potential flow theory

Cilj ovog rada je ispitati racunske neizvjesnosti primjenom razlicitih postupaka analize pouzdanosti brodskog trupa. Za modeliranje granicne cvrstoce brodskog trupa pod zajednickim djelovanjem uzdužnih momenata savijanja na mirnoj vodi i na valovima, kako u progibu tako i u pregibu, preuzete se preporuke Međunarodne organizacije za pomorstvo (IMO) za ocjenu pouzdanosti. Primjer u clanku razmatra tipicnu službu tankera za kemikalije za tek izgrađeni brod i za trup izložen hrđanju. U clanku su uspoređena tri postupka analize pouzdanosti: Metoda prvog reda sa statistickim momentima drugog reda (FOSM), Napredna metoda prvog reda (AFORM) i Monte Carlo simulacija (MCS). U zakljuccima se rezimiraju podaci usporedbi primijenjenih numerickih postupaka za proracun pouzdanosti na elektronickim racunalima te se raspravljaju odgovarajuci postupci za ocjenu sigurnosti brodskog trupa.

Oscillating water column (OWC) devices and oscillating buoys (OBs) are two of the main types of wave energy converters (WECs). In this paper a hybrid oscillating water column-oscillating buoy wave energy converter is proposed, which we have named OWCOB. The oscillating buoy is hinged at the outer wall of the oscillating water column. As waves propagate through the OWCOB, the water column within the OWC chamber moves up and down, producing air flow to propel a turbine. Meanwhile, the oscillation of the OB drives a separate hydraulic system. To solve the wave diffraction and radiation problems of the OWCOB and investigate its energy capture performance, an analytical model is developed based on linear potential flow theory and the eigenfunction matching method. Assuming that the PTOs of the OWC and OB are both linear, the wave power extraction of the OWCOB is evaluated in the frequency domain. Of the two configurations considered, the OWCOB with the OWC opening waveward and the OB hinged leeward is found to have a broader primary frequency band of wave power capture compared to the OWCOB with the OWC opening and the OB on the same side. Further, a thorough sensitivity analysis of power capture is carried out considering the main design parameters (size and submergence of the OWC opening, distance between the OWC and the OB, OB hinge elevation, OB radius), which can form the basis of an optimisation study for a particular wave climate. Importantly, we find that the OWCOB performs generally better than stand-alone OWCs and OBs, not least in terms of frequency bandwidth.

Hydrodynamic analysis is one of the key steps in safety assessment of a structure in waves. Many options are available for answering challenge raised from marine and offshore energy industry, from costly three dimensional CFD to the efficient but not perfect boundary element models. Focus on the boundary element methods, analysis methods for the interaction of waves and structures are discussed. Those boundary element models cover frequency domain and time domain, linear and non-linear. Special attention is pay to the problems encountered in those models and approaches we adopted for their engineering solution.

The study presents an experimental investigation to obtain motion and load measurements of an intact and damaged frigate model in waves. The experimental measurements will show the changes in motion and hull girder loading when a ship hull is damaged and form benchmarking data.

Results are presented of a benchmark and uncertainty assessment study organised by the MARSTRUCT Virtual Institute on global linear wave loads on damaged ship. The study has two aims: to acquire valuable information regarding damage modelling in seakeeping analysis of damaged ships and to contribute to a rational approach for definition of the model uncertainty of linear seakeeping tools. Eight institutes participated in the benchmark, with six codes, representative of important linear seakeeping theories in use nowadays. The benchmark ship is the DTMB 5415 hull, with well documented and accessible data to perform seakeeping analysis and experimental results for motion and global wave loads. The uncertainty analysis is performed using the Frequency Independent Model Error as the uncertainty measure. The analysis is performed for vertical motions, vertical and horizontal global wave load components, and for torsional moments. Uncertainty measures of individual motion and load predictions are presented and compared. In addition, a comparative analysis of linear seakeeping theories is performed and the accuracy of the simplified methods used for the prediction of seakeeping of a damaged ship is assessed. Finally, recommendations are provided for efforts to improve modelling uncertainties in transfer functions of wave loads on damaged ships.

