Research On Fatigue Crack Growth Assessment Technique Of Offshore Platforms

For decades offshore platform plays an essential role in the discovery and exploration of natural oil and gas under the sea.Nowadays,offshore drilling platforms has developed in diverse forms of working units.The jacket platform and the semisubmersible platform are the most used offshore structure in China.Nevertheless,most of the platforms in use are operating beyond designed life span and detected defective.Fatigue has been the principal problem confronting the extension of service life.In recent years the fracture mechanics theory has been popular in the fatigue assessment of fatigue crack life,which is innovated and different from the conventional fatigue calculation and prediction on the occurrence of crack.The main purpose of this book is to establish new methods on assessing and evaluating fatigue cracks in jacket platform and semisubmersible which expired on designed life span or detected flawed.Moreover,this book will also study on the design of fatigue cracking experiment under current circumstances.The major contents are summarized as following,1)The assessment and evaluation method of fatigue crack based on the FAD(Failure Assessment Diagrams)for offshore platform structures is established.Aiming at the fatigue crack of jacket platform,the hot spot stress is analyzed.A more precise and appropriate calculation on hot spot stress is proposed based on ABS standard for the stress concentration factor of tubular joints.The hot spot stress range is extracted from the transfer function on various phases of wave followed by calculation of stress intensity factor.With wave scattered statistics,the FAD assessment curve is used as the failure evaluation standard in the crack growth cycle for calculating the fatigue crack growth life.2)An evaluation system for the fatigue crack growth is developed based on the practices on assessing the tubular joint in jacket platform.Several types of tubular joint are considered,such as X shape,Y shape,T shape,etc.The system will be divided into three functional modules: the first one will be used in the calculation of geometric parameters and crack parameters on the tubular joints,as well as the stress components for the crack evaluation and fatigue growth cycle;the second one is concentrated on evaluating the condition of crack in the tubular joint.Utilizing the stress response under a once-in-a-century wave condition as input along with crack parameters to calculate the assessment points for the FAD assessment,a preliminary result for the acceptability of fatigue crack is provided;the third module combines the inputs from the first two modules to calculate the ultimate fatigue growth life of crack in the offshore structure.3)In this chapter the assessment of fatigue crack growth on semi-submersible is performed.Finite element models of Global structure and local interconnection of semi-submersible with detected crack are created.Strength analysis on the cracked component under various load cases with designed wave method is carried out to confirm the stress response of the fatigue crack joint.Geometric linear extrapolation is employed to calculate structural hot spot stress.FAD assessment follows the previous calculation and ensures the acceptability and reliability of fatigue crack growth.Critical crack size and the fatigue life of crack growth are the results with the assessment process due.4)Sensitivity analysis of the FAD based assessment on fatigue crack growth in Offshore platform is studied in this chapter.Calculation of the linear fatigue cumulative damage ratio of the fatigue crack growth with the S-N curve is executed before comparing to the fatigue failure assessment of FAD.Several factors contributing to the fatigue crack growth such as original crack size,crack growth parameter,fatigue growth rate and seas state data are analyzed as well as the calculating efficiency and reliability.5)Focusing on the interconnection of column and brace of the semisubmersible platform,a specimen of the local structure is designed for the fatigue test under stochastic loading and complex aggressive environment.According to current capacity of equipment,the geometry of specimen and the load cycle for the test are proposed for the test.Considering the effects of corrosion,the crack growth rate under such circumstance is employed in the calculation.The potential critical load cases are provided under unique loading and stochastic loading in the fatigue crack growth.