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Decision support systems for production

scheduling tasks- Part I of a case study:

Analysis and task redesignVincent C. S. Wiers

Published online: 15 Nov 2010.

To cite this article: Vincent C. S. Wiers (1997) Decision support systems for production scheduling tasks- Part

of a case study: Analysis and task redesign, Production Planning & Control: The Management of Operations, 8:

711-721, DOI: 10.1080/095372897234821

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PRODUCTION PLANNI NG & CONTROL, 1997, VOL. 8, NO. 7, 711±721

Decision support systems for production schedulin

tasks ± Part I of a case study: analysis and task redesignVINCENT C. S. WIERS

Keywords decision support systems, production scheduling,task analysis, industrial a pplications

A bstract. The ®rst three steps of a new design method fordecision support systems in production scheduling tasks areapplied to a large bulk terminal. The operational characteristicsof the company largely correspond with those of typical semi-process industries. The design model used consists of ®ve steps:production analysis, task analysis, task redesign, decision sup-port design and decision support implementation. The results of the application of the design model up to t he task redesign stepare described.

1. Introduction

Reports about the implementation of information sys-tems for production scheduling in practice are scarce. Atthe same time, there is a great need for such reports, because successful implementations of practical schedul-ing systems are limited, and seem to depend more onconsulting skills than on applied scheduling theory.Most reports on scheduling give little indication whetherthe system has been implemented in practice, and forthose systems that have been implemented, what imple-mentation issues were encountered. Moreover, even if

implementation issues are discussed, human factors in

implementing a scheduling system are often ignor whereas success or failure of an implementat ion in pr

tice is determined by the human scheduler (not) usthe system.

A review of the applicability of scheduling techniquin practice is given in Wiers (1997) . In this review artica number of unsolved issues regarding the implemention of scheduling systems are discussed, such as the intaction between human cognition and scheduling systemthe motivational factors of the use of systems by humaand the organizational embedding of scheduling systemTo tackle these problems, a framework for decision suport in production scheduling is presented in Wiers a

van der Schaaf ( 1997) . Although this framew ork is ilstrated by a brief description of four case studies, tauthors conclude that a more thorough validationthe framework in practice is required.

In the case study described in this paper, the concedescribed in Wiers and van der Schaaf are integrated idesign method that comprises ®ve steps. The desmodel is applied to a large dry bulk terminal compaAlthough this company visually di ers considerably fr`normal’ production environments, it will be explainthat the operational characteristics of this company a

similar to other manufacturing systems. The applicati

A uthor : Vincent C. S. Wiers, Bolesian, Steenovenweg 19, 5708 HN Helmond, The Netherlane-mail: [email protected].

V i n c e n t C. S. W i e r s holds a masters degree in industrial engineering and management scie from the Eindhoven University of Technology . Since 1993 he ha s been a Ph D student at the sa university . The P hD project integrates k nowledge in the ®eld of cognitive psychology, informattechnology and production planning and control. H is research interests include human factorproduction scheduling, the applicability of OR/AI scheduling techniques in practice, and interaction between these techniques and human schedulers. He has published his work in J ou rna l o f P r od uct io n and O p er at io ns M an ag e m e nt and P roduction P lanning & C ontrol , and has presenhis work at va rious international conferences. His PhD thesis is to appear in 1997.

0953-7287/97 $12.00 Ñ 1997 Taylor & Francis Ltd.

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of the ®rst three steps of the design model is described inthis paper; the ®nal two steps of the application of thedesign model will be described in a separate paper.

This paper is structured as follows: in Section 2, thedesign method used is presented. I n Section 3, an analysisof the production system of the company is presented. I nSection 4, a task analysis of the schedulers in the com-

pany is presented. In Section 5, a redesign of the schedul-ing task is made by describing how parts of thescheduling task can be allocated to a human or to adecision support system. In Section 6, conclusions aregiven.

2. Design method

The methodology used to design the scheduling deci-sion support system is an extension of the framework t hat

has been described in W iers and van der Schaaf ( 1997).In this paper, design variables for decision support sys-tems for production scheduling are derived from charac-teristics of humans and production systems. A designmodel that incorporates the concepts presented inWiers and van der Schaaf is depicted in Figure 1 and will be explained brie¯ y below. As this paper focuses ona case study, a full description of the model used cannot be given here.

