Application of the endurance time method for evaluation of jacket platforms

In this research, seismic performance of Endurance Time (ET) method is considered for assessment of jacket platforms. ET is a new method that can assess the extreme response of the structure at various seismic excitation levels with very low computational costs. For this aim, artificial records have been generated that gradually increase with time. They have been named Endurance Time Acceleration Functions (ETAFs). For determination of the seismic response of the jacket platforms, various nonlinearity such as buckling of the brace members, material nonlinearity, soil structure interaction and fluid structure interaction are important and can be a challenging issue for the ET approach. In this way, a real jacket platform located in the Persian Gulf is studied. Finite element method is utilized to prepare a three dimensional model of this platform with using ANSYS software. Moreover, various nonlinearity sources are considered in this model. Fluid structure interaction is included by using Morison equation that hydrodynamic added damping and added mass are considered by nonlinear drag force and inertia force, respectively. Soil–pile–structure interaction is also considered by near and far field soil effects. Near field soil is modeled by nonlinear spring and elastic solid elements are used to model far field effects. Material nonlinearity is considered by a standard bilinear stress-strain curve with 5% strain hardening and the von Mises yield criterion. Buckling of the brace members is also modeled by the initial imperfections at the mid-span of the braces, as recommended by previous studies. A methodology is also addressed for assessment of this type of offshore structures. For considering the accuracy and the reliability of this approach, the results of the ET method are compared with the typical time history method. In this regard, seven records are selected for soil type C from FEMA 440 and FEMA 695 and scaled to the ELE event such that their spectral accelerations match the ELE spectral acceleration at the main period of the platform. For other excitation levels, scale factors change proportional to the ELE ratio. For example, in this case, the ratio of the ALE spectral acceleration to the ELE one is 1.4; therefore, the scale factors of the ALE event are 1.4 times of the ELE one. ET records are scaled such that the response acceleration spectrum of the ETAF until target time becomes compatible with the ELE spectral acceleration. Initial studies recommend that 10 s is an appropriate target time. Due to linear increase of the excitation of the ET records, each time can be in accordance with the especial level of ELE, for example, 5s and 15s indicate 0.5 and 1.5 times of the ELE event. A comparison between the results of the earthquake records and the results of the ETAFs show that the ET method can accurately estimate engineering demand parameters such as maximum deck displacement, maximum base shear, maximum axial force in the leg and maximum axial force in the brace. The ET method is a vigorous approach that can be successfully estimated the seismic excitation of the buckling initiation. Moreover, the results indicate that despite significant decrease in the computational costs of the ET method, this approach can show appropriate performance.