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Predicting the wave-induced response in the near-future is of importance to ensure safety of ships. To achieve this target, a possible method for deterministic and conditional prediction of future responses utilizing measured data from the most recent past has been developed. Herein, accurate derivation of the autocorrelation function (ACF) is required. In this study, a new approach for deriving ACFs from measurements is proposed by introducing the Prolate Spheroidal Wave Functions (PSWF). PSWF can be used in two ways: fitting the measured response itself or fitting the sample ACF from the measurements. The paper contains various numerical demonstrations, using a stationary heave motion time series of a containership, and the effectiveness of the present approach is demonstrated by comparing with both a non-parametric and a parametric spectrum estimation method; in this case, Fast Fourier Transformation (FFT) and an Auto-Regressive (AR) model, respectively. The present PSWF-based approach leads to two important properties: (1) a smoothed ACF from the measurements, including an expression of the memory time, (2) a high frequency resolution in power spectrum densities (PSDs). Finally, the paper demonstrates that a fitting of the ACF using PSWF can be applied for deterministic motion predictions ahead of current time. more...

A practical estimation methodology of the mean added resistance in irregular waves is shown, and the present paper provides statistical analyses of estimates for ships in actual conditions. The study merges telemetry data of more than 200 in-service container vessels with ocean re-analysis data from ERA5. Theoretical estimates relying on spectral calculations of added resistance are made for both long- and short-crested waves and are based on a combination of a parametric expression for the wave spectrum and a semi-empirical formula for the added resistance transfer function. The theoretical estimates are compared to predictions from an indirect calculation of added resistance relying on shaft power measurements and empirical estimates of the remaining resistance components. Overall, the comparison reveals a bias in bow oblique waves and higher sea states of the spectral estimates as well as the large variance of the empirically derived predictions — particularly in beam-to-following waves. One of the study’s main findings, confirming previous studies but based on a much larger dataset than in earlier similar studies, is that added resistance assessment based on in-service data is complex due to significant associated uncertainties. more...

This paper is concerned with a machine learning-based approach for sea state estimation using the wave buoy analogy. In-situ sensor data of an advancing medium-size container vessel has been utilized for the prediction of integral sea state parameters. The main novelty of this contribution is the rigorous comparison of time and frequency domain models in terms of accuracy, robustness and computational cost. The frequency domain model is trained on sequences of spectral ordinates derived from cross response spectra, while the time domain model is applied to 5-minute time series of ship responses. Multiple deep neural networks were trained and the sensitivity of individual sensor recordings, sample length, and frequency discretization on estimation accuracy was analysed. An Inception Architecture adapted for sequential data yields the highest out of sample performance in both considered domains. Additionally, multi-task learning was employed, as it is known for increased generalization capability and diminished uncertainty. Overall, it was found that the frequency domain method provides both superior performance and significantly less computational effort for training. more...

Extreme value statistics can often be based on the assumption that exceedance events of a high threshold level are statistically independent and identically distributed (i.i.d. process), which further implies the Poisson assumption to be valid. This makes it possible to express the extreme response statistics through the mean up-crossing rate. For non-linear processes, analytic expressions of the mean up-crossing rate do not in general exist. Reliable statistics of mean up-crossing rate based on the brute-force approach, e.g. Monte Carlo simulation (MCS) require long time domain simulations considering a number of different ensemble input. The associated computations can be very time consuming especially when a detailed physical (e.g. hydrodynamic) model is applied. The First Order Reliability Method (FORM) has previously been found efficient for estimation of extreme value prediction of stationary stochastic time domain processes, However, if the non-linearity in a response is significant, the accuracy of the FORM linearized mean up-crossing rate can be limited. more...

Active monitoring of transverse stability in fishing vessels is paramount due to its significant incidence in operational accidents. Access to systems for automatic detection of changes in vessel’s stability related parameters would better support the crew during fishing and navigation operations. The paper presents an initial experimental validation of a signal-based transverse stability monitoring system, which consists of an estimator-detector kernel that solely uses measurements of roll motion to identify changes in vessel’s metacentric height by estimating the roll natural frequency. Its performance is evaluated based on experimental data from a towing tank scale model test campaign. The proposed transverse stability monitoring system well performs by identifying the potential risks and changes in loading condition. more...

