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Over the past decades, the growth of international cruise tourism has been constant. The forecasts made by the Ships and Maritime Equipment Association indicates an annual growth rate of about 7%, leading to the triplication of global cruise passengers in the future (from 19 million passengers in 2010 to more than 54 million in 2035). To cover the future market needs, roughly six or eight new cruise ship buildings per year are planned until 2031, along with an increase in ship size (up to 8,000 persons accommodated onboard). These two facts have led to a growing interest in the passengers' safety, which has been addressed by the International Maritime Organization (IMO) Maritime Safety Committee (MSC) in 2000, with the launch of an initiative for adequate international safety regulation for large passenger ships. The first issue that emerged from such an initiative was the difficulty in safely evacuating passengers toward lifeboats during emergencies (in particular, during a fire or flooding). The solution proposed by the MSC requires that future passenger ships be designed to increase their intrinsic survivability, exploiting the concept that the ship is its own best lifeboat (thus avoiding abandoning the ship as much as possible). Such a solution was formalized in December 2006 through a package of amendments to regulations dedicated to large passenger ships and published in the 82nd session of the MSC, i.e., the Resolution MSC.216(82), commonly known as safe return to port regulations (SRtP).

Batteries of electric vehicles have to be charged by power electronic converters connected to the electric grid. If these power converters are bidirectional they can be exploited to act in support to the grid operation, thus realizing the so called vehicle-to-grid (V2G) systems. At the University of Trieste an experimental V2G apparatus is under construction. Its control system has been developed and the first simulation tests has been performed. The paper describes the V2G experimental apparatus with its control system and reports the results of the preliminary simulation tests.

This paper presents a droop-based dis- tributed control strategy for multi-three-phase machines that provides augmented controllability during power shar- ing transients. The proposed strategy is able to mitigate the mutual interactions among different sets of windings without controlling any sub-space variable, also offering a modular and redundant design. On the contrary, in a centralised configuration sub-spaces would be controlled using the Vector Space Decomposition, but fault tolerance and reliability levels required by the stricter regulations and policies expected in future transportation systems would not be satisfied. The proposed method is analytically com- pared against the state-of-the-art power sharing technique and equivalent models and control design procedures have been derived and considered in the comparison. Uncon- trolled power sharing transients and their effects on mu- tual couplings among isolated sets of windings have been compared against the proposed regulated ones. Experi- mental results on a 22kW nine-phase multi-three-phase synchronous machine rig validate the design procedures showing good agreement with the expected performances.

The paper proposes a control method of a single-phase active power line conditioner (APLC) aimed at correcting the power factor of linear and non-linear loads supplied by distorted voltage utility. The method, that is based on the single-phase p-q theory and utilizes a third-order sinusoidal signal integrator, gets utility unity power factor and, at the same time, overcomes the shortcomings of the solutions pursuing utility sinusoidal current, namely the possible onset of un-damped resonance phenomena and the reduction in the energy delivery capabilities; moreover, the method makes the power factor correction robust against the supply distortion. The excellent performance of the method is substantiated by testing it by means of a hardware-in-the-loop setup.

The all-electric ship concept is becoming a standard for ships with large power requirements. At present, the design of the onboard power system [i.e., the integrated power system (IPS)] is done through a well-proven process. This process relies on historical data and trial-and-error procedures developed in nearly 30 years of design experience to address the ship’s complexity. Nowadays, the evolution in IPSs is pushed forward by the introduction of more demanding requirements, which can be complied with only by using new power system architectures and introducing onboard innovative subsystems. However, introducing innovation by means of a design process defined on the basis of past experience is inefficient. Thus, new design paradigms are needed. In this regard, concepts and tools from other technical areas can be used to achieve a “dependable design process” able to ease the introduction of innovation in ships’ IPSs. In this paper, after a brief presentation of the conventional process and the actual drivers pushing toward its revolution, the dependable design process is proposed. The latter integrates both dependability enforcing techniques and dynamic power system modeling. Insights about the process integration in the overall ship design process are given, while an application example is used to show the effectiveness of the proposed approach.

