To investigate the mechanism of secondary circulations in rip current systems, and to explore the relationship between wave conditions and secondary circulation intensity, a series of numerical experiments is performe...To investigate the mechanism of secondary circulations in rip current systems, and to explore the relationship between wave conditions and secondary circulation intensity, a series of numerical experiments is performed using coupled nearshore wave model and circulation model. In these experiments, the rip currents and secondary circulations generated above barred beaches with rip channels are simulated. A comparison experiment is conducted to investigate the formation and hydrodynamics of the secondary circulations. Model results indicate that the secondary circulations consist of alongshore flows driven by wave set-up near the shoreline, part of the feeder currents driven by the wave set-up over the bars, and onshore flows at the end of the rip channel driven by wave breaking and convection. The existence of the secondary circulation barely affects the rip current, but narrows and intensifies the feeder currents. Three groups of experiments of varying incident wave conditions are performed to investigate the relationship between wave conditions and secondary circulation intensity. The velocity of the alongshore flow of the secondary circulation is sensitive to the variation of the incident wave height and water depth. It is also found that the alongshore flow intensity is in direct proportion to the alongshore variation of the wave height gradient between the bars and the shoreline.展开更多
Compliant offshore structures such as spars,tension leg platforms(TLPs) and semi-submersibles have been dramatically improved in recent years due to their capability for deep water operation. Waves are the most import...Compliant offshore structures such as spars,tension leg platforms(TLPs) and semi-submersibles have been dramatically improved in recent years due to their capability for deep water operation. Waves are the most important environmental phenomenon affecting these offshore structures. Estimation of wave forces is vital in offshore structure design. For large compliant offshore platforms,Morrison's equation is not valid anymore and usually diffraction theory is used. In this research,by using the finite difference method,a detailed analysis of the first-order diffraction of monochromatic waves on a large cylinder as a structural element is performed to solve the radiation and diffraction potentials. The results showed that the developed model is a reliable tool to estimate the wave forces and hydrodynamic coefficients on large structure elements when wave diffraction and radiation are considered.展开更多
The wave force exerted on vertical piles of offshore structures is the main criterion in designing them.In structures with more than one large pile,the influence of piles on each other is one of the most important iss...The wave force exerted on vertical piles of offshore structures is the main criterion in designing them.In structures with more than one large pile,the influence of piles on each other is one of the most important issues being concerned in past researches.An efficient method for determining the interaction of piles is introduced in present research.First the wave force is calculated by the exact method using the diffraction theory,then in the finite difference numerical method the force is calculated by adding the velocity potentials of each pile and integration of pressure on their surface.The results showed that the ratio of the wave force on each of the double piles to a single pile has a damped oscillation around unity in which the amplitude of oscillation decreases with the increase in the spacing parameter.Also different wave incident directions and diffraction parameters were used and the results showed that the numerical solution has acceptable accuracy when the diffraction parameter is larger than unity.展开更多
In early winter it is usual, in cold regions, that ice features approach offshore structures, like offshore platforms, impacting them, in a slow process of constant deformation build up. Interaction follows, in many c...In early winter it is usual, in cold regions, that ice features approach offshore structures, like offshore platforms, impacting them, in a slow process of constant deformation build up. Interaction follows, in many cases, up to the point where ice-failure caused by bending fracture takes place. This supposes very large contact forces that the structure has to resist. Therefore, quantification of these efforts is of vital importance to the structural design of platforms. In several designs, these platforms are constructed with inclined walls so as to cause ice to fail in a flex-compression mode. In such a case the ice feature is analyzed as a beam constituted of a linear elastic material in brittle state with constant ice thickness. The simplification renders the problem solvable in a close form. However, this hypothesis goes against field observations. Marine currents action, wind and the sequence of contacts among features lead to thickness variations. Here this factor is addressed in the construction of a model, for harmonic forms of variation of thickness profile, and the accompanying curvature variations, whose solution determines field variables used to address the failure question. Due to the deformation dependency of the loading, a numerical scheme for the two-point boundary value problem in the semi-infinite space is developed. Failure pressures are computed based on a Rankine locus of failure. Variations of the order of 20% in the failure loads, as compared to the uniform beam model, are observed.展开更多