As shown in F igure 1, the design process is divided in®ve steps: ®rst, analysing the current situation regardithe complexity, uncertainty and ¯exibility of the prodtion system; second, ana lysing the context of the task, the division of autonomy, and the task itself; third, redsign of the task based on the results of the analysis, knoedge about cognitive factors and motivational facto

fourth , design of the decision support system in termsschedule generation functions and information presention functions; ®fth, implementation of the decision suport by building a software system or selecting acon®guring a standard software package.

3. Production analysis

3.1. T he company

The company involved in the project presented in tpaper is a large dry bulk terminal in the harbourRotterdam, the busiest port in the world. Since commsioning in 1973, mainly as an iron ore terminal, the copany has expanded and diversi®ed. Approximately megatons of iron ore and 18 megatons of coal are dcharged annually. The company operates in ®ve shiftsa 24 hour, seven days a week basis. Ship discharge raare up to 140000 tons per day and ship loading rates a

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Figure 1. Design model for decision support in pr oduction scheduling tasks.

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upt to 50 000 tons per day. Annual throughput is about30 megatons.

The physical layout of the production system of thecompany is depicted in Figure 2. The ¯ow of material within the company is depicted in Figure 3. As can beseen in Figure 2, the company is delineated by threeharbours: one for unloading sea vessels (B), one for load-ing sea vessels ( C) and one for loading barges ( A) .Parallel to harbour B is a stockyard that is divided into

seven strips. The most important production equipmentof the company is depicted and numbered in Figure 2.

3.2. Production processes

3.2.1. U nloading sea-goi ng vessels

Harbour B is used to tie up sea vessels that have to be un loaded . The length of habour B is 1050 m which meansthat up to three large sea vessels can be unloaded at thesame time as long as their cumulative length includingsome slack does not exceed the length of the harbour.Therefore, habour B is divided into three quays: west,middle and east. The draught at the eastern quay is23m while the draught in the other two quays is only21.65m. This means that some of the larger vessels canonly be tied up at the eastern quay.

Sea going vessels that are unloaded have a load up toapproximately 170000 tons. These ships arrive from allover the world, e.g. Australia, Africa and South America.The ship’s cargo is usually divided over a number of

holds ( typically 9) . Each hold may have a di erent

type of coal or ore. Di erent types of material have be handled separately to avoid contamination. Asimilar material, even within the same hold, may owned by di erent customers, which often means thit also has to be handled separately. The ship’s captaoften gives instructions how to unload the vessel to avstrain; this means that the holds have to be unloadevenly, leading to smaller unloading batches and®xed unloading sequence.

Ships are unloaded by four unloaders ((1), Figurethat are able to move parallel to harbour B. The t

D ec is ion sup po rt f or pr od uct io n sch ed uli ng 7

Figure 2. Layout of the company: (1) unloaders; (2) stacker/reclaimers; (3) barge loaders; (4) train loaders; (5) ship loader; (6) sil

Figure 3. Material ¯ ow within the company.

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un loaders on the left have a lift capacity of 50 tons each;the two other unloaders on the right have a lift capacityof 80 tons each. The leftmost unloader cannot reach theeastern quay; the rightmost unloader cannot reach the western quay . Moreover, unloader s cannot pass eachother, and a minimum distance has to be retained between unloaders, which means that un loaders cannot

work on adjacent holds. The average un loading capacityis approximately 100000tons per 24 hours. Di erentgrabbers can be attached to an unloader, of which thelargest has a volume of 60 m 3.

At this point it is useful to distinguish between twomajor types of operations in the company: material thatis unloaded and transported to the stockyard, and mate-rial that is unloaded and directly transported to barges orother seagoing vessels. The ®rst type of operation isreferred to as ashore , the second type of operation isreferred to as carry through . This is also depicted inFigure 3.