Medium Voltage DC (MVDC) distribution Power Systems for all-electric ships (AES) can be regarded as functionally composed of three subsystems, namely the power sources, the load centers and the distribution network. Extensive use of power electronics is required for connecting power sources and load centers to the MVDC bus and for protecting the MVDC power system through properly placed DC circuit breakers. In this paper, an overview is given of the power electronics equipment found in the literature and on the market that could be suitable for use in future AES MVDC power systems. Some industrial experiences regarding DC generator systems, energy storage apparatus and solid-state DC circuit breaker prototypes are reported in the paper as examples of state-of-the-art realizations. Different DC/DC converters, which can be employed as solid-state transformers, are also discussed and a structure obtained by combining them is proposed.

The paper deals with the reactive power compensation in single-phase systems by active power line conditioners (APLCs), focusing on the effectiveness of the compensating action when the utility voltage is distorted. The existing compensation approach, based on the achievement of a sinusoidal load current, is first presented, showing that it suffers from some shortcomings. A novel compensation approach, based on the achievement of a unity power factor, is then proposed that has the merit of overcoming the shortcomings of the existing one and, in addition, of outperforming it in terms of power delivery capabilities of the utility. Effectiveness of the proposed compensation approach is corroborated by simulation results.

Electrical propulsion is not a novel concept in marine systems. However, the availability of power electronic converters has proved to be the Key Enabling Technology for electrification of large ships. This paper starts with a summary of EP drives, which led to the birth of all-electric ships. Electric power generation and control systems are then presented, which make it possible to exploit the integrated electrical power system. To ease comprehension of the issues in designing such a system, its conventional design process is given. Then, the reasons that are pushing ahead the research in the shipboard power systems sector are discussed. The need for research in the design methods area is demonstrated through an overview of the latest results of technological research. Finally, a summary of the most significant results on the design tools research is given, including early stage design, dependable-oriented design, and the improvements achievable through software simulators and hardware-in-the-loop are discussed. The goal of this paper is to demonstrate why research on design methods is as important as a technological one, on the basis of the needs concerning the design, integration, and management of future “integrated electrical and electronic power systems” (power systems with power conversion quota approaching 100%).

All Electric Ships (AESs) are becoming a standard for what it concerns large ships with large power requirements (for both propulsion and onboard loads). At present, the design of shipboard complex power system (namely the IPS - Integrated Power System) is done through a well-proven process, relying on historical data and trial-and-error procedures developed in nearly 30 years of design experience. However, the evolution in shipboard power systems (mainly due to more demanding requirements on Quality of Service and safety) is pushing towards new IPS architectures and subsystems, not covered by previous knowledge. Due to that, the conventional design process is becoming inefficient, thus demanding new design paradigms. In this regard, innovative concepts and tools can be adopted to achieve a "new design process". In this paper, after a brief analysis of conventional design process, a proposal about such a new design process is done, integrating both dependability theory and the dynamic power system simulators. Insights about its inclusion in conventional design process are also given, together with an application example, used to show the effectiveness of such a new process.

The switching characteristics of an inverter feeding an induction motor controlled with the direct torque control (DTC) technique are assessed in steady state. At first, the application share of the inverter voltage vectors for the stator flux covering half a sextant is defined and predicted. The prediction indicates that, under operation at fixed inverter dc link voltage and stator flux magnitude, the application share depends only on the supply frequency of the motor and, to a small extent, on the load. Afterwards, the inverter transitions and the corresponding phase commutations within a stator flux sextant are analyzed. The outcome of the analysis permits to compute the commutations of the inverter phases in one turn of the stator flux and, from them, the inverter switching frequency is obtained. Its value is influenced by the sampling interval and the control delay arising from the microprocessor implementation of DTC. For given sampling interval and control delay, it is shown that the inverter switching frequency depends on the same quantities as the application share of the inverter voltage vectors. A comparison with the switching characteristics of an inverter controlled with the space vector modulation technique is carried out. At last, the paper discusses the sensitivity of the switching frequency of an inverter for DTC to the following quantities: inverter dc link voltage, sampling interval and control delay. Throughout the paper simulation and experimental results are given to confirm the theoretical findings.

