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| 2. |
- Li, Zhiyuan, 1974, et al.
(författare)
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Fatigue damage assessment of container ships concerning wave-induced torsion
- 2010
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Ingår i: Proceedings of the ASME 29th International Conference on Ocean, Offshore and Arctic Engineering (OMAE 2010) in Shanghai, China, June 6-11, 2010. - 9780791849101
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Konferensbidrag (refereegranskat)abstract
- The traditional method to assess fatigue damage of ship structures assumes moderate wave amplitudes and linear responses. This method can be questioned when applied on container ships that are characterized by large deck openings, because the low torsion rigidity of this type of ship makes it sensitive to oblique waves. In this paper, the 3D hydrodynamic code WASIM is used to simulate a 4400 TEU container ship operating in the North Atlantic Ocean. Nonlinear wave loads are utilized for direct calculation of the stress histories under severe sea states. The warping stress from wave-induced torsion is separated from the stress components from vertical and horizontal bending. The contribution to fatigue damage accumulation from warping stresses is evaluated. For comparison, the results from the numerical simulations and fatigue calculations are verified with full-scale measurements made on a similar type of container vessel.
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| 3. |
- Mao, Wengang, 1980, et al.
(författare)
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Application of a ship-routeing fatigue model to case studies of 2800 TEU and 4400 TEU container vessels
- 2012
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Ingår i: Proceedings of the Institution of Mechanical Engineers, Part M: Journal of Engineering for the Maritime Environment. - : SAGE Publications. - 1475-0902 .- 2041-3084. ; 226:3, s. 222-234
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Tidskriftsartikel (refereegranskat)abstract
- Ship structures will always be subjected to and suffer from fatigue damage and fracture. However, we can become better at utilising the materials and the structures and operating them more wisely, leading to less maintenance, extended service life, and enhanced safety. In this study, a fatigue model useful for ship routeing is presented. Similar to ship routeing design for ETA and fuel reduction, the objective here is to demonstrate the possibility and benefits of ship route planning leading to a reduction of fatigue damage accumulation. In the context of ship fatigue route planning, uncertainty analysis in fatigue assessments of container ships is discussed. The proposed ship routing fatigue model is employed by 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 spatio-temporal wave model. The results show that the fatigue damage of the studied container ship structures can be decreased by more than 50% if the awareness and knowledge of ship fatigue route planning is employed.
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| 4. |
- Mao, Wengang, 1980, et al.
(författare)
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Assessment of full-scale measurements with regard to extreme hogging and sagging condition of container ships
- 2011
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Ingår i: Proceedings of the ASME Thirtieth International Conference on Ocean, Offshore and Arctic Engineering (OMAE 2011) in Rotterdam, the Netherlands, June 19-24, 2011. - 9780791844342 ; 2
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Konferensbidrag (refereegranskat)abstract
- In the design of a vessel’s ultimate strength the extreme hogging condition is of great concern. Due to special properties of container ship structures, such as large bow flare and overhanging stern, wave-induced slamming makes the ship responses more skewed to sagging conditions. In particular in large sea states, the ratio between maximum sagging and hogging can be quite high. Hence, the sagging condition might be very crucial with respect to a ship’s ultimate strength. In this study, the extreme response caused by hogging and sagging is derived from upcrossing spectrums of ship responses. The Weibull fitting method and Rice’s formula for the computation of the upcrossing spectrum are discussed using full-scale measurements from a container vessel on the North Atlantic trade. The extreme ship responses are therefore predicted using the long-term upcrossing spectrum. In the case where the ship response can be approximately treated as a series of stationary Gaussian processes, the corresponding upcrossings are computed by the explicit Rice’s formula. For the non-Gaussian ship response, it is shown that the 4-moment Hermite transformation is an efficient approach to compute the corresponding upcrossing spectrums. The parameters in the transformation mainly depend on the wave environments and operation profiles. The relations between these parameters and the wave environments are needed if no measurement is available. However, according to the full-scale measurements, it is not possible to find general formulas to estimate the parameters in terms of wave environments or operation profiles for the practical applications.
