Ths document s downloaded from DR-NTU, Nanyang Technologcal Unversty Lbrary, Sngapore. Ttle capacty analyss usng smulaton Author(s) Ctaton Huang, Shell Yng; Hsu, Wen Jng; He, Yuxong; Song, Tancheng; De Souza, Charles; Ye, Rong; Chen, Chuanyu; Nautyal, Stut Huang, S. Y., Hsu, W. J., He, Y., Song, T., Charles, D. S., & Ye, R. (2009). capacty analyss usng smulaton. The Internatonal Conference on Harbor, Martme & Multmodal Logstcs Modellng and Smulaton HMS (2009:Span), pp.1-6. Date 2009 URL http://hdl.handle.net/10220/6754 Rghts
ANCHORAGE CAPACITY ANALYSIS USING SIMULATION Shell Yng Huang (a), Wen Jng Hsu (a), Yuxong He (a), Tancheng Song (b), Charles De Souza (b), Rong Ye (c), Chuanyu Chen (c), Stut Nautyal (c) (a) School of Computer Engneerng, Nanyang Technologcal Unversty, Sngapore. (b) Martme and Port Authorty, Sngapore. (c) Smplus Pte Ltd. (a) {assyhuang,hsu}@ntu.edu.sg, (b) Tancheng.SONG@mpa.gov.sg, (c) {yerong,chuanyu,stut}@smplus.sg ABSTRACT Wth the substantal growth n marne traffc, anchorage space s now n hgh demand n certan hub ports. To provde decson support for port authortes, we have analyzed the usage of anchorages n recent years. The demands on the anchorages arse from the dynamcally changng vessel mx and smlarly complex servce patterns. The utlzaton and the capacty of an anchorage space also depend heavly on the dspatchng and allocaton rules as well as the shape and areas of the anchorage. The complexty of the system studed s therefore beyond the current analytcal tools, and hence smulaton provdes an effectve means for the study. In ths paper, we present a smulaton-based capacty analyss on anchorages. The smulaton model bult s able to match the current scenaros well, and the analyss by smulaton proves very useful n assessng anchorage utlzaton and capacty for future scenaros. There are few publshed results on the capacty studes of anchorage space. In Bugarc and Petrovc (2007), the anchorage as part of a rver termnal for bulk cargo unloadng s smulated where the anchorage s an n-vessel holdng area. The anchorage s abstracted as a Frst-In-Frst-Out queue wth a capacty of n. When the queue reaches ts capacty, ncomng vessels are turned away. A vessel wll leave the queue when a berth s free at the termnal. Fan and Cao (2000) presented a capacty model for an anchorage. The capacty of an anchorage s defned as the maxmum number of vessels that can be accommodated by the anchorage over a perod of tme. The anchorage capacty s a functon of the total area of the anchorage, the average percentages of each type of vessels comng to anchor, the average areas occuped by a vessel of each type and the average duratons of vessel stays of each type of vessels n the anchorage. Keywords: Decson support system, capacty, Crcle packng, Model development 1. INTRODUCTION The Strats of Sngapore play host to an average of about 140,000 vessels annually. These nclude contanershps, general cargo shps, ol tankers, chemcal tankers, ferres, cruse shps and many more. Vessels of dfferent lengths, tonnage and carryng dfferent types of cargoes enter Sngapore Strats and navgable sea space has to be allocated as anchorage space for vessels that requre moorng space for varous reasons. In recent years both vessel traffc volume and vessel sze have shown an upward trend and are expected to ncrease sgnfcantly over the next few decades. It means that the demand on anchorage space wll escalate as well. Smlar to Sngapore, n the Port of New York and New Jersey, the number of vessels has ncreased and ther szes have grown n the past few years (U.S. Envronmental Protecton Agency 2006). The anchorage space has frequently been flled to capacty and the Coast Guard proposes to revse the duraton vessels are authorzed to anchor n specfc anchorage areas. densty 80.7% (a) (b) densty 58.9% densty 78.5% (c) Fgure 1: Dfferent vessel mxes or dfferent anchorage shapes can result n very dfferent utlzaton results. In actual operatons, vessels of dfferent types and szes come and leave anchorages at all tmes and therefore the vessel mx n anchorages are changng dynamcally. It follows that anchorage utlzatons change all the tme. Dfferent vessel mxes wth regard