The increasing complexity of ship's Integrated Power Systems (IPSs) led to the need of a review of conventional design process. Moreover, several stakeholders influence ship design, each of them aiming to protect its own interests, in particular when responsibility of accidents has to be defined. Dependability could be the tool to modify conventional design, transforming it into a dependability oriented design, as to properly manage the inherent system's complexity avoiding future liabilities. In addition to that, dependability could become a new index recognized by all the involved subjects, capable of establishing design quality. This paper addresses motivations to include dependability into internal processes for all the stakeholders involved in ship design process, as well as the motivations to demonstrate internal dependability levels to others subjects. Subsequently, a notional dependability oriented design process, tailored on IPS design, is presented, together with a discussion on the influence of every stakeholder on design steps. This with the aim of explaining dependability impact on design and to demonstrate how modifying conventional process important advantages could be achieved.

Spoke-type interior permanent magnet (IPM) machines are an attractive topology for high performance electric motors, especially designed for vehicle traction applications. In this paper, a special design for a spoke-type IPM motor is presented to enhance motor flux-weakening capability in the operation over a wide speed range. The proposed design consists of a simple and robust mechanical device that includes radially-displaceable rotor yokes, connected to the shaft by means of springs. At high speed, the centrifugal force prevails over the elastic one due to springs, causing the mobile yokes to displace radially and to establish a partial magnetic short circuit between permanent magnets. This increases permanent magnet leakage flux and consequently reduces the air-gap field. As a result, a mechanical flux weakening effect is achieved at high speed, which helps significantly reduce the demagnetizing d-axis current to be injected by the inverter, along with the related copper losses and demagnetization issues. The proposed design is investigated in the paper using an analytical model whose parameters are computed by finite-element analysis (FEA). The effectiveness of the solution being set forth is successfully proven by some testing on a laboratory prototype. Experimental results are compared to analytical predictions showing a satisfactory accordance.

Permanent-magnet (PM) brushless direct current (BLDC) drives are utilized in a variety of applications because of their inherent features and would be even more utilized if their torque performance could be improved. Indeed, when supplied with square-wave currents, the torque–speed characteristic undergoes a drop at high speeds, and a significant torque ripple arises at both low and high speeds due to the commutation phenomena. In this paper, the sinusoidal current supply of the PM BLDC motors is investigated in depth, and an analytical formulation of the torque–speed characteristic, the torque ripple, and the base speed for such a supply are found. Based on the resulting equations, a thorough convenience analysis of the sinusoidal current supply is carried out with respect to the square-wave current in the presence of the commutation phenomena, which is hereafter termed as the real square-wave current supply. Simulation and experimental tests for a study case are given to corroborate the theoretical results. Generation of the sinusoidal current references using Hall sensors is also discussed.

Nowadays, the Medium Voltage Direct Current (MVDC) technology represents a new possibility to renew the shipboard power system of large All Electric Ships (e.g. cruise liners or military vessels.). This new concept is capable of achieving valued advantages, such as simplified system integration, modularity and system efficiency, together with an expected reduction of the space devoted to technical system. For a shipyard interested in keeping a leading position in the market, such benefits will be the main strength points. Therefore, the MVDC Large Ship research program (presented in the paper) has been carried out trying to meet with the industrial needs of one of the partners, whose expertise deeply influenced the research and was deemed indispensable: Fincantieri Shipyard.

Dependability has only recently become a unified theory, developing its own taxonomy, lexicon and methods. This newly formed discipline now regroups concepts defined and developed within international rules corpus in different areas of engineering, such as: aerospace, automotive, computing, etc. Applications to marine sector are, at present, partial, lacking in terms of systematic approach and limited in extension. Dependability techniques provide system analysis methods that classify faults, detail consequences with associated risk index. These indications are quintessential in assessing design progress and effectiveness, in discussing issues and consequent modifications required, in quantifying impact of faults in terms of operations, in directing product and documentation development, rendering easier and less conflictual demonstrating specification adherence. Aim of this work is demonstrating dependable design can be obtained with a bearable effort, implanting dependability techniques on an already present ship design substrate, and that dependability techniques may be profitably employed as project management tools.

DC distribution is being considered as an attractive alternative to its traditional AC counterpart in many fields of applications and, in particular, for shipboard power systems. The adoption of a DC distribution network makes it possible and sometimes necessary to radically revise the technology of the generation systems in many regards. Not only frequency-related constraints are removed, allowing for high-speed smaller generators to be used, but new fault-tolerant configurations, involving power electronics, generation control and management too, become feasible as a natural alternative to the traditional arrangement of AC power plants. In this paper some of these new technological scenarios are outlined, referring to state-of-the-art realizations as well as to novel concept designs presently under development and investigation.

