computers in railways 353 - wit press · between two major logistics systems: sea and land trade....

Designing container yard management systems

M. Paolucci*, V. Recagno* & S. Sacone*

"Department of Communications, Computers and System

Sciences, University ofGenova, Via all'Opera Pia 13,

1-16145 Genova, Italy

*SCIROElectraS.r.L,

Via Fieschi 25/6a, 1-16121 Genova, Italy

[email protected]; [email protected]; [email protected]

Abstract

Freight transportation plays an important role in modern logistics, as oneof the co-operating processes involved in production chains. In thiscontext, an interesting research field is intermodal transport, which isdefined as the movement of goods in the same loading unit or vehicle,using successively several modes of transport without direct handling ofthe goods. In an integrated freight terminal of a transportation networkchanging of either the means or the mode of transport should besupported by suitable freight information systems and involve decisions.In this work, the authors focus on the management of containers in portterminals. In particular, the features of an information system supportingdecision making activities involved in terminal management arediscussed. Finally, the problem of optimising the container allocation onthe terminal yard is investigated.

1 Introduction

A port is a node of an integrated freight transport network where at leastthree different modes of transport flow: i.e., sea, road, and rail. Thus, port

Transactions on the Built Environment vol 34, © 1998 WIT Press, www.witpress.com, ISSN 1743-3509

352 Computers in Railways

container terminals (from now on terminals) can be seen as inter modalfreight transport nodes. Terminals can be classified as logistics systems,since terminal operators have to manage them under precise constraintsrepresented by both time and space. In fact, terminals are the jointbetween two major logistics systems: sea and land trade. In this section, asynthetic and preliminary description of terminal systems is given.

The main function of a terminal is to handle a given set of containers,imported on a vessel, and to split it into several smaller lots to beforwarded by road, rail, and sea. The latter case, namely the transhipment,is usually performed among two vessels with different size and capacity.On the other hand, the same terminal should perform the beforementioned activities in the opposite direction, merging small lots ofcontainers into one bigger (i.e., containers loaded on small vessels, train,and trucks are exported on ships). Time and space constraints can besummarised as follows: time constraints represent the differences of timedistribution for container arrivals, for each mode of transport considered.Space constraints take account of the yard dimensions, which are usuallyhuge in the Northern Range ports (i.e., Rotterdam, Antwerp, etc.), andrather small in the Mediterranean ones (Genova, Barcelona, Marseilles,etc.).

Terminal operators manage the handling of containers flowing in andout the port under the above mentioned constraints, but they are rarelysupported by suitable information. More precisely, trucks carryingcontainers to be unloaded reach the port randomly. Railway operatorsrarely provide suitable logistic information, such as the position ofcontainers on the train. The management aspects of an intermodal nodecan be traced to the management of input and output container flows, alsoconsidering the handling of containers in the yard. In order to maximisethe productivity of ports without adding new handling equipment orinfrastructure, these operations should be synchronised and co-ordinated.To this aim, a real-time mapping of the container yard is needed, in orderto support the yard planner (i.e., the manager and responsible for yardoperations) during such important work. In particular, a decision supportsystem based on real-time mapping should be helpful to improve theperformance of terminals, by aiding the planner during the containerallocation activities.

In the following sections, the authors introduce a new approach tomodel container terminals; this methodology takes into account thephysical characteristics of the site (i.e., the terminal layout), as well as thedescription of the handling equipment. Moreover, a formal description ofthe problem of container allocation in the yard will be detailed.


Computers in Railways 353

2 The layout description

In this section, the description of the main entities included in a containerterminal is given. The layout definition of a terminal includes both theinfrastructures (i.e., berths, yards, warehouses, parking areas, marshallingyards, etc.), and the equipment to handle the containers (i.e., Ship to Shorecranes, Yard cranes, straddle carriers, reach-stackers, front-lift loaders,etc.). A terminal can be described as composed by different subsystems,which co-operate to perform the terminal activities. This subdivision isbased on the analysis of the several terminal components, taking accountof both their respective physical and functional specifications, and theinterfaces between them. In other words, the subsystems could beidentified as self-standing entities in more complex system architecture.More precisely, each subsystem is distinguished through its duty-cycleand spatial context. Under these assumptions, a terminal can be modelledas the superimposition of yard, the quay and the transfer subsystems.