The present paper implements machine learning methods for the prediction of the added-wave resistance of ships in head to beam wave conditions. The study is focused on non-linear regression algorithms namely Random Forests, Extreme Gradient Boosting Machines and Multilayer Perceptrons. The employed dataset is derived from results of three different potential flow methods covering a wide range of operational conditions and 18 hull forms in total. The rational data preprocessing makes up the core part of the paper having its focal point on practical application. Moreover, a rigorous hyperparameter study based on Bayesian optimization is conducted, and the validation of the final models for three case studies against numerical and experimental data as well as two established prediction techniques shows satisfactory generalization in case of the neural network. The tree-based ensemble methods, on the other hand, are not able to generalize sufficiently from the given parameter discretization of the underlying dataset. more...

This article studies a procedure that facilitates short-time, deterministic predictions of the wave-induced motion of a marine vessel, where it is understood that the future motion of the vessel is calculated ahead of time. Such predictions are valuable to assist in the execution of many marine operations (crane lifts, helicopter landings, etc.), as a specific prediction can be used to inform whether it is safe, or not, to carry out the particular operation within the nearest time horizon. The examined prediction procedure relies on observations of the correlation structure of the wave-induced response in study. Thus, predicted (future) values ahead of time for a given time history recording are computed through a mathematical combination of the sample autocorrelation function and previous measurements recorded just prior to the moment of action. Importantly, the procedure does not need input about the exciting wave system, and neither does it rely on off-line training. In the article, the prediction procedure is applied to experimental data obtained through model-scale tests, and the procedure's predictive performance is investigated for various irregular wave scenarios. The presented results show that predictions can be successfully made in a time horizon corresponding to about 8–9 wave periods ahead of current time (the moment of action). more...

This paper presents a novel method for estimating the sea state parameters based on the heave, roll and pitch response of a vessel conducting station keeping automatically by a dynamic positioning (DP) system, i.e., without forward speed. The proposed algorithm finds the wave spectrum estimate from the response measurements by iteratively solving a set of linear equations, and it is computationally efficient. The main vessel parameters are required as input. Apart from this the method is signal-based, with no assumptions on the wave spectrum shape. Performance of the proposed algorithm is demonstrated on full-scale experimental DP data of a vessel in three different sea states at head, bow quartering, beam, stern quartering and following sea waves, respectively. more...

The article presents a practical approach to transform a wave energy spectrum from encounter domain to absolute domain. This problem has its specific relevance, when shipboard sea state estimation is conducted by the wave buoy analogy; notably for some particular implementation solving for the sea state directly in the encounter domain. In this context, the encounter domain is that observed from a ship when it advances in a seaway, whereas the absolute domain is that corresponding to making observations from a fixed point in the inertial frame. Spectrum transformation can be uniquely carried out if the ship sails “against” the waves (beam to head sea) but in following sea conditions there exists no unique solution to the problem. Instead, a reasonable approach valid for practical engineering must be applied, and the article outlines one viable solution that can be used to transform a wave spectrum from encounter to absolute domain. Specifically, two pseudo algorithms are presented, and good performance is achieved with both algorithms when they are tested at different operational scenarios. more...

The international Maritime Organization (IMO) Weather Criterion has proven to be the governing stability criteria regarding minimum metacentric height for e.g., small ferries and large passenger ships. The formulation of the Weather Criterion is based on some empirical relations derived many years ago for vessels not necessarily representative for current new buildings with large superstructures. Thus, it seems reasonable to investigate the possibility of capsizing in beam sea under the joint action of waves and wind using direct time domain simulations. This has already been done in several studies. Here, it is combined with the first order reliability method (FORM) to define possible combined critical wave and wind scenarios leading to capsize and corresponding probability of capsize. The FORM results for a fictitious vessel are compared with Monte Carlo simulations, and good agreement is found at a much lesser computational effort. Finally, the results for an existing small ferry will be discussed in the light of the current weather criterion. 1. Introduction Recently, the International Maritime Organization (IMO) has initiated a thorough revision of the intact stability rules in the framework of goal-based design, e.g., Peters et al. (2013). Several draft guidelines have been issued, e.g., SDC 1/INF.8 (IMO 2013) discussing in details the requirements for the hydrodynamic software to be applied including qualitative and quantitative assessment procedures. The focus so far has been mostly on failure modes related to the change of righting lever in waves, notably parametric rolling and pure loss of stability, whereas the dead ship behavior in beam sea still is based on the existing Weather Criterion issued by IMO (1985) as Resolution A.562. This criterion is based to a large extent on model tests of older hull forms and does not provide any probability of capsize for a given vessel, just a pass/no pass result. Furthermore, the wave environment is not explicitly specified in the criterion thus leading to the same requirement whether the ship is sailing in restricted areas or not. A very detailed and precise description of the drawbacks in the IMO Weather Criterion, as applied to modern ships, is given in Bulian and Francescutto (2004). In a recent study by Tompuri et al. (2015), a detailed investigation on application of the second generation intact stability criteria has been done. However, the criterion for dead ship condition is excluded from this thorough study because the criterion is thought to be still in the early phase of development. more...