In many applications, electric machines are designed for operation over a wide speed range and require important flux weakening capabilities at high speeds. This paper presents and innovative rotor mechanical design that endows an interior permanent magnet (IPM) machine with an intrinsic capability of reducing its own rotor flux at high speeds. Unlike most state-of-the-art flux-weakening techniques, the design does not involve any additional current source nor demagnetization currents to be injected into stator circuits. It relies on a purely mechanical device that establishes a partial magnetic short circuit between rotor permanent magnets. The device is able to self-activate by centrifugal force when the speed exceeds a given threshold that can be fixed by design. The paper presents criteria for dimensioning the new IPM rotor and a mathematical model to predict its performance. The proposed solution is implemented into a motor prototype whose manufacturing and testing are discussed for experimental validation.

Integrated Power Systems (IPS) with DC distribution require the massive use of power electronics for many tasks: 1) the output voltage rectification of generators and the connection of energy storage systems to the IPS, 2) the supply of medium voltage high power users (propulsion drives, bow thrusters, compressors for air conditioning), 3) the adjustment of voltage levels, which is performed by power transformers in AC distribution systems and by DC/DC power converters called Solid-State Transformers (SST) in DC distribution systems. In this paper, an overview is given of the power converters found in the literature and on the market that could be suitable for employment in the mentioned subsystems as parts of a future MVDC IPS. Finally authors propose a structure for an SST to be employed in three-wire MVDC power systems.

A promising solution to improve tomorrow's all-electric ship onboard power system design and performance relies on moving from 50-Hz or 60-Hz AC distribution to a Medium-Voltage DC (MVDC) grid. This paper provides a review of presently-available and under-development technologies for electric generators to be used in perspective MVDC shipboard systems. The inspiring reason for the survey lies in the fact that an MVDC grid design removes the constraints usually imposed on shipboard generation group design and opens the way for a wide variety of non-conventional machine alternatives. In combination of high-speed gas turbines, that can be employed as prime movers, several electric machine topologies, not yet used for ship applications, can be selected to feed MVDC onboard systems. Along with a review of possible generator types suitable for such use, the paper critically reviews some emerging technologies for their fault-tolerance enhancement through conventional and innovative split-phase architectures.

The vector approach in either stationary or synchronous plane is commonly used for the analysis of the AC drives like induction and PM brushless AC drives whilst it has been hardly ever used for the PM brushless DC (BLDC) drives. A possible reason is that they require injection of square-wave currents into the motor phases, which are non-sinusoidal in nature. Recently, the PM BLDC drives have been analyzed by the help of the vector approach in the stationary plane. This paper applies such an approach to an in-depth analysis of the operation of the PM BLDC drives during the current commutations, illustrating the potentialities of the approach in giving a better insight into the current transients and the ensuing torque characteristics. At last, two voltage control strategies proposed to eliminate the torque ripple due to the current commutations are considered, showing that they can be readily understood by the vector approach.

The degree of complexity of Integrated Power Systems (IPS) for All Electric Ships (AESs) points out the need to review the design process from a dependability perspective. This paper suggests the application of dependability concepts and definitions to accomplish to requirements set by recent rules and regulations in the field of threat levels for shipboard electrical equipment. Guidelines about 1) how to properly decompose the IPS into conceptual subsystems and 2) how to apply to analyze them, using the Fault Tree Analysis (FTA) and the Failure Mode and Effect Analysis (FMEA) are presented and discussed. As a case study, the IPS of a new built large AES is chosen.

Permanent-magnet synchronous motors are widely used in vehicular traction applications due to their high efficiency and torque density. A critical point of their operation is the flux weakening needed at high speed, which usually implies injecting a d-axis demagnetizing current by the inverter. This paper presents a critical survey of the alternative approaches found in the literature and patent databases to enhance flux-weakening performance by acting on motor mechanical and electromagnetic design.

