Scientific Journals
Zeszyty Naukowe
Maritime University of Szczecin
Akademia Morska w Szczecinie
2012, 29(101) pp. 182–187 2012, 29(101) s. 182–187
Navigational aspects of ship’s voyage planning taking into
account calculations of ETA (Estimated Time of Arrival)
Nawigacyjne aspekty planowania podróży statku
uwzględniające obliczenia ETA (Estimated Time of Arrival)
Bernard Wiśniewski
1, Tadeusz Korwin-Piotrowski
2, Mirosław Wielgosz
1Szymon Siódmak
11 Maritime University of Szczecin, Institute of Marine Navigation
Akademia Morska w Szczecinie, Instytut Nawigacji Morskiej
70-500 Szczecin, ul. Wały Chrobrego 1–2, e-mail: b.wisniewski@am.szczecin.pl
2 Polish Steamship Company
Polska Żegluga Morska 70-419 Szczecin, Plac Rodła 9
Key words: calculation of ETA, comparison of ships’ routes
Abstract
Since 2009 the Polish Steamship Company has operating on its ships SPOS, which main function is receiving weather information, presentations of this information in the form of synoptic and wave maps with forecasts up to 216 hours. The second function of the system is ability to determine the Great Circle, rhumb line and navigation composite routes and the so-called optimal and recommended by the system route (Optimum High and Wide).
Because for each of these routes it is possible to generate in advance ship’s position for one, two, three days and so on until the destination (usually the roadstead of the destination port), it is likely to compare the various routes. SPOS compares distances (NM), duration of voyages (days and hours), expected average speed, as well as date and time ship reaches the destination (ETA – Estimated Time of Arrival). The reliability of these data is dependent on the ship’s characteristics as a control object or ability to obtain the calm water speed (Vo) at full sea speed propeller revolutions, and reduction of vessel speed due to waves and
wind. The third element affecting accuracy of a voyage plan in terms of initial estimations is the reliability of weather forecasts (waves, wind, currents, ice restrictions, fog, etc.).
Słowa kluczowe: obliczenia ETA, porównanie tras statków Abstrakt
Od 2009 roku Polska Żegluga Morska eksploatuje na swych statkach system SPOS (Ship Performance Opti-misation System), którego główną funkcją jest pozyskiwanie informacji pogodowych, prezentacje tych in-formacji w postaci map synoptycznych i falowania z prognozami do 216 godzin. Drugą funkcją systemu jest możliwość wyznaczania ortodromy, loksodromy, tras żeglugi mieszanej (Composite) oraz tzw. trasy opty-malnej i zalecanej przez system trasy (Optimum High and Wide).
Na każdej z tych tras można wygenerować z systemu przewidywane pozycje statku za dobę, dwie doby, trzy, itd. aż do uzyskania punktu docelowego (zwykle jest to reda portu docelowego), możliwe jest także porównanie poszczególnych tras. Porównuje się drogi (Mm), czas realizacji podróży (dni i godziny), a tym samym przewidywaną średnią prędkość oraz datę i godzinę osiągnięcia punktu docelowego (ETA – Estima-ted Time of Arrival). Wiarygodność tych danych jest zależna od cech statku jako obiektu sterowania, czyli zdolności uzyskiwania prędkości na spokojnej wodzie (Vo) przy eksploatacyjnych obrotach śruby oraz strat
prędkości statku na fali i wietrze. Trzecim elementem wpływającym na dokładność w realizacji podróży względem początkowych przewidywań i planu podróży jest sprawdzalność warunków pogodowych (falowa-nie, wiatr, prądy, ograniczenia przez zjawiska lodowe, mgły, itp.).
ETA’s calculation accuracy in SPOS
In order to calculate more accurate vessel’s arri-val time at the destination port – ETA, what is re-quired from the ships’ masters by ship operators and charterers, using SPOS it is necessary to speci-fy ship’s speed in calm water (Vo) and ship’s speed
curve. In addition, ETA calculation accuracy de-pends on the verifiability of weather forecasts and it is determined by comparing the weather forecasts with actual conditions on a particular date, time and vessel’s position. Designed vessel’s speed in calm water is verified after launching during speed and manoeuverability tests and refines during its opera-tion (such as hull fouling). The fleet of the Polish Steamship Company’s bulk carriers consists of ships maintaining speed on calm water (Vo) in the
limits of 13.0–14.2 knots when ship is fully loaded, and from 13.5 to 14.5 knots in ballast condition. The corresponding value is to be input into SPOS for the specific ship’s type and voyage.
The ship’s speed reduction due to waves and wind is a complex problem that requires a suffi-ciently well-defined speed curve to be able to accu-rately predict the ETA. According to [1] for the purpose of optimizing the ship’s sea route, speed curve should be presented in a form as simple as possible and general.
