Abstract. 1 Introduction

Transcription

1 Ship's movement prediction in the Maritime Traffic Image Advanced System (MATIAS) T. Degre, C. Guedes Scares Institut National de Recherche sur les Transports et leur Securite (INRETS),F Arcueil Cedex, France Institute Superior Tecnico (1ST), Av. Rovisco Pais, 1096 Lisboa, Portugal Abstract The object of this paper is to describe the feasibility study of MATIAS system in which are designed and implemented processing methods to construct on-line the Traffic Image of any regional sea areas - and in particular to predict ships Estimated Time of Arrival (ETAs) - in order to improve the management of maritime transport, especially the logistic aspects. Design and development aspects of the MATIAS system, and the assessment of the quality of MATIAS predictions are described. 1 Introduction The origin of this study is related to two of the conclusions of the last International Symposium on Vessel Traffic Services held in Rotterdam in April 1996 which are: - «The tendency to monitor vessel traffic, even in areas where Vessel Traffic Services (VTS) have not been established, will increase». - «The need for an interactive exchange of information between VTS-centres, both for vessel traffic management and logistic purposes,

2 208 Maritime Engineering and Ports is growing. The development of a so-called Traffic Image in areas where VTS has not been established is a cost-effective alternative for the monitoring of traffic and for a quick and directed response in case of incidents and accidents.» The development of the MATIAS system builds upon the results of the project «Regional Traffic Information System (RTIS)», which was financed by the European Commission under the EURET programme. In this project the concept of Traffic Image in a regional sea area was defined as a dynamic Database composed of three layers of information related to: i) the traffic state, ii) the environmental conditions (wind, waves, currents, etc.) and iii) the state of the resources needed by the ships." It should be noted that the area of interest is a large regional area, like the Mediterranean Basin or the Baltic sea, covered only parsimoniously and locally by VTS centres. The first layer of this Database is of most concern here since its construction implies some complex processing, contrary to the two other layers for which just some "read and write" operations are necessary. More precisely, the availability of this first layer of the Traffic Image must allow at any time to estimate with a good accuracy the ships' identities, characteristics, nature of cargoes, positions, courses and speeds, and to predict also with a good quality their future positions and their ETAs, which is by definition their sailing plans. Obviously, the availability, at any time, of such accurate information on the traffic state is essential for all the actors involved in the management of maritime transport. i) as far as productivity and logistic aspects are concerned, it is well known that accurate ETAs provided at an early stage are crucial information for port authorities and allied organisations (e.g pilotage organisations, tugs companies, stevedores, etc.) in order for them to optimize the management of their own resources and the services they render to the ships. Also external organisations need that information in order to manage efficiently the intervention of other transportation modes. It is also well known that the provision by the shipmasters or by their agents of these ETAs to the ports concerned are sometimes inaccurate or provided too late. For example, in the port of Le Havre, a study made at the end of 1996 showed that less than 15% of the containerships provide spontaneously their ETAs between 18 and 30 hours beforehand, as required. ii) as concerns the safety aspect, it can be also improved by the prediction of ship's positions, the knowledge of which enables the enhancement of the safety of marine traffic in a preventive sense by the recognition and assessment of dangerous traffic situations and to help SAR organisations with remedial measures in case of an accident. Within this frame, the MATIAS project has the global objective to design, implement, and assess processing methods to construct the

3 Maritime Engineering and Ports 209 Transactions on the Built Environment vol 36, 1998 WIT Press, ISSN Traffic Image of any regional sea area in order to improve the management of maritime transport, especially, the logistics aspects. The area of concern is a large one which may only be covered locally by VTS centres. It is a feasibility study of the future system, and due to budget constraints, it has been limited to the feasibility study of the methods related to the construction of the first layer of the Traffic Image only and aiming at answering a shore-based organisation (e.g a VTMIS) the following questions: - who are the ships sailing in the area of interest and what are their estimated positions at any time? _ where will be these ships in the future for any given predicted time scale? - what are their ETAs at any specified locations and in particular when will a ship leave a certain VTS area and when will it arrive at the following VTS area? What are their ETAs at destination, which, as mentionned previously, are crucial information in particular for port authorities, pilotage organisations, tugs companies, stevedores, in order they optimize the management of their own resources and the services they render to the ships. 2 MATIAS architecture As shown on figure 1, MATIAS is composed of three modules: the Man Machine Interface (MMI) module, the Acquisition module and the Prediction module. _ The Man Machine Interface module (M.M.I module), consists of the dialogue and display sub-modules. The dialogue sub-module offers the operator the functions enabling him to interact (by means of the keyboard, mouse, etc.) with the rest of MATIAS system (e.g acquisition by the operator of ship reports and provision of these information to the Acquisition module; pointing out a given ship and request for the ship's characteristics, voyage information, movement information such as route followed, position at current time either estimated or actual, predicted position at a given time, planned route, ETA, etc.). The display sub-module visualizes, in particular, the traffic scene in the considered region, either estimated, real or predicted. _ The Acquisition module acquires data in real time from two main sources: the MMI module, as described above, and distant Databases like the Lloyd's Database (in order to complete as far as posssible the ship's characteristics), the Meteorological Services Databases which provide meteo forecast at regular time intervals and possibly other distant Databases in order to acquire technical ship's characterises (e.g propeller diameter, etc.). This module processes these data, if

