## 1 Introduction
[40] also performed computational simulation of the evolution of wrinkling and its eventual localization using a finite element (FE) shell model, and observed that internal pressure tends to stabilize the structure and increase the wavelength of the wrinkles, while also shifting the initiation of buckling towards the plastic range of the material. Ref. [40] indicates that accurate simulation of the structural stability of tubular shells requires accurate characterization of the nonlinear inelastic properties of the material stress-strain behavior, including yield anisotropies, as well as adequate representation of initial geometric imperfections. Highlighting the significant effect of the material properties on the moment-curvature relationship, [PERSON]. [34] established that for pipes with \(D/t\) ratios less than 200, the mechanical and structural instabilities of the structure are strongly influenced by the interaction of the induced ovalization and the plastic and nonlinear characteristics of the material. This study focuses on the deformational capacity of high-strength API-X80 [41] grade pipelines subjected to various loading conditions. The continuous increase in world energy demand has compelled the hydrocarbon industry to venture into more remote and more environmentally hostile environments in search of fossil fuels. This predicament necessitates the construction and operation of pipelines with the best possible transport efficiency and at the lowest possible cost, therefore initiating a tendency towards using line pipes of larger diameter or maintaining higher operational pressures. High-strength steel grades provide the advantage of forming line pipes with high material yield strength, thus reducing the wall thickness requirement for internal (or external; in the case of deep to ultra-deep offshore environments) pressure containment [42]. [PERSON] and [PERSON] [43] explained that the peak moment strain, and invariably the deformational capacity of line pipes, is strongly influenced by the strain-hardening characteristics of the pipe material. In a numerical study of API-X100 line pipes subjected to axial compression or bending moment, [PERSON] et al. [44] pointed out that high-strength pipe steels tend to have inferior strain hardening properties, and consequently lower deformational capacity, compared to conventional pipeline steels of lower yield strength. However, results of a few experimental studies have shown that by carefully manipulating the thermomechanical control processes (TMCP) of slab reheating, rolling, and cooling during the manufacture of parent steel plates,the microstructural and mechanical properties of steel can be adjusted to produce line pipe steels with improved deformability, toughness, and resistance to strain-aging [45; 46; 47]. [PERSON] et al. [48] explained that the basic morphology of most high-strength pipeline steels is a multiphase structure composed of ferrite, bainite, and martensite. While bainite and martensite ingredients naturally improve the strength and hardness of line pipe steel, ductile and plastic deformation capacities tend to be compromised. Also, variability in manufacturing processes of parent steel plates makes it inevitable for line pipes of the same grade classification to exhibit slight differences in material properties [49; 50]. While manufacturing variability may be regarded as the primary cause of difference in the strain-hardening characteristics of materials with the same grade classification, other factors, such as strain aging, have also been observed to cause significant alteration of the material properties, leading to higher yield strength but lower deformational capacity [51]. Moreover, it has been established that the buckling response of pipes is highly sensitive to the material behavior, especially in the nonlinear range of the stress-strain relationship.