Table 1. Default speed curves in SPOS (on the basis of [2])
Tabela 1. Domyślne charakterystyki prędkościowe w progra-mie SPOS (na podstawie [2])
Wind Wind direction from bow
0 045 090 135 180 Wi nd sp ee d [k no ts] 0 100% 100% 100% 100% 100% 10 100% 100% 100% 100% 100% 20 98% 99% 100% 100% 101% 30 95% 97% 98% 100% 101% 40 85% 90% 95% 100% 102% 50 80% 85% 90% 95% 102%
Sea Wave direction from bow
0 045 090 135 180 Wav e he ig ht [m ] 0 100% 100% 100% 100% 100% 2 98% 98% 100% 100% 100% 4 90% 90% 95% 100% 100% 6 85% 85% 90% 90% 95% 8 65% 70% 80% 85% 90% 10 0% 0% 0% 0% 0%
Default SPOS’s speed curve (Table 1) which is input as a matrix of data presenting the participa-tion of wind speed and direcparticipa-tion from bow, as well as wave height and direction from bow resulting in reduction of ship’s speed. On the basis of series of
analyzes carried out on different voyages made by Polish Steamship Company’s ocean-going vessels, it proved not to be very accurate.
It does not cover the typical operating parame-ters of dry bulk carriers and does not take into ac-count the loading or ballast condition of the ship, as it is commonly known, that ship achieves different speeds depending on whether in loaded or ballast condition.
Such a default speed curve inherent in SPOS does not allow to predict the exact time of arrival at ship’s destination, so that the vessel’s arrival time calculated on the basis of these characteristics significantly differents from the real-time.
For that reason, authors started to analyze the ship’s voyages using SPOS to determine more pre-cise speed curves that may give similar prediction results to the real ETA.
Research data
The collected data contains a collection of real data from fifty-five voyages of ocean going Polish Steamship Company’s bulk carriers both in loaded and ballast condition. Collected data includes in-formation such as date and time of departure at the end of sea passage (ESP), ship departure and arrival draft, quantity and kind of cargo, calm sea speed (Vo), the daily fuel consumption for contracted
speed – main engine revolutions, and the basic de-sign parameters of the ship.
Besides, basic information about the current ship’s voyage is implemented, the research material also includes precise navigation data such as: vessel positions, average course over ground (COG – Course Over Ground), main engine revolutions, average speed over ground (SOG – Speed Over Ground), daily fuel consumption, and the weather condition during the voyage containing the force and direction of wind, sea state and wave direction, and, if available, other weather observations, such as fog (reduced visibility), icing, etc.
With such data it was possible to perform re-lease test of ships’ voyages (composite route) and comparing their results with the real data. In this way, one could evaluate the accuracy of prediction the ship’s ETA by SPOS using authors’ own speed curves.
SPOS basis on knowledge of the geographical coordinates from the actual ship’s voyage, its calm sea speed (Vo), current speed curve and daily fuel
consumption performes calculations on the so-defined variables.
Sample results are contained in these release test (Table 2). Then, authors compared the time differences in predicted ETA by the program to the
actual vessel’s arrival time at each voyage’s stage, as well as total fuel consumption.
The testing stages were chosen to include the route section not longer than 9 days of voyage (216 h) due to the availability of weather forecasts in the system.
When the trip consisted of more than 9 days, SPOS had calculated ETA for 216 h on the basis of the available weather forecasts and then the rest of the way of the vessel’s averaged climatic data basis. Additionally, other possible types of ship’s routes were being compared with composite (real) route, assessing, on the basis of ETA and weather conditions and the route choice’s rightness. All of analyzed routes were described and their results received based on test with SPOS compared with real data from individual voyage’s stages (Table 2).
The subject of this paper is to present the results of ocean-going bulk carriers voyages test release and evaluation of above mentioned results in terms of planning ETA’s accuracy.
Research results
There were at least three parameters affecting accuracy of presented results, two of them
depend-ed on user working with the system. The first of them was charter speed (value of speed defined in charter party), or so called calm sea speed (Vo)
which is usually imposed by ship owner’s operating or chartering department, but also verified by mas-ter on the basis of his experience and knowledge about possible achieved speed.
The second factor was the ship’s speed curve developed and modified by authors on the basis of real voyages’ test data and received results.
Figure 1 presents an example of implemented speed curve used during test for loaded bulk carrier. The third factor affecting the accuracy of re-ceived final results was the weather forecast, which was prepared, developed and sent to ship in digital file form. On the basis of defined speed curves, calm sea speed (Vo) and corresponding weather
forecasts authors carried out tests of bulk carriers real voyages.
For the purpose of this paper two specific voy-ages were selected. Findings present the results of implementing the real ship’s route, track calculated by actual positions (which is called “composite” in SPOS) and other suggested and analyzed tracks, using for this purpose SPOS utilities and weather forecasts.
