required. A supplier located at a larger distance leads to more WIP1 to fill thewhole supply chain. Furthermore, distance also results in a need for higher levels ofbuffer inventories: with longer, more uncertain lead times, buffer inventories must copewith fluctuating demand.

The car industry also has to meet the demand for flexible manufacturing, i.e. the abilityto customize a car, because this reduces the batch sizes and finished goods inventory.Flexible manufacturing requires thus rapid delivery from suppliers in order to avoidhigh inventory levels. This rapid delivery can easily be obtained when the supplier islocated nearby.

A last advantage of having the suppliers and their customers close to each other, isthe fact it enables them to share more information about their processes, quality levelsand cost reduction measures.

1WIP = Work In Progress

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Chapter 3

Means of transport

Before describing the current and future state of the supply chain, the different meansof transport will be outlined in this chapter in order to ensure a better understandingof the remaining part of the thesis.

3.1 Dolly

When new parts are required at the assembly line of VCG, they are picked at DSVand placed in racks. To ease the handling of these racks, a metallic frame on wheels,called a dolly, is welded underneath the racks. As a result, the rack can be moved andpulled without having to use a forklift truck. Figure 3.1 shows three standard racks,each placed on a dolly.

Figure 3.1: Three standard racks + dolly

3.2 Sulky

As mentioned before, the parts that are needed at the assembly line of VCG are pickedat DSV and placed in racks on dollies. The full racks are afterwards pulled by a towtractor from the DSV picking zone to another section in DSV, called DSV outbound,where the racks are prepared to be transported to VCG.

Due to the large distance between DSV picking and DSV outbound, it is more efficientto pull multiple racks at once. In order to accomplish this, each tow tractor pulls a

5

train of sulkies. A sulky is a metallic frame in the shape of the letter “C”. Each ofthese C-frames encloses and pulls a standard rack, which is easy to roll in and out ofthe sulky. Figure 3.2 shows a tow tractor pulling an empty sulky.

Figure 3.2: TOW unit + sulky

3.3 TOW unit at DSV

The tow tractors that are used at DSV to pull the sulkies are called TOW units. TOWunits are three-wheel electric tractors, model TOW310, which are manufactured by theDutch company Spijkstaal. They have a 10 ton towing capacity. Figure 3.3 representsa TOW unit currently used at DSV. The product specifications can be found in AnnexA.

Figure 3.3: TOW unit model 310

3.4 TOW unit at VCG

In VCG, the racks are not placed in a sulky, as it is the case in DSV. As a result, dueto the lack of support, the racks are easily tilted when taking turns. In order to solvethis issue, VCG will use a different type of TOW unit, i.e. a TOW unit automaticallyslowing down from 10km/h to 6km/h when taking a turn. This speed decrease preventsthe TOW unit from tilting the rack when taking a turn.

3.5 TOWex unit

For the transport between DSV and VCG, a different kind of tow tractor is used: theTOWex unit. It is purchased for exterior use, and is a heavier version of the TOWunit previously described. The TOWex unit, more specifically the model 425, is an

6

electric four-wheel tractor, also made by the Dutch company Spijkstaal. It has a 25-30ton towing capacity. Figure 3.4 shows an example of the TOWex unit. The productspecifications can be found in Annex B.

Figure 3.4: TOWex unit model 425

3.6 C-frame

To enable the transport of multiple racks at once between DSV and VCG, C-framesare developed. C-frames are covered wagons, having a frame in the shape of the letter“C”. They are pulled by a TOWex unit. Due to the bad conditions of the road betweenDSV and VCG, the wheels of the dolly cannot carry the racks over the whole distance.Therefore, each C-frame is equipped with five pairs of forks. These forks are used tolift the racks during transport. Upon arrival, the forks are lowered again and the rackscan easily be pulled out of the C-frame. The C-frame also contains suspension forabsorbing shocks when driving over the rough road between DSV and VCG.

In the future state, two C-frames at once will be pulled behind each other for tworeasons:

1. this enables DSV to bring ten racks at once to VCG, versus only five racks whenusing only one C-frame at once.

2. by connecting the rear sides of the two C-frames, the TOWex unit can be con-nected at either the front or the back of the C-frame couple. As a consequence,the C-frame couple can be pulled in two directions, which saves a lot of space atDSV outbound and VCG.

The above described C-frames are designed and manufactured by the German companyFritz, and each C-frame measures 9275x2280x3000mm. A technical drawing of a C-frame can be found in Annex C. Figure 3.5 shows a C-frame couple pulled by a TOWexunit. Figure 3.6 on the other hand shows three racks placed in a C-frame.

Since the C-frames will always be paired in the future situation, a C-frame couple willsimply be called a C-frame in the remainder of this thesis.

7

Figure 3.5: C-frame couple pulled by TOWex unit

Figure 3.6: C-frame loaded with three standard racks

3.7 Low loader

Sulkies and C-frames are very useful on the condition that all the racks have the samesize. Unfortunately, this is not the case at DSV. Therefore a different kind of frame,with a size of 5400x2600x2500mm, is used to transport non-standard racks from DSVto VCG: the low loader.

Low loaders are connected two by two for the same reasons as it is the case for C-frames.A low loader couple has a capacity of carrying seven non-standard racks. Figure 3.7represents a low loader couple pulled by a TOWex unit.

Since the low loaders will always be paired in the future situation, a low loader couplewill simply be called a low loader during the rest of this thesis.

8

Figure 3.7: Low loader couple pulled by TOWex unit

9

Chapter 4

Description of the supply chain

As mentioned in chapter 1, VCG can be subdivided into three sections: the weldingplant, the paint shop and the final assembly. The objective of this chapter is to studythe current and future state of the supply chain from DSV to the third section, thefinal assembly.

Several hours before the assembly line requires new racks of parts, VCG sends a pickingorder through to DSV. Once DSV receives the order, empty racks are manually filledup with the needed parts and brought to DSV outbound. This is the area where partsare leaving the DSV buildings in order to get transported to the final assembly at VCG.In normal circumstances, about 65 minutes before a part is needed at the assemblyline, the part should be at DSV outbound.

The racks, coming from DSV, arrive at VCG in one of the three entry halls (GC01,GC03 or GC10), also called VCG inbound. The racks are then distributed all along theassembly line and empty racks are at the same time picked up and sent back upstreamto DSV inbound. Figure 4.1 shows the path a rack follows from DSV inbound to theassembly line.

Figure 4.1: Floor plan of the path from DSV inbound to the assembly line

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4.1 Current state

The main disadvantage of the current supply chain is twofold: the abundant numberof handling as well as the use of pallets.

First of all, there is too much handling in the current state: when transporting theracks with parts from DSV to VCG, DSV puts the racks on wooden pallets instead ofusing dollies. As a consequence, the use of a forklift truck is required for:

• putting a full rack on a low loader at the DSV picking zone

• taking of the full rack from the low loader again at DSV outbound

• placing the full rack on a lorry at DSV outbound

• loading and unloading the racks at VCG after transport by a lorry

The second disadvantage of the current supply chain is the use of pallets: at normalpace there are 64 standard racks and 49 non-standard racks per hour that have tobe transported to VCG. A pallet has a life span of about 35 to 80 transports, whichimplies that each hour approximately two pallets should be replaced. As a consequence,this process does not only produce a lot of waste, but also contains several risks. Forinstance, in case a pallet would break during forklift transportation, the parts on thepallet can get damaged and can thus interrupt the whole supply chain.

4.2 Future state

Now that the two main disadvantages of the current state have been identified, thefuture state can be designed to minimize these disadvantages. Based on the two prob-lems, described in paragraph 4.1, it is clear that one improvement will be key to buildan efficient future state: the use of a dolly under each rack. This key element shouldsolve both problems.

First of all, it would decrease the number of handling: when all the needed partsare mounted in a rack, the full rack is manually pushed in a sulky and the rack istransported to DSV inbound. At DSV outbound, the rack is manually pulled out ofthe sulky and placed over a pair of forks in the C-frame. When a full C-frame arrivesat VCG, a full rack is coupled to a TOW unit and transported to the assembly line.Because of the short distances between VCG inbound and the assembly line itself,Volvo has decided that the racks will be brought to the line one by one.

Secondly, by simply putting a dolly under the rack, forklift trucks are eliminated fromthe supply chain, meaning that the pallets would also be made redundant. As a resulta lot of waste would be removed from the supply chain. This solution to the palletsproblem would only work in case all the racks would have the same size (1600x1200mmor 1200x800mm). For non-standard racks, it would become impossible to design specialC-frames for transport. Instead, non-standard racks would still have to be transportedon low loaders. Therefore, a few forklift trucks would still be necessary.

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In the following chapters, four different alternatives for the future state will be exam-ined. These four situations are based on two different decisions that could be made inthe future state.

The first decision is related to the stream of empty frames from VCG inbound backto DSV and occurs when a TOWex unit arrives at VCG inbound and parks its fullC-frame or low loader:

• in situations 1 and 2, the TOWex unit can depart to DSV with the first availableempty frame. So for each entry hall there is one common stream of empty framesback to DSV.

• in situations 3 and 4, the TOWex unit must take the same type of frame back toDSV as the one it arrived with. For example, if a TOWex unit arrives at VCGwith a full C-frame, it has to go back to DSV with an empty C-frame, not withan empty low loader. So for each entry hall there are two separate streams ofempty frames back to DSV.

The second decision is related to the unloading of the C-frames:

• in situations 1 and 3, a C-frame is unloaded immediately upon arrival at VCGinbound.

• in situations 2 and 4, the full racks stay in the C-frame, so the C-frame acts asa buffer. The full rack is only taken out of the C-frame by a TOW unit once therack has to be transported to the assembly line.

To conclude this chapter, the four different situations can be represented schematicallyin figure 4.2.

Figure 4.2: Schematic representation of the four different situations

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Chapter 5

The simulation model

Now that the future state of the supply chain is determined, it can be converted intoa simulation model. As already mentioned before in chapter 1, the model will be builtup in a software program called Flexsim. The following chapter consists of two parts:

In the first part, the building of the simulation model will be explained. Starting atDSV and going downstream, the future state can be divided into five parts as can beseen in figure 5.1. For each of these parts, the building of the model will be explainedseparately.

Figure 5.1: Schematic representation of the supply chain

In the second part, the more complex programming will be explained. This program-ming was written and sufficiently tested in smaller models before it was inserted intothe large model. These so-called test models will be explained separately. The testmodels can be found in the electronic version of this thesis.

5.1 The model

Before explaining the model, table 5.1 summarizes the most important Flexsim objectsand their equivalents in reality.

Flexsim object reality

box full rack containing partspallet C-frame or low loadertransporter TOWex unitoperator TOW unit at VCGprocessor at the end the assembly line

Table 5.1: Summary of the most important Flexsim objects

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5.1.1 Within DSV

As already mentioned in chapter 4, VCG sends its picking orders for the needed partsthrough to DSV. Within DSV, the parts are manually placed in racks and sent toDSV outbound, where the racks are transported to VCG. Before moving to the actualconstruction of the model, it is fair to make the following two assumptions:

1. a full rack only contains parts of the same type.

2. when starting to empty a new rack at the assembly line, all the parts from theprevious rack have already been put on the assembly line.

Based on these two assumptions, it is logical to say that not a part, but a rack is thesmallest entity that has to be modeled in the simulations. In Flexsim, these racks arerepresented by boxes, and each type of box is created by a different source in the model.Each source is part of a large group. There are five large groups, whereby the sourcesof three groups create the racks that will be transported to GC01, GC03 and GC10with low loaders. The sources of the other two groups create the racks that will betransported to GC01 and GC10 with C-frames (there are no C-frames going to GC03).

Each source has its own name, starting with the VCG code of the rack. This VCGcode can be found in the table in Appendix D. In the name of the source, the VCGcode is followed by the letter “C” or “L”, depending whether a C-frame or a low loaderis used for the transport to VCG. The final part of the source’s name refers to thenumber of the entry hall the rack will be transported to (GC01, GC03 or GC10). Alittle example:

the window of the left door of a car has VCG code DRE, and has to be brought to GC01with a C-frame. This explains why the name of the source is DRE C01.

