Most of oil and gas offshore platforms are located in the seismic regains. Therefore, Seismic vulnerability evaluation of the offshore platforms is one of the essential vital issues in the structural systems. In this article, jacket type offshore platforms are examined by incorporating the pushover analyses and nonlinear time history analyses, in such a way that, first some push over analyses are performed to detect the more critical members of the jacket platform in consonance with the range of their plastic deformations. Subsequently, nonlinear time history analyses are performed, concentrating on the critical members, to examine the vulnerability of the jacket platform under intensive earthquake loads. Pursuant to the numerical results, the combination of the push over analyses and nonlinear time history analyses proposes a reliable and swift seismic assessment procedure to evaluate the seismic vulnerability of the offshore platforms. Moreover, seismic vulnerability of the offshore structures is dependent on the critical member locations and their load bearing situations in the offshore structures.

jacket-type offshore platforms play an important role in oil and gas industries in shallow and intermediate water depths such as Persian Gulf region. Such important structures need accurate considerations in analysis, design and assessment procedures. Incremental wave analysis for performance-based assessment of jacket platforms is established in 2011 to estimate different limit states and accurate behavior of jacket platform against environmental wave loading. IWA can be appropriate substitute to current pushover practice. The IWA can estimate the collapse-prevention limit state of jacket platforms properly. In addition, an appropriate parameter for ultimate capacity of offshore platforms is introduced which is called Collapse Wave Height (CWH). The CWH can be utilized as a substitute to RSR parameter in the design and assessment of jacket platforms. In this paper, IWA is explained. IWA curves, collapse-prevention limit state and CWH for case study platform is calculated. Effect of deck mass and its distribution on CWH is examined.

Due to the severe environmental condition, steel jacket type offshore platforms are highly vulnerable to damage; so, ensuring its proper performance and detection of probable damage is very important and undeniable. The assessment of existing offshore platforms is relatively a new process and has not yet been standardized as the design has. To do so, seismic assessment of the existing 4-legged steel jacket type offshore platform placed in the Persian Gulf (SPD12) is presented in this paper. Based on an actual platform structure and its mechanical model, the parameters which may affect the rate of shock absorption are analyzed, such as the condition of the site where the platform is located. Assessment is done by finding the seismic damage spread in the structure. Therefore, by using some documents such as FEMA-356 and ATC-40 developed for seismic assessment of buildings, seismic damage of the jacket-type platform is predicted with pushover analysis, and damage spread in this structure is predicted. This method could be very simple and detect damage spread precisely. As a result, knowing the weaker points of the structure which will be the first and third levels of the platforms similar to SPD12 in structural system can help to detect the damaged places, and by improving the capacity of the structure locally, the structural damage spread will be delayed.

Marine vessels may cause accidental damage to the offshore platforms during uploading oil and gas production in the storm conditions or incorrect navigation by ship’s captain. This impact of sea-going vessels is source of local or global deformations in the offshore structures. In this research the reliability indexes of damaged members under vessels impact are estimated for a platform in the phase 19 of the South Pars using the first-order reliability method (FORM). Only deformation of structural members are considered as damage impact and energy dissipation during collision is not addressed in this research. The time consuming Monte Carlo simulation Method (MCS) and Important Sampling (I. S. ) are used to verify the first-order reliability method because the limit state function in FORM is linear-zed by the Taylor series. At first, the wave force is applied into the safe structure, and reliability indexes are estimated for structural members. Then, in addition to the wave loads, the deformations caused by the assumed collision are also taken into account and structural reliability is estimated in the damaged state and corresponding results are compared with each other. Finally, combinations of wave and current, wave directions, and damages scenarios in chord, vertical and horizontal bracing are considered and results are compared.