Contour Line Approach Research Articles

New prediction method for reliability analysis of ocean sustainable energy systems

With the aim of overcoming the shortcomings of traditional approaches, in this paper, an exponential Weibull distribution for fitting the significant wave height data is proposed in order to calculate more accurately the sea-state parameter distribution tails and to extrapolate well. This proposal is applied in predicting the probability distribution tails of a measured ocean wave dataset, and its accuracy is clearly demonstrated. The proposal is subsequently utilised in combination with Monte Carlo sampling to form a new environmental contour method for deriving a 50 year contour line based on the aforementioned measured ocean wave dataset. The derived environmental contour line is compared with that predicted by using a traditional contour line approach. Additionally, the engineering significance of using a more reliable environmental contour line, such as the one derived using the proposed new method for predicting the long-term design force values for an ocean sustainable energy system is highlighted. In conclusion, the newly proposed environmental contour method is recommended to be used when predicting the extreme dynamic response values for the safe and successful design of ocean sustainable energy systems.

Bivariate kernel density estimation for environmental contours at two offshore sites

ABSTRACT This paper proposes a novel environmental contour line approach based on measured ocean wave data at two offshore sites. For implementing the novel environmental contour line approach, we propose the use of bivariate kernel density estimation with rigorous bandwidth selection based on the Scott-Tapia-Thompson estimation method. The environmental contours obtained by using the proposed novel approach have been compared with those obtained by using the traditional Inverse-First-Order Reliability Method (IFORM) with Rosenblatt transformation or principal component analysis, and the effectiveness of our proposed novel approach have been clearly substantiated. The research results in this paper demonstrate that our proposed novel approach can be utilised as an effective tool for generating environmental contours if one has sufficient data extending beyond the return period of interest.

Effects of the asymmetric riser and bilge keel arrangements on FPSO green water assessment

This article aims to report important findings on how the asymmetric riser and bilge keel arrangements affect the motion response and green water assessment by using a real FPSO conversion project. Recently, the authors have proposed a practical approach for short-term and long-term green water prediction. In this paper, the method has been further extended to include the effect of truncated bilge keel by using Morrison elements. Numerical studies are conducted focusing on the effect induced by asymmetric riser arrangement and truncated bilge keels. Comparisons of short-term and long-term results between different models indicate that the FPSO’s motion is significantly affected by asymmetrically arranged appendages and attachments in a complicated way. The relative wave elevation is also affected by appendages and attachments, but not the same trend as the motion response. The effect of the asymmetric arrangement of risers and bilge keel on long-term relative wave elevation response has been captured by both traditional contour line approach and response-based analysis, but some discrepancy identified between the results from the two methods indicates the limitation of the traditional contour line approach.

Measurement of air void system in lightweight concrete by X-ray computed tomography

An industrial lightweight concrete developed for pipe insulation is characterized using X-ray computed tomography to determine the size distribution of air voids within the paste fraction. Strategies to address practical difficulties are presented, particularly discerning air voids in paste from air voids within aggregate, and combining air void size distribution data collected at different resolutions. To address the first issue, a step-wise approach was employed to reduce the problem to a series of easier-to-solve two-phase grayscale threshold problems. The first step isolates a significant number of light weight aggregate particles on 2D slices using an active contour line approach and an effective threshold identified to discern between air void and solid voxels. The threshold is applied globally to the 3D data set, and the air void voxels removed. Next, a significant number of rectangular regions of interest from 2D slices are used to identify a second threshold between the solid portions of the lightweight aggregate and the paste. Finally, a 3D shape based criterion is used to isolate air voids in paste. Given the trade-off between resolution and sample size, it is necessary to conduct multiple scans using different resolutions and sample diameters in order to cover the full range of the air void size distribution. A power law curve is used to unite distributions collected at different resolutions.

Extreme sloshing and whipping-induced pressures and structural response in membrane LNG tanks

The long-term extreme pressure in the membrane LNG tank and structural response of the Mark III containment system are addressed. The effect of hull slamming-induced vibrations on the vertical acceleration is investigated and found to be important in certain situations. Determining structural response due to sloshing requires a stepwise approach considering long-term variation of the sea state, ship motion in a stochastic seaway, fluid motion in the tanks, pressures acting on the tank structure, structural load effects, and their comparison with the appropriate resistance. This paper is focused on the determination of the critical conditions to limit the number of combinations of sea states, vessel heading, and speed that need to be considered in estimating extreme sloshing pressures. In this context, an approximate method is introduced and validated. These sea states are determined by a full linear long-term analysis. The response quantiles suitable for the contour line approach are also found using simplified criteria. The effect of avoiding heavy weather in vessel operation on sloshing and whipping-induced pressures is investigated. Sloshing experiments and various scaling approaches are described. Application of the generalized Pareto distribution and correctly determining its shape parameter is discussed. Structural response of the LNG membrane tank wall is investigated. The importance of accounting for the steel flexibility in calculating sloshing response is shown. A new method for approximating the time history of sloshing impact pressure is suggested.

Influence of Whipping on Long-term Vertical Bending Moment

This paper is concerned with estimating the response value corresponding to a long return period, say 20 years. Time domain simulation is required to obtain the nonlinear response, and long time series are required to limit the statistical uncertainty in the simulations. It is crucial to introduce ways to improve the efficiency in the calculation. A method to determine the long-term extremes by considering only a few short-term sea states is applied. Long-term extreme values are estimated using a set of sea states that have a certain probability of occurrence, known as the contour line approach. Effect of whipping is included by assuming that the whipping and wave-induced responses are independent, but the effect of correlation of the long-term extreme value is also studied. Numerical calculations are performed using a nonlinear, hydroelastic strip theory as suggested by Xia et al (1998). Results are presented for the S-175 containership (ITTC 1983) in head sea waves. The analysis shows that whipping increases the vertical bending moment and that the correlation is significant.