In the table in Appendix D, the mean inter-arrival time for each type of rack is alsoshown. Since the release of the picked racks does not happen at constant speed, vari-ation is added to the model: in Flexsim, each source creates boxes according to anexponential distribution, with the mean equal to the corresponding inter-arrival timefrom the table in Appendix D.

Finally, each type of box in Flexsim also has an item type equal to its VCG code,provided through the OnExit-trigger of the source. As a result, the boxes can alwaysbe recognized during the simulation.

5.1.2 DSV outbound

The main function of DSV outbound is twofold: on the one hand preparing the trans-port of full racks to VCG, on the other hand taking care of the stream of empty racksback to the DSV picking zone.

The first function of DSV outbound is preparing the transport of full racks to one ofthe three entry halls at VCG. In order to do this, both a number of racks from the DSVpicking zone and an empty low loader or C-frame is required. There are two differencesbetween loading a low loader and loading a C-frame:

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1. the loading capacity: a C-frame has capacity for ten racks, while a low loaderhas only space for seven racks.

2. the loading time: the loading time of one rack in either a C-frame or a low loaderis represented in table 5.2. A triangular distribution is used for the loading time,where the minimum and maximum values are -10% and 25% of the mean time1:

min time mean time max time

C-frame 18s 20s 25sLow loader 27s 30s 37.5s

Table 5.2: Loading time of a rack in a C-frame or on a low loader at DSV

As already mentioned before, the racks are represented in Flexsim by boxes. The C-frames and low loaders are represented in Flexsim by pallets. To make the distinctionbetween C-frame pallets and low loader pallets, a different item type has been assignedto each of them: a C-frame pallet receives item type 70, a low loader pallet item type80.

The loading process in Flexsim is modeled by a processor, followed by a combiner. Theprocess time of the processor is the above triangular distribution, whereas the processtime of the combiner is zero. The combiner quantity is set on seven boxes and onepallet for the low loader representation, whereas the C-frame representation has a tento one ratio.

The second function of DSV outbound is taking out the empty racks from a C-frameor low loader, so that the empty racks can be filled up again at the DSV picking zone.However, the empty racks are not represented by an item in the Flexsim model. As aresult, empty pallets arriving at the DSV outbound zone in the Flexsim model, haveto pass a processor first before they can be re-used. One processor uses a triangulardistribution for modeling the emptying of a C-frame, with values equal to ten timesthe values in table 5.2. A second processor uses a triangular distribution for modelingthe emptying of a low loader, with values equal to seven times the values in table 5.2.

5.1.3 Between DSV and VCG

After a C-frame or low loader is fully loaded, a TOWex unit is attached to it at DSVoutbound. This TOWex unit drives the loaded frame from DSV outbound onto theproperty of VCG, until it encounters an intersection. At the intersection, the TOWexunit chooses a direction, depending on which entry hall it should bring the loadedframe to. When the TOWex unit arrives at the correct entry hall of VCG, it parksthe loaded frame and detaches it. Afterwards, an empty frame gets attached to theTOWex unit and driven back to DSV outbound.

The following times have been measured during tests with the C-frames (it is assumedthat the loading and unloading time of a low loader is similar):

1all the minimum and maximum percentages in this thesis are copied from the previous simulationstudy on this topic, performed by Ir. Tim Govaert

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• changing of towing unit: disconnect electrical plug, disconnect towing unit,drive to other C-frame, connect C-frame, connect electrical plug = 90 seconds

• opening C-frame: stepping out of towing unit, opening C-frame 1, loweringC-frame 1, opening C-frame 2, lowering C-frame 2 = 150 seconds

• closing C-frame: stepping out of towing unit, taking racks up C-frame 1, closingC-frame 1, taking racks up C-frame 2, closing C-frame 2 = 120 seconds

In Flexsim, a TOWex unit is represented by a transporter, and a loaded frame isrepresented by a loaded pallet. When a pallet in the model is fully loaded with boxes,it enters queue1. When a transporter becomes available, it picks up the loaded pallet,drives it to one of the VCG entry halls and drops it off. The transporter then picks upan empty pallet, drives it back to the DSV outbound zone of the model and drops thepallet off.

Only the loading and unloading time of a transporter can be inserted in Flexsim. Thevalue of these times can be calculated by using the three above times, measured duringtests with C-frames: the first of the three test times is split up into three times thirtyseconds, and the following times are used in the Flexsim model:

• load time of a TOWex unit = ”closing C-frame” + a part of ”changing oftowing unit” = 120 + 30 seconds = 150 seconds

• unload time of a TOWex unit = ”opening C-frame” + a part of ”changingof towing unit” = 150 + 30 seconds = 180 seconds

• driving time between unloading and loading a TOW-ex unit = 30 seconds

Above model times are again considered as mean times. Similar to the previous studyof Ir. Tim Govaert, the minimum and maximum percentages of the used triangulardistributions are set to -15% and 30%. The result of using these percentages can befound in table 5.3:


load time 127.5 150 195unload time 153 180 234

Table 5.3: Load and unload time of TOWex unit

Now that the loading and unloading of the TOWex units is covered, it is time to studythe path between DSV outbound and VCG inbound that has to be covered by theTOWex units. To model this path in Flexsim, it has been subdivided using networknodes. For each path connecting two network nodes, a speed limit and virtual distancecan be assigned. A floor plan of all the network nodes is presented in figure 5.2.

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Figure 5.2: Floor plan of the network nodes

Finally, in tables ??, ??, 5.6, 5.7, 5.8 and 5.9, the speed limit and virtual distance foreach path between two network nodes is tabulated and some remarks have been added.Below a legend for these remarks has been set up:

• IN: driving within a building, where the speed of a TOWex unit is 4.5 km/h =1.25 m/s

• OUT: driving outside, where the speed of a TOWex unit is 10 km/h = 2.78 m/s

• ONEWAY: driving is only allowed in one way

• GATE: the path between these network nodes represents driving through a gate

• VIRT: for the path between these network nodes, only the time is known. So ifa virtual distance and virtual speed are assigned, the time can be simulated.

The properties for the network nodes from DSV outbound to the intersection arerepresented in table 5.4:

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node path speed limit virtual distance remarks

NN1 - NN2 1.25 20 IN / ONEWAYNN122 - NN1 1 30 IN / ONEWAY / VIRTNN13 - NN122 10000 0.01 IN / VIRTNN2 - NN13 1.25 20 IN / ONEWAYNN2 - NN121 2.78 50 OUTNN121 - NN3 1 10 OUT / GATE / VIRTNN3 - NN4 2.78 144 OUTNN4 - NN5 2.78 437 OUT

Table 5.4: Properties of network nodes between DSV and VCG, before intersection

At the intersection, the speed limits and distances from table 5.5 can be applied:


NN5 - NN6 1 10 OUT / VIRTNN6 - NN7 1 10 OUT / VIRTNN6 - NN8 1 10 OUT / VIRTNN6 - NN9 1 10 OUT / VIRT

Table 5.5: Properties of network nodes between DSV and VCG, on intersection

Starting from the intersection, TOWex units can follow different paths to the differ-ent entry halls. Table 5.6 represents the properties of the network nodes from theintersection to the unloading area for C-frames at GC01:


NN8 - NN10 2.78 35 OUTNN10 - NN11 1 10 GATE / VIRTNN11 - NN22 1.25 37.5 IN / ONEWAYNN22 - NN23 1 30 IN / ONEWAY / VIRTNN23 - NN11 1.25 12.5 IN / ONEWAY

Table 5.6: Properties of network nodes between DSV and VCG, driving to GC01 with C-frame

To reach GC03, the same path can be followed until network node NN10. From NN10to GC03, the properties of the network nodes are presented in table 5.7:

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Table 5.7: Properties of network nodes between DSV and VCG, driving to GC03

In order to arrive at the unloading area for low loaders at GC01, a TOWex unit has todrive straight ahead on the intersection. The properties for the network nodes of thispath are presented in table 5.8:



Table 5.8: Properties of network nodes between DSV and VCG, driving to GC01 with lowloader

To reach GC10, the network node properties of table 5.9 can be applied:


NN26 - NN27 2.78 540 OUTNN27 - NN29 1 10 GATE / VIRTNN29 - NN33 1.25 25 IN / ONEWAYNN29 - NN37 1.25 25 IN / ONEWAYNN33 - NN32 1 30 IN / ONEWAY / VIRTNN37 - NN32 1 30 IN / ONEWAY / VIRTNN32 - NN29 10000 0.01 IN / ONEWAY / VIRT

Table 5.9: Properties of network nodes between DSV and VCG, driving to GC10

5.1.4 VCG inbound

Upon arrival at VCG inbound, a TOWex unit has to park its loaded C-frame or lowloader and connect to an empty frame. This process is very similar to the process atDSV outbound. For example, the time it takes a TOWex unit to couple or uncouplefrom a C-frame or low loader is the same. Nevertheless, there are also some modelingdifferences between the VCG inbound zone and the DSV outbound zone in Flexsim:

1. since the low loaders get unloaded immediately, there is no processor in frontof the separator in Flexsim. Instead, the unload time is directly applied to the

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separator itself: a triangular distribution with values equal to seven times theunloading times of a non-standard rack leads to a minimum time of 189 seconds,a mean time of 210 seconds and a maximum time of 262.5 seconds. The sameprinciple can be applied for the unloading of the C-frame pallets in situations 1and 3.

2. unlike situations 1 and 3, the C-frames are not unloaded immediately in situations2 and 4. In these situations, a rack is only taken out of the C-frame when needed.Therefore, the process time for the C-frame separators in Flexsim equals zero.However, because the C-frames or not unloaded immediately, the C-frame palletin Flexsim cannot be taken back to DSV once it comes out of the separator.Instead, the empty pallet has to stay at the VCG inbound zone until all ofthe ten accompanying boxes are on their way to the assembly line. In orderto accomplish this, some specific programming was needed. This code will beexplained in paragraph 5.2.

5.1.5 Within VCG

At VCG inbound, loaded racks are attached to the TOW units and driven to theassembly line. Since the distances are relatively short, Volvo has decided that thetransport should follow the one-on-one ratio, meaning that a TOW unit only pulls onerack at a time. Arriving at the assembly line, the TOW unit parks the loaded rack,uncouples it and takes an empty rack back to VCG inbound. At VCG inbound, therack is again uncoupled and, in the case of a standard rack, parked in a C-frame.

The following times result from former tests at VCG:

• taking rack out of C-frame with towing unit: driving backwards, connect rack,driving frontwards out the C-frame = 20 seconds

• putting rack in C-frame: driving backwards, position rack, disconnect rack, driv-ing frontwards = 30 seconds

In Flexsim, the transport within VCG is modeled as follows: the boxes arrive at VCGon pallets, carried by transporters. Each full pallet is sent through the separatorand the boxes arrive afterwards at a second queue, representing the buffer area atVCG inbound. From this second queue, boxes are picked up one by one by operators,representing the TOW units. The boxes are afterwards brought to their predestinedqueue, representing the correct assembly line address, which is known by looking atthe item type of the boxes. After a box is placed in its predestined queue, the operatorreturns empty-handed, representing the flow of empty racks.

According to this Flexsim model, the load time and the unload time of the operatorsshould both equal the sum of the two above times resulting from the tests. Thesetest times are again mean times, so a triangular distribution is applied again, withpercentages -10% and 25%. As a result, table 5.10 can be created:

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load time 45 50 62.5unload time 45 50 62.5

Table 5.10: Load and unload time of TOW units at VCG

At the end of the model, the boxes in Flexsim are placed on a processor that representsthe actual assembly line. For each type of rack in Flexsim, the process time on theprocessor equals the mean inter-arrival time from the table in Appendix D.

5.1.6 Flexsim images

Before starting on the actual programming of the model, this paragraph shows threescreenshots of the Flexsim model.