2.1 The yard subsystem

The yard subsystem is in charge for the logical and physical managementof container flowing and stored in the terminal. This system monitors thepositioning of containers in the terminal, gathering those informationrequired by the planner to perform his job. More precisely, the yardsubsystem has to be provided with a suitable monitoring system that mapsthe container positions. In the monitoring system, the information aboutthe position of the containers should be real-time updated. Moreover, theyard subsystem has to schedule the yard resources to complete handlingand re-handling operations on yard-located containers. As a preliminaryconsideration, it is sensible to state that a primary objective of the yardsubsystem is to perform all operation on containers minimising thenumber of handlings. A precise description of this subsystem will begiven in a further section.

2.2 The quay subsystem

The quay subsystem actually involves more areas than just the terminalquays. In fact, it considers the infrastructure and the equipment to unloadand load all means of transport served by the terminal (i.e., ships, trains,and trucks). Nonetheless, in this work the authors will refer to thissubsystem as the quay subsystem, given the major role covered by shipsin the management of modern terminals.

The quay subsystem aims to manage all loading and unloadingoperations on ships, trains and trucks. This is achieved through theoptimised use of quay resources (Ship to Shore cranes, marshalling yard



cranes, front loaders and reach-stackers), under the constraints imposed bythe shipping lists.

2.3 The transfer subsystem

The transfer subsystem interfaces the yard and the quay subsystemsmanaging a fixed amount of terminal transport means. The transfersubsystem is responsible for moving containers from the yard to the quayand vice-versa. The transfer subsystem takes account of terminal trafficmanagement, priorities, and transfer equipment scheduling.

3 Decisional aspects in terminal operations

The management of container terminals involves a large number ofdecisional problems to be solved with optimal, suboptimal, or heuristicapproaches. Such decisions are mainly related to the presence of limitedresources to be shared by different entities, and to the existence of timeconstraints relevant to the movements of transport means and goods toand from the terminal. The resource sets included in the system are:• physical places where means of transport stay to complete theload/unload and commercial operations, i.e., quays, marshalling yardand truck yards. Of course, only quays and railway tracks are limited innumber, thus generating decisional problems in their allocation, whereastruck yards typically have a so high maximum capacity to be consideredas unlimited resources;

* quay resources, intended as equipments to load and unload operations,as defined in SubSection 2.2, dealing with the quay subsystem;

• the container yard, which constitutes the most important resource to bemanaged in the overall system, since the way in which the yard isexploited influences the performance of the whole terminal;

. the yard equipment used to handle containers inside the yard;• the transfer equipment allowing the movement of containers between theyard and the quay;

* human resources needed for the implementation of each terminaloperation.

The time constraints to be fulfilled are relevant to the schedule of thedifferent transportation means and/or to the delivery due-dates ofcontainers. In this framework, many decisional problems can be easilyidentified in the planning of terminal operations. Some of these problemsare listed in the following:- the problem of scheduling the allocation of quays and of thecorresponding equipments to ships;

- the problem of defining the bay-plan of each ship leaving the terminal;- the problem of optimally exploiting the container yard;



- the problem of allocating human resources to the different operations.Each of these problems is crucial in the management of the container

terminal, and its solution affects the performance of the overall system. Afeasible approach to analyse the performance of a terminal container, isthe application of simulation techniques. In [2], a model representing thetypical operations of a multimodal terminal is defined; on the basis ofsuch a model, in [3], the system performance are analysed by means ofextensive simulation runs. The results obtained from simulation are usefulto define new policies with reference to the different decisional aspects, aswell as simulation can be used to test the effectiveness of such newstrategies.