3.2.2. T he conveyor belt system

The material that is unloaded can be dumped on oneof three conveyor belts that run adjacent to the quays of harbour B; these are referred to as qu ay-bel ts . Becausethere are four unloaders and only three quay-belts, thetwo leftmost unloaders often operate as a unit. M oreover,the two leftmost unloaders cannot reach one of the threequay-belts. The quay-belts can be linked to several other

conveyor belts of the conveyor belt system; in total theconveyor belt system contains 47 conveyor belts with atotal length of about 20 km ( note: the conveyor belt sys-tem is omitted in Figure 2) . By linking belts to each otherand to production equipment, over 300 routes can betemporarily created. If a certain routeing has to be con-®gured, the conveyor belt system has to be set up bymoving the ends of individual conveyors.

3.2.3. S torage

Material can be stored in a large stockyard of 100hectare. As can be seen in Figure 2, the stockyard isdivided into seven sections, of which six are edged by ®veconveyor belts. The ®rst strip of the stockyard that is theclosest to harbour B can be reached directly by the un-loaders. This section is used to store material t emporarilyif that material cannot be transported elsewhere at thatmoment. The company prefers not to use this part of thestockyard as its use eventually evokes two handlingoperations instead of one. The other six sections of thestockyard can be reached by ®ve stacker/reclaimers ((2),

Figure 2) that are able to move between these sections.

Because the stockyard is over one kilometre long a unloaders and stacker/reclaimers cannot move very famoving these machines from one end to another may taseveral hours. A stacker/reclaimer is a machine capabof dumping material in the stockyard and excavatimaterial from the stockyard. The material is transportto/from the stacker/reclaimer by a conveyor belt th

runs between sections of the stockyard. Each stackreclaimer can reach two sections of the stockyard; the other hand, some sections can only be reached one stacker/reclaimer.

The average stocking capacity of the stockyard6 megatons and varies according to product mix, i.e. dsity, and pile con®guration. Many small piles take mspace than a few large piles of the same load and densUsually, about 80 types of material are stocked at tsame time in the stockyard. Identical materials for dient customers have to be stocked separately. I t even hapens that two batches of identical material for the sa

customer have to be stocked separately. Ore and coal stocked in separate areas as far as possible to avoid cotamination. Adjacent to harbour C there is anotstockyard which is used mainly to store material thato be loaded on seagoing vessels. There are no stackreclaimers at this stockyard; material is stacked areclaimed by bulldozers and conveyors.

3.2.4. Loading seagoing vessels

Harbour C is used to tie up sea vessels that have toloaded. The length of harbour C is 800m; the draftharbour C is 21.65m. Harbour C is also divided inthree quays: west, middle and east. However, this isless importance in this harbour than in harbour Bthere is only one ship loader ((5), Figure 2), whichable to reach all quays by moving parallel to harboC. The ship loader can be fed by the conveyor belt stem, or it can be fed by moveable conveyor belts thagain are fed by bulldozers. The ship loader has a capcity of 5000 tons per hour. Seagoing vessels typically haa load of about 50000tons. These vessels transport marial that has arrived by sea usually to countries withEurope, such as Germany and Great Britain. Mostonly one type of material has to be loaded into a s vessel.

3.2.5. L oading barges

Harbour A is used to tie up inland shipping barges apushed barges that have to be loaded. The lengthharbour A is 950 m. Because of the relatively small leng

of the vessels that are tied up here, the length of t

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harbour never poses a constraint. Barges typ ically have aload of 1000±3000tons. They are loaded by three bargeloaders ( ( 3) , in Figure 2) that have a capacity of 3500tons per hour. These barge loaders are able tomove parallel to the quay for a short distance in orderto load evenly. Two of the three barge loaders have asmall bu er, which means that if the loader has to stop

for a moment to switch barges or to reposit ion the loader,the production group connected to the loader does nothave to stop operating. Barges are always loaded withone type of material only. Either the material comesdirectly from a seagoing vessel in harbour B, from astacker/reclaimer at the stockyard, or from the silos((6), Figure 2), adjacent to harbour A. The silos are fed by the conveyor belt system and have a capacity of 7000tons each. In the silos, up to six types of coal can be blended on customer speci® cat ion by means of a compu-ter controlled discharge system.

