Muhammad Shazzad Hossain

New mechanism-based design approach for spudcan foundations on single layer clay

Spudcan foundations for offshore mobile drilling rigs are large saucer-shaped foundations that can penetrate several tens of meters into soft sediments. The penetration depth is typically predicted by considering a wished-in-place foundation at different depths and following traditional bearing capacity approaches to assess the depth at which the estimated capacity matches the applied loading. However, the geometry of the spudcan and its progressive mode of penetration lead to soil failure mechanisms that differ markedly from those relevant to onshore practice. This paper presents a new rational design approach for assessing spudcan penetration in single layer clays based on a study combining centrifuge model testing and large deformation finite-element (FE) analysis. The design approach takes account of the evolving failure mechanisms in the soil, which start with cavity formation and surface heave at shallow penetration, gradually transforming to backflow of soil over the spudcan. A detailed FE parametric study has explored the relevant range of normalized strength, strength nonhomogeneity, and spudcan base roughness, with results validated against centrifuge model test data. The penetration response curves are presented in terms of profiles of bearing capacity factors, forming nondimensional design charts along with simplified expressions for convenient use in practice. Comparisons with approaches suggested in the SNAME design code suggest an urgent need to update current practice.

Experimental investigation of installation and pullout of dynamically penetrating anchors in clay and silt

AbstractThis paper reports the results from a series of model tests undertaken to provide insight into the behavior of torpedo anchors during dynamic installation and pullout in lightly overconsolidated kaolin clay and calcareous silt. The tests were carried out in a drum centrifuge at 200g, varying the drop height (hence the impact velocity) and the time delay for consolidation before pullout. The pullout angle at the mudline was also varied to encompass various mooring systems, including catenary (0°), taut leg (45°), and tension leg (∼80°). Two geometries of torpedo anchors were explored, varying the fin and tip geometry. The results demonstrated that the anchor embedment depth increased as the drop height (and hence the impact velocity) increased and the soil undrained shear strength decreased. In stronger silt, the cavity above the installing anchor remained open, whereas in soft clay, it was fully backfilled and replenished. The corresponding anchor embedment depth was also about 0.63 times compared...

The Performance of Dynamically Embedded Anchors in Calcareous Silt

The Omni-Max anchor is a new type of dynamically installed anchor featuring a mooring arm located close to the anchor tip that is free to rotate about the anchor length. An experimental programme has been undertaken on a geotechnical centrifuge to assess the anchor performance in calcareous silt. The testing programme includes (i) anchor drops and measurement of the penetration depth, and (ii) anchor pull and assessment of the anchor trajectory. An anchor embedment model based on strain rate enhanced shearing resistance is capable of satisfactorily predicting anchor embedment using model parameters that are similar to those used for clay. The diving behaviour of the anchor, post keying, is demonstrated, provided that the initial embedment after impact is deep enough to prevent a shallow mechanism to develop during anchor keying.© 2013 ASME

Spudcan Penetration Analysis for Case Histories in Clay

This paper reports advances in the prediction of spudcan resistance profiles during installation and preloading, based on the soil flow mechanisms observed in model tests and numerical analysis, and taking into account strain softening and rate dependency on the soil strength. Deep penetration of spudcan foundations was simulated for 14 case histories reported from different locations in the Gulf of Mexico, where the soils were single layer, predominantly normally to lightly overconsolidated clay. Axisymmetric and three-dimensional analyses were carried out using two large deformation finite-element (LDFE) approaches, modifying the simple elastic-perfectly plastic Tresca soil model to allow strain softening and strain-rate dependency of the shear strength. The field data were compared with the results of the finite-element analyses and with predictions using various design approaches. Excellent agreement was found, particularly when a mechanism-based design approach was used that differentiated between conditions before and after backflow of soil above the penetrating spudcan.

New Design Approach for Spudcan Penetration in Nonuniform Clay with an Interbedded Stiff Layer

AbstractThe paper describes a mechanism-based design approach for assessing spudcan penetration in soft nonuniform clay with an interbedded stiff layer. The proposed approach is developed on the basis of the results from a parametric study through large deformation finite-element analyses and validated against centrifuge test data. This can be taken as a “top-down” approach. Design formulas are proposed for assessing the limiting cavity depth and soil plug height. A direct comparison between the predicted profiles using the proposed design approach and the “bottom-up” approach recommended in an ISO standard confirms the benefit of using the former.