4 210 Maritime Engineering and Ports necessary, in order to make their format or units compatible with the needs of the other modules, records them in the internal Traffic Image Database of MATIAS and lastly provides these data to the Prediction Module. - The Prediction module, which is the heart of MATIAS, is composed of two tool boxes: the prediction tool box and the correction tool box. The inputs to this module are the information provided by the Acquisition module. The outputs provided by this module are at any time, the estimated ships' positions and, on request by the operator for a given ship, her predicted position for a certain time scale of prediction, her ETA or Estimated Time of Passage (ETP) at a specified location. The role of the Correction tool box is to provide to the prediction tool box some correction parameters, if the deviation between a reported position and an estimated one is significant. Depending on the nature of the information provided by the Acquisition module and by the Correction tool box, various possible processing methods, which will be seen in more detail in the following, may be activated by this module in order to estimate or predict the just mentionned output variables. These various methods are contained in the prediction tool box. The global working of MATIAS and the information flow between the three modules which composes it is the following: 1 - Considering MATIAS system at current time t, if no data has been acquired during a time interval [t-dt,t[, where dt is an internal elementary time increment (the order of magnitude being, say, 6 or 10 minutes), the prediction tool box updates the estimated position p(t,t) of all ships present in the system. The estimated ship's positions p(t,t) are sent to the MMI module for display on the chart at t and the new time value t+dt is stored. 2- Suppose now that at t new data has been acquired by the Acquisition module via the MMI module or distant Databases on a given ship (new ship or ship already present in the system) or on weather/sea-state forecast. These data are converted, if necessary, and sent to the Prediction module. Denote as D(t) these data acquired at t by the Prediction module. - if D(t) contains a reported plotted position rp (only for a ship already present in the system), it is sent to the correction tool box which analyses the deviation between it and the corresponding estimated position and provides the prediction methods tool box, if the deviation is significant, with correction parameters CP(t).

5 Maritime Engineering and Ports 211 Transactions on the Built Environment vol 36, 1998 WIT Press, ISSN ftp Data at time t i M. M. i. MODULE - Lloyd N,t+,..., Reoorts DIALOGUE Sub-Module DISPLAY Sub-Module PREDICTION METHODS Tool-Box ** " " P(t, H), ETPs/ETA ) L P(t, t) Figure 1: Global structure of MATIAS In rets/ T.DegrM Dec.97 Based on CP(t), if any, and on D(t), the prediction tool box selects and activates a specific prediction method among the various possible ones, in order to update the basic variables (planned route and speed) on which the calculation of her estimated and predicted positions p(t,t) and p(t, t+) and of her ETPs/ETA are based. Finally, the estimated positions p(t,t) of this reporting ship and of all the other ships present in the system are updated by the prediction tool box.

