Nikos Themelis - Academia.edu (original) (raw)

Papers by Nikos Themelis

Research paper thumbnail of Towards Building an Attained Index of Passenger Ship Fire Safety

Research paper thumbnail of Definitions of Celerity for Investigating Surf-Riding in an Irregular Seaway

Contemporary Ideas on Ship Stability, 2019

As is well-known, if the speed of a ship operating in high, fairly regular, following waves excee... more As is well-known, if the speed of a ship operating in high, fairly regular, following waves exceeds wave celerity, then surf-riding is realized. This motivates one to approach the calculation of the probability of surf-riding in irregular seas as a threshold exceedance problem. However, it is unknown whether such a simple phenomenological rule, using the celerity as threshold, could also be applicable for the ship dynamics associated with a stochastic wave environment. To clarify this, a suitable definition of wave celerity for an irregular seaway needs first to become available. In this chapter, we define celerity as the velocity of propagation of a fixed slope value of the wave profile. This leads to the concept of instantaneous celerity, opening up a window to the literature of instantaneous frequency in signal processing. As it turns out, instantaneous celerity is not always a consistently smooth and bounded curve. Other definitions of local celerity are also conceivable. We tested a few different selections, obtaining time-dependent celerity curves for various types of waves. Relaxing the requirement for a narrow-band spectrum, we offer some clues about the effect of spectrum’s bandwidth on celerity. In a further step, simultaneous treatment of the “wave” and “ship” processes is implemented, in order to investigate the potential of applying a local celerity condition for surf-riding’s prediction. Various patterns of ship motion, before and into surf-riding, are observed.

Research paper thumbnail of Probabilistic Framework for Onboard Fire Safety Fire Ignition Model Specification (D1.2)

Research paper thumbnail of Critical Wave Groups Versus Direct Monte-Carlo Simulations for Typical Stability Failure Modes of a Container Ship

Contemporary Ideas on Ship Stability, 2019

In the second Generation Intact Stability Criteria currently developed at IMO, the process of dir... more In the second Generation Intact Stability Criteria currently developed at IMO, the process of direct stability assessment (DSA) and providing operational guidance (OG) are interlaced with a requirement of performing a large number of numerical simulations. However, extreme roll motions that are generally behind stability failures are rare events as any extreme responses. An additional significant difficulty is that roll response as stochastic process is usually non-Gaussian, therefore, close-form expressions for the probability of extreme roll responses, based on spectral moments, are in general not applicable. A practical approach proposed recently exploits the idea that extreme events occur due to the encountering of extreme wave groups (critical wave episodes). This could alleviate the need for a large number of simulations by focusing on the systematic identification of those deterministic wave sequences that generate unacceptable roll responses. Taking a first step towards a systematic validation process of the wave groups method, the present study compares the exceedance probabilities of 40° roll angle and of g/2 lateral acceleration, computed by the critical wave groups method with Monte-Carlo simulations for a large containership. The nonlinear seakeeping code rolls is used as mathematical model of ship motion. Typical loading conditions where various stability failure modes can occur are examined.

Research paper thumbnail of Wave celerity in a multi-chromatic sea: a comparative study

The expansion of the theory of surf-riding form the regular to the irregular sea is continued. An... more The expansion of the theory of surf-riding form the regular to the irregular sea is continued. An alternative method of instantaneous wave celerity calculation is proposed, based on the Hilbert transform and on the concept of instantaneous frequency. The result is compared against prediction of the same by a previously presented method that was based on the propagation in time-space of a property of the wave profile. The effectiveness of the two methods for the prediction of surf-riding is discussed.

Research paper thumbnail of Surf-Riding in Multi-Chromatic Seas: “High-Runs” and the Role of Instantaneous Celerity

We investigate “high-run” events of ships in following seas. These are cases of ship motion when,... more We investigate “high-run” events of ships in following seas. These are cases of ship motion when, due to waves’ effect, a ship attains abnormally high speed. Investigations are carried out in three directions: firstly, the statistics of high-runs are calculated, exploring in particular their dependence on the wave spectrum and the sea state. Secondly, a rather neglected up to now method, proposed by Grim, for the quantification of the probability of high-run occurrence is implemented. Lastly, the focus is set on the connection of the instantaneous wave celerity with the mean surge velocity during high-run. For its evaluation, two different error metrics are implemented.

