Bounded Elastic Moduli Multiplier Technique for Limit Loads: Part 1 - Theory

Statically admissible stress distributions are necessary to evaluate lower bound limit loads. Over the last three decades, several methods have been postulated to obtain these distributions using iterative elastic finite element analyses. Some of the pioneering techniques are the reduced modulus, r-node, elastic compensation, and linear matching methods, to mention a few. A new method, called the Bounded Elastic Moduli Multiplier Technique (BEMMT), is proposed and the theoretical underpinnings thereof are explained in this paper. BEMMT demonstrates greater robustness, more generality, and better stress distributions, consistently leading to lower-bound limit loads that are closer to elastoplastic finite element analysis estimates. BEMMT also questions the validity of the prevailing power law based stationary stress distributions. An accompanying research offers several case studies to validate this claim.

Bounded Elastic Moduli Multiplier Technique for Limit Loads: Part 2 - Case Studies

An accompanying paper provides the theoretical underpinnings of a new method to determine statically admissible stress distributions in a structure, called Bounded elastic moduli multiplier technique (BEMMT). It has been shown that, for textbook cases such as thick cylinder, beam, etc., the proposed method offers statically admissible stress distributions better than the power law and closer to elastic-plastic solutions. This paper offers several examples to demonstrate the robustness of this method. Upper and lower bound limit loads are calculated using iterative elastic analyses using both power law and BEMMT. These results are compared with the ones obtained from elastic-plastic FEA. Consistently BEMMT has outperformed power law when it comes to estimating lower bound limit loads.

Limit Loads of Bolted Flange Connections

High-pressure applications such as process piping, pressure vessels, risers, pipelines, and subsea production systems use bolted flange connections. Design of flanged joints may be done by design by rules and design by analysis. This paper presents a design by rules method applicable for flanges designed for face-to-face make-up. Limit loads are used to calculate the structural capacity (resistance) of the flanges, bolts, and metallic seal rings. Designers can use the calculation method to size bolted flange connections and calculate the structural capacity of existing bolted flange connections. Finite element analyses have been performed to verify the analytically based calculation method. The intention is to prepare for an ASME code case based on the calculation method presented in this paper.

Impact of Geometrical Imperfections on Estimation of Buckling and Limit Loads in a Silo Segment Using the Vibration Correlation Technique

The paper examines effectiveness of the vibration correlation technique which allows determining the buckling or limit loads by means of measured natural frequencies of structures. A steel silo segment with a corrugated wall, stiffened with cold-formed channel section columns was analysed. The investigations included numerical analyses of: linear buckling, dynamic eigenvalue and geometrically static non-linear problems. Both perfect and imperfect geometries were considered. Initial geometrical imperfections included first and second buckling and vibration mode shapes with three amplitudes. The vibration correlation technique proved to be useful in estimating limit or buckling loads. It was very efficient in the case of small and medium imperfection magnitudes. The significant deviations between the predicted and calculated buckling and limit loads occurred when large imperfections were considered.

Making the Case for the Use of Effective Tension in Casing Design to Eliminate Approximations and Wall Thickness Limitations

Existing plastic limit load equations for casing design include approximations that can result in overly conservative (and costly) well designs and ignore forces that may prove critical for assured integrity in complex operations. The use of effective tension in place of physical tension can simultaneously simplify casing design and eliminate existing approximations and wall thickness limitations. Effective tension has not yet been widely adopted because a rigorous derivation based on axiomatic mechanics and calculus does not exist in the current body of literature. This paper presents a thorough derivation for effective tension in terms of working stresses, with tensile, radial, and tangential stresses all receiving proper treatment. Unlike Barlow’s equation, which is routinely used for casing selection, the analytical result presented here allows fully plastic limit loads to be established for tubulars of any wall thickness, without introducing any approximations. Determining plastic limit loads is a crucial component of designing for well integrity, especially with complex operational loads where classical load equations are too conservative. The methods of transformation detailed in this paper facilitate the use of effective tension, which is often the more efficient primary tension load variable for pipe design, as opposed to physical tension. The rigorous model can also be used to derive strains as functions of loads, which is required for some design problems such as seal effectiveness. Additionally, the formulas can be used to dynamically simulate load changes and dimensional changes of pipe when it deforms, such as expandable casing, deformable ball seats, or when approaching rupture pressure. The elimination of approximations in the equations presented in this paper is significant in today’s well design climate, where integrity is demanded yet excessive cost is not tolerated. The derivation provides a foundation upon which existing casing design can be improved by eliminating wall thickness limitations and dependencies on approximations. This foundation will allow new design techniques to be developed that were not previously achievable due to the inefficiencies inherent in the use of physical tension.

Compliance with strength criteria for a model polymeric-composite grillage taking into account nonlinearity of the filler in different loading scenarios

This study compares different strength criteria in static loading simulation of a polymeric-composite grillage for different loading scenarios, boundary conditions and models of physical material behaviour. The paper discusses a detailed, structurally similar model of grillage made up by three-layered parts taking into account contact interaction (not the joint one) at the boundaries of bearing layers and the filler, with consideration of physically linear and nonlinear model of filler behaviour. The study applies volume-shell FE idealization. The loading (distributed and local)is simulated as per a step-wise procedure until the selected failure criteria are met, i.e. until limit loads (according to various hypotheses) are achieved. The paper gives examples of limit load calculations and their respective states of grillage for different variants of bearing circuit fastening and different loading types. The study yielded the fields of stress-strain parameters and the four principal complex failure criteria. The study discusses the effect of overall grillage compliance, as well as the effect of local (i.e. not affecting the compliant areas of the flooring) and distributed loading upon the limit state pattern of the structure and the level of its bearing capacity. It also estimates the effect of soft non-linearity of the filler upon limit stress-strain state pattern and limit load level, as well as upon the localization of «triggering» zones for non-dimensional criteria.