3.2.6. Loading trains, trucks and the power station

Freight trains are loaded at one of the train loadingstations ((4), Figure 2) at a rate of 2500tons per hour.There is one train loading station for loading ore, and one for loading coal. The maximum train load per station is5000tons at a maximum car capacity of 120tons. Thetrain loading stations can be fed by the conveyor beltsystem; it is also possible to feed train loading stations by bulldozers. A special characterist ic of train loading

is that trains have to depart according to a tight schedule.Trucks can be loaded by means of bulldozers. Theamount of material that leaves by truck is insigni®cantlysmall; loading trucks is therefore regarded as a specialservice to the customer. Lastly, a direct conveyor beltconnection exists between the company and a power sta-tion a few kilometres away.

3.3. O perational characteristics

3.3.1. T ypology of the situation

Although the production system of the company mayseem to be considerably di erent from `normal’ produc-tion systems, we ®nd t hat these di erences largely are of amerely visual nature. Regarding the operational charac-teristics of the company, there is a large similarity between this company and companies within the semi -process industry. The following operational characteris-tics are shared with typical semi-process business ( see alsoFransoo and Rutten, 1994) :

materials involved are process oriented;

capacity is not well de®ned (di erent con®gutions, complex routeings);

resources can be physically linked together teporarily;

large number of process steps;

large number of products ( about 200 prodcodes);

bu

er capacity is limited;

less impact of changeover times than in process ¯industries, but more than in typ ical discrete prodution processes;

material ¯ow can be both convergent and divgent;

long lead times (for unloading operations), mu work-in-process;

production involves manual labour that has toshared by di erent operations.

A number of operational characteristics are not neces

rily typical for semi-process industries. First, materials not transformed into other materials in terms of compotion. However, materials are transformed regardquantity, and the composition of materials is chang by mixing materials from the silos. Second, there are ®xed recipes. Third, production is done only to customorder; however, although not necessary typical, this is nexceptionally unusual for semi-process industries. Foursome of the company’s customers±i.e. ships’ crews±present at the production process. This aspect migappear of minor importance; however, the plann

and scheduling of production is continuously scrutiniz by crew s that all wi sh to leave the harbour as earlypossible. Fifth, instead of the common situation whcustomer orders drive production at the output sidethe material ¯ow, here the customer orders drive tproduction process at the input side of the mater¯ow. This means that the production process is tolarge extent driven by the seagoing vessels that arrat the port. However, some semi-process industries a base production on their material in¯ow.

3.3.2. U ncertainty and ¯exibility

There are many factors causing disturbances in production process. The most important are:

uncertainty in unloading throughput times;

uncertainty in the availability of production equment;

uncertainty in the arrival time of boats and barg

Disturbances can sometimes be compensated for by ¯

ibility in the production system. The most importa


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forms of ¯exibility are:

¯oating unloaders can be hired to increase unload-ing capacity;

alternative routeings can be used to get arounddefective production equipment;

bulldozers can be used to move material to ad jacentsections, so that stacker/reclaimers can be used thatnormally would not be able to reach the section where the material was located;

it is possible, though not preferable, to load bargesin harbour C;

the ®rst strip on the stockyard can be used to storematerial if the material that is unloaded cannot betransported elsewhere.

4. Tasks analysis

4.1. A utonomy

The context of the production scheduling task in thecompany is de®ned by the organization of decision-mak-ing functions regarding production control. The organi-zation of production control and decision-making isdepicted in Figure 4 and is explained below.

In the contracting process, the commercial departmentcommunicates with the customers of the company aboutamounts and rat es of storage and t ransshipment. In some

cases, customers require a minimum amount of di schargeto be realized within a certain time period as soon as aship has arrived at the port. I f this amount is not met, thecompany has to pay demurrage to the customer; on theother hand, if the company discharges faster than agreed upon, hal f the demurrage has to be paid by the cu stomer.

Customers delegate the management of operatioactivities to agents that are situated in the vicinity of port. These agents directly communicate with the planing department of the company. This means that this no direct communication between the customers athe planning department, or between the agents and commercial department. One of the reasons for usi

agents is that customers often are situated in anothpart of the world. Agents provide information to planning department about vessels that are goingarrive at the company.