to szes are able to effectvely utlze the same anchorage space to dfferent degrees. Fgures 1(a) and 1(b) show two vessel mxes producng dfferent utlzaton results n the same anchorage. A vessel mx may be accommodated nto an anchorage wth no problem but the same vessel mx may not be able to ft nto another anchorage of the same area sze but of a dfferent shape. Fgures 1(b) and 1(c) show two anchorages wth the same area but dfferent shapes result n dfferent utlzaton fgures. Ths means statstcs of averages of varous measurements wll not present a clear pcture about peaks and lulls of the demand and anchorage utlzatons and therefore cannot be used as evdence of enough space meetng the demand. A smulaton-based tool wll be the most effectve way to assess quanttatvely anchorage utlzaton levels and to evaluate anchorage capacty. Such a tool wll be very useful for assessng whether the exstng anchorage space s adequate for future scenaros. The tool wll also be useful n evaluatng whether changes n the confguraton of the anchorages and/or changes n certan polces or practce are effectve to satsfy the demand on anchorage space wthout allocatng more physcal space. We defne that an anchorage has reached ts capacty f the probablty of havng n or more vessels not able to fnd anchorage space s greater than a lmt p. Both n and p wll be specfed by the managng authorty of the anchorage. A computer smulaton based plannng tool was developed. Usng ths tool, we are able to provde quanttatve nformaton about nstantaneous, average and maxmum anchorage utlzatons. We also analyze the probabltes of vessel overflows, that s, the probabltes of havng vessels turned away because there s nsuffcent space n the anchorages. The management of the anchorage space may decde that the demand on anchorage space has exceeded ts capacty when the probablty of havng at least a certan number of vessels turned away s more than a threshold value. The tool also allows us to conduct experments to evaluate the effectveness of dfferent anchorage confguratons when space allocated for certan types of vessels s found not enough. The tool allows the user to specfy varous vessel mxes, vessel arrval patterns, and current or future planned anchorage confguratons. The smulaton of the typcal practce was valdated by settng smulaton parameters wth sutable values and comparng smulaton results wth the correspondng statstcs based on a set of hstorcal data n Sngapore. The rest of the paper s organzed as follows. Secton 2 ntroduces a typcal anchorage system and practce n port ctes. It s followed by the descrpton of the archtecture of the smulaton tool for anchorage capacty study n Secton 3. Then Sectons 4, 5 and 6 present the vessel arrval generator, the vessel dspatcher and the anchorage manager respectvely. Secton 7 descrbes the evaluaton of the smulaton tool. Secton 8 presents a method to assess what the space utlzaton s lke when an overflow occurs at an anchorage. Secton 10 concludes our work. 2. A TYPICAL ANCHORAGE SYSTEM Generally, vessels come to a port for varous purposes, such as takng bunkers, gong to shpyards for repar, loadng and unloadng of cargo at a termnal and/or a combnaton of these purposes. Some vessels go to an anchorage before vstng ther termnal/shpyard and some do so after vstng the termnal/shpyard. Other vessels wll vst an anchorage wthout gong to any termnal/shpyard. Some vessels make multple vsts to anchorages for multple purposes wth or wthout vsts to termnals or shpyards. Bunker vessels vst other anchorages to provde bunker servces to other vessels and vst termnals to refll bunker supples n between ther stay n the home anchorages. In the whole port there may be a few areas that are desgnated for anchorng vessels and these areas may be further dvded nto a number of anchorages of dfferent shapes and szes. Each anchorage also has a maxmum depth that lmts the vessels that can anchor n t. Some of the anchorages are reserved for vessels n certan gross tonnage (GT) groups. These anchorages are categorzed to serve dfferent types of vessels wth dfferent purposes of vst. Typcally, shppng agents choose anchorages for ther vessels before they arrve n the port. They take nto consderaton a few factors lke the vessel s purpose of vst, the vessel s type, draft, gross tonnage, the locaton of the vessels entry/ext ponts nto/from the port and the locatons of the termnals/shpyards they vst. After a vessel arrves n the port, the plot or vessel captan chooses an anchorng poston n the anchorage when the vessel needs to go nto the anchorage. If the anchorage of the vessel s choce s full, the plot or the captan wll choose another anchorage whch s desgnated to provde space for the same type of vessels. There s no restrcton on the duraton a vessel s allowed to stay n an anchorage. A careful examnaton of the hstorcal vessel data of a port s carred out to extract patterns and statstcal dstrbutons of vessel calls, vessel vsts to anchorages, arrval tmes and dwell tmes n anchorages. 