Papers by Keyword: Jacket Platform

Abstract: Offshore platforms are marine buildings commonly used for oil and gas exploitation activities. In general, reported failures in the life of offshore structures are fatigue failures resulting from environmental factors, such as random and continuous wave loads. In addition to environmental factors, the determination of the dimensions and thickness of the structure also plays an important role in increasing its strength. This study used Finite Element software with in-place analysis to calculate the strength and deterministic fatigue method for fatigue life analysis. This study aims to analyze the effect of the thickness of the jacket structure members on the strength and fatigue life. The results of the analysis showed that there was an increase in maximum UC and a decrease in fatigue life due to a reduction in thickness in the jacket members, where the initial model had a maximum UC of 0.64 with a fatigue life of 1285.83 years, while the 10% thickness reduction had a UC of 0.71 with a fatigue life of 552.07 years, a thickness reduction of 20% had a UC of 0.79 with a fatigue life of 213.80 years, a thickness reduction of 30% had a UC of 0.90 with a fatigue life of 62.14 years and a thickness reduction of 40% had a UC of 1.04 with a fatigue life 14.71 years. This research is expected to be a reference in designing the jacket structure to determine the optimal dimensions according to the planned fatigue life.

Abstract: Risk-based inspection analysis is a key part of structure integrity management (SIM) process for ensuring the fitness-for-purpose of offshore jacket platforms. For the platforms locates in the South China Sea which often suffer severe sea condition, it is essential to employ an appropriate assessment method to determine the structure strength and increase the platforms’ safety. This paper presents a comprehensive risk-based inspection method for in-service jacket platform structure. Nonlinear pushover analysis is used to carry out the ultimate bearing capacity and failure consequence of the structure, in which the platform ultimate bearing capacity is characterized in terms of the Reserve Strength Ratio (RSR) and Residual Capacity Ratio (RCR). Taking the four limit state equations of structure member and limit state judgment criteria of structure system into consideration, the structure reliability analysis is proposed with the combination of extreme probability model of environment loads. On this basis, a risk matrix will be formed and a risk-based inspection plan can be developed. A case study of jacket platform located in the South China Sea is used to illustrate the application of the structure assessment approach.

Abstract: According to the phenomenon that global damage detection of jacket platform is influenced by structure redundancy of damaged member, the concept of modal redundancy index (MRI) of damaged member is proposed. Through numerical simulation by means of mode strain energy method, the way that MRI of damaged members in different types and positions changed was studied, as well as the relationship between MRI and global damage detection. The numerical simulation result demonstrated that when MRI of pile element is less than 2.1×103, damage can be detected; when MRI of slanted brace element is less than 1.5×105, damage can be detected; when MRI of horizontal brace element is more than 5.8×105, damage cannot be detected. Therefore MRI can be a reference index in engineering application of global damage detection of offshore platforms.

Abstract: Offshore jacket platforms have been widely used in offshore oil and gas exploitation under hostile ocean environments. Finite element models of such structures need to have many degrees of freedom (DOFs) to represent the geometrical detail of complex structures, which leads to more computing power when performing the analysis and what’s more, the incompatibility in the number of degrees of freedom to the experimental models. Therefore, there is a need to simplify the analytical model by reducing the DOFs and in the process, making the essential eigen-properties agree with those of the experimental model is desired. In this paper, a scaled physical experimental model of an offshore jacket platform is simplified using the recently developed model refinement scheme. Mathematically, the procedure to implement the model refinement technique is an application of cross-model cross-mode (CMCM) method for model updating. The master degrees of freedom are chosen according to the placement of accelerometers in the experiment. Upon the completion of the refinement, the improved reduced jacket platform model matches the dynamic characteristics of the experimental model quite well.

Abstract: Jacket structures are designed for a specific water level (LAT). One of the important issues about these structures is the water level rise. For example, the level of water in the Caspian Sea has risen by 2.5m in the last fifteen years and is continuing to rise. The aim of this study is to investigate the behavior of one typical shallow or medium water jacket platform (a four-leg steel jacket platform in 55m water depth) under water level rise and to evaluate utilization factor of different element at different elevations. In this paper the finite element method (ANSYS) was used for the purpose of modeling of the structure. This paper discusses the effect of water level variation acting on the jacket structure and illustrates subsequent results of the stress utilization within the structure elements. The results of time history analysis show that the initial design water depth (before water level rise) has a basic role on the subsequent behavior of structural elements.

Abstract: Jacket-type platforms may be the most common type of offshore structures, and damage localization and severity estimation is important for these structures. This paper employs a multi-hierarchical damage identification method based on BP neural network to detect damages in jacket platforms. Firstly, the damaged storey of the jacket is detected, and the numbers of the elements among the detected storey are then detected. According to this method, the learning samples can be more targeted and the number can be reduced largely. In the end, a jacket model is used to investigate the performance of this method, and the results indicate that this approach is more effective and has higher accuracy than direct diagnosis method.

Abstract: The main mode of pile-soil interaction and the influence factors of pile sinking in offshore platform construction were analyzed and discusses the reasons that penetration resistance increases after stop driving and then continue. To study the drivability of piles of specified driving hammer, taking a jacket platform of Bohai Sea for instance, the results showed that continuous driving will make the open-end steel piles be driven to the designed depth, and driven-on will cause soil plug and pile driving refusal. Based on the results, the paper gives some suggestions about pile driving.