Penetration Mechanism and Kinematic Behavior of Drag Anchors
【摘要】：With resource exploitation reaching deep and ultra-deep waters more often, large floating structures such as the FPSO, Spar and SEMI combining with sufficiently effective and reliable mooring systems are becoming increasingly popular in deepwater offshore engineering. Because of better performances both in pullout capacity and deepwater installation, drag anchors, including the vertically loaded plate anchors, are increasingly applied and play a key role in deepwater mooring systems. The penetration mechanism and kinematic behavior of the drag anchor in seabed soils is not only a complicated engineering problem, but also an interesting topic with lots of scientific meanings. In this dissertation, some fundamental problems that are closely related to the penetration mechanism and kinematic behavior of drag anchors are systematically investigated by synthetically employing theoretical and experimental methods.
Firstly, a contact measurement technique, which is based on the knowledge of the motion characteristic of the drag anchor in soils, is developed and applied in measuring the trajectory of the anchor models. This technique is easier to perform and has little effect on the behavior of drag anchors in soils. The measurement precision and applicability of the technique are verified through specially designed experiments. Being an important application, the measurement technique is adopted to detect the movement direction of the anchor with an arbitrary fluke section. The present work has demonstrated that, through model tests and employing the developed technique, the movement direction of any type of drag anchor can be effectively and easily determined.
Secondly, five fundamental problems closely relevant to the penetration mechanism and kinematic behavior of drag anchors, including the effects of the initial orientation of the anchor, the effects of the shank angle, the relationship between the drag distance and the anchor displacement in soil, the relationship between the fluke orientation and the drag angle at the shackle, and the movement direction of the anchor, are investigated through model experiments. By systematically analyzing the kinematic parameters and trajectories of the anchor, further knowledge of the five fundamental problems, which is believed to be beneficial to improving our understanding on the penetration mechanism and kinematic behavior of drag anchors, is clearly obtained.
Thirdly, an analytical method that is capable of predicting the movement direction of the drag anchor with an arbitrary fluke section in both cohesive and noncohesive soils is developed. The analytical method is verified by experiments and applied to predicting the movement direction of the anchor with rectangular and wedge-shaped sections, and clear prediction results are obtained. Some important features of the movement direction are revealed, especially the step change of the movement direction from the unstable to the stable penetration stages due to the anchor weight. The present work proves that the movement direction of the drag anchor with an arbitrary fluke section can be reasonably determined by the developed analytical method.
Finally, the interactional properties between the drag anchor and the installation line are theoretically and experimentally investigated. Based on the mechanical models for the embedded line and the drag anchor, analytical expressions that can predict the drag force at the shackle in both cohesive and noncohesive soils are derived. Analytical methods that can predict not only the variational property but also the exact value of the drag angle at the shackle during penetration of the anchor are also developed. In order to examine the developed analytical methods, experimental data are adopted to compare the predicted drag force and drag angle at the shackle. Meanwhile, a detailed parametric analysis is performed to explore the effects of different parameters on the drag force and the drag angle, and to determine the reasonable values of several key parameters.