3. SYSTEM ARCHITECTURE Fgure 2 shows the archtecture of the smulaton system. The system conssts of a vessel arrval generator for generatng vessel arrvals to anchorages, a vessel dspatcher for smulatng ncomng vessels choce of anchorages, and for each anchorage there s an anchorage manager for emulatng plots decsons to choose anchorng postons for ncomng vessels n each ndvdual anchorage. At the front end, there s a graphcal user nterface (GUI) for the user to specfy the smulaton parameters and the anchorage specfcaton. There s also an output GUI for dsplayng the utlzaton status of ndvdual anchorages durng smulaton runs and the output statstcs after smulaton runs.
Through the GUI, the tool allows the user to nput the total number of vessel calls for the perod of the smulaton and the dstrbuton of calls among the dfferent vessel types. The user s also able to specfy dfferent anchorage confguratons. The dmensons, depth, and usage for each anchorage and the dstance between any two anchorages can also be defned. Exstng anchorages can be removed and new anchorages can be added. GUI #Vessel Arrvals Vessel Arrval Generator Vessel Arrvals GUI Manager 1 Usage Confguraton Vessel Dspatcher... Fgure 2: System Archtecture Output Statstcs nclude Manager n Output Statstcs Instantaneous utlzaton, ndcatng the occupancy of an anchorage at tme nstance t: Space u (1) t Area where s the ndex of vessels that anchor at the anchorage at that pont of tme; Space denotes the space taken by vessel and Area denotes the area of the anchorage. Average utlzaton, ndcatng the occupancy of an anchorage over the tme perod of nterest: Space Area Dwell u (2) a Duraton where s the ndex of vessels that anchor at the anchorage durng the perod; Space denotes the space taken by vessel and Dwell denotes ts correspondng dwell tme at the anchorage; Area denotes the area of the anchorage and Duraton denotes the smulaton tme perod of nterest, e.g. a day or a month. Number of overflows, ndcatng the occurrences when no sutable anchorage space can be allocated to a vessel. It s an ndcator of the extent the anchorage space s packed and n order to accommodate such extra vessels, certan anchorng rules have to be volated n actual operatons. 4. VESSEL ARRIVAL GENERATOR The demand on anchorages of a port s drven by the arrvals of vessels callng at the port. However, not all vessels callng the port need anchorage space. Meanwhle, some vessels call at anchorages more than once durng a vst. It s of practcal mportance to relate the number of vessel calls at a port to the number of vessel vsts to ts anchorages. The correlaton between the two can be analyzed from hstorcal data. Therefore the frst component n modelng vessel arrvals to anchorages conssts of a translaton mechansm that maps the number of vessel calls to the demand on anchorages, that s, the number of arrvals at anchorages. The second component s a n-statonary Posson Process that assgns an arrval tme to each generated vessel. In the analyss of hstorcal data of a port as an example, the relatonshp between the number of vessel calls and the number of vsts to anchorages s establshed by regresson analyses and we get, for each type of vessels: x b y a (3) where y s the number of vessel calls at anchorages and x s the number of vessel calls at the port of a partcular type of vessels. Wth the knowledge of ths correlaton, t s possble to predct the demand on anchorage space based on the predcted number of vessel calls to the port. Vessel attrbutes lke the vessel length, gross tonnage and draft are expected to affect the anchorage utlzaton so they also need to be generated for each generated vessel accordng to hstorcal dstrbuton of each vessel type. The dstrbutons of vessel lengths an be obtaned from analyss of hstorcal data. It s also found through data analyss that for each type of vessels, vessel gross tonnage and vessel draft are both related to the length of the vessel by Equaton (1) wth dfferent parameter values for a and b. Wth the generated vessel length, the vessel draft and gross tonnage can be generated for each vessel, based on the correlaton found between them n the hstorcal data.