After damage rapid and accurate guidance is necessary to safely recover vessels. An improvement in the understanding of ship behaviour after damage will increase chances of recovery of the vessel and safety for the ship crew and cargo, with corresponding benefits to the maritime environment. Whilst a number of studies have investigated static behaviour of damaged ships with permeability previous research at forward speed considers the damaged compartment to be empty. Novel aspects of this research include the study of a variety of permeabilities and internal arrangements of the damaged compartment for a ship moving in waves. Results from experiments carried out in a towing tank using a segmented ship model show that permeability appears to have a large effect on the pitch and heave motion responses when the vessel is travelling at forward speed, with reduced effect for a stationary ship. Furthermore, results indicate that internal arrangement of compartments has less effect than compartment permeability on the damaged ship responses. Finally, the influence of damaged orifice size on the motion responses is investigated and the results show that at forward speed the responses are significantly affected and that they are largest for the minimum orifice size tested.

The aim of this paper is to investigate the computational uncertainties by applying different methods in reliability analysis of ship hull girder. The International Maritime Organization (IMO) proposals for hull girder reliability assessment are undertaken for probabilistic modelling of ship ultimate strength under combined longitudinal still water and wave bending moments both for hogging and sagging conditions. The example in the paper considers a typical service profile of a chemical tanker in as-built state and in corroded hull condition. The three methods of reliability analysis are compared in the paper: the First Order Second Moment (FOSM), the Advanced First Order Reliability Method (FORM) and the Monte Carlo Simulation (MCS). The conclusions resume the results of comparison of applied methods and discuss the appropriateness of numerical reliability methods on digital computers in ship hull girder safety assessments.

Simones for providing me with the continued guidiance, support, contributions and encouragement throughout my program in the MECE Department of MNSU. Thanks to the Xcel Energy Research Development Fund (RDF) for sponsoring this work. This research would not be possible without this funding. Special thanks to my Kevin Schull for providing critique and engaging discussion during this research; and to Brian Graybill of Nortek Air Solutions, for providing vital industry insights and advice into making this project a success. I am deeply grateful to have the best supporters in the world, my parents; Bassey and Dorathy Umoetok; for providing immense love and support throughout the course of my graduate studies. The support of my siblings, Joshua, Benjamin, Uboho, Sharon and Queen is also highly appreciated including helping to proofread my work. My appreciation also goes to my friends and colleagues, Ramiro, Fahim, Seun, Yacouba, Jeremiah, Devin, Heidi, Joshua, Jon and others too numerous to mention, for valuable interactions, warm friendship and the great times we have shared.

Ship motions after damage are difficult to evaluate since they are affected by complex phenomena regarding fluid and structures interactions. The possibility to better understand how ship behavior in damage is influenced by these phenomena is important for improving ship safety, especially for passenger vessel. In this paper an experimental campaign is carried out on a passenger ferry hull, to show the effects of the water dynamics across damage openings on ship motions. Novel aspects of this research include the study of the effects of the damage position on the ship roll response. The study is carried out for still water and for beam regular waves at zero speed. Results from the experiments carried out underline that the roll behavior of a damaged ship is affected by the position of damage opening and not only by its size. Assuming the same final equilibrium conditions after flooding but characterized by different damage openings it is possible to observe how motions RAOs and roll decay characteristics modify according to the opening locations.

Simones for providing me with the continued guidiance, support, contributions and encouragement throughout my program in the MECE Department of MNSU. Thanks to the Xcel Energy Research Development Fund (RDF) for sponsoring this work. This research would not be possible without this funding. Special thanks to my Kevin Schull for providing critique and engaging discussion during this research; and to Brian Graybill of Nortek Air Solutions, for providing vital industry insights and advice into making this project a success. I am deeply grateful to have the best supporters in the world, my parents; Bassey and Dorathy Umoetok; for providing immense love and support throughout the course of my graduate studies. The support of my siblings, Joshua, Benjamin, Uboho, Sharon and Queen is also highly appreciated including helping to proofread my work. My appreciation also goes to my friends and colleagues, Ramiro, Fahim, Seun, Yacouba, Jeremiah, Devin, Heidi, Joshua, Jon and others too numerous to mention, for valuable interactions, warm friendship and the great times we have shared.

