Wengang Mao

Estimation of Extreme Ship Response

In practice the severity of ship response is measured by high quantiles of long-term distribution of the response. The distribution is estimated by combining the short-term distribution of the response with a long-term probability distribution of encountered sea states. The paper describes an alternative approach, the so-called Rice’s method, based on estimation of expected number of upcrossings of high levels by stress during 1 year. The method requires description of long-term variability of the standard deviation, skewness, kurtosis, and zero upcrossing frequency of ship response. It is assumed that the parameters are functions of encountered significant wave height, heading angle, and ship speed. The relation can be estimated from the measured stresses or computed by dedicated software assuming rigid ship hull model. Then Winterstein’s transformed Gaussian model is used to estimate the upcrossing rates of response during a sea state. The proposed method is validated using the full-scale measurements of a 2,800 TEU container ship during the first 6 months of 2008. Numerical estimation of 4,400 TEU container ship extreme response illustrates the approach when no measurements are available.

Estimation of Weibull distribution for wind speeds along ship routes

In order to evaluate potential benefits of new green shipping concepts that utilize wind power as auxiliary propulsion in ships or of offshore wind energy harvest, it is essential to have reliable wind speed statistics. A new method to find parameters in the Weibull distribution is given. It can be used either at a fixed offshore position or along arbitrary ship routes. The method employs a spatio-temporal transformed Gaussian model for wind speed variability. The model was fitted to 10 years’ ERA-Interim reanalysis data of wind speed. The proposed method to derive Weibull distribution is validated using wind speeds measured on-board by vessels sailing in the North Atlantic and the west region of the Mediterranean Sea. For the westbound voyages in the North Atlantic, the proposed method gives a good approximation of the observed wind distribution along those ship routes. For the eastbound voyages, significant difference is found between the observed wind distribution and that approximated by the proposed method. The suspected reason is attributed to the ship routing decisions of masters and software. Hence, models that consider only the wind climate description need to be supplemented with a method to take into account the effect of wind-aware routing plan.

A generic energy systems model for efficient ship design and operation

There is an environmentally and economically motivated need to reduce the fuel consumption and air emissions of ships. To achieve a reduction in energy consumption, the energy flow in the entire energy system of a ship must be analysed in both the component, or subsystem, level as well as in a holistic way to capture the interactions between the components. Of the currently available energy consumption monitoring and prediction methods or models, no single model or method can be used to assess the energy efficiency of an arbitrary vessel in both the early design phase and during operation. This study presents a new generic ship energy systems model that can be used for this purpose. This new model has two parts: one for the assessment of a ship’s energy consumption based on an ordinary static power prediction and one for advanced operational analysis, considering hydrodynamic and machinery systems effects. A Panamax tanker vessel was used as the case study vessel to prove the versatility of the model for five example simulations for the design and operation of ships. The examples include variations of the main dimensions, propeller design, engine layout and the operational profile on a North Atlantic route. From the results, different areas with a potential for energy savings were identified.

Development of a Spectral Method and a Statistical Wave Model for Crack Propagation Prediction in Ship Structures

As a result of the wide use of high-tensile steel and the imperfect fabrication process in ship construction as well as some uncertainties in ship fatigue design, cracks could be initiated much earlier than expected. The presence of fatigue cracks greatly affects a ships structural safety and serviceability. To ensure structural safety and perform reliable crack inspection and maintenance planning, it is important to know how fast cracks can grow in ship structures. In the current study, the principles of fracture mechanics are used for crack propagation analysis in ships. By taking into account the special properties of a ships stress response, an efficient spectral method is proposed and validated for the prediction of crack propagation in ship structures. In this spectral method, structural stresses are assumed to be narrow-band Gaussian processes. Furthermore, for crack inspection and maintenance planning based on crack growth, it is essential to know the wave environments encountered in a ships future operations. Therefore, a spatiotemporal statistical wave model based on both satellite and buoy measurements is briefly introduced. It is developed to generate wave environments along arbitrary ship routes. Finally, the deck longitudinal stiffener of a 2800TEU containership is used to demonstrate the application of the spectral method and the wave model for the prediction of crack propagation in ships. The route information and operating conditions are taken from full-scale measurements of this ship. In the case study, the scatter of crack propagation associated with the wave environments encountered is also investigated. The results from this investigation indicate possible potentials of crack inspection and maintenance optimization to enable more efficient ship operation.

A comparison of two wave models and their influence on fatigue damage in ship structures

The ship size has been rapidly increased in recent years. Consequently, many aspects of ship structural safety have been concerned, i.e. fatigue strength. Since fatigue cracks are found in the early stage of a ship’s service life, it might be that the classical cumulative fatigue rule is insufficient to consider the effects of many uncertain factors, such as variable wave environment loads. The wave load history hinges on the short-sea history. It means that the accuracy of fatigue assessment is affected by the used wave model. There are several established wave models, which could be used to simulate wave load history as in the real ocean conditions. In this paper, wave load histories of a 2800TEU container ship are generated by two different wave models. It is used for crack propagation analysis in the changing routes and trades. The result in the simulated crack propagation lives is examined and discussed.

