High lift movement in a storage rack system

Baard, Pierre Stephan (2018-03)

Thesis (MCom)--Stellenbosch University, 2018.

Thesis

ENGLISH SUMMARY : With the growing competitive global economy new techniques need to be adopted for distribution of goods. Companies are searching for efficient distribution methods to improve customer service. Due to an increasing demand, distribution centres (DC) must be more versatile to keep up with a higher flow of goods. Working closely with South-Africa's largest single brand retail chain, PEP, it is determined that there is a bottle neck in the pallet storage rack system at their Durban DC. With PEP's current system the rate at which pallets are brought into the storage rack system exceeds that at which pallets can be removed from the storage rack system. This difference, in the storage and retrieval rate of pallets, means that the DC needs large temporary storage areas for pallets waiting to be stored. Furthermore, the slow rate at which pallets are retrieved from the storage rack system slows all processes down the supply chain. This study aims to nd alternative movement logics, slotting congurations and job sequences for storing and removing pallets from the storage rack system in an attempt to increase the rate at which pallets can be stored and retrieved. Five high-lift movement logics are compared namely, same side queueing without paired jobs, opposite side queueing without paired jobs, same side queueing with paired jobs, opposite side queueing with paired jobs and cyclic queueing with paired jobs. The effect of changing the sequence of jobs are also analysed for each high-lift movement logic. The job sequence is determined using heuristics that strive to nd a good sequence. Four tabu search meta-heuristics are used that respectively sequence jobs by randomly swapping jobs, systematically swapping jobs from the top, nding the worst current job pairing and improving it and lastly nding the best current job pairing and improving it. The Hungarian method is also introduced in an attempt to nd a better job sequence. Additionally, the pallet slotting problem is investigated to nd the best storage rack placement conguration based on pallet replenishment cycles. The congurations considered are column pallet slotting (fastest moving inventory (FMI) closest to the picking line), row pallet slotting (FMI closest to the ground) and a stepping conguration with the FMI on the lowest step. The movement logics, slotting congurations and job sequences are incorporated into a DC simulation program, coded in Python 3.4. Using data received from PEP's warehouse management software 10 consecutive shifts are selected for analysis. By simulating di erent combinations it is possible to nd the one that reduces the process time the most. It is found that by changing PEP's current system to one that utilises same side queueing with job pairing the total cumulative completion time is reduced by approximately 39 hours and the distance travelled by high lifts by 45.27 km. This is further improved by adding the row pallet slotting conguration and a job sequencing algorithm. Using this combination reduces the total cumulative completion time by 43 hours.

AFRIKAANSE OPSOMMING : Met die groeiende globale ekonomiese mededinging moet nuwe tegnieke aangewend word vir die distribusie van goedere. Maatskappye soek na doeltreffende distribusie metodes om kliëntediens te verbeter. As gevolg van die toename in vraag moet distribusiesentrums (DS) meer veelsydig word om trend te hou met die vloei van goedere. Deur samewerking met Suid-Afrika se grootste enkele handelsmerkwinkel, PEP, is daar vasgestel dat daar ‘n bottelnek vorm in die stoorrakstelsel by hulle Durban DS. Met PEP se huidige stelsel is die tempo waarteen pallette in die stoorstelsel ingevoer word vinniger as wat pallette uit die stoorstelsel verwyder kan word. Hierdie verskil, in die stoor- en verwyderingstempo van palette, beteken dat die DS gebruik moet maak van groot tydelike bergingsareas vir palette wat wag om gestoor te word. Verder vertraag die stadige tempo waarteen palette uit die stoorrekstelsel gehaal word, alle prosesse wat volg in die voorsieningsketting. Die doel van die studie is om alternatiewe bewegingslogikas, pallet stooringskongurasies en volgordes te vind vir die berging en verwydering van pallette, uit die stoorrakstelsel. Dit word gedoen in ‘n poging om die tempo te verhoog waarteen pallette gestoor en verwyder word. Vyf hyskraan bewegingslogikas word vergelyk, naamlik same side queueing without paired jobs, opposite side queueing without paired jobs, same side queueing with paired jobs, opposite side queueing with paired jobs and cyclic queueing with paired jobs. Terselfdertyd word ‘n analise goed op die effek wat die verandering aan take se volgorde het, vir elke bewegingslogika. Die volgorde van take word bepaal deur gebruik te maak van heuristieke wat probeer om die beste moontlike volgorde te vind. Vier tabusoektogte word gebruik wat onderskeidelik die volgorde verander deur lukraak take om te ruil, stelselmatig take om te ruil deur bo-aan die werkslys te begin, die slegste huidige werksparing te kies en te verbeter en laastens die beste huidige werkparing te vind en te verbeter. Die Hongaarse metode word ook gebruik in ‘n poging om ‘n beter werkreeks volgorde te vind. Daarbenewens word stoorrakkongurasies ondersoek om die beste stoorplek vir pallette te vind. Die kongurasies wat in ag geneem word, is die kolompallet-kongurasies (vinnigste bewegende voorraad (VBV) naaste aan die uitsoeklyne), rypalet-kongurasies (VBV naaste aan die grond) en ‘n trapkongurasie met die VBV op die laagste trap. Die bewegingslogika, stoorrak-kongurasies en taakvolgorde word gebruik in ‘n DS simulasie, wat in Python 3.4 geprogrammeer is. Deur gebruik te maak van data wat vanaf PEP se pakhuisbestuursprogrammatuur ontvang is, word 10 opeenvolgende skofte vir analise gebruik. Deur verskillende kombinasies te simuleer is dit moontlik om vas te stel watter kombinasie die prosesseringstyd die meeste verminder. Die totale kumulatiewe voltooiingstyd verminder met ongeveer 39 ure en die afstand wat die hyskrane aflê verkort met 45.27 km. Dit word verder verbeter deur die rypallet-kongurasies en ‘n taakvolgordealgoritme by te voeg. Deur hierdie kombinasie te gebruik, verminder die totale kumulatiewe voltooiingstyd met ongeveer 43 ure.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/103421
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