The shipbuilding industry in Europe is in crisis due to the recent economic recession and competition from countries with low labour costs. Greek shipyards have been forced to make a series of structural changes which...The shipbuilding industry in Europe is in crisis due to the recent economic recession and competition from countries with low labour costs. Greek shipyards have been forced to make a series of structural changes which have resulted in lower employment levels. Although these changes were gradually creating the conditions for shipyards profitably to re-enter international markets, the current economic crisis and the consequences to the shipping sector have created major problems to the shipyards employment. Perama shiprepair zone, an industrial zone mainly consisted from SMEs （small and medium enterprises） in shipbuilding and shiprepairing sector, has faced major problem for its survivability. In this article, a new approach is presented. Green offshore structures are expected to create competitive advantages in the zone and be a way to utilize available facilities and recover from the existing situation. International experience through case studies is presented followed by economic and technical feasibility for offshore windturbines construction and wave energy devices in shipyards. Finally, a general model shows that sustainable development is possible and shipyards have the adaptability and resources required tbr the production of renewable energy offshore structures.展开更多
In the tourism industry, transportation is the greatest consumer of energy and contributes the largest amount of CO2 emissions （ECCE）. Airplane flights make up between 60% and 70% of all forms of tourism transport. ...In the tourism industry, transportation is the greatest consumer of energy and contributes the largest amount of CO2 emissions （ECCE）. Airplane flights make up between 60% and 70% of all forms of tourism transport. Since airplane travel is the main way for tourists to access islands, airplane travel receives considerable attention in the study of the relationship between island tourism transportation, environment and economy. However, the pa- rameters adopted to estimate ECCE in the literature are usually either out-of-date or taken from papers not written in China. To improve the accuracy of estimates, all the parameters used in this paper are current and were obtained locally. Based on these parameters and a bottom-up approach, a more accurate estimation of ECCE for the off-shore island city of Haikou was obtained in 2012. The results indicate that 24.30% of the city＇s energy con- sumption, 33.89 P J, was due to tourism transportation, while CO2 emissions were 2.54 Mt. It is incorrect to assume that tourism is ＂an industry with no pollution＂. In Haikou, for example, tourism turns out to be the major form of en- ergy consumption in the city. This paper makes several suggestions intended to minimize the negative environ- mental impact from tourism transportation. These include recommending longer stays, a decrease in the number of flights, taxation of airplane emissions, and the setting up an environmental recovery fund.展开更多
Tension leg platform(TLP)is a typical compliant offshore structure for oil/gas resources exploitation.In the design process,the prediction of the free surface elevation is of great importance for the determination of ...Tension leg platform(TLP)is a typical compliant offshore structure for oil/gas resources exploitation.In the design process,the prediction of the free surface elevation is of great importance for the determination of the air gap.So far,the existing researches for predicting the air gap of the TLPs focus on the supporting columns while pay little attention to the horizontal pontoons.For the second order diffraction problem or long incident wave condition,the velocity potential decays slowly with water depth and the effect of pontoons should not be neglected.Herein the effect of pontoons on the diffracted wave field in the vicinity of a TLP platform is investigated in this study.The diffraction of regular waves by a square array of truncated cylinders and a whole TLP structure is studied in detail by using both the linear and the second-order diffraction theory.Numerical calculation is performed for the free surface elevation and wave run up.Numerical results show that the near-trapping phenomenon can occur inside the TLP and leads to significantly increased wave height.To study the effect of pontoons on the free surface elevation comparisons are carried out between the results of these two structures with and without pontoons.It is found that pontoons have an appreciable effect on the diffracted wave field for long incident regular waves and increase the largest response notably when the near-trapping phenomenon occurs at the second-order.展开更多