Transfer functions are often used together with a wave spectrum for analysis of wave–ship interactions, where one application addresses the prediction of wave-induced motions or other types of global responses. This paper presents a simple and practical method which can be used to tune the transfer function of such responses to facilitate improved prediction capability. The input to the method consists of a measured response, i.e. time series sequences from a given sensor, the 2D wave spectrum characterising the seaway in which the measurements are taken, and an initial estimate of the transfer function for the response in study. The paper presents results obtained using data from an in-service container ship. The 2D wave spectra are taken from the ERA5 database, while the transfer function is computed by a simple closed-form expression. The paper shows that the application of the tuned transfer function leads to predictions which are significantly improved compared to using the transfer function without tuning. more...

Shipboard wave estimation has been of interest in recent years for the purpose of decision support. In this paper, estimation of sea state is studied using ship responses and a parametric description of directional wave spectra. A set of parameters, characterising a given wave spectrum is estimated through an optimisation problem using global search basin with proper constraints. The cost function is established based on the difference between the energies of a set of measured ship responses and the corresponding theoretical spectral moments. A partitioning procedure is applied, which is able to separate swell components from wind seas. more...

Ice sounding radars are able to measure ice sheets by profiling their glaciological features from the surface to the bedrock. The current airborne and, in particular, future space-based systems are suffering from off-nadir surface clutter, which can mask the depth signal of interest. The most recent surface clutter suppression techniques are based on multi-phase-center systems combined with sophisticated coherent postprocessing. The performance of the techniques can be improved by accurate direction-of-arrival (DOA) estimates of the surface clutter. This paper deals with data-driven DOA estimation for surface clutter signals, which includes a formulation of the mathematical foundation of spatial aliasing. DOA estimation is applied to data acquired with the P-band POLarimetric Airborne Radar Ice Sounder at the Jutulstraumen Glacier, Antarctica. The effects of spatial aliasing related to a large phase center spacing are analyzed, and an unwrapping procedure is presented and applied to the data. Finally, DOA estimation of full-scene data is analyzed and used to show an along-track and incidence (off-nadir) angle dependent variation of the effective scattering center of the surface return, which is caused by a varying penetration depth. more...

Short-term variability of ship responses is investigated by cross-spectrum analysis. In a steady state condition, it is well known that a certain length of sampled data is required for stable results of the spectral analysis. However, the phase lag between responses, in terms of the phase angle of the cross-spectra, has not been discussed in detail. Using long stationary time series, the transition of amplitudes and relative phase angles of the cross-spectra has been investigated by iterative analyzes with a few seconds of time shifting. In the results, the short-term variability of the relative phase angle was observed. In effect, the variability may compromise the accuracy of the wave buoy analogy. more...

Fault detection and isolation are very important elements in the design of fault-tolerant decision support systems for ship operator guidance. This study outlines remedies that can be applied for fault diagnosis, when the ship responses are assumed to be linear in the wave excitation. A novel numerical procedure is described for the calculation of residuals using the ship's transfer functions which correlate the wave excitation and the ship responses. As tests, multiplicative faults have artificially been imposed to full-scale motion measurements and it is shown that the developed model is able to detect and isolate all faults. more...