Research paper thumbnail of What is Surf-Riding in Irregular Seas ?

The concept of surf-riding in irregular seas is enquired and two calculation schemes are implemen... more The concept of surf-riding in irregular seas is enquired and two calculation schemes are implemented in order to create upper and lower bounds of the probability of surf-riding. The first scheme is focused on the identification of the generation and disappearance of surge equilibria. Due to the time-varying nature of the dynamical system, these are finite-time objects, departing from the conventional notion of an equilibrium. The other scheme is aimed to determine time segments of ship motion where the mean speed is higher than expected. The probability values obtained by the two schemes are compared against each other and conclusions are drawn.

Research paper thumbnail of Numerical Statistical Estimates of Ship Broaching-To

Journal of Ship Research, 2016

Ship "high-run" incidents in irregular following/quartering sea... more Ship "high-run" incidents in irregular following/quartering seas, in particular their correlation with broaching-to behavior, are investigated, aiming to produce statistical evidence about this connection. A simple linear mathematical model of the yaw and sway motions has been combined with a nonlinear surge equation that is often applied in surf-riding investigations, considering multifrequency wave excitation derived from a spectrum. Types of yaw instability are conjectured from the structure of the mathematical model. The concept of instantaneous celerity is used for formally recognizing the occurrence of high-run and a simple method is proposed for its calculation. Problem's rarity (i.e., whether few only or a large number of broaching-to incidents are recorded during long-time simulations) is controlled by using the rudder control gains as "rarity knobs." Expectations of broaching-to, with and without conditioning on high-run incidence, are presented. Histograms of cumulative time of high-run and limiting yaw angle exceedance are produced and the fitting of standard distributions, variants of the normal distribution, are discussed.

Research paper thumbnail of Benchmark study of numerical codes for the prediction of intact stability of ships in extreme seas: Results of "Phase II". International Towing Tank Conference - Specialist Committee on Stability in Waves

Research paper thumbnail of Probabilistic assessment of ship stability. Discussion

Transactions-Society of …, 2007

... THEMELIS Nikos ; SPYROU Kostas J. ; AYYUB Bilal M. ; FRANCESCUTTO Alberto ; BULIAN Gabriele ;... more ... THEMELIS Nikos ; SPYROU Kostas J. ; AYYUB Bilal M. ; FRANCESCUTTO Alberto ; BULIAN Gabriele ; DAG MYRHAUG ; AALL DAHLE Emil ; UMEDA Naoya ; BELENKY Vadim ; ... INIST-CNRS,Cote INIST : 6111 A, 35400019640508.0090. Nº notice refdoc (ud4) : 20601228. ...

Research paper thumbnail of Dynamics of lashed trailers on a ship under roll and pitch excitations

Proceedings of the Institution of Mechanical Engineers, Part M: Journal of Engineering for the Maritime Environment, 2008

The prediction of the dynamic behaviour of lashed trailers placed at various locations on the veh... more The prediction of the dynamic behaviour of lashed trailers placed at various locations on the vehicle deck(s) of a ferry is discussed. Excitations on the lashed trailers are produced by the combined roll and pitch motions, caused by steep waves encountered by the ship from various angles. Firstly, the typical requirements of the IMO's Cargo Securing Manual, as implemented for an existing ferry, are presented. Thereafter the mathematical model that has been developed for the needs of the present study is set out. The characteristics of the lashings, as well as of the trailer support (trestle) and suspension systems, are taken into account. Ship motions are simulated with a well-known panel code, and they are subsequently imported into the mathematical model of lashed trailers. This is a step towards an integrated ‘performance-based’ framework for the investigation of cargo safety on board that is ship and environment specific.