From the ship list, t he stockyard layout, and detaiinformation from agents about ships, a ship scheduleconstructed twice a week. The ship schedule shows tallocation of quays to seagoing vessels, the allocation unloaders to seagoing vessels that have to be un loadand the destination/origin of the material unloadloaded. From the ship schedule, the stockyard layoand detailed information about the contents and unloa

ing sequence of holds, a lot schedule is constructed twa week. The lot schedule contains similar informationthe ship schedule, but at a greater level of detInformation about loading individual barges is matained in an administrative computer system and is nput on the schedule. From the lot schedule, a shift wolist is made which is transferred to the shift foreman.

The operators on the shop ¯oor are allowed to soissues regarding the use of the conveyor belt system, athe allocation of barge loaders to individual bar within one shift. Some schedulers only specify how

transport material from X to Y without indicating trequired con®guration of the conveyor belt system, ain these cases the shift personnel decide how to realize transpositions speci®ed in the schedule.

4.2. S cheduling task analysis

In the scheduling task, two types of schedules amade: a boat schedule and a lot schedule. There is csiderable similarity between the boat schedule and theschedule. The di erence is that the lot schedule is maat a greater level of detail than the boat schedule: in tlot schedule, all information relevant for productiontaken into account, whereas the boat schedule omsome detailed information.

The scheduling activit ies described above are depicin Figure 5. As can be seen in this ®gure, scheduliactivities are often carried out in an iterative mann for example, calculating the unloading time can leadthe scheduler reconsidering the assignment of unloadto boats. The activities within the scheduling task a

described below.

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Figure 4. Production control structure.

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4.2.1. A ssign qu ays

The boat schedule contains a list of boats that aregoing to arrive at the terminal. Barges are not put onthe boat list. The boat s on the boat list are sorted accord-ing to their es t imated t ime of arrival (ETA). Generally,more information about a speci®c boat is available asits ETA approaches. For example, if a boat’s ETA is

two weeks away, the planning department may not know yet what cargo the boat is carry ing, what thetotal load of the boat is, or even what the name of the boat is. One of the reasons for this is that the destinationof cargo can change even if the boat is on its way to theport. Another reason is that some agents prefer to with-hold information from the company for some time.

All seagoing vessels with an ETA that lies within a t imehorizon of three weeks from now, and that have to be un loaded are put in the boat schedule. The boat schedulehas two horizons: the ®rst horizon depicts the occupation

of the quays for seagoing vessels for the coming week, and

the second horizon depicts the same information for two weeks after the ®rst week. The boat schedule updated twice a week . A boat is scheduled as follo®rst, a quay is assigned to the boat. If the draught o boat exceeds 21.65 m, the eastern quay of habour B m

be assigned to the boat . Usually , the sequence of boat sthe boat schedule is arranged FIFO (®rst in ®rst ou

However, the assignment of quays to boats also depenon commercial aspects, such as possible demurraclaims.

4.2.2. Construct unloading sequence

When boats are assigned to quays, detailed informtion about the boats is used to assign unloaders and othproduction equipment to unload the cargo. For ea boat, the following informat ion is needed:

per customer: material type, amount and destintion (ashore or carry through);

per hold: material type, amount.

This information is referred to as hold con®gurat

Furthermore, in many cases the ship’s captain giinstructions how to discharge the boat, w hich limits degrees of freedom in determining an unloadsequence. To unload the individual holds and lots, uloaders have to be assigned in a feasible sequence. Tmeans that an unloading sequence has to be found that d

not con¯ict with the hold con®guration (together wthe instructions of the ship’s captain), and does not co¯ict with the freedom of movement of the unloadersthe allocation of other production equipment.

4.2.3. Assign unloaders to holds

The scheduler assigns unloaders to the holds of t boat. The al loca tion of un loaders is determined by following factors:

the quay assigned to the boat, which determin which unloaders are able to reach the boat ;

assignment of unloaders to other boats;

the hold con®guration leading to an unloadsequence for a boat;

position of the unloaders, which determines tamount of time required to move the unloader pallel to the quay;

constraints and allocation of the conveyor bsystem (in particular quay-belts);

priority of the boat;

destination of the cargo.


Figure 5. Activities within the scheduling task.