DC distribution is an attractive solution for onboard integrated power systems of new-generation all-electric ships, especially thanks to the benefits it offers in terms of reducing generation system size, weight and power consumption. Technologies have already been implemented in full-scale prototypes based on synchronous generators feeding the DC grid through rectifiers. In these proven solutions, the voltage control is allocated either to the generator excitation system or to the rectifier control. This paper investigates a design variant where both voltage control loops exist and act simultaneously. The basic design of the system is presented along with its modeling in the Matlab/Simulink environment. Simulation results obtained from the Simulink models are finally reported illustrating the system behavior during various electrical and electromechanical transients. (6 pages)

PM brushless DC (BLDC) motors should be fed by square-wave currents synchronized with the back emfs to develop a constant torque with the required value. In practice, phase inductances and limited source voltage produce non ideal square-wave currents and unbalanced phase currents during commutations. Such phenomena affect the torque developed by the PM BLDC drives by deviating its value from the requested one and by giving rise to torque ripple. In this paper, an analytical study of the torque characteristics, namely motor torque and torque ripple, of PM BLDC drives is carried out as a function of the motor speed and is supported by experimental data. Torque characteristics are expressed in terms of a quantity distinctive of the motor, thus giving a generalized formulation to the results.

Nominal voltage of the traction motors being used in electric vehicles is typically higher than the voltage generated by the onboard DC sources. Thus, motors of AC type require boost as well as inversion of available DC voltage. This can be done either by a DC-DC boosted inverter (DBI) or by the recently introduced Z-source inverter (ZSI). Since drives with PM brushless DC (BLDC) motor are largely utilized in the traction of light electric vehicles, it is of interest to carry out a comparative analysis of the two supplies when feeding such kind of motor. In this paper, DBI and ZSI supplies are designed for the study case of an electric city scooter propelled by a PM BLDC motor and powered by a fuel cell with the assistance of a battery. Ratings of the transistors and sizes of the passive components for the two supplies are determined as a function of the fraction of the fuel cell power to the nominal power of the traction motor. The design results are discussed.

For future All Electric Ships (AESs), setting up of a dependable Integrated Power System (IPS) requires a dedicated design procedure, more articulated than the traditional ones. As a matter of fact, traditional procedures need to be revised and supplemented using an appropriate approach in order to make the IPS dependable. Such an approach should bear in mind the recently formalized dependability theory and exploit the so-called enforcing techniques (fault prevention, fault tolerance, fault removal, fault forecasting). The aim of this paper is to introduce the basic terminology and formal methods of the dependability theory into the traditional design steps of an IPS for large AESs. Basic guidelines are given to conceptually determine how to proceed in the design of future dependable IPSs.

In this paper two possible design alternatives are investigated for a high-power synchronous motor propulsion drive employing two voltage-source inverters for motor supply. The two arrangements are compared based on their steady-state behavior in both normal and faulty conditions (i.e. with one out-of-service inverter), predicted by time-stepping finite-element simulations. Simulation results found consistent with the results expected theoretically and based on previous studies. Pros and contras of the two design alternatives are highlighted together with the possible design strategies that could help mitigate their drawbacks.

The paper presents the activities ongoing in the framework of a two-year Italian National Research Project aimed at developing a distributed Driving and Steering (DaS) system for Electric Vehicles (EVs). The system is built up around an actuation module embedding a rotary-linear motor and including a power converter and a local Electronic Control Unit (ECU) to supply and control the motor. The actuation module is intended to make a wheel active so as to give it propelling and steering capabilities. The DaS system, in turn, includes a central ECU and a fieldbus to govern and network the system, and a power supply stage. The Project under consideration is focused on a DaS system with two active wheels for a light-duty EV.

Z-source inverters (ZSIs) supply ac loads from a dc source and boost the voltage of the dc source in a single stage. Such an ability is much needed in electric vehicles where the rated voltage of the traction motors is usually higher than the voltage generated by the onboard dc sources. Being the technological trend in the traction drives directed toward the employment of PM brushless motors, the use of a ZSI for their supply is a cuttingedge issue, which is investigated in this paper. The paper starts by formulating the equations of a ZSI in a suitable form. Then the characteristics of the ZSI supply for the two basic types of PM brushless motor, namely the dc and ac types, are assessed with regards to the switch solicitation and utilization. As a comparison, the characteristics of the traditional dc-dc boosted inverter supply are also given.

The paper presents a small-signal stability analysis of voltage control systems employed onboard all-electric cruise liners. To realize the stability analysis, a model of shipboard power station voltage control system has been determined, moving from dynamic models of alternators, voltage regulators and loads. Reduced order models of alternators have been utilized, to simplify at best the analysis. Analysis results have been validated through numerical simulations, using a simulator of power station electromechanic quantities developed at the University of Trieste. Such simulator includes alternators, voltage and frequency regulators, breakers, protections and R-L loads. Numerical results obtained from simulations of both complete and reduced order models fully validate the analytical findings, while computation times are in the order of real-time simulations.