Results are presented of a benchmark and uncertainty assessment study organised by the MARSTRUCT Virtual Institute on global linear wave loads on damaged ship. The study has two aims: to acquire valuable information regarding damage modelling in seakeeping analysis of damaged ships and to contribute to a rational approach for definition of the model uncertainty of linear seakeeping tools. Eight institutes participated in the benchmark, with six codes, representative of important linear seakeeping theories in use nowadays. The benchmark ship is the DTMB 5415 hull, with well documented and accessible data to perform seakeeping analysis and experimental results for motion and global wave loads. The uncertainty analysis is performed using the Frequency Independent Model Error as the uncertainty measure. The analysis is performed for vertical motions, vertical and horizontal global wave load components, and for torsional moments. Uncertainty measures of individual motion and load predictions are presented and compared. In addition, a comparative analysis of linear seakeeping theories is performed and the accuracy of the simplified methods used for the prediction of seakeeping of a damaged ship is assessed. Finally, recommendations are provided for efforts to improve modelling uncertainties in transfer functions of wave loads on damaged ships.

This work is focused on experimental investigation of the hull girder loads on an intact and damaged naval ship DTMB 5415 at zero speed. The experimental campaign was carried out in head and beam regular waves at the University of Strathclyde. The effect of the use of moorings in the model experimental setup was investigated in the context of loads assessment, and the moorings are shown to influence the measured hull girder loads at some wave frequency compared to the free drift case. Therefore the tests in beam seas are performed with free drifting model while the moored model setup was adopted for head seas. The results for ship motions are compared with those from a previous campaign giving an insight into repeatability and uncertainty of measurements. The roll decay of the ship in both intact and damaged conditions is analysed and the linear and quadratic extinction coefficients for the model and the ship scale are reported and detailed discussion on intact-versus-damaged ship roll damping behaviour is given. The results for the hull girder loads are presented for intact and damaged ship. An investigation of the nonlinear effects due to wave height variation in the range wave height to wave length from 1/50 to 1/22 on shear force and bending moment was carried out for a range of wave lengths to ship length ratios from 0.8 to 1.4. The results of the extensive campaign are compared against similar experimental studies forming a benchmark for validation of numerical methods.

This paper discusses uncertainties related to the prediction of loads and responses for ocean and offshore structures in accordance with the findings by the Ocean Engineering Committee of the International Towing Tank Conference (ITTC). The parameters that may cause uncertainties in ocean engineering model tests, full-scale tests and numerical simulations are presented in terms of physical properties of the fluid, initial conditions, model definition, environment, scaling, instrumentation and human factors. Emphasis is given to the uncertainty sources in model tests involving deepwater mooring lines, risers and dynamic positioning systems and the need for quantifying them. A methodology for uncertainty analysis is described according to the International Organization for Standardization (ISO) Guidance for Uncertainties in Measurement (GUM). As an example of application, the combined and expanded uncertainties in the model tests of a moored semi-submersible platform were assessed and quantified in terms of motion responses, air gap and mooring line tensions. It is concluded that the quantification of uncertainties may be challenging in model tests and numerical simulations of ocean and offshore structures. It is particularly challenging in extrapolating model test results to full scale and utilizing complex numerical models, especially if the effects of hydrodynamic nonlinearities are significant.

This paper contains a description of a numerical model for calculating behaviour of ships in waves. There are many models available, but the one described here can be characterised with a set of parameters that have a decisive impact on the final values of roll motion amplitude and frequency. In this paper, it is shown how a fitting of a standard-shape hull characterised by certain readily available parameters affects the final roll and frequency of the motion. In addition, calculations for a flooded tank were made, and a range of results for the maximum dynamic heeling forces from this tank is shown. This calculation can further be verified for a range of hull dimensions and geometries to present a viable method to the industry.