Figure 5.3 shows the whole Flexsim model:

Figure 5.3: Flexsim model of the whole supply chain

Figure 5.4 shows the part of the model that represents DSV:

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Figure 5.4: Flexsim model of DSV

To conclude, figure 5.5 shows the part of the model that represents GC10:

Figure 5.5: Flexsim model of GC10

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5.2 The programming

After the model is built up, some programming is added to control the whole process.Since there are four different situations to be simulated, the code for the differentsituations can sometimes differ a little bit. In this chapter the code for situation 1 willbe explained. When there are big differences with the other situations, these codes willalso be explained.

The code was most of the time tested sufficiently in so-called test models before imple-menting it into the large model. These test models will be explained in the followingparagraphs.

5.2.1 Different paths

When loaded racks arrive at DSV outbound, they are placed in a C-frame or on a lowloader. A C-frame can be transported to two different entry halls at VCG, while a lowloader can be transported to three different entry halls. In the large Flexsim model,these five possibilities can be represented by five different loops:

• going to GC01 on a C-frame: the loop made by queue1, queue2, queue3, queue4.

• going to GC01 on a low loader: the loop made by queue1, queue6, queue3, queue4.

• going to GC03 on a low loader: the loop made by queue1, queue8, queue9, queue4.

• going to GC10 on a C-frame: the loop made by queue1, queue10, queue11,queue4.

• going to GC10 on a low loader: the loop made by queue1, queue12, queue11,queue4.

When programming these five loops, a few difficulties have to be taken into account:

• in Flexsim, the choice which loop to take already has to be made at queue1.

• at VCG, a TOWex unit has to take the first empty frame back to DSV, regardlessthe type of frame it brought to VCG inbound.

• once chosen the right loop, it is not certain that an empty frame will already beavailable at VCG. When a TOWex unit arrives at VCG inbound and no emptyframe is available, the TOWex unit has to wait at VCG until an empty framebecomes available.

Due to all these difficulties, the program code was first created in the test model “2different paths”. In this test model, there are two different loops, each starting andending with the same queue, as it is the case in the big model:

• loop 1: queue1, queue2, queue3, queue4.

• loop 2: queue1, queue5, queue6, queue4.

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The code can be split up into two parts, and for every part a flowchart can be drawn.

Figure 5.6: Test model ”2 different paths”, flowchart 1

Figure 5.6 shows the first flowchart and can be explained as followed: every time a fullpallet enters queue1, the program requests transport from a transporter for one of thetwo different loops, depending on the item type of the pallet. For instance: if the palletin Flexsim represents a C-frame going to GC01 in reality, the item type is 1.

According to the code, the dispatcher gives an idle transporter a sequence of tasks:drive to queue1 (= DSV outbound) and pick up a full pallet, drive to queue2 (= VCGinbound) and unload the pallet, then drive to queue3 where the empty pallets arestored. At the end of this task sequence, a message is sent to the controller asking to

24

complete the task sequence if possible. To accomplish this process, the following codeis used in the “Request transport from”-tab of queue1:

treenode ts = createemptytasksequence( Dispatcher, 0, 0 );

inserttask(ts,TASKTYPE_TRAVEL,current,NULL);

inserttask(ts,TASKTYPE_FRLOAD,item,current,port);

inserttask(ts,TASKTYPE_TRAVEL,outobject(current,1),NULL);

inserttask(ts,TASKTYPE_FRUNLOAD,item,outobject(current,1),

opipno(current,port));

inserttask(ts,TASKTYPE_TRAVEL,Queue3,NULL);

inserttask(ts,TASKTYPE_CALLSUBTASKS, controller, NULL,

10, 1, 1, 1 );

dispatchtasksequence(ts);

}

The controller receives the message and verifies if an empty pallet is available. Ifso, the task sequence is completed: the empty pallet is picked up by the transporterat queue3 (= VCG inbound) and brought back to queue4 (= DSV outbound). Ifthere is no empty pallet available, the waiting transporter is added to a waiting listcalled WaitingOperators3. All this information is inputted in the model by adding thefollowing code in the OnMessage-trigger of the controller:

treenode TaskExecuter = msgsendingobject;

int AmountOfBoxes = content( Queue3 );

treenode FoundProduct2 = NULL;

if( AmountOfBoxes > 0 ){

//search for an available box

for( int j = 1; j <= AmountOfBoxes; j++ )

{

if( getlabelnum( rank( Queue3, j ), "Dispatched" ) == 0 )

{

setlabelnum( rank( Queue3, j ), "Dispatched", 1 );

FoundProduct2 = rank( Queue3, j );

j = 1000;

}

}

}

if( objectexists( FoundProduct2 ) ){

treenode ts = createemptytasksequence( TaskExecuter, 0, 0 );


inserttask(ts,TASKTYPE_FRLOAD,FoundProduct2,Queue3,1);

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inserttask(ts,TASKTYPE_FRUNLOAD,FoundProduct2,Queue4,1);

//already travel back to the first queue


return tonum(ts);

}

else{

treenode WaitingOperators3 = label( current, "WaitingOperators3" );

nodeinsertinto( WaitingOperators3 );

nodeadddata( last( WaitingOperators3 ), 1 );

setnodenum( last( WaitingOperators3 ), tonum( TaskExecuter ) );

treenode ts = createemptytasksequence( Dispatcher, 0, 0 );

inserttask(ts,TASKTYPE_UTILIZE,TaskExecuter,NULL);

return tonum(ts);

}

When an empty pallet enters queue3, the second part of the cycle starts. Figure 5.7shows the flowchart of this second part:

26

Figure 5.7: Test model ”2 different paths”, flowchart 2

The empty pallet enters queue3 and the code in the OnEntry-trigger first examineswhether a waiting transporter is available. If not, the empty pallet stays at queue3 (=VCG inbound) and nothing happens. However, when a transporter is already waiting,the empty pallet is labeled as “Dispatched”, the transporter is removed out of thewaiting list and the task sequence for this transporter is completed, i.e. pick up theempty pallet and bring it back to queue4 (= DSV outbound). The following code isadded to the properties of queue3 to accomplish this process:

if( NumberOfWaitingOperators > 0 ){

setlabelnum( item, "Dispatched", 1 );

treenode ts = createemptytasksequence( TaskExecuter, 0, 0 );

inserttask(ts,TASKTYPE_TRAVEL,current,NULL);

inserttask(ts,TASKTYPE_FRLOAD,item,current,1);


inserttask(ts,TASKTYPE_FRUNLOAD,item,Queue4,1);

//Let’s already travel back to the first queues


27

dispatchtasksequence(ts);

//Remove the TaskExecuter from waiting

freeoperators( Dispatcher, TaskExecuter );

destroyobject( first( WaitingOperators3 ) );

}

5.2.2 Number of parking spaces

One of the main restrictions in the model will be the number of parking spaces availableat DSV outbound and at the three different entry halls of VCG inbound. Due to thecomplexity of the Flexsim model, most of the time the number of parking spaces willequal the sum of the contents of different queues, separators or combiners in the model.

The test model “parking spaces” enables the measuring of the sum of contents ofdifferent objects. This test model shows two production lines and the goal is to knowthe content of the sum of queue7 and queue8 at each moment in time. In order to reachthis goal, two global tables have been added first, named parking spaces and aid file.Next, the following code is added in the OnEntry-trigger of both queue7 and queue8:

int i = gettablenum("aid file", 1,1);

settablenum("parking spaces",i+1,1,gettablenum("aid file",2, 1)+1);

settablenum("aid file",2,1,gettablenum("aid file",2,1)+1);

settablenum("aid file",1,1,gettablenum("aid file",1,1)+1);

This code can be explained as followed: every time a new box enters one of thesequeues, the OnEntry-trigger looks up the previous sum of contents in the global tableaid file, adds one and enters this new content to the table parking spaces. Similarly,whenever a box exits one of these two queues, the OnExit-trigger is activated and theprevious sum of contents minus one is added to the table parking spaces, by using asimilar code.

At the end of the simulation run, the data of table parking spaces is exported to anExcel file. As a result, the maximum number of needed parking spaces can be foundand compared with the current capacity of the investigated hall.

When the sum of the contents for different successive objects has to be found, themodel can even be simplified: in this case it is sufficient to enter the above code in theOnEntry-trigger of the first object, and the similar second code in the OnExit-triggerof the last object.

5.2.3 Unloading a C-frame

When a C-frame is uncoupled from a TOWex unit in situations 2 and 4, the C-frame isnot unloaded immediately, as it is the case with the low loaders. Instead, an unloadedrack is only picked out of a C-frame the moment it is brought to the assembly line

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by a TOW unit. In Flexsim, this means that a pallet can only be released and takenback to DSV when all of the boxes accompanying the pallet are on their way to theassembly line.

To model this, a test model named “unloading C-frame” is created: in this Flexsimmodel, an empty pallet can only leave queue25 when all its corresponding boxes aretaken out of queue20. The properties of queue25 are altered as followed:

• add the line closeoutput(current); to the OnReset-trigger. this way the out-put of queue25 is closed at the beginning.

• set the OnExit-trigger to close output : every time a pallet leaves queue25, theoutput closes until it is opened again by an exterior command.

Afterwards, the center ports of queue25 and queue20 are connected and the followingcode is added to the OnExit-trigger of queue20:

double aantalblokken = content(current);

double aantalpaletten = content(Queue25);

if( aantalblokken - 3*(aantalpaletten-1) == 1){ openoutput(Queue25);

}

In this test model, queue25 only releases the empty pallet when all three boxes thatwere on the pallet are taken out of queue20. After the pallet has been released, theoutput of queue25 is closed again by the OnExit-trigger.

5.2.4 Variable speed operators

Theoretically the TOW units can drive at 10km/h through VCG. Within a factoryhowever, the TOW units are slowed down due to many different factors:

• when taking a turn, a TOW unit is automatically slowed down to 6km/h, toprevent the rack from tilting.

• when arriving at a gate, the TOW unit has to wait for the gate to open.

• sometimes the TOW unit has to slow down for other traffic or personnel.

• ...

Therefore in Flexsim, a variation in maximum speed is built in for the operators rep-resenting the TOW units. In the test model ”variable speed operators”, every time anoperators picks up a box or puts one down, its maximum speed is changed to a ran-dom speed between one and twenty (m/s). This change is accomplished by adding thefollowing code in both the OnLoad -trigger and the OnUnload -trigger of the operator:setvarnum(current,"maxspeed",uniform(1,20));.

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5.2.5 Lead times

Another time to determine, is the time it takes a box to start at DSV outbound and toend at the assembly line. The code used for this calculation is implemented in the testmodel “lead times”. In this model the lead time of every box, created by two sources,is stored in 2 columns, one for each source. In this way it is possible to see which typeof boxes gives the real problem whenever a time limit is exceeded.

The first step in the program is giving each box an item type. In this example theboxes get either item type 101 or 102 (in the large model the item types start at 101and go up to 179, as can be seen from the table in Appendix D).

Secondly, two global tables are added to the model, named lead times and aid file leadtimes. In the first table, there are as many columns as there are item types, and enoughrows are provided. Finally, at the end of the model, the OnEntry-trigger of the sink(or sinks) is provided with the following code:

int j = getitemtype(item)-100;

int i = gettablenum("aid file lead time", 1,j);

settablenum("lead times",i+1,j, time() - getcreationtime(item));

settablenum("aid file lead time",1,j,

gettablenum("aid file lead time",1,j)+1);

Every time a box enters the sink, the creation time of the box is reclaimed and sub-tracted from the current time. This lead time is then added to the designated columnof the lead times table, depending on the item type of the box. By exporting again thetable data to an Excel file at the end of the run, the lead times for the different boxescan be reclaimed.

5.2.6 Steady state

Before arriving at steady state, the model undergoes a transition period for a certainamount of time. During this period, DSV works already at full capacity, but VCGnot yet. It is very important to make sure that all measurements are made withinthe steady state. To estimate the duration of this transition period, the number ofcreated boxes at the beginning of the Flexsim model are put in the first column oftable steadystate by the OnExit-trigger of the sources. The number of destroyed boxesat the end of the Flexsim model are inserted in the second column of table steadystateby the OnEntry-trigger of the sinks.

After each run, the data is exported to an Excel file. In addition, the difference betweenthe number of inputs and the number of outputs at each moment in time is calculated.When this difference stays approximately the same, the steady state is achieved.