Research paper thumbnail of Prediction potential of the parametric rolling behaviour of a post-panamax containership

Research paper thumbnail of Probabilistic Fire Safety Assessment of Passenger Ships

Journal of Ship Research, 2012

A probabilistic framework is proposed for the assessment of passenger ship fire safety that is ba... more A probabilistic framework is proposed for the assessment of passenger ship fire safety that is based on a structured generation of "design fires." The aim is to establish a rational and practical means for evaluating design selections (materials, geometry, sensors, and suppressors) that influence fire safety with reference to the implicit safety objectives attended by the current regulations. Thus, alternative design solutions could be evaluated more comprehensively. Fire growth intensity, restriction of heat release rate resulting from ventilation shortage, occurrence of flashover, and the final decay are key processes modeled in the current approach. They are interfaced with fire suppression triggered by detection. The effect of manual intervention is also assessed. An application is included based on uniform distributions of selected random parameters. Justification for the considered ranges is provided. The effect of assuming different probability distributions is considered. The methodology can be used for producing the probability of containment and extinguishment of fires as well as for judging the performance of an overall or partial ship design solution.

Research paper thumbnail of Lashing of trailers on board RO/RO ships under intensive rolling

We are analysing the lashing of trailers on board a RO/RO ferry from the perspectives of legislat... more We are analysing the lashing of trailers on board a RO/RO ferry from the perspectives of legislative requirements, ship and trailer motion dynamics and lashings' structural strength. We present a mathematical model giving the lashing loads for a trailer placed on the vehicle deck while the ship is performing rolling motion in beam waves. Furthermore we are presenting a finite element approach for determining the stresses that are developed at the lashing points on the trailer. The longer term objective is to produce an integrated model with enough detail that can be used for examining the reliability of a lashing system under realistic conditions .

Research paper thumbnail of Statistical Correlations of Ship High-Run with Broaching-to and Capsize

Journal of Marine Science and Engineering, 2020

In the current paper we are extending our earlier work on the assessment of a ship’s tendency to ... more In the current paper we are extending our earlier work on the assessment of a ship’s tendency to capsize due to broaching-to in a stochastic seaway. Capturing, in a probabilistic context, interferences between different phenomena occurring during ship operation in extreme seas is a challenging task. Estimates of statistical correlations are deduced between high-run events, broaching-to and capsize. A phenomenological approach is adopted in this study for the classification of the targeted motions. Large scale simulations and a direct counting scheme are applied on the basis of a 4 degrees of freedom (4DOF) mathematical model for the coupled surge–sway–yaw–roll (and rudder) motions. Comparison with the results obtained from a previously used 3DOF model for the same scenarios is carried out in order to investigate the effect of roll on high-run’s correlation with broaching-to. Additionally, sensitivity studies are carried out in order to examine the effect of the commanded heading ang...

Research paper thumbnail of Ship stability, dynamics and safety: status and perspectives

With the aim of analysing the current status and possible future perspectives of research in the ... more With the aim of analysing the current status and possible future perspectives of research in the field of ship stability, dynamics and safety, this paper deals with an extensive review of the research work presented at the International Conferences on Stability of Ships and Ocean Vehicles (STAB Conferences) and the International Ship Stability Workshops (ISSW) held during the period 2009-2014. The reviewed material is organised in different sections, corresponding to a set of identified main typical focal macro-topics of research. On the basis of the reviewed material, consolidated research topics are highlighted together with emerging topics, and ideas for possible future research and its needs and focus are provided. Discussion is also provided regarding the link between research and educational aspects.

Research paper thumbnail of Data-driven modelling of ship propulsion and the effect of data pre-processing on the prediction of ship fuel consumption and speed loss

Research paper thumbnail of Evaluation of Different Deep-Learning Models for the Prediction of a Ship’s Propulsion Power

Journal of Marine Science and Engineering

Adverse conditions within specific offshore environments magnify the challenges faced by a vessel... more Adverse conditions within specific offshore environments magnify the challenges faced by a vessel’s energy-efficiency optimization in the Industry 4.0 era. As the data rate and volume increase, the analysis of big data using analytical techniques might not be efficient, or might even be infeasible in some cases. The purpose of this study is the development of deep-learning models that can be utilized to predict the propulsion power of a vessel. Two models are discriminated: (1) a feed-forward neural network (FFNN) and (2) a recurrent neural network (RNN). Predictions provided by these models were compared with values measured onboard. Comparisons between the two types of networks were also performed. Emphasis was placed on the different data pre-processing phases, as well as on the optimal configuration decision process for each of the developed deep-learning models. Factors and parameters that played a significant role in the outcome, such as the number of layers in the neural netw...

