Undrained Load Capacity of Torpedo Anchors in ... - laceo - UFRJ

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total vertical load unable to pull the anchor out. On the contrary, for inclinations higher than 45°, a small amount of soil is mobilized, but the reduction in the surrounding soil stiffness and the high vertical loads are enough to pull the anchor out and well characterize its load capacity. It is worth mentioning that the differences between the observed failure mechanisms make the determination of the anchor hold capacity a difficult task. On the one hand, for inclinations higher than 45°, the failure is clear and the hold capacity is well determined. On the other hand, for inclinations lower than 45°, the failure is not clear and a criterion should be established to determine the load capacity. One possible criterion would be the maximum total displacement reached. However, Figs. 12 to 14 point a significant variation of the maxima displacements achieved in each analysis. When horizontal loads are applied, displacements of about 25cm are verified, while, when vertical loads are applied, these values may be as low as 5cm. Hence, this seems to be not a suitable choice and more studies must be performed to tackle this issue. In this work, in the lack of a defined criterion, the maximum load achieved in each FE analysis is considered to be the hold capacity of the anchor hereafter. Thus, Tables 2 to 4 present the calculated load capacities for soils A, B and C. Load inclination Table 2 – Load capacities for soil A. Load Horizontal capacity component (kN) (kN) Vertical component (kN) 0° 5900 5900 - 15° 6075 5868 1572 30° 6413 5553 3206 45° 6628 4687 4687 60° 5981 2991 5180 75° 5325 1378 5144 90° 5176 - 5176 API 5241 - 5241 Load inclination Table 3 – Load capacities for soil B. Load Horizontal capacity component (kN) (kN) Vertical component (kN) 0° 11719 11719 - 15° 11869 11464 3072 30° 12280 10635 6140 45° 10526 7443 7443 60° 8916 4458 7721 75° 7950 2058 7679 90° 7756 - 7756 API 7163 - 7163 Load inclination Table 4 – Load capacities for soil C. Load Horizontal capacity component (kN) (kN) Vertical component (kN) 0° 20984 20984 - 15° 21608 20872 5593 30° 20414 17679 10207 45° 15694 11097 11097 60° 12958 6479 11222 75° 11850 3067 11446 90° 11456 - 11456 API 10018 - 10018 For soil A, Table 2 shows that the load capacity increases with the load inclination until the maximum value is reached at an angle of 45°. After that, the load capacity is reduced until the minimum value is reached for an inclination of 90°. For soil B, a similar behavior was observed, but the maximum load capacity is obtained at a load inclination of 30° and the minimum at 90°. For soil C, the maximum value occurs at an inclination of 15° and the minimum, again, at 90°. These tables also indicate that the increase of the undrained shear strength leads to an increase in the load capacity of the anchor. However, this increase is higher for lower load inclinations (between 0° and 15°) than for higher inclinations. For lower inclinations, the increase of the undrained shear strength provoked an augment of the load capacity almost in the same proportions, as the failure mechanism is directly related to the lateral resistance of the soil. For higher inclinations, the increase of the undrained shear strength implies the reduction of the adhesion factor, α, Eq. (21), which reduces the effect of the augment of the undrained shear strength by reducing the limiting shear stress on the interface anchor-soil. As, for these inclinations, the load capacity is largely dependent on the adhesion between the soil and the anchor, the load capacity for higher angles is less affected by the increase in the undrained shear strength of the soil. Moreover, the increase of the undrained shear strength amplifies the difference between the horizontal and the vertical load capacities. For soil A, the horizontal capacity is 14% higher than the vertical one. For soils B and C, these differences are, respectively, 51% and 83%. Tables 2 to 4 also present the horizontal and vertical components of each obtained load capacity. These values point that, for inclinations lower than 30°, the horizontal components are close to horizontal load capacity. On the contrary, for higher inclinations, the vertical components are almost coincident with the vertical load capacity. This is consistent with the described failure mechanisms. As a consequence, the load capacities of torpedo anchors subjected to loads with inclinations higher than 30° can be estimated by calculating the vertical load capacity of the anchor 10 Copyright © 2009 by ASME