Onboard sea state estimation is relevant for evaluation of ship operations at sea. Means to obtain the sea state at a fixed position include a traditional wave rider buoy, where motion measurements of the buoy are processed to give the (directional) wave spectrum. The analogy between a ship and a buoy is clear, although the ship is moving with a forward speed and, in general, is characterised by a more complex underwater geometry. Thus, it is possible to obtain an estimate of the wave spectrum at the location of an advancing ship by processing its wave-induced responses similar to the situation of a traditional wave rider buoy. The paper studies the ‘wave buoy analogy’, and a large set of full-scale motion measurements is considered. It is shown that the wave buoy analogy gives fairly accurate estimates of integrated sea state parameters when compared to corresponding estimates from real wave rider buoys. The complete distribution of wave energy is also compared, however, with poorer agreement. Finally, it is shown that the wave buoy analogy, for the studied data, provides, on average, slightly better sea state estimates than a wave radar system. more...

The paper studies parametric wave estimation based on the ‘wave buoy analogy’, and data and results obtained from the training ship Shioji-maru are compared with estimates of the sea states obtained from other measurements and observations. Furthermore, the estimating characteristics of the parametric model are discussed by considering the results of a similar estimation concept based on Bayesian modelling. The purpose of the latter comparison is not to favour the one estimation approach to the other but rather to highlight some of the advantages and disadvantages of the two approaches. more...

Most new advanced ships have extensive data collection systems to be used for continuous monitoring of engine and hull performance, for voyage performance evaluation etc. Such systems could be expanded to include also procedures for stress monitoring and for decision support, where the most critical wave-induced ship extreme responses and fatigue damage accumulation can be estimated for hypothetical changes in ship course and speed in the automatically estimated wave environment. The aim of this paper is to outline a calculation procedure for fatigue damage rate prediction in hull girders taking into account whipping stresses. It is conceptually shown how such a method, which integrates onboard estimation of sea states, can be used to deduce decision support with respect to the accumulated fatigue damage in the hull girder. The paper firstly presents a set of measured full-scale wave-induced stress ranges in a container ship, where the associated fatigue damage rates calculated from a combination of the rain-flow counting method and the Palmgren-Miner damage rule are compared with damage predictions obtained from a computationally much faster frequency fatigue analysis using a spectral method. This analysis verifies the applied multi-modal spectral analysis procedure for fatigue estimation for cases where hull girder flexibility plays a role. To obtain an automated prediction method for the fatigue damage rates it is in the second part of the paper shown how a combination of the full-scale onboard acceleration and stress measurements can be used to calculate sea state parameters. These calculated environmental data are verified by a comparison to hindcast data. In the third part of the paper the full-scale fatigue stress ranges are compared to results from an analytical design oriented calculation procedure for flexible ship hulls in short-term estimated sea states. Altogether, it is conceptually shown that by a combination of the onboard estimated sea state parameters with the described analytical fatigue damage prediction procedure a method can be established for real-time onboard decision support which includes estimates of fatigue damage rates. more...

Mean outcrossing rates can be used as a basis for decision support for ships in severe sea. The article describes a procedure for calculating the mean outcrossing rate of non-Gaussian processes with stochastic input parameters. The procedure is based on the first-order reliability method (FORM) and stochastic parameters are incorporated by carrying out a number of FORM calculations corresponding to combinations of specific values of the stochastic parameters. Subsequently, the individual FORM calculation is weighted according to the joint probability with which the specific combination of parameter values is expected to occur, and the final result, the mean outcrossing rate, is obtained by summation. The derived procedure is illustrated by an example considering the forces in containers stowed on ships and, in particular, results are presented for the so-called racking failure in the containers. The results of the procedure are compared with brute force simulations obtained by Monte Carlo simulation (MCS) and good agreement is observed. Importantly, the procedure requires significantly less CPU time compared to MCS to produce mean outcrossing rates. more...

In this paper nonlinear fault detection for in-service monitoring and decision support systems for ships will be presented. The ship is described as a nonlinear system, and the stochastic wave elevation and the associated ship responses are conveniently modelled in frequency domain. The transformation from time domain to frequency domain has been conducted by use of Volterra theory. The paper takes as an example fault detection of a containership on which a decision support system has been installed. more...