Research paper thumbnail of Detection of surf-riding behavior of ships in irregular seas

Nonlinear Dynamics

ABSTRACT From investigations with ship models tested in steep periodic following waves, it has be... more ABSTRACT From investigations with ship models tested in steep periodic following waves, it has been conjectured in the past that, if the speed is observed reaching (in transient) the celerity value, attraction toward the condition commonly identified as “surf-riding” is already in progress. The nonlinear dynamics of this phenomenon has been investigated in depth also theoretically; however, it is unknown whether the stated simple phenomenological detection rule is meaningful also for more natural sea wave profiles. A practical question that prompted the current investigation is whether the duration of attraction to surf-riding could be properly quantified as a percentage of the total time of exposure to a specified wave environment, so that a probability of surf-riding could be eventually calculated. Celerity is defined in the current work as the velocity of propagation of a suitable local property of the wave profile, such as a certain value of slope. For irregular seas, this leads to the concept of instantaneous celerity. Unfortunately, instantaneous celerity, in general, is not a smooth and bounded curve in time. An alternative definition of celerity fitting to the problem of surf-riding is thus investigated, where the propagated points of the profiles are those with the locally maximum slope. Simultaneous treatment of the “wave” and “ship” processes is implemented, and the potential of a condition based on local celerity for surf-riding prediction is examined. Various patterns of behavior before and into surf-riding are observed and discussed. The paper is a step in the direction of developing a probabilistic method of assessment for the dangerous phenomenon of surf-riding and indirectly, for the evaluation of the class of cross-disciplinary phenomena that obey similar nonlinear dynamics.

Research paper thumbnail of Nonlinear surge motions of a ship in bi-chromatic following waves

Communications in Nonlinear Science and Numerical Simulation

Research paper thumbnail of Towards Building an Attained Index of Passenger Ship Fire Safety

Research paper thumbnail of Definitions of Celerity for Investigating Surf-Riding in an Irregular Seaway

Contemporary Ideas on Ship Stability, 2019

As is well-known, if the speed of a ship operating in high, fairly regular, following waves excee... more As is well-known, if the speed of a ship operating in high, fairly regular, following waves exceeds wave celerity, then surf-riding is realized. This motivates one to approach the calculation of the probability of surf-riding in irregular seas as a threshold exceedance problem. However, it is unknown whether such a simple phenomenological rule, using the celerity as threshold, could also be applicable for the ship dynamics associated with a stochastic wave environment. To clarify this, a suitable definition of wave celerity for an irregular seaway needs first to become available. In this chapter, we define celerity as the velocity of propagation of a fixed slope value of the wave profile. This leads to the concept of instantaneous celerity, opening up a window to the literature of instantaneous frequency in signal processing. As it turns out, instantaneous celerity is not always a consistently smooth and bounded curve. Other definitions of local celerity are also conceivable. We tested a few different selections, obtaining time-dependent celerity curves for various types of waves. Relaxing the requirement for a narrow-band spectrum, we offer some clues about the effect of spectrum’s bandwidth on celerity. In a further step, simultaneous treatment of the “wave” and “ship” processes is implemented, in order to investigate the potential of applying a local celerity condition for surf-riding’s prediction. Various patterns of ship motion, before and into surf-riding, are observed.