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Chapter 6

Results

When the model is built up, simulation runs can be executed to come up with answersto the following questions:

1. how many TOWex units are needed?

2. how many pairs of C-frames are needed?

3. how many pairs of low loaders are needed?

4. how many TOW units are needed at GC01, GC03 and GC10 each?

5. what is the lead time of a rack?

6. how many parking spaces are needed at DSV?

7. how many parking spaces are needed at GC01, GC03 and GC10 each?

In this chapter, an answer to each of these questions can be found. However, it is veryimportant to think about the order in which these questions will be tackled, since dif-ferences in parameters upstream will also influence the outcome of different simulationparameters downstream. The order of the next paragraphs follows the order in whichthe simulations have been executed.

Note: in the simulation results, one run represents a period of one week, equal to423 000 seconds.

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6.1 Situation 1

The first situation that will be discussed is the most basic one. In this situation, uponarrival at VCG inbound, a TOWex unit travels back to DSV with the first emptyframe available, regardless the type of frame. The loaded C-frames are also unloadedimmediately upon arrival at VCG inbound.

6.1.1 Steady state time

Before starting to collect the data, it is crucial to know when the transition periodends, because all the measurements have to be executed within the steady state. Toknow the starting time of this steady state, the model was run three times. Each run,the number of boxes that entered and left the Flexsim model were plotted. Figure 6.1shows one of these runs. The other two runs have a similar outcome.

Figure 6.1: Steady state (situation 1)

As can be seen in figure 6.1, the ending of the transition period is situated somewherebetween 5 000 and 10 000 seconds. For safety’s sake, 10 000 seconds is assumed to bethe beginning of the steady state and thus the starting time of the measurements.

6.1.2 Number of TOWex units

After the steady state time is determined, it is possible to start optimizing the model.First the optimal amount of TOWex units will be determined. Before running thesimulations, it is important that enough C-frames, low loaders and TOW units areavailable, so that a delay in the process can only be due to a shortage of TOWex units.Therefore, enough pallets and operators are added to the Flexsim model.

As mentioned before, the TOWex units are represented in Flexsim by transporters.For each amount of transporters in Flexsim, three simulations have been run, and thefollowing two variables were monitored in Flexsim:

32

1. the average and maximum number of loaded pallets in queue1, representing thenumber of loaded C-frames and low loaders at DSV outbound.

2. the average and maximum staytime of a loaded pallet in queue1.

3. the utilization of the transporters, especially the percentage of the time the trans-porters were idle.

The results regarding the content of queue1 are summarized in table 6.1:

run 1 run 2 run 3 mean

5 transportersavg staytime in queue1 21 293.51max staytime in queue1 26 916.52avg content of queue1 57.21max content of queue1 70

6 transportersavg staytime in queue1 573.55 789.95 1 255.98 873.16max staytime in queue1 2 377.51 2 781.2 3 164.4avg content of queue1 1.76 2.39 3.85 2.67max content of queue1 8 10 11

7 transportersavg staytime in queue1 214.44 206.79 204.19 208.47max staytime in queue1 1 202.87 953.69 1 236.5avg content of queue1 0.7 0.64 0.61 0.65max content of queue1 4 4 4

8 transportersavg staytime in queue1 166.24 165.39 163.96 165.20max staytime in queue1 722.76 644.39 591.4avg content of queue1 0.53 0.5 0.51 0.51max content of queue1 4 4 4

Table 6.1: Properties of queue1 as a function of the number of transporters (situation 1)

The data of table 6.1 can also be represented in graphs, as shown in figure 6.2. It isobvious that seven is the minimal amount of transporters needed: the staytime of aloaded pallet as well as the number of loaded pallets waiting at DSV outbound, risessignificantly when less than seven transporters are used. The advantage of using morethan seven transporters, on the other hand, is minimal.

33

Figure 6.2: Content and staytime of queue1 (situation 1)

The results regarding the utilization of the different transporters in Flexsim are repre-sented in table 6.2. When seven transporters are available, every unit has 16.2% idletime, leaving enough room for unforeseen circumstances.

1 2 3 4 5 6 7 8 mean

6 transp.run 1 3.4% 3.1% 3.7% 5.0% 4.2% 3.7% 3.9%run 2 1.7% 1.5% 2.3% 2.2% 2.6% 2.9% 2.2%run 3 0.6% 0.4% 1.2% 1.3% 0.8% 1.2% 0.9%

MEAN % 1.9% 1.7% 2.4% 2.8% 2.5% 2.6% 2.3%

7 transp.run 1 9.3% 9.7% 10.3% 14.3% 16.1% 18.8% 28.3% 15.3%run 2 8.5% 10.3% 12.8% 13.7% 16.9% 21.7% 28.8% 16.1%run 3 9.3% 11.3% 12.9% 15.3% 15.5% 24.0% 31.9% 17.2%

MEAN % 9.0% 10.4% 12.0% 14.4% 16.2% 21.5% 29.7% 16.2%

8 transp.run 1 11.7% 14.1% 15.3% 19.3% 22.2% 27.7% 37.9% 58.3% 25.8%run 2 11.7% 13.7% 15.4% 17.7% 23.5% 30.6% 38.1% 60.2% 26.4%run 3 12.7% 14.3% 16.0% 21.7% 24.5% 31.0% 41.1% 63.0% 28.0%

MEAN % 12.0% 14.0% 15.6% 19.6% 23.4% 29.8% 39.0% 60.5% 26.7%

Table 6.2: Idle time of the transporters (situation 1)

It can thus be concluded that the optimal number of TOWex units is seven. In all foursituations, the best option is to acquire an extra TOWex unit. By doing so, a back-upTOWex unit is kept stand-by, in case one of the seven TOWex units breaks down.

6.1.3 Number of low loaders and C-frames

Now that the optimal number of TOWex units has been found, simulations can be runto determine the optimal number of low loaders and C-frames.

34

To determine the optimal number of low loader pallets in Flexsim, the following ad-justments have been made to the model:

1. the model contains seven transporters, as this is the optimal amount determinedfrom the previous paragraph.

2. enough empty C-frame pallets are provided at DSV, so that disturbances are onlydue to a shortage of low loader pallets.

3. at the start of the simulation, an empty C-frame pallet and an empty low loaderpallet are provided at GC01 and GC10. An empty low loader pallet is alsoprovided at GC03.

In Flexsim, the boxes representing racks are loaded on an empty pallet by a combiner.In order to load the low loader pallets, there are three combiners available, one foreach entry hall. While varying the number of empty low loader pallets generated bysource10 at the beginning of the simulations, the idle time of these three combinershas been monitored. The idle time of these combiners represents the time that a fullrack, coming from DSV picking, arrives at DSV inbound and no low loader is availableto place the rack on. So the time that a low loader has to wait at DSV outbound forfull racks is not included in this idle time. The results of the simulations are displayedin table 6.3:

# low loaders run 1 run 2 run 3 mean %

6 loading for GC01 13.7% 12.5% 10.3% 12.2%loading for GC03 13.6% 11.6% 11.4% 12.2%loading for GC10 32.2% 27.1% 25.4% 28.2%

17.5%


4.2%


0.6%

9 loading for GC01 0,0% 0,0% 0,0% 0,0%loading for GC03 0,0% 0,0% 0,0% 0,0%loading for GC10 0,0% 0,0% 0,0% 0,0%

0,0%

Table 6.3: Idle time of the low loader combiners (situation 1)

The same principle is used for determining the optimal number of C-frames: in Flexsim,enough empty low loader pallets are provided at DSV, and the number of C-framepallets is varied. During the runs, two combiners are monitored in Flexsim. These are

35

the two combiners that load the boxes on the empty C-frame pallets, one combiner forGC01 and one combiner for GC10. The results of these simulations are summarized intable 6.4:

# C-frames run 1 run 2 run 3 mean %

5 loading for GC01 22.3% 24.2% 19.9% 22.1%loading for GC10 30.6% 31.8% 24.0% 28.8%

25.5%


5.9%


0.5%


0.0%

Table 6.4: Idle time of the C-frame combiners (situation 1)

It can be concluded that eight empty low loaders and seven empty C-frames are theminimal amounts needed at DSV inbound at the beginning of the simulation. Takinginto account the three empty low loaders at the three entry halls of VCG and the twoempty C-frames at GC01 and GC10 at the beginning of the simulation, this brings theminimal amount to ten low loaders and ten C-frames.

Since the empty flow of C-frames and low loaders from an entry hall back to DSV arethe same in this situation, it isn’t necessary to provide both an empty C-frame andan empty low loader at GC01 and GC10. Instead, providing one empty frame at eachentry hall in the beginning of the simulation is sufficient, for example one empty C-frame at GC01 and one empty low loader at GC10. Simulations for this scenario showthat the idle time of a TOWex unit stays the same, and the idle time of the combinersshows a slight but negligible rise. This means that in total nine C-frames and nine lowloaders have to be purchased.

It is again recommended that an extra C-frame and low loader are purchased to keepstand-by in all four situations, in case one of the C-frames or low loaders breaks down.

6.1.4 Parking spaces

After the most optimal number of TOWex units, C-frames and low loaders have beendetermined, it can be investigated if this optimal solution is also possible in practice:are there enough parking spaces at DSV outbound as well as in the three entry hallsat VCG to store all these frames?

The maximum amount of parking spaces for the different halls is summarized in table6.5. In this table, the number of parking spaces for C-frames and low loaders at GC01

36

is given separately. The reason for this is the fact that in the future state, the C-framesand low loaders will enter GC01 through a different gate. C-frames and low loaderswill also have their own parking area at GC01. However, this does not mean that thetwo streams to bring empty pallets back to DSV are also separated, as this is only thecase in situations 3 and 4.

location # parking spaces

DSV 8GC01 C-frames 4

GC01 low loaders 6GC03 3GC10 9

Table 6.5: Maximum amount of parking spaces available at DSV and VCG

As the above restriction on the number of parking spaces is applied to the simulationmodel, it becomes clear that eight parking spaces at DSV is the only requirement thatis not met. In order to make a first estimation for the number of needed parking spacesat DSV, the following reasoning has been applied: in the future state, there are fivedifferent, simultaneous paths (two to GC01, two to GC10 and one to GC03). For eachpath a TOWex unit enters DSV with an empty frame, parks that frame and departswith another loaded frame. Every path needs thus two parking spaces. This reasoningresults in a first rough estimation of ten parking spaces as the minimal amount neededat DSV outbound. A suggested floor plan for ten parking spaces at DSV outboundis given in figure 6.3. This floor plan suggests the addition of two extra gates so thatevery parking space has its own gate, in order to minimize and ease the movementswithin the DSV building.

Figure 6.3: Suggested floor plan DSV inbound

The new restriction of ten parking spaces at DSV outbound is applied to the simula-tion model. Table 6.6 summarizes the gathered data regarding DSV outbound, andrepresents the percentage of time that a certain number of parking spaces is needed.

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needed parking spaces % of the time

≤ 7 0.00%≤ 8 2.61%≤ 9 20.02%10 77.36%

Table 6.6: Needed number of parking spaces at DSV inbound (situation 1)

As can be seen in table 6.6, most of the time ten parking spaces at DSV inbound willbe used. This implies that most of the time a TOWex unit, coming from VCG, willhave to wait until an empty parking space becomes available at DSV.

