Robert Randall, Texas A&M University, Evan Zimmerman, Delmar, Katelyn McCarthy, The University of Texas at Austin, Ching-Hsiang Chen, The University of Texas at Austin, Aaron, Drake, Texas A&M University, Po Yeh, Texas A&M University, Chao-Ming Chi, Texas A&M University, Ryan Beemer, Texas A&M University
The goal of this project was to increase the understanding of DEA performance, and improve the design and application practices, so as to increase the overall reliability of their application for moored MODUs. The research methodology included experimental investigations, data analysis, and engineering interpretations that can be used to develop recommendations for regulatory assessment and engineering design practices. The investigations covered anchor behavior under so-called “in-plane loading” that occurs when anchor loads act within the plane intended by the designers (i.e., the plane of symmetry of the anchor), and under “out-of-plane” loading that can occur when partial failure of the mooring system occur.
Large-scale Test Findings:
- The in-plane results show the tow angle did not have a significant effect on the magnitude of the force. The largest effect on the force was due to the fluke angles.
- The major effect on penetration depth was the fluke angle with the larger fluke angle resulting in the largest penetration depth.
- The out of plane tests generally showed the 50 degree fluke angle produced a larger maximum increase in penetration depth, a higher force at the shank pad eye, and larger roll angles that those measured for the 36 degree fluke angle.
- Detailed measurements of the anchor line direction and anchor trajectory indicated that the anchor line under out-of-plane loading can orient itself into a reverse catenary in an oblique (non-vertical) plane. Accompanying this effect was an apparent increase in the anchor bearing factor Ne. Analytical modeling of the behavior leads to two counteracting trends. If the anchor line lies in an inclined plane, the anchor embedment depth under continued dragging will be reduced. In contrast, a higher bearing factor Ne. will lead to both deeper anchor embedment and greater holding capacity. This finding was in fact supported by some of the small-scale tests, where the anchor can actually dive more deeply under out-of-plane loading than for in-plane loading.
Small-scale Test Findings:
- The small-scale tests include drag embedment tests using thick and thin anchor lines. The use of a thin anchor line led to significantly (50%) greater embedment than that for a thick line. This observed behavior is consistent with theory.
- The initial orientation of the anchor can affect the anchor trajectory during the early stages of embedment, but as drag embedment continues the anchor trajectory converges to a unique path, regardless of initial orientation. A unique trajectory occurs after about 1 fluke length of drag.
- The bearing factor of the small-scale anchor exceeded that of the large-scale anchor by about 10%. This difference is attributed largely to the larger relative fluke thickness of the small-scale anchor. Overall, the scale effect did not appear to be a significant factor in DEA behavior.
- In some tests, after the out-of-plane load was applied, the anchor simply turned into the new direction of applied loading and exhibited behavior similar in all respects to a trajectory typical of in-plane loading.
- Neither the experimental data nor the model simulations indicate (at least for out-of-plane angles up to 30 degrees) a reduction in the initial installation capacity. Model predictions under even the most conservative assumptions indicated continued embedment and increasing holding capacity under out-of-plane loading conditions.
- In spite of this generally positive assessment regarding the effects of out-of-plane loading, it is pointed out that this research is the first attempt both experimentally and analytically to investigate the process. In spite of the insights gained in this research, many questions remain. At this point it is considered reasonable to assume that there will be no reduction of initial installation holding capacity under out-of-plane loading. Additional reserve capacity likely exists, but further studies are recommended before reliable quantitative estimates could be made.
Recommended Future Work:
This research has highlighted a number of areas that merit further investigation. Particularly pressing needs are as follows:
- Effect of Anchor Geometry on Load Capacity. One outcome of this research was to quantify the bearing factor for a generic anchor for various fluke-shank angle settings. In addition, a widened-fluke variant of the original design was tested. The findings indicated that anchor geometry – i.e., fluke-shank angle, fluke thickness, fluke width, shank configuration – all have significant influence on the anchor bearing factor, which controls both anchor trajectory and anchor capacity. Although some variations in anchor geometry were considered in this study, a test matrix encompassing the full range of geometries for anchors used in practice is still needed. Since drag anchors used in practice typically have complex three-dimensional geometries, the most reliable and cost-effective means for evaluating their load capacity characteristics; i.e., bearing factor. A study for evaluating anchor load capacity as a function of anchor geometry would have two main thrusts as outlined in this study: (1) load tests under “pure” loading (translation normal and parallel to fluke, and rotation), and (2) drag embedment tests to evaluate the effective bearing factor during drag embedment.
- Effect of Taut Line Conditions. During a number of tests in this study, particularly the out-of-plane tests, the anchor line transitioned from a “reverse catenary” configuration to a “taut” state for which the anchor line exhibits essentially no curvature. The anchor trajectory model developed and refined in this research implicitly assumes that the anchor line is a reverse catenary; therefore, the model needs to be expanded to accommodate the possibility of a taut anchor line condition. It is important to note that a taut anchor line will affect both anchor trajectory and the effective bearing factor of the anchor. In addition, while the formation of a taut line is relatively uncommon for drag embedment anchors, it can be much more common for vertically loaded anchors (VLAs). Indeed, conventional installation procedures for VLAs involve shortening the mooring line to trigger the shear pin, so taut anchor line conditions may be likely to develop during this process. Therefore, improving our understanding of anchor behavior under taut anchor line condition will be an important step in extending the DEA trajectory-capacity model developed in this study to VLAs. The main thrusts of this recommended research effort would be: (1) analytical studies to formulate a theoretical framework for predicting anchor behavior under taut line conditions, and (2) laboratory model tests of anchor behavior under taut line conditions.
- Effect of Initial Anchor Orientation. The model tests in this study shed a great deal of light on the effect of the initial anchor orientation on DEA trajectory and, very significantly, how a unique trajectory appears to develop that is essentially independent of the initial anchor orientation. The DEA trajectory-capacity model was modified to simulate this behavior, but two tasks remain: evaluating and modifying as necessary the exact form of the equation used to modify the original program, and establishing a reliable means for selecting the model parameters (l and c in the current version) required for this equation. The main thrusts of this recommended research effort would be: (1) additional laboratory model tests where the initial anchor orientation is systematically varied, and (2) analytical studies to support the validation and modification of the model.