| With the increasing development of economy in our country, the demand offossil fuel is growing at an amazing speed. So, accelerating the offshore oil and gasresource should be seen as urgent affairs. As the prevailing structure form to exploreand exploit offshore oil, jacket platform in shallow water area and floating platform indeep ocean will accumulate fatigue damage under the influence of random wave, evenbe damaged gravely and cause serious consequences. Along with the augment ofservice lives of platforms and increasing aggravation of platform aging issue, fatiguedamages have become the main cause of damages happened on offshore structures.So, It is of vital importance to conduct fatigue assessment study for offshore structure.This article summarizes relative theories of fatigue analysis and common fatigueassessment methods. Besides, the domestic and overseas research status of fatigueanalysis method and influence of nonlinear effects on fatigue analysis are discussed.Research work about impact of nonlinear wave force on fatigue assessment foroffshore structure is set as concentration. The main research works and innovations ofthis paper are elucidated as follows:1) Spectral-based method, deterministic method and time-domain analysis methodare studied systematically in the treatise. In addition, Fatigue assessment forjacket platform is conducted and the fatigue lives calculated by different fatigueanalysis methods are compared and analyzed.. If the time-domain method resultsare treated as benchmark, the conclusion that deterministic method over-estimatesfatigue lives of structures and spectral-based method undervalue fatigue lives canbe drawn. At the same time, considering cycle counting correction factor canimprove the accuracy of fatigue analysis results in spectral-based method.2) Nonlinear wave theory is considered according to the application condition of Airy Wave and fifth-order Stokes wave in deterministic method. Then, the jacketfatigue lives are estimated. Moreover, the influence of replacing linear wavetheory by nonlinear theory to fatigue lives is studied and analyzed. Aftercomparing the results, if fifth-order Stokes wave theory is taken into account, theresults are20%larger than the verdicts based on Airy wave theory.3) Generally, linearized wave force spectrum is used to calculate wave force on thejacket leg. Nonlinear drag force spectrum is involved in fatigue calculation in thispaper.In addition, the equation of nonlinear wave force spectrum is deduced.Furthermore, the fatigue assessment for jacket platform is conducted with theconsideration of nonlinear wave force spectrum. The computation verdictsexpound that the fatigue lives are diminished about10%when correction term isconsidered.4) Mooring system of deep water platform is impacted by wave-frequency waveforce. Meanwhile, the influence of difference-frequency wave force is alsoconsiderable, which is often ignored in fatigue calculation. The influence ofsecond-order difference-frequency forces to mooring system of deepwaterplatform is studied in this treatise. First, the time-domain coupled analysis formooring lines of deepwater floating platform is conducted. Second, Based onprevious procedure, tension time-histories induced by difference-frequency waveforce, wave-frequency wave force and sum of the previous two part can berespectively acquired. Third, the fatigue lives can be solved by tensiontime-history, difference-frequency tension time-history and wave-frequencytension time-history. The research results prove that the damage severity causedby difference-frequency wave force occupies a important part in overall damageseverity and cannot be ignored. So, when the fatigue lives of mooring system offloating platforms are assessed, the impact of second-order difference-frequencywave force should be taken into consideration. |
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