Yinghui Tian

Failure Mechanism and Bearing Capacity of Footings Buried at Various Depths in Spatially Random Soil

Abstract The objective of this paper is to demonstrate how the spatial variability of random soil affects the failure mechanism and the ultimate bearing capacity of foundations buried at various depths. A nonlinear finite-element analysis combined with random field theory is employed to explore the vertical capacity of foundations embedded at different depths in random soil. Different possibilities of shear failures resulting from spatial patterns of soil are demonstrated and are used to explain the significant discrepancy between the bearing capacity of the random soil and that of uniform soil. The effect of the spatial pattern of the soil on the development of shear planes is also investigated, with the coefficients of variation for the bearing capacity demonstrated to be closely related to the shear plane length. The results of the statistical variation in the bearing capacity are provided for different embedment depths, and these are also reported as the failure probability of the footing compared wit...

Improving Plate Anchor Design with a Keying Flap

Suction embedded plate anchors have been increasingly applied in deepwater projects because of their efficiency and limited cost. Current industry practice equips the fluke of the anchor with an outwardly rotating keying flap aiming at a reduction of the loss of embedment during keying. However, recent experimental and numerical research has questioned the efficiency of the keying flap currently used in practice because it has demonstrated that the flap does not open during keying as was intended. This paper presents a two-dimensional plain strain large deformation finite-element analysis by assuming the soil as homogenous to investigate the performance of plate anchors equipped with various keying flap designs. The results of the paper agree with previous centrifuge and finite-element analysis that the outwardly keying flap has no effect on the loss of embedment and results in a reduction of the bearing capacity postkeying. The performance of an alternative keying flap design where the flap is rotating inward is also discussed. The new design effectively reduces the loss of embedment during keying and exhibits a bearing capacity higher than the current design.

Considerations on the Design of Keying Flap of Plate Anchors

AbstractOne of the critical issues associated with plate anchor performance and design relates to the reduction of the loss of embedment during the keying process. As deep water offshore sediments typically exhibit an increasing soil strength with depth, the loss of embedment results in a reduction in anchor bearing capacity. A keying flap hinged to the main plate has been developed and adopted by industry with the aim to reduce the loss of embedment by limiting the vertical motion of the anchor. However, uncertainties remain regarding the behavior and the performance of the keying flap. This paper presents a series of numerical analyses performed to investigate the flap rotation mechanism and the condition of activation of the flap. They are compared with existing centrifuge modeling. The numerical results validate the centrifuge observations and demonstrate the nonactivation of the keying flap for typical anchor pull-out conditions.

Pipe-Soil Interaction Model Incorporating Large Lateral Displacements in Calcareous Sand

The use of the plasticity theory offers an attractive framework to encapsulate the behavior of a pipe and the underlying soil in terminology consistent with pipeline structural analysis. Models that express the pipe-soil behavior purely in terms of the loads on a segment of pipe and the corresponding displacements have been suggested, although verification with geotechnical centrifuge experiments has been limited to relatively small lateral displacements (i.e., less than two pipe diameters). Over larger movements, the berms that build up alongside the pipe affect the load-displacement behavior, with existing strain-hardening plasticity models incapable of simulating this transition. This technical note provides experimental evidence of pipe-soil behavior for lateral displacements for up to five diameters. It further presents observations from 20 centrifuge experiments of a prototype 1-m-diameter pipe in calcareous sand. The results are used to validate the modification of a pipe-soil model to include the ...

Incorporating shank resistance into prediction of the keying behavior of suction embedded plate anchors

AbstractThe suction embedded plate anchor (SEPLA) is a promising deepwater anchoring solution that uses a suction caisson to install a plate anchor vertically to a precise depth within the seabed. The plate anchor is then rotated from its vertical position toward an inclination approximately normal to the load applied by the mooring line in a process known as keying. This configuration reduces the embedment depth and, in typically increasing strength with depth soils, diminishes the installed capacity of the anchor. Therefore, an accurate prediction of the keying behavior of SEPLAs is crucial for offshore operations. Recently, plasticity approaches that combine a yield surface written directly in terms of the applied loads on the anchor with an associated flow rule have been suggested. However, the combined loading yield surface has been defined only for a flat square or rectangular plate. None of these approaches directly account for the resistance of the relatively large anchor shank used to attach the ...

