Stability Analysis And Reliability Evaluation Of Deep Sea Top Tensioned Riser Under Multi-frequency Parametric Excitation

Top Tensioned Riser (TTR) is important equipment for offshore oil andgas exploration, its stability is of special importance. Instability excited byparametric excitation is a challenge for practical riser design, the normalexploration may be impacted, and even terrible economic loss orenvironmental pollution can be caused.It‘s critical to effectively predict the parametric excited instability, andto guide the engineering design to avoid the potential disaster. Generally,instability correlated to parametric excitation is investigated based on regularwave, namely single frequency harmonic excitation. However, the real seacondition is stochastic, the excitation on the offshore platform is random andmulti-frequency loading, leading to stochastic tension fluctuation. Thereforethe tension fluctuation assumed to single frequency excitation is impractical,the corresponding correctness would be influenced.In the meanwhile, because of the stochasticity of the sea condition, inaddition to the uncertainty of the structural geometric parameters andmaterial properties, the uncertainty of the design concerning parametricstability is inevitable. Therefore, the prediction based on traditionaldeterministic study method is significantly challenged by the stochasticity.What‘s more, Top Tensioned Riser, which is operated in real seacondition, is excited by both surface wave force and periodic vortexshedding force due to the current overflowing risers, parametric resonancemay be excited thanks to the former factor, while Vortex Induced Vibration(VIV) may occur due to the latter factor. But in real sea condition, instabilityof risers may be caused by both periodic excitations simultaneously,therefore it‘s essential to consider the dynamic stability of risers undercombined excitation. In order to solve the above mentioned problems, three aspects are goingto be developed to further investigate the stability of TTR operated instochastic sea condition:1. Hill‘s equation of a TTR system under multi-frequency excitation isderived based on linear-wave theory using a Pierson-Moskowitz wavespectrum. Analytic results are obtained for the stability limit of the relatedHill‘s equation excitation utilizing Bubnov-Galerkin Method. The features ofthese new limits are explored and compared with Mathieu-type stabilitylimits resulting from traditional single frequency excitation, and theshortcomings of the latter method are discussed. Parametric stability of TTRoperated in real sea condition is predicted based on Hill stability diagram,measures to suppress and eliminate the consequent instability are proposed.2. Considering the uncertainty of various parameters such as theenvironmental conditions of the load, the structural geometric parameters,and material properties. The effects of various random variables on theparametric instability are studied by a sensitivity analysis to find vital factors,influencing parametric instability significantly. A surrogate model isconstructed to model the relationship between the response and vitaluncertain factors, and to conduct the consequent reliability analysis. Thereliability of TTR parametric stability influenced by vital uncertain factors iscalculated, and the potential mechanism is explored. Guidance based onreliability theory is proposed to suppress or eliminate the instability.3. Considering simultaneous parametric and vortex excitation on TTR,the correlated dynamic equation is derived, and the consequent response iscalculated and compared with parametric resonance and VIV. The influenceof various parameters on the dynamic stability of TTR under combinedexcitation explored. The influence of vortex and parametric excitation on theconsequent instability is investigated. Measures to suppress and eliminatethe instability are discussed.