Interaction Between The Drag Anchor And The Installation Line In Seabed Soils

With resource exploitation reaching deep and ultra-deep waters more often, the research of deepwater mooring systems and foundations has become a front and critical topic in the field of the international ocean engineering. Because of better performances both in pullout capacity and deepwater installation, drag anchors are increasingly applied in deepwater mooring systems. During the anchor installation, complicated interaction happens between the anchor and the installation line at the shackle. The penetration performance and kinematic characteristics of the anchor are directly affected by the embedded line, and the complex and irregular shape of the embedded line relates to the anchor. It is at the attachment point or the shackle of the fluke that the anchor and the installation line link together, analyzing the interaction of the anchor and installation line can be equivalently analyzed through investigating the drag force and the drag angle at the shackle. Therefore, studying on the interaction between the drag anchor and the installation line at the shackle is significant to the engineering design and simplified analysis of drag anchors.In this paper, the interactional properties between the drag anchor and the installation line have been theoretically and experimentally investigated. The expression of the drag force at the shackle is theoretically derived by two different ways both for cohesive and noncohesive soils. One is based on the mechanical model for the embedded line. The other is based on the mechanical model for the drag anchor. In order to examine the developed analytical methods, the experimental data are employed to compare the predicted drag force at the shackle. It is concluded that the theoretical expressions can reasonably predict the drag force at the shackle with given suitable values of the parameters as well as the necessary measured data. Meanwhile, a systematic parametric analysis is performed to explore the effects of different parameters on the drag force, and to determine the reasonable values of several key parameters, i.e., the bearing capacity factor N q, the lateral soil pressure factor K , and the angle of interface friction ? . These suggested values are especially meaningful for the analysis of sand-structure interactional problems.Being an important part of the interactional study, 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 theoretically and experimentally investigated. The predicted drag angles at the shackle generally agree well with the measured data at the stable penetration stage of the anchor. During penetration, the drag angle at the shackle keeps always a constant and is actually independent of the anchor and soil properties except the shank angle of the anchor. Besides, the developed analytical method for determining the drag angle at the shackle is examined by comparing with the available data of anchor models reported in the literature. For the drag anchor with rigid shanks, the predicted value of drag angle is bigger than the shank angle and the difference ranges from 2.6°to 8.7°; for the drag anchor with flexible shanks, the predicted value of drag angle exactly equals the shank angle.