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The allocation of unloaders to holds and quay-belts alsodepends on the destination of the material. Numerousconstraints in the conveyor belt system result in the factthat some destinations cannot be reached by some quay- belts. As sta ted earlier, the schedulers will already haveattempted to tackle this problem partially during theallocation of quays to boats. When assigning unloaders

to holds, constraints in the transportation system againhave to be taken into account. The destination of thematerial also depends in the stockyard layout. The mate-rials that are stocked in the stockyard are monitored andregistered continuously by the scheduler that has outsideduty. Registration takes place on a piece of paper thatshows the layout of the stockyard, and on a large boardon the wall of the planning department.

4.2.4. Assign stacker/reclaimer, barge loader and ship loader

The scheduler determines how to transport the un-loaded material to its destination. In the case of lots thatgo ashore , the scheduler decides which stacker/reclaimer will be used to stack the material in the stockyard. Theselection of a stacker/reclaimer to transport material tothe stockyard depends on:

the type of material, which determines in what sec-tion of the stockyard the material should be stocked;

the stockyard layout, which indicates where freespace is available;

the availability of stacker/reclaimers, which is notonly determined by the unloading of boats but also by the loading of barges from the stockyard and theloading of seagoing vessels from the stockyard;

the position of stacker/reclaimers, which determinesthe amount of time required to move the stacker/reclaimer between the sections of the stockyard;

constraints and allocation of the conveyor belt sys-tem.

In the case of lots that are carried through , the schedulerselects a barge loader or allocates the ship loader. Theselection of a barge loader depends on:

the availability of barges, i.e. enough barges have to be present at the same time as the ship is unloaded ;

the availability of barge loaders, which depends onother barges to be loaded.

Scheduling of boats that have to be loaded is much lesscomplex than scheduling boats that have to be unloaded,as there is only one ship loader. Furthermore, the holdcon®guration of the loading boats is much simpler, and insome cases, the schedulers can decide themselves which

material from a ®xed assortment to load.

When assigning production equipment, the schedulhave to take the limited amount of workers into accouThere are some operations that are labour-intensive aif these have to be scheduled simultaneously, a problcould arise. For example, to empty a boat, workers hato be lowered into the hold. Also, activities where budozers are used need extra personnel, e.g. loading a tr

without using the train loading sta tion, loading trucksloading material on the mobile conveyors in harbour

4.2.5. Accept ba rges

The loading of barges is not scheduled in detail; depdent on the allocation of production equipment to othoperations, a ®xed tonnage of barges is accepted per shIf a barge is going to arrive at the port it is put in administrative computer system and scheduled for a p

ticular shift. Within the 8 hours of the shift, the workmon the shop ¯oor may choose which barge to serve whThe loading of trains and trucks also is not scheduleddetail. Trains have to be loaded within the strict tischedule of the railway company; however, this donot usually pose a problem on account of the relativsmall load of trains.

4.2.6. E stimate unloading throughput time

When the production equipment is a ssigned, the sch uler determines the throughput times of the boats thare unloaded. A rough estimate of the unloading specan be made by taking into account the number of uloaders used to discharge a boat, and the destinationthe material, i.e. ashore or carry through. However, unloading capacity depends on many other factors, which the most important ( and tang ible) ones are relato the type of material handled and the type of boinvolved. These are explained below.

M aterial type . The density of ore is higher than thof coal, and therefore, ore can be unloaded fasthan coal. Moreover, some types of material amore dicult to unload than others; if materia wet , sticky and powdery , unloading wi ll probabtake longer than if material is dry, smooth acoarse.

S h ap e o f h o ld s . Some holds have pro®les or othirregular shapes on the inside which means thmaterial may stick between these irregularitOther holds are smooth on the inside which meathat the material will subside during unloadin

Furthermore, the openings of holds vary in size

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these openings are small, unloaders have to be morecareful.

There are other, less tangible factors in¯uencing the un loading capacity, such as weather condi tions, safetyregulations, material heating, personnel, disposition of the captain, etc.

4.2.7. M onitor progress of production

From the completed schedule a list of workorders pershift is made and transferred to t he shift foreman. Duringthe rest of the duty of the scheduler, the progress of production is closely monitored and actions are taken if real production deviates from the schedule. Hence, thescheduler communicates with agents, boats’ crews, barge-masters, shift foremen, shift personnel, safety inspectors,inspectors of weights and measures, the outdoor dutyscheduler, the commercial department, etc.

