Bipul Hawlader

Numerical study of the factors affecting the consolidation of clay with vertical drains

Abstract A methodology has been developed for the consolidation analysis of a unit cell of clay around a single vertical drain. An elasto-viscoplastic constitutive model has been used to capture the time dependent consolidation behaviour of clay. The model considers a void ratio rate dependent yield stress to separate pre- and post-yield consolidation processes. The governing equation of the consolidation of clays with vertical drains has been solved using a robust finite difference scheme. A parametric study has been conducted to identify the influence of viscosity and smear on the consolidation process. It has been shown that viscosity has minimal influence on the consolidation process for closely spaced drains whereas its effects are significant for largely spaced drains. The inner smear zone surrounding the drains was found to be the dominant factor in controlling the overall progress of consolidation. Even small sizes of the smear zone significantly can retard the progress of consolidation. The permeability within the inner smear zone has been found to be more important than its extent. A linear variation of permeability within the inner smear zone using the coefficient of permeability of the undisturbed soil and the remolded soils has been shown to capture the important effects of smear on consolidation.

Numerical Analysis of Large-Diameter Monopiles in Dense Sand Supporting Offshore Wind Turbines

AbstractLarge-diameter monopiles are widely used foundations for offshore wind turbines. In the present study, three-dimensional finite-element (FE) analyses are performed to estimate the static lateral load-carrying capacity of monopiles in dense sand subjected to eccentric loading. A modified Mohr–Coulomb (MMC) model that considers prepeak hardening, postpeak softening, and the effects of mean effective stress and relative density on stress–strain behavior of dense sand is adopted in the FE analysis. FE analyses are also performed with the Mohr–Coulomb (MC) model. The load–displacement behavior observed in model tests can be simulated better with the MMC model than with the MC model. On the basis of a parametric study for different length-to-diameter ratios of the pile, load–moment capacity interaction diagrams were developed for different degrees of rotation. A simplified model, based on the concept of lateral pressure distribution on the pile, is also proposed for the estimation of its capacity.

Penetration of Steel Catenary Riser in Soft Clay Seabed: Finite-Element and Finite-Volume Methods

AbstractThe penetration of steel catenary risers and other cylindrical objects, such as offshore pipelines or T-bar penetrometers, in a soft clay seabed is of practical importance in deepwater oil and gas development. Finite-element (FE) analyses of these large-deformation problems are computationally very expensive. Water can also play a significant role through development of suction behind the riser. The main objective of the present study is to develop an advanced numerical modeling technique to simulate riser–seabed–water interaction near the touchdown zone. Keeping in mind two critical issues, namely the computational cost and modeling of suction, two different numerical modeling techniques are developed. In the first one, the computational fluid dynamics (CFD) approach is used. The CFD modeling is performed using ANSYS CFX 13.0 software. Among the three different types of CFX models developed in the present study, the subdomain modeling technique is found to be the most efficient. In the second num...

Fuzzy rule-adaptive model predictive control for a multivariable heating system

This paper presents design of a rule-adaptive fuzzy model predictive control (MPC) algorithm for controlling temperatures of a multivariable soil-heating process system. The system uses Takagi-Sugeno (TS) type fuzzy model structure. The control objective is to track a desired temperature profile at three locations in the soil sample using three heat sources located at the outer surface of the soil cell. The system recognizes the active fuzzy rules which are recursively adapted for handling the time-variant behavior of the process. For the simulations the soil-heating system is modeled using a general-purpose ABAQUS finite element (FE) program. The dynamic control program is linked to the FE model using a user-defined subroutine. In order to show the effectiveness, the performance of the proposed scheme is compared against the non-adaptive fuzzy model based MPC scheme. A classical non-adaptive MPC is also developed to confirm the superiority of the fuzzy model based MPC controllers

Numerical Modeling of Suction and Trench Formation at the Touchdown Zone of Steel Catenary Riser

Steel catenary risers (SCR) are widely used in deepwater oil and gas production. The riser-seabed-water interaction near the touchdown zone is one of the main concerns in the design of fatigue life of SCR. During upward displacement, suction develops under the riser and a trench might be formed when it separates from the seabed near the touchdown point. In the subsequent downward movement, the riser penetrates through this trench to the seabed. Therefore, modeling of suction and trench formation is very important. In the existing models available in the literature for uplift resistance, these factors are incorporated using empirical relationships. It is also recognized that the available finite-element (FE) modeling techniques for this large-deformation problem are computationally very expensive, although penetration behavior can be simulated. In the present research program, both penetration and uplift behavior are simulated using FE and computational fluid dynamics (CFD) approaches. The simulation results for penetration are presented in Hawlader et al. (2014). In this paper, CFD simulations of uplift resistance, suction and trench formation using ANSYS CFX are discussed. A new model for undrained shear strength of soft clay is proposed that is applicable to a wide range of shear strain rates. The effects of strain rate and strength degradation are incorporated properly in ANSYS CFX and simulations are performed for one penetration-uplift cycle. Comparing with empirical models developed from experimental results and also with FE results for idealized conditions, it is shown that the present CFX model can simulate the suction and uplift resistance. Moreover, the CFX model developed in this study using the subdomain approach is computationally very efficient. The suction under the riser is the main source of uplift resistance for shallow embedments. The parametric study shows that the maximum uplift resistance and depth of trench depend on uplift velocity and the undrained shear strength of clay.

