كاربرد روش تحليل زمان دوام در طراحي و ارزيابي سازه هاي دريايي در مقابل نيروي امواج تصادفي

Application of Endurance Time Method in Design and Assessment of Offshore Structures Subjected to Irregular Waves.

در اين نوشتار، روشي نوين با عنوان تحليل موج دوام (EWA) و كاربرد آن در تحليل، طراحي و ارزيابي سكوهاي دريايي در برابر نيروي امواج تصادفي دريا معرفي شده است. اين روش الهام‌گرفته از رويكرد تحليل زمان دوام (ETA) است كه در مطالعات لرزه ‌يي ساختمان ها مورد استفاده مي گيرد. در روش تحليل موج دوام (EWA)، سازه در معرض توابع قطار موج افزايشي از پيش طراحي‌شده، كه به نحو مناسبي با شرايط امواج ساختگاه همپوشاني دارند، قرار مي گيرد. با شروع تحليل، مقادير شاخص خرابي از مرحله‌ي رفتار كشسان تا مرحله‌ي واژگوني كامل سازه، مستقيماً برحسب پارامتر زمان (و ارتفاع موج متناظر با آن) مورد مطالعه قرار مي گيرد. در اين نوشتار با ساخت 5 نمونه قطار موج افزايشي تصادفي و اعمال آنها به دو سكوي دريايي نوع جاكت، مفهوم روش تحليل موج دوام به تفصيل مورد بررسي قرار گرفته است.

In this paper a novel approach, called Endurance Wave Analysis (EWA), is established to non-linear dynamic analysis and assessment of offshore structures subjected to irregular wave loads. This is, in fact, an extension of the Endurance Time Analysis (ETA) that already presented by previous researchers for the seismic assessment of the onshore structures. With the EWA method, the offshore structure is simulated under a predefined Intensifying Wave Train Functions (IWTFs), corresponding to deteriorating sea states at a specific site. The functions are designed so that the roughness of the sea state represented by them increases over the time and even goes well beyond the design sea state. Using EWA, damage indices such as base shear, drift and allowable stress, or any other desirable parameters, from the linear elastic range of material behavior to permanent deformation or complete collapse level, can be studied directly in terms of the sea state roughness. Structural integrity can then be evaluated based on the EWA results. By definition, Endurance Wave Heights (EWHs) of a structure refers to waves that the structure would be stable against IWTFs according to any arbitrary damage index. Considering spectral features of the sea state, to incorporate waves of different heights and frequencies in a single dynamic analysis, to take into account the irregularity and randomness of the sea waves and requiring relatively short simulation times are among the advantages offered by EWA. It has been demonstrated that EWA, while utilizing a relatively simple methodology, provides a comprehensive insight into the behavior of complicated structural systems against wave actions which themselves are inherently random and complex. It produces detailed and valuable information which can be used for the assessment and design of structures subjected to wave loads. It has been also shown that the EWA approach can be integrated into a performance-based design procedure. For implementing a performance based assessment, it will be necessary to provide appropriate acceptance criteria for offshore structures in different performance levels such as IO, LS and CP. In this paper, the simple concept of the EWA is presented and the method is examined on offshore platform. Different irregular IWTF are generated and employed to evaluate the integrity of the two hypothetical two-dimensional steel jacket type platforms. The results clearly show that the proposed method is very effective for the dynamic non-linear analysis and assessment of offshore structures subjected to sea loads.