Studies on motion response of a vessel is of great interest to researchers, since a long time. But intensive researches on stability of vessel during motion under dynamic conditions are few. A numerical model of vessel is developed and responses are analyzed in head, beam and quartering sea conditions. Variation of response amplitude operator (RAO) of vessel based on Strip Theory for different wave heights is plotted. Validation of results was done experimentally and numerical results was considered to obtain effect of damping on vessel stability. A scale model ratio of 1:125 was used which is suitable for dimensions of wave flume at National Institute of Technology Calicut. Stability chart are developed based on Mathieu"s equation of stability. Ince-Strutt chart developed can help to capture variations of stability with damping.

Although the coupled horizontal-torsional vibrations of open ships have been investigated numerically for decades, the available experimental data in oblique seas seem rare. Model tests, considering natural frequencies of bending and torsional modes, have been conducted by the Centre for Ships and Ocean Structures (CeSOS) in the towing tank and ocean basin. A flexible backbone model was designed with five cutouts on the top side of the aluminum beam to realize approximately torsional stiffness as well as vertical and horizontal bending stiffness. This paper mainly deals with measured bending and torsional vibrations in regular and irregular waves. The damping ratios, mode shapes, and modal moments were derived from experimental data, and a numerical model based on modal superimposition is established according to the measured hydroelastic properties. Some time-domain simulations are carried out considering the structural characteristics, and compared with measured results. The test data in regular waves are presented to show the possible factors of influence on the vibrations. The effect of bending and torsional vibrations on the extreme response values in irregular waves is estimated. The uncertainties in the experiments are discussed and conclusions are presented at the end of this paper.

In the paper, were presented approximations of numerical calculations of ship’s roll on thebasis of main service parameters of the ship. This way were obtained several relationshipswhich make it possible to approximate ship roll in regular and irregular waves by usingthe parameters available in the phase of voyage routing. The relationships were elabora-ted by means of artificial neural networks as well as linear and non-linear regressionmethods. A comparative analysis of the methods regarding approximation accuracyagainst standard data was also performed.Keywords : ship's roll, regular wave, irregular wave, approximation, neural networks, regression methodsIn this paper an attempt to solve the problem has been un-dertaken, aimed at elaboration of a simplified but exact modelof predicting ship’s roll on the basis of main ship service para-meters. METHOD The research in question was limited to approximation ofship’s roll in regular and irregular waves coming from the di-rection p...

Failure of ship hulls may be a result of a combination of various factors. Residual stress caused by welding of the stiffeners on to the steel plates is one of the contributing factors to the failure. This study was completed for a more in-depth look at the residual stress distribution found at a typical weld-stiffener connection of ship hulls. Six small specimens were built and the sizing of the specimens was designed so that they could be accommodated in the test facility available at the Canadian Neutron Beam Centre in the Chalk River Laboratories. The specimens were made out of 9.53 mm thick plate of 350 WT grade structural steel stiffened by L127x76.2x9.53 angle stiffeners. The non-destructive neutron diffraction method was used to collect strain data at locations within the volume of the specimens. The method of neutron diffraction uses the crystal lattice of the sample material as an internal strain gauge. The test results were analyzed to determine the distribution of all th...

The prediction of wave-induced motions and loads is of great importance for the design of marine structures. Linear potential flow hydrodynamic models are already used in different parts of the ship design development and appraisal process. However, the industry demands for design innovation and the possibilities offered by modern technology imply the need to also understand the modelling assumptions and associated influences of nonlinear hydrodynamic actions on ship response. At first instance, this paper presents the taxonomy of fluid structure interaction methods of increasing level of sophistication that may be used for the assessment of ship motions and loads. Consequently, it documents in a practical way the effects of weakly nonlinear hydrodynamics on the symmetric wave-induced responses for a 10,000 TEU Container ship. It is shown that weakly nonlinear fluid structure interaction models may be useful for the prediction of symmetric waveinduced loads and responses of such ship not only in way of amidships but also at the extremities of the hull. It is concluded that validation of hydrodynamic radiation and diffraction forces and their respective influence on ship response should be especially considered for those cases where the variations of the hull wetted surface in time may be noticeable.