Buried footings in random soils: comparison of limit analysis and finite element analysis

The limit analysis and the finite element method are powerful tools for analysing the bearing capacity of foundations. Previous research mainly focused on the foundations in uniform soils. In realistic conditions, soil properties are always varying spatially due to complex physical, chemical, and biological process in earth evolution. This paper investigates the bearing capacity and failure mechanism of footings buried at various depths in clays with spatially variable distribution of undrained shear strength using the lower bound limit analysis, the upper bound limit analysis, and the finite element analysis. Results show that the bearing capacity increases with increasing buried depths in spatially random soils, which is the same as in the uniform soils. The bearing capacity factors calculated using the finite element method, the lower bound limit analysis, and the upper bound limit analysis for a footing in spatially varied soils are all smaller than the corresponding values in uniform soils. The majority of the bearing capacity factors obtained from the finite element method is bounded by those obtained from the lower bound and the upper bound limit analysis. The shear planes show a clearly unsymmetrical manner in spatially varied soils using the three methods, which is different from the symmetrical shear plane in uniform soils.

Application of Statistical Analysis Techniques to Pipeline On-Bottom Stability Analysis

Offshore pipelines are increasingly being employed to transport offshore hydrocarbons to onshore processing facilities. Pipelines laid directly on the seabed are subject to a considerable hydrodynamic loading from waves and currents and must be accurately designed for on-bottom stability. Confidence in the stability of pipelines requires appropriate models for their assessment and, in this paper, particular emphasis is placed on achieving an integrated and balanced approach in considering the nonlinearities and uncertainties in the pipe structure, the reaction of the restraining soil, and the hydrodynamic loading applied. A statistical approach is followed by developing a response surface model for the pipeline maximum horizontal displacement within a storm, while including variability in parameters. The Monte Carlo simulation method is used in combination with the developed response surface model to calculate the extreme response statistics. The benefit of this approach is demonstrated and also used to investigate the sensitivity of the on-bottom pipeline simulation for a variety of model input parameters. These results provide guidance to engineers as to what uncertainties are worth reducing, if possible, before a pipe is designed.

Uniaxial capacities of skirted circular foundations in clay

AbstractResults of finite-element analyses calculating the undrained capacities of skirted circular foundations under uniaxial vertical, horizontal, and moment loading are presented. Parallel finit...

Sustained Uplift of Skirted Foundation in Clay

A series of centrifuge tests have been performed to investigate the uplift behaviour of a shallow skirted foundation resting on clay subjected to various sustained loadings expressed as a fraction of the undrained bearing capacity in compression. Displacements, uplift loads, total and pore pressures underneath the foundation were monitored during testing to provide insight on the development of negative pore pressure (suction) at the foundation base and on the mechanism triggering breakout. The results indicate that the displacement rate and the time the uplift load can be sustained depend on the magnitude of the load, but also on the time the foundation experienced consolidation prior to uplift. Breakout was not observed at a relatively large time scale under low sustained loadings, while the foundation was pulled out directly under high loading levels. At intermediate loadings, breakout was inferred from the sudden increase in displacement rate. The breakout mechanism of the foundation is also discussed based on the observation of pore pressure variations underneath the foundation.The outcomes of the present study have the potential to provide a better understanding of the suction mechanism for offshore foundations and to provide guidelines for offshore retrieval operations.Copyright

A new method to investigate the failure envelopes of offshore foundations

This paper presents an alternative numerical method in addition to the traditional ‘probe test’ to investigate the combined loading failure envelopes of foundations in soil. In the ‘probe test’ method, the foundation is displaced with a specified displacement path and eventually the soil resistance force reaches a stabilised point sitting on the failure envelope in the load space. While the displacement paths are arbitrarily or empirically set, the positions of the stabilised loads on the failure envelope can not be predetermined or planned. This paper’s new method, however, can specify the load paths, which directly shoot onto the failure envelope. This allows the investigation of the failure envelope can be better achieved with planned load paths. In addition, this new method is advantageous in checking the plastic flow conditions (i.e. normality of the failure envelope) as the load path directions are predetermined.Copyright