Theoretical development and validation of a fatigue model for ship routing

Fatigue cracks are observed much earlier than expected in ships and marine structures due to uncertainties in the fatigue design process, such as encountered sea environments and variability in S-N curves, etc. The current study presents the theoretical development and validation of a fatigue model useful for ship routing which may contribute to better utilisation of the materials and structures by more wise operation of them. During the theoretical development of the ship routing fatigue model, various models to estimate fatigue damage intensity, including both cycle counting calculations and spectral approximations, are reviewed. The proposed ship routing fatigue model is a function of operation profiles, i.e., heading angle and ship speed, as well as encountered wave environments (significant wave height), which are easily available in todays commercial routing tool systems. Concerning the characteristics of ship response, the so-called narrow-band approximation is adopted and further simplified in order to estimate the fatigue damage in ships under an arbitrary stationary sea state. Long-term fatigue damage is estimated by a summation of fatigue damages in all encountered sea states during, for example, one voyage or a period of several years. Further, fatigue damage due to wave-induced vibrations (whipping and springing) and fatigue damage caused by various stress components are also studied and discussed. A validation using response from full-scale measurements and numerical analysis is presented. It shows that the proposed model works very well in comparison with the rainflow counting method. Finally, the proposed ship routing fatigue model is applied on real case scenarios, where its applicability in shipping industry is demonstrated.

Application of a ship-routing fatigue model to case studies of 2800 TEU and 4400 TEU container vessels

Ship structures will always be subjected to and suffer from fatigue damage and fracture. However, we can become better at utilizing the materials and the structures and operating them more wisely, leading to less maintenance, extended service life and enhanced safety if we can improve the fatigue damage prediction. In this study, a fatigue model useful for ship routing is presented. Similar to the ship-routing design for the expected time of arrival and fuel reduction, the objective here is to demonstrate the possibility and benefits of ship route planning which lead to a reduction in fatigue damage accumulation. In the context of ship fatigue route planning, uncertainty analysis of the fatigue assessments of container ships is discussed. The proposed ship-routing fatigue model is employed in case studies on two container vessels to illustrate the possible benefits to the fatigue safety of ship structures. Sea environments in the application are obtained from hindcast wave data and a spatiotemporal wave model. The results show that the fatigue damage of the studied container ship structures could be decreased by more than 50% if awareness and knowledge of fatigue in ship route planning are employed.

State-of-the-art within ship weather routing

Increased fuel prices and public awareness of environment impacts from shipping have attracted large efforts in maritime sector to increase its energy efficiency as a factor of competitiveness. Weather routing has become a recognized measure, which can partly help to achieve the targets as well as enhancing safety. A routing system requires a reliable optimization algorithm to consider a ship’s operational costs, expected time of arrival, and cargo safety etc. simultaneously. Hence, the service provided by a weather routing system is highly dependent on a properly selected optimization algorithm and associated input parameters. In this paper the concept of weather routing is broken down into many elements for further analysis. Focus is given to algorithms, constraints and weather forecasts used in the optimized routing plan. Two different aspects of state-of-the-art have been considered. The first is a study of software already in use and the second is a study of methods investigated in the research community. Furthermore, this paper also provides examples of development trends, for example the fatigue based routing, and the risk based routing, as well as its integration with onboard monitoring systems for more reliable weather and ship specific response information.

Estimation of wave loading induced fatigue accumulation and extreme response of a container ship in severe seas

The hydrodynamic analysis of a 4400 TEU container ship with constant forward speed is carried out by the nonlinear numerical code WASIM in the time domain under severe sea states. Straightforward fatigue estimation is performed using the rainflow counting approach based on simulated time series of stresses. The narrow-band approximation, which has been validated in previous work with good accuracy by full-scale measurement of a 2800 TEU container ship, is implemented to estimate the fatigue damage based on the same responses. It is concluded that a slight deviation from the Gaussian process does not influence the fatigue estimation by narrow-band approximation. In addition, extreme response is defined by the level up-crossing approach. The Gaussian crossing model using Rice’s formula is employed to predict the extreme response based on the responses from above numerical analysis. It shows that the Gaussian model is not suitable for this prediction. A more complicated level crossing model is proposed which is based on the Laplace Moving Average method. Its accuracy in prediction of extreme responses is analyzed and presented with good agreement by means of numerical simulations.

Notes on the Prediction of Extreme Ship Response

In this note the relation between two simple approaches to estimate the extreme ship response used when no, or a limited, amount of data are available is discussed. The first one employs the long term distribution of the local maxima of ship response while the second one uses the expected number of upcrossings of a level by the response. It is mathematically demonstrated that the two approaches are equivalent. However, the upcrossing method is more straightforward and convenient for practical applications, particularly for non-Gaussian responses. The full-scale measurements of a 2800 TEU container ship during the first six months of 2008 are used in the comparisons.