Rectangular foundations on a sand embankment over mine tailings

AbstractThis paper reports results from centrifuge model tests that provide insight into the behavior of rectangular foundations (representing earthmoving equipment) during penetration, with freedom in rotation, adjacent to a sand embankment (cover layer), into a weaker clay layer (representing mine tailings). A parametric study was used to explore the relevant range of the setback ratio, slope height ratio, normalized clay strength and its nonhomogeneity, and foundation base geometry. The effect of these nondimensional parameters on the penetration resistance profile is discussed in the context of the likelihood and severity of failure. The failure of a flat-based rectangular foundation was less severe than that associated with a strip foundation, and the corresponding measures reduced with the increasing setback ratio (λ) and slope height ratio (η). No rotational failure occurred for λ/(su,av/γcB)cr>8 (where su,av/γcB is the average normalized strength of the lower clay layer) or η≥0.604, regardless of ...

Extraction of spudcan foundations in single and multilayer soils

AbstractDifficulties are often encountered in a variety of seabed sediments during extraction of spudcan foundations of mobile jack-up rigs. This paper reports results from centrifuge model tests undertaken to provide insight into spudcan foundation behavior during vertical extraction through single, double, and multilayer soils. The model tests included half-spudcan tests against a transparent window, allowing the soil flow to be captured continuously by a digital camera and subsequently quantified through particle image velocimetry (PIV) analysis and separate full-spudcan tests to measure the uplift resistance. The observed soil-failure mechanisms provide insight with respect to the profile of extraction resistance. The mechanisms reveal evolving failure modes at different spudcan extraction depths and, in particular, changes in the failure patterns because of varying layer geometry, soil types, and properties. For single, double, and multilayer clays, suction at the spudcan base, shearing along a trunc...

Numerical Modeling of Spudcan Deep Penetration in Three-Layer Clays

AbstractThis paper reports the results from numerical modeling of spudcan deep penetration through three-layer clays. Two typical seabed strength profiles commonly encountered in the field, and identified as critical for potential punch-through failure, were considered: (1) uniform stiff-soft-stiff clay and (2) nonuniform clay with an interbedded stiff clay layer. Three-dimensional large deformation finite-element (LDFE) analyses were carried out with and without simulating strain softening and strain rate dependency of the shear strength. The results were compared with previously published LDFE results and centrifuge test data. A detailed parametric study was undertaken, varying the relevant range of layer thicknesses (relative to the spudcan diameter), strength ratios, normalized strength, and strength nonhomogeneity. Punch-through and rapid leg penetration (for stiff-over-soft) and squeezing (for the reverse) were demonstrated by the penetration resistance profiles and associated soil failure mechanism...

Installation of Stiffened Caissons in Nonhomogeneous Clays

AbstractA significant difference between predicted and measured installation resistance of stiffened suction caissons was identified due to the existing uncertainty regarding the mobilized soil-flow mechanisms. This paper describes an extensive investigation of stiffened-caisson penetration in nonhomogeneous clays undertaken through large deformation finite-element (LDFE) analysis to provide insight into the soil behavior during installation of a caisson. The soil-flow mechanisms around and between stiffeners, and inside and outside of the caisson, and the corresponding penetration resistances were presented from a parametric study, exploring a range of dimensionless parameters related to stiffened-caisson geometry, caisson roughness, and soil strength nonhomogeneity. The LDFE results were compared with centrifuge test data in terms of the soil-flow mechanisms and penetration resistance profile, with good agreement obtained. Three interesting features in the mobilized soil-flow mechanisms inside the caiss...

Physical and numerical modelling of installation and pull-out of dynamically penetrating anchors in clay and silt

Dynamically penetrating anchors (DPAs) are a recent type of mooring technology that have been shown to provide a cost-effective alternative to other forms of anchoring system in deep waters. This paper reports the results from a series of model tests undertaken to provide insight into the behavior of DPAs during dynamic installation and monotonic pull-out in normally consolidated clay and calcareous silt. The tests were carried out in a drum centrifuge at 200 g, varying the drop height and hence the terminal velocity. The pull-out angle at the mudline was also varied to encompass various mooring systems.This paper also reports the results from finite element (FE) analyses simulating dynamic installation of anchors. The 3D large deformation FE (LDFE) analyses were carried out using the Coupled Eulerian-Lagrangian (CEL) approach in the commercial FE package Abaqus/Explicit. A parametric study was undertaken, exploring a range of anchor geometry in terms of diameter, tip angle and number and length of fins; impact velocity and soil strength.For dynamic installation, two interesting aspects of the soil flow mechanisms were identified: (a) downward soil movement, concentrating around the advancing anchor, being reduced gradually with reducing penetration velocity and more rapidly with increasing number of fins and anchor projected area; (b) mobilisation of end bearing mechanism at the base of the anchor as well as fins, with the latter reduced significantly for shorter fins. The depth of anchor penetration increased as the drop height (and terminal velocity) increases and the soil strength decreases. For static pull-out, the mooring angle at the mudline influenced the pull-out mechanism and the capacity significantly. The anchor rotation and hence the anchor displacement required for attaining maximum capacity reduced as the angle increases.Copyright