Strain Softening and Rate Effects on Soil Shear Strength in Modeling of Vertical Penetration of Offshore Pipelines

Offshore pipelines play a vital role in the transportation of hydrocarbon. In deep seas, pipelines laid on the seabed usually penetrate into the soil a certain amount. These pipelines might experience significant lateral movement during the operational period. The resistance to lateral movement depends on vertical penetration and berm formation around the pipe. Vertical penetration is a large deformation problem. Finite element modeling of vertical penetration of offshore pipeline in soft clay seabed in deep water is presented in this study. The modeling was performed using ABAQUS finite element software. Soil was modeled in an Eulerian framework and the pipe in a Lagrangian framework. Strain softening behavior and strain rate effects on undrained shear strength of clay was incorporated in ABAQUS FE software using user subroutines written in FORTRAN. The variation of undrained shear strength with depth is also considered. The results are compared with centrifuge test results and also with available solutions.Copyright

Influence of Low Confining Pressure in Modeling of Lateral Pipeline/Soil Interaction in Dense Sand

Buried pipelines are extensively used for transporting water and hydrocarbons. Geohazards and associated ground movements represent a significant threat to pipeline integrity that may result in pipeline damage and potential failure. Safe, economic and reliable operation of pipeline transportation systems is the primary goal of the pipeline operators and regulatory agencies. The pipes are often buried at a shallow depth and therefore the behaviour of soil at low stress level need to be considered for proper modeling of the response of pipelines. In this study, finite element (FE) modeling of pipeline/soil interaction is presented, where the stress-stain behaviour of soil at low stress level is implemented. At first, triaxial test results are simulated to validate the proposed model and numerical techniques. Pipeline/soil interaction in plane strain condition is then simulated for lateral loading. The Arbitrary Lagrangian-Eulerian (ALE) method available in Abaqus/Explicit is used for FE modeling. One of the main advantages of this method is that it can simulate large deformation behaviour. The variation of non-dimensional lateral force with non-dimensional displacement is examined for different depth of embedment of pipeline and soil conditions. Finally, shear band formation in soil due to lateral movement of the pipe is presented.Copyright

Steel Catenary Risers at Touchdown Zone: A Fluid Dynamics Approach to Understanding the Water-Riser-Soil Interaction

The critical location for fatigue damage in Steel Catenary Riser’s (SCR’s) often occurs within the Touchdown Zone (TDZ), where cyclic interaction of the riser with the seabed takes place. Riser-fluid-soil interaction at the TDZ is a complex phenomenon. In this study, Computational Fluid Dynamics (CFD) technique is used to investigate the velocity field and suction forces during riser-fluid-soil interaction for a two-dimensional cross section of 0.10 m diameter SCR at TDZ. Numerical simulation shows that the suction forces at the bottom of the riser are high enough to pull the clay upward when it departs from the seabed during the heave action. The influence of suction and water on trench formation mechanism is discussed.Copyright

Impact Drag Forces on Pipelines Caused by Submarine Glide Blocks or Out-Runner Blocks

This paper discusses the forces resulting from the impact of an intact submarine landslide (the glide block or out-runner block region) on a suspended submarine pipeline. Eight physical experiments were conducted using the geotechnical centrifuge facility at C-CORE. In prototype scale, clay chunks equivalent to 12 m × 6 m × 4.5 m (l × w × h) were used to model the glide blocks or out-runner blocks. They had s u ranging from 4 to 7 kPa, and impact velocities ranging from 0.1 to 1.3 m/s. The clay blocks impacted the suspended pipes at a direction normal to the pipe axis. The diameters of the pipes were 0.19 and 0.29 m. Based on these experimental results, a method to estimate the impact drag force on a pipeline caused by a submarine glide block or out-runner block was developed.

Mitigation of Frost Heave of Chilled Gas Pipelines Using Temperature Cycling

A number of recent studies have been undertaken by the pipeline industry to improve understanding and design capability of frost heave issues related to the operation of large diameter chilled gas transmission pipelines through areas of discontinuous permafrost. These studies have included the assessment of methods of reducing the effects of frost heave on pipeline integrity, in particular the onset of significant bending strain due to differential vertical displacement of the pipeline. This paper will present one component of a study to investigate and assess the use of temperature cycling of the product as a means of reducing long-term frost heave of a pipeline system. The use of chillers at compressor station locations is considered a feasible method of controlling the gas temperature in the pipeline. The studies were undertaken using a geotechnical centrifuge as a cost effective means of obtaining physical test data under realistic soil stress conditions. The potential benefits of cyclic temperature operation in reducing pipeline strain are discussed.Copyright