Return period is generally adopted to calculate the design value of environmental condition in offshore structure design. However, it can not make relevant adjustment according to structure＇s, especially the mobile u...Return period is generally adopted to calculate the design value of environmental condition in offshore structure design. However, it can not make relevant adjustment according to structure＇s, especially the mobile unit＇s, life time or its operation areas and usually make the design either insufficiently safe or rather uneconomical. A formula is developed to solve this problem in the case of the design wave height, where the risk, the design life, the distributions of wave heights in operation areas and the operating durations in each area are regarded as parameters. The applications of this method and the comparisons with the general method are presented. The result of this method is considered to be proper.展开更多
基金supported by China’s Public Science and Technology Research Funds Projects of Ocean (No. 200905013-4)by Ministry of Science and Technology of China (No. 2011BAC03B01)
文摘To investigate the mechanism of secondary circulations in rip current systems, and to explore the relationship between wave conditions and secondary circulation intensity, a series of numerical experiments is performed using coupled nearshore wave model and circulation model. In these experiments, the rip currents and secondary circulations generated above barred beaches with rip channels are simulated. A comparison experiment is conducted to investigate the formation and hydrodynamics of the secondary circulations. Model results indicate that the secondary circulations consist of alongshore flows driven by wave set-up near the shoreline, part of the feeder currents driven by the wave set-up over the bars, and onshore flows at the end of the rip channel driven by wave breaking and convection. The existence of the secondary circulation barely affects the rip current, but narrows and intensifies the feeder currents. Three groups of experiments of varying incident wave conditions are performed to investigate the relationship between wave conditions and secondary circulation intensity. The velocity of the alongshore flow of the secondary circulation is sensitive to the variation of the incident wave height and water depth. It is also found that the alongshore flow intensity is in direct proportion to the alongshore variation of the wave height gradient between the bars and the shoreline.
文摘Compliant offshore structures such as spars,tension leg platforms(TLPs) and semi-submersibles have been dramatically improved in recent years due to their capability for deep water operation. Waves are the most important environmental phenomenon affecting these offshore structures. Estimation of wave forces is vital in offshore structure design. For large compliant offshore platforms,Morrison's equation is not valid anymore and usually diffraction theory is used. In this research,by using the finite difference method,a detailed analysis of the first-order diffraction of monochromatic waves on a large cylinder as a structural element is performed to solve the radiation and diffraction potentials. The results showed that the developed model is a reliable tool to estimate the wave forces and hydrodynamic coefficients on large structure elements when wave diffraction and radiation are considered.
文摘The wave force exerted on vertical piles of offshore structures is the main criterion in designing them.In structures with more than one large pile,the influence of piles on each other is one of the most important issues being concerned in past researches.An efficient method for determining the interaction of piles is introduced in present research.First the wave force is calculated by the exact method using the diffraction theory,then in the finite difference numerical method the force is calculated by adding the velocity potentials of each pile and integration of pressure on their surface.The results showed that the ratio of the wave force on each of the double piles to a single pile has a damped oscillation around unity in which the amplitude of oscillation decreases with the increase in the spacing parameter.Also different wave incident directions and diffraction parameters were used and the results showed that the numerical solution has acceptable accuracy when the diffraction parameter is larger than unity.
文摘In early winter it is usual, in cold regions, that ice features approach offshore structures, like offshore platforms, impacting them, in a slow process of constant deformation build up. Interaction follows, in many cases, up to the point where ice-failure caused by bending fracture takes place. This supposes very large contact forces that the structure has to resist. Therefore, quantification of these efforts is of vital importance to the structural design of platforms. In several designs, these platforms are constructed with inclined walls so as to cause ice to fail in a flex-compression mode. In such a case the ice feature is analyzed as a beam constituted of a linear elastic material in brittle state with constant ice thickness. The simplification renders the problem solvable in a close form. However, this hypothesis goes against field observations. Marine currents action, wind and the sequence of contacts among features lead to thickness variations. Here this factor is addressed in the construction of a model, for harmonic forms of variation of thickness profile, and the accompanying curvature variations, whose solution determines field variables used to address the failure question. Due to the deformation dependency of the loading, a numerical scheme for the two-point boundary value problem in the semi-infinite space is developed. Failure pressures are computed based on a Rankine locus of failure. Variations of the order of 20% in the failure loads, as compared to the uniform beam model, are observed.