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| 5. |
- Mao, Wengang, 1980, et al.
(författare)
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Comparison between a fatigue model for voyage planning and measurements of a container vessel
- 2009
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Ingår i: Proceedings of the ASME Twenty-eighth International Conference on Ocean, Offshore and Arctic Engineering (OMAE 2009) on Hawaii, USA, May 31 - June 5, 2009. - 9780791843420 ; 2
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Konferensbidrag (refereegranskat)abstract
- This paper presents results from an ongoing research project which aims at developing a numerical tool for route planning of container ships. The objective with the tool is to be able to schedule a route that causes minimum fatigue damage to a vessel before it leaves port. Therefore a new simple fatigue estimation model, only using encountered significant wave height, is proposed for predicting fatigue accumulation of a vessel during a voyage. The formulation of the model is developed based on narrow-band approximation. The significant response height hs, is shown to have a linear relationship with its encountered significant wave height Hs. The zero up-crossing response frequency fz, is represented as the corresponding encountered wave frequency and is expressed as a function of Hs. The capacity and accuracy of the model is illustrated by application on one container vessel’s fatigue damage accumulation, for different voyages, operating in the North Atlantic during 2008. For this vessel, all the necessary data needed in the fatigue model, and for verification of it, was obtained by measurements. The results from the proposed fatigue model are compared with the well-known and accurate rain-flow estimation. The conclusion is that the estimations made using the current fatigue model agree well with the rain-flow method for almost all of the voyages.
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| 6. |
- Mao, Wengang, 1980, et al.
(författare)
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Crack growth analysis in ship structures using spectral methods
- 2014
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Ingår i: Proceedings of the 7th International Conference on Thin-Walled Structures (ICTWS2014) in Busan, Korea, 28th September - 2nd October, 2014.
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Konferensbidrag (refereegranskat)abstract
- The structural flexibility of modern (container) ship structures subjected to wave loading conditions has increased as a result of larger ship sizes. Cyclic variations of relatively large displacements in these structures may result in local fatigue damage problems. It is known that there are large uncertainties in the prediction of fatigue crack initiation life using fatigue design methods from classification societies. As a consequence, cracks are often observed in ship structures earlier than expected. The existence of cracks in a ship can affect the ship structure’s integrity. Depending on the location of a found crack and its length, it may propagate quite fast to a critical length which could endanger the safety of the entire vessel; cf. MSC Carla and MOL Comfort (under investigation). Furthermore, it may not be realistic to repair all cracks immediately when they have been found in a vessel; some of them may not be critical at all. Therefore, it is of great importance to have knowledge and understanding from studies on fatigue crack growth rate for different wave environments and ship operation conditions. Such studies can assist in decision making for maintenance planning of cracks repair in order to ensure a ship’s structural safety. The most commonly used methods for crack propagation analysis are often too complex to be used in a full-ship crack growth analysis. However, the current study focuses on fatigue fracture analysis methods of ship structures using linear-elastic fracture mechanics (LEFM). The objective is to propose and present a more suitable method for crack growth analysis in ships. By considering the special properties of ship response in a stationary sea state, the structure stress in a ship can be assumed to be narrow band Gaussian process. An explicit formula/simple method is proposed to estimate the crack growth rate under arbitrary sea states. Note that the narrow band assumption might be a bit conservative for the crack growth prediction. Hence, the crack propagation analyses, which are carried out using the code FRANC2D, are carried out using the narrow band approximation. Results are presented from the proposed method, which shows that the method can be efficiently used to compute the crack growth rate in ship structures. Furthermore, uncertainties associated with the crack propagation analysis, such as mean stress effect and crack closure, are also addressed.
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| 7. |
- Mao, Wengang, 1980, et al.