6 212 Maritime Engineering and Ports - if D(t) doesn't contain a reported position (a ship delivering other information, or information on weather/sea-state forecast affecting all present ships), then the prediction methods tool box updates the estimated position p(t,t) of all ship present in the system. In any case, the estimated ship's positions p(t,t) are sent to the MMI module for display on the chart at t, and the time value t+dt is stored. 3- Independently of the 2 steps described above, the operator may select at any time t any ship present in the system. He can ask for any time in the future t+ her predicted position p(t,t+), or her ETPs/ETA. 3 Basic Principles of the Prediction module The basic principles of the Prediction Module are to recreate by calculation a complete ship's sailing plan, when this sailing plan has not been reported by the shipmaster, was incomplete or has not been followed by him, and to update it as soon as new information (on the ship, on weather forecast) is acquired. From the definition of a sailing plan, this recreation is a two-steps process: i) creation of a planned route, i.e of the succession of Way Points (WPs) from the actual ship's position to her destination, ii) estimation of the ship's ground speed on this route as a function of the ship's characteristics and the weather/sea-state conditions. From these basic variables, ETPs at the next WPs (as well as at other particular points of the route), ETA at destination and estimated/ predicted ship's positions can be easily derived by a simple extrapolation technique. Three fundamental cases of availability and quality of a reported ship's sailing plan lead to different actions: a) when a complete sailing plan has been reported, i.e a planned route and an ETA provided, and as long as the ship doesn't deviate from this sailing plan, then an extrapolation technique is directly used to derive the estimated and predicted ship's positions and her ETPs. b) when an incomplete sailing plan has been reported with a planned route but without the provision of an ETA, or when a complete one has been reported but for which the correction tool-box has detected significant deviation in time (or speed) on the followed planned route, then speed estimation method is used in order to estimate ship's speed. Two types of speed estimation method are possible: - an elementary one by using speed estimators issued from external or internal databases. In particular, the estimator of ship's speed could be her service speed or derived from past ship's speed recorded in MATIAS. These basic estimators could be possibly weighted by some

7 Maritime Engineering and Ports 213 weighting factor as a function of foreseeable weather conditions, geographical constraints, etc. on the allocated planned route. - the use, if enough characteristics are acquired on the ship, of a ship hydrodynamic model. These two types of speed estimation method will be described in more detail in the next sections. c) when no sailing plan at all has been reported or when an incomplete or a complete one has been reported but for which the Correction tool-box has detected significant deviation with regard to the planned route and if no safe planned route has been found in a Route Database, then an Optimization method is applied based on classical developments for optimum on-board ship weather routeing decisions. 4 Estimation based on mean speed In this experiment, a container ship, the Nedlloyd Korrigan (Lloyd's n ) from the French shipowner company COM, has been asked to report her sailing plan before departing and to report her positions during her movement. During her voyage (Damietta - La Spezia), the ship has been made 25 position reports. In order to have some interesting but not too complex situations, the conditions of sailing were the following: i) the ship's initial planned route was followed, ii) the ship's ETA (which was unknown) was different fom the Actual Time of Arrival (under-estimation of about 3%) and was not updated during the trip, iii) the ship didn't sail at a regular speed all along her voyage. If the ship doesn't deviate from her reported planned route, it is sufficient, in order to estimate the ship's ETA (or her voyage duration), to estimate the ship's overall mean speed from the origin to the destination of her voyage. The speed estimator is set, for each report i, to the mean speed Vi^ processed up to report i, included. Mathematically, as Vm, converges with time towards the overall mean speed, if this convergence is fast, it will mean that the ETA could be estimated rather soon with a good accuracy. The results show that as soon as i= 4, i.e after a sailed distance equal to 289 nm which is about 20% of the total sailed distance, the estimation of the ship's ETA is around 2% accuracy. The constraint with this estimator is that it needs at least one reported ship's position or observation, which is not always the case. 5 Estimation based on a hydrodynamic model

8 214 Maritime Engineering and Ports The prediction of the voyage duration is a complicated problem due to the effect of weather on the speed made by the ships. The added resistance of ships in waves reduces their net speed in moderate and bad weather, and the same can be said of the effect of currents and wind. The effect of the weather also depends on the ship type and on the captain's navigation strategy. Some ships may be navigating with a substantial reserve of power and they may have the strategy to keep the planned arrived times in which case they will use their additional power to compensate for the reduced speed due to weather effects. However others may be operating in a policy of keeping the engine rating at an economic level of rotations, and thus the real speed will be the one that results from the interaction with the weather elements. Finally, others may already being operated close to their maximum available output, in which case there is no margin available to overcome the weather constraints. The general objective of the mathematical hydrodynamic model is to predict the speed reduction of a vessel as a result of the environmental conditions to which she is subjected. The important parameters of the environmental conditions are the wind velocity and direction, the characteristics of the sea state, as described by the significant wave height, mean wave period and direction, and finally the current velocity and direction. Basically the ship's speed will be obtained by equating the propulsive forces to the resistance forces. The propulsive forces are dependent of the power setting applied to the engine, which will be reflected on the rpm of the propeller and the propeller main characteristics. In fact, some ships have engines with power reserves of 20 to 40%, so that they can prevent this situation. This is generally the case of the container ships, which have rigorous time schedules to fulfil. On the other hand, tankers and bulk carriers generally do not have much power reserves since they work with more flexible schedules, and consequently an increase in the resistance will result in a speed reduction. The resistance to propulsion of a ship can be divided into resistance in still water and the added resistance resulting from ship motions in waves. The later arises from the effect of several factors like added resistance directly caused by wind and wave action, indirect effect of waves associated with ship motions and rudder action. Another phenomena affecting the performance of a ship is the reduced propulsive efficiency caused by the increase of propeller loading, propeller racing or air drawing and reduced hull efficiency.