An estimate of the on-site wave spectrum can be obtained from measured ship responses by use of Bayesian modelling, which means that the wave spectrum is found as the optimum solution from a probabilistic viewpoint. The paper describes the introduction of two hyperparameters into Bayesian modelling so that the prior information included in the modelling is based on two constraints: the wave spectrum must be smooth directional-wise as well as frequency-wise. Traditionally, only one hyperparameter has been used to control the amount of smoothing applied in both the frequency and directional ranges. From numerical simulations of stochastic response measurements, it is shown that the optimal hyperparameters, determined by use of ABIC (a Bayesian Information Criterion), correspond to the best estimate of the wave spectrum, which is not always the case when only one hyperparameter is included in the Bayesian modelling. The paper includes also an analysis of full-scale motion measurements where wave spectra estimated by the Bayesian modelling are compared with results from ocean surface measurements by satellite and from a wave radar. The agreement is found to be reasonable. more...

Reliable estimation of the on-site sea state parameters is essential to decision support systems for safe navigation of ships. The wave spectrum can be estimated from procedures based on measured ship responses. The paper deals with two procedures—Bayesian Modelling and Parametric Modelling—which both use complex-valued frequency response functions (FRF) to estimate the wave spectrum. It is therefore interesting to investigate how the filtering aspect, introduced by FRF, affects the final outcome of the estimation procedures. In order to do this, extensive numerical simulations—with known wave parameters—are carried out for a large container vessel. The study shows that filtering has an influence on the estimations, since high-frequency components of the wave excitations are not estimated as accurately as lower frequency components. more...

This paper presents a trend analysis for prediction of sea state parameters onboard ships during voyages. Given those parameters, a JONSWAP model and also the transfer functions, prediction of wave induced ship responses are thus made. The procedure is tested with full-scale data of an in-service container ship. Comparison between predictions and the actual measurements, implies a good agreement in general. This method can be an efficient way to improve decision support onboard ships. more...

Airborne radar ice sounding is challenged by surface clutter masking the depth signal of interest. Surface clutter may even be prohibitive for potential space-based ice sounding radars. To some extent the radar antenna suppresses the surface clutter, and a multi-phase-center antenna in combination with coherent signal processing techniques can improve the suppression, in particular if the direction of arrival (DOA) of the clutter signal is estimated accurately. This paper deals with data-driven DOA estimation. By using P-band data from the ice shelf in Antarctica it is demonstrated that a varying penetration depth influences the DOA. more...

In general, two main concepts can be applied to estimate the on-site directional wave spectrum on the basis of ship response measurements: (1) a parametric method which assumes the wave spectrum to be composed by parameterised wave spectra; or (2) a non-parametric method where the directional wave spectrum is found directly as the values in a completely discretised frequency-directional domain without a priori assumptions on the spectrum. The paper outlines the theory of these two concepts, and it is shown how to deal with the speed-of-advance problem for operating ships. In addition, the methods include an equivalence of energy in the governing equations and, as regards the parametric concept, a frequency-dependent spreading of the waves is introduced. The paper includes an extensive analysis of full-scale measurements for which the directional wave spectra are estimated by the two ship response-based methods. Hence, comparisons are made between these estimates and, moreover, the agreement with the corresponding directional wave spectra produced by the wave radar system WAVEX is studied. The agreement between the two methods is reasonable, as well is the agreement between the results of these methods and those of WAVEX. It is difficult to propose one of the ship response-based methods in favour of the other, since they perform equally well. more...

An accurate estimation of the ocean wave directional spectrum at the location of an advancing ship is very useful for the ship master to improve operation and safety in a seaway. Research has been conducted to obtain sea state estimates by the Wave Buoy Analogy. The method deals with processing the ship’s wave-induced responses based on different statistical inferences including parametric and non-parametric approaches. This paper considers a concept to improve the estimate obtained by the parametric method for sea state estimation. The idea is illustrated by an analysis made on full-scale response measurements carried out on a 9400 TEU container ship. The process of fitting a standard JONSWAP spectrum takes into account an analysis of residuals formed by the difference between measured and estimated amount of energy of the response(s). A couple of motion responses and also bending moment are considered as the input of the estimation process. A comparison is made between the results and also with some inhand outputs from other estimation sources, e.g., wave radar measurements and sea surface elevation by microwave sensors. The discussed and analyzed procedure could also lead to an automatic selection of response combinations for sea state estimation. more...