(författare)
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Development of a fatigue model useful in ship routing design
- 2011
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Ingår i: The Society of Naval Architects and Marine Engineers (SNAME) - TRANSACTIONS. - 0081-1661. ; 118:1, s. 497-511
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Tidskriftsartikel (refereegranskat)abstract
- In this paper, a simple fatigue estimation model using only encountered significant wave height, is used for predicting fatigue accumulation of a vessel during a voyage. The formulation of the model is developed based on the narrow-band approximation. It is assumed that the significant response range, hs, has a linear relationship with encountered significant wave height, Hs. The mean stress up-crossing frequency, fz, is represented by the corresponding encountered wave frequency and it is expressed as a function of Hs. The capacity and accuracy of the model is illustrated by application to one container vessel’s fatigue damage accumulation in an amidships detail, operating in the North Atlantic during 2008. For this vessel, all the necessary data needed in the fatigue model, and for verification of it, was obtained by measurements. The results from the proposed fatigue model are compared with the well-known and accurate rainflow analysis. Influence of nonlinearities, e.g. whipping, on fatigue damage predictions and extreme responses is also discussed.
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| 8. |
- Mao, Wengang, 1980, et al.
(författare)
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Development of a fatigue model useful in ship routing design
- 2010
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Ingår i: Journal of Ship Research. - 1542-0604 .- 0022-4502. ; 54:4, s. 281-293
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Tidskriftsartikel (refereegranskat)abstract
- In this paper, a simple fatigue estimation model, only using encountered significant wave height, is used for predicting fatigue accumulation of a vessel during a voyage. The formulation of the model is developed based on the narrow-band approximation. It is assumed that the significant response range, hs, has a linear relationship with encountered significant wave height, Hs. The mean stress up-crossing frequency, fz, is represented by the corresponding encountered wave frequency and it is expressed as a function of Hs. The capacity and accuracy of the model is illustrated by application on one container vessel’s fatigue damage accumulation in an amidships detail, operating in the North Atlantic during 2008. For this vessel, all the necessary data needed in the fatigue model, and for verification of it, was obtained by measurements. The results from the proposed fatigue model are compared with the well-known and accurate rainflow analysis. Influence of nonlinearities, e.g. whipping, on fatigue damage predictions and extreme responses is also discussed.
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| 9. |
- Mao, Wengang, 1980, et al.
(författare)
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Estimation of wave loading induced fatigue accumulation and extreme response of a container ship in severe seas
- 2010
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Ingår i: Proceedings of the ASME 29th International Conference on Ocean, Offshore and Arctic Engineering (OMAE 2010) in Shanghai, China, June 6-11, 2010. - 9780791849101 ; 2
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Konferensbidrag (refereegranskat)abstract
- 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.
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| 10. |
- Mao, Wengang, 1980, et al.
(författare)
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Fatigue variation in ships due to the variability of environmental loads
- 2012
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Ingår i: Proceedings of the ASME Thirty-first International Conference on Ocean, Offshore and Arctic Engineering (OMAE 2012) in Rio de Janeiro, Brazil, July 1-6, 2012. - 9780791844892 ; 2
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Konferensbidrag (refereegranskat)abstract
- The design and analysis of structural strength against fatigue failure always includes large uncertainties. It is crucial to understand and identify the most important uncertainties that affect the performance, functionality and service life of an engineering structure - in particular when it comes to the safety aspect, which may involve the risk of loss of human lives. In maritime industry, it is known that due to various sources of uncertainties in ship fatigue design, some ships may survive (the occurrence of fatigue cracks which may endanger the structural integrity) much longer than their designed life, while other ships develop fatigue cracks far too early. The current investigation presents some of the most important uncertainties and their effects on the accuracy of fatigue assessments in a container vessel. The study emphasizes the analysis of the fatigue damage variation when the ship is sailing on different routings between two ports. A fatigue model developed by the authors for ship fatigue routing application is employed to estimate the long term fatigue damage. In this model, only a few parameters, i.e. the encountered significant wave height and operational profiles, are needed. The procedure of using this model for a ship fatigue routing design is described in detail. Its potential use and benefits are demonstrated in a case study by a 2,800 TEU container ship using both full-scale measurements and hindcast wave data. It is shown that awareness and careful fatigue routing design can reduce fatigue damage significantly by up to 50%!
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