and dividing this value by the sine of this angle. For lower inclinations, this approach is not suitable since the failure mechanism is different. Finally, Tables 2 to 4 also present the vertical load capacity estimated by the API [10]. When comparing the values from the FE model to the API [10] ones, differences of –1.3% (soil A), +7.6% (soil B) and +12.7% (soil C) are found. Despite the good agreement found, a discussion on these differences is necessary. The first aspect to observe is that the difference grows as the participation of the lateral friction in the load capacity of the anchor decreases. According to API [10], the total load capacity of the anchor embedded in soil A is 94.5% due to the lateral friction; in soil B, 92.5%; and, in soil C, 89.7%. Assuming that the load capacity is given by the sum of the lateral friction and the top resistance, the main difference between the values predicted by API [10] and the FE ones relies on how the top resistance is evaluated. The model proposed by API [10] considers that this resistance is given by an empirical constant multiplying the soil cohesion in the considered depth. This empirical constant (usually 9) was calibrated for piles with no flukes and under pure compression and was not incorporated to the FE model since the model is supposed to directly account for it. Therefore, the FE model values, when compared to the API ones, overestimate the top resistance of the anchor. This requires further experimental investigation. Effect of the number of flukes Here, FE analyses were performed to evaluate the effect of the number of flukes on the load capacity of a torpedo anchor. These analyses were carried out considering only soil B and torpedo anchors with 0, 3 and 4 flukes. In all these analyses, the applied loads are in the symmetry plane indicated as plane 1 in Fig. 10. It is important to mention that the reduction in the number of flukes from 4 to 3 leads to a decrease in the total lateral contact area of the anchor of about 16% and, from 4 to 0, 58%. Figure 16 presents the obtained load capacities. This figure points that the load capacities of the anchors with 4 flukes are about 1200kN higher than the ones of the anchors with 3 flukes for inclinations between 30° and 90°. This difference corresponds to an increase of about 15% in each load capacity and is quite close to the increase of the lateral contact area in the anchor with 4 flukes, as the failure mechanism is governed by the vertical load capacity of the anchor. For lower inclinations, this difference increases to 1600kN, which corresponds to values 16% higher in each load capacity and are again quite similar to the augment in the lateral contact area. Furthermore, a similar behavior was verified when comparing the load capacities of the anchors with 4 and no flukes. The values predicted for the anchor with no flukes are between 48% and 56% lower than the ones of the anchor with 4 flukes for all inclinations. This difference corresponds to reductions between 4000kN and 6600kN in the load capacities. Load capacity (kN) 14000 12000 10000 8000 6000 4000 2000 0 4 Flukes 3 Flukes 0 Flukes 0 10 20 30 40 50 60 70 80 90 Angle (degrees) Figure 16 – Load capacities for different inclinations considering a torpedo anchor with 3 and 4 flukes and soil B. Therefore, not only the vertical load capacity is increased by the presence of the flukes and the consequent augment in the lateral contact area of the anchor, but also the lateral load capacity is increased by nearly the same amount. This leads to the conclusion that the lateral load capacity is also directly related to the lateral contact area of the anchor. Effect of the flukes width In this set of analyses, the width of the flukes was modified to 0 (no flukes), 0.15m, 0.45m and 0.90m. All analyses were performed considering the anchor with 4 flukes and soil B. Again, the applied loads are in the plane of symmetry 1 presented in Fig. 10. For the sake of comparisons, it is worth mentioning that the reduction of the flukes width from 0.90m to 0.45m decreases the total lateral contact area of the anchor to 29%, while the reduction from 0.90m to 0.15m leads to a reduction of the lateral contact area of 48% and, finally, from 0.90m to 0m, a reduction of 58%. Figure 17 presents the obtained load capacities. This figure shows that the load capacities are extremely dependent on the width of the flukes. For all inclinations, a decrease in the width of the flukes provoked a reduction in the load capacity. The observed reductions are, again, directly related to the variation of the total lateral contact area. For instance, the load capacities obtained in the analyses with 0.45m, 0.15m and no flukes are, respectively, about 23%, between 37% and 43% and between 48% and 56% lower than the ones calculated for the anchor with 0.90m flukes. 11 Copyright © 2009 by ASME
Page 1 and 2: Proceedings of the ASME 2009 28th I
Page 3 and 4: E G = (7) 2⋅ K ( 1+υ) = 1000 ⋅
Page 5 and 6: Anchor modeling The torpedo anchor
Page 7 and 8: The FE mesh of the anchor interacts
Page 9: Load (kN) 14000 12000 10000 8000 60
Page 13: Comparisons with API [10] values sh

Undrained Load Capacity of Torpedo Anchors in ... - laceo - UFRJ

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