Roll stabilisation of motorised vessels plays an important part in reducing passenger discomfort and increasing safety and cargo capacity. Passive free surface tanks are considered a low-cost stabilising method, which is efficient at all speeds without increasing hull resistance. In this study, a mathematical model for evaluating the performance of a passive free surface tank is established. This is done by coupling a roll model to a fluid flow model. As a numerical example, the seakeeping abilities of a container vessel are evaluated. The necessary methods for performing the simulation are presented and the design of a free surface tank is explained. The effects of the passive free surface tank are evaluated and a significant damping effect is observed, particularly in cases with resonant roll. more...

In Denmark, the maritime engineering competences requested by the industry have changed in the past one to two decades. The typical naval architects do no longer find them selves working in the ship-building industry but rather in the industry of ship operators, consultancies, class societies, etc. This means that universities educating maritime engineers need to reflect the changes in the curricula for their maritime engineering students. Topics and issues regarding this matter have recently been addressed in a survey made in the Danish maritime industry. The survey concludes that the demand for maritime engineers in the industry is considerably larger than the output from the technical universities. Moreover, it sets forth a series of recommendations to the industry as well as to the universities to facilitate meeting the demand for maritime engineers in Denmark in the future. The recommendations are outlined together with work commenced at the Technical University of Denmark (DTU) to update the curricula for DTU’s maritime engineering students. Thus, DTU offers an education reflecting a large share of the recommendations in the curricula.Copyright © 2012 by ASME more...

In February 2011, the Polarimetric Airborne Radar Ice Sounder (POLARIS) was flown in Antarctica in order to assess the feasibility of a potential space-based radar ice sounding mission. The campaign has demonstrated that the basal return is detectable in areas with up to 3 km thick cold ice, in areas with up to several hundred meters thick warm shelf ice, and in areas with up to 700 m thick crevassed glacier ice. However, major gaps in the basal return are observed, presumably due to excessive absorption, scattering from ice inclusions in the firn, low basal reflectivity, and the masking effect of the surface clutter. Internal layers are observed down to depths exceeding 2 km. The polarimetric data show that the internal layers are strongly anisotropic at a ridge, where the ice flow is supposed to be highly unidirectional. In case of space-based ice sounding, the antenna pattern cannot offer sufficient surface clutter suppression, but improved clutter suppression has been demonstrated with novel multi-phase-center techniques. more...

This article proposes a method to estimate a ship’s relative heading against the waves. The procedure relies purely on ship-board measurements of global responses such as motion components, accelerations and the bending moment amidships. There is no particular (mathematical) model connected to the estimate, and therefore it is called a ‘blind estimate’. The approach is in this introductory study tested by analysing simulated data. The analysis reveals that it is possible to estimate a ship’s relative-heading on the basis of shipboard measurements only. more...

The paper deals with estimation of sea state parameters on the basis of time histories of ship responses. The focus is on the Bayesian estimation concept, where the outcome is controlled by a set of hyperparameters, which theoretically must be optimised to provide the optimum solution in terms of sea state parameters. The paper looks into the possibility of fixing the hyperparameters since this will increase the computational efficiency of the method. Sensitivity studies with respect to the hyperparameters are made for both synthetic data and full-scale data.Copyright © 2010 by ASME more...

The paper presents results of the wave buoy analogy applied to synthetic data including measurement noise (white noise). The wave buoy analogy is of particular interest in the context of on-site wave estimations to be used in onboard decision support systems. In the study, Bayesian modelling is introduced as the underlying methodology. The analysis concerns an LNG vessel with forward speed and estimations are made of a rather comprehensive set of wave excitations. In general, reasonable agreements with the true wave parameters are observed. The paper forms parts of the background of another paper, Nielsen and Iseki [17], submitted also to OMAE 2010.Copyright © 2010 by ASME more...

