| As the exploration of oil and gas moving into deep water area, the platform of offshore oil and gas exploitation faced with increasingly harsh environment. Collapse of the platform will cause serious casualties and great financial losses. As the important component of offshore oil and gas exploitation platform, mooring system plays a key role in keeping the platform safe in operation. The increasingly deep water depth presented new requirements on the mooring system, while the mooring system is developing gradually from conventional gravity base structure into catenary system that constituted by anchor and chain. Due to convenient for installation and reliable in operation, gravity installed plate anchors and suction embedded plate anchors have been widely deployed in offshore engineering. However, the performance of these two plate-type anchors (plate anchors) in deep-water clay application has not been fully cognized. The posture of the anchor at the state of installation completed is different from the posture of the anchor at the state of the bearing force. The anchor will experience a keying process to reach the final bearing force state. The keying process will result in embedment loss of the anchor, thus to decrease the anchor pullout capacity, while the pullout capacity is the main issue of anchor design and the hot issue and basic problem in researches of offshore engineering. Therefore, the research on the keying process and pullout capacity are theoretically important and is beneficial to application.Taking the gravity installed plate anchor (GIPLA) and suction embedded plate anchor (SEPLA) as objects, this thesis adopted plasticity analysis approach to investigate the keying process and pullout capacity of the anchors in clay. Results were compared with that from three-dimensional finite element (3D-LDFE) analyses. The effect of pullout inclination and padeye offset angle on the embedment and pullout capacity and the coupled effect of these two factors were studied using plasticity analysis. Based on these studies, the condition of application and the direction of anchor optimization design to enhance the anchor pullout capacity are found.The first part of the thesis studied the keying behavior of the GIPLA in clay. Due to the complex geometry, the current researches of the GIPLA were experimentally based. Most of them were the prediction of penetration depth and the others are pullout model tests. Numerical analyses of the anchor pullout capacity and keying behavior in clay have never been found in publications. In order to probe into the regular pattern and inner mechanism of the GIPLA "diving" behavior when pulled upward, a plasticity model was built based on small strain finite element analyses and least squares regression scheme. The anchor keying/rotation process was studied using plasticity analysis, results from these two methods were compared and good agreement is achieved. Then the plasticity analysis was used to carry out parametric study of the pullout capacity and embedment loss in terms of load inclination, padeye offset and soil strength profile. Results indicated that the gravity installed anchor "diving" after keying process and the pullout capacity increased when the loading angle and padeye offset were set up properly.The second part of the thesis investigated the keying behavior of the SEPLA in clay. SEPLA has a keying flap which can rotate relative to the fluke. The effectiveness of the flap has been studied by many investigators. However, these researches simplified the anchor in a considerable degree due to the complex geometry of the SEPLA, which will result in great error. This thesis carried out accurate simulation of the keying process of the anchor and investigated the actual effectiveness of the keying flap with the actual structure and application condition of the SEPLA thoroughly considered. Finally, the "diving" potential of the SEPLA was investigated using parametric study and the anchor optimization design was presented. |
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