Table 1 shows the percentage dfferences between the generated numbers of arrvals to anchorages for each type of vessels and the numbers from the port s hstorcal data n 2006. Ths ndcates the success of the translaton scheme that maps vessel calls to the demand on anchorages. Table 1: Comparng generated demand on anchorage wth hstorcal data Vessel type % dfference Vessel type % dfference TA 1.3 FR 2.0 VLCC -1.4 CO 6.4 CH/LPG/LNG -0.2 BA -0.7 CTNR 0.7 BK 0.6 BC -0.3 Dfferent dstrbutons may also be specfed by the user through the nput GUI f t s necessary. Arrval of vessels to a port s a complcated dynamc process that s dependent on shppng lnes plannng and s very senstve to economc actvty fluctuatons. Consderng the whole port as a complete servce faclty, the arrval process would be best modeled as a Posson process. Ths s based on the facts that: (1) even though the arrvals of ndvdual vessels are scheduled, when the vessel traffc to the whole port s consdered, the dstrbuton of the nter-arrval tmes becomes random and fts well the exponental dstrbuton; (2) the unpredctable weather condtons and possble delays n servce by other ports of call further randomze the arrvals. Statstcal tests on hstorcal data confrmed that the nter-arrvals for each vessel type agreed reasonably well wth the exponental dstrbuton so the Posson dstrbuton for the arrval process s sutable n our study. 5. VESSEL DISPATCHER As descrbed n Secton 2, shppng agents choose anchorages for ther vessels. Therefore the role of the vessel dspatcher s to smulate the shppng agent s choce of anchorage for ts ncomng vessel. Modelng vessels choce of anchorages s complcated, as a number of parameters have to be taken nto consderaton, for example, purpose of vessel call, vessel type, vessel gross tonnage and vessel draft. Moreover, each type of vessel may choose any of a few canddate anchorages. There are no fxed regulatons and rules for the selecton among these canddates so t may be based on agents preferences. Agents preferences are unknown to us, may be numerous and may not be general. It s unrealstc to summarze a comprehensve set of rules that emulate the preference of clents when they select among the canddate anchorages. There are also some unforeseen crcumstances n actual operatons that would affect the choce of anchorage. Therefore, nstead of applyng rules to emulate ther (agent, plot, control centre) anchorage choces, we propose to mne the hstorcal data of a port to fgure out how ts man anchorages were used as the end results of ther choces. From the mnng results, the probablty (weghtage) dstrbuton for each type of vessels n choosng varous anchorages can be obtaned and the process of choosng an anchorage based on these weghtages for a vessel s shown n Fgure 3. Vessel arrval Check purpose of call Get correspondng lst of canddate anchorages Flter canddates wth nadequate GT or depth Choose an anchorage based on weghtages Enough space? Remove the canddate from the lst Add backup anchorages to the lst of canddates Sort canddates by dstance to the frst choce Choose the nearest canddate anchorage Enough space? Remove the canddate from the lst Lst empty? : dverson occurs Flter canddates wth nadequate GT or depth Done Done Overflow Fgure 3: Process of choosng an anchorage for a vessel 6. ANCHORAGE MANAGER In most ports, plots and captans are free to choose the anchorng postons for ther vessels n an anchorage. Therefore the