文摘The shipbuilding industry in Europe is in crisis due to the recent economic recession and competition from countries with low labour costs. Greek shipyards have been forced to make a series of structural changes which have resulted in lower employment levels. Although these changes were gradually creating the conditions for shipyards profitably to re-enter international markets, the current economic crisis and the consequences to the shipping sector have created major problems to the shipyards employment. Perama shiprepair zone, an industrial zone mainly consisted from SMEs （small and medium enterprises） in shipbuilding and shiprepairing sector, has faced major problem for its survivability. In this article, a new approach is presented. Green offshore structures are expected to create competitive advantages in the zone and be a way to utilize available facilities and recover from the existing situation. International experience through case studies is presented followed by economic and technical feasibility for offshore windturbines construction and wave energy devices in shipyards. Finally, a general model shows that sustainable development is possible and shipyards have the adaptability and resources required tbr the production of renewable energy offshore structures.
基金National Natural Science Foundation of China(41101044)Key Project of National Social Science Foundation of China(15AGL012)
文摘In the tourism industry, transportation is the greatest consumer of energy and contributes the largest amount of CO2 emissions （ECCE）. Airplane flights make up between 60% and 70% of all forms of tourism transport. Since airplane travel is the main way for tourists to access islands, airplane travel receives considerable attention in the study of the relationship between island tourism transportation, environment and economy. However, the pa- rameters adopted to estimate ECCE in the literature are usually either out-of-date or taken from papers not written in China. To improve the accuracy of estimates, all the parameters used in this paper are current and were obtained locally. Based on these parameters and a bottom-up approach, a more accurate estimation of ECCE for the off-shore island city of Haikou was obtained in 2012. The results indicate that 24.30% of the city＇s energy con- sumption, 33.89 P J, was due to tourism transportation, while CO2 emissions were 2.54 Mt. It is incorrect to assume that tourism is ＂an industry with no pollution＂. In Haikou, for example, tourism turns out to be the major form of en- ergy consumption in the city. This paper makes several suggestions intended to minimize the negative environ- mental impact from tourism transportation. These include recommending longer stays, a decrease in the number of flights, taxation of airplane emissions, and the setting up an environmental recovery fund.
基金supported by the National Natural Science Foundation of China(Grant Nos.11072052,51221961)the National Basic Research Program of China("973"Project)(Grant No.2011CB013703)
文摘Tension leg platform(TLP)is a typical compliant offshore structure for oil/gas resources exploitation.In the design process,the prediction of the free surface elevation is of great importance for the determination of the air gap.So far,the existing researches for predicting the air gap of the TLPs focus on the supporting columns while pay little attention to the horizontal pontoons.For the second order diffraction problem or long incident wave condition,the velocity potential decays slowly with water depth and the effect of pontoons should not be neglected.Herein the effect of pontoons on the diffracted wave field in the vicinity of a TLP platform is investigated in this study.The diffraction of regular waves by a square array of truncated cylinders and a whole TLP structure is studied in detail by using both the linear and the second-order diffraction theory.Numerical calculation is performed for the free surface elevation and wave run up.Numerical results show that the near-trapping phenomenon can occur inside the TLP and leads to significantly increased wave height.To study the effect of pontoons on the free surface elevation comparisons are carried out between the results of these two structures with and without pontoons.It is found that pontoons have an appreciable effect on the diffracted wave field for long incident regular waves and increase the largest response notably when the near-trapping phenomenon occurs at the second-order.
基金the National Great Project of Scientific and Technical Supporting Programs Funded by Ministry of Science & Technology of China During the 11th Five-year Plan (No. 2006BAA01A25)
文摘Return period is generally adopted to calculate the design value of environmental condition in offshore structure design. However, it can not make relevant adjustment according to structure＇s, especially the mobile unit＇s, life time or its operation areas and usually make the design either insufficiently safe or rather uneconomical. A formula is developed to solve this problem in the case of the design wave height, where the risk, the design life, the distributions of wave heights in operation areas and the operating durations in each area are regarded as parameters. The applications of this method and the comparisons with the general method are presented. The result of this method is considered to be proper.