Research paper thumbnail of Probabilistic Framework for Onboard Fire Safety Fire Ignition Model Specification (D1.2)

Research paper thumbnail of Critical Wave Groups Versus Direct Monte-Carlo Simulations for Typical Stability Failure Modes of a Container Ship

Contemporary Ideas on Ship Stability, 2019

In the second Generation Intact Stability Criteria currently developed at IMO, the process of dir... more In the second Generation Intact Stability Criteria currently developed at IMO, the process of direct stability assessment (DSA) and providing operational guidance (OG) are interlaced with a requirement of performing a large number of numerical simulations. However, extreme roll motions that are generally behind stability failures are rare events as any extreme responses. An additional significant difficulty is that roll response as stochastic process is usually non-Gaussian, therefore, close-form expressions for the probability of extreme roll responses, based on spectral moments, are in general not applicable. A practical approach proposed recently exploits the idea that extreme events occur due to the encountering of extreme wave groups (critical wave episodes). This could alleviate the need for a large number of simulations by focusing on the systematic identification of those deterministic wave sequences that generate unacceptable roll responses. Taking a first step towards a systematic validation process of the wave groups method, the present study compares the exceedance probabilities of 40° roll angle and of g/2 lateral acceleration, computed by the critical wave groups method with Monte-Carlo simulations for a large containership. The nonlinear seakeeping code rolls is used as mathematical model of ship motion. Typical loading conditions where various stability failure modes can occur are examined.

Research paper thumbnail of Wave celerity in a multi-chromatic sea: a comparative study

The expansion of the theory of surf-riding form the regular to the irregular sea is continued. An... more The expansion of the theory of surf-riding form the regular to the irregular sea is continued. An alternative method of instantaneous wave celerity calculation is proposed, based on the Hilbert transform and on the concept of instantaneous frequency. The result is compared against prediction of the same by a previously presented method that was based on the propagation in time-space of a property of the wave profile. The effectiveness of the two methods for the prediction of surf-riding is discussed.

Research paper thumbnail of Surf-Riding in Multi-Chromatic Seas: “High-Runs” and the Role of Instantaneous Celerity

We investigate “high-run” events of ships in following seas. These are cases of ship motion when,... more We investigate “high-run” events of ships in following seas. These are cases of ship motion when, due to waves’ effect, a ship attains abnormally high speed. Investigations are carried out in three directions: firstly, the statistics of high-runs are calculated, exploring in particular their dependence on the wave spectrum and the sea state. Secondly, a rather neglected up to now method, proposed by Grim, for the quantification of the probability of high-run occurrence is implemented. Lastly, the focus is set on the connection of the instantaneous wave celerity with the mean surge velocity during high-run. For its evaluation, two different error metrics are implemented.

Research paper thumbnail of What is Surf-Riding in Irregular Seas ?

The concept of surf-riding in irregular seas is enquired and two calculation schemes are implemen... more The concept of surf-riding in irregular seas is enquired and two calculation schemes are implemented in order to create upper and lower bounds of the probability of surf-riding. The first scheme is focused on the identification of the generation and disappearance of surge equilibria. Due to the time-varying nature of the dynamical system, these are finite-time objects, departing from the conventional notion of an equilibrium. The other scheme is aimed to determine time segments of ship motion where the mean speed is higher than expected. The probability values obtained by the two schemes are compared against each other and conclusions are drawn.

Research paper thumbnail of Numerical Statistical Estimates of Ship Broaching-To

Journal of Ship Research, 2016

Ship "high-run" incidents in irregular following/quartering sea... more Ship "high-run" incidents in irregular following/quartering seas, in particular their correlation with broaching-to behavior, are investigated, aiming to produce statistical evidence about this connection. A simple linear mathematical model of the yaw and sway motions has been combined with a nonlinear surge equation that is often applied in surf-riding investigations, considering multifrequency wave excitation derived from a spectrum. Types of yaw instability are conjectured from the structure of the mathematical model. The concept of instantaneous celerity is used for formally recognizing the occurrence of high-run and a simple method is proposed for its calculation. Problem's rarity (i.e., whether few only or a large number of broaching-to incidents are recorded during long-time simulations) is controlled by using the rudder control gains as "rarity knobs." Expectations of broaching-to, with and without conditioning on high-run incidence, are presented. Histograms of cumulative time of high-run and limiting yaw angle exceedance are produced and the fitting of standard distributions, variants of the normal distribution, are discussed.

