| Spar platform is one of the major equipments in deep sea petroleum's exploitation and the mooring ropes are the crucial part of the platform. This paper focuses on the fatigue assessment methods on spar platform mooring ropes. For the first time, the theory of random dynamic response spectrum is applied on this area and the exploratory studies of fatigue assessment on spar platform mooring ropes are carried. These studies could serve as reference in the deep sea rope design and have significant impact in platform safety area and the theory and technical developments of the deep sea platform.This paper summarized the related theories of fatigue analysis and the fatigue assessment methods on offshore platform structure, analyzes the current research on offshore fatigue assessment and deep-sea mooring ropes in the world.Based on the characteristics of recent oil and gas resources developments in South China Sea ("S7"), Norway Classification Society ("DNV") SESAM software is applied to set up three simplified classical spar - finite element Structure Model, Morison Model and Panel Model. The analysis of spar response, in which the wave force is calculated under diffraction theory, is carried out. Ultimate response has been analyzed on the basis of global response of platform. Consequently the transform function of ropes stress is acquired and the short-term and long-term response is predicted.The analysis for reliability and fatigue of mooring ropes based on the global response in frequency domain is made up with two methods, i.e. S-N curve approach and crack growth approach. In the fatigue analysis, suppose if the long-term stochastic load process is divided in to k short-term cases which are in Rayleigh distribution. The stress spectras of the short-term cases are determined on the basis of the transfer functions of the mooring ropes' stress. Apply the Miner model and the Wirsching method to calculate the fatigue reliability of the spar platform mooring ropes and comparing with the methods of fatigue assessment stated in the rule of major classification societies. In the calculation, the rainflow correction is taken in to account. Then the fatigue reliability analysis is carried out by establishing the fatigue crack growth rate mode and applying the Paris formula. Working out the relating fatigue assessment programs to calculate the fatigue life of mooring ropes under the S7 condition, the figures on fatigue life of integral Spar platform mooring ropes (till structure damage) and from first crack to the mooring ropes structured damage are given, which enable us to give a overall evaluation on the figures life of the mooring ropes.This paper presented the method of fatigue assessment and reliability analysis of spar platform mooring ropes structure by applying the random dynamic response spectrum theory. Combining the merits of time domain method and frequency domain method, considering the nonlinear factor of the mooring ropes, the fatigue life could be calculated efficiently and exactly by the method. And in the next studies, other load-factors including erosion shall be taken into consideration as well. |
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