Charles Aubeny

Behavior of Suction Embedded Plate Anchors during Keying Process

Suction embedded plate anchors (SEPLAs) allow for accurate positioning, thus providing an attractive alternative to traditional drag embedment anchors. This paper presents an analytical model for predicting the behavior of SEPLAs during the keying process, with a specific focus on predicting the loss of embedment depth as the anchor rotates from its initial vertical position to its target orientation perpendicular to the direction of loading. The soil is idealized as an incompressible, rigid-plastic material obeying an associated-flow rule. A generalized plastic limit analysis is employed to estimate the trajectories and corresponding capacities of SEPLAs under different loading conditions. The effects of soil resistance on the shank and anchor interaction with the anchor chain are also considered in the model. The SEPLA design commonly features a hinged flap; the effect of the flap is examined theoretically by comparing the solutions for the SEPLA with and without flap. Predicted solutions are shown in r...

Undrained Capacity of Plate Anchors under General Loading

Under general conditions of loading, a plate anchor is subjected to six degrees of freedom of loading, three force components and three moment components. Prediction of the anchor performance under general conditions of loading requires realistic estimates of the anchor pullout capacity for each individual load component as well as the interaction effects when these loads are applied in combination. This paper presents an analysis of plate anchor capacity under these general conditions of loading. The study considers a range of plate width-to-length ratios ranging from 1:1 to 2:1. The anchor capacity estimates and interaction relationships were developed based on finite-element studies and upper bound plastic limit analyses. Interaction relationships developed from the numerical and analytical studies were fitted to a simple six degrees-of-freedom yield locus equation.

Mechanics of Drag Embedment Anchors in a Soft Seabed

This paper presents an analysis of the load capacity and trajectory of a drag embedment anchor in a soft seabed. Anchor capacity relationships are developed for an idealized anchor comprising a rectangular fluke and a cylindrical shank. Geometric variables considered for the anchor include fluke length, fluke thickness, shank length, angle between fluke and shank, and shank thickness. Parametric studies are presented investigating the effect of these variables on anchor capacity and performance. A method of anchor trajectory prediction during drag embedment is developed by considering anchor behavior in conjunction with the mechanics of the anchor line. The anchor trajectory simulations indicate that an equilibrium condition rapidly develops during embedment in which the rate of anchor rotation is identical to the rate of change in the anchor line uplift angle at the shackle point. At the equilibrium state, the anchor load capacity normalized by soil strength remains constant and the anchor is in a state ...

Interaction Model for Steel Compliant Riser on Soft Seabed

The use of catenary steel-compliant-riser (SCR) systems has increased as hydrocarbon production has moved progressively farther offshore and into deeper waters. The issue of fatigue damage caused by cyclic interaction of a riser with the seabed has gained prominence with the widespread use of SCRs and with the lengthening of the spans. The problem involves a number of complex factors, including trench configuration, nonlinear soil stiffness, breakaway of the riser from the seafloor, and degradation of soil resistance during cyclic loading. This paper presents a soilinteraction model capable of modeling these complexities, using input parameters that can be obtained with reasonable expenditure. Model simulations for typical offshore soft-soil conditions indicate that the model is capable of realistic predictions of cyclic bending moments. The degradation of soil resistance has a major effect on cyclic bending moments, particularly when uplift motions at the riser touchdown point (TDP) are large.

Effect of Rate-Dependent Soil Strength on Cylinders Penetrating Into Soft Clay

This paper presents a predictive model for undrained penetration of cylinders into soft seafloor soils. The penetration depth will depend on the velocity of the cylinder as it touches down at the seafloor, and the net deceleration of the cylinder as it is acted on by forces of self-weight, soil buoyancy, and soil-shearing resistance. The soil-shearing resistance force increases as a function of penetration depth and, due to the dependence of undrained shear strength on strain rate, penetration velocity. This paper presents finite element (FE) simulations that quantify both effects and form the basis of a simplified soil-resisting force model. Strain-rate effects are modeled within a framework of rate-dependent plasticity, with shearing resistance increasing semilogarithmically with increasing strain rate above a certain threshold strain rate. With all forces acting on the cylinder, estimated penetration depths are predicted from simple equations of motion for a single particle. Comparisons to laboratory tests involving penetration of cylinders into soft reconstituted marine clay show reasonable agreement between model predictions and measurements.