The paper elaborates on the probabilistic assessment of a simplified model for the rolling of a ship in a stochastic seaway. The model can be easily integrated with a probabilistic tool which enables evaluations of numerical simulations by the first order reliability method (FORM) and by Monte Carlo simulation (MCS). Results are presented for synchronous roll as well as parametric roll, where e.g. mean outcrossing rates have been calculated. FORM offers an efficient approach for the computations, although the approach should be applied with care in cases of parametric roll. The paper also touches on issues such as ergodicity and transient versus stationary stages in the roll realisations.Copyright © 2009 by ASME more...

Accurate position measurements are extremely valuable in the shipping industry for various reasons such as safety (collision avoidance), fuel saving (weather identification), punctuality (route prediction), etc. Although GNSS (Global Navigation Satellite System) receivers installed on-board the ships are proven to be highly accurate, the data collection process may occasionally be problematic, mainly due to the complexity of the measurements and the decimal precision that is required. Data was collected from 3 years of operations of 228 Maersk Line container vessels and an analysis reveals that there is a substantial amount (≈20%) of historical position measurements sent to shore that does not reflect reality. With this study, the authors initially identify the source of the faulty logged position measurements, then they categorize them into segments and finally they apply an interpolation methodology in order to validate and correct them by using AIS (Automatic Identification System) data. more...

In the present study, model drift – mostly driven by biofouling – is mitigated by applying continual learning and the hydrodynamic performance of a large LNG tanker trading in worldwide service is assessed in parallel. The available auto-logged sensor data spans five years including two dry docking intervals. An adaptive training methodology of an artificial neural network is established, and the drift score is derived from simulated data under sea trial conditions. In addition, hindcast metocean data from ERA5 is included in the model’s feature space for capturing environmental conditions. Crucially, it was found that the obtained drift estimate is in general accordance to ISO 19030 results, proving the method’s validity. Still, the methodology itself is subject to considerable uncertainty. Finally, limitations and possible extensions of the proposed methodology are presented. more...

Real time estimation of waves and ship responses using onboard measurements has been under investigation in recent years. This has been done using different methods, including parametric and non-parametric models. Since none of the methods are believed to be fully accurate, it is important to assign an uncertainty measure to the waves and responses that are being estimated. In this paper, a parametric model approach based on moments of responses is considered for wave estimation. A method based on linear error propagation is introduced to assess the uncertainty of wave estimations. The uncertainty of response calculation based on the estimated wave is also quantified. more...

Complex marine operations are moving further from shore, into deeper waters, and harsher environments. The operating hours of a vessel are weather dependent, and good knowledge of the prevailing weather conditions may ensure cost-efficient and safe operations. This paper considers the estimation of the peak wave frequency of the on-site sea state based on the vessel’s motion in waves. A sea state can be described by significant wave height, peak wave frequency, wave direction, and often wind speed and direction are added as well. The signal-based algorithm presented in this paper is based on Fourier transforms of the vessel response in heave, roll and pitch. The measurements are used directly to obtain an estimate of the peak frequency of the waves. Experimental results from model-scale offshore ship runs at the Marine Cybernetics Laboratory (MCLab) at NTNU demonstrate the performance of the proposed sea state estimation algorithm. © 2016 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works. more...

Onboard decision support systems (DSS) are used to increase the operational safety of ships. Ideally, DSS can estimate — in the statistical sense — future ship responses on a time scale of the order of 1–3 hours taking into account speed and course changes. The calculations depend on both operational and environmental parameters that are known only in the statistical sense. The present paper suggests a procedure to incorporate random variables and associated uncertainties in calculations of outcrossing rates, which are the basis for risk-based DSS. The procedure is based on parallel system analysis, and the paper derives and describes the main ideas. The concept is illustrated by an example, where the limit state of a non-linear ship response is considered. The results from the parallel system analysis are in agreement with corresponding Monte Carlo simulations. However, the computational speed of the parallel system analysis proved slower than expected. Moreover, it is important that the failure surface of the limit state is smooth, otherwise the parallel system analysis may not be applicable.Copyright © 2008 by ASME more...

It is possible to obtain estimates of the sea state at the specific position of an advancing vessel by processing measurements of the vessel’s wave-induced responses. The analogy to a wave rider buoy is clear, although the situation of an advancing ship is more complex due to forward speed. The paper studies the ‘wave buoy analogy’, and a large set of full-scale motion measurements is considered. It is shown that the wave buoy analogy gives fairly accurate estimates of sea state parameters when compared to estimates from real wave rider buoys. more...