In this paper a high-power electric drive based on a quadruple-three-phase synchronous motor and four load commutated inverters (LCI) is investigated to evaluate its feasibility and performance. Based on accurate simulation analyses, the advantages of the proposed solution compared to the technologically-proven dual three-phase scheme are highlighted in terms of torque quality and motor efficiency improvement.

A simple fault-tolerant induction motor drive with v/f speed control included in the electrical auxiliary equipment of railway locomotives is presented in the paper. The employment of fault tolerant on-board electrical auxiliary equipment increases the dependability and then the quality of the railway service. The fault tolerance technique consists of some emergency countermeasures applied in case of fault. It includes a fault diagnosis procedure based on the analysis of the current vector, the connection of the stator winding neutral point to the dc-link middle point and the variation of the v/f law, taking into account the contribution of the zero-sequence impedance and the need for a flux weakening in order to avoid saturation. Some experimental results are reported showing the improved performance of the motor drive under fault conditions by means of the proposed remedial strategy.

RTAI extension to Linux operating system makes it possible to perform Hard Real-Time (HRT) controls using a commercial PC and utilizing open-source code. This paper details a software installation process, along with the suitable hardware chosen for realizing motion controls using Linux-RTAI. Advantages offered by the possibility of designing and developing motion controls using the powerful tools of high level object-languages (i.e. Simulink) are described. It is proven that the combination of Simulink as a development environment and Linux-RTAI PC as a controller platform brings advantages that can be greatly appreciated in industrial motion control applications, such as: very reduced time for development, diagnostics, design of controls; adequate control performance; cost reductions offered by the open-source status of the operating system.

The paper deals with a direct torque control (DTC) scheme for Induction Motor (IM) drives where flux and torque of the motor are estimated by the IM current model instead of by the voltage model. The scheme is intended to enable operation of the DTC IM drives at zero speed. As a return, its implementation requires the knowledge of speed, rotor time constant and inductive parameters of the motor. In the paper the performance of the proposed scheme is studied for a drive equipped with an incremental encoder and commanded with a constant flux. Using the position information delivered by the encoder, a suitable representation of the current model is formulated that makes the drive operation feasible at true zero speed. Operating with a constant flux command, the variations in the rotor time constant dominate over the other parameters; their effect on the drive behavior is analyzed in steady state founding that the motor flux and torque are deviated from the references of a quantity, which is a function of the torque reference. Experimental results are given to substantiate both the capabilities of the scheme of developing the full torque at a standstill and the theoretical findings on the drive behavior under rotor time constant mismatch.

Power quality is a key issue in island, shipboard integrated electric power systems (IEPS). However, it is not of easy accomplishment, due to the complexity of shipboard power station (generators are many and differ by sizes, prime movers, control systems, etc.) and to the intrinsic weakness of the shipboard grid. This paper presents a new approach for controlling voltage and reactive power generated in the power station of an all electric ship (AES). A shipboard voltage and VAr integrated regulator (WIRE) is proposed. The WIRE is aimed at controlling alternators voltage, main busbar voltage and jointly optimizing the reactive power generated by each alternator. WIRE project reunions into an integrated device the most advanced control functions presently implemented in smartest land power stations. WIRE is also endowed with an embedded power-station/IEPS real-time simulator, suitable for performing factory functional, diagnostic and hardware-in-the-loop (HIL) tests. The paper will show how WIRE can improve power quality into an AES, while respecting marine constraints like: reduced room availability, redundancy, reliability, optimal and safe utilization of generators, fast commissioning, diagnostics.

Electric propulsion for cruise liners has now become a standard owing to its several advantages. However propulsion drives have a large impact on the ship power system. This gives rise to the requirement for designers to understand in depth the effects of propulsion drives on the ship network in different operating conditions. For this purpose computer simulations have proved to be a very useful tool. In the paper a typical propulsion drive for an all-electric cruise liner is described and simulation results are shown. Owing to the model complexity, simulations turn out to be computationally quite heavy. For this reason a simplified model has been developed. Simulation results, from the ship network viewpoint, proved to be close to the detailed model ones.