role of the anchorage manager n the smulaton tool s not to assgn anchorng postons for vessels but to smulate plots decsons n choosng an anchorng poston. We present an algorthm here to smulate how plots choose an anchorng poston n an anchorage. When a vessel anchors n an anchorage, the space t occupes s more than the wdth and length of the vessel. Due to wnd and current, a vessel may be at dfferent postons at dfferent tmes wthn the space of a crcle. A mnmum safety clearance also needs to be mantaned between any two anchorng ponts at all
tmes. So each vessel wll occupy a crcular space wth a radus of the length of the vessel plus half the mnmum safety clearance. In ths way, the dstance between the two anchorng ponts s at least the sum of the two vessels length plus the mnmum safety clearance. In a hub port where anchorage space s hghly contested, plots and captans usually anchor ther vessels close to at least one of the anchored vessels, so that the anchorng space s better utlzed than f a completely random poston s chosen. Four typcal anchorng scenaros are dscussed for elaboratng how anchorng postons are decded n the system based on the usual practce of the plots and captans. Fgure 4 shows these scenaros where a sold-boundary crcle represents a vessel that has already anchored, and a dash-boundary crcle represents the canddate choces for anchorng an ncomng vessel. The four scenaros are 7. EVALUATION To evaluate whether the system works correctly and accurately, we use the hstorcal data of a port as an example. We set the smulaton parameters to sutable values and confgure the anchorages accordngly. The yearly average utlzatons of ndvdual anchorages were used as the ndcators for comparng the outputs of the smulaton tool wth hstorcal statstcs. Fgure 6 summarzes the results. Search for vesselsde corners Search for snglevessel cuts Vessel Arrval Search for twovessel corners Search for twosde corners Type 1: 50% Type 2: 20% Type 3: 20% Type 4: 10% Type 1: vessel-sde corner formed by a border lne of the anchorage and an exstng vessel. Type 2: sngle-vessel cut s a poston next to one exstng vessel only. Type 3: two-vessel corner formed by two exstng vessels. Type 4: two-sde corner formed by two border lnes of the anchorage. Randomly pck a poston based on the slected scenaro Anchorng poston found? Other scenaros tred? Type 1 Type 2 Type 2 Done Fgure 5: Anchorng algorthm full Type 1 60% 50% 40% Ave. Utlzaton Type 4 30% 20% Type 3 10% 0% 1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 Index Fgure 4: Four typcal anchorng scenaros Type 4 In a survey conducted among 10 plots (experenced and not so experenced), t was found that about 50% of them preferred Type 1, 20% preferred Type 2, another 20% preferred Type 3, and the remanng 10% preferred Type 4. Therefore the dstrbuton was appled to the smulaton tool when modelng the plots choce for anchorng postons for these typcal scenaros. Ths s shown n Fgure 5. Average of 2002-2006 data System output Fgure 6: Comparng System Output and Hstorcal Statstcs Most of the utlzaton fgures from system output ft well wth the hstorcal averages. te that n Secton 4, we have confrmed that the generated numbers of arrvals to anchorages for each type of vessels match the numbers from the hstorcal data. It was therefore accepted that f the parameters of the smulaton tool s set correctly, t s able to emulate the operatons of an anchorage system well.