The prediction of wave-induced motions and loads is of great importance for the design of marine structures. Linear potential flow hydrodynamic models are already used in different parts of the ship design development and appraisal process. However, the industry demands for design innovation and the possibilities offered by modern technology imply the need to also understand the modelling assumptions and associated influences of nonlinear hydrodynamic actions on ship response. At first instance, this paper presents the taxonomy of fluid structure interaction methods of increasing level of sophistication that may be used for the assessment of ship motions and loads. Consequently, it documents in a practical way the effects of weakly nonlinear hydrodynamics on the symmetric wave-induced responses for a 10,000 TEU Container ship. It is shown that weakly nonlinear fluid structure interaction models may be useful for the prediction of symmetric waveinduced loads and responses of such ship not only in way of amidships but also at the extremities of the hull. It is concluded that validation of hydrodynamic radiation and diffraction forces and their respective influence on ship response should be especially considered for those cases where the variations of the hull wetted surface in time may be noticeable.

Accurate prediction of ship structural response is a key element in the design stage. The conventional design philosophy of predicting loads using empirical relations or first principal based calculation is very well proven. But in case of novel/complex ship structure, conventional methods may not be able to predict the accurate structural response behaviour. In this context, various classification society rules/standards contain an advance framework to deal with such complex designs. However, to assess the structural strength in veridical manner, it is essential to perform the structural analysis using direct hydrodynamic loads. But, this kind of analysis is not straight forward and only few guidelines explained the methodology. Present paper outlines the available numerical methods for hydrodynamic load computation and its applications in Finite Element (FE) analysis of ships. It proposes a direct calculation procedure of determining structure response of ship which can be utilize...

The purpose of the paper is to compare two simplified methods that may be used for modeling wave-induced motion and resulting structural loads of ship damaged in collision or grounding accident. The first method is the added mass method, in which it is assumed that the mass of the flooded seawater becomes integral part of the ship mass and moves with the ship. The second approach is the lost buoyancy method, where structure of damaged tanks and all of its contents are removed from the vessel. The former method is applicable for small damages while the latter is more suited for large damage extents. Response amplitude operators of vertical motions and hull girder vertical wave bending moments at amidships are calculated by the state-of-the-art linear 3D panel hydrodynamic code and compared to those for intact ship as the effect of damage is often neglected in computation of motions and wave loads of damaged vessel. An attempt of verification of procedures is done by comparison with seakeeping experiments on damaged warship, described in available literature.

The article introduces a spectral procedure for sea state estimation based on measurements of motion responses of a ship in a short-crested seaway. The procedure relies fundamentally on the wave buoy analogy, but the wave spectrum estimate is obtained in a direct-brute-force-approach, and the procedure is simple in its mathematical formulation. The actual formulation is extending another recent work by including vessel advance speed and short-crested seas. Due to its simplicity, the procedure is computationally efficient, providing wave spectrum estimates in the order of a few seconds, and the estimation procedure will therefore be appealing to applications related to realtime, onboard control and decision support systems for safe and efficient marine operations. The procedure's performance is evaluated by use of numerical simulation of motion measurements, and it is shown that accurate wave spectrum estimates can be obtained for all wave directions in short-crested waves, taking the wave system to be composed by both wind generated sea and swell. Furthermore, the procedure is tested using full-scale motion data obtained from sea trials. Good wave estimations are achieved as compared to corresponding results from a free-floating (classical) wave buoy.

This paper discusses uncertainties related to the prediction of loads and responses for ocean and offshore structures in accordance with the findings by the Ocean Engineering Committee of the International Towing Tank Conference (ITTC). The parameters that may cause uncertainties in ocean engineering model tests, full-scale tests and numerical simulations are presented in terms of physical properties of the fluid, initial conditions, model definition, environment, scaling, instrumentation and human factors. Emphasis is given to the uncertainty sources in model tests involving deepwater mooring lines, risers and dynamic positioning systems and the need for quantifying them. A methodology for uncertainty analysis is described according to the International Organization for Standardization (ISO) Guidance for Uncertainties in Measurement (GUM). As an example of application, the combined and expanded uncertainties in the model tests of a moored semi-submersible platform were assessed and quantified in terms of motion responses, air gap and mooring line tensions. It is concluded that the quantification of uncertainties may be challenging in model tests and numerical simulations of ocean and offshore structures. It is particularly challenging in extrapolating model test results to full scale and utilizing complex numerical models, especially if the effects of hydrodynamic nonlinearities are significant.