In order to find out whether seven TOWex units would still be the optimal numberin case the parking restriction is applied, a close look needs to be taken at the statecharts of the different transporters and the average content and staytime of queue11 inFlexsim:

Figure 6.4: Flexsim output 7 transporters with restriction on parking spaces (situation 1)

In figure 6.4, a new state has entered the chart: the state blocked represents the timethat a transporter, coming from VCG and arriving at DSV, has to wait until an emptyparking space becomes available. Due to this waiting period, the percentage of meanidle time of a transporter decreases, and the average waiting time and content ofqueue1 slightly increases. A comparison of these three variables with and without therestriction of parking spaces is made in table 6.7. The mean idle time of a transporterhas decreased by 50%, but is still acceptable. The average waiting time and content

1queue1 represents the number of loaded C-frames and low loaders at DSV inbound waiting fortransportation

38

of queue1 are slightly elevated but are also quite good. Therefore it can be concludedthat seven is still the optimal number of TOWex units.

variable without restriction with restriction

mean idle time transporter 16.2% 8.36%average waiting time queue1 208.47 249.54

average content queue1 0.65 0.74

Table 6.7: Variables regarding the optimal number of TOWex units (situation 1)

6.1.5 Lead times

As already mentioned before, VCG inbound plays the buffer between the part of thesupply chain that is pushed and the part of the supply chain that is pulled, and thereforeWIP is stored at VCG inbound. Therefore it is not useful to measure the lead timefrom beginning to end, but it is rather adequate to measure the lead time starting atDSV outbound and arriving at VCG inbound. This time has to be smaller than 3 900seconds (= 65 minutes) minus the time needed to move the rack from VCG inbound tothe assembly line, assuming worst case scenario. When the lead times are investigated,it can be noticed that this is not the case. For example a rack of type 148, which is thewiring for the servo steering brought to GC03, has a maximum lead time of 5 481.57seconds. It is most important that the root cause for these high lead times is found.

The first possible root cause is the duration of the transport between DSV and oneof the entry halls of VCG in worst case scenario. Therefore the worst case times arecalculated from the moment the racks are mounted on the low loader or in the C-frame,to the moment all the racks are unloaded at one of the entry halls. The worst scenarioin the simulation model can be described as followed:

1. an empty pallet is loaded with boxes.

2. when the pallet is loaded, it arrives at queue1. At that moment, all the trans-porters just started to load another full pallet from queue1

3. all the transporters have to deliver their loaded pallet to the farthest entry hall(GC10). The transporters then return from GC10 with an empty pallet, unloadit at DSV outbound and drive back to queue1. This process takes in total 2 034seconds.

4. finally, one of the transporters picks up the loaded pallet, transports it to itsdestination and drops it of.

The worst case time for each of the five different paths is represented in table 6.8:

39

type of frame + destination worst case lead time

C-frame to GC01 445 + 2 034 + 1 005 = 3 484 secondslow loader to GC01 457.5 + 2 034 + 1 040.5 = 3 022 secondslow loader to GC03 457.5 + 2 034 + 1 143.5 = 3 635 secondsC-frame to GC10 445 + 2 034 + 1 213 = 3 692 secondslow loader to GC10 457.5 + 2 034 + 1 225.5 = 3 717 seconds

Table 6.8: Worst case lead times (situation 1)

The times displayed in table 6.8 are worst case scenario, and thus highly unlikely tooccur. However, they are still much lower than the simulated times in the model. Adifferent root cause will thus have to be discovered.

A second possible root cause might be the number of TOWex units. Therefore anotherTOWex unit is added to the model, bringing the total to eight TOWex units. The leadtimes for simulations with seven and eight TOWex units have been compared. Seetable 6.9 for some examples. Nevertheless, there are no large differences, so it is highlyunlikely that the number of TOWex units causes the highly excessive lead times.

type 7 TOWex units 8 TOWex units

136 4 618.34 4 632.7148 5 481.57 5 474.26164 3 680.35 3 518.25

Table 6.9: Lead times depending on number of TOWex units

A third root cause can be the loading of racks in an empty frame: suppose there is anempty pallet at DSV inbound, representing an empty C-frame, ready to be filled upwith ten boxes. A first box is created by a source and put on the empty pallet. Atthis time the lead time measurement for this box starts running. However, if it takesa long time before the other nine boxes are created, the lead time for the first box willbe very high. Indeed, looking at the maximum staytime of a pallet on a combiner, itcan be noticed that some boxes have a very high staytime at the combiners:

type + destination run1 run2 run3 mean

C-frame to GC01 2 404.78 2 218.69 2 475.48 2 366.32low loader to GC01 4 338.67 4 407.47 4 832.98 4 559.71low loader to GC03 5 791.09 5 033.06 6 142.73 5 655.63C-frame to GC10 3 824.56 3 384.01 3 046.63 3 418.40low loader to GC10 3 620.09 3 620.61 5 011.81 4 084.17

Table 6.10: Maximum staytime at the combiners

It can be concluded that the loading of the racks on an empty pallet is the main rootcause of the high lead times in the simulation model. Unfortunately this problem falls

40

beyond the boundaries of this thesis, as this is a scheduling project for DSV picking.However, the current and future situation don’t differ much in this respect, so thescheduling program of DSV will normally already solve this problem.

6.1.6 Number of TOW units at VCG

As final part of the study for this situation, the number of TOW units at each entryhall of VCG is investigated. However, in contrast to the transport between DSV andVCG, the full racks are not pushed downstream to the assembly line. Instead, theitems are pulled from the end of the supply chain. Therefore the study of the optimalnumber of TOW units requires a somewhat different approach than the study of theTOWex units: besides the idle time of the operators and the properties of the queuerepresenting the loading area of the entry hall, the response to variability within themodel will also be a crucial factor. In the following paragraphs, each of the three entryhalls will be discussed separately.

Number of TOW units at GC01. For each number of operators at GC01, threesimulation runs are executed in Flexsim. During each run, the following data is ac-quired:

• the mean idle time of an operator.

• the average content of queue169. This queue represents the buffer area for fullstandard racks at GC01.

• the average content of queue171. This queue represents the buffer area for fullnon-standard racks at GC01.

The results of these simulations are summarized in table 6.11:

41


4 operatorsmean idle time operator 0.3% 3.5% 2.8% 2.2%avg nbr. standard boxes 84.37 22.80 103.28 70.15avg nbr. non-standard boxes 51.49 61.34 61.24 58.02


6 TOW operatorsmean idle time operator 41.5% 42.8% 42.8% 42.4%avg nbr. standard boxes 47.09 38.95 29.60 38.55avg nbr. non-standard boxes 45.42 15.58 31.68 30.89


Table 6.11: Gathered data for optimal number of TOW units at GC01 (situation 1)

It is very hard to draw conclusions from table 6.11, besides the fact that more operatorsresult in more idle time per operator, which is a quite logical relation.

However, a different approach may be more successful. Figures 6.5, 6.6, 6.7 and 6.8each show 2 charts. The left chart of each figure draws the content of queue169 at eachmoment in time in Flexsim. As already mentioned, queue169 represents the bufferarea for fully loaded standard racks. So the left chart represents the number of fullyloaded standard racks stored in GC01 at each moment in time. Reasoning by meansof analogies, the right chart of each figure represents the number of fully loaded non-standard racks, stored in GC01 at each moment in time.

Figure 6.5: Content vs. time chart for GC01, 4 operators

In case 4 operators are deployed, the content of both queues keeps growing, as can beseen in figure 6.5. This indicates that the number of operators is insufficient.

42


When there is an approximately constant arrival rate of boxes at GC01, 5 operatorswill be sufficient. However, when the arrival rate suddenly rises, the 5 operators arenot capable of cutting back the corresponding content rise. This can be deducted fromthe right chart of figure 6.6.


As can be seen in figure 6.7, the disadvantages of having only four or five operatorsdisappear when having six operators.


43

When comparing figures 6.7 and 6.8, it becomes clear that using seven operators insteadof six does not hold a real advantage towards the reaction time on a rise of the arrivalrate. Only the mean idle time per operator increases in Flexsim. That is why six isthe optimal number of TOW units at GC01.

Number of TOW units at GC03. After the optimal number of TOW units forGC01 is determined, the same principle can be applied for GC03. Since the number oftransports to GC03 is much less than the number of transports to the other two entryhalls, the optimal number of TOW units will also be less.

For this study, similar data as in GC01 can be obtained:

• the mean idle time of an operator.

• the average content of queue151. This queue represents the buffer area for fullyloaded non-standard racks at GC03.

For several number of operators, three simulation runs have been executed. Table 6.12summarizes all the gathered data:


1 operatormean idle time operator 24.3% 25.8% 24.2% 24.8%avg nbr. non-standard boxes 17.85 6.13 16.24 13.41

2 operatorsmean idle time operator 61.2% 61.1% 62.6% 61.6%avg nbr. non-standard boxes 11.75 19.41 11.13 14.10

Table 6.12: Gathered data for optimal number of TOW units at GC03 (situation 1)

The content vs. time charts for queue151 are represented in figures 6.9 and 6.10. Asmentioned before, queue 151 represents the buffer area for fully loaded non-standardracks at GC03. This means the content vs. time charts for queue151 represents thenumber of fully loaded non-standard racks, stored in GC03 at each moment in time.

As there are only non-standard racks being transported to GC03, the two charts infigures 6.9 and 6.10 do not represent the charts for standard and non-standard boxes.Instead, two charts of the same figure represent two different runs for the same exper-iment.

44

Figure 6.9: Content vs. time chart for GC03, 1 operator


As can be seen in the figures 6.9 and 6.10, the response time for a sudden rise of thearrival rate is similar for one or two TOW units, so the optimal number of TOW unitsfor GC03 equals one.

Number of TOW units at GC10. To end the simulations of situation 1, theoptimal number of TOW units at GC10 is determined, similar to the study of theoptimal number of TOW units at GC01. To know the optimal number of operators atGC10, the content vs. time charts are drawn for queue225 and queue157. These twoqueues represent the buffer area for fully loaded standard and non-standard racks atGC10.

When only three operators are deployed, the content of both queues keeps growing,which indicates that the use of three operators is insufficient. These charts are repre-sented in figure 6.11:

45


Similar to GC01, four operators will be sufficient when the arrival rate of full boxes issteady. However, as can be deducted from the right chart in figure 6.12, four operatorsare not capable of cutting back a sudden rise of the buffer followed by an increase ofthe arrival rate:


From figures 6.13 and 6.14, it can be concluded that five operators is the optimalnumber for GC10.

Figure 6.13: Content vs. time chart for GC10, 5 TOW units

46

Figure 6.14: Content vs. time chart for GC10, 6 TOW units

The advice that was given for the TOWex units is also applicable to the TOW units,for each of the four situations: it is highly recommended that an extra TOW unit iskept stand-by, in case one of the TOW units at VCG would break down.

Now that all the simulations for situation 1 have been completed and all the previouslyasked questions have been answered, it is time to look for solutions to the other situ-ations. A lot of conclusions made for situation 1 will also be applicable to the otherthree situations. However, there are also some questions for which the answer dependson the situation. These questions will be dealt with in the following chapters.

47

6.2 Situation 2

The next situation that will be discussed is situation 2: in this situation, a TOWex unitcan depart at VCG inbound with the first empty C-frame or low loader that becomesavailable, as in situation 1. The only difference with situation 1 is that in this situation,full C-frames are not unloaded immediately upon arrival at one of the entry halls ofVCG. Normally this would imply that more C-frames will be needed.

6.2.1 Steady state

To know the beginning of the steady state, the number of inputs and outputs fromthe Flexsim model are again exported to Excel and designed in figure 6.15. Fromthis figure, it can be concluded that 10 000 seconds is again a safe estimation of thebeginning of the steady state and the beginning of the information gathering.

Figure 6.15: Steady state (situation 2)

6.2.2 Number of TOWex units

In order to determine the optimal number of TOWex units, the same two variables asin situation 1 were monitored in Flexsim: the content of queue1 and the utilization ofthe transporters. Table 6.13 summarizes the properties of queue1 throughout severalruns:

48


5 transportersavg staytime in queue1 21 264.15max staytime in queue1 26 696.73avg content of queue1 56.93max content of queue1 70

6 transportersavg staytime in queue1 1 177.14 558.16 756.79 830.7max staytime in queue1 3 299.04 2 202.44 2 621avg content of queue1 3.57 1.69 2.27 2.51max content of queue1 11 7 9



Table 6.13: Properties of queue1 as a function of the number of transporters (situation 2)

Figure 6.16 shows two graphs representing the data in table 6.13. These graphs showthat seven is again the minimal amount of transporters needed. The advantage of usingmore than seven transporters is again minimal.