A further approach to the management of complex systems is thedevelopment of decision support systems. A few examples of decisionsupport systems for port terminals, or better container terminals, can befound in the literature ([4], [5]). In this work, one of the above mentioneddecisional problems is considered, namely the optimal allocation ofcontainers in the yard Such a problem is faced by defining a suitablemodel and by stating an optimisation problem, relevant to the containerunloading and positioning operations. Details about the proposed modeland the statement of the considered problem can be found in the followingsections.

4 The system architecture

The proposed yard management system would provide real timeinformation on the yard status. This can be obtained by defining a suitabledata structure and identifying the relevant events that update such data.The information to be managed are relevant to four main types of objects,that is, yard cells (space resources), yard equipments, containers andtransport means accessing the terminal. The events involving such objectsbelong to three classes: a) arrivals and b) departures of containers, c)transfers of containers internal to the yard It is essential to track everyaction performed inside the terminal to provide the user with a reliableimage of the yard status, allowing an effective use of decision supporttools, such as simulation, or procedures to optimise the resource usage.

The basic architecture proposed for the management system is depictedin figure 1. In such a structure a key role is played by the user interfacethat should provide information with different levels of details; inparticular a graphic interface should be available where graphic itemscorrespond either to objects or class of objects in the database. Eachcontainer or mean of transport accessing the terminal should be identified,determining an initial status; then, on the basis of event occurrences, thestatus is updated The terminal personnel and/or the equipment should beprovided with communication tools which act as management system



sensors. It is very important to adopt reliable communication tools andprocedures, even if quite simple. In fact, the identification of objectsentering the terminal, which in general may be a complex and expensiveprocess, could be performed only once; then, the only information neededcorrespond simply to the variations of the position (status) of objects. Tofurther simplify the procedures and devices for container identification,the information included in the containership bay-plans, which areassumed available before the beginning of the unloading operations,should be provided as an input to the system.

Graphic ObjectInterface 4 w

91

Decision SupportModule

/Arrivals /Departures/~~

/intra-yard /vfovementy

YardCommunication

System

C2L

ObjectDBMS

""""•x

Figure 1: The architecture of the yard information management system

Finally, for the proposed architecture the use of an object orientedDBMS is suggested. This choice would make the definition of theprocedures to interact with the system objects easier, in particular throughthe communication system and the GUI.

5 The container positioning problem

A key factor for unloading and positioning operations is time, that is,containers should be picked up from the ship hold and positioned on theterminal yard avoiding unnecessary handlings and tool idle times. Thisfact could trivially lead to the simpler decision of unloading containerswithout a particular order or location criteria. However, such a policycould produce a poor quality container allocation, namely, a yard statusthat would require a great number of intermediate handlings in order toretrieve the containers when they have to continue their travel todestination. The problem is then to determine the optimal position ofunloaded containers in order to make the successive loading operationseasier, that is, to locate containers such that most of them will be directlyavailable when they should leave the terminal. The problem of containerunloading and positioning at a vessel arrival at the terminal is considered.Several information are assumed known at the containership arrival:



« the containership bay-plan (i.e., the detailed location of containers inthe ship hold);

+ for each container, the delivery date (i.e., the date at which thecontainer is expected to leave the terminal), the weight, the destinationand/or the shipper;

• the terminal yard status, that is, which cells are available and withpossible constraints.The problem has been mathematically modelled as described in the

following. Two types of containers have been considered: 20' containers(corresponding to the TEU standard) and 40' containers. Then, the setA={j: j=l,...,C}, of C containers included in the bay-plan of a ship to beunloaded is partitioned into the subset, B, of 40' containers, and thesubset, S, of 20' containers. For each container the delivery date and theweight are known. Such two pieces of information are used to determinethe appropriate location of containers on the yard as they define a partialpriority order for the containers that should be located, and, in general,among all the containers on the terminal yard In particular, delivery datehas priority with respect to weight. Such a partial order can be modelledassigning a synthetic (integer) delivery date dj, je A, to each container.