Research paper thumbnail of Benchmark study of numerical codes for the prediction of intact stability of ships in extreme seas: Results of "Phase II". International Towing Tank Conference - Specialist Committee on Stability in Waves

Research paper thumbnail of Probabilistic assessment of ship stability. Discussion

Transactions-Society of …, 2007

... THEMELIS Nikos ; SPYROU Kostas J. ; AYYUB Bilal M. ; FRANCESCUTTO Alberto ; BULIAN Gabriele ;... more ... THEMELIS Nikos ; SPYROU Kostas J. ; AYYUB Bilal M. ; FRANCESCUTTO Alberto ; BULIAN Gabriele ; DAG MYRHAUG ; AALL DAHLE Emil ; UMEDA Naoya ; BELENKY Vadim ; ... INIST-CNRS,Cote INIST : 6111 A, 35400019640508.0090. Nº notice refdoc (ud4) : 20601228. ...

Research paper thumbnail of Dynamics of lashed trailers on a ship under roll and pitch excitations

Proceedings of the Institution of Mechanical Engineers, Part M: Journal of Engineering for the Maritime Environment, 2008

The prediction of the dynamic behaviour of lashed trailers placed at various locations on the veh... more The prediction of the dynamic behaviour of lashed trailers placed at various locations on the vehicle deck(s) of a ferry is discussed. Excitations on the lashed trailers are produced by the combined roll and pitch motions, caused by steep waves encountered by the ship from various angles. Firstly, the typical requirements of the IMO's Cargo Securing Manual, as implemented for an existing ferry, are presented. Thereafter the mathematical model that has been developed for the needs of the present study is set out. The characteristics of the lashings, as well as of the trailer support (trestle) and suspension systems, are taken into account. Ship motions are simulated with a well-known panel code, and they are subsequently imported into the mathematical model of lashed trailers. This is a step towards an integrated ‘performance-based’ framework for the investigation of cargo safety on board that is ship and environment specific.

Research paper thumbnail of Prediction potential of the parametric rolling behaviour of a post-panamax containership

Research paper thumbnail of Probabilistic Fire Safety Assessment of Passenger Ships

Journal of Ship Research, 2012

A probabilistic framework is proposed for the assessment of passenger ship fire safety that is ba... more A probabilistic framework is proposed for the assessment of passenger ship fire safety that is based on a structured generation of "design fires." The aim is to establish a rational and practical means for evaluating design selections (materials, geometry, sensors, and suppressors) that influence fire safety with reference to the implicit safety objectives attended by the current regulations. Thus, alternative design solutions could be evaluated more comprehensively. Fire growth intensity, restriction of heat release rate resulting from ventilation shortage, occurrence of flashover, and the final decay are key processes modeled in the current approach. They are interfaced with fire suppression triggered by detection. The effect of manual intervention is also assessed. An application is included based on uniform distributions of selected random parameters. Justification for the considered ranges is provided. The effect of assuming different probability distributions is considered. The methodology can be used for producing the probability of containment and extinguishment of fires as well as for judging the performance of an overall or partial ship design solution.

Research paper thumbnail of Lashing of trailers on board RO/RO ships under intensive rolling

We are analysing the lashing of trailers on board a RO/RO ferry from the perspectives of legislat... more We are analysing the lashing of trailers on board a RO/RO ferry from the perspectives of legislative requirements, ship and trailer motion dynamics and lashings' structural strength. We present a mathematical model giving the lashing loads for a trailer placed on the vehicle deck while the ship is performing rolling motion in beam waves. Furthermore we are presenting a finite element approach for determining the stresses that are developed at the lashing points on the trailer. The longer term objective is to produce an integrated model with enough detail that can be used for examining the reliability of a lashing system under realistic conditions .

Research paper thumbnail of Statistical Correlations of Ship High-Run with Broaching-to and Capsize

Journal of Marine Science and Engineering, 2020

In the current paper we are extending our earlier work on the assessment of a ship’s tendency to ... more In the current paper we are extending our earlier work on the assessment of a ship’s tendency to capsize due to broaching-to in a stochastic seaway. Capturing, in a probabilistic context, interferences between different phenomena occurring during ship operation in extreme seas is a challenging task. Estimates of statistical correlations are deduced between high-run events, broaching-to and capsize. A phenomenological approach is adopted in this study for the classification of the targeted motions. Large scale simulations and a direct counting scheme are applied on the basis of a 4 degrees of freedom (4DOF) mathematical model for the coupled surge–sway–yaw–roll (and rudder) motions. Comparison with the results obtained from a previously used 3DOF model for the same scenarios is carried out in order to investigate the effect of roll on high-run’s correlation with broaching-to. Additionally, sensitivity studies are carried out in order to examine the effect of the commanded heading ang...