Figure 6.16: Content and staytime of queue1 (situation 2)

The second variable to monitor is the utilization of the transporters, represented intable 6.14. When seven transporters are deployed, every transporter has 16% idletime, leaving enough room for unforeseen circumstances.

49

1 2 3 4 5 6 7 8 mean

6 transp.run 1 1.2% 1.0% 2..3% 1.9% 2.2% 2.0% 1.8%run 2 3.2% 3.0% 3.1% 4.3% 4.2% 5.4% 3.9%run 3 2.1% 2.7% 2.7% 3.4% 3.8% 4.8% 3.3%

MEAN % 2.2% 2.2% 2.7% 2.53.2% 3.4% 4.1% 3.0%


MEAN % 9.0% 11.1% 11.9% 13.6% 16.7% 20.4% 29.4% 16.0%


MEAN % 12.1% 13.7% 15.8% 19.1% 24.0% 30.1% 40.3% 62.3% 27.2%

Table 6.14: Idle time of the TOWex units (situation 2)

As an example, the output of a Flexsim-run with seven transporters is shown in figure6.17:

Figure 6.17: Flexsim output 7 TOWex units

50

6.2.3 Number of C-frames, low loaders and parking spaces

After the optimal number of TOWex units for situation 2 has been decided to beseven, the next paragraph will now investigate the optimal number of C-frames andlow loaders. Since this investigation is connected to the optimal number of parkingspaces, the two simulations are being handled simultaneously.

First remark that can be made is that the only difference between situations 1 and 2 isthe flow of C-frames. Hence, the optimal number of low loaders and the correspondingnumber of parking spaces for situation 2 will stay the same as in situation 1.

The second remark is that a different approach has to be taken than the one usedfor situation 1, since an abundance of pallets has to be available in situation 2 to tideover the transition period.

The only difference with situation 1 is that in situation 2 a C-frame has to stay at theentry hall of VCG until all its full racks are replaced by empty racks. So in situation2, two extra events can occur:

1. a TOWex unit has to wait at the entry hall to uncouple its fully loaded C-frameuntil an empty parking space becomes available.

2. a TOWex unit has to wait at the entry hall because no empty frame is availablefor transport back to DSV.

These two extra events will take up a percentage of the total time, and it is known fromsituation 1 that the total idle time of a TOWex unit is 8.36%. To prevent disturbancesin the parts supply, the two extra events combined should take up less than 8.36% ofthe time. This is the only way to ensure that only the idle time of the TOWex unit isrearranged.

In a first instance, the percentages for the two extra events are acquired for GC01:the content of queue332 equals the number of C-frames containing full racks at GC01.Therefore, for each run, the percentages of the time that queue332 contains a certainamount of pallets is monitored. These percentages are summarized in table 6.15:

GC01 run 1 run 2 run 3 run 4 run 5 mean %

content > 1 53.47% 70.59% 43.18% 69.51% 92.32% 65.81%content > 2 21.92% 41.14% 22.51% 41.69% 50.71% 35.59%content > 3 3.88% 22.12% 7.64% 17.60% 14.39% 13.13%content > 4 0.21% 8.33% 2.91% 6.32% 1.66% 3.89%content > 5 0.00% 0.32% 0.15% 1.19% 0.00% 0.33%content > 6 0.00% 0.00% 0.00% 0.00% 0.00% 0.00%

Table 6.15: Non-empty C-frames at GC01 (situation 2)

Table 6.16 can be built up for GC10 in a similar way:

51


content > 1 56.10% 77.07% 27.22% 58.14% 87.20% 61.15%content > 2 25.08% 63.45% 8.49% 32.99% 67.33% 39.47%content > 3 12.85% 34.85% 0.68% 16.65% 62.56% 25.52%content > 4 2.65% 19.09% 0.00% 10.15% 46.41% 15.66%content > 5 0.00% 9.53% 0.00% 1.96% 28.72% 8.04%content > 6 0.00% 1.49% 0.00% 0.00% 2.99% 0.90%content > 7 0.00% 0.00% 0.00% 0.00% 0.00% 0.00%


The best way to explain the percentages is by using the following examples:

• at GC01, in 13.13% of the total time, there were more than three non-emptyC-frames. Assume that Volvo decides to implement situation 2 and that threeextra C-frames are acquired for GC01. This would imply that, in comparison tosituation 1, a TOWex unit would spend 13.13% of its total time more on waitingfor an empty C-frame becoming available at GC01.

• take the example again of the 13.13%. Suppose three extra parking spaces wouldbe provided for C-frames at GC01. This would mean that a TOWex unit wouldspend 13.13% of its total time waiting on a parking space becoming available atGC01.

These examples show that the mean percentages of table 6.15 and 6.16 equal thepercentages of the two extra events. As already mentioned, the percentages of thesetwo extra events for GC01 and GC10 combined should take up less than 8.36% of thetime of a TOWex unit. This means that the sum of the four percentages, two for GC01and two for GC10, should be less than 8.36%:

• it is the easiest to start with GC10: there are maximum three parking spacesoccupied in situation 1 and GC10 has a maximum of nine parking spaces. Thismeans that there is room for six more C-frames. Looking up the mean percentagefor six C-frames in table 6.16, it can be concluded that in 0.90% of the time aTOWex unit has to wait to park a full C-frame.

• Since 8.36% should not be exceeded, it can be seen from table 6.16 that therealso should be at least six extra C-frames provided for GC10. Providing morethan six C-frames would be useless because there are only six parking spaces. Sosix extra C-frames are provided, implying that 0.9% of the time a TOWex unithas to wait for an empty C-frame to become available.

• as space is scarce at GC01, it is optimal to minimize the number of extra C-framesand parking spaces, without exceeding 8.36%. The optimal solution consists offive extra parking spaces and five extra C-frames.

• this brings the total percentage to 2.46%. This means that 2.46% of a TOWexunit’s idle time of situation 1 is replaced by waiting to park a full C-frame orwaiting for an empty C-frame at GC01 or GC10.

52

To summarize this study, table 6.17 gives a summary of the total number of low loaders,C-frames and parking spaces of situation 2:

situation 1 extra GC01 extra GC10 total

C-frames 9 5 6 20low loaders 9 9parking DSV 10 10parking GC01 (C-frames) 2 5 7parking GC01 (low loaders) 2 2parking GC03 2 2parking GC10 3 6 9

Table 6.17: Number of C-frames, low loaders and parking spaces (situation 2)

6.2.4 Number of TOW units at VCG

To finalize the study of situation 2, the number of TOW units at each entry hall ofVCG is investigated. However, in this part of the supply chain there is no differencebetween situations 1 and 2. Hence, the optimal number of TOW units at each of thethree entry halls of VCG is the same as in situation 1: six TOW units at GC01, oneTOW unit at GC03 and five TOW units at GC10. As in situation 1, an extra TOWunit should also be kept in stand-by in case one of the TOW units breaks down.

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6.3 Situation 3

Now that situations 1 and 2 are discussed, we start the situations where the emptystreams of low loaders and C-frames are separated. Situation 3 is similar to situation1. The only difference is that in situation 1, a TOWex unit at VCG can depart withthe first empty frame that becomes available at VCG. In this situation, situation 3, aTOWex unit has to depart to DSV with the same type of frame as the full frame itbrought to VCG. For example: if a TOWex unit arrives at GC01 with a full C-frame,it has to travel back to DSV with an empty C-frame, not with an empty low loader.Because there are a lot of similarities between situations 1 and 3, only the differenceswill be discussed.

6.3.1 Number of low loaders and C-frames

There will be no difference between situations 1 and 3 regarding the number of TOWexunits and the number of TOW units. However, because the two streams back to DSVare now separated, it is no longer sufficient to provide one empty frame at GC01 andGC10 each. One empty frame of each type has to be provided at each of the entryhalls. So in the beginning an extra low loader has to be provided at GC01 and an extraC-frame has to be provided at GC10 to keep the total flow undisturbed. Indeed, whensimulations are done with these two extra frames, the results are similar to those insituation 1.

6.3.2 Parking spaces

Since the two different types of frame already entered GC01 through a different gate,the optimal number of parking spaces was already determined separately for C-framesand low loaders in situation 1.

For GC10 however, the optimal number of parking spaces, being three, was determinedfor the two types of frames together. When simulations are done in this situation, withtwo separate flows for empty frames from GC10 back to DSV, the results are a littledifferent: instead of three parking spaces needed, as in situation 1, there are now amaximum of four parking spaces needed at GC10. This does not cause an immediateproblem because nine parking spaces are available at GC10. However, it will give quitedifferent results in the last situation, situation 4.

To conclude this situation, table 6.18 gives a summary of the differences betweensituations 1 and 3:

situation 1 situation 3

number of C-frames 9 10number of low loaders 9 10number of parking spaces at GC10 3 4

Table 6.18: Differences between situations 1 and 3

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6.4 Situation 4

The last situation is a combination of situations 2 and 3: the full C-frames are notunloaded immediately upon arrival at one of the entry halls of VCG, as in situation2, and the transports of empty racks back to DSV with low loaders and C-frames isseparated, as in situation 3.

6.4.1 Number of TOWex units, C-frames, low loaders andparking spaces

For situation 4, a similar technique can be used as in situation 2. Situation 2 assumedthe same optimal amounts as in situation 1, and calculated the number of extra C-frames needed.

In situation 4, the same two extra events occur as in situation 2:

1. a TOWex unit has to wait at the entry hall to uncouple its fully loaded C-frameuntil an empty parking space becomes available.

2. a TOWex unit has to wait at the entry hall because no empty frame is availablefor transport back to DSV.

To determine the number of extra C-frames needed at GC01 and GC10, the sum ofthe percentages for these two extra events should be less than the mean percentage ofidle time for a TOWex unit.

First of all, the required data is gathered from the Flexsim model with parking restric-tions, and summarized in tables 6.19, 6.20, 6.21 and 6.22:

1 2 3 4 5 6 7 8 mean %


MEAN % 6.5% 7.3% 7.2% 8.6% 9.4% 10.0% 9.5% 8.4%


MEAN % 11.3% 12.4% 14.2% 16.0% 18.0% 21.5% 25.5% 36.6% 19.4%

Table 6.19: Idle time of the transporters (situations 3 and 4)

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content > 1 53.47% 70.59% 43.18% 69.51% 92.32% 65.81%content > 2 21.92% 41.14% 22.51% 41.69% 50.71% 35.59%content > 3 3.88% 22.12% 7.64% 17.60% 14.39% 13.13%content > 4 0.21% 8.33% 2.91% 6.32% 1.66% 3.89%content > 5 0.00% 0.32% 0.15% 1.19% 0.00% 0.33%content > 6 0.00% 0.00% 0.00% 0.00% 0.00% 0.00%



content > 1 56.10% 77.07% 27.22% 58.14% 87.20% 61.15%content > 2 25.08% 63.45% 8.49% 32.99% 67.33% 39.47%content > 3 12.85% 34.85% 0.68% 16.65% 62.56% 25.52%content > 4 2.65% 19.09% 0.00% 10.15% 46.41% 15.66%content > 5 0.00% 9.53% 0.00% 1.96% 28.72% 8.04%content > 6 0.00% 1.49% 0.00% 0.00% 2.99% 0.90%content > 7 0.00% 0.00% 0.00% 0.00% 0.00% 0.00%


DSV 10GC01 C-frames 2GC01 low loaders 2GC03 2GC10 4

Table 6.22: Number of parking spaces needed (situation 3)

Let’s suppose seven TOWex units are deployed. This means that the two extra eventscan take up maximum 8.4% of the total time, as can be seen in table 6.19. At GC10there is a maximum of nine parking spaces, and table 6.22 states that four of themare already needed for situation 3. So a maximum of five parking spaces can be usedfor non-empty C-frames and five extra C-frames can be ordered. This would alreadytake 8.04% of the time of the TOWex unit to wait for a free parking space at GC10,according to table 6.21. From the same table, it can be seen that another 8.04% ofthe time a TOWex unit waits at GC10 for an empty C-frame. The sum of thesetwo percentages, resulting in 16.08%, clearly exceeds the maximum of 8.4%, so sevenTOWex units will not be enough in situation 4.