Refined Model for Inclined Load Capacity of Suction Caissons

Suction caissons used as mooring anchors for offshore structures can, depending on design concept, be subjected to pullout forces ranging from nearly vertical for tension leg platforms, to intermediate inclination angles for taut mooring systems, to nearly horizontal for catenary moored systems. Hence, the ability to understand and predict suction anchor pullout resistance for a full range of load orientations is becoming of increasing importance. A previous paper by the authors presents a plastic limit analysis for estimating the load capacity of suction anchors over a full range of load inclination ranging from horizontal to vertical. The model was capable of predicting load capacity for various load attachment (padeye) depths, caisson aspect ratios, and soil undrained strength profiles that vary linearly with depth. Loading conditions are assumed to be undrained; therefore, a purely cohesive soil is assumed. The original analysis assumed full adhesion on the boundaries of the caisson; i.e., a skin resistance coefficient α equal to unity. However, actual values of this coefficient are less than unity, with specific values varying according to soil conditions and the method of caisson installation. To overcome the limitation of the original model, this paper presents a modified formulation that allows a skin resistance less than unity. The modified formulation develops an interaction relationship between vertical and horizontal soil resistance on the sides of the caisson that is applicable for any skin resistance conditions ranging from no to full adhesion.Copyright

Analytical Model for Vertically Loaded Anchor Performance

AbstractVertically loaded anchors (VLAs) are installed in a manner similar to drag-embedded anchors, except that after initial drag installation the anchor shank is released to enhance pullout capacity. Because of their lightness and relatively low installation cost, VLAs offer an attractive alternative for mooring systems. However, existing design procedures are largely empirically based, and uncertainties with regard to anchor trajectory and load capacity have limited their application to temporary mooring systems. This paper presents a design model for predicting VLA performance during drag installation, shank release, and subsequent loading beyond the installation load. Model-based parametric studies on the effects of VLA fluke–shank angle and anchor line angle at the mudline are also presented. Some aspects of the model have been benchmarked against limited available field data, but a broader validation is still needed.

Penetration of cylindrical objects in soft mud

The depth of penetration of objects impacting into soft sediments in the seafloor will depend on a number of factors including sediment shear strength, impact velocity, orientation of the object, and the density of the object. A critical step in developing a model for predicting depth of penetration is establishing a database of impact penetration measurements under closely controlled conditions. Toward this end, researchers at Texas A&M University constructed a 4-ft deep by 6-foot diameter test basin for conducting impact tests. The penetration experiments used medium to high plasticity sediments collected from the Gulf of Mexico seafloor. Sediments were processed to achieve uniformity and shear strength characteristics were measured using miniature vane tests. Impact tests were conducted using an apparatus that could vary the object density, impact velocity and orientation. Test results were evaluated based on a simplified model that used plasticity theory to describe instantaneous soil resistance and a simple kinetic model to describe motions of the object. The test results indicated that sediment shear resistance is a critical factor controlling penetration depths.

Two-Dimensional Moisture Flow–Soil Deformation Model for Application to Pavement Design

Seasonal cycles of wetting and drying in highway subgrade soils induce corresponding cycles of soil swelling and shrinkage that contribute to roughness and loss of serviceability of pavements. The design of defensive measures to mitigate the effects of this process requires a realistic model of moisture diffusion and associated swelling and shrinkage of soil. A Windows-based two-dimensional finite element program, FLODEF, is presented; it performs a sequentially coupled flow-displacement analysis for the prediction of vertical movement in highway subgrade soils. The theoretical formulations of moisture flow and stress-deformation components of the model are introduced first. Parametric studies illustrate capabilities of the model to evaluate the effectiveness of vertical moisture barriers, horizontal barriers, soil replacement or improvement, and paved medians in reducing swell-shrink deformations beneath highway pavements.

Simplified Analysis of Unsteady Moisture Flow Through Unsaturated Soil

The moisture diffusion properties of unsaturated soils control the rate of infiltration of surface moisture into the soil mass and hence are critical to a wide variety of civil structures, including pavements, structures, retaining walls, and slopes. Because of the dependence of permeability on suction and the nonlinearity of the suction–moisture relationship, the analytical formulation for flow through unsaturated soils is highly nonlinear. An approximate linear analysis of this problem, which was originally proposed by Peter Mitchell, was investigated. One advantage of this approximate analysis is that it can provide the practical basis for measuring soil moisture diffusion characteristics in laboratory tests. A second advantage is that the linear formulation provides an analytical tool accessible to practitioners. Mitchell originally based his formulation on a relatively restrictive assumption on the permeability-versus-suction relationship. An approach to circumventing that restriction is proposed. The findings of a laboratory test program that uses Mitchell’s formulation to estimate a soil’s moisture diffusion characteristics are presented. Finally, some simple analytical predictions demonstrate the practical significance of the soil moisture diffusion properties.