5. Task redesign

In the task redesign phase, activities within the scheduling task are allocated to a decision support system or tothe human scheduler. The task allocation is based onthe task analysis described in the previous section and knowledge about human cognition and mot ivat ion ( see

Figure 1; for an elaborate description of the cognitive andmotivational factors involved, see Wiers and van derSchaaf 1997). Note that task allocation decisions arenot of a black and white nature; it is also possible thata decision support system supports the human scheduler toa certain extent while the human carries out the activity( Higgins, 1996) .

Regarding cognitive factors , questions that should beasked for each activity are:

Is the activity of a routine nature?

Does the activity require problem-solving activities?

Does the activity require the simultaneous consid-eration of many elements?

Does the activity require laborious calculations?

Does the activity require communication withhumans?

Activities within the scheduling task that are of a routinenature and require the simultaneous consideration of many elements and laborious calculations may be allo-cated to a decision support system. Activities within thescheduling task that require problem-solving activitiesmay be supported by a decision support system while the

human remains the principal controller. Activities within

the scheduling task that require communication whumans should be allocated to the human scheduler.

Regarding motivational factors , questions that shouldasked for each activity are:

Does the activity require much expertise?

Is the activity of a critical nature?

Is the activity of an ill-de®ned nature?

Can the performance of the activity easily measured?

Activities within the scheduling task that require muexpertise, and that are of a critical and ill-de®ned natu will result in the human scheduler demanding to becontrol, i.e. without the visible existence of a decisisupport system. If the performance of an activity ceasily be measured, it can be fed back to the schedulthereby comparing the actual performance to the perfmance that could have been reached if the system h

been used. In this way, the con®dence of the schedulerthe system increases. However, in most scheduling tasperformance cannot be measured objectively, and highhuman con®dence in a decision support system conly be achieved by increasing the transparency of system.

5.1. Assign qu ays to boats

Start times of boats are determined by assigning boto quays and by making a preliminary assignment unloaders to boats. Because the boat schedule is mamanually, generating and maintaining the boat schedis very labour-intensive; changes in information regaing boats often lead to laborious updating activities.

The task of making changes can be supported bydecision support system. The need for making chanhas to be identi®ed by the human scheduler becauthis is often triggered by communication between tagent and the scheduler. Making the change itself h

to be supported by the decision support system as tis of a routine nature and requires laborious calculatioA decision support system also will be able to provoverview on the schedules, as many elements±i.e. boalots±have to be considered simultaneously.

From a cognitive viewpoint, the decision-making pcess of designing alternatives of the new boat schedcan also be supported. However, designing alternatschedules requires much expertise and is of a critical aill-de®ned nature. Therefore, from a motivational stanpoint, decision support regarding the (re)generation

boat schedules shou ld be implemented wi th caut ion.


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5.2. R eview hold con®guration

The hold con®guration of a boat restricts the freedomof the scheduler in unloading the boat. As stated inSection 4, di erent lots have to be unloaded separatelyto avoid contamination, and holds have to be unloadedevenly to avoid strain. For each hold con®guration, the

scheduler has to construct an unloading sequence.Devising an unloading sequence is a routine and labor-ious task and can be allocated to a large extent to adecision support system.

5.3. Assign unloaders to holds

Assigning unloaders to boats/holds requires problem-solving activities from the human scheduler. Many cri-teria, both well- and ill-de®ned have to be taken intoaccount when assigning unloaders. Hence, assigning

un loaders requires much expertise. Moreov er, assign ing un loaders to boats is a critical task as it largely deter-mines the productivity of the unloading process.Therefore, assigning unloaders to boats is an activitythat needs the human scheduler as principal controller.However, because many elements have to be taken intoaccount simultaneously, a decision support system could be used to support the human scheduler when assign ing un loaders, i.e. by providing overv iew. Also, it could be used to warn the scheduler if impossible schedules have been made ( e.g. in the case where unloaders would have

to pass each other). From a motivational standpoint, thedecision support system components that support thisactivity should be transparent.