Research paper thumbnail of Ship stability, dynamics and safety: status and perspectives

With the aim of analysing the current status and possible future perspectives of research in the ... more With the aim of analysing the current status and possible future perspectives of research in the field of ship stability, dynamics and safety, this paper deals with an extensive review of the research work presented at the International Conferences on Stability of Ships and Ocean Vehicles (STAB Conferences) and the International Ship Stability Workshops (ISSW) held during the period 2009-2014. The reviewed material is organised in different sections, corresponding to a set of identified main typical focal macro-topics of research. On the basis of the reviewed material, consolidated research topics are highlighted together with emerging topics, and ideas for possible future research and its needs and focus are provided. Discussion is also provided regarding the link between research and educational aspects.

Research paper thumbnail of Data-driven modelling of ship propulsion and the effect of data pre-processing on the prediction of ship fuel consumption and speed loss

Research paper thumbnail of Evaluation of Different Deep-Learning Models for the Prediction of a Ship’s Propulsion Power

Journal of Marine Science and Engineering

Adverse conditions within specific offshore environments magnify the challenges faced by a vessel... more Adverse conditions within specific offshore environments magnify the challenges faced by a vessel’s energy-efficiency optimization in the Industry 4.0 era. As the data rate and volume increase, the analysis of big data using analytical techniques might not be efficient, or might even be infeasible in some cases. The purpose of this study is the development of deep-learning models that can be utilized to predict the propulsion power of a vessel. Two models are discriminated: (1) a feed-forward neural network (FFNN) and (2) a recurrent neural network (RNN). Predictions provided by these models were compared with values measured onboard. Comparisons between the two types of networks were also performed. Emphasis was placed on the different data pre-processing phases, as well as on the optimal configuration decision process for each of the developed deep-learning models. Factors and parameters that played a significant role in the outcome, such as the number of layers in the neural netw...

Research paper thumbnail of Detection of surf-riding behavior of ships in irregular seas

Nonlinear Dynamics

ABSTRACT From investigations with ship models tested in steep periodic following waves, it has be... more ABSTRACT From investigations with ship models tested in steep periodic following waves, it has been conjectured in the past that, if the speed is observed reaching (in transient) the celerity value, attraction toward the condition commonly identified as “surf-riding” is already in progress. The nonlinear dynamics of this phenomenon has been investigated in depth also theoretically; however, it is unknown whether the stated simple phenomenological detection rule is meaningful also for more natural sea wave profiles. A practical question that prompted the current investigation is whether the duration of attraction to surf-riding could be properly quantified as a percentage of the total time of exposure to a specified wave environment, so that a probability of surf-riding could be eventually calculated. Celerity is defined in the current work as the velocity of propagation of a suitable local property of the wave profile, such as a certain value of slope. For irregular seas, this leads to the concept of instantaneous celerity. Unfortunately, instantaneous celerity, in general, is not a smooth and bounded curve in time. An alternative definition of celerity fitting to the problem of surf-riding is thus investigated, where the propagated points of the profiles are those with the locally maximum slope. Simultaneous treatment of the “wave” and “ship” processes is implemented, and the potential of a condition based on local celerity for surf-riding prediction is examined. Various patterns of behavior before and into surf-riding are observed and discussed. The paper is a step in the direction of developing a probabilistic method of assessment for the dangerous phenomenon of surf-riding and indirectly, for the evaluation of the class of cross-disciplinary phenomena that obey similar nonlinear dynamics.

Research paper thumbnail of Nonlinear surge motions of a ship in bi-chromatic following waves

Communications in Nonlinear Science and Numerical Simulation