This paper contains a description of a numerical model for calculating behaviour of ships in waves. There are many models available, but the one described here can be characterised with a set of parameters that have a decisive impact on the final values of roll motion amplitude and frequency. In this paper, it is shown how a fitting of a standard-shape hull characterised by certain readily available parameters affects the final roll and frequency of the motion. In addition, calculations for a flooded tank were made, and a range of results for the maximum dynamic heeling forces from this tank is shown. This calculation can further be verified for a range of hull dimensions and geometries to present a viable method to the industry.

Jacket platforms are life bloods of oil-rich countries and one of the well-studied offshore structures subjected to environmental loads. In the Persian Gulf, wave loads are the dominant load in designing procedure. Vibration control of jacket platforms is always one of the important issues for demanding requirements such as production activity, safety and serviceability. Tuned Mass Damper (TMD) is one of the well-established control systems in this context. The present study investigates the dynamic behavior of the Ressalat platform located in the Persian Gulf under the wave loads. An Active Tuned Mass Damper (ATMD) is employed for vibration control, while taking the actuator saturation into account. A fuzzy controller is applied to calculate the control force. Moreover, the Harmony Search Algorithm (HSA) is examined to optimize the power of the actuator in the control system. Results obtained through this study indicate superior performance of the proposed ATMD in comparison to the TMD, leading to significant reduction in vibration of the Ressalat platform under studied wave loads.

Studies on motion response of a vessel is of great interest to researchers, since a long time. But intensive researches on stability of vessel during motion under dynamic conditions are few. A numerical model of vessel is developed and responses are analyzed in head, beam and quartering sea conditions. Variation of response amplitude operator (RAO) of vessel based on Strip Theory for different wave heights is plotted. Validation of results was done experimentally and numerical results was considered to obtain effect of damping on vessel stability. A scale model ratio of 1:125 was used which is suitable for dimensions of wave flume at National Institute of Technology Calicut. Stability chart are developed based on Mathieu"s equation of stability. Ince-Strutt chart developed can help to capture variations of stability with damping.

Results are presented of a benchmark and uncertainty assessment study organised by the MARSTRUCT Virtual Institute on global linear wave loads on damaged ship. The study has two aims: to acquire valuable information regarding damage modelling in seakeeping analysis of damaged ships and to contribute to a rational approach for definition of the model uncertainty of linear seakeeping tools. Eight institutes participated in the benchmark, with six codes, representative of important linear seakeeping theories in use nowadays. The benchmark ship is the DTMB 5415 hull, with well documented and accessible data to perform seakeeping analysis and experimental results for motion and global wave loads. The uncertainty analysis is performed using the Frequency Independent Model Error as the uncertainty measure. The analysis is performed for vertical motions, vertical and horizontal global wave load components, and for torsional moments. Uncertainty measures of individual motion and load predictions are presented and compared. In addition, a comparative analysis of linear seakeeping theories is performed and the accuracy of the simplified methods used for the prediction of seakeeping of a damaged ship is assessed. Finally, recommendations are provided for efforts to improve modelling uncertainties in transfer functions of wave loads on damaged ships.

A system for forecasting the operability of ships was developed by extending a wave forecasting system with the ship responses to the forecast sea states. The system produces 4-day wave forecasts in an implementation for the Portuguese Exclusive Economic Zone, where the vessels' seakeeping performance is forecast, although the method presented could well be applied to any other coastal region. The proposed procedure requires the knowledge of the transfer functions for different ship speeds and headings. The computations are made for two different types of fishing vessels that currently operate in the Portuguese coast. Ship motions and derived responses are evaluated with a strip theory code and compared with the prescribed values of relevant seakeeping criteria that represent the limit conditions that still allow standard operational conditions for those vessels. For each forecast sea state at each grid point at a given time, the response spectrum is calculated and operability indexes are computed and compared with the maximum allowed values for a specific operation criterion. The results are presented as operational index maps for various areas of the Portuguese coast, and the information provided by system will help ship masters to plan the operation of their ships, namely to decide the areas to operate in anticipation. In this way, they can identify areas in which the level of operationally of their ships will be degraded due to weather conditions and avoid them.

