## 1 Introduction  
Due to the self-weight, cargo weights, buoyancy forces, wave forces and other local loads, the ship hull girder is subject to alternating overall bending moment, and a hogging or sagging condition will result. Because of the overall bending of the hull girder, i.e., hogging or sagging, the structure members in the deck or the bottom will be subject to longitudinal compressive loads [1]. Among structural members, the stiffened plate is a basic structural member of ship hull structures which has a small addition of weight in the form of stiffeners, while generating a large increment in the strength of the structure [2]. The main role of this type of structural component on the deck and bottom is to resist lateral load and also in-plane compression. Ultimate strength is a critical and fundamental ability of a ship in ship structural design. The ultimate strength assessment of the stiffened plates has been investigated in many studies; experimentally, analytically and numerically. Experimentation has been the most reliable approach for investigating the behavior of structure members. [PERSON] et al. tested ultimate strength of a series of stiffened plates to investigate the influences of stiffener cross-section [3]. The nonlinear Finite Element Analysis (FEA)-based commercial program ANSYS was used for analyzing the ultimate strength and collapse behavior of stiffened plates subject to axial compression load, and comparison of the numerical results against experimental ones showed a good agreement [4]. [PERSON] et al. investigated, both experimentally and numerically, the behavior of stiffened plates under combined in-plane compression and lateral pressure, and the commercial program ABAQUS was employed for the nonlinear FEA to capture realistic behaviors of stiffened plates under combined loads [5]. By comparing the FEM results, [PERSON] analyzed the Flexural-torsional Buckling of Angle-bar Stiffened Plates [6; 7]. [PERSON] et al. carried out a parametric study on the buckling behavior of stiffened plates under compressive load using the nonlinear FEA-based commercial program ABAQUS [8]. [PERSON] et al. employed three numerical methods, namely, ANSYS nonlinear finite element method, DNV Panel Ultimate Limit State (PULS) method, and Maestro ALPS/ULSAP method, to analysis the ultimate strength of stiffened plates under combined biaxial compression and lateral pressure [9; 10]. To better understand the accuracy of common methods in simulating the collapse behavior and predicting the ultimate strength of ship structural members, a benchmark study was carried out by the ISSC'2000 committee VI.2 [11]. In the benchmark calculations, a variety of methods, including the analytical method, empirical formulae, nonlinear FEA and Idealized Structural Unit Method (ISUM) [12], were employed to analyze the ultimate strength of stiffened plates, and the results showed that the nonlinear FEA with large elastoplastic deflection was the most accurate method for progressive collapse analysis. [PERSON] and [PERSON] [13] showed that both welding residual stresses and initial geometrical imperfections reduce the compressive buckling and ultimate strength of plates. [PERSON] and [PERSON] et al. [14; 15] investigated the effect of residual stresses and initial imperfections on the ultimate strength of plates and stiffened plates, and concluded that welding-induced distortions and residual stresses residual stress were the major factors that reduce the structural strength. It is known that there are residual stresses in tension in areas close to the welds, further away from which there is a corresponding residual stress field in compression to maintain equilibrium. The most effective approaches, for dealing with the residual stress modelling for the ultimate strength assessment, are those by using direct prescribed pre-stresses by using a moving heat source, simulating the welding processes and obtaining the residual stresses [15].