Research On Fatigue Life Evaluation Of Drilling Ship’s Moonpool Structure

With the continuous development of science and technology,mankind’s demand for resources is increasing.As a new type of energy,the advantages of natural gas hydrate have gradually emerged.The natural gas hydrates on the earth are mainly distributed in the ocean.In order to meet the needs of energy extraction,natural gas hydrate drilling and production ships came into being.In the process of drilling and mining operations,the moonpool structure has the characteristics of stress concentration and is susceptible to complex loads,such as lowfrequency linear loads during self-propulsion and operation,and fluid sloshing loads in the moonpool caused by hull motion.This area is prone to fatigue damage and needs to be considered during the fatigue strength assessment process.This paper aims to use numerical simulation methods to study the load characteristics and structural response characteristics of the moonpool structure under the combined action of linear and non-linear factors,so as to carry out the fatigue strength evaluation of the moonpool structure.The main research content includes the following aspects:(1)Based on the design wave method for the moonpool area structure,the fatigue hot spots are screened.Shear force,vertical bending moment,torque and horizontal bending moment are selected as the main control load,the load response of the drilling and production ship under different wave directions and frequency ranges is calculated,and the relevant parameters corresponding to the response extreme values are selected as the design wave parameters.The wave load corresponding to the design wave is loaded on the finite element model of the structure,and the high stress area is selected as the fatigue hot spot,so as to carry out the mesh refinement work for the hot spot.(2)The method of calculating the hot spot stress time history under linear fatigue load is explained,and the irregular wave of the operating sea state is equivalent to the superposition of several regular waves.Based on the three-dimensional linear potential flow theory,by solving the stress response of the hot spot in the moon pool area under the action of a series of regular waves,the superposition idea is used to convert the result in the frequency domain into the stress response time of the hot spot under the action of irregular waves.(3)The calculation method of hot spot stress time history in the moon pool area under nonlinear sloshing load is studied.Through the research of CFD calculation method,the grid independence of wave simulation is verified,and the reasonable grid division size is determined.Based on two methods of equivalent method and decomposition combination method,a certain number of wave altimeters are arranged in the moon pond to study the fluid movement inside the moon pond,and certain measuring points are arranged to study the hydrodynamic pressure distribution on the side wall of the moon pond.By loading the finite element model of the moon pool area,the stress response time history of each hot spot is obtained based on the transient response analysis,and the results obtained by the two methods are compared.(4)The research on fatigue strength evaluation of moon pool structure is carried out.Based on the rain flow counting method,the stress range and cycle times of the structure under different loads are obtained,then the appropriate SN curve is selected,and the fatigue damage of the hot spots is obtained through the linear cumulative damage theory.The damage superimposition and the damage under the linear load and the nonlinear load are respectively used.The method of superposition of stress time history is evaluated,then the similarities and differences of the two methods are compared,and the fatigue life of the hot spot is finally obtained.