8. ASSESSING SPACE UTILIZATION NEAR CAPACITY The space taken by a vessel n an anchorage s represented as a crcle wth the vessel length plus a safety margn as the radus and to ensure safety, any two crcles are not allowed to overlap. Ths means that the maxmum utlzaton of an anchorage s lower than 100%. For example, n a scenaro llustrated by Fgure 7, the anchorage cannot accommodate any vessels longer than 120m, although ts utlzaton at that pont s 61.55%. However t s able to accommodate smaller vessels by fllng them n the gaps n between exstng vessels. Therefore, the achevable maxmum utlzaton of an anchorage s decded by a few parameters, namely anchorage sze, shape and vessel mx (of sze), whle vessel mx s the combned effect of anchorage dspatchng rules and vessel arrval patterns. As these parameters vary n dfferent anchorages, there s no sngle maxmum utlzaton fgure applcable to all anchorages. Assessng space utlzatons near capacty for anchorages of dfferent shapes and szes, and recevng dfferent vessel mx, s of great mportance n evaluatng anchorng space usage under future traffc scenaros. Table 2: Space utlzaton when overflow occurs Utlzaton Percentage Utlzaton Percentage 5% 0.00% 55% 20.24% 10% 0.00% 60% 48.57% 15% 0.00% 65% 24.26% 20% 0.00% 70% 0.80% 25% 0.00% 75% 0.00% 30% 0.00% 80% 0.00% 35% 0.02% 85% 0.00% 40% 0.14% 90% 0.00% 45% 0.99% 95% 0.00% 50% 4.97% 100% 0.00% Assessng space utlzaton near capacty manly nvolves two tasks, namely, generatng hgh but realstc vessel traffc to ndvdual anchorages so as to create suffcent and representatve overflow nstances, and recordng the nstantaneous utlzaton at the ponts of overflow. One smple way to generate such traffc s by dsablng all but one anchorage so that all assocated vessels wll be drected to that partcular anchorage. As an example, Table 2 summarzes the overflow nstances at one of the anchorages. Columns 1 and 3 n the table show the space utlzaton fgures when an overflow occurs at the anchorage. Columns 2 and 4 show the percentages of overflows that occur wth the correspondng utlzaton fgures. As shown n Table 2, for that partcular anchorage and the vessel traffc, overflow could occur at the anchorage when utlzaton s as low as 35%, although most of the tme t occurs only when utlzaton s at a hgher level of around 60%. Ths confrms that even at the same anchorage, the maxmum possble utlzaton vares wth the sze of ncomng vessel and the vessel mx at that tme. On average, overflow can happen when utlzaton reaches 57.10%. It should be noted that for a dfferent anchorage and dfferent vessel mx, overflows may occur at a dfferent utlzaton level. From our experments for 17 anchorages, we found that ths utlzaton fgure vares from 35.1% to 61.1% 9. CONCLUDING REMARKS We have developed a reconfgurable tool for assessng the capacty of anchorages. It allows the user to nput varous vessel mxes and volumes n vessel arrvals for current or future scenaros. It also allows the user to specfy current or planned anchorage confguratons, e.g. to add, remove or change the szes and shapes of the anchorages. The user can also change the types of vessels that use an anchorage. Our system proves to be a useful decson support tool for assessng the mpacts of dfferent vessel demands, anchorage confguratons, anchorng practces and polces. We used ths tool to assess the anchorage utlzaton fgures when vessel overflow occurs and we can also use the tool to compute the probablty that a certan number of vessels cannot be accommodated n an anchorage. For future work, we plan to desgn and evaluate algorthms for mprovng anchorage utlzatons. ACKNOWLEDGMENTS We thank the Martme and Port Authorty of Sngapore for supportng the project. REFERENCES Bugarc, U. and Petrovc, D., 2007. Increasng the capacty of termnal for bulk cargo unloadng. Smulaton Modellng Practce and Theory, 15 (10), 1366-1381. Fan, H. S. L. and Cao, J., 2000. Sea space capacty and operaton strategy analyss system. Transportaton Plannng and Technology, 24 (1), 49 63. U.S. Envronmental Protecton Agency, 2006. Regulatons: Port of New York. DOI= http://www.epa.gov/fedrgstr/epa- IMPACT/2006/vember/Day-16/19314.htm AUTHORS BIOGRAPHY Shell Yng Huang and Wen Jng Hsu are currently Assocate Professors n SCE, NTU. Yuxong He was a PhD canddate n SCE, NTU. They have done many projects n martme R & D funded by Port Authorty, termnal operators and a shppng company. Tancheng Song s Assstant Drector n Technology Dvson of MPA. Charles De Souza s controller n Plotage Exam and Marne Projects of MPA, Sngapore. Rong Ye, Chuanyu Chen and Stut Nautyal are postgraduates and graduate from NTU and dd projects n martme R & D n NTU. They are now the cofounders and drectors of SmPlus Pte Ltd.