By deploying eight TOWex units, the idle time per TOWex unit increases from 8.4% to19.4%, which is higher than 16.08%. As space is also scarce at GC01, it is optimal tominimize the number of extra C-frames and parking spaces, without exceeding 19.4%- 16.08%. The optimal solution consists of five extra parking spaces and five extra

56

C-frames. This brings the sum of the four percentages for the extra two events to16.74%, which is smaller than the mean idle time percentage of a TOWex unit.

Table 6.23 gives a summary of the total number of TOWex units, low loaders, C-framesand parking spaces for situation 4:

situation 3 extra GC01 extra GC10 total

TOWex units 7 1 8C-frames 10 5 5 20low loaders 10 10parking DSV 10 10parking GC01 (C-frames) 2 5 7parking GC01 (low loaders) 2 2parking GC03 2 2parking GC10 4 5 9

Table 6.23: Number of TOWex units, C-frames, low loaders and parking spaces (situation4)

57

Chapter 7

Recommendation

Before giving any recommendations, table 7.1 summarizes again the results for the foursituations:

number of ... situation 1 situation 2 situation 3 situation 4

TOWex units 7 7 7 8C-frames 9 20 10 20low loaders 9 9 10 10parking spaces DSV 10 10 10 10parking spaces GC01 (C-fr) 2 7 2 7parking spaces GC01 (LL) 2 2 2 2parking spaces GC03 2 2 2 2parking spaces GC10 3 9 4 9TOW units at GC01 6 6 6 6TOW units at GC03 1 1 1 1TOW units at GC10 5 5 5 5

Table 7.1: Total summary

From table 7.1, it becomes clear that situation 1 is the most optimal situation. This isthe situation where full C-frames are unloaded immediately upon arrival at VCG, anda TOWex unit can depart back to DSV with the first empty frame available, regardlessthe type of frame. This result can be explained quite easily:

• the main purpose of a C-frame is a succession of transport of full racks fromDSV to VCG, and transport of empty racks from VCG back to DSV. Whenfull C-frames are unloaded immediately upon arrival at VCG, they can be usedstraightaway for transport back to DSV. However, when C-frames are used tostore full racks at VCG, there will be a need of additional C-frames to assurethat the flow between DSV and VCG is kept continuous.

• by separating the flow of C-frames and low loaders back to DSV, the variabilityof the process is raised. Therefore more frames are needed to compensate thisvariability. That is why in situation 1, the concept of risk pooling is applied inorder to reduce the variability of the process.

58

Chapter 8

Sensitivity analysis

Now that the most optimal situation is chosen, it is possible to run a few “what-if”scenario’s in order to find an answer on the following question: how resistant is theoptimal situation against non-expected disruptions in the process?

8.1 Failure of a TOWex unit

The first disruption that can happen to the process is that a TOWex unit is down fora while. When this situation occurs, it is important to know whether the other sixTOWex units can cope with the arrival rate of full frames at DSV. In order to obtainthe answer to this question, the content of queue11 is being measured while one of theTOWex units breaks down for a certain amount of time.

There are a few things that have to be taken into account when simulating such adisruption:

• all the measurements have to be done in the steady state.

• the downtime of a TOWex unit has to be large enough to have an effect on theprocess: simulations show that half an hour has no impact on the process, butan hour of downtime does has its impact.

• the measurements have to be done over a smaller period of time than a wholeweek in order to be able to distinguish small irregularities in the process. Onthe other hand the measurements have to be long enough to check for delayedirregularities.

• therefore the measurements start at 100 000 seconds and stop measuring at 130000 seconds, including a one hour breakdown starting at 102 000 seconds.

In two out of five simulation runs no irregularities could be discovered. In the otherthree runs there was a slight raise of the content, as can be derived from figure 8.1:

1the content of this queue represents the number of full frames at DSV, waiting to be transportedto VCG

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Figure 8.1: Raise of the content of queue1 during a one hour breakdown of TOWex unit

Table 8.1 contains all the data, gathered from each of the five runs2. As shown in thetable, it takes the process on average 1 303 seconds, or 22 minutes, to recover from aone hour breakdown of a TOWex unit. However, simulation shows that this breakdownhas no significant effect on the assembly line itself.

run 1 run 2 run 3 run 4 run 5 mean

start breakdown 102 000 102 000 102 000 102 000 102 000start irregularity 102 539.03 / 103 550.5 / 104 662.75end breakdown 105 600 105 600 105 600 105 600 105 600end irregularity 108 413.26 / 108 197.41 / 106 707.8

recovery time 2 813.26 0 2 597.41 0 1 107.8 1 303.69

Table 8.1: Time needed to recover from a one hour breakdown of a TOWex unit

2the recovery time is measured from the end of the breakdown until the end of the irregularity inthe content of queue1

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8.2 Failure of a TOW unit at VCG

The effects of a failure of a TOW unit at VCG can be investigated in a similar wayas in the previous paragraph. For each of the three entry halls, a one hour breakdownis enforced at one of the TOW units, starting at 102 000 seconds. However, it isvery important to find an answer to the following question: what variables should bemonitored for gathering data, to be able to draw the right conclusions? Some answerscan already be formulated:

1. the number of full racks at VCG inbound waiting to be transported to the assem-bly line: full racks arrive at VCG inbound in batches of seven or ten (dependingwhether they are brought in with C-frames or low loaders). As a consequence,small increases can be the result of new TOWex units arriving at VCG. Theseincreases are not necessarily a direct result of a fall-out of a TOW unit.

2. the content of the processors, representing the assembly line, at the end of theFlexsim model: the boxes, representing the racks, are created at DSV accordingto an exponential distribution. Hence a decrease in the content of a processor canbe the result of a high inter-arrival time between two boxes at DSV, and shouldagain not immediately be interpreted as a result of the fall-out of a TOW unit.

It becomes clear that the consequences of the failure of a TOW unit are very difficultto assess.

However, when looking at the minimal lead times in table 8.2, it can be seen that theseare much lower than the maximum lead time of 3 900 seconds, or 65 minutes, enforcedby VCG. This extra time can be used as a buffer for unforeseen circumstances.

push-chapter pull-chapter total total [minuts]

GC01 C-frames 1 255 378 1 633 28GC01 low loaders 1 303 390 1 693 29GC03 1 406 254 1 660 28GC10 C-frames 1 463 254 1 717 29GC10 low loaders 1 488 315 1 803 31

Table 8.2: Minimum lead times (situation 1)

61

Chapter 9

Conclusion

By implementing situation 1, the disadvantages of the current supply chain will besolved:

• the number of handling will strongly decrease due to the increased mobility ofthe racks.

• the use of pallets will heavily reduce due to the highly diminished usage of forklifttrucks.

To conclude, table 9.1 summarizes the optimal amounts of situation 1:

number of ... situation 1

TOWex units 7C-frames 9low loaders 9parking spaces DSV 10parking spaces GC01 (C-fr) 2parking spaces GC01 (LL) 2parking spaces GC03 2parking spaces GC10 3TOW units at GC01 6TOW units at GC03 1TOW units at GC10 5

Table 9.1: Summary of situation 1

62

List of Figures

1.1 Floor plan of VCG and its three sections . . . . . . . . . . . . . . . . . 11.2 Floor plan of DSV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

3.1 Three standard racks + dolly . . . . . . . . . . . . . . . . . . . . . . . 53.2 TOW unit + sulky . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63.3 TOW unit model 310 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63.4 TOWex unit model 425 . . . . . . . . . . . . . . . . . . . . . . . . . . . 73.5 C-frame couple pulled by TOWex unit . . . . . . . . . . . . . . . . . . 83.6 C-frame loaded with three standard racks . . . . . . . . . . . . . . . . . 83.7 Low loader couple pulled by TOWex unit . . . . . . . . . . . . . . . . . 9

4.1 Floor plan of the path from DSV inbound to the assembly line . . . . . 104.2 Schematic representation of the four different situations . . . . . . . . . 12

5.1 Schematic representation of the supply chain . . . . . . . . . . . . . . . 135.2 Floor plan of the network nodes . . . . . . . . . . . . . . . . . . . . . . 175.3 Flexsim model of the whole supply chain . . . . . . . . . . . . . . . . . 215.4 Flexsim model of DSV . . . . . . . . . . . . . . . . . . . . . . . . . . . 225.5 Flexsim model of GC10 . . . . . . . . . . . . . . . . . . . . . . . . . . . 225.6 Test model ”2 different paths”, flowchart 1 . . . . . . . . . . . . . . . . 245.7 Test model ”2 different paths”, flowchart 2 . . . . . . . . . . . . . . . . 27

6.1 Steady state (situation 1) . . . . . . . . . . . . . . . . . . . . . . . . . 326.2 Content and staytime of queue1 (situation 1) . . . . . . . . . . . . . . . 346.3 Suggested floor plan DSV inbound . . . . . . . . . . . . . . . . . . . . 376.4 Flexsim output 7 transporters with restriction on parking spaces (situ-

ation 1) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 386.5 Content vs. time chart for GC01, 4 operators . . . . . . . . . . . . . . 426.6 Content vs. time chart for GC01, 5 operators . . . . . . . . . . . . . . 436.7 Content vs. time chart for GC01, 6 operators . . . . . . . . . . . . . . 436.8 Content vs. time chart for GC01, 7 operators . . . . . . . . . . . . . . 436.9 Content vs. time chart for GC03, 1 operator . . . . . . . . . . . . . . . 456.10 Content vs. time chart for GC03, 2 operators . . . . . . . . . . . . . . . 456.11 Content vs. time chart for GC10, 3 operators . . . . . . . . . . . . . . . 466.12 Content vs. time chart for GC10, 4 operators . . . . . . . . . . . . . . . 466.13 Content vs. time chart for GC10, 5 TOW units . . . . . . . . . . . . . 466.14 Content vs. time chart for GC10, 6 TOW units . . . . . . . . . . . . . 476.15 Steady state (situation 2) . . . . . . . . . . . . . . . . . . . . . . . . . 486.16 Content and staytime of queue1 (situation 2) . . . . . . . . . . . . . . . 49

63

6.17 Flexsim output 7 TOWex units . . . . . . . . . . . . . . . . . . . . . . 50

8.1 Raise of the content of queue1 during a one hour breakdown of TOWexunit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

64

List of Tables

5.1 Summary of the most important Flexsim objects . . . . . . . . . . . . . 135.2 Loading time of a rack in a C-frame or on a low loader at DSV . . . . . 155.3 Load and unload time of TOWex unit . . . . . . . . . . . . . . . . . . . 165.4 Properties of network nodes between DSV and VCG, before intersection 185.5 Properties of network nodes between DSV and VCG, on intersection . . 185.6 Properties of network nodes between DSV and VCG, driving to GC01

with C-frame . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185.7 Properties of network nodes between DSV and VCG, driving to GC03 . 195.8 Properties of network nodes between DSV and VCG, driving to GC01

with low loader . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195.9 Properties of network nodes between DSV and VCG, driving to GC10 . 195.10 Load and unload time of TOW units at VCG . . . . . . . . . . . . . . 21

6.1 Properties of queue1 as a function of the number of transporters (situ-ation 1) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

6.2 Idle time of the transporters (situation 1) . . . . . . . . . . . . . . . . . 346.3 Idle time of the low loader combiners (situation 1) . . . . . . . . . . . . 356.4 Idle time of the C-frame combiners (situation 1) . . . . . . . . . . . . . 366.5 Maximum amount of parking spaces available at DSV and VCG . . . . 376.6 Needed number of parking spaces at DSV inbound (situation 1) . . . . 386.7 Variables regarding the optimal number of TOWex units (situation 1) . 396.8 Worst case lead times (situation 1) . . . . . . . . . . . . . . . . . . . . 406.9 Lead times depending on number of TOWex units . . . . . . . . . . . . 406.10 Maximum staytime at the combiners . . . . . . . . . . . . . . . . . . . 406.11 Gathered data for optimal number of TOW units at GC01 (situation 1) 426.12 Gathered data for optimal number of TOW units at GC03 (situation 1) 446.13 Properties of queue1 as a function of the number of transporters (situ-

ation 2) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 496.14 Idle time of the TOWex units (situation 2) . . . . . . . . . . . . . . . . 506.15 Non-empty C-frames at GC01 (situation 2) . . . . . . . . . . . . . . . . 516.16 Non-empty C-frames at GC10 (situation 2) . . . . . . . . . . . . . . . . 526.17 Number of C-frames, low loaders and parking spaces (situation 2) . . . 536.18 Differences between situations 1 and 3 . . . . . . . . . . . . . . . . . . 546.19 Idle time of the transporters (situations 3 and 4) . . . . . . . . . . . . . 556.20 Non-empty C-frames at GC01 (situation 4) . . . . . . . . . . . . . . . . 566.21 Non-empty C-frames at GC10 (situation 4) . . . . . . . . . . . . . . . . 566.22 Number of parking spaces needed (situation 3) . . . . . . . . . . . . . . 56