5.4. Assign stacker/reclaimers, barge loaders, ship loaders

The activity of assigning stacker/reclaimers, bargeloaders, ship loaders is of a similar nature to the activityof assigning unloaders to boats/holds. An additional factor that has to be considered in this activity is theavailability of the conveyor belt system. Hence, decisionsupport in this activity should be of the same nature asdescribed in the previous section.

As described in Section 4.1, checking the availability of individual components of the conveyor belt system isoften delegated to the operators on the shop ¯oor.Therefore, from the schedulers’ viewpoint, it is not neces-sary for a decision support system to handle the avail-ability of the conveyor belt system. However, it has beenagreed in the company that schedules that a re transferredto the shop ¯oor, while not necessarily decisive, should atleast be feasible. Therefore, a decision support system

should be able to handle the availability of the conveyor

belt system in an invis ible manner; the system does require the scheduler to assign individual conveyor be but warns the scheduler if impossible combinationsoperations are scheduled.

5.5. E stimate unloading throughput time

Based on the assignment decisions made in the p vious described activities, an estimate of the through ptime of unloading operations is made. As describedSection 4.2.6, unloading throughput times are hardgrasp in the company. Estimates are based on a numbof well- and ill-de®ned factors, and the result of tprocess is an estimate of the number of tons that adischarged per shift. As soon as the estimate is macalculating the throughput time becomes a laboriotask, as the total tonnage of a boat has to be divided the estimate, and the scheduler needs to know

remaining tonnage per boat at the end of each shDuring the unloading process the real unloading spemay deviate from the estimate, and the scheduler hasperform the calculations again. Therefore, a decision suport system should calculate the unloading time fogiven estimate of the unloading speed. Furthermothe decision support system may make a preliminaestimate of the unloading speed based on a small numbof factors that have proven to be important in the estimtion process. The scheduler then is able to deviate potively or negatively from this estimate, based on

knowledge about a speci®c instance.

5.6. Accept ba rges

Based on the assignments of production equipmthat are made in the previous activities, a certain amouof capacity remains to load barges. Based on this amou barges are accepted and scheduled for a speci®c shWithin this shift, the operators on the shop ¯oor cdecide which barge to serve ®rst. Because the activto accept barges requires communication with humait should be allocated to the human scheduler. Howevthe scheduler can be supported in determining amount of capacity remaining for barge loading.

5.7. M onitor progress of production

The progress of production is monitored closely by scheduler on duty, and deviations from the schedule aidenti®ed and acted upon. This activity requires muproblem solving by the human scheduler, and it oft

involves dealing with ill-de®ned factors that cause real

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to deviate from scheduled production. Many disturbances are ident i®ed by communicat ing with humans.These activities are often of a critical nature as the pro-gress of production may be endangered if the problemsidenti®ed are not solved quickly. I f changes to the schedule have to be made, many elements have to be consid-ered simultaneously, and laborious calculations may be

the result. In this activity, a decision support systemcould be used to support the human scheduler by provid-ing overview and by facilitating updates of the schedule.From a motivational standpoint, the decision supportsystem components that support this activity should behighly transparent.

6. Conclusions

A case study has been presented in which a newmethod for the design of decision support in production

scheduling tasks is applied. The methods consists of ®vesteps including an analysis of the production system, atask analysis, a redesign of the task, and the design andimplementation of a decision support system. In part I of

the case study described in this paper, the ®rst three stof the method have been presented. Part II of this castudy will be described in a separate paper, and will foon the ®na l two steps of the design method: design of decision support system and implementation of the desion support system.

References

F r a n so o , J. C., and R u t t e n, W. G. M . M., 1994, A typoloof production control situations in process industrI nternational J ournal of O perations and P roduction M anagem

14( 12) , 47±57.H i g g i n s, P. G., 1996, Interaction in hybrid intelligent sched

ing. International J ournal of H uman F actors in M anufacturing

185±203.W i e r s, V. C. S., and Sc h a a f, T. W. v a n d e r , 1997, A fram

work for dec ision support in production scheduling ta

P roduction P lanning & Co ntrol , 8, 6, pp. 533±544.W i e r s, V. C. S., 1997, A review of the applicability of OR aAI scheduling techniques in practice. To appear in O M E G

T he I nternational J ournal of M anagement S cience , 25( 2), pp. 1153.


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