Table 2. Analyzed routes and their results received based on test with SPOS compared with real data from individual voyage’s stages
Tabela 2. Analizowane trasy i otrzymane rezultaty w oparciu o testy w programie SPOS i ich porównanie z poszczególnymi etapami podróży
No. Ship’s name condition Loading
Voyage’s number and ports
Speed curve
Real data (dd-mm-yy
hh:mm UTC) Vo [knots] Vmean [knots]
SSP (start sea passage)
ESP (end of sea passage) Stage of voyage Time [h] Distance [Mm] Consumed fuel [t] 1 2 3 4 5 6 7 8 9 10 11 1 DELIA Ballast 122 BWB-03 09-03-2010 16:45 23-03-2010 22:30 I 342 14.5 13.0 Durban II 282 4433.5 449.21 Rosario III 178 2 DIANA Loaded 118 V2L 27-02-2009 11:00 19-03-2009 00:40 I 470 13.5 12.9 Valparaiso II 334 6081.3 607.1 Durban III 275
No. Kind of route
ETA testing result[h] Time difference [%] Total consumed
fuel [t] I stage II stage [∆t] II stage III stage [∆t] III stage I stage II stage III stage
12 13 14 15 16 17 18 19 20 21 1 Great Circle 337 60 271 164 165 –3.22 –6.03 –8.99 414.7 Rhumb Line 331 60 265 164 162 406.5 Composite 331 60 265 164 162 406.8
Optimum High & Wide 326 60 261 164 161 400.7
2 Great Circle 441 136 314 195 264 –2.77 –1.80 –1.45 569.3 Rhumb Line 445 136 328 195 270 588.2 Composite 457 136 328 195 271 590.6
Fig. 1. Percentage speed reduction due to wind and sea for loaded bulk carrier (on the basis of [3])
Rys. 1. Procentowy udział strat prędkości statku maso-wego z ładunkiem od wiatru i fali (na podstawie [3])
The shortest of all the analyzed voyages lasted 202 hours when the longest 729 hours. Best results were obtained for the shortest routes due to the fact that the analysis of voyage time were based on data from current 9-days weather forecasts and analyses derived from SPOS (A + 216 hours) without using mean climatic data.
The m/v “Delia” voyage from Durban / Repub-lic of South Africa (start sea passage – SSP on 09.03.2010 / 1645 UTC) to Rosario / Argentina (end of sea passage – ESP at La Plata roads on 23.03.2010 / 2330 UTC) in ballast condition lasted 342 hours (Table 2, no. 1).
The voyage’s first stage was calculated at the current weather forecast up to 216 hours available in the system, when the remaining part of the route – 126 hours was calculated by the program for average climatic data. Voyage time calculated by SPOS had to take 331 hours, when it really took 342 hours as reported from ship. This shows time difference of “minus” 3.22%, what should be read, that ship reached its destination 11 hours later than
predicted by SPOS. This discrepancy may be due to the fact that part of the ship’s voyage was calculat-ed for the average weather data only (seasonal climatic data).
At the second comparing stage the routes started after 60 h from the beginning of sea passage. The theoretical calculated time of passage to the desti-nation port would take 265 hours, when in reality the ship completed passage after 282 hours. At this calculation’s stage the difference in ETA was prob-ably also influenced by seasonal average weather data used in SPOS.
At the third stage, after 164 hours from depar-ture time, the result obtained in SPOS was 162 h until the end of voyage, but in fact it lasted 178 h. It shows the difference of “minus” 8.99 % of time. This testing’s stage confirms the inaccuracy of SPOS forecasts, because according to the ship log book the ship on 22.03.2010 encountered stormy weather with waves 4–6 meters high and wind 8– 10°B, while SPOS forecasts indicated wind speed less than 30 knots and wave heights up to 3.5 m.
Figure 2 and 3 present predicted ship’s positions on different analyzed routes and weather impact on 22.03.2010 on the basis of weather forecasts dated 16.03.2010 and 22.03.2010, which were found in-consistent with the actual observations from the ship. Therefore, in this trip one can see greater dif-ferences in ship’s arrival time to the destination port calculated by SPOS compared to the actual data.
Authors conclude that these errors in predicting vessel’s arrival time by SPOS were affected by the fact that the calculations in the first two stages were partly carried out on the basis of average seasonal weather conditions, and in the last stage the ETA’s determination accuracy was influenced mainly by not very relevant SPOS weather forecasts.
Weighted mean voyage time difference in 3 stages was found “minus” 5.49%, what means that ship arrived 5.49% of voyage time later than pre-dicted.
The second example (Table 2, no. 2) concerns the voyage m/v “Diana” (“Delia’s” sister ship) in loaded condition. The analysis of voyage which lasted 470 hours, in fact, has been showing at every analyzed stage a smaller percentage difference in voyage time. The range of time differences is kept within the limits of “minus” 2.77% to “minus” 1.45%. For this passage SPOS weather forecasts used for the calculations were close to the actual weather conditions.