65

6.23 Number of TOWex units, C-frames, low loaders and parking spaces (sit-uation 4) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

7.1 Total summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58

8.1 Time needed to recover from a one hour breakdown of a TOWex unit . 608.2 Minimum lead times (situation 1) . . . . . . . . . . . . . . . . . . . . . 61

9.1 Summary of situation 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

66

Appendix A

Product specifications of a TOWunit, model TOW310

67

mod

el Motor typeSteering standing/sitting

DC Seriesitting

wei

ghts

Towing capacityEmpty weight without batteryWeight with standard batteryAxle load frontAxle load rear

kNkgkgkgkg

100800

1500535965

whe

els Wheel base

Track F/Rmmmm

1559-/887

dim

ensi

ons

Total length (without coupling)Total widthHeight of couplingStep heightSitting heightTotal heightTurning radiusWorking aisle width (90º)Road clearance centre wheelbase

mmmmmmmmmmmmmmmmmm

22701020355320850

131024501765142

perf

orm

ance

s

Speed without loadSpeed with max. loadDrawbar pull, 1 min.Drawbar pull, 5 min.Drawbar pull, 60 min.

km/hkm/h

NNN

10-1810

850060002500

driv

e

Brakes: service/parkingBrakes: hydr./electr./mech.Brakes: frontBrakes: rearSuspension F/RShock absorbersBattery tensionBattery capacityBattery weight, abt.E-motor 60 min.SpeedcontrolTransmission

VoltAh5kgkW

1:

foot/hand hydr.

disc + 4 piston calipers2x disc + 4 piston calipers

springsyes48

4507008.75

electronic19.6

Spijkstaal Elektro B.V. Tel. 0181 - 45 70 30Postbus 9 Fax. 0181 - 62 39 583200 AA Spijkenisse www.spijkstaal.nl

PRODUCT SPECIFICATON MODEL 310

Appendix B

Product specifications of a TOWexunit, model TOW425

69

mod


DC Seriesitting

wei

ghts


kNkgkgkgkg

2501730297013031667

whe

els Wheel base

Track F/R (IC10/IC40 tyres)mmmm

22201100

dim

ensi

ons

Total length (without coupling)Total widthTotal height + cab.Sitting height from cab.floorHeight of couplingStep heightLoading platformLoading heightTurning RadiusWorking aisle width (90º)Road clearance centre wheelbase

mmmmmmmmmmmmmmmmmmmmmm

311012801900560415365

960x700790

40102590155

perf

orm

ance

s Speed without loadSpeed with max. loadDrawbar pull, 1 min.Drawbar pull, 5 min.Drawbar pull, 60 min.Max. ramp with max. load

km/hkm/h

NNN%

22-2812

15000110005000

3

driv

e


VoltAh5kgkW

1:

foot/hand hydr./mech.

discdrum

springsyes80

450124016.5

electronic24.4


PRODUCT SPECIFICATON MODEL 425 K

mod


DC Seriesitting

wei

ghts


kNkgkgkgkg

2501785318514011784

whe

els Wheel base


23851100

dim

ensi

ons



327512801900560415365

960x700790

43402670155

perf

orm

ance


km/hkm/h

NNN%

22-2812

15000110005000

3

driv

e


VoltAh5kgkW

1:


discdrum

springsyes80

540140016.5

electronic24.4


PRODUCT SPECIFICATON MODEL 425 M

mod


DC Seriesitting

wei

ghts


kNkgkgkgkg

2501830348015311949

whe

els Wheel base


25351100

dim

ensi

ons



342512801900560415365

960x700790

43202800155

perf

orm

ance


km/hkm/h

NNN%

22-2812

15000110005000

3

driv

e


VoltAh5kgkW

1:


discdrum

springsyes80

645165016.5

electronic24.4


PRODUCT SPECIFICATON MODEL 425 L

Appendix C

Technical drawing of a C-frame

73

Appendix D

Transport details of the requiredparts and racks

75

VCG

Code

itemtype

flexsimitem description in dutch

entry hall at

VCG

type of

transport

distance entry

hall - assembly

line

# racks /

day

inter

arrival

time

VKP 101 Bodemkabel GC01 C-frame 229 66,0 1282

BST 102 Booster GC01 C-frame 215 33,0 2564

KDL 103 Deurlijn kitting L GC01 C-frame 135 39,6 2137

KDR 104 Deurlijn kitting R GC01 C-frame 115 39,6 2137

DLL 105 Deurlijst onder links P1 - P2 GC01 C-frame 421 17,6 4807

DLR 106 Deurlijst onder rechts P1 - P2 GC01 C-frame 190 17,6 4807

RDL 107 Deurrubbers Trim F links GC01 C-frame 217 11,0 7691

RDR 108 Deurrubbers Trim F rechts GC01 C-frame 240 11,0 7691

DRE 109 Deurruit LA GC01 C-frame 218 22,5 3760

DRF 110 Deurruit RA GC01 C-frame 217 22,5 3760

VIN 111 Haaivin P1 + P3 GC01 C-frame 149 16,5 5128

LTF 112 Luchtfilter GC01 C-frame 264 41,3 2051

MSL 113 Middenlijst L GC01 C-frame 102 11,7 7256

MSR 114 Middenlijst R GC01 C-frame 118 11,7 7256

MKI 115 Motorkapisolatie GC01 C-frame 230 16,1 5268

116 Pedaalsteunblokken GC01 C-frame 245 11,0 7691

PLC 117 Plenumcover GC01 C-frame 228 36,7 2308

ITB 118 Tussenschot isolatie GC01 C-frame 199 24,1 3512

ITS 119 Tussenschot isolatie Binnen GC01 C-frame 198 41,3 2051

VBK 120 Versnellingsbakkabel GC01 C-frame 244 11,0 7691

VRP 121 Versnellingspook GC01 C-frame 230 33,0 2564

WCR 122 Watercontainer + resonator GC01 C-frame 237 66,0 1282

ZPY 123 Zijpaneel L P3 GC01 C-frame 182 26,7 3165

ZPZ 124 Zijpaneel R P3 GC01 C-frame 192 26,7 3165

KEL 125 Ergo kitting L GC01 C-frame 205 22,0 3846

KER 126 Ergo kitting R GC01 C-frame 219 22,0 3846

PRV 127 Pedaalset GC01 C-frame 220 41,3 2051

KLP 128 Klapdeurpaneel P12 GC01 C-frame 215 13,7 6155

DAL 129 Daksierlijst links + A-stijl Y413 GC01 LL 441 9,6 8841

DAR 130 Daksierlijst rechts + A-stijl Y413 GC01 LL 167 9,6 8841

DRC 131 Deurruit LV P1+ P3 GC01 LL 160 36,7 2308

DRD 132 Deurruit RV P1+ P3 GC01 LL 165 36,7 2308

DML 133 Drempellijst links GC01 LL 205 33,0 2564

DMR 134 Drempellijst rechts GC01 LL 222 33,0 2564

GAB 135 Gordel achterbank GC01 LL 284 24,8 3419

GBS 136 Gordel B-stijl GC01 LL 268 24,8 3419

ICL 137 Luchtgordijn L GC01 LL 175 24,8 3419

ICR 138 Luchtgordijn R GC01 LL 171 24,8 3419

KBM 139 Motorkabel GC01 LL 210 49,5 1710

ACL 140 AC-leiding P1 - P2 GC03 LL 151 33,0 2564

BEN 141 Benzineleidingen P1 GC03 LL 145 24,1 3512

CPA 142 Coolerpack GC03 LL 182 16,7 5064

DRK 143 Drijfas L Kort GC03 LL 167 22,0 3846

DRL 144 Drijfas R lang GC03 LL 135 22,0 3846

KAT 145 Katalysator GC03 LL 138 33,0 2564

MTL 146 Motorsteun L GC03 LL 215 30,0 2820

MTR 147 Motorsteun R GC03 LL 213 30,0 2820

SVO 148 Servoleidingen P1 GC03 LL 150 36,1 2342

ALL 149Achterlichten S40-S60 D-stijl C30

V50 XC60 LinksGC10 C-frame 179 33,0 2564

ALR 150Achterlichten S40-S60 D-stijl C30

V50 XC60 RechtsGC10 C-frame 208 33,0 2564

BPA 151 B-stijlpaneel L GC10 C-frame 182 39,6 2137

BPB 152 B-stijlpaneel R GC10 C-frame 193 39,6 2137

DDL 153 Deurdrempels L GC10 C-frame 135 49,5 1710

DDR 154 Deurdrempels R GC10 C-frame 165 49,5 1710

MAT 155 Extra mattenset GC10 C-frame 120 26,1 3248

HVT 156 Handvat koffer P12 GC10 C-frame 178 5,1 16540

INB 157 Instructieboekje GC10 C-frame 122 2,4 34959

KTA 158 Klapdeur P3 Kitting GC10 C-frame 93 14,9 5697

KFM 159 Koffermat GC10 C-frame 145 7,3 11653

KLL 160 Koplamp P1 + P3 links GC10 C-frame 149 24,8 3419

KLR 161 Koplamp P1 + P3 rechts GC10 C-frame 188 24,8 3419

QGL 162 Q-glas L P3 GC10 C-frame 125 8,9 9495

QGR 163 Q-glas R P3 GC10 C-frame 85 8,9 9495

TFL 164 Trim F Kitting L GC10 C-frame 170 22,0 3846

TFR 165 Trim F Kitting R GC10 C-frame 156 22,0 3846

VMY 166 Vloermat achter P3 GC10 C-frame 187 26,7 3165

AST 167 Airbag & stuurwiel GC10 C-frame 165 49,5 1710

HPL 168 Hoedenplank P11 GC10 C-frame 215 10,9 7769

TAP 169 Klapdeurpaneel P3 GC10 C-frame 93 12,5 6754

CPC 170 C-stijlpaneel L GC10 LL 147 61,9 1368

CPD 171 C-stijlpaneel R GC10 LL 317 61,9 1368

TAL 172 Klapdeur P14 GC10 LL 190 24,8 3419

TAY 173 Klapdeur P3 GC10 LL 65 38,2 2216

VLP 174 Vloerluik P12 + P14 GC10 LL 145 28,1 3017

VLY 175 Vloerluik P3 GC10 LL 150 14,9 5697

VMP 176 Vloermat Achter P11 - P12 GC10 LL 180 58,3 1452

VMX 177 Vloermat Achter P14 GC10 LL 215 14,1 5982

VML 178 Vloermat LV GC10 LL 205 33,0 2564

VMR 179 Vloermat RV GC10 LL 218 33,0 2564

Bibliography

[1] Fowler J.W. Brown S., Chance F. and Robinson J. A centralized approach tofactory simulation. In Future Fab International. Future Fab International, 1997.

[2] Levy D.L. Lean production in an international supply chain. Sloan Management,1997.

[3] Wolfgang Kuhn. Digital factory: simulation enhancing the product and productionengineering process. In WSC ’06: Proceedings of the 38th conference on Wintersimulation, pages 1899–1906. Winter Simulation Conference, 2006.

[4] Averill M. Law and Michael G. McComas. Simulation of manufacturing systems.In WSC ’99: Proceedings of the 31st conference on Winter simulation, pages 56–59,New York, NY, USA, 1999. ACM.

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