The focus of this paper is on the application of a time domain hybrid method to the calculation of the motions and wave loads on a semi-submersible, which has been investigated extensively in ITTC benchmark studies in the past. Numerical results include hydrodynamic coefficients, motion responses and drift forces. Good agreement has been observed between the results of the present method, other numerical codes and experimental data, which confirm the capability of the developed numerical approach to deliver meaningful predictions for practical applications to offshore structures.

In this paper, a nonlinear time domain method based on the impulse response function concept is presented. For the simulation of the six DOF nonlinear motions of a ship in seaways, the equations of motion are derived by application of the momentum conservation theorem and numerically solved in the time domain. Hydrodynamic coefficients are determined by linear potential theory, whereas certain nonlinear terms are considered in the equations of motion, such as the excitation by large amplitude waves, the exact restoring forces and moments resulting from the actual wetting of the ship hull geometry and the semi-empirical nonlinear viscous damping. Finally, nonlinear inertia terms are retained when considering large angles of motions. The validation of the developed theoretical method and computer code has been conducted by the study of well-established benchmark ship models, namely the S-175 hull, the DTC hull and the KVLCC2 hull. Good agreement has been observed between the results of the present method, other numerical codes and available experimental data, which confirm the capability of the developed numerical approach to deliver reliable predictions.

This paper discusses uncertainties related to the prediction of loads and responses for ocean and offshore structures in accordance with the findings by the Ocean Engineering Committee of the International Towing Tank Conference (ITTC). The parameters that may cause uncertainties in ocean engineering model tests, full-scale tests and numerical simulations are presented in terms of physical properties of the fluid, initial conditions, model definition, environment, scaling, instrumentation and human factors. Emphasis is given to the uncertainty sources in model tests involving deepwater mooring lines, risers and dynamic positioning systems and the need for quantifying them. A methodology for uncertainty analysis is described according to the International Organization for Standardization (ISO) Guidance for Uncertainties in Measurement (GUM). As an example of application, the combined and expanded uncertainties in the model tests of a moored semi-submersible platform were assessed and quantified in terms of motion responses, air gap and mooring line tensions. It is concluded that the quantification of uncertainties may be challenging in model tests and numerical simulations of ocean and offshore structures. It is particularly challenging in extrapolating model test results to full scale and utilizing complex numerical models, especially if the effects of hydrodynamic nonlinearities are significant.

Enhancing safety at sea through specification and quantification of uncertainties related to description of the environment and predictions of loads and responses is currently one of the main concerns of the Shipping and Offshore Industry. These uncertainties play an important part in risk assessment for the design and operation of marine and offshore structures. Whereas measured values are used in the process of validating modelling techniques and associated assumptions both measurements and predictions in principle have errors associated with them. For example, high uncertainty of environmental description may lead to significant risk impact and several authors have previously been able to demonstrate the importance of modelling uncertainties in the calculation of loads and responses. To characterise the accuracy of a quantity it is necessary to distinguish between systematic (bias) and precision (random) errors with reference to the true value. Specification of uncertainties is n...

The focus of this paper is on the application of a time domain hybrid method to the calculation of the motions and wave loads on a semisubmersible, which has been investigated extensively in ITTC benchmark studies in the past. Numerical results include hydrodynamic coefficients, motion responses and drift forces. Good agreement has been observed between the results of the present method, other numerical codes and experimental data, which confirm the capability of the developed numerical approach to deliver meaningful predictions for practical applications to offshore structures.

The applicability of an efficient 3D numerical method for the evaluatfion of the seakeeping performance and structural loads of advanced highspeed vessels is investigated. A semi-empirical approach to account for the viscous flow effects on ship's behavior at speed is presented and discussed. Numerical results are compared with model tests for two high-speed monohull vessels with satisfactory agreement. q

The applicability of an efficient 3D numerical method for the evaluatfion of the seakeeping performance and structural loads of advanced highspeed vessels is investigated. A semi-empirical approach to account for the viscous flow effects on ship's behavior at speed is presented and discussed. Numerical results are compared with model tests for two high-speed monohull vessels with satisfactory agreement. q