Statistical summary of all analyzed voyages made by ocean-going Polish Steamship Company’s bulk carriers is contained in table 3. It should be 30 40 50 60 70 80 90 100 110 0 10 20 30 40 50 W in d dire ctio n fro m b ow [ º]
Wind speed [knots] 0º 45º 90º 135º 180º 30 40 50 60 70 80 90 100 0 2 4 6 8 10 W av e direc ti on fro m b ow [ º]
Wave height [meters] 0º
45º 90º 135º 180º
noted that despite the increase in the mean value in % for the subsequent stages of voyage planning accuracy of ETA can be increased at each stage of the trip, due to the distance shortening to the desti-nation port. The least accurate results comparing to the actual data were obtained in the third stage of the voyage testing.
It can be assumed that mainly inaccurate weath-er forecast in relation to the actual weathweath-er had significantly influenced these results. Decisive
in-fluence on the final voyage result has the choice of route before going out to sea. Although, the analy-sis of both above mentioned voyages clearly shows, that every single voyage, at every stage has still to be reviewed to optimize the route in weather condi-tions aspects, because the daily analysis of changes in hydro-meteorological conditions is crucial for the success of the whole route planning and moni-toring, and affects the economic performance of the company [4].
Fig. 2. Predicted ship’s positions on different analyzed routes and weather impact on 22.03.2010/1200 UTC on the basis of weather forecast on 16.03.2010/1200 UTC (on the basis of [2])
Rys. 2. Prognozowane pozycje statku na różnych trasach i wpływ warunków hydrometeorologicznych dnia 22.03. 2010 r. o godz. 12:00 UTC na podstawie prognozy pogody z dnia 16.03.2010 r. z godz. 12:00 UTC (na podstawie [2])
Fig. 3. Predicted ship’s position and weather impact on 22.03.2010/900 UTC on the basis of weather forecast on 22.03.2010/1200 UTC (on the basis of [2])
Rys. 3. Prognozowana pozycja statku i wpływ warunków hydrometeorologicznych dnia 22.03.2010 r. o godz. 19:00 UTC na pod-stawie prognozy pogody z dnia 22.03.2010 r. z godz. 12:00 UTC (na podpod-stawie [2])
Table 2. Mean, maximum and minimum values, standard devi-ation at specific voyage’s stages (on the basis of [3])
Tabela 2. Średnia, odchylenie standardowe, wartość max i min na poszczególnych etapach podróży (na podstawie [3])
Mean [%] I stage II stage III stage
3.25 3.58 4.07
SD [%] 2.35 2.31 4.68
MAX [%] 7.33 6.21 5.24
MIN [%] –9.74 –11.18 –8.99
For the first testing stage (at the start of sea passage) the predicting accuracy of ETA obtained using SPOS was within 3% of passage time. For the shortest analyzed route, authors achieved accuracy of 6 hours, which should be considered good, espe-cially for ocean passages. With increasing distances between the ports, the value of 3% seems to be not very satisfactory, but it must be taken into account that this accuracy is particularly affected by weath-er forecast, which now cannot be exactly predicted over ten days in advance. It should be also noted that speed curves used by authors in SPOS require further research and are still to be improved, what will undoubtedly appear in more accurate results of ETA calculations.
Conclusions
In the research process three different speed curves were exploited, which based on results of
analyses that will still be elaborated. Differences between real weather conditions and forecasted conditions can considerably influence the accuracy of predicting ship’s ETA, which are the reasons for those results.
For tested shorter routes more similar results to real data were achieved than for longer routes.
Using SPOS with its tools and own speed curves, one can determine more accurately the ves-sel’s real speed and ETA at the destination port, than on the basis of average seasonal conditions and the expected vessel’s average speed used by the master.
References
1. WIŚNIEWSKI B.: Problemy wyboru drogi morskiej statku.
Wydawnictwo Morskie, Gdańsk 1981.
2. Program SPOS Fleet Management, version: 7.0.0.1. Meteo Consult BV, The Netherlands, 2009.
3. Maritime University of Szczecin research data, the analyz-es of real ships’ (equipped with SPOS) voyaganalyz-es and their evaluation. Report from scientifical research „Zintegrowa-ny system programowania tras statków na oceanach w aspekcie bezpieczeństwa statku, ładunku i ludzi”, Grant No. N509495437/170/INM/2010, Szczecin 2012.
4. WIŚNIEWSKI B., SIÓDMAK S.: Wykorzystanie programu komputerowego SPOS w pogodowym planowaniu podróży statku. 14th International Scientific and Technical
Confer-ence on Marine Traffic Engineering 12–14.10.2011, Świnoujście.