Short-term variability of ship responses is investigated by cross-spectrum analysis. In a steady state condition, it is well-known that a certain length of sampled data is required for stable results of the spectral analysis. However, the phase lag between responses, in terms of the phase angle obtained from the (coupled) cross-spectra, has not been discussed in detail. In a previous but similar study, the authors pointed out that the short-term variability of the relative phase angle of the cross-spectra is harmful to sea state estimation using the wave buoy analogy, i.e. ship as a wave buoy. In the present report, an investigation based on the multi-variate auto-regressive model method has been applied to the same data. In the results, it has been observed that the short-term variability of the relative phase angle can be reduced. more...

Short-term variability of ship responses is investigated by cross-spectrum analysis. In a steady state condition, it is well known that a certain length of sampled data is required for stable results of the spectral analysis. However, the phase lag between responses, in terms of the phase angle of the cross-spectra, has not been discussed in detail. Analyzing long stationary time series by direct Fourier transform, the authors pointed out that the short-term variability of the relative phase angle of the cross-spectra was very harmful to sea state estimation of the wave buoy analogy. In this report, the same investigation based on the correlation (Blackman-Tukey) method was applied to the same data. In the results, it was observed that the short-term variability of the relative phase angle was reduced to a certain extent. more...

The reliable and accurate determination of prevalent sea state conditions during a vessel’s passage is of high scientific and practical importance not only due to crew and cargo safety, but also due to increasing energy efficiency demands. The present paper deals with the prediction of integral sea state parameters using 5-minute time series samples of wave-induced ship motions and builds upon deep neural networks. The study is based on synthetic motion histories of heave, pitch and roll of a container vessel generated from transfer functions and a parametric wave spectrum. The multi-dimensional search space of the four free variables: significant wave height, zero-upcrossing period, relative wave direction and advance speed, is discretized by Latin Hypercube Sampling. In order to comply with the assumption of moderate wave steepness, an empirical wave breaking constraint is incorporated into the spectral simulation procedure. Both a residual network and a multichannel convolutional LSTM (Long Short Term Memory) network are employed and compared to a state of the art architecture. In the frequency domain, a multilayer perceptron is used for comparison and trained on features obtained from cross-spectral analysis. The results of the simulation-based study show sufficient accuracy and generalization for all models in both time and frequency domain. Lastly, limitations of the present study are pointed out and it stressed that the applicability of the presented methodology to real in-situ measurement data is an important part of future work. more...

This paper deals with uncertainty modelling of wave-vessel transfer functions used to calculate or predict wave-induced responses of a ship in a seaway. Although transfer functions, in theory, can be calculated to exactly reflect the behaviour of the ship when exposed to waves, uncertainty in input variables, notably speed, draft and relative wave eading, often compromises results. In this study, uncertling is applied to improve theoretically calculated transfer functions, so they better fit the corresponding experimental, full-scale ones. Based on a vast amount of full-scale measurements data, it is shown that uncertainty modelling can be successfully used to improve accuracy (and reliability) of theoretical transfer functions. more...

Reliable estimation of the on-site sea state parameters is essential to decision support systems for safe navigation of ships. The sea state parameters can be estimated by Bayesian Modelling which uses complex-valued frequency response functions (FRF) to estimate the wave spectrum on the basis of measured ship responses. It is therefore interesting to investigate how the filtering aspect, introduced by FRF, affects the final outcome of the estimation procedures. The paper contains a study based on numerical generated time series, and the study shows that filtering has an influence on the estimations, since high frequency components of the wave excitations are not estimated as accurately as lower frequency components. Moreover, the paper investigates how the final outcome of the Bayesian Modelling is influenced by the accuracy of the FRF. Thus, full-scale data is analysed by use of FRF calculated by a 3-D time domain code and by closed-form (analytical) expressions, respectively. Based on comparisons with wave radar measurements and satellite measurements it is seen that the wave estimations based on closed-form expressions exhibit a reasonable energy content, but the distribution of energy appears to be incorrect.Copyright © 2007 by ASME more...