Andreas P. Christoforou

Stick-Slip and Bit-Bounce Interaction in Oil-Well Drillstrings

Drillstring vibrations and in particular stick-slip and bit-bounce are detrimental to oil-well drilling operations. Controlling these vibrations is essential because they may cause equipment failures and damage to the oil-well. A simple model that adequately captures the dynamics is used to simulate the effects of varying operating conditions on stick-slip and bit-bounce interactions. It is shown that the conditions at the bit/formation interface, such as bit speed and formation stiffness, are major factors in shaping the dynamic response. Due to the varying and uncertain nature of these conditions, simple operational guidelines or active rotary table control strategies are not sufficient to eliminate both stick-slip and bit-bounce. It is demonstrated that an additional active controller for the axial motion can be effective in suppressing both stick-slip and bit-bounce. It is anticipated that if the proposed approach is implemented, smooth drilling will be possible for a wide range of conditions.

Impact dynamics and damage in composite structures

A methodology for damage resistance and tolerance assessment of composite structures under low-velocity impact is presented. It is based on a priori knowledge of the impact response, using characterization curves that relate the pertinent impact parameters in normalized form, together with appropriate failure criteria. It is demonstrated that, for constant impact energy, the nature of the impact response (e.g., impact force vs impact duration) varies according to the impactor and structure characteristics, influencing the type of damage and the extent of damage degradation. This indicates that the force is also a key parameter in impact damage assessment. The methodology and results presented in the study will be valuable in designing model tests leading toward impact damage resistant and tolerant composite structures.

Characterization of impact in composite plates

The nature of impact response of composite plates is studied through normalization of the governing equations. The key parameters which govern the transition from locally dominated to globally dominated responses are identified and used to characterize the impact response. Numerical and experimental results show that the type of response can be predicted through a functional relationship of the normalized impact force and two nondimensional parameters.

On the impact between a rigid sphere and a thin composite laminate supported by a rigid substrate

Abstract The impact response of a thin composite laminate supported by a rigid substrate is addressed. An impact model based on a static contact law which incorporates damage effects is used in simulations to investigate the effect of impact parameters on the impact response. Damage is found to affect the response significantly. It is shown that the impact energy is not a sufficient parameter to describe impact where the impact parameters other than the impact velocity and the impactor mass are varying. The results show that the impact model presented in the current study completely describes the local impact response of composite laminates.

On the contact of a spherical indenter and a thin composite laminate

Abstract An analytical solution for the contact between a rigid sphere and a thin composite laminate supported on a rigid substrate is presented. The contact force-deformation, including damage effects, is obtained from an axisymmetric formulation of the contact problem. The material is assumed to be transversely isotropic and following an elastic-perfectly plastic stress-strain law. Damage is predicted using a maximum shear failure criterion. Experimental data previously reported in the literature agree well with the analysis.

An inverse solution for low-velocity impact in composite plates

Abstract An inverse model for estimating the mass and velocity of the impactor in low-velocity impact of composite plates is presented. In the model, a least-squares optimization technique is used to optimally reconstruct the force–time history by comparing direct model simulations of the impact response with experimental measurements. Guidelines for better initial guesses and faster direct models that are based on an impact characterization procedure are provided to the inverse algorithm. Three sets of experimental data covering a wide range of impact response are used to validate the model with excellent results.

Low-Velocity Impact Response of Structures With Local Plastic Deformation: Characterization and Scaling

This paper presents a methodology for the characterization and scaling of the response of structures having different shapes, sizes, and boundary conditions that are under impact by spherical objects. The objectives are to demonstrate the accuracy of a new bilinear contact law that accounts for permanent indentation in the contact zone, and to show the efficacy of a characterization diagram in the analysis and design of structures subject to impact. The characterization diagram shows the normalized functional relationship between the maximum impact force and three nondimensional parameters that cover the complete dynamic spectrum for low-velocity impact. The validity of using the bilinear elastoplastic contact law is demonstrated by both finite element (FE) and Rayleigh-Ritz discretization procedures for simply-supported plates. The efficacy of the characterization diagram, which was developed using simple structural models, is demonstrated by the FE simulations of more complicated and realistic structures and boundary conditions (clamped, stiffened plates, and cylindrical panels). All of the necessary parameters needed for the characterization are ‘measured’ using the FE models simulating real-world experiments. Impact parameters are varied to cover the complete dynamic spectrum with excellent results.

Impact Response of Adaptive Piezoelectric Laminated Plates

Semi-analytical models for the impact response of composite plates having distributed active and sensory piezoelectric layers are presented. An exact Ritz model is utilized based on a mixed-e eld piezoelectric laminate theory thatencompassesbothdisplacementandelectricdisplacemente eldsthroughthelaminate.Thegoverningequations of motion of a simply supported plate and a low-velocity impactor are formulated including impactor dynamics and contact law. State feedback linear quadratic regulator and output feedback controllers are implemented to actively reduce impact force. Numerical results quantify the electromechanical response of a composite plate with piezoelectric sensors impacted by three different mass impactors. The impact response of an actively controlled plate is also shown, and the possibility to improve critical impact parameters is demonstrated.

Low-energy impact of adaptive cylindrical piezoelectric–composite shells

Abstract A theoretical framework for analyzing low-energy impacts of laminated shells with active and sensory piezoelectric layers is presented, including impactor dynamics and contact law. The formulation encompasses a coupled piezoelectric shell theory mixing first order shear displacement assumptions and layerwise variation of electric potential. An exact in-plane Ritz solution for the impact of open cylindrical piezoelectric–composite shells is developed and solved numerically using an explicit time integration scheme. The active impact control problem of adaptive cylindrical shells with distributed curved piezoelectric actuators is addressed. The cases of optimized state feedback controllers and output feedback controllers using piezoelectric sensors are analyzed. Numerical results quantify the impact response of cylindrical shells of various curvatures including the signal of curved piezoelectric sensors. Additional numerical studies quantify the impact response of adaptive cylindrical panels and investigate the feasibility of actively reducing the impact force.

Normalized Impact Response and Damage in a Thin Composite Laminate Supported by a Rigid Substrate

The local impact response and damage of thin composite laminates are investigated. The impact model uses a static contact law which incorporates damage effects due to local contact stresses. A major difference between this contact law and the others used previously is that the damage is accounted for during the loading rather than during the unloading phase which is expected to yield a more reasonable prediction of the maximum impact force. Normalization of the governing differential equations and initial conditions yields a single governing parameter, which is a dimensionless group of all the pertinent impact parameters (i.e., impact velocity, impactor mass, material and geometric properties). The introduction of this parameter not only simplifies the problem but also provides the necessary scaling rules for model testing.