9 Maritime Engineering and Ports 215 In calm water one can decompose resistance into two independent components, a frictional one arising because of the viscosity of the water and a wave component. A ship travelling on the free surface will create a steady pressure field that results in a steady wave system. The wave resistance is proportional to the energy spent to generate this wave system. The procedure to determine the reduced ship speed when additional environmental resistance is acting on the ship can be obtained using one of the following three different assumptions: i) the ship is travelling at constant RPM; ii) the ship is travelling at constant torque; and iii) the ship is travelling at constant delivered power. These three assumptions are related to the main engine characteristics and with the captain strategy for the navigation plan. To cover different ship types and conditions the numerical procedure is able to use alternatively any of the methods. In order to implement the proposed hydrodynamic method in a global VTMIS system, some additional information is required about the environmental conditions, which will change in time and is dependent of the area where a ship is travelling. The system will update this type of information at every instant that the calculations of speed reduction are needed. Comparisons have been made between the speeds reported on board and the speeds predicted by the mathematical ship model, for six voyages of a tanker crossing the Atlantic and the Mediterranean in a total of 66 days of voyage. Good agreement was found between the values of the predicted and recorded speeds, and it was also shown that to obtain a good accuracy on the predictions of the positions of the ships, it is important to account for the delaying effects due to wind and waves. (Fonseca, Ramos and Guedes Soares, 1998). 6 Conclusions In this paper, processing methods which have been implemented in MATIAS in order to estimate or predict on-line, from a shore-based organisation and in a wide regional sea area, the first layer of Traffic Image, i.e the traffic state and in particular ship's ETAs, have been described. The originality of the designed system is that various prediction methods may be selected and activated for a given ship all along her voyage, depending: _ on one hand on the nature, quality and value of the data related to this ship and to the environmental conditions acquired by the Acquisition module.

10 216 Maritime Engineering and Ports - on the other hand, on the corrections parameters provided by the Correction tool box. More accurate information is acquired on a given ship and less complex will be the selected prediction method for this ship. In particular, we have seen the importance of the availability and the quality of a reported ship's sailing, the acquisition of which, as long as it is followed by the ship, implying the choice of a simple extrapolation technic. Combined with that and in the same way, worse will be the weather forecasts and more the selected method will be sophisticated. In good weather f.i, the service speed is sufficient in order to estimate the ship's speed whereas in bad weather conditions, the use of a ship hydrodynamic model seems unavoidable. Here again, to apply this method, we have seen the importance of the avalability of some important ship's characteristics and loading conditions, which is not often the case and which make us work under degraded conditions. However, the results obtained are globally encouraging and in case of inaccurate and/or none or late provisions of ETAs by the shipmasters themselves, the processed ETAs and the predicted ship's positions obtained by these methods, when provided to the interested shore-based organisations (port authorities and allied organisations e.g pilotage organisations, tugs companies, stevedores, etc. on one hand, SAR organisations on the other), may undoubtely improve the logistic and the safety aspects of marime transport. 7 Acknowledgements This project has been performed within the COMFORTABLE project, which has been partially funded by the DG VII (Transport) of the Commission of the European Communities under contract n : WA- 95-SC.198. It has been carried out between 1996 and 1998 by the following organisations: INRETS (France) as Task Leader, 1ST (Portugal), MARAN (The Netherlands), TUC (Greece), PTMM (Italy) and ERAAM (France). 8 References C. GUEDES SOARES, N. FONSECA, J. RAMOS, "Prediction of Voyage Duraction with Weather Constraints", Proceedings International Conference on Motions and Manoeuvrability>», Royal Institute of Naval Architects, London, 18 & 19 February, 1998, paper 4, pp