The relative position of containers in the bays of a ship hold imposes aset of precedence constraints on the container unloading order. Twoopposite cases can be assumed: a weakly constrained one, where eachcontainer stack included in the bays is considered independent from theothers, or a strongly constrained one, where the unloading order is totallyimposed by the bay-plan. The model proposed for such constraints resultto be general, that is, able to deal with any of the above two cases.

The container yard can be modelled as a 3D grid composed by i=l,...,Ncells (or places) on which containers can be stacked up to L tiers, indexedby k=l,...,L. Each cell can contain a container jeS, whereas two adjacentcells at the same tier should be used to locate a container j'eB. The setCB={(i,h)}of pairs of adjacent cells suitable to locate a 40' container isassumed a-priori defined In addition, it has be assumed that the pair inCB cannot overlap. The number of TEU equivalent containers is thenT=ISI+2IBI, whereas C=ISI+IBI.

The unloading and location problem corresponds in general todetermine an order for the container unloading and the container locationon the yard such that constraints imposed by the structure of the bay-plansand by the yard status are satisfied, aiming at minimising the number ofviolation of the precedence partial order defined by the synthetic deliverydates. Such a problem is a combinatorial one which, in general, can beassociated with the problem of finding a set of simple maximal cuts in agraph. Consider, in fact, the particular case of unloading and locating asingle bay stack using two yard cells; then, a dgraph G=(V,A) can bedefined by associating a node to each container and an arc (vj,Vj)e A when



the container corresponding to v, precede container YJ in the bay stack andfor the associated (synthetic) delivery dates, d; and dj, the relation di<dj istrue. The problem of locating the containers on the two cells consists offinding the simple max cut for G, that is, a node partition Vi, V%, such thenumber of arcs (Vi,Vj) such that V;E Vi and VjE V% is maximum. The simplemax cut problem for general non-planar graphs is a well-known NP-Complete problem [1]. However, the particular structure of the graphsassociated with the instances of the container problem could be exploitedto define suitable algorithms, which, in the worst case, could be based onapproximate or heuristic techniques. A possibility that the authors arecurrently investigating is to define a mathematical formulation of theproblem and then analyse it in order to introduce a hierarchy ofrelaxations.The problem can be formulated as a linear integer programming

problem introducing the two following sets of variables:

• Xjftt, j=l,..,C, i=l,...,N, k=l,...,L, t=l,...,C, XjibE {0,1} such that x l ifcontainer j is the t-th to be unloaded and is located on cell i at tier k,

and 0 otherwise. The index t is introduced to determine the sequence ofcontainer unloading operations. In general, the values that can beassumed by k may vary from cell to cell depending on the initial status(e.g., if the first 3 tiers of the i-th cell are initially occupied,

keLi={4,5}). The number of these variables is at most C*LN;* %,h, (q,h)eP, where P={(q,h): dq3dh, q=l,...,C, h=l,...,C, q#h},

Zqh€ {0,1 } such that Zqh=l if container q is located in a lower tier thancontainer h on the same yard cell, but dq<d*,. The number of suchvariables is at most C(C-l), and it actually corresponds to the numberof order relations derived from the delivery dates defined in set P.The following sets of constraints can be introduced:

a) each container of set S should be allocated in a single position (a tier ina yard cell), whereas each container of set B takes 2 positions:

N L C N L CI I Z*j&i=l YleS I I %Xjikt=2VjEB (1)i=lk=lt=l i=ik=lt=l

b) a single cell tier is occupied with the t-th unloading operation if acontainer belonging to S is processed, whereas if it belongs to B twosimultaneous operations occur:

I X %Xj%=l Vt = l,...,C I I I Xjit=2Vt = l,...,C (2)i=lk=ljeB

c) the precedences imposed by the bay plan on unloading operations arerespected. Knowing the ship bay-plan, a set O of ordered pairs (q,j)



representing the fact that container q should be processed beforecontainer j can be defined, and then the following set of constraints:

C N L t-1 N LZ Z Z Xjfcw< I I I Xqaa V(q,j)e QVt = 2,...,C (3)w=ti=lk=l z=ii=ik=l

To take into account the cases of two 20* containers stacked over a 40'one, an additional set O'={(p,q,j)} can be defined, where (p,q,j)represents the fact that containers p and q should be unloaded beforecontainer j. Then a further set of constraints is imposed:

C L N N t - l N L N LZ Z (Z Xjikw + Z Xjhkw) Z ( Z Z Xqikz + Z Zw=tk=l i=l h=l z=l i=lk=l u=ll=l

(4)

V(p,q,j)eO'Vt = 2,...,C

d) a yard stack tier can be used only after the lesser tiers have beenoccupied This is ensured by the following constraints:

t-1 CZ Z XM,k-l,z Yje AViVt = 2,...,CVk = 2,...,L (5)z=0h=l

e) 40' containers are located according to the two types of conditions:cells (i,h)eCB are occupied simultaneously by a 40' container

Vje B V(i,h)e CBVkVt (6)

undo* cells (i,h)e CB 20' containers should not be located:

Xjikt^l- Z Z Xqi,k-i,z Vje B VtVkV(i,h)e CB (7)z=OqeS

and analogously for x ;f) a final set of constraints is introduced to identify the violations to the

container delivery dates:

C C k-1 C LZ Xq&i < Z Zxhift + 1- Z Zxbift + Zqb _\t=l t=l f =1 t=lf=l W

V(q,h)e PVi = l,..NVk = 2,...,L

It can be verified that variables Zqi equal to 1 only if q is located in thesame yard cell as h but in a lower tier.



At least the following two types of objectives can be considered:1. minimising the number of violations to the delivery date order

min I 2.% (9)V(q,h)eP

2. minimising the number of yard cells used:N C C

mini I Ixjiit (10)i=l j=l t=l

6 Conclusions

The possibility of providing multimodal container terminals with ad-hocdesigned information systems, is addressed in this paper. The activity ofsuch information systems involves two major functions: the first objectiveis that of maintaining real-time updated information about the state of theterminal components, whereas the second objective consists in supportingdecision making activities involved in terminal management. In this work,the authors focused on the management of the container yard, whichconstitutes a crucial component of the overall container terminal. Inparticular, the architecture of the yard information management systemhas been described and a mathematical model for the container unloadingand positioning operations on the yard has been presented. Basing on theproposed model, the problem of optimising the container allocation on theterminal yard has been stated, also discussing possible solutionprocedures. Work is in progress now to extend the definition of theinformation system to overall container terminals and to investigatefurther decisional aspects involved in the management of such workingframeworks.

References

[1] Garey M.R. & Johnson D.S., Computer and Intractability - A guide to theTheory oj~NF -Completeness, W.H. Freeman, San Francisco, 1979.

[2] Mazzucchelli M., Recagno V., & Sciutto G., Evaluation of interportperformance: a state automaton approach, Proc. 6* Int. Vehicle Navigationand Information Systems, Seattle, WA, USA, 1995.

[3] Mazzucchelli M., Recagno V., & Sciutto G., Integrated freight terminalsimulation tool: design and implementation, Proc.Sth IFAC/IFIP/IFORSSymposium on Transportation Systems, Chania, Greece, 1997.

[4] Van Hee K.M., Huitink B., & Leegwater O.K., Portplan, decision supportsystem for port terminals, European Journal of Operational Research, 34,pp.249-261, 1988.

[5] Van Hee K.M. & Wijbrands RJ. Decision support system for containerterminal, European Journal of Operational Research, 34, pp.262-272, 1988.


computers in railways 353 - wit press · between two major logistics systems: sea and land trade....

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